contents · 11/1/2011 · concrete materials 4 cement 4 aggregates 7 properties of concrete 10 ......

P0048459, Learning Resource S3, CPCCBC5004A, Ed 1 1 of 53

© State of New South Wales, Department of Education and Communities 2011 Version 1 November 2011

Contents

Contents 1

Introduction 3

Concrete materials 4

Cement 4

Aggregates 7

Properties of concrete 10

Compressive strength 10

Tensile or flexural strength 10

Durability 10

Workability 11

Cohesiveness 11

Concrete Testing 12

Sampling 12

Slump testing 12

Compression testing 14

Proportioning and mixing 17

Design strength 17

Target strength 17

Specification of concrete 17

Batching 19

Bulking of aggregates 20

Mixing 20

Premixed concrete 20

Slump 21

Admixtures 21

Air entraining admixtures 21

Water reducing agents 22

Transporting and placing of concrete 24

2 of 53 P0048459, Learning Resource S3, CPCCBC5004A, Ed 1


Transporting concrete 24

Methods of transporting concrete 25

Placing concrete 26

Compacting 27

Curing 27

Reinforced concrete 30

Basic principles 30

Design of reinforced concrete 31

Formwork 34

Basic requirements 34

Supervision 35

Materials 36

Surface treatments 37

Stripping times 38

Back propping 39

Off-form finishes 40

Joints in concrete construction 41

Prestressed and post stressed concrete 44

Finishing concrete 49

Initial finishing 49

Final finishing 49

Floating 49

Concrete finish class 50

Activity 1 51

Activity 2 52

Summary 53



Introduction

Concrete is one of the world’s most abundant building materials. It use dates

back to Roman times when limestone mortar was produced by heating

limestone and grinding the stone into a powder and then mixed with water

to form a paste that set both hard and quickly. It was during this era of

limestone mortar, that the first concrete was produced when the Romans

added sand, crushed stone or brick or broken tiles to the limestone mortar.

However, this concrete was severely limited since the mortar would dissolve

on contact with water. So it was a great achievement when a ‘sand’ (really a

volcanic ash) was discovered which, when mixed with lime and rubble,

hardened and could be used under water as well as in ordinary building.

This material was called ‘pozzulan’ since it was produced near the village of

Pozzuoli.

This ‘cement’ opened the way to a much greater use of mortars and

concrete; however, with the fall of the Roman Empire, the use of concrete

seems to have declined and not much is recorded about it until the mid

eighteenth century. It was not until 1845 that the real prototype of our

modern Portland cement was made.

So concrete is hardly a new material, but new aspects of concrete

technology are being investigated all the time and indeed the material has

been the source of an enormous amount of research for many years.

The ability of plastic concrete to be moulded into any shape probably makes

it one of our most versatile building materials and it is difficult to imagine a

building project today which does not make use of it in some manner.



Concrete materials

Concrete is a composite material which consists of a ‘binder’ (Portland

cement and water, commonly referred to as the paste) and aggregate. The

paste will also usually contain some entrapped air.

Aggregates are generally classified into two groups:

fine aggregates which consist of sand with particle sizes less than 5

mm

coarse aggregates—generally crushed rock of varying sizes but

greater than 5 mm

In properly made concrete each particle of aggregate, whether large or

small, is completely surrounded by paste, and all spaces between the

aggregate particles are completely filled with paste. The aggregates may be

considered as inert materials, while the paste (cement and water) is the

active cementing medium which binds the aggregate particles into a solid

mass.

In a given quantity of concrete, aggregate occupies approximately 75 per

cent of the volume while the remaining 25 per cent is taken up by cement

paste and air voids. Air voids will remain in even well compacted concretes

but usually occupy less than 2 per cent of the total volume unless an air

entraining agent has been used.

Fine (sand) Coarse (gravel, crushed

stone, slag etc)

Cement and water Voids (max 1–2%)

Aggregate Paste

Figure 1 - Composition of concrete

The setting or hardening process of concrete takes place through the

chemical reaction of the cement and water. This process is called

‘hydration’ and is characterised by the release of heat.

Cement Portland cements are hydraulic cements manufactured from carefully

selected raw materials under closely controlled conditions to ensure a high

degree of uniformity in their performance. In Australia, all Portland cements

are made to meet the requirements of AS3972–2010 Portland and Blended

Cements.



This standard covers five types of Portland cements which can be grouped

under the headings general purpose and special purpose.

General purpose cements:

- Type GP - general purpose Portland cement

- Type GB - general purpose blended cement.

Special purpose cements:

- Type HE - high early strength cement

- Type LH - low heat cement

- Type SR - sulphate resisting cement.

In general, Portland cement is produced by grinding together Portland

cement clinker and calcium sulphate.

General purpose cements

Type GP

General purpose cement is suitable for all uses where special properties are

not required. It is used for concrete products and building work where early

stripping for forms is not required.

Type GB

Blended cement consists of a mixture of Portland cement and pozzulands

such as fly ash and blast furnace slag. Blended cements generally have a

slower rate of strength gain and less heat of hydration when compared to

normal Portland cements; however, with continuous curing, they may

achieve higher long-term strength.

Special purpose cements

Type HE

Type HE cement is used where high strength is required at an early stage;

for example, where it is required to move forms as soon as possible or to put

concrete into service as quickly as possible (e.g. vehicle crossings). It is also

used in cold weather construction to reduce the required period of protection

against low temperatures.

Type LH

Type LH cement is intended for use in massive concrete structures such as

dams. In such structures the temperature rise resulting from the heat

generated during hardening of the concrete is likely to be a critical factor



Type SR

Type SR—sulphate resisting cement has better resistance to attack by

sulphates in ground water than other types because of its special chemical

composition.

White and off-white cements

White and off-white cements are true Portland cements. White cement is

made from selected raw materials and by processes which introduce no

colour, staining or darkening to the finished product. Off-white cement is in

general use in cottage construction but white cement usually proves cost

prohibitive. Portland cement is generally available in 40 kg bags; that is, 25

bags to the tonne.

High alumina cement

High alumina cement is not a Portland cement. If mixed with Portland

cement it can give a rapid or ‘flash’ set. It is characterised by a very high

rate of strength development accompanied by a high heat of hydration and

by a greater resistance to sulphate and weak acid attack than Portland

cements. Curing conditions require very close control for 24 hours after

placement.

Storage of cement

Cement will retain its quality indefinitely if it does not come in contact with

moisture. If it is allowed to absorb appreciable moisture it will set more

slowly and its strength will be reduced. Therefore, storage of bagged cement

requires storage facilities to be as airtight as possible, and the floor should

be above ground level to protect against dampness. The bags should be

tightly packed to reduce air circulation, but they should not be stacked

against outside walls. If they are to be held for a considerable period the

stacks should be covered with tarpaulins or water-proof building paper.

Doors and windows should be kept closed. A ‘first-in-first-out’ rotation of

bags should be maintained at all times.

Setting and hardening

Setting is the initial stiffening of the cement paste during the period in

which the concrete loses its plasticity and before it gains much strength.

This period is affected by the water content of the paste and the temperature.

The more water in the paste the slower the set, and the higher the

temperature the faster the set.

Hardening is the gain in strength which takes place after the paste has set. It

is affected by the type of cement used and the temperature. High

temperatures cause more rapid hardening.



Water

Water used for mixing good concrete should be free of deleterious amounts

of acids, alkalis and oil. Water containing decayed vegetable matter is

particularly to be avoided, as this may seriously interfere with the setting of

the cement. Water suitable for drinking will generally be suitable for

concrete making.

Aggregates Aggregates used in concrete should consist of clean, hard, durable particles

strong enough to withstand the loads to be imposed upon the concrete. In

general they should consist of either natural sands or gravels or crushed

rocks, although some manufactured aggregates such as blast furnace slag

and expanded shale and clays can be equally satisfactory. Commonly used

crushed rocks include basalt, granite, diorite, quartzite and the harder types

of limestone. Unsatisfactory materials include slate, shale and soft

sandstone.

Materials such as vermiculite and perlite and other lightweight materials are

unsatisfactory as aggregates for structural concrete as they lack strength.

In general, therefore, concrete aggregates should be:

strong and hard enough to produce concrete of the required

compressive strength and to resist abrasion and wear

durable to withstand the effects of weather and the cycles of wetting

and drying

chemically inert so that they will not react with the cement and cause

deterioration of the concrete

clean and free from impurities such as organic matter which can

inhibit the setting and hardening of the cement

free from silt and clay which, if present in excessive quantities, can

weaken the concrete

free from pieces or wood or coal which weaken the concrete and

cause blemishes

free from weak, soft particles which reduce the strength and break

down when exposed to the weather

free from surface coatings of clay or other weak material which

weaken the bond between the aggregate and the cement paste

Grading

Both coarse and fine aggregates should contain a range of particle sizes.

Graded aggregates produce more workable concretes which are less prone to

segregation and bleeding.



Particle shape and surface texture

The particle shape and surface texture of aggregates affect the workability.

For workability, particles should be smooth and rounded. On the other hand,

angular materials result in greater strength, so that, in the final analysis,

there is little or no difference in effectiveness. The ultimate decision is one

of economics and availability.

Maximum size of aggregates

The greatest economy is achieved when the largest maximum size aggregate

is used. The factors limiting size are the availability, transporting and

placing equipment to handle the larger sizes, and the clear spacing between

reinforcing bars and the clear spacing between the reinforcement and the

formwork.

Manufactured aggregates

Blast furnace slag

If sound and free from excessive quantities of ferrous iron, blast furnace

slags are satisfactory concrete aggregates. Generally they are angular in

shape and require a higher percentage of fines to produce workable

concrete.

Some slags contain quantities of anhydrited lime which, if undetected, can

hydrate and cause cracking of the concrete. Unsound slags can be detected

by soaking in water for two weeks, at which time they will show signs of

disintegration.

Lightweight aggregates

Expanded shale aggregates produce concrete having approximately two-

thirds the density of those made with dense aggregates, but with comparable

strengths. Lightweight aggregates may be smooth and rounded or harsh and

angular, depending on the method of manufacture.

Testing of aggregates

Since aggregates comprise up to 75 per cent of the volume of concrete, their

properties are obviously important. These properties include size and

grading as well as cleanliness.

The testing of concrete aggregates is generally carried out to determine:

the presence of organic or other deleterious material which may

severely limit the strength of the concrete

the resistance to abrasion, which may limit the durability of the

concrete

the presence of any alkalis which may react with the cement and

cause expansion of the aggregate



Conclusion

Good concrete can be made from a wide variety of aggregates provided

these are clean and free from harmful impurities. As the quality of concrete

becomes higher, the quality of the aggregate becomes more important and

factors such as grading more critical. Good aggregates, although sometimes

higher in initial cost, are generally more economical because of the higher

quality and lower overall cost of the concrete they produce.



Properties of concrete

There are several properties of concrete which affect its quality. These are:

compressive strength

tensile strength

durability

workability

cohesiveness

Let’s examine these properties in detail.

Compressive strength Compressive strength remains the common criterion of concrete quality and

will frequently form the basis of mix design. For fully compacted concrete

made from sound clean aggregates the strength and other desirable

properties under given job conditions are governed by the net quantity of

mixing water used per bag of cement. This relationship is known as the

water/cement ratio, that is, the quantity of water in the mix to the amount of

cement present.

Example: A concrete mix having a water/cement ratio of 0.5:1 would

require 10 litres (10 kg) of water for each 20 kg bag of cement.

The ultimate strength of concrete depends almost entirely on the

water/cement ratio, for as the ratio increases the strength of the concrete

decreases.

Tensile or flexural strength This is the measure of the concrete’s ability to resist flexural or bending

stresses.

The tensile or flexural strength of concrete is dependent on the nature, shape

and surface texture of the aggregate particles to a much greater degree than

does the compressive strength.

Durability Concrete may be subject to attack by weathering or chemical action. In

either case the damage is caused largely by the penetration of water or

chemical solutions into the concrete and is not confined to action on the



surface. The resistance to attack may therefore be increased by improving

the watertightness of the concrete. This is achieved by lowering the

water/cement ratio, assuming the concrete is fully compacted.

Workability The workability of concrete, or the effort required to handle and compact it,

depends on several factors, as follows:

Water/cement ratio: The higher the water/cement ratio, the more

workable concrete becomes. However, the water/cement ratio should

be fixed by considerations other than workability (e.g. strength and

durability), and should not be increased beyond the maximum

dictated by these considerations.

Cement content: The cement paste in concrete acts as a lubricant,

and at a fixed water/cement ratio, the higher the cement content, the

more workable the concrete becomes. It follows then that any

adjustments to increase workability should be made by increasing

the cement and the water content at a constant water/cement ratio.

Grading of aggregates: Grading tends to produce more workable

concrete.

Particle shape and size of aggregates: Smooth, rounded aggregates

will produce more workable concrete than rough, angular

aggregates. Also, for a given water/cement ratio and cement content,

workability increases as the maximum size of the aggregate

increases.

Traditionally concrete with a slump of 85mm or so was specified and

ordered. More recently design standards have been changed to permit a

higher slump level, to improve workability, and reduce WHS related issues

for concretors.

Cohesiveness The cohesiveness of concrete means the ability of plastic concrete to remain

uniform, resisting segregation (separation into coarse and fine particles) and

bleeding during placing and compaction.

Concrete in the plastic state should be cohesive to prevent ‘harshness’ of the

mix during compaction, and to avoid segregation of the coarse and fine

components during handling. Segregation may occur during transporting

over long distances, discharging down inclined chutes into a heap, dropping

over the reinforcement or falling freely through a considerable height and

placing in formwork which permits leakage of mortar. Maximum

cohesiveness usually occurs in a fairly dry mix, so as a rule the wetter the

mix the more likely it is to segregate. Segregation can, however, occur in

very dry mixes.



Concrete Testing

Concrete is tested on the site or in the laboratory to determine its strength

and durability or to control its quality during construction. These tests help

the engineer or job supervisor to determine whether the concrete is as

specified and that it is safe to proceed with the job or whether adjustments

should be made to the mix.

These tests must be carried out carefully and in the correct manner or the

results may be misleading and cause unnecessary delays while they are

being checked. Worse still, faulty tests may result in either substandard

concrete being accepted or even good concrete being rejected.

There are several ways in which testing can be carried out:

by sampling

by slump testing

by compression testing

Sampling To make a composite sample from the discharge of a mixer or truck, three

or more approximately equal portions should be taken from the discharge

and then remixed on a non-absorbent board. The sample portions should be

taken at equal intervals during the discharge and none should be taken at the

beginning or the end. The concrete at these points may not be truly

representative of the whole mix.

When sampling freshly deposited concrete, a number or samples should be

taken from different points and recombined to make a composite sample.

Care should be exercised to make certain the sample is representative by

avoiding places where obvious segregation has occurred or where excessive

bleeding is occurring.

Slump testing The slump test is a measure of the consistency or mobility of concrete and is

the simplest way of ensuring that the concrete on the site is not varying. It

should be done often as an overall control on the various factors that can

affect the result. Chief among these factors is the water content of the mix,

variation of which can result in varying strengths of concrete. A consistent

slump means that the concrete is under control. If the results vary it means



that something else has varied, usually the water, which can then be

corrected.

Equipment

To carry out the slump test, the following equipment is required:

A standard slump cone.

A bullet pointed steel rod or tamping rod.

A rule.

The slump cone is made from sheet metal and is 300 mm high, 200 mm in

diameter at the bottom and 100 mm in diameter at the top. It should be fitted

with footrests at the bottom and with handles by which it can be lifted.

The tamping rod is 600 mm long, 16 mm in diameter and bullet pointed.

All the equipment must be assembled before your begin testing.

Figure 2 - Slump test equipment

Method

To make the test, you should follow these steps.

1 Moisten the inside of the slump cone and place it large end down on a

clean level surface. Hold it firmly in place with a foot on each footrest.

2 Fill the cone, in three approximately equal layers, with concrete from

the sample.

Each layer should be tamped down exactly 25 times with the tamping

rod, which must be allowed to penetrate each layer.

3 The strokes must be uniformly distributed over the whole surface of the

layer and not worked up and down continuously in one place.



4 After the top layer has been compacted, the surface of the concrete is

struck off level with the top of the cone and any surplus concrete is

removed from around the base.

5 The cone should then be lifted, carefully but firmly, straight up so that

the concrete is allowed to subside. Lift the cone smoothly and quickly

but do not jerk, twist or take off at an angle lest a false result be

obtained.

6 To measure the slump, invert the cone and place it alongside the

slumped concrete. Lay the tamping rod on top of the cone and measure

the amount of slump, measuring to the highest point of the concrete.

The slump is recorded to the nearest 10 mm.

Figure 3 - Slump test

Types of slump

In practice, concrete can slump in three ways:

True slump: the concrete subsides but more or less retains its conical

shape.

Shear slump: the concrete subsides but one side shears or falls away.

Collapsed slump: the concrete collapses completely.

If the concrete collapses or shears away, repeat the test.

Compression testing The strength of concrete is determined by making specimens, curing them,

and then crushing them to ascertain their strength. The preparation of

specimens is most important as a badly prepared specimen will nearly

always give a low result.



Compressive test specimens are normally cylinders 150 mm in diameter and

300 mm high.

Equipment

Moulds in cylindrical shapes

Tamping rod

Rule

Mineral oil

Moulds for the cylinders should be made of metal and be rigid enough to

retain their shape during preparation of the specimen. They should be fitted

with a base plate which can be fitted securely to the mould to prevent loss of

the cement paste.

Method

1 Before filling with concrete, the mould should be clean and coated

inside with a very light film of mineral oil.

2 Place the mould on a level surface and fill with concrete from the

sample in three equal layers. Rod each layer 25 times with a bullet

pointed rod 600 mm long and 16 mm in diameter, allowing each stroke

to penetrate the previous layer.

In this case it is necessary that the concrete be fully compacted and it

may be necessary to rod each layer more than 25 times. The rodding

must be distributed over the whole surface of each layer and not merely

in one place. The concrete in the mould may be compacted by vibration

if suitable vibrators are available.

3 After the specimen has been moulded, it should be stored in a place

where it will be undisturbed for 18–24 hours, kept moist and at a

temperature of between 21°C and 24°C. After 24 hours the specimen

should be removed from the mould and again stored under moist

conditions and at the correct temperature. This is called curing.

4 For transport to the laboratory, the specimens should be packed in moist

sand or hessian so that they will remain moist and be undamaged during

transit.



Figure 4 - Preparation of a concrete specimen for compression testing



Proportioning and mixing

Design strength The designer of a concrete structure determines during the design stage, the

concrete properties that are necessary to ensure that the structure performs

in the desired manner. Since compressive strength is usually the most

important property required and since most other desirable properties are

directly related to it, it is usual for the designer to specify the minimum

compressive strength required, usually at 28 days. The ‘design strength’ is

the minimum strength required by the designer.

Target strength The mix designer must design a mix which will produce concrete with a

strength in excess of the design strength for the following reasons:

It is known that when a series of compressive tests are made from

samples of concrete taken from time to time through the course of a

job, the results will be scattered to either side of an average value,

even though all the concrete is made to the same specification. This

means that the concrete produced is never completely uniform in

quality—some is always weaker than the average strength and some

is always stronger.

Since the designer has specified the minimum strength required, the

mix designer must aim at an average strength, between the target

strength and the design strength.

Generally, a target strength 33 per cent higher than the design strength

meets the requirements of the building codes.

Specification of concrete In writing the specification to ensure that the concrete has the properties

required, the designer has two alternatives:

specify the concrete by strength (the usual method)

specify concrete by proportions



Concrete specified by strength

Figure 5 - Strength development of Cement1

The designer specifies the minimum compressive strength required in the

concrete and the age at which the concrete should have this strength, usually

28 days.

Figure 6 - Water : cement ratio – the effect of adding water to concrete2

The ratio of water to concrete by weight gives a good indication of the likely

final strength concrete. As W/C ratio increases the concretes strength

decreases. (See Figure 6 - Water : cement ratio – the effect of adding water

to concrete)

1 Nikulski B, Materials 1 Subject Notes, 2007, Unpublished




Figure 7 - Effect of curing3

Concrete specified by proportions

In this case, the designer specifies the materials to be used and the

proportions to be used. Designers use knowledge and experience as a basis

for ensuring that concrete of the desired strength is produced, and the job

supervisor is responsible for the correct materials being used in the specified

proportions. The responsibility for the concrete strength and other properties

remains with the designer.

Batching All materials, including water, should be accurately measured to ensure that

concrete of uniform quality is produced.

The method used to measure the quantities of different materials required

for a mix is called batching by mass. Mass batching is very accurate and

reduces the danger of variations of quality of concrete between one batch

and another.

Batch proportions are often specified in relation to the bag of cement; for

example, one 20 kg bag of cement to so many kilograms of coarse aggregate

and so many kilograms of fine aggregate with perhaps 10 L or 10 kg of

water. Even though the solid materials are measured by mass, it is quite

common for water to be measured by volume from a graduated tank above

the mixer. Provided that the tank is accurately graduated there is no loss of

accuracy as 1 L of water has a mass of 1 kg and is not subject to variation.




With mass batching, there is no need to make allowance for the bulking of

damp sand but allowance must be made for the non-absorbed water held by

the aggregates as this moisture forms part of the mixing water.

Equipment for mass batching ranges from simple inexpensive platform

scales to large and elaborate types, while some large types of concrete

mixers have mass batching devices built into them.

Bulking of aggregates Volume proportions are always specified on the assumption that the

aggregates are loose packed and dry. Most aggregates contain some

moisture and sand exhibits a property described as ‘bulking’ when moist;

that is, sand when moistened increases in volume. This property makes sand

difficult to gauge accurately by volume measurement and is, in fact, the

principal reason why batching by mass rather than by volume is the

preferred method.

Mixing The aim of mixing concrete is to obtain a uniform mixing of all the concrete

materials and to ensure that each particle of aggregate is adequately coated

with cement paste.

Mixing time

Short mixing times, although increasing production, produce patchy, non-

uniform concrete.

Excessive mixing is generally uneconomical and may cause undesirable

grinding of the aggregates particularly if they are on the soft side.

The minimum mixing time allowed by AS3600–2001 Concrete Structures is

11

2 minutes.

Premixed concrete Premixed concrete is used almost universally on residential building sites.

The use of premixed concrete has advantages which include:

Better quality control is possible at a large plant than under most site

conditions.

Less labour is required.

Premixed concrete is controlled by AS1379–2007 Specification and

Manufacture of Concrete, which should be referred to for information on

methods of ordering, mixing and delivery.



Slump The slump of a batch of concrete at the time of discharge should be

expressed as the average of two tests, one on concrete sampled at the one-

quarter point of the batch volume and the other on concrete sampled at the

three-quarter point.

The concrete should be considered to comply with the specified slump if:

when the specified slump does not exceed 75 mm the average of two

tests is within 12 mm of the specified slump; and

when the specified slump exceeds 75 mm the average of two tests is

within 12 mm of the specified slump.

Admixtures An admixture may improve the properties of concrete. Admixtures are

available in both solid and liquid forms. The general nature of the

admixture should be known before adding it to the concrete mixture in case

it may impair strength or durability.

Accelerators

Accelerators increase the rate of reaction between cement and water in the

mix.

Calcium chloride

The amount of calcium chloride accelerator used should not exceed 2% by

weight of cement when its temperature is between 50C and 200C.

Stannous chloride

Stannous chloride is an expensive accelerator that must be fresh and the

concrete thoroughly compacted.

Triechanclamine

Small amounts of triechanclamine accelerator may be used at 0.5% to 0.4%

by weight of cement. It may increase the shrinkage. If used excessively it

can produce rapid setting.

Air entraining admixtures Air entraining admixtures are soluble salts of wood resins, fatty acids,

soluble salts or sulphate or sulphonated hydrocarbons. These are used to

develop microscopic bubble systems by agitation in mixer. This improves



workability and durability to reduce bleeding and decrease segregation.

Bubbles provide lubricating and plasticising effects which allows for less

water without loss of slump.

Air cells remain separate entities in hardened concrete and act as barriers to

normal entry of water and water-borne salts via capillary pores. They also

provide expansion chambers to withstand extreme temperature changes.

They improve volume of air by 3 to 5%. Excess air entrained can cause

serious loss in strength.

Set retarders

Hydroxylated carboxylic acids and their salts, certain sugars and

carbohydrates can be used to retard the onset of setting. These are useful

additives to extend the time between mixing, placing and finishing from 1 to

3 hours. They leave more water in the concrete for workability until it is

placed when hydration can continue.

Water reducing agents Water reducing agents add strength. Intermixing of cement and water is

minimal due to differing surface temperatures and energies.

Strength is improved at all ages and the strength is due to physico-chemical

effects on hydration rather than to the use of less water.

Super plasticisers

Super plasticisers improve workability. They make concrete almost self-

levelling. The duration of effectiveness of super plasticisers is 20 to 90

minutes and then the concrete returns to its original behaviour.

Waterproofing

Tests have proven that waterproofing admixtures are largely ineffective so

waterproof sheeting is still needed under slabs. Transmission through walls

and upright structures or into concrete floors may be effected to some

degree by these additives. Talc, fullers earth, some silicates, substances

from saps, fatty acids, ammonium and BU + YL stearates are used for

waterproofing. Most cause a reduction of strength

Workability agents

Workability agents improve cohesiveness, for easier placement and better

compaction. They reduce permeability by filling voids between particles.

They can also be used for mixes deficient in fines. They are finely divided

providers and include hydrated lime, bentonite, talc, clay and pulverised

stone.



Pigments

When pigments are used in concrete, cement content should be increased by

10 to 15% by weight. Colour lightens when concrete is dry. Special curing

is needed for consistency of colour. Either a layer of washed sand or curing

compound containing matching colour should be used.

Expanding agents

To counteract the effects of shrinkage, settlement and bleeding expanding

agents can be used. They are used to provide maximum bearing, base

plates, and steel columns for under-pinning work. In grouting for cavity

joints, ducts containing pre-stressed concrete members that are post-

tensioned. Wax is used to facilitate pumping and reduces bleeding.



Transporting and placing of concrete

The care taken in the production of good quality concrete is to some extent

nullified unless the mixed concrete is transported from the mixer to the

forms, placed and compacted satisfactorily.

Transporting concrete Irrespective of the methods used to transport, place and compact the freshly

mixed concrete, the following requirements are basic to good practice:

The concrete must be transported, placed and compacted with as

little delay as possible.

The concrete must not be allowed to dry out before compaction.

There must be no segregation of the materials.

The concrete in the forms should be fully compacted.

Dangers of poor transporting practice

Delay

Stiffening of concrete begins as soon as the cement and water are

intermingled. This stiffening increases with time, and therefore, the time

which elapses after mixing has an adverse effect on the workability of the

mix. Under normal conditions, the amount of stiffening which takes place in

the first 30 minutes after mixing is not significant, and if the concrete is kept

agitated, up to one and a half hours can normally be allowed to elapse

between mixing and compacting.

Drying out

Concrete is designed to have a workability which will allow it to be fully

compacted with the equipment available. If it is allowed to dry out during

transportation or placing, it will lose workability and full compaction may

not be possible.

Segregation

Segregation can occur if unsuitable methods are used to transport, place and

compact plastic concrete and results in the hardened concrete being non-

uniform with weak and porous honeycomb patches.



Inadequate compaction

The strength, durability and permeability of the hardened concrete all

depend on the concrete being fully compacted in the forms. Inadequate

compaction results in an appreciable loss of strength.

Methods of transporting concrete There are several methods of transporting concrete:

barrows

hoists

trucks

chutes

pumps

pipelines

Barrows

These are the most basic of the vehicles used in this country for transporting

concrete but are still in considerable use. They are particularly suited for

smaller jobs and for larger jobs with short hauls.

The number of barrows should be sufficient to take the full mix from the

mixer in order to minimise waste of time and avoid confusion.

Hoists

The hoist is a commonly used means of elevating concrete. Proprietary hoist

towers ranging in height from about 4.5 m to 45 m can be made. These

hoists can operate an elevating platform on to which one or two barrows of

concrete can be wheeled.

Trucks

Trucks are in general use for transporting concrete from a central mixing

plant to scattered jobs or to various parts of a large project. In ordinary

trucks, wet concrete is liable to segregate and dry mixes are liable to

compact.

Premix firms have overcome the problem of segregation during transport by

the use of agitator trucks for wet mixes and by truck-mounted mixers which

transport a dry batch and mix it when approaching the site.

Chutes

Unless special care is taken to ensure that the discharge is vertical at the end

of the chute and that long chutes are adequately protected to prevent drying



out, this can be one of the most unsatisfactory methods of transporting

concrete.

The slope of chutes should be sufficient to allow the flow of the lowest

slump concrete being used on the job. A baffle at the end of the chute should

direct the concrete into a vertical downpipe at least 600 mm long to prevent

segregation of the concrete on discharge from the chute.

Pumps and pipelines

Pumps and pipelines enable concrete to be transported across congested

sites and where space is limited. The maximum horizontal distance concrete

can be pumped is 500 m. Vertical pumping in excess of 120 m may be

achieved but heights are normally kept below 30 m. Maximum length

cannot be combined with maximum height.

Curves and rises should be limited as they reduce the maximum pumping

distance. A 90° bend, for example, is equivalent to about 10 m of straight

pipe. Each metre rise in elevation is equivalent to about 5 m of straight

horizontal pipe, although this value depends on pipe size and concrete

velocity. With very slow rates of pumping in large pipes this equivalent

value can be as high as 30 m.

The output of a conventional 100 mm pump varies between about 10 and

100 m3

per hour, depending on type of pump and conditions.

Concrete for pumping must be of medium workability with a slump of 70

mm to 120 mm and must be free from any tendency to segregate. The

introduction of fly ash to the concrete improves pumpability and workability

of the mix, and therefore adds appreciably to the distance concrete can be

pumped.

Placing concrete Certain precautions must be taken when placing concrete, to ensure that:

formwork and reinforcement is not damaged or dislodged

the concrete is free from segregation

other qualities of the concrete are not impaired

The following is a summary of some of the most important points of good

placing practice:

Concrete should be placed vertically and as near as possible to its

final position. If spreading is necessary it should be done with

shovels and not by causing the concrete to flow.

Concrete should not be dropped into the forms from an excessive

height as this can cause damage and segregation. The height to fall

should be kept to a minimum and should not exceed 1.8 m unless a

drop chute or a vertical funnel is used.

Placing should start from the corners of formwork and from the

lowest level if the surface is sloping.



Each load of concrete should be placed against the face of the

previously deposited concrete, not away from it.

If stone pockets occur, the stones should be shovelled from the

pocket and tamped or vibrated a into sandy section.

Concrete should be deposited in horizontal layers and each layer

should be compacted before the next is placed. Each layer should be

placed in one continuous operation and before the previous layer has

hardened.

As the top of a lift is neared, drier mixes should be used to allow for

the water gain which begins to form on the surface.

To minimise the pressure on forms with high lifts, the rate at which

the concrete rises should not exceed 1.5 m per hour in warm weather

and 600 mm per hour in cold weather.

Concrete should not be placed during heavy rain without overhead

shelter to prevent the rain washing the surface of the concrete.

Compacting It is essential that concrete be properly compacted to ensure maximum

density. Air holes must be eradicated, voids between aggregate particles

must be filled and all aggregate particles must be coated with cement paste.

Thorough compaction results in:

maximum strength

watertight concrete

sharp corners

a good bond to reinforcement

protective cover to reinforcement

a good surface appearance

Vibration

Concrete is usually vibrated to achieve good compaction. There are three

types of vibrators:

immersion vibrators

form vibrators

surface or screed vibrators

The immersion vibrator is driven either electrically, mechanically or

pneumatically and is probably the most efficient type of vibrator as it

vibrates the concrete directly by immersion in the concrete. They are

particularly suited to the compaction of large volumes of concrete.

Curing Concrete hardens as a result of the chemical reaction that occurs between

cement and water which is called hydration. Hydration occurs only if water

is available and if the concrete's temperature stays within a suitable range.



After placing concrete, the concrete surface needs to be kept moist for a

period of time to permit the hydration process. This period is referred to as

the curing period and is usually 5-7 days after placing conventional

concrete.

While it is true that concrete increases in strength and other desirable

properties with age, this is so only so long as drying is prevented. The

hydration of cement is a chemical reaction and this reaction will cease if the

concrete is permitted to dry. Evaporation of water from newly placed

concrete not only stops the process of hydration, but also causes the

concrete to shrink, thus creating tensile stresses at the drying surface; and if

the concrete has not developed sufficient strength to resist these stresses,

surface cracking may result.

As in many other chemical reactions, temperature affects the rate at which

the reaction between the cement and water progresses; the rate is faster at

high temperatures than at lower temperatures.

It follows then that concrete should be protected so that moisture is not lost

during the early hardening period and should also be kept at a temperature

that is favourable to hydration.

Curing methods

Curing methods can be classified as follows:

The supply of additional moisture to the concrete during the early

hardening period.

Sealing the surface to prevent loss of moisture from the concrete.

Ponding

On flat surfaces, concrete can be cured by building an earth or sand dyke

around the perimeter of the concrete surface in which a pond of water is

retained.

Ponding is not only a very efficient method of preventing water loss from

the concrete but also maintains a uniform temperature in the concrete.

Sprinkling

Sprinkling can be either continuous or intermittent. If intermittent, care must

be taken to ensure that the concrete does not dry between applications of

water. A fine spray of water applied continuously through a system of spray

nozzles provides a constant supply of moisture and prevents the possibility

of cracking or crazing caused by alternate cycles of wetting and drying.

Wet coverings

A 50 mm thick layer of earth or sand, straw or hessian or other moisture

retaining material spread over the surface of the concrete and kept

constantly moist so that a film of water remains on the surface of the

concrete throughout the drying period has proved very satisfactory.



Waterproof paper and plastic sheets

Strips of waterproof paper or plastic sheeting spread over the surface of the

concrete prevents the evaporation of the water from the concrete. The edges

of the sheeting should be overlapped and sealed with sand, tape or by

weighting down with planks or other heavy objects. An important advantage

of this method is that periodic additions of water are not required.

Curing compounds

Liquid membrane forming curing compounds sprayed over the surface of

moist concrete retard or prevent the evaporation of moisture from the

concrete. Some curing compounds prevent the bonding of fresh concrete to

hardened concrete and should not be used for instance on the base slab of a

two-course floor since the top layer may be prevented from bonding. The

adhesion of resilient floor coverings to concrete floors may also be affected

by some curing compounds.

Curing of vertical surfaces

Vertical surfaces can be satisfactorily cured by:

leaving the forms in place. If wooden forms are used, they must be

kept moist by sprinkling

draping hessian over the surface and keeping it moist

constant sprinkling or hosing of the surface

Length of curing period

For most structural purposes, the curing time for concrete varies from a few

days to two weeks according to conditions; for example, lean mixes require

longer curing time than rich mixes and temperature affects the curing time

as does the type of cement used.

Since all the desirable properties of concrete are improved by curing, the

curing period should be as long and as practicable in all cases.



Reinforced concrete

Basic principles Concrete, like any other building material, has limitations, mainly because

of the fact that while it is strong in compressive strength, it is comparatively

weak in tensile strength. To overcome this weakness in tension, concrete

which is to be subjected to tensile stresses is reinforced with steel bars or

mesh which is so placed that it will resist such stresses.

The designing and detailing of reinforcement is the job of the designing

engineer and will not be dealt with in any great detail here, but it is

important that those who supervise the fixing of reinforcement on the job

have an appreciation of the basic principles of reinforced concrete. They can

then understand why it is necessary that reinforcement be correctly handled

and fixed in the positions indicated on the job drawings.

Figure 8 - Types of stress found in a structure

Reinforced concrete is so designed to combine the concrete and steel into

one structural entity in such a way as to make the best use of the

characteristics of each of these materials.

The aim of reinforced concrete design is to combine the steel reinforcement

with the concrete in such a manner that just enough steel is included to resist

the tensile stresses and excess shear stresses while the concrete is used to

resist the compression stresses.

Steel and concrete combine together successfully because:



the bond between concrete and steel directly counteracts any

tendency for the concrete to stretch and crack in a region subjected

to tension

with temperature changes, concrete and steel expand and contract the

same amount. If this were not so, the different expansion rates would

break the bond between the two materials and so prevent the transfer

of tensile stresses to the steel

concrete has a high fire-resistance and protects the steel from the

effects of fire

A broad understanding of stresses and the methods of indicating the

particular stress on drawings is essential.

Design of reinforced concrete In order to be effective, the tensile reinforcement must be prevented from

sliding in the concrete. The adhesion or bond between the concrete and the

steel is related to the surface area of the steel embedded in the concrete.

Adequate anchorage is effected by extending the rods past the critical points

(where no longer required to resist tensile and shear stresses) and by the use

of:

standard hooks

plain rods extended into the supports (rarely used)

deformed bars (rolled with lugs or projections)

The three environment phases

In the course of time, the environment surrounding the reinforcement

changes.

Before the concrete is cast, the steel bars are exposed to atmospheric

rusting, which is due to the simultaneous presence of water and

oxygen (air).

The bars are surrounded by freshly mixed concrete which although it

contains water, is normally so alkaline that it prevents further

corrosion of the steel.

For a very long time the bars are encased in solid concrete which is

slightly permeable, may crack, and may itself be modified by

chemical attack.

The surface condition of reinforcement shall comply with the following

requirements.

At the time concrete is placed, reinforcement shall be free from mud,

oil, grease and other non-metallic coatings and loose rust which

would reduce the bond between the concrete and the reinforcement.



The prevention of corrosion

There are three ways of reducing or preventing the corrosion of the steel in

reinforced concrete.

One is to use more cement with or without a greater thickness of

concrete cover so as to preserve the high alkalinity around the

reinforcement.

Another is to put a protective coating of some additional material on

the reinforcement.

Finally, rust resistant alloy steels or even non-ferrous metals may be

used.

The likelihood of corrosion

If the reinforcement were to be surrounded by a minimum thickness of 60

mm of impermeable uncracked concrete, even a moderately aggressive

environment will cause corrosion in due course. In dry, unpolluted air the

protection of 25 mm of concrete cover should maintain the required

alkalinity of the concrete in contact with the steel. These specifications are,

however at risk due to the effects of workmanship, tensile cracking of the

concrete, and the porosity of the aggregate, and in some circumstances it

may not be possible to meet them. The best protection against corrosion is to

ensure specified cover with well compacted concrete.



Figure 9 - Positioning of main reinforcement to resist tensile stresses in beams



Formwork

Basic requirements In its plastic state, concrete can be readily moulded into any desired shape.

As any inaccuracy or blemish in the formwork will be reproduced in the

finished concrete, it is essential that the forms be designed and constructed

so that the desired size, shape, position and finish of the concrete is

obtained. Although the formwork is a temporary structure, it will be

required to carry heavy loads resulting from the mass of the freshly placed

concrete and construction loads of materials, workers and equipment. The

formwork must therefore be substantial enough to carry these loads without

fear of collapse or deflection, and within the confines of AS1509, SAA

Formwork Code.

As the cost of formwork can amount to about one-third of the total cost of a

concrete structure, efficiency in its construction can become an important

factor in the overall economy of the job.

Good formwork

The guiding principles for the production of good formwork are:

quality

safety

economy

Quality

First quality formwork should be:

Accurate: True to the shapes, lines and dimensions required by the

contract drawings.

Rigid: Forms must be sufficiently substantial so as to prevent any

movement, bulging or sagging during the placing of the concrete.

Tight-jointed: If joints are not tight, they will leak mortar. This will

leave blemishes in the shape of fins on the surface of the concrete

and may result in honeycombing of the concrete close to the leaking

joint.

Well-finished: The quality of the finish of the concrete is dependent

on the finish of the forms. Nails, wires, screws and so on should not

be allowed to mar the surface of the finished concrete.



Safety

Strength: For the safety of the workers and of the structure, the

formwork must be strong enough to withstand not only the mass of

the wet concrete but also the live loads of workers, materials and

equipment. It is impossible to over emphasise how important this

aspect of safety really is.

Soundness: Materials must be of good quality and durable enough

for the job. The time will come, no doubt, when it will be essential to

use for structural load-bearing members, only timber that has been

tested with the mechanical stress grading process.

Economy

For economy, formwork should be:

Simple: Formwork should be designed for simplicity of erection and

removal.

Easily handled: Shutters and units should be light enough to permit

easy handling.

Standardised: Where standardisation of formwork is possible, the

ease of assembly and the possibility of reuse serve to lower the

formwork cost.

Reusable: Formwork should be designed for easy removal and in

sections that are reusable. This will minimise the amount of waste

material and thus decrease the cost of the formwork.

Supervision The field supervisor’s work falls into four categories:

Control: The supervisor must ensure that formwork is constructed in

accordance with the specifications and working drawings and must

check that all dimensions are within the allowable tolerances.

Planning: The supervisor might also play a part in planning the work

so as to achieve an efficient cyclic program of assembly, concreting,

removal and restoring.

Safety: The supervisor must play a leading role in ensuring adequate

safety precautions to protect workers. There will be many occasions

where she or he should seek the counsel of the site engineer.

Workmanship: The supervisor must ensure that formwork is

constructed to a high standard of quality.

Some of the deficiencies which can lead to form failures are:

Premature removal of forms or props.

Inadequate bracing and poor splicing of multiple storey timber

props. Splices should have long cleats at the joint on all four sides

and be well nailed.

Failure to control the rate of placing concrete in deep forms without

regard to the effect of temperature changes.



Failure to regulate properly the placing of concrete on horizontal

forms and prevent unbalanced loadings.

Failure to check the adequacy of footings for falsework to prevent

settlement in unstable ground.

Failure to inspect formwork during concreting to detect any

abnormal deflections or signs of imminent failure.

Failure to provide adequately for lateral pressure on formwork.

Props not plumb.

Locking devices on metal props not locked or inoperative.

Overturning by wind.

Damage in excavations by reason of embankment failure.

Failure to check that the drawings are being interpreted correctly.

Points which are related to workmanship are:

Joints or splices in sheathing, plywood panels and bracing should be

staggered.

Tie rods or clamps should be in the correct numbers and locations.

Tie rods or clamps should be properly tightened.

The connections of props and stays to joists, stringers and wales

must be adequate to resist any uplifts or twisting at joints.

Form coatings should be applied before placing of reinforcement and

should not be used in such quantities as to run onto bars.

Bulkheads for control and construction joints should preferably be

left undisturbed when forms are stripped, and removed only after the

concrete has cured sufficiently.

Bevelled inserts to form keyways at contraction joints should be left

undisturbed when forms are stripped, and removed only after the

concrete has cured sufficiently.

Wood inserts for architectural treatment should be partially split by

sawing to permit swelling without applying pressure to the concrete.

The loading of new slabs should be avoided in the first few days

after concreting.

Formwork must not be treated roughly or overloaded if reuse is

desired.

Materials Formwork can be constructed in many different types of materials. Details

about each type follow.

Timber

Partially seasoned softwoods, such as Oregon or pine, dressed where in

contact with the concrete, make good formwork. Fully seasoned timber will

swell excessively when wet and green timber will warp and shrink during

hot weather.



Plywood

Varying in thickness from 5 mm to 20 mm, plywoods give a large area of

joint-free surface. Plastic coated plywood (plasply) can be used to give a

smooth grainless surface to the finished concrete. Plywood can be bent to

produce curved surfaces.

Hardboard (Masonite formboard)

Hardboard has many of the features of plywood but requires more support

and cannot be curved so easily.

Steel

Steel is relatively costly but it can withstand repetitive reuse. Steel framing

and bracing can be used in conjunction with timber and plywood panel

systems. There are a number of proprietary steel formwork systems

available.

Surface treatments

Preparation of forms for concreting

All debris, particularly chippings, shavings and sawdust, must be removed

before the concrete is placed and the surfaces which are to be in contact with

the concrete must be cleaned and thoroughly wetted or, alternatively, treated

with a suitable composition. Compositions that have not been approved by

the engineer or architect must not be used.

Temporary openings must be provided at the bases of columns and wall

forms and at other points where necessary to allow cleaning and inspection

immediately before the placing of the concrete.

Surface coatings for forms

Any material used as a surface coating for forms must:

act as a separating agent to allow the release of the forms without the

concrete sticking to their surfaces

act as a sealer to prevent the forms absorbing water from the

concrete

not stain or disfigure the finished concrete surface

not prevent the adhesion of render or other similar surface finishes

not reduce the active life of the forms



Wood and plywood forms

A number of form oils suitable for timber forms are marketed commercially.

These are designed to penetrate the surface to some extent and leave the

surface of the form only slightly greasy to the touch. For plywood, apart

from the commercially produced oils, a mixture of linseed oil and kerosene

is satisfactory. Plywood may also be coated with shellac, lacquer, resin-

based products or plastic compounds which almost totally exclude water

from the plywood, thus preventing the grain from rising. Such coatings

require little or no oiling.

Metal forms

Form oils suitable for timber forms are not always suitable for metal forms.

Paraffin-based form oils and petroleum-based oils blended with synthetic

castor oil, silicone or graphite have proved successful on metal forms.

Stripping times The time of the removal of forms is generally specified by the architect or

engineer in the contract documents or made subject to this person’s approval

because of the danger to the structure if forms are stripped before the

concrete has developed sufficient strength. Forms can usually be safely

stripped when the concrete has developed about two-thirds of its 28-day

strength. However, the earliest possible removal of forms is desirable for the

following reasons:

To allow the reuse of forms as planned.

In hot weather, to permit curing to begin.

To permit any surface repair work to be done while the concrete is

still ‘green’ and favourable to good bonding.

Additional information is available in AS

Remember safety is paramount, and it is much better to be sure than sorry.

Vertical forms can generally be removed before the forms to the soffits of

beams and slabs.

Where stripping times have not been specified, Table 1 - Times for stripping

formwork and supports may be used as a guide to appropriate stripping

times when using normal Portland cement.



Table 1 - Times for stripping formwork and supports

Location and type of formwork

Average temperature of concrete during the period before stripping

21°C to 32°C 4°C to 21°C

Days Days

Beam sides, walls and

unloaded columns

1–2 2–5

Heavily loaded columns,

tunnel linings supporting

unstable material, and other

heavily loaded structures

7–10 10–14

Slabs, including flat slabs

and flat plates, with props

left under

3–7 7–20

Removal of props from

under slabs

7–14 14–21

Beam and girder soffits

(with props left under) and

arch soffits

7–10 10–14

Removal of props from

under beams

10–14 14–28

Information on required formwork stripping times for reinforced concrete

slabs continuous over formwork supports not supporting structures above

is provided in AS 3600:2009 Table 17.6.2.4 if not provided in project

documentation.

Information on required formwork stripping times for reinforced concrete

slabs and beams not supporting structures is provided in AS 3600:2009

Table 17.6.2.5 if not provided in project documentation.

Also as a guide only information on for multistorey formwork stripping

times with and without back propping is provided in AS 3610:1995 Table

5.4.3 and clause 5.4.4 if not provided in project documentation.

Back propping Builders must consider a number of issues when planning the construction

of multistorey reinforced concrete buildings. On the one hand concrete

curing to achieve desired strength must be achieved prior to formwork

removal. However builders will want to reuse formwork as soon as possible

on upper levels to minimise cost. Careful consideration must be given to

suitable formwork stripping times. Advice from a structural engineer about

the suitable formwork stripping time is recommended. Options are available

and include the implementation of back propping to accelerate formwork

reuse. AS3610 states that undisturbed support requirements for multistorey



formwork systems are to be in accordance with project documentation and

formwork documentation.

Formwork is supported by false work, and typically in multistorey

formwork, ply sheets are supported by joists which are supported by

bearers. Once a new slab has been cast, and after the required number of

days delay, back propping can be implemented by:

Fixing the required number of back props to the underside of the

plywood directly (not to joists or bearers supporting ply) before any

supports or ply or anything else is removed.

Then, and only then, remove the rest of the supports, beams, joists and

ply with no back props holding it up.

New back props are then placed under the bare concrete.

Then the props that are left holding up just the ply sheets are removed

one at a time, the ply is removed and the prop is put back against the

concrete before the moving on to the next one.

In this way the slab is always supported.

Activity 1

Look up AS3610:1995 Formwork for concrete and particularly Table

5.4.3 and clause 5.4.4. Determine the impact that back propping has

on formwork removal by answering the following questions:

1. If back propping is used should the minimum number of levels

of undisturbed supports be increased or decreased?

2. If back propping is used can the maximum number of levels of

support which may be back propped exceed half of the total

number of levels of support at the time of the pour?

3. Can stacked materials be placed on any of the supported floors?

4. What is the minimum time between pause of successive floors?

5. What are the minimum temperature requirements?

Off-form finishes It is economical for the structural concrete to form the surface finish. Where

special characteristics such as smoothness, pattern, texture, intricate detail

and so on are required, extra special care must be taken in the selection of

form materials and in the form construction.

Smooth surfaces

Most sheathing and lining materials are available in grades smooth enough

to produce a blemish free concrete surface. The correct choice of form oil is

important in achieving the desired smoothness.



Wood grain finishes

A surface simulating wood grain can be produced by casting the concrete

against a plywood form liner which has had the grain revealed by wire-

brushing or sand-blasting. Sometimes an exposed grain plywood is available

ready-made for this purpose. To produce a rough board marked surface,

sawn boards are used for sheathing. These boards may be sprayed with

ammonia to raise the wood fibres and accentuate the grain markings.

Textured and patterned surfaces

These finishes are obtained by lining the forms with liners such as striated

plywood, rubber matting and moulded plastic. The liners are either nailed or

fixed with a waterproof glue to the inside surfaces of the forms.

Joints in concrete construction Interruptions to the placing of concrete will inevitably occur when pouring

large quantities. Irrespective of the length of these interruptions, if the

concrete is allowed to stiffen to the extent that it cannot be worked, then a

joint must be made. Other cases will occur when it is necessary, for

structural reasons, to break the continuity of placing and to form a joint.

Joints can be of two general types:

Construction joints: These aim at bonding the new concrete to the

hardened concrete in such a manner that the concrete appears to be

monolithic and homogenous across the joint and allows for no

relative movement of the concrete on either side of the joint.

Control joints: These allow for relative movement on either side of

the joint, thus they can be either construction joints or expansion

joints.

Construction joints

In practice, it is very difficult to obtain a perfect bond at a joint and a plane

of weakness will always occur at a construction joint. For this reason, they

should be avoided wherever possible.

While unscheduled interruptions are often unavoidable during placing,

making an unplanned construction joint necessary, some breaks in the

continuity of placing may be foreseen either in the design stage or just

before commencement of construction, thus allowing the position of many

joints to be planned. Good planning will aim to interrupt placing in a

position suitable for a control joint and so eliminate the need for a

construction joint.



Location of construction joints

Where construction joints are necessary in structural members they should

be made where the shear forces are at a minimum. The joint should be at

right angles to the axis of the member so that axial forces act normally to the

joint and do not tend to cause sliding along a weakened plane.

Concrete for columns should be poured continuously to just below the soffit

of the beam, drop panel or capital, and the concrete left for at least two

hours to settle before fresh concrete is placed. The whole floor system

around the head of the column should then be cast in one operation after

suitable preparation of the joint.

Construction joints in beams should be made in the middle third of the span

and on no account should they be made at or near the supports or over any

other beam, column or wall since shearing stresses are usually very high at

these positions.

When a construction joint is required in a floor slab it should be made near

the middle of the span.

Making vertical construction joints

When making a construction joint in a beam or slab, the concrete must not

be allowed to assume its natural angle of repose, but should be taken up to a

suitable stop board so as to form a vertical joint. To assist the transfer of

load across the joint, either dowels or a keyway to aid mechanical bonding

may be used at about mid-depth of the beam or slab. This is recommended

in sections over 150 mm deep. Reinforcement must not be cut at a

construction joint but must be left continuous in the member.

Figure 10 - Making a vertical construction joint

Preparation of construction joints

The correct method of preparation and making of construction joints is

detailed in AS3600 1994 Concrete Structures Code.



Watertight construction joints

A correctly made horizontal construction joint in a wall should not require

sealing, but if the joint is to be in contact with water and particularly if

subjected to hydraulic pressure, effective sealing will be necessary because

of the tendency of the joint to open up as the concrete shrinks. This can best

be carried out by using a water stop. PVC water stop membranes extending

into the concrete equally each side of the joint and welded or glued together

at the ends to form a continuous diaphragm are commonly used.

Contraction joints

A contraction joint is a concrete joint made so that the concrete is free to

shrink away from the joint while all other relative movement across the joint

face is prevented.

As concrete sets, hardens and dries out, it shrinks. If no provision is made to

relieve the drying-shrinkage tensile stresses within the concrete, cracking

will occur when these stresses exceed the tensile strength of the concrete. If

the concrete is completely unrestrained, cracking will not occur, but very

few structures are completely unrestrained.

Contraction joints are most needed in unreinforced concrete structures

because reinforcement considerably increases the tensile strength of

concrete, restrains overall shrinkage movement and prevents the formation

of large shrinkage cracks.

Location of joints

Contraction joints should be located where it can be expected that the

severest concentration of tensile stresses will occur, such as:

Where abrupt changes in cross section occur.

On irregularly shaped floors and slabs (e.g. T, H, L and U shapes), to

divide them into rectangular shapes.

Where structures are weakened by openings.

In long structures such as walls and road pavements, which are not

sufficiently reinforced to prevent the formation of shrinkage cracks.

In large areas of pavement or slab on the ground.

Construction of joints

A vertical plane of weakness is purposely formed in the slab or wall.

Vertical movement is controlled by forming a keyed joint or by using non-

ferrous dowels with one end capped and coated so that they are free to slide.

The bond between new and existing concrete at a contraction joint must be

broken.

Dummy contraction joints

A dummy contraction joint is a plane of weakness built into a structure by

means of a groove, either sawn or formed with a grooving tool.



This joint functions as a contraction joint by localising shrinkage cracks to

beneath the groove. The irregularity of the crack serves to transfer loads

across the joint and prevents relative movement in the plane of the joint.

Since this type of joint is an alternative to a full depth contraction joint, the

location should be the same as for contraction joints.

Expansion joints

An expansion joint is formed by creating a gap between the two surfaces of

the concrete to allow for expansion. The gap is usually filled with a

compressible filler and all relative movement in the plane of the joint is

prevented.

Expansion joints are generally provided in structures exceeding 30 m length,

in unreinforced or lightly reinforced road pavements and as sliding joints

between a roof slab and a supporting wall.

Prestressed and post stressed concrete

Prestressing

The basic principle of prestressing concrete is very simple. If a material has

little tensile strength it will fracture immediately its own tensile strength is

exceeded, but if such a material is given an initial compression, then, when

load-creating tension is applied, the material will be able to withstand the

force of this load as long as the initial compression is not exceeded. You are

already familiar with the properties of concrete that result in a material of

high compressive strength but low tensile strength. By inserting steel

reinforcing bars of the correct area into a concrete member, and fixed in a

predetermined pattern, ordinary concrete can be given an acceptable amount

of tensile strength. Prestressing techniques are applied to concrete in an

endeavour to make full use of the material’s high compressive strength.

Tendons of strand can be used singly or in groups to form a multi-strand

cable. The two major advantages of using strand are:

providing a large prestressing force in a restricted area

the production of long flexible lengths that can be stored in drums

thus saving site space and reducing site labour requirements by

eliminating site fabrication self help exercise.

A prestressing force inducing precompression into a concrete member can

be achieved by anchoring a suitable tendon at one end of the member and

applying an extension force at the other end which can be anchored when

the desired extension has been reached. Upon release, the anchored tendon,

in trying to regain its original length, will induce a compressive force into

the member. Figure 8 shows a typical arrangement in which the tendon



including the compressive force is acting about the neutral axis and is

stressed so that it will cancel out the tension induced by the imposed load

W. The stress diagrams show that the combined or final stress will result in

a compressive stress in the upper fibres equal to twice that of the imposed

load. The final stress must not exceed the characteristic strength of the

concrete as recommended and if the arrangement given in the figure is

adopted the stress induced by the imposed load will only be half its

maximum.

Figure 11 - Prestressing principles

To obtain a better economic balance the arrangement shown in figure y is

normally adopted where the stressing tendon is placed within the lower third

of the section. The basic aim is to select a stress that, when combined with

the dead load, will result in a compressive stress in the lower fibres equal to

the characteristic strength of the concrete and a zero stress in the bottom

fibres. Note, however, that this is the pure theoretical case and is almost

impossible to achieve in practice, but provided any induced tension

occurring in the lower fibres is not in excess of the tensile strength of the

concrete used, and acceptable condition will exit.



Figure 12 - Alternative prestressing arrangement

Post-stressing

Concrete is cast around ducts in which the stressing tendons can be housed.

The stressing is carried out later. When the stress required has been

reached, the tendons are anchored at their ends to prevent them returning to

their original length thus inducing the compressive force. The anchors used

form part of the finished component. The ducts for housing the stressing

tendons can be formed by using flexible steel tubing or inflatable rubber

tubes. The void created by the duct will enable the stressing cables to be

threaded prior to placing the concrete, or they can be positioned after the

casting and curing of the concrete has been completed. In both cases the

remaining space within the duct should be filled with grout to stop any

moisture present setting up a corrosive action and to assist in stress

distribution. A typical arrangement is shown in Figure 10.



Figure 13 - Post-stressing principles

Poststressing is the method usually employed where:

stressing is to be carried out on site

curved tendons are required

the complete member is to be formed by joining together a series of

precast concrete units

where negative bending moments are encountered

Figure 11 shows various methods of overcoming negative bending moments

at fixed ends and for continuous spans.

Figure 14 - Overcoming negative bending moments by using post-stressing



Figure 12 shows a typical example of the use of curved tendons in the cross

members of a girder bridge. Another application of post-tensioning is in the

installation of ground anchors.

Figure 15 - Structural uses of prestressed concrete



Finishing concrete

Initial finishing Immediately after placing and vibrating a screed is used to quickly level the

concrete. The screed board is moved forward with a sawing motion, and

concrete shovelled up to and away from the front of the screed as necessary.

After initial screed the area should be checked for level and adjusted where

necessary. Overworking the surface should be avoided.

Final finishing Edging, jointing, floating, trowelling and brooming should be delayed as

long as possible, within reason, before final set. The correct timing is

determined by a variety of factors such as concrete temperature and age,

type of cement, admixture type, and quantities of water, cement and

admixtures used. Weather conditions, depth of pour, type of aggregate, type

of substrate and the like also influence the time for final finishing.

Excessive surface moisture: Cement should not be used to dry up surface

moisture as this will cause surface cracking later. Instead mopping or

dragging with hessian are preferable.

Dry and windy conditions resulting in cracking: accelerated evaporation

due to hot windy weather can result in setting that is too rapid for

satisfactory finishing, and even surface cracking. Due to the amount of time

it takes to finish concrete, and the impact adverse weather can have,

typically builders pour concrete slabs early in the morning. RC concrete

piers on the other hand are often poured in the afternoon where such issues

are less critical.

Floating After necessary delay the surface is floated with a wood float. This smooths

irregularities in the surface following screeding, by pushing large aggregate

below the surface and removing imperfections.



Concrete finish class

AS3610:2010 Formwork for concrete at Table 3.2.1 sets out the applicable

surface classes for finished concrete. These concrete classes are often

referred to in specifications, and must be achieved by builders.

Class 1 – is the highest class that is recommended for use in special features

of buildings of a monumental nature.

Class 2 – has a consistently good quality that is intended to be viewed in

detail.

Class 3 – has good visual quality that is intended to be viewed as a whole.

Class 4 – had good general alignment and where texture is not important.

Class 5 – where alignment and texture are not important.



Activity 1

In this activity you will inspect the concrete finish of a number of

nearby reinforced concrete structures. Find three different reinforced

concrete structures which have some visible concrete elements, such

as slabs columns or beams

1. Determine the concrete class achieved for each of the visible

concrete elements, using the class groups from AS3610

Table 3.2.1. For example inspect the finish in a reinforced

concrete car park in a big shopping centre, to that in the

columns or façade in an office building, to that in block of

units. Which has the highest standard of finish, and what is

its corresponding finish class?

2. AS3610 section 5.2.1 states that the surface appearance of

concrete should be evaluated by assessment of the extent of

blowholes, grout loss, honey combing and surface treatment

with a viewing distance of at least 2m or more if that is the

items normal viewing distance. The standard gives other

measures for undulations, flatness, out of plumb, stepping

and whether out of plumb. Now reconsider your evaluation

of the concrete finish of inspected concrete above based on

these characteristics. Would you make any changes to your

classification?



Activity 2

Obtain copies of the following documents and review in relation to

topics covered.

www.concrete.net.au/publications/pdf/Long-span%20Floors.pdf

http://www.rta.nsw.gov.au/doingbusinesswithus/downloads/contractor-

ohs/tipsheets_dl1.html and obtain Formwork PDF

www.construct.org.uk/bpg/BPGEarly_Striking.pdf

http://www.concrete.net.au/publications/pdf/Long-span%20Floors.pdf

http://www.rta.nsw.gov.au/doingbusinesswithus/downloads/contractor-ohs/tipsheets_dl1.html

http://www.rta.nsw.gov.au/doingbusinesswithus/downloads/contractor-ohs/tipsheets_dl1.html

http://www.construct.org.uk/bpg/BPGEarly_Striking.pdf



Summary

Concrete is a composite material, comprised of Portland cement and water

(known as the paste) and aggregate. Aggregate occupies approximately 75

per cent of the volume of the concrete while the paste and voids occupy the

remainder. General purpose (type GP) is the most commonly used cement in

the building industry.

Water and aggregates used in concrete should be free of any deleterious

materials, and aggregates should also be hard and durable.

Compressive strength is the common criteria of concrete quality and is

dependent on the water/cement ratio. Concrete is tested on site for

consistency (the slump test) and off site, following strict curing procedures,

to determine the compressive strengths at 28 days (the compression test).

In residential building, concrete is delivered to the site ‘ready mixed’ in

nearly all cases except where only a small quantity is required and then will

usually be mixed on site using bags of premixed cement and aggregate.

Good practice for the transport and placing of concrete must be followed to

ensure a strong, dense and watertight product. It must be properly cured to

allow an increase in strength with age. The first seven days are particularly

important in allowing the chemical process of hydration to proceed

unheeded.

Reinforced concrete combines steel and concrete, making use of the best

properties of both materials to produce a product used universally on

virtually all building projects. The tensile strength of the steel is combined

with the compressive strength of concrete as a building material. This

strength, combined with its ability to assume any desired shape and its

resistance to fire, makes concrete a very valuable and adaptable material for

the building industry.

contents · 11/1/2011 · concrete materials 4 cement 4 aggregates 7 properties of concrete 10 ......

Documents