Concrete Technology Lecture Notes Ordinary Diploma in Civil Engineering

© Julius Ngabirano 0

REPUBLIC OF UGANDA

MINISTRY OF EDUCATION AND SPORTS

UGANDA TECHNICAL COLLEGE - BUSHENYI

LECTURE NOTES FOR

CONCRETE TECHNOLOGY AND PRACTICES

Prepared by: Julius Ngabirano (B Sc. CIV ENG (MUK), CCA, GMUIPE) ©2011

© Julius Ngabirano i

TABLE OF CONTENTS

LIST OF TABLES --------------------------------------------------------------------------------------- v

LIST OF FORMS --------------------------------------------------------------------------------------- vi

LIST OF FIGURES------------------------------------------------------------------------------------ vii

REFERENCES ------------------------------------------------------------------------------------------viii

PREAMBLE ---------------------------------------------------------------------------------------------- ix

YEAR ONE ----------------------------------------------------------------1

CHAPTER 1: INTRODUCTION ------------------------------------------------------------------ 1

1.1 Definition------------------------------------------------------------------------------------------- 1

1.2 Limitations of concrete --------------------------------------------------------------------------- 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 Concrete materials -------------------------------------------------------------------------------- 3

1.5 Batching of ingredients --------------------------------------------------------------------------- 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 aggregates ------------------------------------------------------------------------------------ 5

2.3 Coarse aggregates --------------------------------------------------------------------------------- 6

2.3.1 Functions of aggregates---------------------------------------------------------------------------- 6

2.3.2 Qualities of good aggregates ---------------------------------------------------------------------- 7

2.4 Testing 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 aggregates ----------------------------------------------------------------------------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 Page ii

3.3 Chemical composition of cement ---------------------------------------------------------------15

3.4 Setting and hardening of cement ---------------------------------------------------------------16

3.4.1 Functions of the various cement compounds -----------------------------------------------------16

3.4.2 False set--------------------------------------------------------------------------------------------17

3.5 Types of cement ----------------------------------------------------------------------------------17

3.5.1 Common types of cement --------------------------------------------------------------------------17

3.5.2 Special cements------------------------------------------------------------------------------------18

3.6 Admixtures ---------------------------------------------------------------------------------------19

3.6.1 Precautions taken when using admixtures -------------------------------------------------------19

3.7 Transportation and storage of cement ---------------------------------------------------------20

3.8 Physical properties of cement -------------------------------------------------------------------20

3.8.1 Consistence of standard paste --------------------------------------------------------------------20

3.8.2 Setting time ----------------------------------------------------------------------------------------21

3.8.3 Soundness ------------------------------------------------------------------------------------------21

3.8.4 Fineness of cement --------------------------------------------------------------------------------22

3.8.5 Strength of cement ---------------------------------------------------------------------------------22

CHAPTER 4: WATER-----------------------------------------------------------------------------24

4.1 Functions of water -------------------------------------------------------------------------------24

4.2 Quality of water for concrete works -----------------------------------------------------------24

4.3 Water-cement ratio ------------------------------------------------------------------------------24

4.4 Sea water ------------------------------------------------------------------------------------------25

CHAPTER 5: FRESH CONCRETE--------------------------------------------------------------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 Concrete 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 General principles in the use of concrete mixers----------------------------------------------31

5.6 Types of concrete mixers ------------------------------------------------------------------------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: WORKING WITH CONCRETE – 1 --------------------------------------------35

6.1 Transporting concrete ---------------------------------------------------------------------------35

6.1.1 Sampling concrete for test purposes --------------------------------------------------------------35

© Julius Ngabirano Page iii

6.2 Placing concrete ----------------------------------------------------------------------------------36

6.3 Compaction of concrete -------------------------------------------------------------------------37

6.3.1 Manual (hand) compaction -----------------------------------------------------------------------37

6.3.2 Machine compaction ------------------------------------------------------------------------------37

6.4 Concreting in hot weather-----------------------------------------------------------------------38

6.5 Cold weather concreting-------------------------------------------------------------------------39

6.6 Research-------------------------------------------------------------------------------------------40

6.6.1 Research (maintenance of vibrators) -------------------------------------------------------------40

6.6.2 Research (formwork) ------------------------------------------------------------------------------40

6.6.3 Research (steel reinforcements) ------------------------------------------------------------------40

CHAPTER 7: HARDENED CONCRETE -------------------------------------------------------41

7.1 Concrete curing ----------------------------------------------------------------------------------41

7.2 Durability of concrete----------------------------------------------------------------------------41

7.3 Strength of hardened concrete------------------------------------------------------------------41

7.3.1 Test for compressive strength ---------------------------------------------------------------------42

7.4 Other properties of hardened concrete --------------------------------------------------------43

7.5 Concrete defects ----------------------------------------------------------------------------------43

YEAR TWO-------------------------------------------------------------- 46

CHAPTER 8: MATERIAL AND CONCRETE TESTS - PRACTICE -----------------------46

8.1 Review ---------------------------------------------------------------------------------------------46

8.2 Practical tests -------------------------------------------------------------------------------------46

CHAPTER 9: WORKING WITH CONCRETE - 2 --------------------------------------------47

9.1 Concrete 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 concrete --------------------------------------------------------------------------------48

9.2.1 Floating --------------------------------------------------------------------------------------------48

9.2.2 Trowelling -----------------------------------------------------------------------------------------49

9.3 Yield of a concrete mix --------------------------------------------------------------------------49

9.3.1 Determination of yield of a concrete mix ---------------------------------------------------------50

CHAPTER 10: INTRODUCTION TO PRESTRESSED CONCRETE ------------------------51

10.1 Introduction --------------------------------------------------------------------------------------51

10.2 Methods of prestressing concrete---------------------------------------------------------------51

10.3 Comparison of prestressed and reinforced concrete beams ---------------------------------51

10.4 Applications of prestressed concrete -----------------------------------------------------------52

CHAPTER 11: CONCRETE MIX DESIGN ------------------------------------------------------53

11.1 Definition------------------------------------------------------------------------------------------53

11.2 Types of Mixes------------------------------------------------------------------------------------53

11.3 Trial mixes ----------------------------------------------------------------------------------------53

11.4 Considerations in mix proportioning ----------------------------------------------------------53

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11.5 Mix design procedure----------------------------------------------------------------------------54

CHAPTER 12: PRECAST PRODUCTS IN UGANDA ------------------------------------------56

12.1 Research-------------------------------------------------------------------------------------------56

APPENDIX -----------------------------------------------------------------------------------------------57

A1: Forms----------------------------------------------------------------------------------------------57

A2: Tables----------------------------------------------------------------------------------------------59

A3: Figures---------------------------------------------------------------------------------------------60

© Julius Ngabirano v

LIST OF 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/mm2) of concrete mixes made with a free-

water/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

© Julius Ngabirano vi

LIST OF FORMS

Form A101: Cube crushing strength result sheet ------------------------------------------------57

Form A102: Concrete mix design form-----------------------------------------------------------58

© Julius Ngabirano vii

LIST OF 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 A306: Relationship between compressive strength and free water/cement ratio .......... 64

Figure A307: Cement manufacture (dry and wet processes) ................................................. 64

© Julius Ngabirano viii

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

© Julius Ngabirano ix

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

September 2011

© Julius Ngabirano Page 1

YEAR ONE

CHAPTER 1: 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 concrete

Concrete has some limitations which should be realized by both the designers and the builder. The

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. These 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,


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 interstit ial 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 subsequent

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/m3.

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.


1.4 Concrete materials

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 crushed stones are the commonly used fine aggregates. These 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. These 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 ingredients

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. The 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.


1.5.2 Batching by weight

This is where materials are weighted on site and batched according to their weights in the correct

proportions. This 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

density

Proportion by

volume

Mass of per

bag of cement

Volume of per

bag of cement

Proportion

by weight

kg/m3

k g kg/m3

Cement 1440 1 0.0347 50.0 1.00

Sand 1600 2 0.0694 111.0 2.22

Coarse aggregates 1360 4 0.1388 188.8 3.78

Assumed volume of a 50kg bag of cement = 0.0347 m3

Table 1-1: Conversion from volume to weight proportions


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. The 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/m3. 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. Their 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 aggregates

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


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. The 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 aggregates

These are aggregates that are retained on a 5.0 mm sieve. They 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.


2.3.2 Qualities of good aggregates

i. 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.

ii. Aggregates should be hard and tough to resist forces of abrasion, impact, crushing, etc.

Usually strong aggregates make strong concrete.

iii. Aggregates should be durable

iv. They should not be liable to any form of shrinkage, swelling and decomposition.

v. Aggregates should be sound and thus they should not be thin and elongated to avoid

breakages.

vi. Aggregates should be inert, they should not react to water component in cement so as not to

take part in the hydration process

vii. They should have a rough surface and a regular shape to promote bonding

viii. For dependable results, the coefficient of expansion and thermal conductivity of aggregates

should be equal or nearly equal to that of cement

2.4 Testing 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 litt le aggregates from different places rather than a sample from

one place only. Under favorable conditions, at least 10 proportions should be drawn 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.


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. The 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.


Bulking test

Apparatus:

� Flat bottomed cylinder

� Steel rule

� Steel rod

Method

i. Fill loosely packed damp sand in a cylinder about 2 3� of its capacity

ii. Measure the depth of the sand D

iii. Pour the sand on a tray

iv. 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.

v. 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: Percentage bulking against moisture content for different sizes of sand


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. They 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.


vii. Then tabulate your results as shown below and plot the percentage by weight of aggregates

passing each sieve against the sieve sizes. The sieve sizes are plotted on the horizontal axis

with a logarithmic scale to base 10.

Results

BS Sieve

Mesh size Weight retained % retained

Cumulative %

pas sing

Cumulative %

retained

4.76 mm 0 0 100 0

2.4 mm 1.6 0.8 99.2 0.8

1.2 mm 5 2.5 96.7 3.3

600 μm 27.4 13.7 83 17

300 μm 98 49 34 66

150 μm 50 25 9 91

Pan 18 9 100

TOTAL 200

Ma terial retained % by weight pas sing

0

10

20

30

40

50

60

70

80

90

100

% b

y w

eig

ht

of

aggr

egat

es

pass

ing

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.


0

10

20

30

40

50

60

70

80

90

100

% b

y w

eig

ht o

f a

ggr

egat

es

pas

sin

g

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. This type of concreting is used 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 aggregates

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.


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. The 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.


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 t ightness

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, t ippers, 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.

� The wet process

� The dry process

3.2.1 The wet process

i. Grounded limestone and clay are mixed in the appropriate ratios and mixed with water to

form slurry in wash mills.

ii. The slurry is sieved and any coarse particles are returned to the wash mills for re-grinding.

iii. 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).

iv. 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 150 to the horizontal.

v. 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 0C and undergoes

successive chemical reactions

vi. Sintering occurs between 1300 – 1400 0C and the material fuses into small balls known as

cement clinker.

vii. 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.

viii. 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.


ix. An automatic packing plant fills paper bags with a standard weight of 50 kg.

3.2.2 The dry process

i. 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.

ii. The dry powder (raw meal) is pumped to a blending silo where final proportioning is done to

ensure the correct chemical composition.

iii. 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%.

iv. The raw meal is then pre-heated to about 800 0C to remove all the moisture.

v. It is then fed to the rotary kiln and the subsequent operations are the same as those in the wet

process of manufacture.

3.2.3 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 material has a higher moisture content

Low energy required for burning due to a 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 %

Al2O3 3 - 8 %

Fe2O3 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


Name of compound Oxide composition Abbreviation

Tricalcium Silicate 3CaO.SiO2 C3S

DiCalcium Silicate 2CaO.SiO2 C2S

TriCalcium Aluminate 3CaO.Al2O3 C3A

TetraCalcium Aluminoferrite 4CaO.Al2O3.Fe2O3 Ca4AF

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 Setting 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. The term setting is used to describe this stiffening state. Once set, cement paste

gradually develops strength and forms hardness. This 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;

2C3S + 6H C3S2H3 + 3Ca(OH)2

2C2S + 4H C3S2H3 + Ca(OH)2

C3A + 6H C3AH6

Hydrate abbreviation Hydrate name

C3S2H3 Calcium silicate Hydrate

C3AH6 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. The 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. To 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.


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 C3S by aeration at moderately high humidity.

3.5 Types of cement

3.5.1 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

This 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. This 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. The 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.


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 This is manufactured by grinding a mixture of Portland cement clinker and blast furnace slag. The

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. The cement is highly resistant to sea water, sulphates, acids and oils. These 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.


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 water-

proofing properties and can be stored in damp conditions for a long time without deterioration.

3.6 Admixtures

These are suitable additives used to change the properties of cement to achieve other specific

properties. They 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. Their use results in longer

setting times, slower strength gain, enables long transit t imes 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 bubbles 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


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 properties 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.


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. These 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. Time 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 added 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

� Temperature

� 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. Test for

soundness is done using Le Chatelier apparatus. This apparatus (shown below) consists of a small

brass cylinder split along its generatix. Two 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.


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 0C 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. The 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. Thus, 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


→→→→ 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


CHAPTER 4: WATER

4.1 Functions of water

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. There are therefore two main sources of voids in concrete.

♥ Entrapped air bubbles

♥ Water required for lubrication which later evaporates

4.2 Quality of water for concrete works

Generally, clean water suitable for drinking should be used. The presence of impurities such as

suspended solids, organic matter and salts adversely affects the setting, hardening and durability of

concrete.

4.3 Water-cement ratio

The definition of the term water – cement ratio needs clarification. The difficulty arises from the

presence, in a batch of concrete, of water from different possible sources:

1. Water absorbed in the aggregate 12

2. Surface water on the aggregates 13

3. Water added during mixing 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

17�

117

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 added during mixing

is equal to:

14 � 12 6 1


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 Sea water

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.


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. There 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. Consistency: this is the measure of the wetness or fluidity or the ability of fresh concrete to

flow. This is measured by slump.

ii. Mobility: is the ease with which a given mix can flow into and completely fill the formwork

or moulds

iii. 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

→→→→ Two-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. The mould (slump cone) used in slump test is a frustum of a cone 300 mm high


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 t imes 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

subsided 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


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. The 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. This 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. Thus 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.


� 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. The 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.

ii. Water-cement ratio

iii. Presence of admixtures

iv. Aggregate size, shape, texture, grading and absorption characteristics

v. Ratio of coarse to fine aggregates

vi. Temperature: increase in temperature speeds up evaporation as well as hydration

vii. Humidity: affect the rate of loss of water through evaporation

viii. Wind velocity: affect the rate of loss of water through evaporation

ix. Time: 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.

5.3 Concrete 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.


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 t ight 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


� Turn 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

Time 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 allowed to mix for about 2 – 5 minutes and concrete is then ready for discharge. The

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 General principles in the use of concrete mixers

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.


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 tilt ing 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 Types of concrete mixers

Types of concrete include:

� Non tilt ing drum mixers

� Tilting 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-tilt ing 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


5.6.2 Tilting drum mixers

Small t ilt ing 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 tilt ing drum mixers

are provided with a loading skip similar to that for a non-tilt ing 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. The 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.

General 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.


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


CHAPTER 6: WORKING WITH CONCRETE – 1

6.1 Transporting concrete

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

� Segregation 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 graded.

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 m3. 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


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 t imes from sunlight and wind.

�� The 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. Test 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.


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 concrete

This process consists essentially of the elimination of entrapped air. The 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 workabilit ies. 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. The 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;

� Internal / immersion / pocket / poke vibrators

� External / framework vibrators


� Vibrating tables

� Surface vibrators

� Vibrating rollers

Care should be taken not to make excessive use of vibrators otherwise the concrete becomes non-

homogeneous.

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. The 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. The

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: To 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 t ight 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. The table is then made to vibrate. They 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 Concreting in hot weather

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


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. 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.

ii. Using a cement with a lower heat of hydration

iii. 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.

iv. Cooling the aggregates by spraying with water or liquid nitrogen. However, this is more

difficult and less effective.

v. 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.

vi. Keeping all mix constituents under shade where possible to reduce their temperatures in the

stockpile

vii. Injecting liquid nitrogen after mixing of concrete in order to cool the concrete

viii. Restring the time between mixing and placing of the concrete. A given concrete batch should

be placed as soon as the mixing is complete

ix. Initiating curing immediately after final tamping and continue until an appropriate surface

insulation system is fully in place

x. Providing approved surface insulation continuously over all exposed surfaces to prevent

droughts and to maintain uniform temperature through the concrete mass

xi. If the surface exhibits crack after compaction, it should be re-tamped to close the cracks while

the concrete is still in plastic stage.

xii. Using high-insulation formwork or surface insulation to reduce heat ingress when temperature

gradients are critical.

6.5 Cold weather concreting

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. 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

ii. 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.

iii. Conserve heat: Surfaces of concrete can be covered with good insulating materials (such as

thick timber formwork). Any cold winds should be kept off.

iv. Heating the building: This can be achieved by use of hot air blowers but great care must be

taken to avoid drying the fresh concrete.


v. 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.

vi. In cold climates with frequent freeze/thaw conditions, the concrete may need an air-entraining

admixture for long term durability.

vii. Try to keep concrete as much above 10 9 (preferably at room temperature) as possible for the

first few days.

6.6 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


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 Concrete 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. The 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. The 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 hardened concrete

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:


�� 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

♥♥♥♥ Temperature

♥♥♥♥ Length of curing period

�� Test conditions:

♥♥♥♥ The higher the moisture content at t ime 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 1

14 � 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 t imes 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 0C. The mould is

then stripped off and the cube cured in water at 19 – 21 0C 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/(mm2s) 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.


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 cubes, use of moulds that do not comply with

specifications, wrong placement of cubes in the testing machine and also machine fault .

b) Tests 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 Other properties of hardened concrete

i. 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 0C, 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 0C, 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.

7.5 Concrete defects

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


� 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'. To 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 hitt ing 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. This 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.


� 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 bleeding.

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. They 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.


YEAR TWO

CHAPTER 8: MATERIAL AND CONCRETE TESTS

- PRACTICE

8.1 Review

So far, the procedures and equipment for the following tests has already been covered in the previous

topics.

� Taking samples of aggregate and quantities required for laboratory tests

� Grading tests for aggregate

� Testing aggregates for suspected organic impurities

� Testing 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. The

Lecturer shall assess the performance of the student and this shall be incorporated in the student’s

continuous assessment.


CHAPTER 9: WORKING WITH CONCRETE - 2

9.1 Concrete joints

In order to prevent concrete structures from damage caused by plastic shrinkage, thermal shrinkage,

settlement, movement etc, concrete joints are required. There 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. The

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. These 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.


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 expansion) joints? Isolation joints isolate slabs or concrete structure from other parts of structure. The 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 concrete

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.


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. The 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 concrete 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 (m3). 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.


'�� 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/m3.

��8 �� .��" ��/ �� 1��/� �� 5��/ ��.��" �� ! 1��/� �� .�8 ��

@��

A��" �� = � ?�� 5��/� .� ��/

��8 �� / ��

B��; 8�" �A��"

)�� "�"

)�� <�=�� /�� 5��/� ! ��8

<�=�� /�� 5��/�

Sometimes, the yield of a concrete mix may also be quoted in terms of the quantity of concrete

produced by a unit of cement say one bag of cement.

A��" � @��

1��/� �� '��1��/� ��

� �� .� ��

Considering a standard bag of cement of 50 kg,

A��" � @��

1��/� �� "50

� � �� .� ��


CHAPTER 10: INTRODUCTION TO PRESTRESSED

CONCRETE

10.1 Introduction

Prestressing concrete is a method for overcoming concrete's natural weakness in tension. It is a

combination of high strength concrete and steel strands to make a very strong structural material that

is used in construction of roof slabs, bridges and railroad ties. Structural elements produced have a

longer span than is practical with ordinary reinforced concrete. Prestressing tendons (generally of

high tensile steel cable or rods) are used to provide a clamping load which produces a compressive

stress that offsets the tensile stress that the concrete compression member would otherwise experience

due to a bending load.

10.2 Methods of prestressing concrete

Prestressed concrete can be created using two different methods; pre-tension and post-tension

� Pre-tensioning method involves stretching high tensile steel strands between abutments located at

both ends of the concrete casting bed/mould. After the strands are taught, concrete is poured into

the mould where it surrounds and adheres to the strands. Once the concrete is dry it will have

bonded to the steel. After the concrete has reached the desired strength the strands are released,

resulting in the concrete to develop a slight arch that makes it more resistant to heavy loads. Pre-

tensioned beams can resist very high stress without cracking while columns don't buckle under the

weight of heavy loads. Thin concrete pads are prestressed to keep them from bowing under

normal weight.

� Post-tensioning method involves applying compression after the concrete has been poured and

hardened to the required initial strength. The concrete is poured around a curved duct that has had

steel strands ran through it . Upon curing, tension is applied to the strands using hydraulic jacks.

The strands are then wedged into place so the tension remains after the hydraulic jacks have been

removed. Post-tension concrete is used as monolithic slabs for structures in areas with expansive

soils such as clay and is also highly efficient for constructing buildings with more elaborate

design work.

10.3 Comparison of prestressed and reinforced concrete beams

Generally, a prestressed concrete beam of a given span and section can carry a very great load

compared to a reinforced concrete beam of the same span and cross section. As already seen,

prestressed concrete beams produced have a longer span than is practical with ordinary reinforced

concrete.

Consider prestressed beams spaced at 4 m center to centre, spanning 12 m and designed to support a

load of 4 kPa. Below are the sections for ordinary reinforced and prestressed beams designed for this

given span and loading.


Figure 10-1: Comparison of prestressed and reinforced concrete beams

Table 10-1: Comparison of prestressed and reinforced concrete beams using Figure 10-1

Material Reinforced

concrete design

Prestressed concrete

design

Material savings from reinforced

concrete design to prestressed concrete design

Concrete 0.288 m3/m 0.18 m

3/m 37.5 %

Reinforcing steel 42 kg/m 6.20 kg/m 66 %

Prestressing steel -----------

8.47 kg/m 66 %

10.4 Applications of prestressed concrete

� Prestressed concrete is the predominating material for floors in high-rise buildings and the

entire containment vessels of nuclear reactors.

� Un-bonded post-tensioning tendons are commonly used in parking garages as barrier cable.

Also, due to its ability to be stressed and then de-stressed, it can be used to temporarily repair

a damaged building by holding up a damaged wall or floor until permanent repairs can be

made.

� Prestressed concrete has the advantage of crack control

� Lower construction costs especially for high rise buildings.

� Thinner slabs are important in high rise buildings in which floor thickness savings can

translate into additional floors for the same (or lower) cost

� Fewer joints since post-tensioned structural elements usually have a larger span

� Prestressing can also be accomplished on circular concrete pipes used for water transmission.

High tensile strength steel wire is helically-wrapped around the outside of the pipe under

controlled tension and spacing which induces a circumferential compressive stress in the core

concrete. This enables the pipe to handle high internal pressures and the effects of external

earth and traffic loads.


CHAPTER 11: CONCRETE MIX DESIGN

11.1 Definition

Mix design can be defined as the process of selecting suitable ingredients of concrete and determining

their relative quantities with an aim of producing as economically as possible concrete of certain

minimum properties. Therefore, a concrete mix should be designed such that it has the minimum

possible production cost and meets the required specifications. The topic of concrete mix design is

beyond the level of Ordinary Diploma students but has been included just to provide a highlight of the

procedure.

11.2 Types of Mixes

Nominal Mixes Their proportions are fixed to just ensure adequate strength and are published in the relevant Codes.

They offer simplicity and usually have a margin of strength above that specified. However, due to

variability of materials the nominal concrete for a given workability varies widely in strength.

Standard mixes

In these mixes, proportions of ingredients are also published in Codes. They give acceptable

proportions.

Designed Mixes In these mixes, performance (e.g. strength and any other properties necessary for durability) of

concrete is specified by the designer but the mix proportions are determined by the producer of

concrete, except that the minimum cement content can be laid down. This is most rational approach to

the selection of mix proportions with specific materials in mind possessing more or less unique

characteristics. The approach results in the production of concrete with the appropriate properties

most economically. For the concrete with undemanding performance, nominal or standard mixes may

be used.

11.3 Trial mixes

A mix design results into a better approximation of the target mix proportions and this approximation

is the trial mix. Adjustments can be made on the original (initial) trial mix in order to suit the design

requirements. There are three courses of action on trial mixes:

→ To use the trial mix proportions in the production mixes

→ To modify the trial mix proportions slightly in the production mixes

→ To prepare further trial mixes incorporating major changes to the mix proportions

The following tests are carried out on the trial mixes:

♥ Slump test or V-B consistometer test

♥ Determination of the weight per cubic meter of fresh concrete

♥ Making and curing test cubes for compression testing

11.4 Considerations in mix proportioning

The following factors are considered in choice of mix proportions:


� Compressive strength: The mean compressive strength required at a specific age (28 days)

determines the nominal water-cement ratio of the mix and the degree of compaction.

� Quality control: This refers to the control of the variations in the properties of the mix

ingredients and also the control of the accuracy of all those operations which affect the

strength and consistence of concrete. The control of these variations need be established

during mix design.

� Durability: This requirement determines the water – cement ratio to be used.

� Workability: This has to be established depending on the size of sections to be concreted,

spacing and amount of reinforcements and method of compaction.

� Maximum size of aggregates: This is governed by size of sections and spacing of

reinforcements. Compressive strength tends to increase with a decrease in size of aggregate

� Type and grading of aggregates: These influence the workability of the mix, water-cement

ratio and aggregate-cement ratio.

11.5 Mix design procedure

The mix design process is divided into five stages:

i. Selection of target water/cement ratio

ii. Selection of free water content

iii. Determination of the cement content

iv. Determination of total aggregate content

v. Selection of fine and coarse aggregate contents

Stage 1: Selection of target water/cement ratio

If the previous information concerning the variability of strength tests comprises less than 40 results,

the standard deviation to be adopted can be obtained from �� )301. If the previous information

is available consisting of 40 or more results, the standard deviation of such results may be used

provided that this value is not less than the appropriate value obtained from

�� )301_Figure_A301; _Relationship. The margin M can then be computed from:

< � � · �

Where:

M = the margin

k = a value appropriate to the percentage defectives permitted below the characteristic

strength. A value of k=1.64 is recommended.

s = standard deviation

The target-mean strength can then be determined from:

�4 � �7 6 <

Where:

�4 = the target mean strength

�7 = the specified characteristic strength

M = the margin

A value is then obtained from �� )201 for the strength of a mix made with a free water/cement

ratio of 0.5 according to the specified age, the type of cement and the aggregate to be used. This

strength value is then plotted on �� )306 and a curve is drawn from this point and parallel to

the printed curves until it intercepts a horizontal line passing through the ordinate representing the

target mean strength. The corresponding value for the free water/cement ratio can then be read from

the x-axis. This should be compared with any maximum free water/cement ratio that may be specified

and the lower of the two values used.


Stage 2: Selection of free water content

The free water content can be determined from �� )202 depending upon the type of and

maximum size of the aggregates to give a concrete of the specified slump or V-B time.

Stage 3: Determination of cement content

Cement content can be determined from the relationship:

'�� 5��

�� 5��

The resulting value should be checked against any maximum or minimum values specified and an

appropriate choice made. If the calculated cement content is below the specified minimum, this

minimum value must be adopted. As a result , either the free-water/cement ratio of the mix may be less

than that determined in stage 1 or the free water content may be greater than that determine in stage 2.

This will result in a concrete that has a mean strength somehow greater than the target mean strength,

or workability somehow higher than the initially chosen, depending on the choice made. On the other

hand, if the design indicates a cement content that is higher than a specified maximum, then it is

probable that the specification cannot be met simultaneously on strength and workability requirements

with the selected materials. Consideration should then be given to changing the type of cement, type

and maximum size of aggregates or the level of workability of the concrete.

Stage 4: Determination of total aggregate content

This requires an estimate of the density of fully compacted concrete and this is obtained

from �� )302. If no information is available regarding the relative density, approximate values

of 2.6 and 2.7 can be used for uncrushed and crushed aggregates respectively. The total aggregate

content can then be obtained from:

?�� " ��" �� "�8# � ! 17 ! 1XY Where:

D = the wet density of concrete

W\ = the cement content

W]^ = the free water content

Stage 5: Selection of fine and coarse aggregate contents

With a known fine aggregate grading (obtained from the grading curve of the actual fine aggregates to

be used in conjunction with �� )203), appropriate zone can be located any of

�� )303, )304, )305 depending on the maximum coarse aggregate size. Since the free

water/cement ratio is known, the percentage (proportion) of fine aggregates can also be established

from the appropriate zone on Figure A305. The fine and coarse aggregate contents can then be

computed from:

0�� .��.��

'�� ! ��

The best proportions will depend on the aggregate shape and concrete usage. It should be noted that

the above designed mix will be a satisfactory trial mix but need to be tested and adjusted in order to

achieve the desired results. Samples from the deigned mix can be tested and the results checked with

the desired ones.


CHAPTER 12: PRECAST PRODUCTS IN UGANDA

12.1 Research

Research and compile your own notes about the various precast concrete products that are made in

Uganda and those that are imported into the country


APPENDIX

A1: Forms

Form A101: Cube crushing strength result sheet

Client:…………………………….……….…...……..Job:……………….………Site:……..…….………………...

Ref. No.

Date received

Date tested

Age

(days)

Dimensions

(LxBxH mm)

Weight

(kg)

Density

(kg/m3)

Load

(kN)

Crushing strength

(N/mm2)

Remarks

Tested by: ……………..……………….……. Date:………....……….. Checked by: ………………………………


Form A102: Concrete mix design form

Concrete mix design form

Job title: ……………………………………………………………………………...………

Stage Item Reference or

calculat ion

Values

1 1.1 Characterist ic st rength Specified ___________N/mm2

at _______________day s

Prop ortion defective ___________________%

1.2 Standard deviation From grap h _____________N/mm2 or no data ___________N/mm

2

1.3 Margin Calculated ( k=_________ ) Х _______ Х __________ = __________N/mm2

Specified ________________________N/mm2

1.4 Target mean s trength Calculated _____________+________________=_________N/mm2

1.5 Cement typ e Specified OPC/SRPC/RHPC

1.6 Aggregate ty pe: Coarse Cushed / Uncrushed

Aggregate ty pe: Fine Cushed / Uncrushed

1.7 Free water / cement rat io From Table

and grap h

___________

_____________________

1.8 Maximum Free water /

cement ratioSpecified ______________________

2 2.1 Slump or Vebe t ime Specified Slump _______________mm or Vebe time _____________s

2.2 Maximum aggregate size Specified __________________mm

2.3 Free water content From Table __________________________________________ kg/m3

3 3.1 Cement content Calculated _______________ /_____________=______________kg/m3

3.2 Maximum Cement content Specified _________________kg/m3

3.3 Minimum Cement content Specified __________________kg/m3

use 3.1 if ≤ 3.2 and ≥ 3.3

use 3.3 if ≥ 3.1 kg/m3

3.4 Modified free water cement rat io ________________________________________

4 4.1 Relat ive dens ity of

aggregates (SSD)___________________________known / as sumed

4.2 Concrete density From gragh ________________________kg/m3

4.3 Total aggregate content Calculated _______________−_______________−____________=____________kg/m3

5 5.1 Grading of fine aggregates Percentage p assing 600µm sieve __________________________%

5.2 Proport ion of fine aggregatFrom grap gh __________________________%

5.3 Fine aggregate content _________________Х __________________ = kg/m3

5.4 Coarse aggregate content _________________−___________________ = kg/m3

Cement Water Fine aggreg

kg (kg or l) (kg)

10 mm 20 mm 40 mm

Per m3 (to the nearest 5kg) _____________________________ __________ _________ __________ __________

Per t rial mix of ________m3

______________________________ __________ __________ __________ __________

Use the lower value

(kg)Quantities

Coarse aggregate


A2: Tables

Table A201: Approximate compressive strength (N/mm2) of concrete mixes made with a free-

water/cement ratio of 0.5

Type of cement Type of coarse aggregate Compressive strength (N/mm2)

Age (days)

3 7 28 91

Ordinary Portland Cement (OPC) or

Sulphate Resisting Portland Cement (SRPC)

Uncrushed 18 27 40 48

Crushed 23 33 47 55

Rapid Hardening Portland Cement (RHPC) Uncrushed 25 34 46 53

Crushed 30 40 53 60

Note: 1 N/mm2 = 1 MN/m2 = 1 MPa

Table A202: Approximate free water contents (kg/m3) required to give various levels of workability

Slump (mm) 0-10 10-30 30-60 60-180 Vebe time ( seconds) > 12 6-12 3-6 0-3

Maximum size of aggregate (mm) Type of aggregate

10 Uncrushed 150 180 205 225

Crushed 180 205 230 250

20 Uncrushed 135 160 180 195

Crushed 170 190 210 225

40 Uncrushed 115 140 160 175

Crushed 155 175 190 205 Note: when coarse and fine aggregates of different types are used, the free water content is estimated by the formula:

0� 5�� 2 3� 1̀ 6 13� 17 where 1̀ and 17 are the free water contents appropriate to the type of fine

and coarse aggregates respectively.

Table A203: BS 882:1973 Grading requirements for fine aggregates

BS Sieve size Percentage by weight passing sieves

Grading zone 1 Grading zone 2 Grading zone 3 Grading zone 4

9.5 mm 100 100 100 100

4.75 mm 90-100 90-100 90-100 95-100

2.36 mm 60-95 75-100 85-100 95-100

1.18 mm 30-70 55-90 75-100 90-100

600 µm 15-34 35-59 60-79 80-100

300 µm 5-20 8-30 12-40 15-50

150 µm 0-10* 0-10* 0-10* 0-15*

*For crushed stone sands, the permissible limit is increased to 20%


A3: Figures

Figure A301: Relationship between standard deviation and characteristic compressive strength

Figure A302: Graph of estimated wet density of fully compacted concrete (specific gravity is given for saturated, surface dry aggregates)


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)


Figure A306: Relationship between compressive strength and free water/cement ratio

Figure A307: Cement manufacture (dry and wet processes)

Raw meal

pre-heater Rotary kiln

Pulverised

coal

Clinker cooler

Diagramatic illustration of the dry process for manufacture of cement

Blending

silo

Ball millCrusher

Limestone Shale

Gypsum

Ball mill

Cement

silo

Packing plant,

Bulk transport, etc.

Cold air

Clay Water

Wash

millClay

slurry

Wash

mill

Chalk

Water

Blending

Diagramatic illustration of the wet process for manufacture of cement

Slurry

tankRotary kiln

Pulverised coal

Cold air

Clinker

cooler

Gypsum

Ball

mill

Cement

silo

Packing plant,

Bulk transport,

etc.

Concrete Technology Lecture Notes Ordinary Diploma in Civil Engineering

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