Concrete Introduction...Concrete is a composite material which consists of a ‘binder’ (Portland cement and water, ... surrounded by paste, and all spaces between the aggregate

Concrete Introduction

Concrete is one of the world’s most abundant building materials. Its 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 in detail, UNIT CPCCBS6001, Ed 1 1 © New South Wales Technical and Further Education Commission, 2015

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 seven 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 GL - general purpose limestone cement - Type GB - general purpose blended cement.

• Special purpose cements:

- Type HE - high early strength cement - Type LH - low heat cement

2 Concrete in detail, UNIT CPCCBS6001, Ed 1 © New South Wales Technical and Further Education Commission, 2015

- Type SR - sulphate resisting cement - Type SL – shrinkage limited 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 GL General purpose limestone cement contains Portland cement and may contain limestone and/or other minor additional constituents.

Type GB

Blended cement consists of a mixture of Portland cement and pozzulans 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.

Type SL

Type SL—shrinkage limited cement when designed in conjunction with aggregate type, water content, admixtures, etc. may produce a concrete which has reduced shrinkage.

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 anhydrated 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 concreters.

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, the most common being:

• slump testing

• compression testing

Sampling for testing of concrete 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.

3. Each layer should be tamped down exactly 25 times with the tamping rod, which must be allowed to penetrate each layer.

4. The strokes must be uniformly distributed over the whole surface of the layer and not worked up and down continuously in one place.

5. 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.

6. 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.

7. 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.

3. 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.

4. 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.

5. 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.

The strength of concrete is normally shown a prefix (e.g. N) and a number (20, 25, 32, 40 and 50). N stands for normal class concrete. The number is the corresponding characteristic strength of the concrete in Megapascals (MPa). Therefore, an N32 concrete mix is a normal class concrete with a characteristic strength of 32 MPa at 28 days. Standard strength grades are N20, N25, N32, N40 and N50.

Special Class Concrete (S) is specified to have certain properties or characteristics different from or additional to those of Normal Class concrete.

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 2 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.

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


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 AS3600–2009 Concrete Structures is 112 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 supply 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 50°C and 200°C.

Stannous chloride Stannous chloride is an expensive accelerator that must be fresh and the concrete thoroughly compacted.

Triethanolamine Small amounts of triethanolamine 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.

Concrete is measured in cubic metres (m3). A standard concrete truck has the capacity to carry 7 m3. Minimum order is generally 0.6 m3, but there is usually a premium charge for quantities less than 7 m3. Concrete is ordered in 0.2 m3 increments.

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 as possible 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.

Concrete Cancer When concrete cracks, water penetrates through causing the steel reinforcement inside to corrode (rust). Rusting steel then sheds its skin forcing the layers of rust to push away the concrete surrounding it. This results in large or small pieces of concrete falling away and


allowing steel reinforcement to become even more corroded and may cause devastating wear on both the steel and the 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 AS3610 Formwork for concrete.

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 3610

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.


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 3.3.1 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.


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.


Documents

Concrete Introduction...Concrete is a composite material which consists of a ‘binder’ (Portland cement and water, ... surrounded by paste, and all spaces between the aggregate