Mix Design.ppt

CONCRETE MIX DESIGN

BUILDING TECHNOLOGY AND MANAGEMENT

INTRODUCTION Concrete is obtained by mixing cement, fine aggregate, coarse aggregate, water and admixtures in required proportions. The mixture when placed in forms and allowed to cure becomes hard like stone.

The hardening is caused by chemical action between water and the cement due to which concrete grows stronger with age.

It is the most widely-used man-made construction material in the world. CONCRETE MIX DESIGN

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CONCRETE MAKING MATERIALS • • • •

Cement Aggregates Water Admixtures

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CEMENT Cement and water forms the „active‟ component of concrete, while the inactive group comprises the fine and coarse aggregates. The cement and water form a paste that hardens and bonds the aggregates together. Types of Cement Although around 18 types of cements are recognized by BIS, more commonly used ones are: • Ordinary Portland Cement 33, 43, & 53 grade OPC, • Blended Cements (PPC and PSC). • Sulphate Resisting Cement (SRC), • Low Heat Portland Cement (LHPC), • Hydrophobic Portland Cement, • Colored Cement (White Cement). CONCRETE MIX DESIGN

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AGGREGATES • They are cheaper than cement and impart greater volume stability and durability to concrete. • The main purpose is to provide bulk to the concrete. • Some of the aggregates may be chemically active. CLASSIFICATION • Geological origin – natural and artificial • Size – fine, coarse and all-in • Shape – rounded, irregular, angular, flaky and elongated • Unit weight – normal-weight, heavyweight and lightweight

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WATER • The most important and least expensive ingredient of concrete. • One part of the water is used in the hydration of cement to form the binding matrix. • The remaining water affords lubrication and workability to the concrete. • Water-cement ratio depends on the grade of concrete, its workability, durability, nature and type of aggregates etc. • Potable water is ideal for concreting. • Seawater may be used in PCC.

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ADMIXTURES Added to the concrete immediately or during mixing to modify its properties in the fresh or hardened state.

Types: • Accelerators - speed up the initial set of concrete. • Retarders – delay the setting time of concrete mix. • Plasticizers and Super-plasticizers - water reducers. • Air entraining admixtures • Water proofers • Pigments • Corrosion-inhibiting chemicals • Antifungal admixtures

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TYPES OF CONCRETE MIXES 1.

NOMINAL MIX Mixes of fixed proportions, IS:456-2000 permits nominal mixes for concretes of strength M20 or lower

2.

DESIGN MIX Designed on the basis of requirements of the concrete in fresh and hardened states.

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TRIAL MIXES Prepared to verify whether the Design Mix would perform as per the assumptions. If appreciable variation exists, the available alternatives are: 1. directly employ the trial mix proportions at the site 2. modify the trial mix proportions on the basis of intuition and employ the revised proportions at the site 3. prepare further trial mixes incorporating changes in the proportions based on the feedback generated from the previous mix.

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CONCRETE MIX DESIGN DEFINITION

“ Mix Design is the science of determining the relative proportions of the ingredients of concrete to achieve the

desired properties in the most economical way.”

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PRINCIPLES OF MIX DESIGN 1

The environment exposure condition for the structure

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The grade of concrete, their characteristic strength‟s and standard deviations

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The type of cement

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The types and sizes of aggregates and their sources of supply

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The nominal maximum sizes of aggregates

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Maximum and minimum cement content in kg/m3

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Water cement ratio

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The degree of workability of concrete based on placing conditions CONCRETE MIX DESIGN

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Air content inclusive of entrained air

10 The maximum/minimum density of concrete 11 The maximum/minimum temperature of fresh concrete

12 Type of water available for mixing and curing 13 The source of water and the impurities present in it.

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IS 456:2000

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IS 456:2000

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IS 456:2000

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FACTORS DEFINING THE CHOICE OF MIX PROPORTIONS 1. 2. 3. 4. 5.

Compressive Strength Workability Durability Type, size and grading of aggregates Aggregate-cement ratio

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COMPRESSIVE STRENGTH Abram‟s Law log F = log A1 – x log B1 where F is the compressive strength A1, B1 are constants and x is the w/c ratio by weight

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WORKABILITY “that property of freshly mixed concrete which determines the ease and homogeneity with which it can be mixed, placed,

consolidated and finished.”

DURABILITY

“the resistance to weathering action due to environmental conditions such as changes in temperature and humidity, chemical attack, abrasion, frost and fire.”

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REQUIREMENTS AND TESTS OF MATERIALS REQUIRED FOR MIX DESIGN ASSIGNMENT # 1 TO BE SUBMITTED BY 07-02-2011 Roll #s 01-21 – Cement Roll #s 22-42 – Aggregates Roll #s 43-63 – Water and Admixtures

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METHODS OF CONCRETE MIX DESIGN 1. 2. 3. 4. 5. 6. 7. 8.

American Concrete Institute Committee 211 method Bureau of Indian Standards Recommended method IS 10262-82 Road note No. 4 (Grading Curve) method Department Of Environment (DOE - British) method Trial and Adjustment Method Fineness modulus method Maximum density method Indian Road Congress, IRC 44 method

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American Concrete Institute Method of Mix Design (a) Data to be collected : (i ) Fineness modulus of selected F.A. (ii ) Unit weight of dry rodded coarse aggregate. (iii ) Sp. gravity of coarse and fine aggregates in SSD condition (iv ) Absorption characteristics of both coarse and fine aggregates. (v ) Specific gravity of cement. (b) From the minimum strength specified, estimate the average design strength by using standard deviation. (c) Find the water/cement ratio from the strength and durability points of view. Adopt the lower value. CONCRETE MIX DESIGN

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(d) Decide the maximum size of aggregate to be used. Generally for RCC work 20 mm and pre-stressed concrete 10 mm size are used. (e) Decide workability in terms of slump for the given job. (f ) The total water in kg/m3 of concrete is determined, corresponding to the selected slump and selected maximum size of aggregate. (g ) Cement content is computed by dividing the total water content by the water/cement ratio.

(h) Select the bulk volume of dry rodded coarse aggregate per unit volume of concrete, for the particular maximum size of coarse aggregate and fineness modulus of fine aggregate. CONCRETE MIX DESIGN

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( j ) The weight of C.A. per cubic meter of concrete is calculated by multiplying the bulk volume with bulk density. (k ) The solid volume of coarse aggregate in one cubic meter of concrete is calculated by knowing the specific gravity of C.A. (l ) Similarly the solid volume of cement, water and volume of air is calculated in one cubic meter of concrete.

(m) The solid volume of FA is computed by subtracting from the total volume of concrete the solid volume of cement, CA, water and entrapped air. (n) Weight of fine aggregate is calculated by multiplying the solid volume of fine aggregate by specific gravity of F.A.

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ACI METHOD: DESIGN EXAMPLE 1 Design a concrete mix for the construction of an elevated water tank. The specified design strength of concrete (characteristic

strength) is 30 MPa at 28 days measured on standard cylinders. Standard deviation can be taken as 4 MPa. The specific gravity of FA and C.A. are 2.65 and 2.7 respectively. The dry rodded bulk density of C.A. is 1600 kg/m3, and fineness modulus of FA is 2.80. Ordinary Portland cement (Type I) will be used. A slump of 50 mm is necessary. C.A. is found to be absorptive to the extent of 1% and free surface moisture in sand is found to be 2 per cent. Assume any other essential data.

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1 Mean Cylinder Compressive Strength Assuming 5 per cent of results are allowed to fall below specified design strength, The mean strength, fm= fmin + ks = 30 + 1.64 x 4 = 36.5 MPa

2

Water/cement ratio  

Strength criteria Durability criteria

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Strength Criteria

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Durability Criteria

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3

Mixing Water Content

Slump = 50 mm Maximum size of aggregate = 20 mm Concrete is non air-entrained

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The mixing water content is 185 kg/m3 of concrete The approximate entrapped air content is 2%.

The required cement content =185/0.47 = 394 kg/m3

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Bulk Volume of C.A.

Maximum size of C.A = 20mm Fineness Modulus of F.A. = 2.80 Find the dry rodded bulk volume of C.A.

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The dry rodded bulk volume of C.A. = 0.62 per unit volume of concrete

The weight of C.A. = 0.62 x 1600 = 992 kg/m3

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The first estimate of density of fresh concrete = 2355 kg/m3

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Weight of FA

The weight of all the known ingredients of concrete Weight of water

= 185 kg/m3

Weight of cement = 394 kg/m3 Weight of C.A.

= 992 kg/m3

Weight of F.A.

= 2355 – (185 + 394 + 992) = 784 kg/m3

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Alternatively, the weight of F.A. can be determined by the more

accurate absolute volume method Absolute Volume = weight/(1000ρ)

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m3

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Total absolute volume = 697 x 103 cm3

Absolute volume of F.A. = (1000 – 697) x 103 = 303 x 103 Weight of FA = 303 x 2.65 = 803 kg/m3

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7 Estimated quantities of materials per cubic meter of concrete are:

Cement = 394 kg F.A = 803 kg C.A = 992 kg Water = 185 kg

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Proportions

C : F.A : C.A : water 394 : 803 : 992 : 185 1 : 2.04 : 2.52 : 0.47 Weight of materials for one bag mix in kg = 50 : 102 : 126 : 23.5

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9 The above quantities is on the basis that both F.A and C.A are in saturated and surface dry condition (SSD conditions).

FA has surface moisture of 2% Total free surface moisture in FA = (2/100 x 803) = 16.06 kg/m3 Weight of F.A in field condition = 803 + 16.06 = 819.06 kg/m3

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C.A absorbs 1% water Quantity of water absorbed by C.A. = (1/100 x 992) = 9.92 kg/m3 Weight of C.A in field condition = 992 – 9.92

= 982 kg/m3 Change in Water Content

Water contributed by F.A Water absorbed by C.A. Extra water contributed by aggs. Total water content

= 16.06 kg = 9.92 kg = 16.06 – 9.92 = 6.14 kg = 185.00 – 6.14 = 179 kg/m3

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Quantities of materials to be used in field, corrected for free surface moisture in F.A and absorption characteristic of C.A

Cement

= 394 kg/m3

F.A.

= 819 kg/m3

C.A.

= 982 kg/m3

Water

= 179 kg/m3

Field density of fresh concrete = 2374 kg/m3

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2. A mix with a mean 28-day compressive strength of 35 MPa and a slump of 50 mm is required, using OPC. The maximum size of well shaped, angular aggregate is 20mm, its bulk density is 1600 kg/m3 and its specific gravity is 2.64. The available fine aggregate has a fineness modulus of 2.60 and a specific gravity of 2.58. No air-entrainment is required.

Water/Cement ratio

As durability criteria is not mentioned, consider strength alone

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Strength Criteria

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Water/cement ratio =0.48 Slump = 50 mm

Max size of aggregate = 20 mm

Determine water content

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Composition of concrete (kg/m3)

Water

=

185

Cement

=

386

CA

=

1020

FA

=

738

Density of Concrete

=

2329 (kg/m3)

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ROAD NOTE No. 4 METHOD OF MIX DESIGN Proposed by the Road Research Laboratory, UK (1950) Procedure: 1. The average compressive strength of the mix to be designed is obtained by applying control factors to the minimum compressive strength 2. Water/cement ratio is read from compressive strength v/s w/c

ratio graph 3. Proportion of combined aggregates to cement is determined from tables, for maximum size 40 mm and 20 mm

4. If the aggregate available differs from the standard gradings, combine FA and CA so as to produce one of the standard gradings

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5. The proportion of cement, water, FA and CA is determined from knowing the water/cement ratio and the aggregate/cement ratio. 6. Calculate the quantities of ingredients required to produce 1 m3 of concrete, by the absolute volume method, using the specific gravities of cement and aggregates.

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DRAWBACKS OF ROAD NOTE NO. 4 METHOD 1. Cannot be used directly for the design of air - entrained concrete

2. No recommendations for durability or strength, regarding the water/cement ratio 3. The design tables refer to mixes in which the FA and CA are of the same shape 4. In selecting aggregate/cement ratio, only 3 shapes of aggregates and 4 gradings are recommended.

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DOE METHOD OF MIX DESIGN Can be used for concrete containing fly ash. PROCEDURE 1. Target mean strength is calculated 2. Select water/cement ratio, from the type of cement and CA. Compare this with the ratio from durability conditions. 3. Decide the water content for required workability 4. Compare the cement content with the minimum cement content value and adopt the higher value 5. Find out the total aggregate content 6. Determine the proportion of FA using the appropriate FA% v/s CA size graph, and find the weight of CA and FA 7. Work out a trial mix.

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BIS RECOMMENDED MIX DESIGN METHOD The BIS recommended mix design procedure is covered in IS 10262-82. In line with IS 456-2000, the first revision IS 10262-2009 was published, to accommodate some of the following changes: • Increase in strength of cement • Express workability in terms of slump, rather than the compacting factor • Extend the W/C ratio v/s compressive strength graph

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Modifications in IS 10262-2009

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MIX DESIGN BASED ON IS RECOMMENDATIONS Based on IS 10262:1982

Procedure: 1. Target mean strength for mix design:

fck* = fck + tS where fck = characteristic compressive strength at 28 days S = standard deviation t = a statistical value depending on the risk factor.

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2. Selection of Water/Cement ratio

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3. Estimation of Entrapped Air

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4. Selection of Water Content and Fine to Total Aggregate ratio

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5. Calculation of Cement Content: cement by mass = Water content/Water cement ratio To be checked against the minimum cement content for the requirement of durability and the greater of the two values to be adopted.

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6. Calculation of aggregate content:

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7. Actual quantities required for mix Adjust the mix for deviations from assumed conditions

8. Check the calculated mix proportions

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DESIGN EXAMPLE: BIS RECOMMENDED METHOD

Grade M20 (a ) Design stipulations

(i ) Characteristic compressive strength required in the field at 28 days - 20 MPa (ii ) Maximum size of aggregate - 20 mm (angular) (iii ) Degree of workability - 0.90 compacting factor

(iv ) Degree of quality control - Good (v ) Type of Exposure - Mild

(b) Test data for Materials (i ) Specific gravity of cement - 3.15 (ii ) Compressive strength of cement at 7 days - Satisfies the requirement of IS: 269–1989 (iii )

1. Specific gravity of coarse aggregates - 2.60 2. Specific gravity of fine aggregates - 2.60

(iv ) Water absorption: 1. Coarse aggregate - 0.50% 2. Fine aggregate - 1.0% (v ) Free (surface) moisture: 1. Coarse aggregate - Nil 2. Fine aggregate - 2.0%

Design Procedure 1.

Target mean strength of concrete fck* = fck + tS

fck = 20, t = 1.64 , S = 4

fck* = 26.6 MPa

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2. Selection of Water/Cement Ratio

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Durability Criteria: Mild Exposure Conditions

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W/C ratio from strength considerations = 0.50 W/C ratio from durability considerations = 0.55

Adopt the lower value

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3. Selection of water and sand content

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Adjustments in Water and Sand Contents

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4. Determination of cement content Water-cement ratio = 0.50 water = 191.6 kg/m3

Cement = 191.6/0.50 = 383 kg/m3

Is this satisfactory for „mild‟ exposure condition?

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5. Determination of coarse and fine aggregate contents Specified max. size of aggregate = 20 mm Corresponding entrapped air = 2%

fa = 546 kg/m3, Ca = 1188 kg/m3

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Final Mix Proportions Water

Cement

FA

CA

191.6

383

546

1188

0.50

1

1.425

3.10

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