Construction Materials in civil engineering
March 12, 2017 | Author: Enes Karaaslan | Category: N/A
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CE 212 Engineering Materials 2009-2010
Introduction The structure of materials can be described on dimensional scales 1. The molecular level 2. Materials structural level 3. The engineering level
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1. The molecular level o o o o
Smallest scale (atoms, molecules or aggregation of molecules) Realm of materials science Particle sizes: 10-7 – 10-3 mm Examples: crystal structure of metals, cellulose molecules in timber, calcium silicate hydrates in hardened cement paste, variety of polymers
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o o
o
Atomic models used for description of the forms of physical structure (regular or disordered) Chemical and physical factors determine material properties o Chemical composition and/or the rate of chemical reactions determine material properties such as porosity, strength, durability, etc. Mathematical and geometrical models are employed to deduce the way materials behave
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2. Materials structural level – –
Up in size from the molecular level Material considered as a composite of different phases Phase I Phase II
Examples: Entities within the • Cells in timber material structure • Grains in metals • Concrete Deliberate mixing of • Asphalt disparate parts • Fiber composites • Masonry - regular composition • Concrete • Asphalt
particles such as aggregates distributed in a matrix such as hydrated cement or bitumen
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o Particle sizes : 5x10-3 mm (wood cell) - 225 mm (brick length) o Individual phases of the material can be recognized independently o More general information can be derived from examination of the individual phases of the material (Multiphase models allow prediction of material behavior)
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Three aspects must be considered while formulating the models; a) Geometry: Particulate or disperse phase scattered or arranged within the matrix or continuous phase model considers shape and size distribution and concentration b)
State and properties: Structure of material is affected from chemical and physical states of phases. Behavior of material is affected by properties of phases
c) Interfacial effects: Existence of interfaces between phases may introduce additional modes of behavior. (strength: failure of material being controlled by band strength at an interface)
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3. The engineering level o Total material is considered o Material is considered as continuous and homogenous o Average properties for the whole volume of material body o This level is recognized by construction practitioners
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Representative cell: minimum volume of the material that represents the entire material system Dimensions for cells: 10-3 mm for metals – 100 mm for concrete – 1000 mm for masonry Isotropic material: properties same for all directions – unit cell is a cube Anisotropic material: properties change with dimensions – unit cell is a parallel pipe Technical information on materials used in practice comes from tests on specimens of the total material Strength and failure tests Deformation tests Durability tests
provide technical information used in practice
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CONCRETE
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CONCRETE A composite of mineral particles (aggregates) distributed in a matrix of hardened cement paste (mixture of powder cement and water at the beginning) Versatile, comparatively cheap and energy efficient Great importance for all types of construction throughout the world Concrete is fresh and plastic at the beginning (throughout some time after mixing of constituent materials) Final properties of the hardened state of concrete have been gained slowly through time
Properties change with time
50-60 % of ultimate strength is developed in 7 days, 8085 % in 28 days
Increases in strength have been found in 30 year old concrete 11
History of concrete History of concrete is very old Mixtures of lime, sand and gravels have been found in Eastern Europe, in Egypt and in Ancient Greek and Roman times This dates from about 5000 BC
Romans; first concrete with a hydraulic cement (lime + volcanic ash from near Pozzuoli) Active silica and alumina in ash reacts chemically with lime Similar materials still known as pozzolona
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Roman structures; Foundations and columns of aqueducts
In arches of the Colosseum
In the dome of the Pantheon Fig. 1: www.wonderquest.com/fountain-octopus-enzyme.htm Fig. 2: http://www.artchive.com/artchive/r/roman/roman_colosseum.jpg Fig. 3: www.dolceroma.it/images/common/dove/pantheon.jpg
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In 1756, John Smeaton Mixture of burnt clay bearing limestone & Italian pozzolana for producing a suitable hydraulic cement to be used in construction of Eddystone Lighthouse
Picture ref: http://www.scienceandsociety.co.uk/results.asp?image=10307921 Extra info: http://en.wikipedia.org/wiki/Eddystone_Lighthouse
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In 1790, James Parker Patented “Roman cement” from calcareous clay burnt in a kiln and ground to a powder In 1824, Joseph Aspdin Patented “portland cement” an artificial mixture of lime and clay bearing materials used in repairs of Thames Tunnel in 1828 In 1890s improvement in kiln technology reduced the cost of Portland cement production. Then widespread production and use started worldwide
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CONSTITUENT MATERIALS OF CONCRETE Portland Cements Raw materials; Clay and calcareous stones Silica from clay + lime from calcareous stone (SiO2) (shale) (CaO) (Chalk or limestone) Al2O3, Fe2O3, MgO, K2O also exist in clay
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Cement manufacturing process; simple but involves high temperatures
Chalk + clay reduced to 75μm or less and mixed Blend fed into upper end of inclined long (up to 250m), 6m diameter rotating kiln which is heated to 1500°C at lower end Raw materials Fuel + air
20°C Drying
250°C
650°C
950°C
Preheating Calcining decomposition CaCO 3 of clay minerals CaO + CO 2
Fig. The processes taking place in a Portland cement kiln in the wet process
1250°C
1500°C
Clinker
Burning or clinkering combination of oxides to produce calcium silicates, calcium aluminates and calcium aluminoferrites 17
At 600°C, CaCO3 in chalk decomposes to give quicklime (CaO) and gaseous CO2 Fusion reactions start at 1200°C Calcium silicates, 2CaOSiO2 or 3CaOSiO2 Form as a result of these reactions Calcium aluminates, 3CaOAl2O3 Other oxides act as a flux Clinker particles (a few mm) emerge from kiln After cooling, 3-4% gypsum (CaSO42H2O) is added to clinker Mixture is ground to powder (2-80μm size), (300m2/kg specific surface)
www.cement.org/tech/cct_port_cem_prod_tech.asp
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Diagrammatic representation of the wet process of manufacture of cement
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Diagrammatic representation of the dry process of manufacture of cement
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Composition; Principle oxides in cement CaO (lime):C SiO2 (silica): S Al2O3 (alumina): A Fe2O3 (iron oxide): F Four main compounds (phases) formed in fusion process: Tricalcium silicate: 3CaO.SiO2 (C3S) Dicalcium silicate: 2CaO.SiO2 (C2S) Tricalcium aluminate: 3CaOAl2O3 (C3A) Tetracalcium aluminoferrite: 4CaOAl2O3Fe2O3 (C4AF)
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Each cement grain consists of an intimate mixture of these compounds. Direct chemical analysis is not possible to determine the amounts. Instead BOGUE formulas are used that were calculated from the results of oxide analysis.
If A/F > 0.64 % C3 S = 4.07C − 7.60S − 6.72A −1.43F − 2.85S
S = SO3
% C2S = 2.87S − 0.754C3S % C3 A = 2.65A −1.69F
% C4 AF = 3.04 F
If A/F ≤ 0.64 C3 S = 4.07C − 7.60S − 4.48A − 2.86F − 2.85S
C2 S = 2.87S − 0.754C3S
C2 F + C4 AF = 2.1A + 1.70F 22
The approximate range of oxide composition that can be expected for Portland Cements Compositions of Portland Cements Oxides (% by wt) CaO
Range 60-67
SiO2 Al2O3
17-25 3-8
Fe2O3 Na2O + K2O
0.5 – 6.0 0.2 – 1.3
MgO Free CaO
0.1 – 4.0 0–2
SO3
1–3
Principle oxides: CaO & SiO2
∼ 3 to 1by wt.
Principle oxides: C3S & C2S
∼ 75 – 80 % by wt. 23
Composition of cement depends on quality and proportions of raw materials (limestone and clay) Relatively small variations in oxide composition result in considerable changes in compound composition
Properties of compound cement constituents Rate of reaction
Cementing value Early
Final
Heat of hydration
Sulfate resistanc e
C3S
Medium
High
High
Medium
Medium
C2S
Slow
Medium
High
Low
High
C3A
Flash
Low
Low
Very high
Very slow
C4AF
High
Low
Low
High
Medium
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Typical Portland Cements Oxides (% by wt)
A
B
C
D
CaO
66
67
64
64
SiO2
21
21
22
23
Al2O3
7
5
7
4
Fe2O3
3
3
4
5
Free CaO
1
1
1
1
SO3
2
2
2
2
Potential compound composition (% by wt) C3S
48
65
31
42
C2S
24
11
40
34
C3A
13
8
12
2
C4AF
9
9
12
15
Typical or High-Early Low Heat average P.C Strength P.C P.C
Sulphate Resisting P.C
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Türk Çimento Standardları Standard No TS 3441 TS 19
İsim Portland Çimentosu Klinkeri Portland Çimentosu
TS 3646 TS 21
Erken Dayanımı Yüksek Çimento Beyaz Portland Çimentosu
TS 10157 TS 10158 TS 26 TS 20
Sülfatlara Dayanıklı Çimento Katkılı Çimento Traslı Çimento Yüksek Fırın Cüruflu Çimento
TS 809 TS 640 TS 22 TS 12139
Süper Sülfat Çimentosu Uçucu Küllü Çimento Harç Çimentosu Portland Cüruflu Çimento
TS 12140
Portland Kalkerli Çimento
TS 12141 TS 12142
Portland Silika Füme Çimento Kompoze Çimento
TS 12143
Portland Kompoze Çimento
TS 12144
Puzolanik Çimento
TS 23 TS 24
Çimento Numune Alma Metodları Çimentoların Fiziki ve Mekanik Deney Metodları Çimentonun Kimyevi Analiz Metodlar
TS 687
Notasyon PÇ 32.5 PÇ 42.5 PÇ 52.5 EYÇ 52.5 BPÇ 32.5 BPÇ 42.5 SDÇ 32.5 KÇ 32.5 TÇ 32.5 CÇ 32.5 CÇ 42.5 SSÇ 32.5 UKÇ 32.5 HÇ 16 PCÇ/A PCÇ/B PLÇ/A PLÇ/B PSFC KZÇ/A KZÇ/B PKÇ/A PKÇ/B PZÇ/A PZÇ/B
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ENV 197-1’e Göre Çimento İçinde Bulunabilecek Malzemeler (Materials in cement) Malzeme PÇ Klinkeri
Kısaltma K
Granüle Yüksek Fırın Cürufu
S
Doğal Puzolan Endüstriyel Puzolan Silisli Uçucu Kül
P Q V
Kireçli Uçucu Kül
W
Pişirilmiş ŞeyL
T
Kalker
L
Silis Dumanı
D
Minör İlave Bileşen Kalsiyum Sülfat Katkılar
F
Sınırlamalar C3S+C2S ≥ % 66.7 CaO/SiO2 ≥ 2.0 MgO ≤ % 5 Camsı faz miktarı ≥ % 66.7 CaO + SiO2 + MgO ≥ % 66.7 (CaO + MgO)/SiO2 > 1.0 Reaktif SiO2 ≥ % 25 KK ≤ % 5 Reaktif CaO ≤ % 5 Reaktif SiO2 ≥ % 25 % 5 ≤ Reaktif CaO ≤ % 15 Reaktif SiO2 ≥ % 25 KK ≤ % 5 Hacim Genl. < 10mm σ28 ≥ 25N/mm2 Hacim Genl. < 10mm CaCO2 ≥ % 75 Kil miktarı ≤ 1.2 g / 100g Organik Madde Miktarı ≤ % 0.2 Amorf SiO2 ≥ % 85 KK ≤ % 4 Özgül yüzey (BET) ≥ 15m2/g
% 5 ≤ Reaktif CaO ≤ % 15 Reaktif SiO2 ≥ % 25 KK ≤ % 5 Hacim Genl. < 10mm
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TS EN 197-1 Çimento Tipleri ve Kompozisyonları (Types of cement and compositions) Çimento Adı Tipi I
II
Notasyon K
Portland Çimentosu Portland Cüruf Çimentosu
I II/A-S II/B-S Portland Silis Dumanı Ç II/A-D II/A-P II/B-P Portland Puzolan Çimentosu II/A-Q II/B-Q II/A-V II/B-V Portland Uçucu Kül Çimentosu II/A-W II/B-W II/A-T Portland Pişirilmiş Şeyl Çimentosu II/B-T II/A-L Portland Kireçtaşı Çimentosu II/B-L II/A-M Portland Kompoze Çimento II/B-M
95-100 80-94 65-79 90-94 80-94 65-79 80-94 65-79 80-94 65-79 80-94 65-79 80-94 65-79 80-94 65-79 80-94
III/A III/B III/C IV/A IV/B V/A V/B
35-64 20-34 5-19 65-89 45-64 40-64 20-39
III
Yüksek Fırın Çimetosu
IV
Puzolanik Çimento
V
Kompoze Çimento
S
D
P
Q
6-20 21-35 -
6-10 -
6-20 21-35 -
6-20 21-35 -
65-79
V 6-20 21-35 6-20
W
T
L
MİB
6-20 21-35 -
6-20 21-35 -
-
0-5 0-5 0-5 0-5 0-5 0-5 0-5 0-5 0-5 0-5 0-5 0-5 0-5 0-5 0-5 0-5 0-5
6-20 21-35
21-35
36-65 66-80 81-95 18-30 31-50
-
-
-
11-35 36-55 18-30 31-50
-
-
-
0-5 0-5 0-5 0-5 0-5 0-5 0-5 0-5
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EN 197-1 The 27 products in the family of common cements
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Table cont.
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TS EN 197-1 Çimentolarında Aranan Kimyasal Koşullar Özellik Kızdırma Kaybı (%) Çözünmeyen Kalıntı (%) SO3 (%)
Cl (%)
Çimento Tipi CEM I CEM III CEM I CEM III CEM I CEM II CEM IV CEM V CEM III* Bütün tiplert
Dayanım Sınıfı Bütün Sınıflar
Sınır ≤5
Bütün Sınıflar
≤5
32.5, 32.5R, 42.5
≤ 3.5
42.5R, 52.5, 52.5R
≤ 4.0
Bütün Sınıflar Bütün Sınıflar
≤ 0.10
* CEM III/C %4.5’e kadar SO3 içerebilir. t CEM III % 0.1’in üstünde Cl- içerebilir. Bu durumda Cl- miktarı belirtilir.
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EN 197-1 Chemical Requirements given as characteristic values
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TS EN 197-1 Dayanım Sınıfları Dayanım Sınıfı 32.5 32.5R 42.5 42.5R 52.5 52.5R
2G ≥ 10 ≥ 10 ≥ 20 ≥ 20 ≥ 30
Basınç Dayanımı Sınırları (N/mm2) 7G 28G ≥ 16 ≥ 32.5, ≤ 52.5 ≥ 32.5, ≤ 52.5 ≥ 42.5, ≤ 62.5 ≥ 42.5, ≤ 62.5 ≥ 52.5 ≥ 52.5
EN 197-1 Mechanical requirements given as characteristics values
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Hydration
(Cement + Water) paste mixture
Fluidity or consistency remains constant for an initial period after mixing
Initial set (2-4 hours after mixing): Mix starts to stiffer, fluidity is lost at a faster rate
Final set (max 10 hours after mixing): Mix is completely stiff, hardening and strength gain starts
Rate of strength gain is fast for the first 1-2 days . It continues with a decreasing rate in time
initially fluid
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Hydration reactions are exothermic A: High but very short peak lasts only a few minutes Dormant period: cement is inactive (2-3 hours) B: Broad peak after final set C: Sharp peak (seldom) after one or two days
Rate of heat output (J/kg/sec)
A
B C
Dormant period
0.1
1.0
10
100
Time after mixing (hours, log scale) 35
Rate of Heat Evolution
Heat of Hydration Final set hydrolysis
I
C3S reacts
III
nucleation dissolution
II
IV diffusion control
V
Initial set
Time
Stage I
Rapid Heat Evolution
(12% 39
3) C4AF reacts similarly over same time scales Reaction products are similar to C3A products (has little effect on the overall cement behavior) 4) C3S and C2S react to form bulk of hydrated material after these initial reactions are completed. They are responsible for most of properties of hardened cement
2C 3 S + 6 H → C 3 S 2 H 3 + 3CH
(Reaction of C3S - faster)
Most of the main peak “B” is due to this reaction
2C 2 S + 4 H → C 3S 2 H 3 + CH
(Reaction of C3S - slower)
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FLY ASH (cured for 5 years)
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FLY ASH (cured for 5 years)
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Fracture surface of 24 hour old cement paste, showing C-S-H and ettringite
aggregate Transition zone
Bulk cement paste
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Calcium silicate hydrate (C-S-H) is responsible for strength and other properties
After 1 day CSH dominates, thus; Ca(OH)2 production enhanced
Typical hydration product development in Portland cement paste 44
Development of strength of pure compounds from Portland cement 45
Increasing temperature accelerates reactions of hydration. Reactions stop completely below -10ºC
Fresh cement and water
Initial set
Two or three days old paste
Mature paste
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Microstructural development during HYDRATION of cement 1) After mixing fresh cement particles dispersed throughout mix water as single grains or small flocs. Spacing depends on w/c 2) During dormant period, ettringite is formed at cement surface as sharp needles or rods. 3) At the end of dormant period, ettringite from adjacent cement particles has began to interfere and C-S-H with a spicular crumpled-foil form has started to appear. Solid layers of foil are a few molecules thick 4) During subsequent hydration, a dense continuous gel of C-SH is formed between particles, resulting in increasing strength. Also large hexagonal crystals of CH are formed. Some larger pores remain unfilled between grains, and fresh unhydrated cement is left in center of grains.
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Model – Hydration of cement paste
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Pore size distribution in 28-day-old hydrated cement paste 49
Structure of hardened cement paste
Residue of unhydrated cement, at center of original grains Hydrates, mainly calcium silicates (C-S-H); also calcium aluminates, sulphoaluminates and ferrites Crystals of calcium hydroxide (calcite) Unfilled spaces between cement grains, called capillary pores
C-S-H occupy about 75 % of volume of HCP C-S-H govern mechanical properties C-S-H structure: from poorly crystalline fibers to crumpled sheet-like network of colloidal scale Extremely high specific surface: 100-700 m2/g (~ 103 times higher than cement particles) Spaces between C-S-H particles: gel pores: ~ 0.5-5nm ~ 27 % of C-S-H weight Note: Don’t confuse gel pores with capillary pores (on the average about 2 orders of magnitude larger) 50
Strength of hardened cement paste Strength arise from Van der Waals forces between hydrate layers Quantitative estimate of unhydrated cement, hydrated gel and capillary pores was done by Powers in 1950s. Important futures of his model are: 1) Hydration takes place at constant volume Vcem+water = Vunh.cem+gel+cap.pores 2) Same gel is produced at all stages of hydration regardless of type of cement and water/cement ratio Constants are; a) Chemically combined water: ~23% by wt of cement b) Relative density of gel solids = 2.43 c) Relative density of gel+pores = 1.76
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Gel occupies (including the pores) a space about 1.8 times that of unhydrated cement For too small a space, hydration stops when products grow to fill this space (complete hydration never occurs) For too large a space, 100% hydration doesn’t fill the space (capillary pore) For hcp in water: at w/c = 0.38 → 100% hydration fills space completely and no capillaries form
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(a)
(b)
Composition of hydrated cement paste at the final stage of hydration after prolonged storage a) in water, b) sealed
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For sealed hcp, self-desiccation occurs at low w/c because of insufficiency of water, hydration stops before it is affected by lack of space for gel. Break-even point for w/c is 0.44. The curves show the final stage (100% hydration) which is rarely achieved. Therefore, hcp contains less cement gel and more unhydrated cement and capillaries than those shown in the figures Unhydrated cement, not detrimental to strength, results in self-healing property.
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Water in hardened cement paste
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Water vapour: in partially filled larger voids Capillary water:: in capillary pores; bulk water free from attractive forces of solid surfaces. In voids > 50nm (large capilleries) It is free water, and removal does not cause shrinkage In voids < 50nm (small capillaries) capillary tension forces dominate and removal of water may result in shrinkage Adsorbed water: On solid surfaces under influence of surface attractive forces up to 5 molecular layers (~ thickness of 1.3 nm) Lost on drying to 30% RH and this contributes mainly to shrinkage Interlayer water: In gel pores < 2.6nm under influence of two surfaces very strongly held. Lost on drying at elevated temperatures and/or to 10% of RH. Causes in considerable shrinkage (Van der Waals forces pull solid surfaces closer together) Chemically combined water: combined with fresh cement in hydration reactions. Not lost on drying. Heating to very high temperatures evolves this water through decomposition of paste. 56
ADMIXTURES Chemicals added immediately before or during mixing Significantly change fresh, early age or hardened properties to advantage Used in small quantities (1-2 % by wt of cement)
Plasticizers
Workability aids; ↑ fluidity or workability of concrete at same w/c Water reducers; ↓ w/c and thus ↑ strength and durability at same
workability
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1) 2)
Normal plasticizers; based on lignosulphonates or hydroxycarboxylic acids Superplasticizers; modified lignosulphonates or based on sulphonated melamine or naphtalene formaldehydes – Great increases in workability (flowing concrete) (segregation occurs if used with high doses of normal plasticizers or high water contents) – Great decreases in w/c (down to 0.2 at normal fluidity) and thus very large increases in strength (high-strength or highperformance concrete)
58
Plasticizers adsorbe on cement particle surfaces, giving slight negative charges to the surface and thus particles repel each other, breaking up any flocs and causing a better dispersion and wetting of particles This results in increased fluidity and slight increase in strength at same w/c ratio Plasticizers may cause retardation of setting time and also may entrain 1-2% air into concrete
59
Accelerators Increased rate of hardening and enhanced early strength May allow early removal of formwork May reduce curing time for concrete placed in cold weather CaCl2 is a popular accelerator. It may cause increased creep and shrinkage Prohibited in R.C and P.S.C due to corrosion of steel in presence of chloride ions
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Typical effects of calcium chloride admixture on (a) setting times, and (b) early strength of concrete 61
Retarders Delay setting time Counteracts accelerating effect of hot weather (especially for long transportation distances) Avoids cold joints and discontinuities by controlling setting in large pours Sucrase and citric acid and calcium lignosulphonate are examples
62
Air entraining agents Organic materials which entrain controlled amount of microscopic (less than 0.1mm) bubbles into cement paste of concrete Bubbles preserve stability during mixing, transporting, placing, compaction and setting and hardening Note; entrained air and entrapped air are different
Air entrainment is done for providing freeze-thaw resistance to concrete In winter time, water in capillary pores expands on freezing resulting in disruptive internal stresses. Successive cycles of freezing and thawing may lead to progressive deterioration. Entrained air, uniformly dispersed in hcp with a spacing factor of not more than 0.2mm, provide a reservoir for water to expand Entrained air volumes of 4-7% by vol. of concrete is required to provide effective protection. 63
Secondary effects Increase in workability due to lubricating affect of small air bubbles About 6% decrease in strength for each 1% of air. However, improvement in workability may allow to partially offset the loss in strength by reducing water content and thus w/c ratios
Air bubble
hydrophilic
Organic substances reduce surface tension of water and bubbles form during mixing Long chain molecules have hydrophilic and hydrophobic ends They align themselves radially on surface of air bubble with hydrophilic ends in water and hydrophobic ends in air. Thus they provide air stability
hydrophobic 64
Cement replacement materials (CRM) Mineral additives that partially replace portland cement Could be by-products from other industries (Economically advantageous) They enhance concrete properties in a variety of ways
Pozzolanic behavior A pozzolanic material is one which contains active silica (SiO2) and is not cementitious in itself but will, in a finely divided form and in presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form cementitious compounds Pozzolanic reaction (Secondary reaction) S + CH + H →
C - S - H
65
Types of Cement Replacement Materials 1.
Fly ash (pulverized fuel ash); ash from pulverized coal used to fire power stations
2.
Ground granulated blast furnace slag (ggbs); slag from scum formed in iron smelting in a blast furnace, ground to a powder
3.
Condensed silica fume; sometimes called microsilica; very fine particles of silica condensed from waste gases given off in production of silicon metal
4.
Natural pozzolans; some volcanic ashes
5.
Calcined clay and shale; clay and shale minerals heat treated
6.
Rice husk ash; ash from controlled burning of rice husks after rice grains have been separated
Dictionary definitions: Pulverize; to reduce to dust or powder, as by pounding or grinding Smelt ; to melt or fuse (ores) in order to separate the metallic constituents. Husk; the dry external covering of certain fruits or seeds
66
Typical composition and properties of cement replacement materials Oxide SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O Specific gravity, (gr/cm3) Particle size (μm) Specific Surface (m2/kg)
Fly ash Low lime High lime 48 40 27 18 9 8 2 4 3 20 1 4 -
Ggbs
Silica fume
PC
36 9 1 11 40 -
97 2 0.1 0.1 -
20 5 4 1 64 0.2 0.5
2.1
2.9
2.2
3.15
10-150
3-100
0.01-0.5
0.5-100
350
400
15000
350
67
High lime fly ash and ground granulated blast furnace slag are not true pozzolanas. They have certain self-cementing due to high CaO content. They may be used at high substitution rates (up to 90%) Low lime fly ash is used at most 40% replacement Silica fume is used at most 25% replacement (needs superplasticizer to maintain workability) Particles of artificial pozzolanas are smooth surfaced and spherical (Thus they improve workability)
68
Compressive Strength (N/mm2)
100% P.C
70% P.C + 30% F.A
w/c or w/(c+fa) = 0.47
Age (days)
Pozzolanic reaction and then early strength development is slow With silica fume, delay is much less due to high surface area and active silica content At later ages concretes with cement replacement materials exceed strength of Portland cement only concretes Slower pozzolanic reaction reduces porosity Pozzolanic reaction enhances transition zone between aggregate and cement paste
69
Aggregates Disadvantages of hardened cement paste (hcp); 1. Dimensional instability (high creep and shrinkage) 2. High cost Remedy to disadvantages; Put aggregates into cement paste → Produce concrete Aggregates occupy about 70-80% of total concrete volume
70
Objective; Use as much aggregate as possible Use largest possible aggregate size Use a continuous grading of particle sizes from sand to coarse stones Thus; Void content of aggregate mixture Amount of hcp required
Coarse agg. Minimized
Fine agg.
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Concrete composite models a) Two-phase model for describing deformation behavior Coarse aggregate dispersed in mortar matrix Coarse and fine aggregate dispersed in hcp matrix b) Three-phase model for considering cracking and strength Aggregates + hcp + transition or interfacial zone (∼ 50 μm) (cracking and failure starts from interfacial zone, the weakest phase)
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Types of aggregates 1. According to origin 2. Accroding to size 3. According to density or specific gravity 1. According to origin; a) Natural aggregates from natural sand and gravel deposits and crushed rocks b) Specifically manufactured aggregates such as fly ash pellets, granulated blast furnace slag 2. According to size; a) Fine aggregate; Particle size from 0 to 4 mm Ex; natural sand and crushed sand b) Coarse aggregate; Particle size from 4 to 16 or 32 mm Ex; gravel, crushed limestone
Dictionary definition: Pellet; small, rounded or spherical body
73
3. According to density or specific gravity; a) Normal density aggregates; natural aggregates, Examples; gravels, igneous rocks (basalt, granite), sedimentary rocks (limestone, sandstone) • Mineral composition is not that important • Specific gravities; 2.55 – 2.75 gr/cm3 • Concrete density; 2250 - 2450 kg/m3 • Gravels from deposits in river valleys or shallow coastal waters are directly used after washing and grading, particles are round • Bulk rock sources (granite, basalt, limestone) require crushing giving angular and sharp particles
Dictionary definitions: Igneous; produced under conditions involving intense heat, as rocks of volcanic origin or rocks crystallized from molten magma Sediment; mineral or organic matter deposited by water, air, or ice. 74
b) Lightweight aggregates; pumice (a naturally occurring volcanic rock), artificial lightweight aggregates (sintered fly ash, expanded clay or shale, foamed slag) • To produce lower density concretes (less than 2000 kg/m3) advantages; reduced self-weight, better thermal insulation • Reduced specific gravity (less than 2.0 gr/cm3) due to voids in particles • Reduced strength of concrete due to increased porosity c) Heavyweight aggregates; minerals like barytes to barium sulphate ore and steel shots • To produce high density concrete (3500 to 4500 kg/m3) (For nuclear radiation shielding)
Dictionary definition: Sintering; to form a coherent mass by heating without melting. Barytes; a white or colorless mineral (BaSO4); the main source of barium Ore; a mineral or natural product serving as a source of some nonmetallic substance
75
Properties of aggregates Grading or particle size distribution (Why do we need that?) Overall objective – To calculate suitable grading for good workability and stability (continuous grading → low void content)
How much sand? How much crushed stone?
76
Grading or particle size distribution (Sieve analysis)
– Aggregate samples dried, weighed and passed through a stack of the sieves • Sieve sizes in mm (0.25, 0.50, 1, 2, 4, 8, 16, 31.5) – Weight of aggregate retained on each sieve measured and converted to percentage retained and then to cumulative – Then plotted against the sieve size to obtain grading curve
Sieve Sieve Shaker
100 90 80 70 60 50 40 30 20 10 0
0.25
0.5
1
2
4
8
16
31.5
77
Grading curves Standards for aggregate define limits inside which the grading curves for coarse and fine aggregate must fall 100 90 80 70 60 50 40 30 20 10 0
0.25
0.5
1
2
4
8
16
31.5
78
Table. Outline of TS 706 Limits for Concrete Aggregates 1. Limits for Grading (with square opening) Sieve size 0-8 mm 0-16 mm (mm) A8 B8 C8 A16 B16 63 31.5 100 100 16 100 100 100 60 76 8 61 74 85 36 56 4 36 57 70 21 42 2 21 42 57 12 32 1 5 11 21 3 8 0.25 2. Limits for Quality (max. %) Property Deleterious 1. Clay lumps Substances 2. Soft particles 3. Coal and Lignites 4. Mud and clay Sulphate Soundness
1. With Na2SO4 2. With MgSO3
Abrasion
1. Los Angeles 2. Impact
C16
A32
0-31.5 mm B32
100 88 74 62 49 18
100 62 38 23 14 8 2
100 80 62 47 37 28 8
Fine Aggregate 1.00 1.00 3.00
C32 100 89 77 65 53 42 15
A63 100 61 46 30 19 11 6 2
0-63 mm B63 100 80 64 50 38 30 24 7
C63 100 90 80 70 59 49 39 14
Coarse Aggregate 0.25 5.00 1.00 0.50
15.00 22.00
18.00 27.00 50 45
4.00 Freeze and Thaw (DIN 4226) Organic Impurities: The aggregate may give yellow or lighter color in a 3% solution of, NaOH, but not dark colors
79
Example problem Determine mix proportions of sand and crushed stone such that fineness modulu of mixture will be 4.30.
Sieve size (mm) Material Passed (%) Sand (%) Crushed stone (%)
0.25 0.50 18 0
23 0
1
2
4
8
16
31.5
28 0
48 0
60 5
90 40
100 60
100 100
Fineness modulus; Sum of the cumulative percentages retained on the sieves of the standard series Fineness modulus ↑
coarser material
80
Fineness Modulus = -----------------------------------------------For sand = --------------------------------------For crushed stone = -----------------------------------
81
Mix proportions by vol: a (for sand), b (for crushed stone) Using law of simple mixtures; m1a + m2b = mm
Sieve size (mm) 0.25 Material Passed (%) Sand (0.63) 11.3 Crushed stone (0.37) 0 Mixture 11
a + b = 1 3.33 a + 5.95 b = 4.30
a = 0.63 b = 0.37
0.50
1
2
4
8
16
31.5
14.5 0 15
17.6 0 18
30.2 0 30
37.8 1.9 40
56.7 14.8 72
63 22.2 85
63 37 100
82
Limiting grading curves 100 90 80 70 60 50 40 30 20 10 0
0.25
0.5
1
2
4
8
16
31.5
83
Grading curves of sand and crushed stone 100 90 80 70 60 50 40 30 20 10 0
0.25
0.5
1
2
4
8
16
31.5
84
Grading curve of the mixture (sand+crushed s.) 100 90 80 70 60 50 40 30 20 10 0
0.25
0.5
1
2
4
8
16
31.5
85
In case of more than two aggregate fractions a +b + c =1
Can be extended to as many equation as number of size fractions which provides full conformity to the desired grading curve
Pi1 a + Pi 2 b + Pi 3 c = Pi m
Pj1 a + Pj2 b + Pj3 c = Pjm
Ideal (desired) grading curves TS 500 grading curve
⎛ di di +4 Pi = 20⎜ ⎜D Dmax ⎝ max Fuller parabola
⎞ ⎟ ⎟ ⎠
where; Pi = % passing from ith sieve di = opening size of ith sieve Dmax= Max particle size (sieve size through which 100% of aggregate passes)
di Pi = 100 Di 86
OTHER PROPERTIES OF AGGREGATES 1) Porosity and absorbtion Normal weight aggregates contain pores (typically 1-2 % by volume) Particles can absorb and hold water
Aggregate Moisture Conditions
Completely (oven dry) All pores empty
Absorb some of mix water in fresh concrete
Air dry Pores partially filled
Saturated surface dry Saturated or wet excess water All pores full but no excess water
Field conditions
Possible only in lab. conditions
Field conditions
No absorbtion and no addition
Add to mix water in fresh concrete
Absorb some of mix water in fresh concrete
87
Amount of water available for cement hydration, i.e. non-absorbed or free water is of prime importance Therefore, to ensure the required free water/cement ratio, it is necessary to allow for the aggregate moisture condition When calculating the amount of mix water; If aggregate is drier than SSD, extra water must be added If it is wetter, then less mix water is required
2) Elastic properties and strength Elastic properties of aggregates have major influence on elastic properties of concrete Strength of normal weight aggregates are higher than hcp and do not have major influence on strength of normal strength concrete In high-strength concrete (greater than 70-80 MPa), strength of aggregates and effect of transition zone between aggregate and hcp become seriously important
88
3) Surface characteristics Surface texture have greater influence on the flexural strength than on the compressive strength of the concrete (rougher texture results in a better adhesion) Surface cleanliness is also important for adhesion (surface should be kept clear of the materials such as mud, clay etc.) Better adhesion » stronger interface between aggregate and hcp Stronger interface zone » higher mechanical performance
89
Fresh state / early age properties of concrete Fresh state properties enormously affect hardened state properties Fresh concrete: from time of mixing to end of time concrete surface finished in its final location in the structure Operations: batching, mixing, transporting, placing, compacting, surface finishing Treatment (curing) of in-placed concrete 6-10 hours after casting (placing) and during first few days of hardening is important
90
Main properties of fresh concrete during mixing, transporting, placing and compacting •
Fluidity or consistency: capability of being handled and of flowing into formwork and around any reinforcement, with assistance of compacting equipment
•
Compactability: air entrapped during mixing and handling should be easily removed by compaction equipment, such as poker vibrators
•
Stability or cohesiveness: fresh concrete should remain homogenous and uniform. No segregation of cement paste from aggregates (especially coarse ones)
Fluidity & compactability known as workability Higher workability concretes are easier to place and handle but obtaining higher workability by increasing water content decreases strength and durability 91
Compaction of concrete
Finishing of concrete
92
Workability measurement methods 1. 2. 3. 4. 5.
Slump test Mini-slump test Compacting factor test Vebe test Flow table test
1. Slump test - simplest and crudest test (standardized in ASTM C 143
and EN 12350-2)
Fill concrete into frustum of Hand tap concrete a steel cone in three layers In each layer
Lift cone up Define slump as downward Movement of the concrete 93
Define slump as downward movement of the concrete Lift cone up Fig; http://www.arche.psu.edu/thinshells/module%20III/concrete_material_files/image002.gif Fig; http://myphliputil.pearsoncmg.com/media/nccer_carpentry_2/module03/fg03_00900.gif
94
True
Valid slump measurement 0-175 mm
Shear
Mixes having tendency to segregate – repeat test
Collapse
Slumps greater than 175 mm - self-leveling concrete
Consistency grade
Slump (mm)
Recommended method of compaction
Stiff, K1
0 - 60
Mechanical compaction like vibration
Plastic, K2
60 – 130
Mechanical or hand compaction (rodding, tampering)
Flowing, K3
130 – 200
Hand compaction or no compaction
Self compacting, K4
≥ 200
No compaction
95
2. Mini-slump test • Used for workability testing of cement pastes • Mini slump cone is a small version of slump cone • The cone is placed in the center of a piece of glass, paste is cast into cone and then the cone is lifted to measure the average spread of paste.
w/b : 0.2 sp:%2
w/b : 0.22 sp:%2
96
3. Compacting factor test (to distinguish between low slump mixes) Upper hopper 1. Concrete is placed in an upper hopper 2. Dropped into a lower hopper to bring it to a standard state and then allowed to fall into a standard cylinder. 3. The cylinder and concrete weighed (partially compacted weight) 5. The concrete is fully compacted, extra concrete added and then conrete and cylinder weighed again (fully compacted weight)
Compacting factor =
Lower hopper
approx. 1m
Cylinder
weight of partially compact concrete weight of fully compact concrete
97
4. Vebe test
1. A slump test is performed in a container 2. A clear perspex disc, free to move vertically, is lowered onto the concrete surface 3. Vibration at a standard rate is applied Vebe time is defined as the time taken to complete covering of the underside of the disc with concretecontainer 98
5. Flow table test
(to differentiate between high workability mixes) 1. A conical mould is used to produce a sample of concrete in the centre of a 700 mm square board, hinged along one edge 2. The free edge of the board is lifted against the stop and dropped 15 times Flow = final diameter of the concrete (mean of two measurements at right angles)
99
Correlations between compacting factor, Vebe time and slump
Some degree of correlation between the results exist, however the correlation is quite broad since each tests measures the response to different conditions
100
EARLY AGE PROPERTIES OF CONCRETE A) Behavior of fresh concrete after placing and compacting 1. Segregation and Bleeding From placing to final set, concrete is in a plastic, semi-fluid state Heavier particles (aggregates) have tendency to move down (SEGREGATION) Mix water has a tendency to move up (BLEEDING)
101
BLEEDING A layer of water (~ 2 % or more of total depth of concrete) accumulates on surface, later this water evaporates or re-absorbed into concrete
Water-rich pockets Mix water
Surface laitance
Cement and aggregates
Other effects of bleeding; Surface laitance; water rich concrete layer hydrating to a weak structure (not good for floor slabs that need to have hard wearing surface) Water-rich pockets; upward migrating water can be trapped under coarser aggregate particles causing loss of strength and local weakening in transition zone
102
2. Plastic settlement Cracks
Horizontal reinforcing bars may put restraint to overall settlement of concrete Then plastic settlement cracking can occur Vertical cracks form along line of the bars, penetrating from surface to bars
Plan
Reinforcing bars
Section
103
3. Plastic shrinkage
On an unprotected surface, bleed water evaporates. If rate of evaporation > rate of bleeding, then surface dries (water content reduces on surface) and plastic shrinkage (drying shrinkage in fresh concrete) will occur Restraint of walls of concrete causes tensile strains in near surface region Fresh concrete has almost zero tensile strength, thus, plastic shrinkage cracking results cracking is in fairly regular “crazing” form Plastic shrinkage cracking will be increased by greater evaporation rates of the surface water which occurs, i.e. with higher concrete or ambient temperatures, or if the concrete is exposed to wind 104
4) Methods of reducing segregation and bleed and their effects
CAUSES OF BLEEDİNG
REMEDIES
Poorly graded aggregate with a lack of fine material with particle size < 300µm
1. İncrease sand content 2. Air entrain concrete as substitute for fine materials
High workability mixes
Provide high workability with superplasticizers rather than high water contents
Use very fine materials such as silica fume
105
REMEDIES for PLASTIC SETTLEMENT or PLASTIC SHRINKAGE CRACKS
Revibrate surface region, particularly in large flat slabs
Apply good curing that stops moisture loss from surface as soon as after placing is possible and for first few days of hardening
106
B) Curing Curing; proctection of concrete from moisture loss from as soon after placing as possible, and for the first few days of hardening
Curing methods Spraying or ponding surface of concrete with water Protecting exposed surfaces from wind and sun by windbreaks and sunshades Covering surfaces with wet hessian and/or polythene sheets Applying a curing membrane, a spray-applied resin seal, to the exposed surface to prevent moisture loss
107
i) Effect of curing temperature
Hydration reactions between cement and water are temperaturedependent and rate of reaction increases with curing temperature At early ages rate of strength gain increases with curing temperature (higher temperatures increases rate of reaction, thus more C-S-H and gel is produced at earlier times, achieving a higher gel/space ratio and thus higher strength) At later ages, higher strength are obtained from concrete cured at lower temperatures (C-S-H gel is more rapidly produced at higher temperature and is less uniform and hence weaker than produced at lower temperatures)
Standard curing temperature is 22 ± 1 º C Hydration proceeds below 0 º C, stop completely at -10 º C
108
ii) Maturity Cement hydration depends on both time and temperature
Maturity = Σ t. (T + 10)
→ shows correlation with strength
T= -10 ºC is datum line At T= -10 ºC, hydration reactions stop, no maturity developed t (hours), T (ºC ) Useful in estimating strength of concrete in a structure from strength of laboratory samples cured at different temperatures
109
DEFORMATION OF CONCRETE Causes of deformation of concrete 1) Environmental effects e.g. moisture movement and heat 2) Applied stresses e.g. short and long term
110
The response of concrete to a compressive stress applied in a drying environment Load removal
Strain (contraction)
Load application
Elastic recovery
Creep strain
Creep recovery
Elastic strain Shrinkage strain t1
t2
Time
111
I → before t1→ net contraction in volume known as shrinkage due to drying, t1→ stress is applied and held constant, t2→ stress is removed (without stress it follows dotted extension beyond t1,difference between solid and dotted curves shows effect of loading)
II → on loading → immediate strain response (proportional to stress for low stress level)
III → Compressive strain increases at a decreasing rate, this increase, after allowing for shrinkage represents creep strain
IV → Upon unloading, immediate strain recovery is less than immediate strain on loading.
V → Time-dependent creep recovery
112
1) Drying shrinkage
Loss of capillary water, adsorbed water and interlayer water results in a net volumetric contraction called shrinkage Shrinkage is expressed as a linear strain through determination of length change
113
Swelling with continuous immersion
swelling
shrinkage Initial drying shrinkage
first drying drying wetting
drying wetting
drying wetting
reversible shrinkage
Time
Maximum shrinkage occurs on first drying, considerable part of this is irreversible There is a continuous but small swelling of the hcp on continuous immersion in water As the strength of the hcp increases → less shrinkage and swelling Low w/c; high degree of hydration; age - decreases porosity - increases strength - decreases shrinkage
114
Mechanisms of shrinkage and swelling 1)
Capillary tension
Free water surfaces in capillary pores will be in surface tension and upon drying due to drop in ambient vapor pressure free surface becomes more concave and surface tension increases
Kelvin’s equation
p0, (r = ∞) p, r
2T P ln( ) = Rθρ r P0 P0: vapor pressure over a plane surface T : surface tension of liquid R : gas constant θ : absolute temperature ρ : density of the liquid r : the radius of curvature
p1, r = d/2
d
115
p0, (r = ∞) p, r
Tension within water near meniscus = 2T/r which must be balanced by compressive stresses in surrounding solid.
p1, r = d/2
If evaporation ↑ compressive stresses ↑ shrinkage ↑ Exposing hcp to a steadily decreasing vapor pressure, pores gradually empty starting with the widest first
d
Higher w/c pastes will shrink more
116
2) Surface tension or surface energy Surface of both solid and liquid materials is in a state of tension To increase surface area, work should be done against this tension force Surface tension forces induce compressive stresses=2T/r which are important in hcp whose average particle size is very small Adsorption of water molecules onto the surface of the particles, reduces surface energy and reduces balancing internal compressive stresses leading to an overall volume increase, i.e. swelling
117
3) Disjoining pressure Adsorbed water layer
Capillary water
Capillary tension
Disjoining pressure in region of hindered adsorption
Water inside gel pores is under influence of surface forces 5 molecules (1.3nm) thick adsorbed water forms on solid surface at saturation which is under pressure from the surface attractive forces. In regions narrower than twice this thickness (~ 2.6nm) interlayer water will be in hindered adsorption resulting in development of swelling or disjoining pressure which is balanced by tension in interparticle bond On drying, thickness of adsorbed water layer drops and reduces disjoining pressure. This results in an overall shrinkage
118
4) Movement of interlayer water Interlayer water has an intimate contact with solid surfaces , to move interlayer water, high energy is needed Movement of interlayer water likely results in significantly higher shrinkage than movement of equal amount of free or adsorbed water
119
Opinion is divided on the relative importance of the above mechanisms and their relative contribution to the total shrinkage
Suggested shrinkage mechanisms Source Powers (1965) Ishai (1965) Feldman and Sereda (1970) Witmann (1968)
Relative humidity (%) 0 10 20 30 40 50 60 70 80 90 100 < ------------- capillary tension ----------->
< --capillary tension and surface energy -> < -------- disjoining pressure -------------->
120
1) Drying shrinkage of concrete a) Effect of mix constituents and proportions Drying shrinkage of concrete < drying shrinkage of cement paste
Aggregate → dimensionally stable Aggregates put restraint to shrinkage deformation of hcp in concrete Degree of restraint depends on; aggregate volume concentration modulus of elasticity of aggregate In hcp; unhydrated cement grains also act as a restraint
121
Influence of aggregate content in concrete on the ratio of the shrinkage of concrete to that of neat cement paste
Shrinkage ratio
Normal concretes have a shrinkage of 10-30 % of that of neat paste
Range for normal concrete
Aggregate content (% by volume)
122
Shrinkage after 1 year of drying (microstrain)
Normal density aggregates have higher stiffness (higher E – modulus) They give more restraint to concrete Lightweight aggregate concretes tend to have higher shrinkage
Lightweight concrete
Normal-weight concrete
Elastic modulus of aggregate (kN/mm2) 123
Combined effects of aggregate volume ratio and stiffness
ε c = ε p (1 − g ) n sh
sh
ε c = shrinkage strain of concrete sh
ε p = shrinkage strain of hcp sh
g = aggregate volume content
n=
3 (1 − μ p ) 1 + μ p + 2 (1 - 2 μ a )
Ep Ea
= 1. 2 − 1. 7
μ p = poissons ratio of hcp μ a = poissons ratio of aggregates
E p = modulus of elasticity of hcp E a = modulus of elasticity of aggregates
124
b) Effect of specimen geometry Size and shape of concrete specimen influence rate of drying and degree of restraint from the core, e.g. a member with a large surface area to volume ratio will dry and shrink more rapidly Restraint from central core of a concrete element which has higher moisture content than the surface puts the surface into tension. Thus, under these tensile stresses, surface cracking may occur C3A and sulphate content of cement affect the shrinkage, also, alkali content and fineness has a significant effect
125
2) Autogenous shrinkage
Continued hydration with an adequate supply leads to slight swelling of cement paste Conversely, with no moisture movement to or from the cement paste, self desiccation leads to removal of water from the capillary pores and
autogenous shrinkage
Its magnitude is an order of magnitude less than that of drying shrinkage More pronounced in concretes with low water-to-cement ratios
126
3) Carbonation shrinkage Carbonation shrinkage is not a result of loss of water, its cause is chemical
CO2 + H2O → H2CO3 H2CO3 + Ca (OH)2 → CaCO3 + 2H2O
H2O is → weight of paste ↑ released shrinkage accompanies, strength ↑ and permeability ↓
Explanation; Ca(OH)2 is dissolved from stressed region resulting in shrinkage CaCO3 crystallizes in pores reducing permeability and increasing strength Max carbonation at 25-50 % RH If saturated, H2CO3 can not penetrate concrete If dry H2CO3 can not form
127
Thermal Expansion Cement paste and concrete expand on heating Thermal expansion coefficient is needed in two main situations; 1. To calculate stresses due to thermal gradients arising from heat of hydration 2. To calculate overall dimensional changes in structures
Thermal expansion of cement paste Coefficient of thermal expansion of hcp=10 -20 x 10-6 / ºC The value depends on moisture content Disturbance of equilibrium between water vapor, free water, freely adsorbed water, water in areas of hindered adsorption and forces between layers of gel solids will determine the behavior of cement paste upon being heated.
128
Thermal expansion of concrete
Coefficient of thermal expansion of most rocks = 6 - 10 x 10-6 / ºC Therefore, coefficient of thermal expansion of concrete is less than that of hcp Since aggregate occupies 70- 80 % of concrete volume, effect of humidity is very much reduced, therefore, we assume a constant coefficient of thermal expansion for concrete. This value depends on concrete mix proportions, cement paste content and aggregate type. At temperatures higher than ~ 60 ºC, differential stresses set up by different thermal expansion coefficients of paste and aggregate can lead to internal microcracking
129
Stress-strain behavior Elasticity; hcp has a near linear compressive stress-strain relationship, modulus of elasticity can be determined from stressstrain data
E p = E g (1 - p c )3 E p = modulus of elasticity of paste E g = modulus of elasticity when Pc = 0
pc = capillary porosity
(represents the modulus of elasticity of the gel)
130
Elastic modulus, (kN/mm2)
Effect of w/c and age on the elastic modulus of hcp
30
20
Age (days) 60 28 14 7 3
10
0.20
0.30
0.40
0.50
0.60
water/cement ratio by weight
131
Models for concrete behavior Concrete is a composite multiphase material Elastic behavior depends on elastic properties, relative proportions and geometrical arrangement of; a) unhydrated cement, b) cement gel, c) water, d) fine aggregate, e) coarse aggregate The model for the concrete behavior requires the following 1. The property values for the phases a. The elastic modulus of the aggregate, Ea b. The elastic modulus of the hcp, Ep c. The volume concentration of the aggregate, g. 2. A suitable geometrical arrangement of the phases
Several models (unit cubes) are proposed to predict average behavior Assumption for model analysis; 1) Applied stress remains uniaxial and compressive throughout the model 2) Effects of lateral continuity between layers can be ignored 3) Any local bond failure or cracking does not contribute to deformation 132
Hansen’s models; Model I
Matrix
Aggregate
Matrix (hcp) and distributed phase (aggregates) arranged parallel with direction of loading. Phases undergo same strain Known as; Parallel-phase model Equal strain model
g
Strain compatibility; εc = εa = εp Equilibrium; Total force = Σ force on each phase Expressed in terms of stresses and area σ.1= σa.Va + σp.(1-Va) 133
Constitutive relations; both phases and concrete are elastic
σ c = ε c Ec
σ a = ε a Ea
σ p = ε p Ep
Substituting into equilibrium equation
ε c E c = ε a E aVa + ε p E p (1 − Va )
From compatibility equation
E c = E aVa + E p (1 − Va )
134
Model II σ Matrix (hcp) and distributed phase (aggregates) arranged in series with direction of loading Phases are subjected to same stress Known as; Series - phase model Equal stress model
g
Equilibrium;
σc = σ
a
= σ
p
Strains; Total displacement = Σ displacements in each of the phases expressed in terms of strain εc = ε a.Va + ε p. (1-Va) Substituting from constitutive and equilibrium equations and rearranging gives
1 Va (1 − Va ) = + Ec E a Ep 135
Counto’s model; Aggregate set within hcp complying with volume requirements Greater resemblance to concrete Combination of Hansen’s two models
(1 − Va ) Va 1 = + Ec Ep Ea Va + E p (1 − Va )
136
The effect of volume concentration of aggregate on elastic modulus of concrete calculated from simple two-phase models
All aggregate
All paste
Ec
Ea Model
A
Models A & B give upper and lower boundaries to concrete modulus of elasticity. Model C gives intermediate values.
C B
Ep 0
0.5
1.0
Volume concentration of aggregate, Va (g)
137
Prediction of elastic modulus of concrete (Ec) from the modulus of the cement paste (Ep) and the aggregate (Ea) for 50 % volume concentration of aggregate
5
Ec/Ep
4
Model A
3
C
2
B 1
1
5
10
Ea/Ep
138
Observed stress-strain behavior of concrete Aggregate
Stress-strain behavior of both aggregates and cement paste is substantially linear almost up to maximum
Stress, N/m2
40 30
Concrete Cement paste
20
Composite concrete with intermediate stiffness is markedly non-linear
10
0
1000
2000
3000
Strain (μs) Stress-strain relationships for cement paste, aggregates and concrete “typical behavior of hcp, aggregate and concrete”
139
First cycle Second cycle
Stress
Working stress
A
B
C
0
x
Unloading/loading cycles show substantial ,but diminishing hysteresis loops. Explanation lies in contribution of microcracking to overall concrete strains Transition zone is a region of weakness and ever before loading some microcracks occur in this zone. Bleeding, drying and thermal shrinkage determine the number and width of these cracks. As stress level increases, these cracks increase in number, length and width, causing progressively increasing nonlinear behavior
Strain
Stress-strain relationships for cement paste, aggregates and concrete “typical short term behavior of concrete” 140
Elastic modulus of concrete A
Stress
B
C
Different definitions for elastic modulus, -A: slope of tangent to the curve at any point (tangent modulus) -B: initial tangent modulus -C: slope of the line between the origin and a point on the curve (Secant modulus) According to the testing method, - static modulus of elasticity - dynamic modulus of elasticity
Strain
141
• •
Static modulus of elasticity: secant modulus is calculated from readings of strain at a stress at 40% of ultimate strength. Cylindrical or prismatic specimens are used and loaded longitudinally with a static load Dynamic modulus of elasticity: dynamic test is applied to a prismatic specimen and dynamic elastic modulus is calculated as 2 2 n: fundamental resonant frequency d l: length of specimen
E = 4n l ρ
ρ: density of concrete
Dynamic modulus of elasticity approximates to the initial tangent modulus (line B). It is higher than secant modulus
142
Modulus of elasticity increases with age and decreasing w/c. Thus, increasing compressive strength of concrete results in increased modulus of elasticity
Proposed relations for mean moduli;
Ed = 31 + 0.16 f cu
f cu (MPa),
E (GPa)
Ec = 20 + 0.2 f cu Ed= dynamic modulus of elasticity Ec= static modulus of elasticity fcu= ultimate compressive strength (28 day strength)
143
Poisson’s ratio for water-saturated cement paste, 0.25 ≤ μ ≤ 0.30 on drying it reduces to 0.2 μ is largely is dependent of w/c, age and strength Anson proposed the relation;
μ c = μ p (1 − Va ) for
n
μp = 0.22, n = 0.42
μc = poisson ratio of concrete μp = poission ratio of cement paste Va = volume of aggregates
144
Creep Magnitude of creep strains is as great or greater than elastic strains on loading. Therefore, they have a significant influence on structural behavior Creep and shrinkage are interdependent, creep is higher while the concrete is simultaneously drying
145
Strain
εsh: free shrinkage of unloaded
εsh
concrete in drying condition
(a) Free shrinkage (no stress)
εbc: basic creep under loading with no drying (sealed concrete)
εdc: drying creep under loading and
εbc (b)
drying condition
Basic creep (stress, no loss of moisture)
εdc (drying creep) εbc εsh Time
(c)
εcr
Total strain
Total creep strain; εcr= εbc + εdc Total strain; εtot= εsh + εcr = εsh + εbc+ εdc
Total creep stress and drying
146
Factors effecting creep 1. 2. 3. 4. 5.
Moisture content before loading: completely dried concrete has very small creep Level of applied stress: creep increases almost linearly with applied stress up to stress/strength of 0.40 to 0.60 Concrete strength: increasing strength decreases creep Temperature: increasing temperature increases creep significantly Aggregate volume concentration: aggregate is inert in creep. Hence, creep of concrete is less than that of cement paste
Neville proposed a relationship: εcr= εcrp (1-Va)n n = 1.7 – 2.1 6.
Elastic modulus of aggregate: increasing modulus of aggregate decreases creep of concrete
147
Mechanisms of creep 1)
Moisture diffusion: applied stress changes internal stresses and upsets the thermodynamic equilibrium in hcp. Moisture then moves from smaller to larger pores, resulting a. Pressure drop in capillary pores b. Adsorbed water gradually moving from zones of hindered adsorption c. Interlayer water diffusing slowly out of the gel pores
2)
Structural adjustment: stress concentrations in hcp may cause consolidation due to a. Viscous flow with adjacent particles sliding past each other b. Local bond breakage followed by reconnection nearby
3)
Microcracking: defects and cracks existing in concrete before loading propagate and form new cracks and this contributes to creep strains
4)
Delayed elastic strain: active creeping component (water in capillary or gel pores) act parallel with elastically deforming component (unhydrated cement particles, gel particles, calcium hydroxide crystalls) Load will be transferred to inert material which then deforms elastically
148
Overall Outline • • • • • • • •
Introduction Concrete Bituminous materials Masonry Polymers and polymer composites Cement-based fiber composites Metals Timber 149
Chapter Outline • CONCRETE – Cement – Admixtures – Aggregates – Strength and failure of concrete – Durability of concrete – Statistical quality control in the production of concrete – Property composition relations for concrete and concrete mix design
150
STRENGTH AND FAİLURE OF CONCRETE 1)
2)
Strength tests a) Compressive strength test b) Tensile tests i. Direct tension test ii. Flexural test iii. Splitting tensile test Factors influencing strength a) Transition zone b) Water to cement ratio c) d) e)
3)
i.
Prediction of concrete strength
Effect of age Effect of humidity Effect of aggregate properties, size and volume concentration Cracking and fracture in concrete
151
STRENGTH AND FAILURE OF CONCRETE Strength of concrete is most important because structural elements must carry imposed loads safely In loading test: max stress = strength of specimen Under compressive loading, at max stress, test specimen is still whole (with extensive internal cracking). Complete breakdown subsequently occurs at higher strains and lower stress
Also, strength is related to: elastic modulus, durability (permeability) Different types of loading in structures result in different modes of failure Relevant strengths: Compressive, tensile, torsional (shear), fatigue, impact, strength under multiaxial loading
http://www.ame-geolab.com/CRIB%20MATERIAL%20PHOTOGRAPHS%20testing%20013.jpg
152
1. Strength tests a) Compressive strength test: Cubes (100x100x100 mm or 150x150x150 mm) Cylinder: (D= 100mm, H= 200mm or D=150mm, H= 300mm) Steel moulds are used, concrete compacted top surface smoothed Steel Platens
1. Cubes or cylinder are placed between steel platens of loading machine 2. Load is increased to its ultimate value in a few minutes
- Cubes are loaded on two parallel smooth steel moulded surfaces - Cylinders are loaded on top and bottom surfaces. Top surface is a trowelled surface. Therefore it must be capped either with plaster paste or with molten sulfur
153
Cracking patterns during testing of concrete specimens in compression Cracking at approx. 45° to axis near ends
Cube (H/D=1)
Cracking pattern of cubes and cylinders show that CUBES are not in uniaxial loading This is due to end restraint because of friction between loading platens and concrete surface. This induces lateral tensile stress in both platen and concrete due to Poisson effect. Concrete is in a triaxial stress state with consequent higher strength compared to uniaxial stress case IN CYLINDERS, there is a centre portion not affected from end restraint. Thus, this central zone is in uniaxial stress state. Therefore,
cylinder strength ≅ 0.80 cube strength
Cylinder (H/D=2) Cracking parallel to loads away from ends
154
Slenderness ratio (h/d) affects strength Avoid h/d < 1 for test specimens h/d = 1 for cubes, h/d = 2 for cylinders
1.8 1.6
1.4 Relative str.
Height
1.2
diameter
1.0 0.8 0
1
2
3
4
Height/diameter ratio General relationship between height/diameter ration and compressive strength of a concrete cylinder, (Gonnerman, 1925) 155
b. Tensile tests Tensile behavior of concrete can be evaluated by; i. Direct tension test ii. Flexural test iii. Splitting tensile test
P
i. Direct tension test Results are not very dependable (due to eccentric loading and failure at grips) not very much used fd = P/A
P
Cross-sectional Area A 156
ii) Flexural test
End area = bxd
P
fb
L
Prism specimen (100x100x400mm or 150x150x600mm)
+ Stress distribution
PL Modulus of rupture = f b = bd 2
Load is applied at third points Failure occurs when flexural tensile crack at bottom of beam propagates upwards throughout beam Modulus of rupture > direct tensile strength 157
iii) Splitting tensile test tension compression
d
fs =
2P πdl
Stress distribution
Cylinder specimen (φ150mm, height 300mm) Placed on its side in a compression testing machine and loaded across its vertical diameter Stress distribution on a plane of vertical diameter is a near uniform tension Failure occurs by split or crack along this plane
For the same compressive strength: Modulus of rupture > cylinder splitting >
direct tension 158
Tensile strength (N/mm2)
Typical relationships between tensile and compressive strengths of concrete
8 Modulus of rupture
6
Cylinder splitting
4 2
0
Direct tension
10
20
30
40
50
60
70
Compressive strength (N/mm2)
159
STRENGTH AND FAİLURE OF CONCRETE 1)
2)
Strength tests a) Compressive strength test b) Tensile tests i. Direct tension test ii. Flexural test iii. Splitting tensile test Factors influencing strength a) Transition zone b) Water to cement ratio c) d) e)
3)
i.
Prediction of concrete strength
Effect of age Effect of humidity Effect of aggregate properties, size and volume concentration Cracking and fracture in concrete
160
2. FACTORS INFLUENCING STRENGTH a) Transition Zone
Transition zone around aggregate particles
hcp Aggregate
Transition zone (~50μm wide) is the weakest phase, cracking and failure initiate in this zone Due to drying shrinkage, cracks are present before loading As loading increases in compression or tension, cracks in this zone start propagating into hcp, resulting in paths through concrete
Typical crack path through normal strength concrete
Use 3-phase model when considering strength of concrete 161
b) Water/cement ratio: Strength of concrete depends on; Strength of hcp Strength of aggregates Strength of transition zone Strength of hcp is governed by Porosity (indirectly depending on w/c) degree of hydration Strength of transition zone is dependent on; w/c
162
i) Prediction of Concrete Strength Various relationships are given by several researchers
Abrams; f c =
Graf;
k1 k2W / C
f cc fc = KG (W / C ) 2
when w/c (by wt) k1 & k2 empirical constants depending on age, curing regime, type of cement, entrained air, test method, aggregate type and size where fcc = compressive strength of cement KG = empirical constant related to testing condition KG = 4-8, average = 6
163
Feret; c fc = K F ( )2 c+w+v
Bolomey;
C fc = K B ( − k `) W +v
when c, w, v → volumes of cement, water, and air voids in 1cm3 of concrete KF = empirical constant depending on age, type and amount of cement KF = 80 – 300 MPa for 7 days strength KF = 150 MPa for 28 days strength KF = 180 MPa
where c, w → weights of cement and water v → volume of air voids KB = empirical constant depending on age, type and amount of cement KB = 7 – 35 MPa for 7 days strength KB = 15 MPa for 28 days strength KB = 19 MPa k`=0.5 164
Mix design methods use one of these formulas or curves or tables based on these equations to estimate w/c required for a given strength To achieve a homogenous, cohesive concrete without significant segregation w/c < 1 0.4 < w/c < 0.7 → produce concretes of normal to medium strength (20-50 MPa) w/c ≈ 0.20 – 0.30 → high or very high strength concretes are produced (70 - 150 MPa) By using superplasticizer to achieve adequate workability By incorporating silica fume at 5-10 % cement replacement to improve properties of transition zone while also increasing strength of cement paste to a limited degree By selecting aggregate having high inherent strength and good bond characteristics With this approach, at w/c ≤ 0.26, compressive strengths > 130 MPa are achieved and placed with conventional mixing, transporting and compaction methods, but with extreme care given to high standards of site practice and supervision and quality control
165
c) Effect of age Hydration reaction continues in time with decreasing rate Thus even after years, in presence of moisture, there will be some strength increase
d) Effect of humidity
Curing in water results in higher strength compared to air curing Moisture all through the life of concrete provides higher strengths
166
e) Effect of aggregate properties, size and volume concentration Aggregate strength becomes important in high strength concretes. WHY?? With some carbonate and siliceous aggregates, structure and chemistry of the transition zone is influenced by aggregate mineralogy and surface conditions. Increased surface roughness improves bonding due to mechanical interlocking. Thus concretes with crushed rock aggregates have about 15 % higher strengths than concretes with smooth gravel aggregates Larger maximum aggregate size reduces strength at lower w/c. In high strength concretes, Dmax is limited to 8 or 16 mm. WHY??? Increasing volumetric proportion of aggregate in the mix, at a constant w/c, produce a relatively small increase in concrete strength. There is a maximum limit to aggregate content (~ 80) for practical concretes
167
3. CRACKING AND FRACTURE IN CONCRETE
Stress (% ultimate)
100 Stage 4 75 Stage 3 50 30
Stage 2
Transverse Longitudinal Volumetric
Stage 1 Strain
Stress – strain behavior of concrete under compressive loading: a) from Glucklich (1965); b) From Newman (1966).
168
STRENGTH AND FAİLURE OF CONCRETE 1)
2)
Strength tests a) Compressive strength test b) Tensile tests i. Direct tension test ii. Flexural test iii. Splitting tensile test Factors influencing strength a) Transition zone b) Water to cement ratio c) d) e)
3)
i.
Prediction of concrete strength
Effect of age Effect of humidity Effect of aggregate properties, size and volume concentration Cracking and fracture in concrete
169
Stage 1: Below 30% of ultimate load transition zone cracks remain stable, stress-strain curve is approximately linear Stage 2: Between 30-50% of ultimate load. Cracks increase in length, width and number. However, they still remain stable, nonlinearity is observed. Stage 3: Above 50% of ultimate load, cracks start to spread into matrix, towards 75% of ultimate load, cracks become unstable and curve further deviates from linearity Stage 4: Unstable crack growth and propagation is frequent, leading to high strains. Transverse strains start increasing faster than longitudinal strains resulting in an overall increase in volume
170
Stress
Coarse aggregate
Mortar
hcp
Concrete
Strain
Typical stress-strain characteristics of aggregate, hardened cement paste, mortar and concrete under compressive loading
171
Chapter Outline • CONCRETE – Cement – Admixtures – Aggregates – Strength and failure of concrete – Durability of concrete – Statistical quality control in the production of concrete – Property composition relations for concrete and concrete mix design
172
Durability of concrete Outline • • •
•
Transport mechanisms through concrete Flow processes Degradation of concrete 1) Durability against freezing and thawing 2) Durability against chemical action 3) Durability against very high temperatures Durability of steel in concrete – –
Principles of corrosion of steel in concrete Two stages of corrosion damage • •
Carbonation induced corrosion Chloride induced corrosion
173
Durability of concrete Durability: ability of a material to remain serviceable for at least the required lifetime of the structure Concrete is not inherently of high durability Degradation of concrete arises from;
Environment to which concrete is exposed Internal causes within concrete
Most important factor is the rate at which moisture, air or other aggressive agents can penetrate the concrete
174
Transport mechanisms through concrete
Porous impermeable material
Porous permeable material
High porosity, low permeability Low porosity, high permeability
Hcp and concrete contains pores of varying types and sizes. Rate of flow (permeability) depends on not only porosity but the degree of continuity of pores and their size 175
Flow process depend on degree of saturation of hcp or concrete At very low humidities: moisture is in vapor state and adsorbed on dry surfaces of the paste
with increasing humidity; adsorption is completed. Flow is taking place through the pore as a direct vapor movement due to pressure gradient
Vapour flow
Liquid flow
Adsorbed phase 176
At humidity sufficient for water condensation: vapor flow is enclosed through a shorter path
At increasing humidity: condensed water zone extend and flow is augmented by transfer in adsorbed layer
177
At increasing humidity: liquid flow under pressure gradient in incompletely saturated state
Finally at high humidities: liquid flow under pressure gradient in completely saturated state
178
Flow processes Movement of a fluid under a pressure differential – i.e. permeation Movement of ions, atoms or molecules under a concentration gradient, i.e. diffusion Capillary attraction of a liquid into empty or partially empty pores, i.e. sorption
179
Flow processes 1) Flow or movement of a fluid under a pressure differential Darcy’s law
∂h ux = − K ∂x
For flow in x direction, ux = mean flow velocity
∂h ∂x
= rate of increase in pressure head in x-direction
K = coefficient of permeability (m/sec)
180
2) Movement of ions, atoms or molecules under concentration gradient, process of diffusion governed by Fick’s law
∂c P = −D ∂x For flow in x direction,
P = transfer rate of substance per unit area normal to x-direction
∂c = concentration gradient ∂x D=diffusivity (m2/sec)
181
3) Adsorption and absorption of a liquid into empty or partially empty pores by capillary attraction
x = St 1 / 2 where, x = depth of penetration S= sorptivity (mm/sec1/2) t = time (sec)
182
Primary transport mechanisms in the various exposure zones of a concrete offshore structure
Exposure zone
Primary transport mechanisms
Atmospheric
Gas diffusion Water vapour diffusion
Splash
Water vapour diffusion ionic diffusion
Tidal
Water vapour diffusion Water absorption ionic diffusion
Submerged
Ionic diffusion Water permeability
183
Coefficient of permeability (x10-13m/sec)
Permeability; measure of the ability of a material to transmit fluids
Coefficient of permeability
10-6
10-8
10-10
10-12 0
10
20
30
Age (days) The effect of hydration on the permeability of cement paste (w/c =0.7)
10 9 8 7 6 5 4 3 2 1 0
10
20
30
40
Capillary porosity (%) The relationship between permeability and capillary porosity of hardened cement paste 184
Permeability (x 10-13 m/sec)
10 8 6 4 2
0.3
0.5
0.7
Water/cement ratio
The relationship between permeability and water/cement ratio of mature cement paste (% 93 hydrated) 185
Degradation of Concrete 1)
Durability against freezing and thawing (important in cold climates) Damage occurs due to;
freezing and thawing cycles of water in capillary pores and entrapped air voids of the cement paste water in the pores of aggregates may also freeze and affect the durability of concrete against frost action water in the gel pores is adsorbed on CSH surfaces and does not freeze until temperature drops to about -78 ºC. After capillary water has frozen, it posses a lower thermodynamic energy than still-liquid gel water, which therefore tends to migrate to supplement capillary water and thus increase disruption
Water – cement ratio is a controlling factor of durability of concrete against freezing and thawing cycles because its magnitude determines the amount and size of the capillary pores in the cement paste. For this reason, water-cement ratio is limited in specifications for durability of concrete against frost action
186
Max. values of w/c for durability against frost action Climatic condition Types of structures Thin elements Medium size elements Exterior faces of mass concrete (large elements)
In air 0.49 0.53 0.58
Severe In water Fresh Salty 0.44 0.40 0.49 0.44 0.48 0.44
In air 0.53 -
Mild In water Fresh Salty 0.49 0.40 0.53 0.44 0.53 0.44
187
Air-entrainment into concrete increases the durability of concrete against freezing and thawing. Optimum percentage of air-entrainment is about 4-6 % by volume. Air entrainment is achieved through some chemical admixtures and thus use of air-entraining admixtures has been a rule for concretes subjected to severe climatic action Freezing of the water in the pores of the aggregates may also damage concrete via damaging aggregate particles. Durability of aggregate against frost action depends upon its pore characteristics and saturation degree. Indirect or direct tests of freezing and thawing are used to measure durability of aggregates
188
Number of freeze-thaw cycles for 25% weight loss
4000
3000 Air entrained 2000 Not air entrained 1000
0.4
0.5 0.6 0.7 Water/cement ratio
0.8
The effect of air entrainment and water/cement ratio on the frost resistance of concrete moist-cured for 28 days (US Bureau of Reclamation, 1955) 189
Therefore, to make concrete durable 1)
Cement paste should be made durable
2) 3)
Use durable aggregates Apply good curing to concrete
(↓ w/c, air-entrain, pozzolanic materials)
Concrete should be tested for durability against frost action Prismatic specimens are subjected to 300 cycles of freezing at -17 ºC and thawing at + 4 ºC, a cycle completed in 4 hours. Specimens are tested non-destructively from time to time by measuring dynamic modulus of elasticity through resonance frequency testing. Decrease in Ed with respect to control concrete indicates the degree of durability against frost action
190
Freeze – thaw cabin
191
2) Durability against chemical action
Concrete in industrial buildings Concrete containing contaminated aggregates Concrete in contact with sulphate soils or sulphate ground water may be subjected to the harmful effects of some chemical compounds
Degree of action of certain chemicals in concrete Chemical pH value CO2 in water, mg/lt NH4 in water, mg/lt Mg+2 in water, mg/lt SO4-2 in water, mg/lt SO4-2 in soils, mg/kg
Weak 6.5 – 5.5 15 – 30 15 – 30 100 – 300 200 – 600 2000 - 5000
Degree of Action Strong 5.5 – 4.5 30 – 60 30 – 60 300 – 1500 600 – 3000 > 5000
Very Strong < 4.5 > 60 > 60 > 1500 > 3000 -
192
Sulphates can react with hydrated aluminate phases in hardened cement paste to produce ettringite through reaction;
C3 A.CS .H18 + 2CH + 2S + 12H → C3A3CSH 32 3CaOAl 2 O3 CaSO4 18H 2 O + 2Ca (OH ) 2 + 2 SO3 + 12 H 2 O → 3CaOAl 2 O3 3CaSO4 32 H 2 O This is an expansive reaction, causing disruption
Reactions of other forms of sulphates can occur with CH in hcp forming gypsum again with an increase in volume;
Na 2 SO4 + Ca(OH ) 2 + 2 H 2 O → CaSO4 2 H 2 O + 2 NaOH MgSO4 + Ca(OH ) 2 + 2 H 2 O → CaSO4 2 H 2 O + Mg (OH ) 2 Gypsum Severity of attack depends on the type of sulphate
Damage of attack by MgSO4 > Damage of attack by Na2SO4 > Damage of attack by CaSO4 193
Deterioration due to sulphates decreases with;
⇓ C3A content Higher cement content and lower water-cement ratio. WHY?? Cement replacement materials decreases permeability and improves resistance to sulphates
194
To resist the weak action;
Concrete properly designed and well consolidated and cured w/c should be less than 0.60
To resist strong action;
Concrete should be properly designed and well consolidated w/c should be less than 0.50 Lateral dimension of the structural elements should be enlarged to ensure some protective cover Special cements & cement replacement materials should be used
To resist very strong action;
In addition to all above, protect the concrete by applying a protective cover
195
3) Durability against very high temperatures Concrete maybe subjected to very high temperatures above 1000ºC in the case of; Fire of buildings Pavements of air field Furnaces or chimneys of industrial buildings Rate of degradation depends on; Maximum temperature Concrete constituents Size of element
196
For the cement paste a) at 105ºC, capillary and gel water has been lost and shrinkage occurs b) at 250-300ºC, aluminous and ferrous constituents of cement loose crystal water c) at 400-700ºC, siliceous constituents loose crystal water and shrink meanwhile Ca(OH)2 looses its water and converts to CaO. After cooling, if it is wetted, it again converts to Ca(OH)2 and expand. These may result in severe cracking in hcp.
197
For the aggregates a) b) c) d)
Aggregates expand very mildly with increasing temperature At 550ºC, some siliceous aggregates may expand excessively due to a change in crystal structure At 900ºC, limestone aggregates may be calcined (separated from CO2) Basalt, blast furnace slag, crushed firebrick usually don’t change their volume extensively up to and well above 1000ºC
198
To produce a concrete durable against medium temperatures (500600ºC)
A proper design must be achieved Use limestone aggregates
To produce a concrete durable against higher temperatures (9001100ºC)
A refractory concrete should be designed Stabilizing agent (shamotte earth, powdered silica, etc.) should be used in place of some fine aggregate Use basalt, blast furnace slag or crushed firebrick
On the other hand, aluminous cement is very profitable for refractory concrete
199
Durability of steel in concrete
Steel exist in concrete for reinforcing the concrete to compensate for weakness of concrete under tensile and shear stresses Sound concrete provides excellent protective medium for steel. If protection is broken, steel is left vulnerable to corrosion. Corrosion products, being expansive, caused cracking and/or spalling of concrete, exposing steel to more rapid corrosion
http://cce.oregonstate.edu/structural/images/corrosion_1.JPG
200
Principles of corrosion of steel in concrete O2 Fe2O3.H2O Fe(OH)2 Oxygenated water
Metal
2OH-
Fe++
1/2O2 H2O Fe Anodic area
2e-
Spacial arrangement of corrosion reactions of iron in moist air or oxygenated water
Cathodic area
Anode reaction; 2Fe → 2Fe++ +4eCathode reaction; 4e + O2 + 2H2O → 4OH at some distance from surface 2Fe++ + 4OH -→ 2Fe (OH)2 (Ferrous hydroxide (black rust)) Followed by 4Fe(OH)2 + O2 → 2Fe2O3H2O + H2O (Ferric hydroxide (red rust)) 201
Electrolyte is the pore water in contact with steel Normally highly alkaline (pH = 12-13) due to Ca(OH)2 from the cement hydration and the small amounts of Na2O and K2O in cement In such a solution, anodic reaction gives out Fe3O4 instead of Fe++. Fe3O4 is deposited at metal surface as tightly adherent thin film and stops any further corrosion (Steel is said to be passive) Concrete passivation may be broken by; A loss of alkalinity by carbonation Chloride ions
202
Different forms of damage from steel corrosion
cracking
spalling
lamination
corner effects
203
Corrosion
Two stages of corrosion damage
t0 = time for depassivating agents to reach steel and initiate corrosion t1 = time for corrosion to reach critical levels, sufficient to crack concrete
Level of corrosion to cause concrete cracking
t0
t1
age
204
Carbonation induced corrosion Carbonation first occurs on the surface and progresses inwards Reaction is a diffusion controlled process
x = k t 1/ 2
k = constant x = carbonation depth t = time
High quality, well cured concrete is subjected to limited carbonation (only 20 – 30mm after years)
205
Chloride induced corrosion Common sources of corrosion
Calcium chloride (an accelerating admixture) Contamination in aggregates Sea water for coastal and marine structures Road deicing salts (on bridge decks)
First two enter into concrete during mixing and steel may never be passivated (t0=0) Last two have to penetrate the concrete cover sufficiently to depassivate the steel (t0=fin Transport mechanisms are permeability, diffusivity, sorptivity
206
If protection against corrosion can not be guranteed, then
Use corrosion inhibiting admixture (calcium nitrate) Use corrosion resistant stainless steel bars or epoxy coated bars Cover concrete surface by a protector Apply cathodic protection
207
Chapter Outline • CONCRETE – Cement – Admixtures – Aggregates – Strength and failure of concrete – Durability of concrete – Statistical quality control in the production of concrete – Property composition relations for concrete and concrete mix design
208
Statistical quality control in the production of concrete OUTLİNE
• TS 500 • TS EN 206-1 • Conformity criterion
209
Statistical quality control in the production of concrete Concrete quality will show variation due to factors such as; variations in concrete making materials, production and measuring methods and human, Compressive strength of concrete, which is an excellent measure of not only the mechanical strength, but also some other properties of concrete, is taken as a basis for investigating the quality of the concrete When a concrete structure is designed, the designer will first decide about the compressive strength of the concrete. This strength shall be selected among the values given in the standards of that country, taking into account Level of importance of structure Materials Equipment Production facilities
210
TS 500:
Requirements for design and construction of reinforced concrete structures → revised on Feb., 2000, April 2002
TS EN 206-1: Concrete, Ready - Mixed concrete → obligatory standard is adopted to accord with EU countries (March, 2004) TS 11222: Ready - Mixed concrete → Revised on Feb., 2001
(abrogated after acceptance of TS EN 206 – 1)
Problem; Different conformity conditions and rules are defined by above standards
TS 500 strength requirements
TS 500 (2000) (Design principles for reinforced concrete buildings) Presents concrete design strengths (fcd) as; C14, C16, C20, C25, C30, C35, C40, C45, C50 C14
28 day compressive strengths of concrete cylinder specimens in MPa (D=15cm, H=30cm).
TS500 specifies that these strength values are lower limits with confidence degree of 90%. That is, only 10% of the specimens taken from the produced concrete may have compressive strengths below these design strengths
212
TS 500 strength requirements
Calculation of aim strength (given by TS 500 (1984)) (not included in TS 500 (2000))
Considering test variation in conrete strength obeys normal probabilistic distribution law, the standard deviation (σ) for the concrete production plant should be known for this purpose tσ
fcd
fca
213
TS 500 strength requirements
TS 500 (1984) Confidence limits in normal distribution Confidence Degree of Degree of parameter risk reliability (t) (r) (1-r) 0.00 0.50 0.50 0.67 0.25 0.75 1.00 0.16 0.84 1.28 0.10 0.90 1.65 0.05 0.95 1.96 0.025 0.975 2.33 0.01 0.99 3.00 0.001 0.999
Risk is P (fc ≤ fcd) Reliability is P (fc > fcd) Then required average strength (aimed or targeted strength) to be used in mix design calculations is given for 90% confidence as: fca = fcd + 1.28 σ
fcd = design strength fca = aim strength
214
TS 500 strength requirements
If standard deviation (σ) is not known, then aimed strength is obtained by increasing the design strength by Δf as given for BS14 & BS16 → Δf = 4MPa for BS18 - BS30 → Δf = 6MPa for BS35 - BS50 → Δf = 8MPa Ex; for a design strength (fcd) of BS25, strength to be aimed in mix design is calculated as: fca = 25+6 = 31MPa
215
TS 500 strength requirements
TS 500 specifies to take
3 test specimen for 1 unit (1 unit = 100m3 of concrete or 450m2 of area ) 3 specimens → 1 group 3 groups → 1 party P (G1, G2, G3) Compressive strengths of these specimens at 28 days after standard curing are evaluated statistically and arithmetical mean (fcm) is determined for the sample space.
Acceptance or rejection of concrete is made on two basis: Mean value (fcm) and minimum value (fcmin) are determined
for acceptance fcm ≥ fcd + 1.0 (MPa) (mean value of each party) fcmin ≥ fcd – 3.0 (MPa) (Minimum arithmetical mean calculated from groups of each party)
216
TS EN 206-1 strength requirements
TS EN 206-1
Concrete design strengths C8-C100 (16 concrete classes are defined) Strengths are given as C20/25
C 20 / 25
28 day compressive strengths of concrete cylinder specimens in MPa (D=15cm, H=30cm). 28 day compressive strengths of concrete cube specimens in MPa (a=15cm).
Lightweight concrete design strengths LC8 – LC80 (14 concrete classes are defined)
LC 8 / 9
28 day compressive strengths of concrete cylinder specimens in MPa (D=15cm, H=30cm). 28 day compressive strengths of concrete cube specimens in MPa (a=15cm).
217
Minimum rate of sampling for assessing conformity Production
3
First 50m of production
Minimum rate of sampling Subsequent to first 50m3 of production a Concrete with production Concrete without production control certification control certification 3 1/200 m or 2/production 1/150 m3 or week 1/production day
Initial (until at 3 samples least 35 test results are obtained) 1/400 m3 or 1/production Continuous b (when at least 35 week results are available) a Sampling shall be distributed throughout the production and should not be more than 1 sample within each 25 m3 b Where the standard deviation of the last 15 test results exceeds 1,37σ, the sampling rate shall be increased to that required for initial production for the next 35 test results
218
TS EN 206-1
Conformity criteria for compressive strength Production
Initial Continuous
Number “n” of test results for compressive strength in the group
Criterion 1
Criterion 2
Mean of “n” results (fcm) N/mm2
Any individual test result (fci) N/mm2
3 15
≥ fck + 4 ≥ fck + 1.48 σ
≥ fck - 4 ≥ fck - 4
219
TS EN 206-1 performance requirements
Two definitons are given for concrete; 1) Designed concrete; concrete for which the required properties and additional characteristics are specified to the producer who is responsible for providing a concrete conforming to the required properties and additional characteristics 2) Prescribed concrete; concrete for which the composition of the concrete and the constituent materials to be used are specified to the producer who is responsible for providing a concrete with the specified composition
220
TS 500 vs. TS EN 206-1 TS EN 206-1 is more specific by means of durability regulations than TS 500. Limit values and conformity criterions are given for properties other than strength The number of samples to be tested is smaller in TS EN 206-1. This can be a disadvantage.
Ref; Selçuk Türkel, Kamile Tosun, The evaluation of TS EN 206-1, TS 500 and TS 11222 standards from the view point of concrete
221
Chapter Outline • CONCRETE – Cement – Admixtures – Aggregates – Strength and failure of concrete – Durability of concrete – Statistical quality control in the production of concrete – Property composition relations for concrete and concrete mix design
222
Property composition relations for concrete mix design OUTLINE • •
Workability Concrete mix design calculations a) Preliminary design b) Trial batch production and measurement
223
Property composition relations Required properties
Workability Compressive strength Deformation (elastic, creep, shrinkage, thermal) Permeability Durability against freezing & thawing, chemical attack, fire
224
THE MIX DESIGN PROCESS
225
Workability Empirical equation that relate the mix water to workability
W = α (10 − m) where, W = amount of mixing water, dm3/m3 α = coefficient depending on consistency & type of aggregate m = fineness modulus of the aggregate mixture
Consistency grade
αave
Natural sand Natural sand & Sea sand & & gravel crushed stone crushed stone
Stiff
28-30
33
37
Plastic
31-33
37
40
Fluid
36-40
43
47 226
Concrete mix design calculations a) Preliminary design Geometrical compatibility
a+c+ w+v =
A
δa
+
C
δc
+
W
δw
+ v = 1m3 = 1000dm3
227
Strength requirement; Abrams Graf Feret Bolomey
fc =
k1 k2
W /C
f cc fc = K G (W / C ) 2
fc = K F (
c )2 c+ w+v
C fc = K B ( − k `) W +v
(taken by weight)
(taken by weight)
(taken by volume)
(taken by weight)
228
Constraint on air content; Assumption for volume of entrapped air voids ∼2-3 % for stiff consistency ∼1-2 % for plastic consistency ∼0-1 % for fluid consistency Solve for C, A, W, V where C+A+W = Δth
229
b) Trial batch production and measurement Preliminary design → first approximation to actual values
Trial batch produced based on proportions obtained from preliminary
design. Only water requirement is adjusted till required workability is obtained This is done by adding mix water incrementally to the mixture of dry ingredients (cement + aggregates) in the mixer Workability is tested by slump test throughout incremental addition of mix
water. Unit weight of the trial batch is measured (Δm). Strength specimen (cylinders or cubes) are cast from the trial batch with desired workability. Specimens are tested at 28 days to check whether strength requirement is satisfied or not. If not mix proportions are altered.
230
Summary Determine w, c, a, v considering given requirements Calculate theoretical weight (Δth) Cast trial batch Measure actual unit weight (Δac) Calculate actual proportions using Δth, Δac
231
Example (Concrete mix design) Given: Portland cement (fcc = 41MPa, δc=3,15kg/m3) Aggregate : mixture of sand & sandy gravel
Sieve size (mm) Sand Sandy - gravel
0.25 8 1
0.50 23 3
1 45 7
2 60 12
4 100 20
8 100 55
16 100 100
δ 2.60 2.65
Desired properties: Fresh concrete: plastic consistency Hardened concrete: BS16 Restrictions: For the aggregate mixture, m=4.07 Assume v=1% Cmin=300kg/m3
232
Overall Outline • • • • • • • •
Introduction Concrete Bituminous materials Masonry Polymers and polymer composites Cement-based fiber composites Metals Timber 233
Chapter Outline BITUMINOUS MATERIALS – – – – – –
Introduction Sources of bitumen Chemistry and Molecular structure Types of bitumen Aggregates Strength and Failure • Modes of breakdown • Evaluation of road condition – Viscosity of bitumen – Factors affecting deformation of bituminous mixes – Property composition relations for bituminous mixes
234
BITUMINOUS MATERIALS Bituminous materials include all materials consisting of aggregate bound with either bitumen or tar. Mineral dust called “filler” is also used. Bituminous materials are used in highway engineering to construct flexible pavements. Factors determining the type of bituminous mixtures Bitumen content Bitumen grade Aggregate grading Aggregate size
picture: www.saocl.com
235
SOURCES of BITUMEN 1)
Natural deposits (types of deposit range from almost pure bitumen to bitumen-impregnated rocks and bituminuous sands with only a few per cent bitumen) • Rock asphalt (Porous limestone or sandstone impregnated with bitumen with 10% content, “Val de Travers”, Switzerland, “Tar sands” of North America) • Lake asphalt (Bitumen lake with mineral matter dispersed throughout the bitumen, Trinidad lake (55 % bitumen, 35 % mineral matter, 10 % organic matter)
2)
Refinery bitumen
236
2) Refinery bitumen: residual material left after the fractional distillation of crude oil Crude petroleum
Products of distillation
Petrol
Kerosene
Diesel oil
Lubricating oil
Bitumen
Fluxing/blending
Cutback bitumen
Penetration grade bitumen
Fig. Preparation of refinery bitumens
237
Chemistry and molecular structure Bitumen is a complex colloidal system of hydrocarbons which is soluble in trichloroethylene Constituents of bitumen are Carbenes; fraction insoluble in carbon tetrachloride Asphaltenes; fraction insoluble in heptane Maltenes; fraction soluble in heptane
238
Types of bitumen 1) Penetration grade bitumen: refinery bitumen with a range of viscosities Penetration test classifies the bitumen according to hardness. For road bitumen penetration grades is from 15 to 450. Softening point, on the other hand, specifies the viscosity 2) Oxidized bitumens: air is introduced into bitumen under pressure Oxygen(from air) reacts with some compounds to result in more asphaltene with higher molecular weight. Thus, hardness of bitumen is increased with reduced ductility and temperature dependence. Mostly used for industrial applications such as roofing and pipe coatings
239
3) Cutbacks: penetration grade bitumens for which the viscosity is temporarily reduced by dilation in a volatile oil. After application, oil evaporates and bitumen turns to its former viscosity Curing time defines
Slow curing Medium curing
Cutbacks
Rapid curing
240
Bitumen droplet (dispersed phase) Water Phase (continuous Phase)
Repulsion between particles (like charges repel)
Attraction of particle to substrate
Adhesion of bitumen to aggregate surface
Surface active agent Opposite charges attract
4) Emulsions: two-phase system made up of two immiscible liquids, bitumen being dispersed as fine globules in water. Emulsifier provides dispersal of bitumen globules. Emulsifier in a hydrocarbon chain with cationic and anionic functional group. Hydrocarbon chain has affinity for bitumen where as ionic part has affinity for water. Each droplet carries a like charge, depending on charge of ionic part of emulsifier. Cationic emulsions are positively charged. Anionic emulsions are negatively charged. Cationic emulsions aid adhesion of bitumen on negatively charged aggregate surfaces.
241
Aggregates Aggregates make up about 92 % of bituminous materials Coarse aggregates → retained on 2.36mm sieve, Fine aggregates → passes 2.36mm sieve but is retained on 75µm sieve. Filler → passes 75µm sieve Open textured aggregate mixtures: Grading is continuous and provides a dense packing of particles. Strength and resistance to deformation are largely determined by aggregate grading with bitumen acting as adhesive Dense graded aggregate mixtures: Aggregate grading is still important but properties are determined largely by the matrix of fines and bitumen Aggregate particles must have sufficient strength for surfacing materials, they must be resistant to abrasion and polishing. Shape and surface texture are also important. 242
Strength and Failure Purpose of the road is to distribute applied load from traffic to a level which the underlying subgrade can bear There are two forms of failure: Road surface may deteriorate through breakdown of surface material so that skidding resistance drops Road structure deteriorates – gradual and develops with the continued application of wheel loads
243
Modes of breakdown Case I
Case II Nearside wheel track
rut depth
Bituminous material
Granular material
Subgrade soil
Permanent deformation
moving wheel load
Asphalt layer Granular layer
fatigue crack εt , critical tensile strain
εz ,
critical vertical subgrade strains
Subgrade
Fatigue cracking and critical strains
244
Two modes of breakdown Case I - Permanent deformation occurs in wheel tracks. Rutting is associated with deformations in all layers of pavement. It is linked to loss of support from subgrade soil. Case II - Cracking appears along wheel tracks, cracking is caused by tensile strain developed in bound layers as each wheel load passes. It is a fatigue failure.
245
Evaluation of road condition If failure can be defined, life of a road can be determined provided that loading can be assessed and performance of the materials evaluated. Complete collapse of roads are not encountered. Therefore, failure of roads must be
identified in terms of serviceability and/or repairability.
It is accepted that if cracking is visible at surface, road is regarded as being at critical condition or as having failed. For no visible cracking, if rut depth reaches 20mm, road is regarded as having failed
246
Viscosity of bitumen Viscosity of a liquid is the property that retards flow so that when a force is applied to the liquid, the higher the viscosity, the slower will be the movement of the liquid. The viscosity of bitumen is dependent upon both its chemical composition and its structure. There are two most common measures of viscosity Softening point: Temperature at which a bitumen reaches a specified level of viscosity Standard tar viscometer: (used to measure the viscosity of tars) time taken for 50ml of the tar to run out of a cup through a standard orifice
224
247
Another test to evaluate viscosity Penetration test: commonly applied to bitumen for material characterization. The test measures the hardness of bitumen which is related to viscosity. It measures depth to which a needle penetrates a sample of bitumen under a load of 100 gr over a period of 5 sec at a temperature of 25ºC
Bitumen is viscoelastic; therefore, the penetration will depend on the elastic deformation and viscosity Bitumens are thermoplastic materials so that they soften as the temperature rises but become hard again when the temperature drops. Susceptibility of bitumen to temperature is determined from the penetration value and softening point temperature
Viscoelasticity describes materials that exhibit both viscous and elastic characteristics when undergoing plastic deformation
248
Factors affecting deformation of bituminous mixes 1) Bituminous viscosity: when a stress is applied to a bituminous material, both the aggregate particles and the bitumen will be subjected to the stress. Aggregate particles, being hard and stiff, while undergo negligible strain, whereas the bitumen, being soft, will undergo considerable strain. Deformation is associated with movement in the bitumen and the extent of the movement will depend on its viscosity. 2) Aggregate: bituminous mixtures utilizing a continuously graded – aggregate rely mainly on aggregate particle interlock for their resistance to deformation. Thus, grading and particle shape of aggregate are major factors governing deformation. Characteristics of fine aggregate are important in gap-graded materials. WHY?? 3) Temperature: permanent strain increases with temperature due to the reduction in viscosity and stiffness of bitumen.
249
Property composition relations for bituminous mixes McKesson – Frickstad (California) formula P = 0.015a + 0.03b+ 0.17c where, a = % of aggregate retained on No 10 (2mm) sieve b = % of aggregate remaining between No 10 (2mm) and No 200 (0.075mm) sieves c = % of aggregate passing No 200 (0.075mm) sieve P = % of bitumen by weight of the bituminous mixture In this equation, amount of bitumen is related to the surface area of the aggregate groups.
250
In lab., some tests have been done to determine the optimum bitumen content Compression test Stability test (Marshall test, Hubbard-Field test, Hveem test) Triaxial test
251
Marshall test (most common)
Asphalt sample
Objective To determine an optimum binder content from a consideration of mix strength (stability), mix density, mix
deformability (flow)
Cylindrical samples of 10cm diameter and 6.25cm height specimen is placed between the crescent shaped testing heads of testing machine and the force is applied from the side surfaces, subjecting the sample to an internal shearing towards the stress free plane surfaces. Loading speed is 5cm/min. Resultant maximum load and the corresponding deformations are measured
Required properties are 1) minimum stability of 225kg (2207N), 2)a maximum flow of 0.5cm, 3) air voids (3-5%) and 4) degree of saturation of aggregate voids with bitumen (75-85%).
Test is repeated with mixes of different bitumen contents and bitumen content fulfilling all requirements is determined as OPTIMUM BITUMEN CONTENT
252
Analysis of mix design data from the Marshall test In compacted bituminous mixture Existing unit weight
Unit weight
d= Air voids
weight in air (weight in air) - (weight in water)
Theoretical max, unit weight
Gm = Marshall stability
Pbit
δ bit
100 ( Pagg ) i
+∑ i
(δ agg ) i
Air void content Flow
V= 5
6
7
8
Gm − d x100 Gm
Bitumen Content (%) 253
Example Properties and mix proportions of materials used for a bituminous mixture are as follows Material Asphalt Crushed stone
Density (δ) 1.04 kg/dm3 2.70 kg/dm3
Mix proportions (P) 10 % by wt 90 % by wt
Following data are obtained from a lab. specimen made using this mix: weight in air = 111.95 gr weight in water = 61.16 gr
254
Calculate unit weight of the lab. specimen Calculate the theoretical maximum unit wt. of the mix Calculate volume percentage of voids in the lab. specimen This mix is placed on the road and consolidated under a roller. After consolidation a core sample is taken and the unit wt. of this core sample is determined to be 2.13kg/dm3. The specified degree of compaction for this pavement being at least 95% of the compaction of a laboratory specimen, indicate if this bituminous pavement is acceptable?
255
Solution: a)
d=
b)
Gm =
c)
v =
d)
specified unit weight =
256
Overall Outline • • • • • • • •
Introduction Concrete Bituminous materials Masonry Polymers and polymer composites Cement-based fiber composites Metals Timber 257
Brickwork and Blockwork (Masonry) Masonry; One of the oldest building material Used by mankind for more than 6000 years Ancient civilizations of Middle East, Greeks and Romans used masonry Many of mudbrick work has been lost. However, stone structures such as Egyptian pyramids, Greek temples and many structures from fired clay bricks have survived for thousands of years The Romans used both fired clay bricks and hydraulic mortar
258
These are four main techniques for achieving stable masonry a) b) c) d)
Dry stone walls: Irregularly shaped laminar pieces are placed by hand in an interlocking mass Ashlar: Medium to large blocks are made to a few sizes and assembled to a basic grid pattern either without or with mortar having very thin joints Normal brickwork: Small to medium units of different sizes are assembled to a basic grid pattern and mortar is used as a packing material Random rubble walls: Irregularly shaped and sized pieces are bonded together with adherent mortar
259
(a)
(b)
(c)
(d) 260
Materials Sand: particles with sizes from about 10mm diameter down to 75μm diameter obtained from riverbeds, sea beaches, older deposits from alluvial and glacial action. Most common sands are based on silica (SiO2)
Sand should be free of clay particles Mortar sand should have particles smaller than 5mm, Good range of particle size distribution is also needed (for good packing) Naturally occurring sands should be sieved and washed Very flaky and vary absorbent sand particles are not desired
261
Aggregates: Natural aggregates, sintered fly ash pellets, expanded clay and foamed slag Binds mixtures of sands, aggregates, fillers to make Binders: mortar for masonry Organic plasticizers: For improving plasticity or workability of mortars. These plasticizers also entrain air as small bubbles Latex additives: Synthetic copolymer plastics may be produced in the form of a “latex”, a finely divided dispersion of the plastic in water usually stabilized by a surfactant such as a synthetic detergent. Solid content is ~ 50% of dispersion. They increase adhesion of mortar to all substrate. They increase tensile strength and durability
262
Mortar Mortars are composed of cementing material, sand and water. There are three types depending on type of binder: 1) Cement mortar 2) Mixed mortar 3) Lime mortar The sand to be used in mortar must be free of mud, clay, organic matter. It must have a good grading in size range of 0 - 4mm. The composition of the mortar is chosen in such a way to obtain a good workability. For this purpose, all the voids of the aggregate must be filled with the paste of the cementing material and all the sand particles must be surrounded by a film of this paste. Existence of smaller particles in the sand is advantageous because; they fill the voids between larger particles and reduce the need for cementing material If the water content of the paste is high, then the strength of the mortar will be lowered, if the cementing material is in excessive, the mortar is said to be too rich, and it tends to crack due to shrinkage. Taking all these factors into account, certain mortar compositions have been accepted as a result of many years of experience. 263
Types and Mix Proportions of Masonry Mortars (TS 2848)
Class of Mortar
A B
C D E
Volume Proportions of Mix Type No
Type of Mortar
Sand
Cement
1 2 3 4 1 2 3 1 2 3 -
Cement Cement Mixed Mixed Mixed Mixed Cement Mixed Mixed Mixed Mixed Lime
3 4 4 4 4 7-9 5 5 6-8 6-8 2-3 3
1 1 1 1 1 1 1 1 1 1 -
Masonry Lime cement putty ½ 2 1 -
1 2 1
Minimum compressive Powder strength Lime (MPa) ½ 1 3 -
15 15
5
2 0.5
The last column shows the expected minimum compressive strength of the mortar at the age of 28 days 264
Natural stones;
Rocks from magnetic, volcanic, sedimentary or metamorphic origin make up the natural building units for masonry. Magnetic and volcanic origin stones, being crystalline, give good quality materials. Examples are: granite, diorite, rhyolite, basalt, andesite, trechite, etc. However, care should be taken for mica, feldspar and other nondurable minerals contained in these rocks
Sedimentary rocks are not crystalline but layered. They may contain voids and fossils. Examples are: conglomerate, breccia, sandstone, shale, limestone, chert. Sandstones and limestones make good stones for masonry.
Metamorphic rocks are those who experienced a change in their structure. Good quality stones are gneiss, quartzite and marble. However, slate and shale are of poor quality.
241
265
Properties of Natural Stones
Type of Stone Basalt Granite Andesite, Diorite Dense limestone Travertene
Comp. Str. (MPa) 90 - 165 75 - 145 55 - 75 55 - 102 32 - 97
Unit wt. (kg/dm3) 3.35 2.65 2.56 2.70 2.5 – 2.60
Water absorption (%) by wt. 0.5 – 2.0 0.6 0.2 0.2 – 2.0 0.4 – 2.4
266
Fired Clay Bricks Bricks are made by forming the unit from moist clay by pressing, extrusion or casting followed by drying and firing (burning) to a temperature usually in the range of 850-1300ºC During firing process complex chemical changes occur and clay particles are bonded together by sintering (transfer of ions between particles at points where they touch) or by partial melting to a glass. Most important properties of bricks are compressive strength, size tolerances and unit weight. A low unit weight is desired to reduce the dead load and to provide heat and sound insulation
267
Requirements for Factory made Bricks (TS 705) Definitions
Dimension (mm)
Normal brick (NT) Modular brick (MT) Brick block (BT)
190x190x50 190x90x85
Type of Brick
Solid Brick (DOT/20) Solid Brick (DOT/12) Solid Brick (DOT/8) Brick with vertical holes (DDT/15) Brick with vertical holes (DDT/10) Brick with vertical holes (DDT/6) Brick with horizontal holes (YDT/3.6) Brick with horizontal holes (YDT/2.4) Solid clinker brick (DOK/30) Clinker brick with holes (DEK/30)
Unit wt (max) (kg/dm3) 1.80 1.80 1.80 1.20 1.20 1.20 0.8 0.50 1.80 1.80
Tolerances for dimensions (mm) 50 ± 2 85 ± 3, 90 ± 3 190 ± 3 or 190 - 2 Comp. Str. Mean (MPa)
Min (MPa)
20 12 8 15 10 6 3.6 2.4 30 30
16 9.5 6.5 12 8 4.5 3 2 25 25
λ
kcal mh D C
Absorption max (%)
0.75
-
0.34
-
0.32
-
0.75
8
268
Concrete Units
Used in building partition in houses since 1920s Concrete unit is poured into a mould and vibrated and demoulded after setting. It is a labor demanding and slow process Concrete is filled into a mould (die) and a dynamic presshead compacts the concrete into the die. Then, green concrete is ejected on to a conveyer system and taken away to cure either in air or after in stream Thus solid and hollow bricks and blocks are produced either in dense concrete or as a porous open structure by using gap-graded aggregates and not compacting fully Block sizes are 200x200x40 (mm), 150x200x400 (mm) They contain usually two large vertical holes. TS 406 classifies them into strength levels of 2.5, 5, 7.5, 10 and 15 MPa. Maximum unit weight is not to exceed 1.60 kg/dm3
269
Masonry construction Basic method of construction; units are laid one on top of another in such a way that they form an interlocking mass in at least the two horizontal dimensions Mortar needs to be firmly dry for dense low absorption units. While high absorption units need a sloppy wet mortar Walls and columns are built by laying out a plan at foundation level and masonry rises up layer by layer The foundation layers are horizontal It is essential to maintain the verticality Thickness of mortar joints must be kept constant Joint color and shape influence the appearance
270
Reinforced and post-stressed masonry forms
Reinforced pocket wall
(a)
Pockets formed as built, shuttered then filled with concrete to bond in the reinforcement
(b)
(a)Bed joint reinforced (b)Reinforced pocket walls
271
Bars either bonded in with mortar as the brickwork is rasied or grouted in with concrete in lifts
Ties at 5/m2
Concrete grouted cavity Width = 2 x cover +bar
Vertical or horizontal or both rebar reinforcement
(c)
(d)
(c) Grouted cavity (d)Quetta bond
272
Bond patterns Bond (interlocking) between units is achieved as Stretcher bond or half bond Third quarter bond Soldier course
Stretcher or half bond Soldier course Quarter bond
Half brick bonds
273
Typical block/brick cavity wall
274
Flat shear ties at 5/m2
Collar-jointed brick wall
Mortar filled collar joint < 25 mm
275
Bonded wall types
Header a) English
c) Heading
Closer
Stretcher
b) Flemish garden wall
d) Rat-trap 276
Structural behavior Unreinforced masonry is; good at resisting compression forces, moderate to bad at resisting shear forces very poor when subjected to direct tension However, reinforced masonry is good also at resisting tension forces Any masonry under compressive stress also resists bending since the compressive prestress in the wall must be overcome before any tensile strain can occur Most of small masonry structures are still designed using experiencebased design rules. Strength of masonry elements are predicted from strength and/or other characteristics of materials used in masonry construction Then check is made against worst loading conditions obtained from past data. A factor of safety is applied to allow for statistical uncertainty in the material characteristics and loads. Relatively high (3-5) safety factors are used due to high variability and brittle failure mode
277
Forces on walls Compressive
Tensile Lateral (flexural) Shear (in plane)
Shear (normal)
278
Mechanisms for resisting bending forces in masonry cantilevers
Disturbing force e.g. due to wind load
Restoring force Generated by mass acting at centre of gravity
Crack
Gravity mechanism
Generated by tension stresses in the bed joints
Flexural resistance 279
Compressive loading Masonry is most effective under compressive load If a load or force is put on a wall at a point, it would logically spread or toward from the point of application in a stretcher bonded wall since each unit is supported by the two units below it.
Such a compressive force causes elastic shortening (strain) of the masonry. As a result of Poisson’s ratio effects, a tension strain and hence a stress is generated normal to the applied stress.
280
Stability: slender structures For masonry wall and columns, if ratio of height to thickness (h/d = slenderness ratio) is small, then the strength will depend largely on the strength of the constituent materials. Other factors that affect the strength of masonry are • Degree of workmanship • Thickness of joints • Type of arrangement of bricks 10 190
90
90 10 190
1
1 brick length column 2
281
Some empirical formulas for masonry design for h/d ≤ 10
Brocker formula
σ max = 3 σ mortar 2 σ unit Graf formula
σ max
σ mortar ⎛ σ unit ⎜ 4 + 10 ⎝ = h 16 + 3( ) d
⎞ ⎟ ⎠ +e
σ : in kgf/cm2 e : + 10 for good workmanship 0 for medium workmanship - 5 for poor workmanship
282
Table a - Allowable Stresses for Masonry (DIN 1053) Type of wall Walls with irregularly shaped natural stones Walls with natural Stones with horizontal joints Walls with stones with more than 3-plane faces Walls with matural stone blocks
Brick Walls
Type of mortar Lime Mixed Cement Lime Mixed Cement Lime Mixed Cement Lime Mixed Cement
Lime Mixed Cement
Comp. strength of stone (MPa) 20 30 50 80 0.2 0.2 0.3 0.4 0.2 0.3 0.5 0.7 0.3 0.5 0.6 1.0 0.3 0.4 0.6 0.8 0.5 0.7 0.9 1.2 0.6 1.0 1.2 1.6 0.4 0.6 0.8 1.0 0.7 0.9 1.2 1.6 1.0 1.2 1.6 2.2 0.8 1.0 1.6 2.2 1.2 1.6 2.2 3.0 1.6 2.2 3.0 4.0 Comp. strength of bricks (MPa) 2.5 5 7.5-10 15 25 0.3 0.4 0.6 0.8 1.0 0.5 0.7 0.9 1.2 1.6 0.6 1.0 1.2 1.6 2.2
120 0.6 0.9 1.2 1.0 1.6 2.2 1.6 2.2 3.0 3.0 4.0 5.0 35 _ 2.2 3.0
Allowable stresses in this table are acceptable for h/d ≤ 10. If 10 < h/d ≤ 20, reduced allowable stresses for slender walls should be used (Table b). Slender walls with h/d > 20 are not permitted for masonry.
283
Table b - Reduced allowable stresses for slender masonry, in N/mm2 h/d 12 14 16 18 20
0.3 -
0.4 -
0.5 0.3 -
Allowable stress of unslender wall (MPa) 0.6 0.7 0.8 0.9 1.0 1.2 1.6 2.2 0.4 0.5 0.6 0.6 0.7 0.8 1.1 1.5 0.3 0.3 0.4 0.4 0.5 0.6 0.8 1.0 0.3 0.3 0.3 0.4 0.6 0.7 0.3 0.4 0.5 0.3
3.0 2.0 1.4 1.0 0.7 0.5
4.0 3.0 2.2 1.4 1.0 0.7
5.0 4.0 3.0 2.2 1.4 1.0
Linear interpolation for intermediate values permitted Allowable stresses for slender walls h/d > 10
284
Movements and elastic moduli of masonry materials Masonry component material Granite Limestone Marble Sandstone Slate Mortar Dense concrete brick/blockwork LWAC blockwork AAC blockwork Calcil brickwork Clay brickwork
Thermal expansion coefficient α (per ºC x 10-6) 8 - 10 3-4 4-6 7 – 12 9 – 11 10 – 13 6 - 12 8 - 12 8 8 – 14 5-8
Reversible moisture movement (%) 0.01 0.07 0.02 – 0.06 0.02 – 0.04 0.03 – 0.06 0.02 – 0.03 0.01 – 0.05 0.02
Irreversible moisture movement (%) - 0.04 – 0.1 - 0.02 – 0.06 - 0.02 – 0.06 - 0.05 – 0.09 - 0.01 – 0.04 0.02 – 0.10
Modulus of elasticity E (N/mm2) 20 – 60 10 – 80 35 3 – 80 10 – 35 20 – 35 10 – 25 4 – 16 3–8 14 – 18 4 - 26
285
Example; A masonry wall of 190 mm thickness (1 brick length) and 3.04 m height will be built of solid bricks DOT/12 according to TS705 and mixed mortar C-3 according to TS2348. Determine the maximum load that could be permitted on a 1m length of this wall.
Solution;
Slenderness ratio =
h 304 = = 16 > 10 d 19
from Table a σall= 0.9 MPa (for σbrick = 7.5 – 10 for mixed mortar) from Table b σall= 0.3 MPa (for σall = 0.9 MPa for unslender wall and h/d=16) Mixed load on 1m length of wall
Pmax = σ all ⋅ A = 0.3 × 10 6 N / m 2 × (0.19 × 1m 2 ) = 57 kN / m
286
Overall Outline • • • • • • • •
Introduction Concrete Bituminous materials Masonry Polymers and polymer composites Cement-based fiber composites Metals Timber 287
Polymers and polymer composites Polymeric materials produced by combining a large number of small molecular units (monomers) by polymerization to form long chain molecules 1) Thermoplastic polymers Long chain molecules held together by weak Van der Waals forces cycle of softening by heating and hardening by cooling can be repeated almost indefinitely. However, with each cycle, the material tends to become more brittle 2) Thermosetting polymers Epoxies and phenolics are principle examples cross linking exists between chains 3) Foamed polymers Two-phase system of gas dispersed in solid polymer, produced by adding a blowing agent to molten resin. Gas is released and causes polymer to expand. Small gas cells increase the volume of resin many times. They can be either thermoplastic or thermosetting. 4) Elastomers Long-chain polymer molecules made of coiled and twisted chains. Flexible material that undergoes very large deformations. Volcanization provides rigidity and hardness. 288
Fibers for polymer composites Load transfer from matrix to fibers results in high-strength, highmodulus material. Glass, carbon, boron fibers are examples of amorphous and crystalline fibers used in polymeric matrices.
1) Glass fibers
Manufactured by drawing molten glass from an electric furnace through platinum bushings at high speed. Filaments cool from liquid state at about 1200ºC to room temperature in 10-5 seconds.
Molten glass Platinum bushing 204 filaments
Strands
Sizing process Winding 289
Four types of glass are used for fibers - E-glass of low alkali content (most used one in composites in construction industry) - A-glass of high alkali content (previously used in aircraft industry) - Z-glass developed for reinforcing cement-based materials because of its high resistance to alkali attack - S2-glass fiber used for extra high strength and high modulus applications in aerospace
290
2) Carbon fibers Previously produced from pitch obtained by destructive distillation of coal Synthetic fiber polyacrylonitrile is spun and concurrently stretched so that molecular chains are aligned parallel to the fiber axis. Fiber is then heated under tension to 250ºC in oxygen environment where it gains strength. Carbonization starts when polymer is heated in inert atmosphere. Higher stiffness is obtained through greater heat energy given to carbon filament 3) Aramid fibers (Tradename - Kevlar) Most successful commercial organic fiber developed by DuPont Comp. Poly-parabenzamide fibers E=130GPa 2 forms of Kevlar fiber Kevlar 29: high strength and intermediate modulus Kevlar 49: high modulus and same strength as Kevlar 29 (preferred for high-performance composite materials)
291
Mechanical properties Polymer properties Mechanical properties are highly dependent on network of molecular units and on the lengths of cross-link chains. Curing process determines the length of cross-linked chains. Composites are heat cured to maximize mechanical properties Mechanical properties of thermoplastics arise from properties of monomer units and high molecular weight
292
Mechanical properties of common thermosetting and thermoplastic polymers
Thermosetting Polyester Epoxy Phenolic Thermoplastics Polyvinylchloride (PVC) Acrylonitrile butadiene styrene (ABS) Nylon Polyethylene (highdensity)
Specific weight
Ultimate tensile strength (GPa)
Modulus of elasticity in tension, GPa
Coefficient of linear expansion 10-6/ºC
1.28 1.30 1.35 - 1.75
45 – 90 90 – 110 45 – 59
2.5 – 4.0 3.0 – 7.0 5.5 – 8.3
100 – 110 45 – 65 30 – 45
1.37
58.0
2.4 – 2.8
50
1.05
17 – 62
0.69 – 2.82
60 – 130
1.13 – 1.15 0.96
48 – 83 30 - 35
1.03 – 2.76 1.10 – 1.30
80 – 150 120
293
Fiber properties Composite materials have high specific strength and high specific stiffness achieved by the use of low-density fibers with high strength and modulus values
Carbon fiber Type I Type II Type III E – Glass S2 – Glass fiber Kevlar fibers 29 49
Specific weight
Ultimate tensile strength (GPa)
Modulus of elasticity in tension, (GPa)
1.92 1.75 1.70 2.55 2.47
2.00 2.41 2.21 2.40 4.6
345 241 200 72.4 88.0
1.44 1.45
2.65 2.65
64 127
294
Polymer composites Mechanical properties of polymers can be greatly enhanced by incorporating fillers and/or fibers into resin formulations For structural applications, such composites should 1) Consist of two or more phases 2) Be manufactured by combining separate phases such that dispersion of one material in the other achieves optimum properties of the resulting material 3) Have enhanced properties composed with those of individual components
295
In fiber-reinforced polymer materials, primary phase (the fiber) uses plastic flow of secondary phase (the polymer) to transfer load to fiber. This results in a high-strength, high-modulus composite. High strength and high modulus properties of fibers are associated with very fine fibers with diameters of 7-15μm. Fibers are usually brittle. Polymers may be ductile or brittle and generally have low strength and stiffness By combining two components a bulk material is produced with a strength and stiffness dependent on fiber volume fraction and fiber orientation. Interface between fiber and matrix plays a major role in physical and mechanical properties of composite material Load is transferred from fiber to fiber through the interface and the matrix
296
Assumptions made for the analysis of composite materials The matrix and the fiber behave as elastic materials Bond between fiber and matrix is perfect and consequently there will be no strain discontinuity across the interface Material adjacent to fiber has the same properties as the material in bulk form Fibers are arranged in a regular or repeating array
Properties of interface region are very important for fracture toughness of the material Weak interface → low strength and stiffness → high resistance to fracture (ductile) Strong interface → high strength and stiffness → low fracture toughness (very brittle)
297
Elastic properties of continuous unidirectional laminae 3
E22, σ22 2
E11, σ11 1
Basic laminate
298
1) Longitudinal stiffness The orthotropic layer has three mutually perpendicular plane of property symmetry, characterized elastically by four independent elastic constants. E11= modulus of elasticity along fiber direction E22= modulus of elasticity in the transverse direction ν12 = Poisson’s ratio i.e. strains produced in direction 2 when specimen is loaded in direction 1. G12= longitudinal shear modulus ν21 = Poisson’s ratio, i.e. obtained from E11 ν21 = E22 ν12 If line of action of a tensile or compressive force is applied parallel to fibers of a unidirectional lamina, εm = εf provided bond is perfect As both fiber and matrix behave elastically then σf = Ef εf and σm = Em εm where εf = εm
299
As Ef > Em, stress in fiber must be greater than stress in matrix Thus fibers carry major part of applied load Composite load Pc = Pm + Pf σc Ac = σm Am + σf Af 3 σ c = σ m V m + σ f Vf where A = area of the phase V = volume fraction of the phase 2 Vc = volume of composite = 1 for perfect bond σ11 1 εc = εm = εf From above equation Ec εc = Em εc Vm = Ef εc Vf Ec = Em Vm = Ef Vf
Ec = E11 = Em (1-Vf) + EfVf (law of mixtures)
300
2) Transverse stiffness Applied load transverse to fibers acts equally on fiber and matrix; therefore, 3 σf = σm εf = σ22/Ef and εm = σ22/Em ε22 = Vf εf + Vm εm or ε22 = Vf σ22/Ef + Vm σ22/Em (substitute σ22= E22ε22) E22 = Ef Em / [ Ef (1-Vf) + EmVf ] to take into account of Poisson contraction effects1
σ22 2
E22 = Em`Ef / [ Ef (1-Vf) + Em`Vf ] where Em`= Em / (1-νm2)
301
Elastic properties of short-fiber composite materials As aspect ratio (l/d) decreases, effect of fiber length becomes more significant When a composite containing uniaxially aligned discontinuous fibers is stressed in tension parallel to the fiber direction there is a portion at the end of each finite fiber length and in surrounding matrix where stress and strain fields are modified by discontinuity Efficiency of the fiber to stiffen and to reinforce the matrix decreases as the fiber length decreases
302
Diagrammatic representation of the deformation field around a discontinuous fiber embedded in a matrix Resin
Uniform load
Uniform load
Fiber
Deformation of matrix Shear stress at interface
σf,max
Tensile stress in fiber
lc/2
lc/2
The critical transfer length over which fiber stress is decreased from maximum to zero at the end of fiber is referred to as half the critical length of fiber
303
To achieve the maximum fiber stress, fiber length must be equal to or
greater than the critical value lc
Reinforcing efficiency of short fibers is less than that for long fibers. Orientation of short fibers in a lamina is random and therefore lamina is assumed to be isotropic on a macro scale. Rule of mixture for long-parallel fiber case is modified by inclusion of a fiber orientation distribution factor n
Ec = E11 = ηEf Vf + Em Vm
η = 0.375 for a randomly oriented fiber array = 1.0 for unidirectional laminae when tested parallel to fiber = 0 for unidirectional laminae when tested perpendicular to fiber = 0.5 for a bidirectional fiber array
304
Failure of the fiber composite σ
σ
σfu
σfu
fiber
)f V (1
d le l ro t + on c Vf er σ fu b Fi =
σ′ m
σf σcu
composite
σc
σmu
matrix
σmu σ′m σm
σ cu
σ′m ε εfu
σ mu − σ m′ Vcr = σ fu − σ m′
ε
Vmin
0 VminVcr
mat rix
σcu = controlle σmu (1 d -Vf ) 1
Vf
σ mu − σ m′ = σ fu + σ mu − σ m′ 305
- If failure is due to breaking of fibers (fiber controlled, Vf > Vmin, then σcu > σmu) σcu = σ′m Vm + σfu Vf = σ′m (1-Vf) + σfu Vf - If failure is due to matrix flow (matrix controlled, Vf Vcr, fibers become effective in increasing the strength of composite above that of matrix. From practical point of view, there is no meaning to use a fiber ratio less than this critical value. 306
For discontinuous fibers lc =
rσ fu
τ my
or
σ fu lc = d 2τ my
and
l α= lc
for which tensile strength in the fiber will be reached at the same force as the yield strength of matrix on the interface If α > 1, then matrix will yield first before fiber breaks in tension
307
Average value of tensile stress in the fiber is found as −
σ f = σ f (1 −
1− β
α
−
)
σf = η = fiber efficiency factor σf
where β is related to stress distribution on fiber. β= 0.5 for fully plastic stress distribution β = 1.0 for fully elastic distribution
308
Strength of composite for discontinuous parallel fibers
σ cu = σ fuV f (1 − d
1− β
α
) + σ ' m (1 − V f )
If α > 5 and β = 0.5 then
σ cu
d
σ cu
for Vf > Vmin
≥ 0.9
If fibers are not too short, their discontinuity may be neglected
309
Example Properties of matrix and fiber materials for a glass fiber reinforced composite with continuous fibers of 30% volume fraction are as follows
Polymer matrix Glass fibers
E (GPa) 2 140
σfu (MPa) 200 2800
εu 0.10 (yield) 0.02
a) Determine the modulus of elasticity of the composite for tension parallel to fibers b) Assuming linear elastic behavior of fibers until failure and elastic-plastic behavior of matrix, determine ultimate tensile strength of composite c) What minimum fiber volume ratio should be used to obtain a reinforcing effect
310
Durability of polymer composites Polymer composites change with time and most significant factors are Elevated temperatures Fire Moisture Adverse chemical environments Natural weathering when exposed to sun’s ultra-violet radiation
311
Temperature Fluctuating temperatures have a greater deterioration effect on GRP. Difference in coefficients of thermal expansion of glass and resin may cause debonding. Exposed to high temperatures a discoloration of the resin may occur composite becoming yellow. Both polyster and epoxy show this effect. As a result of exposure to high temperatures, the composite becomes brittle.
Fire A composite material must meet appropriate standards of fire performance Some mineral filler, calcium carbonate can improve mechanical properties. Aluminum trihydrate and antimony trioxide are used as fillers to enable flame-retardant properties
312
Moisture Cross-linked polymers absorb water which may cause a decrease in strength and modulus of elasticity. Absorption of water by polyesters and epoxies leads to swelling of the laminate. Water will also cause some surface flaws on fibers and reduce strength, long-term water absorption may cause weakening of the bond between fiber and polymer
313
Weather
Natural weathering causes some deterioration of GRP composites. Sunlight degrades both polyester and epoxy resins. As a result of discoloration, loss of light transmission occur. UV absorbers and stabilizers are added to resin formulations. A rise of temperature accelerates chemical reaction and hence degradation. Weathering can affect mechanical properties of GRP composites through surface debonding. Because weathering is a surface effect, thickness of laminate becomes important. 3mm thick laminate shows 12-20% reduction in flexural strength after 15 years exposure, while 10mm laminate shows only ~ 3% reduction after 50 years exposure
314
Use of polymers and polymer composites Use of polymers and polymer composites in construction industry falls into three categories; Non - load - bearing Unreinforced polymers Semi - load – bearing Load - bearing Fiber – reinforced polymers
315
Selection of most appropriate resin should be done considering particular end use since every material does not possess all the following characteristics
High light transmission Infinite texture possibilities Minimum maintenance requirements Infinite design possibilities Resistance to water and corrosion High specific strength High impact resistance
316
Disadvantages of polymers in construction are High cost of materials. However, low density and reduced foundation size make them competitive Low stiffness and strength (use in composites) Poor scratch resistance Degradation under UV light (stabilizers used) Low resistance to fire and high temperatures (additives used) Non-load bearing thermoplastic polymers such as polyethylene have
been used to manufacture pipes for transportation of water, oil and gas
317
Thermoplastic polymers known as geosynthetics are used in soil environment in five categories Geotextiles •
Main fibers used in geotextiles are polyethylene, polypropylene, polyester and polyamide. Fibers are the load-bearing elements in geotextiles and framing technique determines the structure and physical and mechanical characteristics of the system
Geomembranes •
Manufactured in impermeable sheet form from thermoplastic polymers or bituminous materials either reinforced or unreinforced. Matrix can be reinforced by textiles.
Geo-linear elements •
Long, slender strips or bars consisting of a unidirectional filament fiber core made from a polyester, aramid or glass fiber in a polymer sheath of a low-density polyethylene or a resin. Fiber provides strength and deformation characteristics and matrix protects fiber and provides bonding with the soil
318
Geogrids • Grid-like structures of thermoplastic polymer material. They form a quasi-composite system in conjunction with soil grid structure in fiber and soil is assumed to be the matrıx • Two forms – Cross-laid strips – Punched thermoplastic polymer sheets
Geocomposites • Hybrid systems. Two or more different types of thermoplastic polymer systems. They form a drainage passage along the water course with polymer core as drainage channel and geotextile skin as the filter
319
Marine applications Dominant materials for pleasure craft. Effective in replacing wood Truck and automobile systems Sports – car bodies and truck cabs Strength, stiffness, toughness, corrosion resistance and highquality finish are physical and mechanical properties that must be satisfied. However, economy is the crucial consideration Aircraft and space applications Used for making components in aircraft industry
The fin of European airbus is a component made from a sandwich construction with carbon fiber/epoxy resin face material. The Westland helicopter rotor blades are made from carbon fiber composite material
320
Pipes and tanks for chemicals Critical consideration is the corrosive resistance and ease of fabrication of complex shapes Civil engineering structures
Applications have started with GRP structures A dome structure erected in 1968 in Benghazi, Libya Roof structure in Dubai Airport built in 1972 1970-1980 prestigious buildings in UK
(Morpeth School, Mondial House, Covent Garden Flower Market etc.)
These buildings are built as a composite system, with either steel or reinforced concrete structural system and GRP composite as load-bearing infill panels In 1970, a classroom system, using only GRP, by folding flat plates into a folded plate system so that stiffness is provided by the structural shape. In 1980’s more ambitious structural elements were produced
321
Mondial house (clad with FRP panels) FRP radar dome (25m diameter,
factory made FRP sandwich panels are bolted together on site)
FRP modular classroom 322
MATERİAL
Relative density
Diameter thickness ratio (microns)
Length E (mm) (GPa)
Tens. Str. (MPa)
Failure strain (%)
Volume in composite (%)
Mortar matrix
1.8-2.0
300-5000
-
10-30
1-10
0.01-0.05
85-97
Concrete matrix
1.8-2.4
10000-20000 -
20-40
1-4
0.01-0.02
97-99.5
Asbestos
2.55
0.02-30
5-40
164
200-1800
2-3
5-15
Carbon
1.16-1.95
7-18
3-cont. 30-390
600-2700
0.5-2.4
3-5
Glass
2.7
12.5
10-50
70
600-2500
3.6
3-7
0.96 0.96
900 20-50
3-5 Cont.
5 10-30
200 > 400
>4
2-4 5-10
0.91
20-100
5-20
4
-
-
0.1-0.2
1-3
3-8
2-6
12-40
700-1500
-
2-3
7.86
100-600
10-60
200
700-2000
3-5
0.3-2.0
Polyethylene HDPE filament High modulus Polypropylene (Monofilament) Polyvinyl alcohol (PVA) Steel
Performance is controlled by vol. fraction of fibers properties of fibers and matrix bond between the two 323
Table shows that elongations at break of all fibers are two or three orders of magnitude greater than strain at failure of matrix. Hence matrix cracks before fiber strength is approached Most organic fibers have modulus of elasticity less than five times that of the matrix. Low modulus fibers are used in situations where matrix is expected to be uncracked. Large Poisson’s ratio of these fibers may cause debonding and pull-out, woven meshes or networks of fibers are necessary for efficient composites. Steel fibers of varying cross-sections or bond ends provide anchorage; glass fiber bundles may be penetrated with cement hydration products to give effective bonding
Dmax of mortars or concrete affect efficient fiber distribution Concrete with a Dmax = 10mm is preferred. Dmax > 20mm is never allowed. To avoid shrinkage, at least 50% by volume of inert filler (aggregate, fly ash, or limestone dust) should be used 324
Structure of fiber-matrix interface Properties of fiber reinforced cementitious materials depends on microstructure of interface Interface is initially water-filled zone which develops to a microstructure different than bulk matrix There are 3 layers in interface; a) Very thin (less than one micron) Ca(OH)2 rich rather discontinuous, directly in contact with fiber b) Massive Ca(OH)2 layer c) Porous zone (up to 40 microns) consisting of CSH and some ettringite
325
Structure and post-cracking composite theory In hardened cement-based composites, fibers bridging the cracks contribute to the increase in strength, failure strain and toughness. Limited volume fraction of low modulus fibers do not contribute to the modulus of elasticity of the composite
326
σfuVf
σcu σ ′f
f
V fu
=σ
σcu
composite
σ′
σmu
Fiber controlled
cu
fiber
σ
σfu
σcu σfu
σ cu
= σm
(1u
σ′ fV f + V) f
) ked σ = σ′ V c a r cu f f (unc
cracked)
fVf
σmu
σ′f = σmu σ′f < σmu
matrix
0
εmu
εfu
ε
0
Vmin Vcr Vf
Vf
1.0
σmu (1 – Vf) + σ′f Vf 327
Modulus of elasticity of the composite 1) Where the matrix and fibers behave elastically Ec = Ef Vf + Em (1-Vf) 2) After the point where the matrix starts cracking
⎛ dσ f Ec = ⎜ ⎜ dε ⎝ f
⎞ ⎟.V f ⎟ ⎠
328
Ultimate tensile strength 1) Matrix fails and later composite carries stress at a decreased level. This mode of failure is called a “matrix controlled” failure. This occurs under the condition that Vf < Vcr
σcu = σmu (1-Vf) + σ’f Vf (uncracked) σcu = σ’f Vf (cracked) σ’f = stress in the fiber when matrix cracks (ε = εmu) 2) Matrix fails and later composite carries stress at an increased level. This mode of failure is called “fiber controlled” failure. This occurs under the condition that Vf > Vcr, σcu = σfu Vf Then the critical volume fraction for fibers is found as
Vcr =
σ fu
σ mu + σ mu − σ ' f 329
On the other hand, there is a fiber volume ratio that determines the strengthening effect of fibers in the composite
Vmin
σ mu = σ fu
It is observed that Vcr is a function of σ’f If σ’f = σmu, then Vcr = Vmin If σ’f < σmu, then Vcr < Vmin
330
When discontinuous fibers are used there is a critical length for fibers so that critical bond shear stress along the fiber-matrix interface does not lead to bond failure, that is, fibers will break rather than pull-out
⎛ σ fuν tan φ ⎞ r ⎟⎟ Lcr = ln⎜⎜ υ tan φ ⎝ c − n tan φ ⎠ where, r = radius of fiber υ = Poisson’s ratio of fiber c = cohesion φ = Angle of fraction between matrix and fiber n = normal pressure exerted on the surface of fiber due to shrinkage of matrix
331
Determination of critical length of discontinuous fibers is done through experimentation When discontinuous fibers of short length are used (L < Lcr) Then we may have a third mode of failure which is due to reaching the bond strength at the fiber-matrix interface. In this case the strength of the composite is determined not by fiber strength but the adhesion between the fiber and matrix
332
Example
It is planned to reinforce a concrete matrix of tensile strength σmu = 8 MPa with polymer fibers to increase its tensile strength. Fiber diameter is 0.25mm. Fiber tensile strength is 80 MPa and fiber-matrix bond strength is 0.2 MPa. Assuming constant bond shear stress distribution along the interface, calculate critical fiber length If discontinuous parallel fibers of 40mm length are used and a composite tensile strength of 11MPa is required, what fiber volume ratio will be necessary? In this case what will be the failure mode? If the moduli of elasticity of concrete and fibers are 20GPa and 50GPa, respectively, plot stress-strain diagram of composite in part b) For a continuous use of above fibers, and for the purpose of obtaining a reinforcing effect of the fibers for any volume ratio, what should be the lower limit of fiber modulus of elasticity. Calculate critical fiber ratio for value of Ef in c
333
Overall Outline • • • • • • • •
Introduction Concrete Bituminous materials Masonry Polymers and polymer composites Cement-based fiber composites Metals Timber 334
Metals, their differences and uses Metals in Civil Engineering
Ferrous (iron + carbon)
Cast iron (C > 1.7 %)
Steel (C < 1.5 %)
Non-ferrous
Structural steel (C ≈ 0.25 %)
Aluminium
Copper
335
1) Cast irons
Major consumption in pipes and fittings In civil engineering for tunnel segments and mine shaft tubing Carbon content is greater than 2 %. Hard and brittle.
2) Steel Steel is obtained through carbon reducing operations. Carbon steel is an alloy of iron and carbon. Amount of carbon within the lattice determines the properties of the steel. Alloys containing less than 0.008 % carbon are classed as irons. Steel has a carbon content less than 2.0 % C ≤ 0.25 % ⇒ mild steel, low carbon steel (structural steel is in this category) 0.3 % ≤ C ≤ 0.6 % ⇒ medium carbon steel, carbon steel C > 0.6 % ⇒ high carbon steel
Normally, Mn and S are added to steel during production If elements other than Mn and Si are added ⇒ alloy steels If elements like Cr and Ni are added ⇒ stainless steels 336
2.1 Structural steel Four grades of structural steels Grade Min.ten.st.(MPa) 40 400 43 430 50 500 55 550 All structural steels are readily weldable. For higher grades care is needed in welding
2.2 Heat treated steels Properties of steel (C > 0.3 %) can be varied by heat treatment Heating to high temperature Fast cooling by quenching in oil or water Followed by reheating to about 650ºC (tempering) Fast cooling produces ⇒ hard, brittle microstructure (known as martensite) Used only for cutting tools, no use in structural engineering 337
2.3 Stainless steels
Ferrow alloys: containing at least 12% Cr and some Ni and Mo. Chromium produces a stable passive oxide film.
Three basic types (grouped according to metallurgical structure):
1. Martensitic: 13% Cr, low carbon, hard, used for cutlery, unweldable
2. Ferritic: 13% Cr, low carbon, unweldable 3. Austenitic: 18% Cr and 8% Ni, low carbon, most resistant to pitting type of corrosion, weldable
338
3. Aluminium and alloys
Used both structurally and decoratively in cladding, roofing, window frames, window and door furniture. Lightness of aluminium is an advantage in structures with high selfweight / live load. Such as roofs, footbridges and long span structures High durability of aluminium makes it usable in polluted and coastal areas High cost limits its use High initial cost maybe offset by reduced maintenance Structural sections are produced by extrusion
Eal = 70GPa & Est = 210 GPa ⇒ Aluminium deflects more under same load However, specific moduli (E/ρ) is comparable (E/ρ)al = 20, (E/ρ)st = 29
339
4. Copper and alloys Used in applications where high thermal and electrical conductivity needed such as domestic water services, heating, sanitation 4.1 Brasses; copper – zinc alloys ⇒ enhanced strength and corrosion resistance 4.2 Bronzes; copper – tin alloys ⇒ high corrosion resistance
340
Design for minimum weight To find what is the least amount of material needed to carry design loads
P
Consider a cantilever beam of length L End deflection is
PL3 δ= 3EI
P = Applied load L = length of beam E = modulus of elasticity I = second moment of area
PL3 δ= 3Ex 4
, x is a unit of length
341
weight is W = ρAL = ρLx2 A = area of the cross-section ρ = density of the material of which the beam is made
W2 x = 2 2 ρ L 4
⎛ PL W = ⎜⎜ ⎝ 3δ
5
⎞ ⎟⎟ ⎠
1/ 2
ρ E 1/ 2
for a given set of design condition (P, L, δ), W is minimized by minimizing
ρ E1/ 2
342
Materials in decreasing order of efficiency in bending Timber
4.55
Aluminium
10.0
GFRP
11.25
Titanium
12.86
Steel
17.17
Concrete
21.82
Copper
25.60
Timber is oustanding Then comes Al and GFRP These compete in applications where weight is costly (aircraft, sailing dinghies, racing cars, small-scale building, squash rackets, golf clubs, tables and chairs)
343
Overall Outline • • • • • • • •
Introduction Concrete Bituminous materials Masonry Polymers and polymer composites Cement-based fiber composites Metals Timber 344
TIMBER Timber has been one of the basic materials of construction since the earliest days of human kind. Today it has been largely superseded by concrete and steel. However, the use of timber remains extensive. Wood – A Nature Made Composite Wood contains; 60 % cellulose (C6H10O5)n 28 % lignine (C41H36O6) 12 % pectine (C6H8O6)n & some others
345
Internal structure of wood is made up of groups of long and slender cells, called “grain”, that look like a group of thin pipes. The walls of the cells are made up of “ligno-cellulose” and they are bonded to each other with “pecto-cellulose”. The length of the cells is between 1-6 mm and their diameter is about 1/100 of their length Depending on internal structure, the trees may be separated into three groups; 1)
Softwoods without any vessels or pores for the sap like pine, fir, spruce, cedar and cypress 2) Hardwoods with diffused vessels or pores like maple, beech, poplar 3) Hardwoods with ring-porous structure like oak, ash etc. Even though chemical composition is the same, differences in internal structure result in differences of properties. Unit weights vary between 0,398 – 0,720 kg/dm3 and flexural strengths between 65 – 110 MPa.
346
Table 1. Mechanical properties of trees in Turkey in air-dried condition and parallel to grains
Type of tree Poplar (kavak) Fir (köknar) Spruce (ladin) Yellow pine (sarıçam) Cedar (sedir) Black pine (karaçam) Red pine (kızılçam) Oak (meşe) Beech (kayın)
Unit weight (kg/dm3) 0,398 0,431 0,436 0,515 0,523 0,568 0,577 0,690 0,720
Compressive Flexural str. str. (MPa) (MPa) 41 69 36 71 31 69 38 65 45 77 48 110 38 73 55 94 53 105
Even though chemical composition is the same, differences in internal structure result in differences of properties.
347
Properties of timber change with - direction - moisture content Largest properties are obtained in the direction parallel to grain and in dry condition which is the condition in which wood is usually utilized Being an organic material, wood is affected by the moisture in the atmosphere, by the ultraviolet rays of sun and deteriorated by parasites such as fungus or worms. Wood has to be protected by coatings and poisonous preservatives Its anisotropical nature and the limitations of size make its use difficult. To correct those deficiencies, artificial wood products like plywood, fiberboard, etc. are manufactured and used to a great extend
348
Comp. Str. 100 (MPa)
Composite strength – moisture relation for soft - woods with unit wt = 0.420 kg/dm3
90 80 70
60 50 40 30 20 10 10
20
30
40
50 60
70
80
w (%)
349
Timber specification (TS 647) Allowable stresses
Strength (MPa) Tensile // Compressive // Compressive ⊥
3rd class Pine Oak 6.0 7.0 2.0 3.0
2nd class Pine Oak 8.5 10.0 8.5 10.0 2.0 3.0
1st class Pine Oak 10.5 11.0 11.0 12.0 2.0 3.0
Modulus of elasticity Pine: // 10 GPa ⊥ 0.3 GPa Oak: // 12.5 GPa ⊥ 0.6 GPa
350
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