Concrete technology Lecture notes - Ordinary Diploma in Civil Engineering
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Concrete technology Lecture notes - Ordinary Diploma in Civil Engineering...
Description
REPUBLIC OF UGANDA
MINISTRY OF EDUCATION AND SPORTS UGANDA TECHNICAL COLLEGE - BUSHENYI
LECT URE NOT ES FOR
CONCRETE TECHNOLOGY AND PRACTICES
© Julius ©2011
Ngabirano
Prepared by:
Julius Ngabirano (B Sc. CIV ENG (MUK), CCA, GMUIPE) 0
TABLE OF CONTENTS LIST O F TABLES --------------------------------------------------------------------------------------- v LIST O F FO RMS --------------------------------------------------------------------------------------- vi LIST O F FIGURES------------------------------------------------------------------------------------ vii REFERENCES ------------------------------------------------------------------------------------------viii PREAMBLE ---------------------------------------------------------------------------------------------- ix
YEAR
ONE ----------------------------------------------------------------1
CHAPTER 1: INTRO DUCTIO N ------------------------------------------------------------------ 1 1.1 Definition------------------------------------------------------------------------------------------- 1 1.2 Limitations of concre te --------------------------------------------------------------------------- 1 1.3 Cement concrete----------------------------------------------------------------------------------- 1 1.3.1 Mass (plain) concrete ------------------------------------------------------------------------------ 1 1.3.2 Reinforced concrete -------------------------------------------------------------------------------- 2 1.3.3 Light weight concrete ------------------------------------------------------------------------------ 2 1.3.4 Normal weight concrete---------------------------------------------------------------------------- 2 1.3.5 Heavy weight concrete ----------------------------------------------------------------------------- 2 1.4 Concre te mate rials -------------------------------------------------------------------------------- 3 1.5 Batching of ingre dients --------------------------------------------------------------------------- 3 1.5.1 Batching by volume--------------------------------------------------------------------------------- 3 1.5.2 Batching by weight --------------------------------------------------------------------------------- 4 1.5.3 Conversion from volume to weight proportions--------------------------------------------------- 4 CHAPTER 2: AGGREGATES --------------------------------------------------------------------- 5 2.1 Introduction --------------------------------------------------------------------------------------- 5 2.2 Fine aggregate s ------------------------------------------------------------------------------------ 5 2.3 Coarse aggre gates --------------------------------------------------------------------------------- 6 2.3.1 Functions of aggregates---------------------------------------------------------------------------- 6 2.3.2 Qualities of good aggregates ---------------------------------------------------------------------- 7 2.4 Te sting aggregates -------------------------------------------------------------------------------- 7 2.4.1 Sampling -------------------------------------------------------------------------------------------- 7 2.4.2 Bulking of sand ------------------------------------------------------------------------------------- 8 2.5 Grading of aggregates ---------------------------------------------------------------------------10 2.5.1 Grading test for fine aggregates (sieve analysis) ------------------------------------------------10 2.6 Quality of aggre gates ----------------------------------------------------------------------------12 2.6.1 Simple test for organic impurities (Colorimetric test) -------------------------------------------13 CHAPTER 3: CEMENT ---------------------------------------------------------------------------14 3.1 Introduction --------------------------------------------------------------------------------------14 3.2 Manufacture of cement --------------------------------------------------------------------------14 3.2.1 The wet process------------------------------------------------------------------------------------14 3.2.2 The dry process------------------------------------------------------------------------------------15 3.2.3 Comparison of the wet and dry processes of cement manufacture ------------------------------15 3.2.4 Cement manufacturing industries in Uganda ----------------------------------------------------15 © Julius Ngabirano i
3.3 3.4 3.4.1 3.4.2 3.5 3.5.1 3.5.2 3.6 3.6.1 3.7 3.8 3.8.1 3.8.2 3.8.3 3.8.4 3.8.5
Chemical composition of cement ---------------------------------------------------------------15 Se tting and hardening of cement ---------------------------------------------------------------16 Functions of the various cement compounds-----------------------------------------------------16 False set--------------------------------------------------------------------------------------------17 Type s of cement ----------------------------------------------------------------------------------17 Common types of cement--------------------------------------------------------------------------17 Special cements------------------------------------------------------------------------------------18 Admixture s ---------------------------------------------------------------------------------------19 Precautions taken when using admixtures -------------------------------------------------------19 Transportation and storage of cement ---------------------------------------------------------20 Physical prope rties of cement -------------------------------------------------------------------20 Consistence of standard paste --------------------------------------------------------------------20 Setting time ----------------------------------------------------------------------------------------21 Soundness------------------------------------------------------------------------------------------21 Fineness of cement --------------------------------------------------------------------------------22 Strength of cement---------------------------------------------------------------------------------22
CHAPTER 4: WATER-----------------------------------------------------------------------------24 4.1 Functions of wate r -------------------------------------------------------------------------------24 4.2 Quality of wate r for concrete works -----------------------------------------------------------24 4.3 Wate r-ce ment ratio ------------------------------------------------------------------------------24 4.4 Se a wate r------------------------------------------------------------------------------------------25 CHAPTER 5: FRESH CO NCRETE--------------------------------------------------------------26 5.1 Introduction --------------------------------------------------------------------------------------26 5.2 Workability ---------------------------------------------------------------------------------------26 5.2.1 Slump test ------------------------------------------------------------------------------------------26 5.2.2 Compacting factor test ----------------------------------------------------------------------------28 5.2.3 The Vebe (V-B consistometer) test----------------------------------------------------------------28 5.2.4 Factors affecting workability ---------------------------------------------------------------------29 5.3 Concre te stability---------------------------------------------------------------------------------29 5.3.1 Segregation ----------------------------------------------------------------------------------------29 5.3.2 Bleeding--------------------------------------------------------------------------------------------30 5.4 Mixing concrete ----------------------------------------------------------------------------------30 5.4.1 Hand mixing ---------------------------------------------------------------------------------------30 5.4.2 Mixing by machine --------------------------------------------------------------------------------31 5.5 Ge neral principles in the use of concre te mixe rs----------------------------------------------31 5.6 Type s of concre te mixe rs ------------------------------------------------------------------------32 5.6.1 Non-tilting drum mixers---------------------------------------------------------------------------32 5.6.2 Tilting drum mixers--------------------------------------------------------------------------------33 5.6.3 Split drum mixers----------------------------------------------------------------------------------33 5.6.4 Reversing drum mixers ----------------------------------------------------------------------------33 5.6.5 Forced action mixers------------------------------------------------------------------------------33 5.6.6 Continuous mixers---------------------------------------------------------------------------------33 5.7 Research-------------------------------------------------------------------------------------------34 5.7.1 Research (maintenance of concrete mixers)------------------------------------------------------34 CHAPTER 6: WO RKING WITH CO NCRETE – 1 --------------------------------------------35 6.1 Transporting concre te ---------------------------------------------------------------------------35 6.1.1 Sampling concrete for test purposes--------------------------------------------------------------35 © Julius Ngabirano
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6.2 6.3 6.3.1 6.3.2 6.4 6.5 6.6 6.6.1 6.6.2 6.6.3
Placing concrete ----------------------------------------------------------------------------------36 Compaction of concre te -------------------------------------------------------------------------37 Manual (hand) compaction -----------------------------------------------------------------------37 Machine compaction ------------------------------------------------------------------------------37 Concre ting in hot we athe r-----------------------------------------------------------------------38 Cold we athe r concre ting-------------------------------------------------------------------------39 Research-------------------------------------------------------------------------------------------40 Research (maintenance of vibrators) -------------------------------------------------------------40 Research (formwork) ------------------------------------------------------------------------------40 Research (steel reinforcements) ------------------------------------------------------------------40
CHAPTER 7: HARDENED CO NCRETE -------------------------------------------------------41 7.1 Concre te curing ----------------------------------------------------------------------------------41 7.2 Durability of concrete----------------------------------------------------------------------------41 7.3 Strength of hardene d concre te------------------------------------------------------------------41 7.3.1 Test for compressive strength ---------------------------------------------------------------------42 7.4 Othe r prope rties of hardene d concre te --------------------------------------------------------43 7.5 Concre te defe cts ----------------------------------------------------------------------------------43
YEAR
TWO-------------------------------------------------------------- 46
CHAPTER 8: MATERIAL AND CONCRETE TES TS - PRACTICE -----------------------46 8.1 Re vie w---------------------------------------------------------------------------------------------46 8.2 Practical tests -------------------------------------------------------------------------------------46 CHAPTER 9: WO RKING WITH CO NCRETE - 2 --------------------------------------------47 9.1 Concre te joints -----------------------------------------------------------------------------------47 9.1.1 Construction joints --------------------------------------------------------------------------------47 9.1.2 Contraction (control) joints-----------------------------------------------------------------------47 9.1.3 Expansion joints -----------------------------------------------------------------------------------48 9.1.4 Guidelines in placement of isolation (contraction and expansion) joints-----------------------48 9.2 Finishing concre te--------------------------------------------------------------------------------48 9.2.1 Floating --------------------------------------------------------------------------------------------48 9.2.2 Trowelling -----------------------------------------------------------------------------------------49 9.3 Yield of a concre te mix --------------------------------------------------------------------------49 9.3.1 Determination of yield of a concrete mix---------------------------------------------------------50 CHAPTER 10: INTRO DUCTIO N TO PRESTRESSED CO NCRETE ------------------------51 10.1 Introduction --------------------------------------------------------------------------------------51 10.2 Me thods of prestressing concre te---------------------------------------------------------------51 10.3 Comparison of prestre sse d and reinforce d concre te be ams ---------------------------------51 10.4 Applications of prestresse d concre te -----------------------------------------------------------52 CHAPTER 11: CONCRETE MIX DESIGN ------------------------------------------------------53 11.1 Definition------------------------------------------------------------------------------------------53 11.2 Type s of Mixes------------------------------------------------------------------------------------53 11.3 Trial mixes ----------------------------------------------------------------------------------------53 11.4 Conside rations in mix proportioning ----------------------------------------------------------53 © Julius Ngabirano
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11.5
Mix design proce dure----------------------------------------------------------------------------54
CHAPTER 12: PRECAST PRO DUCTS IN UGANDA------------------------------------------56 12.1 Research-------------------------------------------------------------------------------------------56 APPENDIX -----------------------------------------------------------------------------------------------57 A1: Forms----------------------------------------------------------------------------------------------57 A2: Tables----------------------------------------------------------------------------------------------59 A3: Figures---------------------------------------------------------------------------------------------60
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LIST O F TABLES
Table 1-1:
Conversion from volume to weight proportions ................................................. 4
Table 10-1:
Comparison of prestressed and reinforced concrete beams using Figure 10-1 ... 52
Table A201:
Approximate compressive strength (N/mm 2 ) of concrete mixes made with a freewater/cement ratio of 0.5............................................................................... 59
Table A202:
Approximate free water contents (kg/m3 ) required to give various levels of workability................................................................................................... 59
Table A203:
BS 882:1973 Grading requirements for fine aggregates................................... 59
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LIST O F FO RMS
Form A101:
Cube crushing strength result sheet ------------------------------------------------57
Form A102:
Concrete mix design form-----------------------------------------------------------58
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LIST O F FIGURES
Figure 2-1:
Quartering method of sampling aggregates....................................................... 8
Figure 2-2:
A riffler........................................................................................................... 8
Figure 2-3:
Experimental determination of the percentage bulking of sand ........................... 9
Figure 2-4:
Percentage bulking against moisture content for different sizes of sand............... 9
Figure: 2-5:
Grading curve for a sample of aggregates....................................................... 11
Figure 2-6:
Typical grading curve for gap graded aggregates............................................ 12
Figure 3-1:
The Vicat apparatus ...................................................................................... 20
Figure 3-2:
Le Chatelier apparatus .................................................................................. 22
Figure 4-1:
Compressive strength versus water-cement ratio ............................................. 25
Figure 5-1:
The slump cone ............................................................................................. 27
Figure 5-2:
Slumps of various concrete mixes ................................................................... 28
Figure 10-1:
Comparison of prestressed and reinforced concrete beams .............................. 52
Figure A301:
Relationship between standard deviation and characteristic compressive strength........................................................................................................ 60
Figure A302:
Graph of estimated wet density of fully compacted concrete (specific gravity is given for saturated, surface dry aggregates) ................................................... 60
Figure A303:
Relationship between proportion of fines (percentage of fine aggregates of the total aggregates) and free water/cement ratio for various workabilities (maximum coarse aggregate size 10 mm) ....................................................... 61
Figure A304:
Relationship between proportion of fines (percentage of fine aggregates of the total aggregates) and free water/cement ratio for various workabilities (maximum coarse aggregate size 20 mm) ....................................................... 62
Figure A305:
Relationship between proportion of fines (percentage of fine aggregates of the total aggregates) and free water/cement ratio for various workabilities (maximum coarse aggregate size 40 mm) ....................................................... 63
Figure A306:
Relationship between compressive strength and free water/cement ratio .......... 64
Figure A307:
Cement manufacture (dry and wet processes) ................................................. 64
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REFERENCES
Alan Everett (1989), Materials, Mitchell’s N. Jackson, R. K. Dhir (1988), Civil Engineering Materials, Fourth edition, Macmillan Singh, Engineering Materials British Standards Publications from Ministry of Works and Transport – Uganda Publications from Uganda Institution of Professional Engineers Internet
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PREAMBLE
This book has been solely written for use by students undertaking a programme leading to the awards of Ordinary Diploma in Civil and Building Engineering and Ordinary Diploma in Water Engineering. However, it may also be of great use to students pursuing a Higher Diploma in the same disciplines.
Special appreciations are extended to my lovely parents and all those who have been an advantage to my career, professional and moral development.
Julius Ngabirano B. SC. CIV. ENG. (MUK), CCA (MUK), GMUIPE
Se ptembe r 2011
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YEAR
CHAPTER 1:
ONE
INTRODUCTION
1.1 Definition Concrete is an artificial building and structural material obtained by mixing particles of a semi-inert material (aggregates), binding material (cement) and water in correct proportions. The reasonable amount of water added is used to hydrate the binding material. Concrete is presently one of the most popular materials for the structural parts of buildings and other civil engineering works. When reinforced with steel, it has a very high capacity for carrying loads. Depending upon the cementing or binding materials used, we use lime concrete or cement concrete. Cement concrete is most common. 1.2 Limitations of concre te Concrete has some limitations which should be realized by both the designers and the builder. T he main limitations are:
It has a low tensile strength: This can be avoided by reinforcing the concrete by use of steel bars or wire fabric.
Thermal movements: During setting and hardening of concrete, the temperature is raised by the heat of hydration of cement it then gradually cools. T hese temperature changes can cause severe thermal strength and early cracking. This can be prevented by providing expansion and contraction joints.
Drying shrinkage and moisture movements: Concrete shrinks as it dries out and even when hardened, it also expands and contracts with wetting and drying. These movements necessitate provision of contraction joints at intervals.
Creep: concrete gradually deforms under load and this can be prevented by using reinforcements both horizontally and vertically.
Permeability: concrete is permeable and thus joints can form ingress of water. This can be prevented by; Use of admixtures Proper compaction of concrete Increase on the cement aggregate ratio etc.
1.3 Cement concrete This is classified under the following headings: Mass / plain concrete Reinforced concrete 1.3.1 Mass (plain) concrete This is concrete which is strong when it is on a firm ground and the load to be carried is not too heavy. Mass concrete is quite strong in compression but weak in tension. To make it strong in tension, © Julius Ngabirano
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steel bars (reinforcements) are imbedded in concrete and this then becomes reinforced concrete. When the concrete has no steel reinforcement, it is called mass / plain concrete. 1.3.2 Reinforced concrete This is concrete with extra strength created so that it counteracts with the tension and makes the concrete stronger. This is achieved by adding steel bars (usually of different shapes and/or sizes), wire fabric and expanded metal. Depending on the density, concrete can also be grouped into three types: Light weight concrete Normal weight concrete Heavy weight concrete 1.3.3 Light weight concrete This is concrete with a low density practically lower 1850 kg/m3 . The use of low density concrete is governed primarily by economic considerations and can be achieved by; Omitting the fine aggregates from the mix so that a large number of interstitial voids is present Use of light weight aggregates e.g. burnt clay products, slates, shale, pumice, volcanic ash etc. Introduction of large voids within concrete. This type concrete is commonly called aerated or foamed or cellular or gas concrete. Uses of light weight concrete include: It is commonly used in multi-storeyed structures Used in precast floors It is used in roof limits Among the advantages of reducing the density of concrete is the use of smaller sections with a corresponding reduction in the size of foundations. The formwork is also designed to withstand a lower pressure than would be in ordinary concrete. The total weight of materials to be handled during construction is reduced with a subse quent increase in productivity. Light weight concrete also gives a better thermal insulation than ordinary concrete. 1.3.4 Normal weight concrete This is the type concrete got from heavy aggregates e.g. sand, gravel, crushed stones. The density is in the range 2200 – 2600 kg/m 3 . Uses of normal weight concrete include: It is used for radiation protection (shielding against x-ray in radioactivity) It is used in massive engineering works It is used in construction of bridges and dams. 1.3.5 Heavy weight concrete This is concrete made of very heavy aggregates e.g. iron ore, barites and steel punching. While much concrete used in radiation protection is of normal weight, the use of high density concrete becomes necessary when the thickness of the shield is governed by the space pace available. Therefore, if there is limited space for the shield thickness, heavy weight concrete becomes the best alternative. © Julius Ngabirano
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1.4 Concre te mate rials Concrete forming materials shall be carefully selected so as to get quality concrete. It has already been introduced that cement concrete materials include cement, fine aggregates, coarse aggregates and water. Cement Normally, Ordinary Portland cement is used. However, for special conditions other types of cement suiting particular requirements are used. Cement being hygroscopic attracts moisture quickly and sets. Therefore storage of cement should be carefully attended to and no set or partially set cement should be used since it will have already lost its original strength. Fine aggregates Sand and cr ushed stones are the commonly used fine aggregates. T hese should be clean and must contain neither animal nor vegetable matter nor lumps of clay. Coarse aggregates Stone ballast, gravel and brick ballast are commonly used. T hese should be clean, free from organic matter and should be well graded i.e. they should have particles of various sizes so that voids of bigger particles are filled up by the particles of smaller sizes. Water Only good and clean water should be used for making concrete. It should be free from silt, salts or any other organic matter. Generally water that is good for drinking is good enough for concrete works. 1.5
Batching of ingre dients This is the measurement of the concrete materials (cement, aggregates and water) in their correct proportions. This done through the following methods: Batching by volume Batching by weight
1.5.1 Batching by volume This is commonest method in Uganda and is where a batch box is used to measure the ingredients. The basis of batching by volume is generally 1 part of cement to n parts of sand (fine aggregates) and 2n parts of coarse aggregates. The course aggregates are usually (but not always the case) twice the sand where as the ratio of sand to cement depends upon the desired strength of the concrete. T he mass of one bag of cement is 50 kg and is about 34.5 liters. When cement is taken out of bags, it becomes loose, showing a considerable increase in volume. As such, batching concrete by taking into account the volume of loose cement is likely to result in less cement being mixed in the concrete. Therefore, in batching ingredients by volume, materials corresponding to the whole number of cement bags should only be used. The convenient method it use an open measuring gauge box with a capacity of 34.5 liters. Batches of fine and coarse aggregates required could then be measured in multiples of these boxes in accordance with the required proportions of the ingredients. In measuring (bulking) sand, due allowance should be made otherwise the concrete would be under-sanded. The advantages of volume batching include: It is easy to carry out It is economical on small sites However, this method of batching is less accurate.
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1.5.2 Batching by weight This is where materials are weighted on site and batched according to their weights in the correct proportions. T his is done by using weighing machines. Provided that weighing machines on site retain their accuracy, errors in proportioning are negligible. Weighing machines need careful maintenance and regular calibration if reasonable accuracy is to be maintained. It has an advantage of being more accurate than batching by volume. However it is not easy to arrange on smaller or isolated sites. 1.5.3 Conversion from volume to weight proportions If we assume that a 50 kg bag of cement occupies 0.0347 m3 , then the equivalent proportions of a nominal 1:2:4 mix are 50 2 0.0347 4 0.0347
The table below shows how a 1:2:4 nominal mix can be converted into a mix by weight when the usual assumptions for the bulk densities of cement, sand and coarse aggregates are made. Loose dry Proportion by Mass of per Volume of per Proportion density volume bag of cement bag of cement by weight 3
Cement Sand Coarse aggregates
kg /m 1440 1600 1360
1 2 4
Assumed volume of a 5 0kg bag of cement = 0.0347 m Table 1-1: Conversion from volume to weight proportions
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kg 0.0347 0.0694 0.1388
kg /m 50.0 111.0 188.8
1.00 2.22 3.78
3
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CHAPTER 2:
AGGREGATES
2.1 Introduction Aggregates are inert materials mixed with a binding material like cement, lime and water in the preparation of mortar or concrete. Aggregates constitute 70-75% of the total volume of mass concrete. Therefore the properties of concrete at large extent depend on the properties of aggregates used. Aggregates can be natural or artificial: → Natural aggregates are formed from the naturally occurring materials/rocks. T he Uganda construction industry entirely uses natural aggregates. During this course of study, though not repeatedly specified, the term aggregates shall refer to natural aggregates unless otherwise specified. Natural aggregates are usually of normal weight as they produce concrete of density within the usual range of 2200 – 2600 kg/m 3. Natural aggregates include barite, iron ore etc. → Artificial aggregates are manufactured from industrial products and are usually of light weight or heavy weight. Artificial aggregates include expanded clay, shale, and slate, steel punching, sheared bars etc. The advent of these aggregates has been attributed by the growing shortage of naturally-occurring aggregates in some countries like UK. It should be noted that some artificial aggregates are manufactured from waste materials that could have otherwise been discarded. Aggregate can also be of light, normal, and heavy weights. → Light weight aggregates have a high porosity which results in a very low density. They produce concrete of low density practically lower than 1850 kg/m3 . When used, they reduce the dead weight of structure allowing the use of smaller supporting members and reduction of foundation costs. They also improve on the thermal insulation. They include pumice which is a volcanic rock, clinker which is a well burnt fused furnace residue, diatomite, fibre, blast furnace slag, expanded clay, slates, shale etc. → Normal weight aggregates produce concrete of density in the range of 2200 – 2600 kg/m3 . These include sand, gravel, crushed stones obtained from superficial deposits of rivers, lakes, seas or excavated from soil deposit (pit sand). → Heavy weight aggregates have a very high density and produce concrete of density greater than 2600 kg/m3 . They are used in production of heavy concretes for nuclear and radioactive shielding and include magnetite, barites (Barium Sulphate), iron ore and steel punching. T heir specific gravities are greater than 4. Aggregates can also be crushed or uncrushed depending on how they are produced. Uncrushed aggregates are reduced to its present size by natural agents while crushed aggregates are obtained by a deliberate fragmentation (breaking) of rock. Depending on the particle sizes, aggregates are divided into two classes: ♥ Fine aggregates ♥ Coarse aggregates 2.2 Fine aggregate s These are aggregates that pass through a 5 mm sieve and are entirely retained on a 0.15 mm sieve. Most commonly used fine aggregates are sand, crushed stone, ash and surkhi. Sand
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It consists of small grains of silica and is formed by disintegration of rocks caused by weather. Good quality sand should have the following requirements; Should be hard, durable, clean and free from organic matter Should not contain harmful impurities such as salts, alkalis or other material which will affect hardening and attack requirements In natural sand, the amount of clay, fine silt and fine dust should not be more than 4% by weight. There are different types of sand: i.
Pit sand / quarry sand: It is found as deposits in soil and has to be excavated out. T he grains are generally sharp and if angular and free from organic matter and clay, it is extremely good for use in mortar and concrete. ii. River sand: It is obtained from the banks and beds of the rivers and may be fine or coarse. iii. Sea sand: It consists of fine rounded grains of brown colour and is collected from sea beach. It usually contains salts and therefore should be thoroughly washed to remove the salts. Crushed stone It is obtained from crushing waste stone from quarries to the particle size of sand. When crushed from good quality stone, it produces an excellent fine aggregate. Surkhi This is obtained from powdered broken brick (burnt brick). It is commonly used in the preparation of lime mortar. 2.3 Coarse aggre gates These are aggregates that are retained on a 5.0 mm sieve. T hey range between 5 - 19 mm diameter / size. Mostly commonly used coarse aggregates are stone ballast, brick ballast, gravel and/or shingle. Stone ballast Stones that are free from undesirable mineral constituents and are not soft or laminated are broken and screened to have stone ballast for use in concrete. Brick ballast Where aggregates from natural resources are either not available or expensive, broken brick are used as a coarse aggregate in lime concrete. Only well burnt good bricks should be used. Gravel and shingle These are obtained from river beds, quarries and sea shores. Being hard and durable, these are extensively used in the preparation of concrete. 2.3.1 Functions of aggregates They provide the skeleton and strength to concrete They reduce on the material costs. Generally, aggregates occupy a large percentage of concrete and are less costly than cement. They help in restraining the amount of drying shrinkage on the setting and expansion of concrete. They determine the density of concrete i.e. by using high or low density aggregates. They offer resistance to wear by abrasion They impart special properties to concrete e.g. fire resistance and thermal insulation Fine aggregates fill in voids within coarse aggregates and also reduce on cement consumption.
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2.3.2 i.
ii. iii. iv. v. vi. vii. viii.
2.4
Qualities of good aggregates Cleanliness: impurities such as dust, clay, organic matter prevent proper bonding between cement paste and aggregates. This would affect the strength of concrete. The percentage of impurities should not exceed 3%. In case of dust, aggregates should be washed before use. Aggregates should be hard and tough to resist forces of abrasion, impact, crushing, etc. Usually strong aggregates make strong concrete. Aggregates should be durable They should not be liable to any form of shrinkage, swelling and decomposition. Aggregates should be sound and thus they should not be thin and elongated to avoid breakages. Aggregates should be inert, they should not react to water component in cement so as not to take part in the hydration process They should have a rough surface and a regular shape to promote bonding For dependable results, the coefficient of expansion and thermal conductivity of aggregates should be equal or nearly equal to that of cement Te sting aggregates
2.4.1 Sampling When a sample of aggregates is taken for testing, it should be a representative sample of the whole stock pile thus it is vital to correct little aggregates from different places rather than a sample from one place only. Under favorable conditions, at least 10 proportions should be dra wn from different parts of the stock pile. And all these portions should be combined to form the main sample to be sent to the laboratory for testing. There are two ways of obtaining (and / or reducing) the size of a sample, each essentially dividing the sample into two similar parts; Quartering method Riffling method Quartering method Procedure: i. Mix the main sample thoroughly and in the case of fine aggregates, dampened them to avoid segregation. ii.
Heap the material into a cone and then turn over to form a new cone.
iii.
Repeat step (ii) at least two times, the material each time being deposited at the apex of the cone so that the fall of the particles are evenly distributed around the circumference.
iv.
Flattened the final cone and then divide it into quarters.
v.
One pair of diagonally opposite quarters is discarded and the remainder forms the sample for testing. If the resultant sample is too large, it can still be reduced by steps iii and iv. Care must be taken to include all the fine material in the appropriate manner.
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Figure 2-1:
Quartering method of sampling aggregates
In the above case, quarters 1 and 3 forms the sample for testing. Alternatively, 2 and 4 can also form the sample. Riffling method This is done using a riffler. This is a box with a number of parallel vertical divisions, alternate ones discharging to the left and to the right. The sample is discharged into the riffler over its full width and the two halves are collected into two boxes at the bottom of the chutes on each side. One half is discarded and riffling of the other half repeated until the sample is reduced to the desired size. Below is the figure showing a riffler.
Figure 2-2:
A riffler
2.4.2 Bulking of sand The volume of a given mass of sand is dependent on the moisture content. The volume is at minimum when sand is either dry or wet and maximum when sand is damp. Films of water are formed on the particles and surface tension tends to hold them apart causing an increase in volume (bulking). Therefore, damp sand has more volume than dry sand or wet sand and fine sands are more bulky than coarse sands. When batching damp sand by volume, an allowance for bulking should be made otherwise the concrete or mortar mix will be under-sanded. T he allowance is made by increasing the apparent volume of sand and this depends on the percentage of moisture present and on the fineness of the sand. Finer sands bulks considerably more and reaches maximum bulking at higher water content than coarse sand. Bulking of sand can therefore be defined as an increase in volume of a given weight of sand caused by the films of water pushing the sand particles apart. © Julius Ngabirano
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Bulking test Apparatus: Flat bottomed cylinder Steel rule Steel rod Method i. ii. iii. iv. v.
Fill loosely packed damp sand in a cylinder about 23 of its capacity Measure the depth of the sand D Pour the sand on a tray Half fill the cylinder with water and gradually return the damp sand to it while carefully stirring with a steel rod to expel all air bubbles. Note the depth of the sand d ! "# · 100 "
Therefore, during volume batching, the volume of damp sand used will have to be increased by the above percentage. When expressed as a factor (not a percentage), it is sometimes called the bulking factor. ! "# & "
Figure 2-3:
Experimental determination of the percentage bulking of sand
Figure 2-4:
Percentag e bulking against moisture content for different sizes of sand
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Therefore, the following adjustments are adopted to account for bulking of sand: → During volume batching, the volume of damp sand to be used will have to be increased by the percentage bulking. → The amount of water present in the aggregates should be determined. The amount added to the mix will have to be decreased by the weight of free moisture present in aggregates in order not to alter the water-cement ratio. → During weight batching, the weight of damp sand to be used will also be increased. 2.5 Grading of aggregates This refers to the particle size distribution of aggregates. It is obtained by sieve analysis which involves dividing a sample of aggregates into fractions, each (fraction) consisting of particles within specific size limits, these (limits) being the openings of standard test sieves. This is done by sieving the aggregates through a series of standard test sieves and followed by calculating the percentage by volume passing the various sieves. From the results obtained, aggregates can be described as: → Well graded aggregates → Poorly graded (uniformly graded) aggregates → Gap graded aggregates Well graded aggregates consist of particles ranging from the smallest size to the largest size. Poorly (uniformly) graded aggregates consist of particles of almost the same size. This is likely to cause the mix to be harsh and therefore difficult to compact. Gap graded aggregates consist of particles of extreme sizes with the intermediates sizes missing. A danger of segregation is likely to occur when too many intermediate sizes are missing. Well graded aggregates are best because they interlock properly and leaving minimum volume of voids to be filled with the costly cement. T hey flow together easily giving a workable mix, enabling the use of a lower water cement ratio resulting into a strong concrete. 2.5.1 Grading test for fine aggregates (sieve analysis) Apparatus BS sieves of the following sizes: 4.76 mm, 2.40 mm, 1.20 mm, 600 µm, 300 µm, 150 µm. Pan Physical balance Tray Procedure i. Weigh 200 g of sand ii.
Stand the sieve of the larger mesh size in tray and put the weighed sample on to the sieve
iii.
Shake the sieve horizontally in all directions for at least 2 minutes until no more than a trace of sand passes. Ensure that all sand passing through fall on to the tray.
iv.
Weigh any material retained in the sieve
v.
Pass materials corrected in the tray via the sieve of mixed smaller mesh size as in ii and iii and weigh any material retained.
vi.
Repeat the procedure for the retaining sieves in the order of diminishing mesh size.
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vii.
Then tabulate your results as shown below and plot the percentage by weight of aggregates passing each sieve against the sieve sizes. T he sieve sizes are plotted on the horizontal axis with a logarithmic scale to base 10.
Results BS Sieve
Ma terial retained
Mesh size
Weight retained
4.76 mm 2.4 mm 1.2 mm 600 μm 300 μm 150 μm Pan
0 1.6 5 27.4 98 50 18
TOTAL
200
% retained
0 0.8 2.5 13.7 49 25 9
% by weigh t pas sin g Cumulative % Cumu lative % pas sin g retain ed
100 99.2 96.7 83 34 9
0 0.8 3.3 17 66 91 100
1 00
% by weight of aggregates passing
90 80 70 60 50 40 30 20 10 0 BS Sieve Sizes
Figure: 2-5: Grading curve for a sample of aggregates
Gap graded aggregates As already explained, gap grading is a type of grading in which one or more intermediate size fractions are omitted. Gap graded aggregates have a grading curve similar to the one shown below. The graph below shows limited percentage of particles of size between 2.4 mm and 300 µm where the graph is almost horizontal are present.
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% b y weight o f aggregates p assing
100 90 80 70 60 50 40 30 20 10 0
BS Sieve S izes
Figure 2-6:
Typical grading curve for gap graded aggregates
Gap graded aggregates can be applied in: Preplaced concrete: This is an operation in which the first stage involves placing and compacting coarse aggregates in the formwork. In the second stage, remaining voids are filled with mortar. Preplaced concrete is also referred to as prepacked concrete or intrusion concrete or grouted concrete. T his type of concreting is use d where ordinary concreting methods may not easily apply and also in sections containing a large number of embedded items that have to be precisely located. Since coarse and fine aggregates are placed separately, the danger of segregation is eliminated. Exposed aggregated concrete. In this case, a pleasing finish is obtained since a large quantity of only one size of coarse aggregates becomes exposed after treatment. 2.6 Quality of aggre gates Aggregates are usually ordered from quarry site by informing the supplier of the required sizes. It is however advisable to first visit the quarry site and inspect the rock type from which the aggregates are being obtained from. This will help in ascertaining the quality of the aggregates. Aggregates need to be handled carefully to avoid segregation and breakage on site. Different sized aggregates should be handled and stockpiled separately and remixed only when being fed to the concrete mixer in the desired proportions. To avoid breakages, coarse aggregates should be lowered into bins by means of rock ladders and not dropped from excessive height. Good aggregates for use should be: ♥ Free from impurities that interfere with the processes of hydration of cement ♥ Free from coatings that could prevent development of good bond between the aggregate and the cement paste ♥ Free from certain individual particles which are weak or unsound in themselves ♥ Non-reactive with the cement paste ♥ Sound i.e. should be able to resist excessive changes in volume as a result of changes in physical conditions. ♥ Should be of the required strength It is better to check the quality of aggregates by making actual test cubes.
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2.6.1 Simple test for organic impurities (Colorimetric test) A mixture of aggregates and 3% solution of NaOH is placed in a bottle and vigorously shaken to allow intimate contact necessary for chemical reaction. It is then left for 24 hours and the color of the solution noted. T he greater the organic content, the darker the color. Yellow implies a harmless organic content whereas a darker color implies a rather high organic content. When a darker color is obtained, further tests e.g. compressive strength tests are carried out and the results compared with concrete of the same mix proportions but made with aggregates of known quality.
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CHAPTER 3:
CEMENT
3.1 Introduction Cement is a finely ground powder that when mixed with water, a chemical reaction (hydration) takes place which in time produces a very hard and binding medium for the aggregate particles. It is the binding material and the most expensive component of concrete. Generally, cement performs the following functions: ♥ Provide lubrication of the fresh plastic mass ♥ Fill the voids between the particles of the inert aggregates and thus produces water tightness in the hardened product. ♥ To give strength to the concrete in its hardened state. 3.2 Manufacture of cement Cement is made primarily from calcareous materials such as from limestone, and from alumina and silica found as clay or shale. Procedure Quarrying of raw materials (limestone and clay) Transportation of raw materials to factory by Lorries, tippers, conveyor belts, etc. Cleaning of raw materials to remove dirt, leaves, etc. Crushing of raw materials to smaller sizes Mixing of raw materials (clay and limestone) in the ratio of 1:3 Further grinding of raw materials and storage There are two methods of the manufacture of cement. 3.2.1 i. ii. iii. iv.
v.
vi. vii. viii.
The wet process The dry process
The wet process Grounded limestone and clay are mixed in the appropriate ratios and mixed with water to form slurry in wash mills. The slurry is sieved and any coarse particles are returned to the wash mills for re-grinding. The slurry is fed into large storage tanks where it is agitated to prevent settlement and samples are taken for testing for the correct chemical composition (1:3). The slurry is fed into the upper end of a large rotary kiln which rotates slowly about its axis. This kiln is a refractory-lined steel cylinder of up to 7.5 m diameter and 230 m long and is inclined at about 15 0 to the horizontal. The slurry enters the kiln at the cooler end and by rotation of the kiln in conformation with the shape, it is subjected to a gradual rise in temperature up to 1500 0 C and undergoes successive chemical reactions Sintering occurs between 1300 – 1400 0 C and the material fuses into small balls known as cement clinker. The clinker is passed through coolers and then to ball mills where it is ground to the required fineness during which 3.5% gypsum is added to control the rate of setting of the cement. From the ball mills, cement is passed through a separator and fine particles are blown by an air current to the storage silos where it is packed for sale. The coarse particles are passed through the mill once again. © Julius Ngabirano
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ix.
An automatic packing plant fills paper bags with a standard weight of 50 kg.
3.2.2 i. ii. iii. iv. v.
3.2.3
The dry process The raw materials are crushed and fed in the correct proportions into a grinding (ball) mill where they are dried and reduced in size to a fine powder called raw meal. The dry powder (raw meal) is pumped to a blending silo where final proportioning is done to ensure the correct chemical composition. In the blending silo, the material is then aerated to obtain a uniform mixture. The aerated mixture will behave almost like a liquid with a moisture content of about 0.2%. The raw meal is then pre-heated to about 800 0 C to remove all the moisture. It is then fed to the rotary kiln and the subsequent operations are the same as those in the wet process of manufacture. Comparison of the wet and dry processes of cement manufacture Wet process
Dry process
Mixing and grinding are done in water Mixing and grinding are done in a dry condition Requires more energy for burning since the Low energy required for burning due to a material has a higher moisture content relatively low moisture content Size of the rotary kiln is larger The rotary kiln is smaller since the raw meal contains no moisture to be driven off and it is already pre-heated Relatively expensive Economical especially when materials are comparatively dry 3.2.4 Cement manufacturing industries in Uganda The Cement manufacturing industries in Uganda are Hima and Tororo cement industries. The major limestone deposits at Hima and Tororo have provided the raw materials for Uganda’s Portland cement industry. There are also a number of limestone deposits found at Muhokya in Kasese, Dura, kaku, Bududda, Metu in Moyo and Moroto. 3.3 Chemical composition of cement The following reactions take place in the kiln;
Loss of water in raw materials (dehydration) Loss of carbondioxide from limestone (decarbonation) leaving Calcium Oxide. Creation of the following oxides (shown against the oxides are their approximate composition limits of cement)
Oxide Content in cement (%) CaO 60 - 67 % SiO2 17 - 25 % Al2 O3 3 - 8% Fe2 O3 0.5 - 8 % MgO 0.1 - 4% Alkalis 0.2 - 1.3 % SO3 1 - 3% Fusion and chemical combination of these oxides. Four main compounds are formed in cement clinker as a result of chemical combination of the oxides mentioned above
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Name of compound Oxide composition Abbreviation Tricalcium Silicate 3CaO.SiO2 C3 S DiCalcium Silicate 2CaO.SiO2 C2 S TriCalcium Aluminate 3CaO.Al2 O3 C3 A . T etraCalcium Aluminoferrite 4CaO.Al2 O3 Fe2 O3 Ca4 AF The properties of cement and concrete depend on the abundancy of these compounds. If some CaO remains uncombined, it causes cracking in concrete. 3.4 Se tting and hardening of cement When water is added to cement, a cement paste is formed. This paste gradually changes from a fluid to a rigid state. T he term setting is used to describe this stiffening state. Once set, cement paste gradually develops strength and forms hardness. T his is referred to as hardening. The process of setting and hardening is caused by the selective hydration of cement compounds. Hydration includes all the reactions of cement and water; 2C3 S + 6H
C3 S2 H3 + 3Ca(OH) 2
2C2 S + 4H
C3 S2 H3 + Ca(OH)2
C3 A + 6H
C3 AH6
Hydrate abbreviation C3 S2 H3 C3 AH6
Hydrate name Calcium silicate Hydrate Calcium Aluminate Hydrate
3.4.1 Functions of the various cement compounds ' ( It actively hydrates with water to form Calcium silicate Hydrate and Calcium Hydroxide which have the cementing properties. It causes early strength development between 0 – 14 days by reducing the setting time and quickening hardening. T he Calcium Hydroxide provides the alkali medium for protection against corrosion of steel reinforcement. ' ) The reaction of ' ) with water is very violent and leads to rapid stiffening of the paste, a phenomenon called flash set. In order to control this rapid hydration, calculated amounts of gypsum '(*+ .2,- * are added to the cement clinker during cement manufacture. ( ./ The durability of concrete in sulphate medium is governed by the TriCalcum Aluminate content. Sulphate ions are combined with DiCalcium ions to form an expansive compound which causes disintegration of the structure. T o guard against sulphate attack, cement with small amounts of TriCalcum aluminate should be used. '+ )0 It hydrates in the same way as ' ) and lowers the temperature during hydration and is responsible for imparting a grey colour to cement.
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Note: Rapid hardening cement arises from a high ' ( content. Low rate of strength development and low heat cement is due high '- ( and low ' ( content. A very low ' ( content increases resistance to sulphate attack. 3.4.2 False set This is a name given to the abnormal premature stiffening of cement within a few minutes of mixing with water. It differs from flash set in that no heat is evolved and remixing the cement paste without addition of water restores the plasticity of the paste until it sets in the normal manner and without a loss of strength. Causes of false set include; → Dehydration of gypsum during grinding in the cement manufacture → Presence of alkalis in cement → Activation of C3 S by aeration at moderately high humidity. 3.5 3.5.1
Type s of cement Common types of cement
1. Ordinary Portland cement (OPC) This is the most common cement in use. It is suitable for use in general construction works where there is no exposure to sulphates. This cement is unsound due to presence of free lime. 2. Rapid hardening Portland cement T his generally has high tricalcium silicate content which when combined with the finest grinding contributes towards a high early strength. It is applicable when rapid strength development is desired e.g. When formwork is to be removed early for reuse When sufficient strength for further construction is wanted as quickly as possible. Desirable for construction at low temperatures due to a high rate of heat liberation. However, the rapid gain of strength implies a high rate of heat development and therefore this cement should not be used in mass construction or large structural sections. Its soundness and chemical composition is similar to that of OPC. 3. Extra rapid hardening Portland cement It has a higher rate of strength development than rapid hardening Portland cement. Its strength is about 25% greater at 1 or 2 days. T his cement is obtained by inter-grinding a regulated amount of Calcium Chloride with rapid hardening Portland cement and is suitable for cold weather concreting or when early strength is required but is not permitted for reinforced concrete due to risks of corrosion. It should strictly be stored under dry conditions, and be used within one month of dispatch from the factory. 4. Ultra-high early strength Portland cement This is produced by separating the finest particles from rapid hardening Portland cement during manufacture. T he result is cement with a very high specific surface (very fine particles). Its high fineness and very high proportion of gypsum gives it a very rapid rate of strength development which is more than that of rapid hardening Portland cement. It has no admixtures and is therefore suitable for use in reinforced and prestressed concrete.
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3.5.2
Special cements
5. High Alumina cement This is produced by fusing together in a furnace a mixture of limestone and bauxite (Aluminium Ore). It is very expensive and has the highest rate of strength development and with good resistance to sulphate attack. It has a slow initial setting time but the final set follows the initial set more rapidly than I the case of Portland cements. 6. Low heat Portland cement It has a high proportion of Dicalcium silicate mainly at the expense of the tricalcium silicate and is therefore slow in hardening and produces less heat than other cements. 7. Sulphate resisting Portland cement It has a better performance in resisting sulphate attack than ordinary Portland cement due to the reduction in the tricalcium aluminate content. 8. White Portland cement It has the same properties as ordinary Portland cement but is manufactured from raw materials containing less than 1% Iron. Its cost is about 3 – 4 times that of ordinary Portland cement. Architecturally, it helps to achieve the desired finish (colour) and also avoid staining. When coloured pigments are used, they should not affect the development of the cement strength. 9. Coloured Portland cements They are made by adding suitable pigments to ordinary Portland cements in case of deep colours and to white Portland cement when pale shades are required. 10. Air-entraining cements These are Portland cements to which an air entraining agent has been interground during the manufacturing process. 11. Portland blast-furnace cement T his is manufactured by grinding a mixture of Portland cement clinker and blast furnace slag. T he proportion of blast furnace slag is made not to exceed 65% of the weight of the mixture. It has a low heat of hydration, longer setting time, and requires a relatively low energy during manufacture. 12. Portland - Pozzolanic cements They are produced by grinding together a mixture of Portland cement and pozzolana which may be a naturally active material such as volcanic ash or pumicite or an active product such as pulverized fly ash, burnt clay or shale. Pozzolanic cements have a high resistance to chemical disintegration than the base Portland cement which they contain. They have a low heat of hydration, good resistance to sulphate attack and produces concrete with low permeability. 13. Super-sulphated cements They are made by grinding a mixture of 80 – 85% granulated blast-furnace slag with 10 – 15% calcium sulphate and about 5% of Portland cement clinker. The Portland cement clinker acts as an activator. T he cement is highly resistant to sea water, sulphates, acids and oils. T hese cements should not be mixed with Portland cements because the lime released by hydration of an excess amount of Portland cements interferes with the reaction between the slag and the calcium sulphate.
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14. Hydrophobic cement This is a Portland cement, to which an additive has been introduced, giving the cement particles a protective coating which inhibits the hydration of the cement. The protective coating (film) around the cement particles is broken during the mixing of concrete and normal hydration occurs but early strength is rather low. It has a better workability than ordinary Portland cement and improved waterproofing properties and can be stored in damp conditions for a long time without deterioration. 3.6 Admixture s These are suitable additives used to change the properties of cement to achieve other specific properties. T hey can be classified as: i. Accelerating admixtures are added to concrete either to increase the rate of early strength development or to shorten the time of setting or both. They can be applied in concrete works during rainy season, emergency repair work and for early removal of formwork. ii. Retarding admixtures slow the rate of hydration of concrete and are used on large or difficult works where partial setting before placing is completely undesirable. T heir use results in longer setting times, slower strength gain, enables long transit times but do not affect the long term mechanical properties of concrete. They are applicable in large concrete pours, sliding formwork and hot weather concreting. iii. Water reducing admixtures (plasticizers) increase the workability of fresh concrete, allowing it to be placed more easily with less consolidating effort and low water content. iv. Superplasticizers (high-range water-reducing admixtures) are a class of plasticizers which have fewer deleterious effects when used to significantly increase workability. They are more effective than plasticizers, produces flowing concrete with very high slump (more than 175 mm) for use in heavily reinforced structures, in placements where adequate consolidation by vibration cannot be readily achieved and in production of high-strength concrete at water – cement ratios ranging from 0.3 to 0.4 v. Air-entraining agents (admixtures) add and distribute tiny air bubbles in the concrete, which will reduce damage during freeze-thaw cycles thereby increasing concrete durability. They also improve workability and reduce bleeding and segregation of fresh concrete. However, these tiny air bubble s reduce strength of hardened concrete. vi. Pigments like ferrous oxides are used to change color of concrete for aesthetic reasons. vii. Bonding agents are used to create a bond between old and new concrete. viii. Pumping aids improve pumpability, thicken the paste and reduce dewatering of the paste. ix. Water proofing admixtures are common in construction of water retaining structures. 3.6.1 Precautions taken when using admixtures Check the job specifications Ensure that you use the correct admixture ♥ Never use an admixture from an unmarked container ♥ Keep containers closed to avoid accidental contamination Add the correct dosage ♥ Avoid adding ‘a little bit extra’ ♥ Always use a dispenser and wash it thoroughly at the end the day If possible/recommended, add the admixture to the mixing water Manufacturer’s recommended dosage is usually adequate Trial mixes are important to determine most effective dosage
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3.7 Transportation and storage of cement Cement for small jobs is usually packed in 50 kg bags and transported to the site by Lorries. Where large quantities are used, and cement silos are installed on the site, transport in bulk is more economical. Transport of cement is entirely a matter of keeping it dry and it is necessary to stack the bags under a shade or whatever cover is available. Every effort should be made to prevent moisture from coming into contact with the bags at any point and it is advisable to provide a raised floor covered with water proof material. Cement should be stacked in such a way that the bags first delivered can be used first. 3.8 Physical prope rties of cement The following are the physical properties considered when selecting cement to use for a particular purpose: Fineness Soundness Setting time Specific gravity Consistency Strength 3.8.1 Consistence of standard paste Standard consistency is that consistency at which the Vicat plunger penetrates to a point 5 – 7 mm from the bottom of Vicat apparatus mould. Below is the figure showing a Vicat apparatus.
Figure 3-1:
The Vicat apparatus
Apparatus Vicat apparatus Balance Gauging trowel Procedure Weigh approximately 400 g of cement and mix it with a weighed quantity of water. The time of gauging should be between 3 to 5 minutes. Fill the Vicat mould with the cement paste and level it with a trowel. Lower the plunger gently till it just touches the cement surface. Release the plunger allowing it to sink into the paste. Note the reading on the gauge. Repeat the above procedure taking fresh samples of cement and different quantities of water until the reading on the gauge is between 5 and 7 mm.
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Reporting of Results Express the amount of water (that produces a paste with a reading of 5 – 7 mm) as a percentage of the weight of dry cement. The usual range is 26 – 33 % 3.8.2 Setting time Cement has two setting times i.e. initial and final setting times. T hese setting times are measured using the Vicat apparatus. Apparatus Vicat apparatus Balance Gauging trowel Procedure for determination of initial set i. Prepare cement paste of standard consistency ii. Start stop-clock immediately after addition of water to cement iii. Fill the paste into a special mould and strike level. iv. Position the mould beneath the vicat needle, lower the needle gently to get contact with the surface of the paste, then release it observing the degree of penetration v. Repeat (iv) at intervals with the needle at different points on the surface until penetration is not beyond a point 5±0.5 mm from the base plate (bottom). vi. T ime from commencement of the addition of the mixing water to this condition gives the initial setting time of cement in hours and minutes. A minimum time of 45 minutes is prescribed for Ordinary Portland cement and rapid hardening Portland cement (BS 12 : 1978). Procedure for determination of final set i. Immediately after the initial setting time, and with the stop clock continuing, change the needle in the vicat apparatus. ii. The needle is fitted with a metal attachment hollowed out so as to leave a cutting edge 5 mm in diameter and set 0.5 mm behind the tip of the needle. iii. Continue as in step (v) of the initial set procedure. iv. Final set is said to have taken place when the needle, gently lowered to the surface of the paste, makes an impression on it but the circular cutting edge fails to do so. v. The final setting time is determined from the time when mixing water was adde d to the cement and should not be more than 10 hours for ordinary, rapid hardening, low heat, and blast-furnace Portland cements. Natural factors that may affect the setting time T emperature Humidity Wind velocity 3.8.3 Soundness This is the ability of a cement paste to resist changes in volume during hydration. Cement which exhibits expansion is said to be unsound. It is caused by presence of impurities liable to react with moisture resulting in expansion which can cause cracking, spalling or disintegration. T est for soundness is done using Le Chatelier apparatus. This apparatus (shown below) consists of a small brass cylinder split along its generatix. T wo indicators with pointed ends are attached to the cylinder on either side of the split. Hence, any expansion of the cement causes widening of the split. This widening is greatly magnified and can easily be measured. © Julius Ngabirano
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Figure 3-2:
Le Chatelier apparatus
Apparatus Le Chatelier apparatus Balance Water bath Procedure
Place the cylinder on a glass plate and fill it with a cement paste of standard consistence Cover the cylinder with another glass plate and place a small weight on this covering glass sheet Immerse whole assembly in water at about 20 – 25 0 C for 24 hours Measure and record the distance between the two indicators (d1 ) Immerse the mould in water again (at the temperature prescribed above) and gradually bring it to boiling point within a period of 25 – 30 minutes After boiling for at least one hour, the assembly is taken out and allowed to cool After cooling, the distance between the indicators is again measured (d2 ). The increase (d2 – d1 ) represents the expansion of the cement, and for Portland cements it should be less than 10 mm.
Note: If the expansion exceeds this value (10 mm), another Le Chatelier test is made after the cement has been spread and aerated for 7 days. T he expansion of the aerated cement must not exceed 5 mm. Any cement which fails to satisfy at least one of these tests should not be used. 3.8.4 Fineness of cement This refers to the surface area of cement particles available for hydration. T hus, for a rapid development of strength, higher fineness is required. Ho wever, the cost of grinding to a higher fineness is considerable and also the finer the cement, the more rapidly it deteriorates on exposure to the atmosphere. Finer cement leads to a stronger reaction with alkali-reactive aggregates and makes a paste, though not necessarily concrete, exhibiting a higher shrinkage and a greater proneness to cracking. However, finer cement bleeds less than coarse cement. The water content of a paste of standard consistence is greater for finer cement but conversely, an increase in fineness improves the workability of a mix. Fineness can be measured using the Lea and Nurse Permeability apparatus. 3.8.5 Strength of cement The test for compressive strength of concrete is commonly used in preference to that of mortar or neat cement because: → Structures are mainly designed to resist concrete in compression © Julius Ngabirano
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→ The strength depends on the adhesion and strength of aggregates used → Pure tension is also not usually tested since it is rather difficult to apply to concrete or mortar
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CHAPTER 4:
WATER
4.1 Functions of wate r Water has two functions in a concrete mix namely: ♥ To enable the chemical reactions which cause setting and hardening to take place (hydration) ♥ To lubricate the mixture of aggregates and cement in order to facilitate placing and compaction. Other uses of water on a site include: ♥ Curing concrete ♥ Washing aggregates and concrete equipments As in other chemical reactions, the cement and water combine in definite proportions. Concrete containing a small proportion of water produces a greater strength but is exceedingly difficult to compact. Extra water is therefore needed to lubricate the concrete. It is important that water added for lubrication purposes is kept to a minimum. A low water content is also necessary for imperviousness, resistance to frost, resistance to chemicals and abrasion and to minimize drying shrinkage. If concrete is not fully compacted, numerous bubbles of air may be entrapped, resulting in further voids. T here are therefore two main sources of voids in concrete. ♥ Entrapped air bubble s ♥ Water required for lubrication which later evaporates 4.2 Quality of wate r for concrete works Generally, clean water suitable for drinking should be used. T he presence of impurities such as suspended solids, organic matter and salts adversely affects the setting, hardening and durability of concrete. 4.3 Wate r-ce ment ratio The definition of the term water – cement ratio needs clarification. T he difficulty arises from the presence, in a batch of concrete, of water from different possible sources: 1. Water absorbed in the aggregate 2. Surface water on the aggregates 3. Water added during mixing
12 13 14
Water from sources (2) and (3) together provides what might be termed as the free water in the mix and therefore we can adopt that: 5 !
13 6 14 1 17 17
Where 17 denotes the weight of the cement. In this equation, it is assumed that the aggregates are damp and internally saturated. If the aggregates are dry, the water that should be adde d during mixing is equal to: 14 12 6 1 © Julius Ngabirano
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12 will be the water required for internal saturation of the aggregates. The relationship between water-cement ratio, richness of the mix, grading of aggregates, workability and strength of concrete was first studied by Professor Duff Abrams in America. The conclusions drawn from his work led to the formulation of Abrams water-cement ratio law stated as follows: “with given concrete materials and conditions of test, the quantity of mixing water is used to determine the strength of concrete, so long as the mix is of workable plasticity”. The law implies that with fully compacted concrete, sound aggregates and given cement, the strength depends on the ratio of water to cement. Increases in water-cement ratio have adverse effects on such properties as permeability, resistance to frost action, resistance to abrasion, tensile strength, creep, modulus of rapture and shrinkage. Below is a graphical representation of compressive strength versus water-cement ratio for a fully compacted concrete.
Figure 4-1:
Compressive strength versus water-cement ratio
4.4 Se a wate r Sea water does not normally reduce the strength of Portland cement concrete and may be used for plain (unreinforced) concrete. However, the salts present usually lead to efflorescence. In reinforced concrete, these salts promote the corrosion of steel.
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CHAPTER 5:
FRESH CONCRETE
5.1 Introduction Care need to be taken at construction sites while working with concrete in order to obtain finished concrete of the required structural and architectural quality. Errors whether through lack of competence or inattention to detail may be costly to be corrected later or even impossible to be corrected. T here are two basic and desirable properties of fresh concrete: → Workability → Stability 5.2 Workability This generally refers to the ease with which a concrete mix can be handled from the mixing point up to the finally compacted shape. It can clearly be understood through three characteristics: i. ii. iii.
Consistency: this is the measure of the wetness or fluidity or the ability of fresh concrete to flow. T his is measured by slump. Mobility: is the ease with which a given mix can flow into and completely fill the formwork or moulds Compactability: is the ease with which a given mix can be fully compacted to remove all air voids.
Workability is a property of fresh concrete or mortar which determines the ease and homogeneity with which it can be mixed, placed, compacted (consolidated) and finished. For fresh concrete to be acceptable, it should be:
Be easily mixed and transported Be uniform throughout a given batch and between batches Be of consistency so that it can completely fill the forms for which it was designed Have the ability to be compacted without excessive loss of energy Not segregate during placing and consolidation Have good finishing characteristics
Workability → → → → → → → →
can be measured by the following methods: Slump test Compacting factor test Remoulding test Vebe test Flow test Ball penetration test Nasser’s K-probe test T wo-point test
5.2.1 Slump test This is the most common test and is the measure of the consistency of the concrete. It is used on site as a check on variations of the materials being fed to the mixer. For example, an increase in slump may mean that water content has unexpectedly increased or a change in the quantities of the aggregates added. T he mould (slump cone) used in slump test is a frustum of a cone 300 mm high
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Figure 5-1:
The slump cone
Apparatus → Slump cone → Steel tamping rod (16 mm diameter) → Ruler Procedure The internal surface of the mould is thoroughly cleaned and applied with a light coat of oil. The mould is the placed on a smooth, horizontal, rigid and non-absorbent surface. The mould is then filled in three equal layers with freshly mixed concrete, each approximately to one-third of the height of the mould Each layer is tamped 25 times by the rounded end of the tamping rod (strokes are distributed evenly over the cross section). After tamping the top layer, the top surface is struck off level by means of a screeding and rolling motion of the tamping rod. Then immediately clean off any leakages and any other concrete around the base of mould. The mould is removed from the concrete immediately by raising it slowly and carefully in the vertical direction. The difference in level between the height of the mould and that of the highest point of the subside d concrete is measured. This difference in height in millimeters is called the slump of the concrete. Reporting of Results The slump measured should be recorded in millimeters. Any slump specimen which collapses or shears off laterally gives incorrect results and if this occurs, the test should be repeated with another sample. If the test is repeated and the specimen again shears, the slump should be measured and the fact that the specimen sheared, should be recorded. Limitations of the slump test It has no unique relationship with workability. It only detects changes in workability It occurs under self weight of concrete only and does not reflect behaviors under dynamic conditions such as vibrations Results are unreliable for leaner mixes e.g. in a shear slump Only suitable for concrete in which the maximum aggregate size is less than 40 mm
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Slumps of various concrete mixes
Figure 5-2:
Slumps of various concrete mixes
5.2.2 Compacting factor test This measures the degree of compaction achieved by a standard amount of work and thus offers a reasonably more reliable assessment of the workability of a concrete i.e. it is more reliable than slump test. T he compacting factor apparatus consists of two hoppers each in the frustum of a cone and one cylinder, the three arranged one above the other. The hoppers have hinged doors at the bottom. Procedure The sample of concrete is placed in the upper hopper up to the brim. It is placed gently so that there is no compaction at this stage. The bottom door is opened so that the concrete falls into the lower hopper. The bottom door of the lower hopper is opened and the concrete is allowed to fall into the cylinder. The excess concrete remaining above the top level of the cylinder is then cut off with the help of floats slid across the top of cylinder. Concrete in the cylinder is weighed. T his will be the weight of partially compacted concrete. The cylinder is filled with a fresh sample of concrete and vibrated to obtain full compaction. The concrete in the cylinder is weighed again. This weight is known as the weight of fully compacted concrete. The compacting factor can then be calculated from the formula. ' .
1 / . 8 . " 1 / 8 . "
Limitations of the compacting factor test Only suitable for concrete in which the maximum aggregate size is less than 40 mm The procedure for placing concrete in the measuring cylinder is totally different from that employed on site 5.2.3 The Vebe (V-B consistometer) test This test measures the time taken to transform a standard cone of concrete to a compacted flat cylindrical mass by means of vibration and is measured in seconds. The treatment of concrete in this test is comparable to the method of compacting concrete in practice and is sensitive to changes in consistency, mobility and compactability as well as the variations in aggregate characteristics such as shape and texture. T hus the results are reliable and suitable for a range of mixes. Procedure A conventional slump test is performed, placing the slump cone inside the cylindrical part of the consistometer The glass disc attached to the swivel arm is turned and placed on the top of the concrete in the pot. © Julius Ngabirano
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The electrical vibrator is switched on and a stop-watch is started, simultaneously Vibration is continued till the conical shape of the concrete disappears and the concrete assumes a cylindrical shape. When the concrete fully assumes a cylindrical shape, the stop-watch is switched off immediately. T he time in seconds is noted. The consistency of the concrete should be expressed in Vebe seconds, which is equal to the time in seconds recorded above.
Limitations of the compacting factor test Apparatus is expensive and requires electric power supply Its accuracy tends to decrease with increasing size of aggregates, above 20 mm the results become somehow unreliable Requires good experiences in handling 5.2.4 Factors affecting workability As already seen in the previous topics, workability can be influenced by: i.
Fineness of cement: the workability of concrete decreases as the fineness of the cement increases as a result of increased specific area. Finer particles have a larger surface area and therefore require more water. Water-cement ratio Presence of admixtures Aggregate size, shape, texture, grading and absorption characteristics Ratio of coarse to fine aggregates T emperature: increase in temperature speeds up evaporation as well as hydration Humidity: affect the rate of loss of water through evaporation Wind velocity: affect the rate of loss of water through evaporation T ime: freshly mixed concrete loses workability with time due to loss of water. Water can be lost through absorption by aggregates, evaporation or in hydration reactions.
ii. iii. iv. v. vi. vii. viii. ix.
5.3 Concre te stability This refers to the cohesion of a concrete mix. The two common features of concrete are segregation and bleeding. 5.3.1 Segregation It is defined as the separation of the constituent materials of a heterogeneous mixture so that their distribution is no longer uniform. Large and fine particles in a mix become separated and this is due to poor aggregate grading and improper care in concrete handling. Specifically, factors that affect segregation include:
Jolting of concrete during transportation Dropping concrete from excessive heights during placing Over-vibration Difference in size of the particles (large maximum particle size) High specific gravity of coarse aggregates increases segregation Decrease in amount of fine particles Particle shape and texture Extreme (very low or very high) water-cement ratio Presence of admixtures e.g. air entrainers reduces the danger of segregation
A less cohesive mix has a greater tendency to segregation. Segregation results in blemishes, porous layers and honey-combing. These adversely affect the hardened concrete. © Julius Ngabirano
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5.3.2 Bleeding During compaction and until concrete has hardened, there is a natural tendency for the solid particles to exhibit a downward movement and displace some water which then rises to the surface and may leak through the joints in formwork. This separation of water from a mix is called bleeding. This causes the concrete at or near the top surface to be weaker and less durable. Bleeding can be reduced by avoiding over-vibration, use of rich mixes, increasing the fineness of cement and use of specific admixtures like air entrainers. 5.4 Mixing concrete The main objectives of mixing concrete are: ♥ To coat the surface of all aggregates with a cement paste ♥ To blend all the ingredients of concrete into a uniform mass There are two methods used in mixing concrete namely; Hand mixing Machine mixing 5.4.1 Hand mixing A batch to be mixed by hand should be in relatively small (affordable) amounts. The equipment consists of a mixing platform, two shovels, a metal-lined or wooden measuring box and a graduated bucket (or any container with a known capacity such as the common 20 litre jerycan) for measuring water. The mixing platform used should be level, water tight and clean before use. It can be; An abandoned concrete slab
A concrete packing lot that can be cleaned after use.
Wooden platform having tight joints to prevent loss of water. A platform made of brickwork or stone masonry with joints sealed to prevent water loss Procedure for hand mixing Place the measured quantities of course and aggregates on a raised ground on site.
Measure the correct portions of cement
Put it on top of a heap of aggregates and spread evenly with a mixing shovel.
A measured amount of water is then added while mixing.
However, the following is the most commonly used hand mixing procedure on construction sites in Uganda: Place the measured quantity of sand (fine aggregates) on the clean platform and spread it out in a layer of uniform thickness Place cement over the sand and spread out uniformly. Mix the fine aggregates with cement using a shovel
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T urn the mix from side to side as many times as possible to produce a uniform color throughout. Workers doing the mixing face each other from opposite sides of the heap and work from the outside to the center. After the uniform color is got, spread the mixture on the platform and pour course aggregates on top Use the watering can or a hose to add water while mixing. Care should be taken to ensure that neither water by itself nor with cement can escape. When all the water has been absorbed, the mixing is continued until the mix is of uniform consistency. No soil or other extraneous material must be allowed to become included in the concrete. Advantages of hand mixing It is cheap for smaller jobs It is the best alternative for unskilled personnel Disadvantages of hand mixing T ime consuming It is sometimes hard to get a uniform mix It is costly for big jobs (in terms of labour) 5.4.2 Mixing by machine This involves drum types of machines and each drum has its own capacity chosen to meet the required quantities on a particular job and the speed to which each batch can be laid. Water is first added and this moistens the drums and removes any concrete adhering to the sides. The remaining materials are then measured into the drum in their correct proportions. The loaded drum is allowe d to mix for about 2 – 5 minutes and concrete is then ready for discharge. T he concrete is released from the drum depending on the type of the drum. It is released to the cart or wheel barrows or dumper and driven to the site for placing. Advantages of machine mixing It is very fast Produces a better mixture Disadvantages
High initial costs
May result in poor workmanship
Requires skilled personnel to operate the machine 5.5
Ge neral principles in the use of concre te mixe rs i. It is an advantage to feed cement, sand and coarse aggregates in the mixer simultaneously and in such a way that the flow of each extends over the same period.
ii.
The water should enter the mixer at the same time and over the same period like the other materials. With many mixers, this is not possible since the rate of flow is limited. In such case it is advisable to start the flow of water earlier.
iii.
Mixing should continue until the concrete is of uniform consistency and colour. © Julius Ngabirano
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iv.
The mixer shouldn’t be loaded beyond its rated capacity. Overloading results in spillage of materials and less satisfactory mixing, in addition to imposing undue strain on the mechanical parts.
v.
The mixer should be set up accurately so that there is access to the rotating drum and the mixture inside except in the case where the tilting drum type is horizontal.
vi.
For satisfactory performance, the mixer should be capable of producing a uniform concrete throughout the batch. This is to prevent the risk of honey combing resulting from an even distribution of stones and sand in any parts of the batch. It is advisable to discharge the whole batch into a suitable container specially made to receive the fresh concrete rather than to discharge in small separate quantities for example into wheel barrows.
vii.
The mixer should be run at a correct speed as stated by the manufacturer. The speed should be checked regularly.
viii.
Some cement mortar from the first batch of concrete mixed is left on the blade and drum. In order to avoid difficulties in placing due to shortage of fines, an extra 10% each of cement and sand should be added for the first batch.
ix.
Regular cleaning at the end of each spell of mixing is necessary to prevent concrete building up, especially if stiff mixes are in use. Considerable amount of concrete adhering to the blade or in the surface of the drum reduce the efficiency of the mixing.
x.
Badly worn and bent blades should be replaced since they decrease efficiency. Also wear of the inlet and discharge chutes eventually results in loss of materials and should be solved by suitable repairs.
xi.
After cleaning, grease or oil should be rubbed off the mixer to decrease adherence of the concrete
5.6 Type s of concre te mixe rs Types of concrete include: Non tilting drum mixers T ilting drum mixers Split drum mixers Reversing mixers Forced action mixers Continuous mixers 5.6.1 Non-tilting drum mixers This normally has a single drum (mixing chamber) rotating about the horizontal axis. The blades in the drum are arranged in such a way to work concrete towards discharge end of the mixer in order to provide a rapid rate of discharge. The drum has 10 similar blades arranged around the periphery (around the perimeter). Non-tilting drum mixers are available in sizes of 200 – 750 litres normal batch capacities. Disadvantages Slo w rate of discharge Concrete is susceptible to segregation Large sized aggregates tend to stay in the mixer so that the discharge starts as mortar and ends as a collection of coated stones © Julius Ngabirano
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5.6.2 Tilting drum mixers Small tilting drum mixers commonly used for types of building works are generally available in the sizes of 100, 150, 175 and 200 litres batch capacity. Those of capacity up to 150 litres of mixed concrete are often loaded by shoveling straight into the drum while medium sizes tilting drum mixers are provided with a loading skip similar to that for a non-tilting drum mixer. T ilting drum mixers usually have a conical or bowl-shaped drum with vanes inside. T ilting drum mixers are the most suited type for concrete with large size aggregates and because of their large and rapid discharge, they are suitable for low workability concrete. Advantages Rapid rate of discharge Discharge is always good as all the concrete can be tipped out Limited chances of segregation Suitable for low workability concrete Most suited for concrete with large size aggregates 5.6.3 Split drum mixers Normally they are 2 m3 capacity. Their distinctive feature is that the drum is separated into two halves along a vertical plane allowing the mixed concrete to be discharged. 5.6.4 Reversing drum mixers Mixers in this category rotate in one direction for mixing and the reverse direction for discharge. It has two types of blades i.e. one type for mixing and the other for discharging. When the drum is reversed after mixing is completed, the concrete is discharged quickly. 5.6.5 Forced action mixers These are widely used for precast concrete manufacture. T he common type of pan mixers with a rotating pan is fitted with a mixing stand of cast, paddles mounted eccentrically to the pan. The stars revolves either in the same direction as the pan or in the counter direction but at a greater speed. Forced action mixers are available in sizes from 200 litres up to 2 m3 normal batch capacity. 5.6.6 Continuous mixers This is the easiest type of concrete mixer. The materials are mixed and transported to the discharge and blades on the inside of the drum. The concrete is discharged steadily as a continuous stream and also produces good quality concrete.
Ge neral note : Rules for feeding ingredients into the mixer depend on the desired properties of the mix and type of mixer. Generally, a small amount of water should be fed first, followed by all the solid materials, preferably fed uniformly and simultaneously in to the mixer. If possible, a greater part of the water should also be fed during the same time, the remainder of the water being added after the solids. With some drum mixers, however, when a very dry mix is used, it is necessary to feed first some water and the coarse aggregate, as otherwise its surface does not become sufficiently wetted. Moreover, if coarse aggregates are absent to begin with, sand or sand and cement become lodged in the head of the mixer and do not become incorporated in the mix. This is called head pack.
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5.7
Research
5.7.1 Research (maintenance of concrete mixers) Read and make your own notes about the general care and maintenance of concrete mixers
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CHAPTER 6:
WORKING WITH CONCRETE – 1
6.1 Transporting concre te The transport of concrete from the mixing point or plant to the point at which it is to be placed must comply with some requirements: Transport must be rapid so that concrete does not dry out or lose its workability or plasticity Se gregation must be reduced to a minimum in order to avoid non–uniform concrete. Transport should be organized so that during placing of any particular section, delays will not result in the formation of construction joints. In order to reduce segregation, the following should be observed: → Where possible, concrete carries should be equipped with pneumatic tires and the surfaces over which they travel should be as smooth as possible. On common construction sites, wheel barrows are made to move over timber well fixed to provide smooth movement. → Concrete should not be allowed to drop from a considerable height. → Concrete should be placed directly in the position in which it is to remain and must not be allowed to flow or be worked along the formwork. Vibrators should not be used to spread a heap of concrete over a large area. → Aggregates used should be well grade d. Good concrete for transport should be well graded well mixed, well put on transporting equipments and not at a distance. Various methods are available for transporting concrete but the most common ones include: ♥ Wheel barrows, head pans ♥ Pumpers ♥ Lorries ♥ Conveyor belt ♥ Concrete pumps with tubes ♥ Chutes Evaporation of water from concrete in hot dry regions during transport can be quite serious and the only alternative is to provide some cover to the transporting medium. 6.1.1 Sampling concrete for test purposes This refers to procedures for obtaining a representative of a freshly mixed concrete on which tests are performed to determine conformance with quality requirements. Samples should preferably be random and at least 0.03 m 3. However, smaller samples may be permitted for routine tests such as slump test but the sample size is dictated by maximum aggregate size. Below are the sampling rules:
Sampling should be performed as concrete is delivered from the mixer. Sample by collecting two or more portions taken at regularly spaced intervals during discharge of the middle of the batch. No sample should be taken before 10% or after 90% of the batch has been discharged. Due to the difficulty of determining the actual quantity of © Julius Ngabirano
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concrete discharged, the intent is to provide samples that are representative of widely separated portions, but not the beginning and end of the load. As routine tests such as slump tests are not readily adaptable to sampling the concrete at two or more regularly spaced intervals during discharge of the middle portion of the batch, the sample may be taken after at least one-quarter cubic meter of concrete has been discharged. Combine the portions into one sample for testing purposes. Do not obtain portions of the composite sample from the very first or last part of the batch discharge Take care not to restrict the flow of concrete from the mixer, container or transportation unit so as this would to cause segregation. Immediately transport the sample to the place where test specimens are to be molded or where the test is to be made, and remix as needed to insure uniformity and compliance. The sample shall be protected at all times from sunlight and wind. T he elapsed time between obtaining the first and final portions of the composite samples shall be as short as possible, but in no instance shall it exceed 15 minutes. T est for slump should be started within 5 minutes while molding of specimens for strength tests shall be within 15 minutes after the sampling. Concrete used in one test may not be reused for any other test. It may be returned to the forms if the maximum time from batching has not been exceeded or adverse conditions have not caused its excessive drying. Make the sample as representative as possible and guard against segregation during sampling 6.2 Placing concrete After mixing, concrete should be placed before setting time tends e.g. for Ordinary Portland Cement the concrete setting time is between 30 minutes to 1 hour. Concrete is quickly transported to the place of laying and the mode of transport depends upon the magnitude of the work. It is very essential to essential that neither during transport nor placing, there is any segregation of aggregates. Factors to consider during placing i. Concrete should be placed as soon as possible ii.
Concrete should be deposited in thin horizontal layers and compacted thoroughly
iii.
Concrete should be continuously poured to avoid joints and improper bonding.
iv.
Concrete should be thoroughly worked into position i.e. all corners of the formwork and no space should remain.
v.
Concrete should never be dropped from a height as this would cause segregation of aggregates.
vi.
Protect fresh concrete from temperature extremes during and after placement
vii.
Coordinate the placing and compaction rates so that concrete is not deposited faster than it can be compacted properly
viii.
When placing concrete on slopes, always deposit the concrete at the bottom of the slope first and then proceed up the slope.
ix.
When placing slabs, place concrete at the far end first and then subsequent batches against the previous one. Avoid placing in separate/isolated heaps.
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x.
When constructing walls and/or beams, place the first batch of each layer at the ends of the section and then proceed towards the centre to prevent water from collecting at the formwork ends and corners.
In case concrete has more water or it has been laid in thick layers, then on compaction water, fine particles of aggregates and tiny particle of cement come to the top forming a layer of weak substance (scum) called laitance. This weak substance should be broken down and removed and the surface coated with a richer mix before fresh concrete is placed on it. 6.3 Compaction of concre te This process consists essentially of the elimination of entrapped air. T he reasons for compacting concrete include; To remove voids / air holes To increase strength Improving on the texture. Compaction brings fine material to the surface and against the formwork to produce the desired finish To make concrete air-tight When compacting, it is important that the reinforcement bars are properly embedded and should not be disturbed since the strength of a concrete member depends on proper reinforcement location. Formwork should also not be damaged or displaced. Compaction can be done manually (by hand) or mechanically (use of vibrators). The two methods require mixes of different workabilities. A mix that is too dry cannot be sufficiently worked by hand and conversely a very wet mix should not be vibrated to avoid the risk of segregation. 6.3.1 Manual (hand) compaction This method requires spades, sticks or tampers. To consolidate concrete with a spade or stick, insert the spade or stick along the surface of the formwork, through the fresh concrete (layer just placed) and into the concrete layer underneath. T he extension of the spade/stick up to the concrete layer underneath is to avoid a plane of weakness between the two layers there by forming a monolithic concrete element. Continue “ spading/sticking” until the coarse aggregates disappear into the concrete. Advantages of manual compaction Fair on small jobs Convenient on mixes with a high workability Disadvantages of manual compaction In concrete mixes with a low workability, compaction is hard to attain Slo w It is economically expensive on large projects 6.3.2 Machine compaction This is done by use of vibrators and is the best method because; It is more economical on large projects Faster Most of the desirable concrete properties can be attained It makes it possible to use less workable mixes resulting in increased strength. There are different types of vibrators namely; © Julius Ngabirano
Internal / immersion / pocket / poke vibrators External / framework vibrators Pag e 37
Vibrating tables Surface vibrators Vibrating rollers Care should be taken not to make excessive use of vibrators otherwise the concrete becomes nonhomogeneous. Internal vibrators They are the most common and consist of a rod (poker) which when inserted in concrete gives vibrations to it resulting in the consolidation of concrete. Care should be taken not to let it touch the reinforcement which is likely to get displaced. Internal vibratos have a higher efficiency than other types of vibrators since all the energy is transmitted directly to the concrete. During operation, lower the vibrator into the fresh concrete vertically (at points not more than 450 mm apart) and allow it to descend by gravity. T he vibrator should not only pass through the layer just placed but also penetrate into the underneath concrete layer (if still plastic or can be brought again to a plastic state) to ensure good bond between the two layers. One is able to know that he has compacted properly when a thin film of mortar appears along the formwork near the vibrator, the coarse aggregates disappears into the concrete and/or the paste begins to appear near the vibrator head. T he time required for vibration depends on the consistence of the mix and may be up to 2 minutes. Then, gradually withdraw the vibrator at approximately the same gravity rate that it descended so that the hole left by the vibrator closes fully without any air being trapped. Note: T o avoid the possibility of segregation, neither vibrate a mix that you can consolidate manually nor that with a high workability and do not use vibrators to move concrete in the form. External vibrators These are usually rigidly attached to the formwork by means of a clamp and they cause a vibratory motion of the formwork which distributes the vibrating forces into the concrete. In other words, both formwork and concrete vibrate. When these external forces are used, you should ensure that the formwork is strong/rigid and water tight to avoid distortion and leakage of grout. These vibrators are usually adopted when it is impossible to insert a manual or internal vibrator for example into heavily reinforced or small and narrow sections. They are not as efficient as internal vibrators since a considerable amount of energy is absorbed by the formwork. Vibrating tables These are commonly used in the laboratory and involve clamping the formwork with concrete on to the vibrating table. T he table is then made to vibrate. T hey have an advantage of offering uniform treatment (compaction) throughout the entire specimen. Surface (pan) vibrators These are in the form of plates which are used for the consolidation of mass concrete especially in road construction and floor slabs. It applies vibration through a flat plate directly to the top surface of the concrete. Vibrating rollers They are common in road construction works for compacting thin slabs. 6.4 Concre ting in hot we athe r Hot weather causes high temperatures of concrete and an increased rate of evaporation from the fresh mix. A higher temperature of fresh concrete leads to a more rapid hydration and hence an accelerated setting leading to a lower strength of hardened concrete. Also rapid evaporation may also cause © Julius Ngabirano
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plastic shrinkage and crazing and subsequent cooling of the concrete would result in tensile stresses. Curing methods such as the wetting of heated concrete elements exposed to prolonged and direct radiation, which induce temperature gradients within the concrete mass are strongly prohibited. For large pours, extra precautions should be taken to reduce concrete temperature gradients and to prevent the loss of surface moisture. Such practical measures include but are not limited to: i.
ii. iii. iv. v.
vi. vii. viii. ix. x. xi. xii.
Reducing the cement content preferably by the use of admixtures (but not below that required for the durability) so that the heat of hydration does not unduly aggravate the effects of a high ambient temperatures. Using a cement with a lower heat of hydration Cooling of mixing water and/or replacing part or whole of the added water with ice. It is however essential that the ice melts completely before the mixing has been completed. Cooling the aggregates by spraying with water or liquid nitrogen. However, this is more difficult and less effective. Providing shade to the fresh concrete surface to prevent heat gain from direct radiation. If not protected from the sun, the night cooling that follows is likely to cause cracking due to temperature differences. Keeping all mix constituents under shade where possible to reduce their temperatures in the stockpile Injecting liquid nitrogen after mixing of concrete in order to cool the concrete Restring the time between mixing and placing of the concrete. A given concrete batch should be placed as soon as the mixing is complete Initiating curing immediately after final tamping and continue until an appropriate surface insulation system is fully in place Providing approved surface insulation continuously over all exposed surfaces to prevent droughts and to maintain uniform temperature through the concrete mass If the surface exhibits crack after compaction, it should be re-tamped to close the cracks while the concrete is still in plastic stage. Using high-insulation formwork or surface insulation to reduce heat ingress when temperature gradients are critical.
6.5 Cold we athe r concre ting The temperature of concrete should never fall below 59 before and during placing or below 49 until it has hardened. When the atmospheric temperature falls below about 49, one should take all necessary steps to prevent freezing. When water freezes, it expands and can crack hardened concrete. The following measures are recommended: i.
ii.
iii. iv.
Increase heat evolved by cement by: Use of rapid-hardening Portland cement or ultra-high early strength Portland cement. Adding an accelerator which must not contain calcium chloride for reinforced or prestressed concrete or concrete containing an embedded metal Heating the ingredients: Materials to be used should be warm. Water temperature should not exceed 829 and frozen aggregates should never be used. Aggregates can be heated by use of steam via steam pipes. To avoid flash set of the cement, aggregates and mixing water should be mixed before the cement is added so that their temperature is unlikely to exceed about 309. Conserve heat: Surfaces of concrete can be covered with good insulating materials (such as thick timber formwork). Any cold winds should be kept off. Heating the building: T his can be achieved by use of hot air blowers but great care must be taken to avoid drying the fresh concrete. © Julius Ngabirano
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v.
vi. vii.
6.6
Heating the formwork: Concrete must never be placed in frost covered formwork or frozen ground. Formwork can be heated by a low pressure wet steam or hot air with a fine water spray. In cold climates with frequent freeze/thaw conditions, the concrete may need an air-entraining admixture for long term durability. Try to keep concrete as much above 10 9 (preferably at room temperature) as possible for the first few days.
Research
6.6.1 Research (maintenance of vibrators) Research and make your own notes about the general care and maintenance of vibrators 6.6.2 Research (formwork) Read and compile your own notes about the following in relation to formwork: Design, erection and removal of formwork. Qualities of good formwork. Common materials used as formwork in Uganda. Effects of poor formwork on concrete Functions of formwork 6.6.3 Research (steel reinforcements) Read and compile your own notes about the following in relation to steel reinforcements: Functions and quality of reinforcements Positioning of reinforcements Corrosion of reinforcements Concrete cover and fire resistance
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CHAPTER 7:
HARDENED CONCRETE
Generally, hardened concrete should be durable, with sufficient strength and impermeable. After concrete has been worked, it is then properly cured to achieve these desirable properties. 7.1 Concre te curing This is the name given to the procedures taken for promoting the hydration cement and it consists of the control of temperature and moisture movement to and from the concrete. T he major objective of curing is to keep concrete saturated due to the fact that hydration of cement takes place in water filled capillaries. The period of curing should be a minimum of 7 days for Ordinary Portland Cement. With slower-hardening cements, a longer period is desirable. Concrete curing can therefore be achieved by addition of moisture or prevention of moisture loss or both. In practice, the following methods may be used: ♣ Oiling (inside surface) and wetting of formwork before casting ♣ The forms may also be wetted during hardening ♣ Keeping concrete in contact with a source of water e.g. by spraying, flooding, etc. A continuous water supply is more efficient ♣ After stripping off formwork, concrete may be sprayed and wrapped with polythene sheets or other suitable covering. ♣ Large surfaces of concrete such as road slabs need to be protected even prior to setting. Since the concrete is mechanically weak before setting, a suspended cover in case of dry weather or during rains ♣ Covering the concrete with wet sand or earth, sawdust or straw ♣ An impermeable membrane or water proof paper may also be used. Provided the membrane is not punctured or damaged, it will effectively prevent evaporation of water from the concrete but will not allow ingress of water to replenish that lost by self-desiccation. T he membrane is formed by sealing compounds applied after free water has disappeared from the concrete surface. However, this method is expensive and reduces the rate of hydration Improper curing can impart adverse effects on hardened concrete through: • Reduced durability and strength • Scaling • Poor abrasion resistance • Cracking etc. • Increasing permeability 7.2 Durability of concrete This is the resistance to deterioration processes that may occur as a result of interaction with its environment (external) or between the constituent materials or their reaction with the contaminants present (internal). This property is controlled by the strength of concrete. 7.3 Strength of hardene d concre te This is the maximum load or stress the hardened concrete can carry. Strength can be compressive or tensile. Compressive strength is commonly used in concrete technology since most of the structural elements in civil engineering works are designed to withstand compressive forces. Factors that affect concrete strength include: © Julius Ngabirano
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Constituent materials ♥ Water-cement ratio: the higher the water-cement ratio, the lower the compressive strength ♥ Cement characteristics: both fineness and chemical composition affect strength especially at early stages ♥ Richness/leanness of the mix ♥ Aggregate grading, surface texture (to facilitate bonding), shape and strength ♥ Maximum size of aggregates ♥ Cement-aggregate (fine and coarse) ratio ♥ Presence of admixtures Method of preparation and placing in order to achieve a proper and fully compacted mix Curing conditions ♥ Presence of moisture during curing ♥ T emperature ♥ Length of curing period T est conditions: ♥ The higher the moisture content at time of test, the lower the strength. ♥ Specimen size and shape ♥ Method of loading Age It should be noted that the direct tensile strength of concrete varies between 18 and 114 of its compressive strength but the tensile strength measured in bending is usually about 50% greater. 7.3.1 Test for compressive strength Preparation of test specimen ♠ Assemble a mould of internal dimensions 150 by 150 by 150 mm and apply a thin layer of oil on inside surface. Oil prevents bonding between the mould and concrete. ♠ Fill the mould with concrete in three layers, each layer being tamped at least 35 times by a steel rod. Finish the top of the concrete by means of a trowel. ♠ The cube is then stored undisturbed for about 24 hours at about 18 – 22 0 C. The mould is then stripped off and the cube cured in water at 19 – 21 0 C up to the time of testing. In order to determine the quality of the concrete in the actual structure, the cubes are cured under conditions similar to those existing in the actual structure. Procedure of testing ♠ Weigh each specimen and measure its dimensions ♠ Immerse the specimens in water for a minimum of 5 minutes to ensure that they are wet during testing. ♠ Ensure that all testing-machine bearing surfaces are clean and any loose material is removed from the surfaces of the test cube ♠ Carefully centre the cube on the lower platen of the testing machine and ensure that the load will be applied to two opposite cast faces of the cube ♠ Apply and increase the load continuously at a nominal rate of 0.2 - 0.4 N/(mm 2 s) until no greater load can be sustained. On manually controlled machines, as failure is approached the loading rate is decreased. ♠ Record the maximum load applied to the cube and the type of failure.
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Results Calculate the compressive strength (in N/mm2 ) of each cube from the formula: ' . ; /
# '
- #
Note: a) Unsatisfactory failures are usually caused by insufficient attention to the procedures above. For example, it may be due to badly made c ube s, use of moulds that do not comply with specifications, wrong placement of cubes in the testing machine and also machine fault. b) T ests of six cubes are required c) The strength of concrete in the actual structure is usually less than that of the test specimen d) The results can be tabulated in a form similar to the one in Appendix A1 (Form A101). 7.4
Othe r prope rties of hardene d concre te Appearance: Variations in the appearance of concrete surfaces may result from: • Materials e.g. coloured cements, aggregates (colour, shape, texture and grading) • Formwork • Works done on the surface after casting • Exposure to atmosphere • Abrasion ii. Permeability: Use of a low water:cement ratio and ensuring thorough compaction produces concrete with a very high resistance to water penetration. Some admixtures can also contribute to impermeability. iii. Chemical resistance: This generally increases with increase in the crushing (compressive) strength. iv. Frost resistance: Water (ice) in pores or cracks may expand due to freezing and damage concrete. Air entrainment admixtures are able to form discontinuous pores which improve resistance to frost. v. Abrasion: Resistance to abrasion depends on the hardness of aggregates and ability of the mortar to retain them. vi. Fire resistance: Up to about 120 0 C, the strength of ordinary concrete increases, but there is a serious loss of strength at higher temperatures. Generally, the survival of reinforced concrete depends upon the protection afforded to steel reinforcements by concrete cover. Once the cover spalls off, steel conducts heat readily and failure is rapid. Above 900 0 C, over 85% of the strength will have been lost, depending on the composition of the concrete. vii. Moisture movement: Concrete shrinks when it dries and expands when wetted, the greater part of the initial drying shrinkage being irreversible. Excessive moisture movements may cause distortions and cracks. Moisture movements increases with richness of mix, water:cement ratio, permeability and when aggregates which are not rigid are used. Proper reinforcement detailing reduces moisture movements in reinforced concrete. i.
7.5 Concre te defe cts Some defects are obvious only to a trained eye while others may be are obvious to anyone. Below are some of the defects that may occur in concrete: Cracking Colour variation Crazing Dusting Rain damage Spalling © Julius Ngabirano
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Efflorescence Honeycombing Blistering
1. Cracking These are of different types and can greatly reduce concrete strength. Repair depends on the type and extent of the cracks. Various causes of cracking include: Use of weak formwork Partial compaction of concrete Wrong curing procedures Ground movement or settlement (poor foundation) Overloading of the structure when steel reinforcements are not fixed properly as 2. Colour variation These are differences in colour across the surface of concrete and appear as patches of light and dark. This may be caused by use of uneven concrete mix, variable curing conditions across the surface, applying different materials to the surface as a 'driers'. T o hide (repair) these variations, a surface coating can be applied. 3. Crazing This appears as a network of fine cracks across the surface of concrete. Crazing is caused by minor surface shrinkage in rapidly drying conditions (low humidity and hot temperatures, or alternate wetting and drying.) Prevention is by proper finishing and curing procedures. Repair may not be necessary because crazing will not weaken concrete. However, if the crazing looks too bad then a surface coating of paint or other overlay sealer can be applied to cover and/or minimize the effect of the cracks. 4. Dusting It appears as a fine powder on the concrete surface which comes off on your fingers and is caused by finishing before the bleed water has dried, finishing during rains., not curing properly, exposure of concrete to severe abrasion or using concrete of a very low grade. In repair, where surface dusting is minimal the application of a surface hardener can be beneficial. If the surface is showing significant wear distress it is essential to remove all loose material and then apply a suitable topping. 5. Rain damage In this case, the surface has bits washed away or many small dents or exposed aggregates and can be caused by heavy rain hitting exposed fresh concrete while it is setting or rainwater being allowed to run across the concrete surface. Repair • If concrete has not hardened and damage is minimal the surface can be refinished taking care not to allow excess water into the concrete. • If the concrete has hardened it may be possible to grind or scrape the minimal amount of the surface layer and apply a topping layer of new concrete. T his should only be done with great care. 6. Spalling In this case the slab edges and joints chip or break leaving an elongated cavity. Causes Edges of joints break because of heavy loads or impact with hard objects. As concrete expands and contracts the weak edges may crack and break. Entry of hard objects into joints. Poor compaction of concrete at joints. Prevention Design the joints carefully. © Julius Ngabirano
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Keep joints free from rubbish. Keep heavy loads away from the joints and edges until they have properly hardened. Ensure proper compaction. Repair Scrape, chip or grind away the weak areas until you reach sound concrete, making sure you brush the old concrete clean of any loose material. Then refill the area with new concrete or repair mortar and compact, finish and cure the new patch carefully. 7. Efflorescence This is a white crystalline deposit sometimes found on the surface of concrete soon after it is finished. This can be prevented by using clean, salt-free water and washed sands avoiding excessive blee ding. Remove efflorescence by dry brushing (without using a wire brush) and washing with clean water or a dilute solution of hydrochloric acid. 8. Honeycombing In this defect, too much coarse aggregate appears on the surface. It is caused by poor compaction, segregation during placing, paste leakage from formwork and use of a poor concrete mix (e.g. with limited fine aggregates causing a rocky mix). If it has occurred, it can be repaired by rendering (covering the surface with a layer of mortar). However, if honeycombing happens throughout the concrete section, the concrete may need to be removed and replaced. 9. Blistering Blisters are hollow bumps on the concrete surface filled with either air or bleed water. T hey occur when the fresh concrete surface is finished while trapped air or bleed water under the surface. They usually appear in thick slabs or on hot days when the surface is prone to drying out. In order to avoid blisters, after placing and initial finishing, leave the concrete as long as possible before final finishing and cure properly. Repair can be done by grinding off the weakened layer to an even finish.
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YEAR
CHAPTER 8:
TWO
MATERIAL AND CONCRETE TESTS - PRACTICE
8.1 Re vie w So far, the procedures and equipment for the following tests has already been covered in the previous topics.
T aking samples of aggregate and quantities required for laboratory tests Grading tests for aggregate T esting aggregates for suspected organic impurities T esting sand for bulking Setting time of a cement paste Sampling of concrete for test purposes Slump test for workability Compacting factor test for workability Making test cubes Compressive strengths tests for cubes
8.2 Practical tests Practical tests shall be done in the laboratory by each individual student. The students are therefore expected to understand the procedures and inquire where necessary to avoid wrong results. T he Lecturer shall assess the performance of the student and this shall be incorporated in the student’s continuous assessment.
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CHAPTER 9:
WORKING WITH CONCRETE - 2
9.1 Concre te joints In order to prevent concrete structures from damage caused by plastic shrinkage, thermal shrinkage, settlement, movement etc, concrete joints are required. T here are 3 common types of concrete joints:
Construction joints Contraction (control) joints Expansion joints
These joints need to be sealed so that they are not left empty. 9.1.1 Construction joints Construction joints are formed where concrete placement operations end for the day or where one structural element is cast against a previously cast concrete. Generally, they are made before and after interruptions in the placement of concrete or through the positioning of precast units. Locations are usually predetermined so as to limit the work that can be done at one time to a convenient size, with least impairment of the finished structure, though they may also be necessitated by unforeseen interruptions in concreting operations. Depending on the structural design, they may be required to function later as expansion or contraction joints, or they may be required to be soundly bonded to the first so as to maintain complete structural integrity. Construction joints may run horizontally or vertically depending on the placing sequence prescribed by the design and extends entirely through the concrete element. The following should be preferably observed during placement of construction joints:
Joints should be straight either vertical or horizontal In columns, they should be made as near as possible to the beam haunching. In beams and slabs, it should be within the middle third of the span. Vertical joints should be formed against temporary but rigid stop-boards which must be designed to allow reinforcements pass through while simultaneously avoiding mortar leakage. Laitance (scum of cement and very fine material) must not be allowed to form on horizontal joint surfaces, preferably by use of a drier mix. If present, the scum can be removed by brushing or hacking (in case of hardened concrete). The cleaned surface may be wetted to reduce absorption of water from the fresh concrete by hardened concrete. Alternatively, a thin grout of cement can be brushed over the surface. T he new concrete must then be placed within 30 minutes. Ensure thorough compaction and no segregation of new concrete along the joint plane.
9.1.2 Contraction (control) joints Contraction joints are purposely installed joints designed to regulate cracking that might otherwise occur due to the unavoidable, often unpredictable, contraction of concrete. T hese joints are often called control joints because they are intended to control crack locations. The necessary plane of weakness may be formed by reducing the concrete cross-section by tooling or saw cutting a joint within 24 hours of placing. Contraction joint movement is supposed to be small.
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9.1.3 Expansion joints Expansion joints are designed to prevent the crushing and distortion of the abutting concrete structural units that might otherwise occur due to the transmission of compressive forces that may be developed by expansion, applied loads, or differential movements arising from the configuration of the structure or its settlement. Expansion joints are made by providing a space over the entire cross section between abutting structural units. Expansion joint movement may be high (up to 30 % of joint width). Qn: What is the significance of isolation (contraction and e xpansion) joints? Isolation joints isolate slabs or concrete structure from other parts of structure. T he presence of isolation joints allows independent vertical or horizontal movement between adjoining parts of the structure. Otherwise, the structure may experience cracking owing to the restrained movement caused by directional connection between adjoining concrete structures. 9.1.4
Guidelines in placement of isolation (contraction and expansion) joints Always follow the guidelines for maximum spacing. If in doubt use closer spacing than recommended. This is particularly important on decorative concrete surfaces. For slabs wider than footpaths the longest side of any section should be no longer than 1.5 times the width of the shorter side. Always put a joint at any change of direction. Pay particular attention to re-entrant corners. Never place a joint at an acute angle to the concrete edge. Always make a turn with the joint to arrive at right angles to the edge. That is don't leave sections with pointed ends, they always crack. The trick is to set a line square off the sloping edge. This averages the angle.
9.2 Finishing concre te The finishing process is aimed at providing the final concrete surface. 9.2.1 Floating This has the following purposes: ♠ To embed aggregate particles just below the surface ♠ Remove slight imperfections (high or low spots) ♠ Compact the concrete at the surface in preparation to other finishing operations. If a smoother surface is required, the surface should be worked sparingly with wood or aluminium floats. An aluminium float gives the finished concrete a much smoother surface than a wood float. In order to achieve the desired results, the following should be noted during floating: To avoid cracking and dusting of the finished concrete, begin aluminium floating when the water sheen disappears from the freshly placed concrete surface. Do not use cement or water as an aid in finishing the surface. Begin floating immediately after screeding while the concrete is still plastic and workable. However, do no overwork the concrete when it is still plastic because you may bring an excess of water and paste to the surface which forms a thin, weak layer that will quickly wear off during use. To remove a coarse texture in the final finish, you usually have to float the surface a second time after it partially hardens.
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9.2.2 ♥
♥
♥ ♥ ♥ ♥
♥
Trowelling If a dense smooth finish is desired, steel trowelling must follow floating. Trowelling should begin after the moisture film or sheen disappears from the floated surface and when concrete has hardened enough to prevent fine material and water from being worked on the surface. This step should be delayed as long as possible since trowelling too early tends to reduce durability. However, a longer delay for trowelling results in a surface becoming too hard to finish properly. Trowelling should leave the surface smooth and free from marks and ripples. Spreading dry cement on a wet surface to take up excess water is not a good practice where a wear-resistant and durable surface is required. Wet sports must be avoided if possible and if they do occur, finishing operations should not be resumed until the water has been absorbed or evaporated or has been mopped up. A fine textured, un-slipperly surface can be obtained by trowelling lightly over the surface with a circular motion immediately after the first regular trowelling. In this process, the trowel is kept flat on the surface of the concrete. Where a hard steel-trowelled finish is required, follow the first regular trowelling by a second one. T he second trowelling should begin after the concrete has become hard enough so that no mortar adheres to the trowel, and a ringing sound is produced as the trowel passes over the surface. During this final trowelling, the trowel should be tilted slightly and heavy pressure exerted to thoroughly compact the surface.
9.3 Yield of a concre te mix This is the volume of a freshly mixed, unhardened concrete made from a known quantity of ingredients. It is sold on a volume basis (m 3 ). If ready mix concrete has been batched by mass, it is necessary to convert the plant scale readings to volume for sale. The volume of freshly mixed and unhardened concrete in a given batch can be determined from the total mass of the batch divided by the density of the concrete. Concrete yield problems (shortages) may occur due to • • • • • •
Miscalculating form volumes or slab thicknesses. A fractional error may result in more concrete being used than was previously ordered. Form deflection or distortion under the weight of the fresh concrete Irregular sub-grades which require extra concrete, or sub-grade settlement under pressure from the fresh concrete Waste, spillage, loss of some entrained air, settlement of wet mixes and use of excess concrete in incidental mud sills or footings An over yield can be an indication of a problem if the excess concrete has been caused by excess air or aggregates or if the forms have not been properly filled. Differences between the batched weights of ingredients being outside permitted ranges
These shortages can be prevented by:
Generally avoiding all the above causes Proper and accurate measurements Constructing formwork to withstand the pressure of fresh concrete without deflection or distortion For slabs on grade, the sub-grade should be level and well compacted Include an allowance of 4 – 10% to account for concrete waste, spillage, over-excavation and other factors. Some jobs may require a larger allowance for contingencies than others. Always check the concrete yield by measuring the concrete unit weight. Repeat these tests if problems arises. © Julius Ngabirano
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' 8 "
? 5 / " " 8 /
9.3.1 Determination of yield of a concrete mix In the equations below, density is the unit weight of concrete in kg/m 3 . 8 . " /
1 / 5 / . " ! 1 / . 8 @
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B ; 8 " )
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