CONCRETE

byIkmalzatul Abdullah

ADVANTAGES OF CONCRETE:

•Ability to be cast - many different shapes and types of

structures, offsets other disadvantages.

•Economical - on-site preparation, local materials,

unskilled labor.

•Durable - maintenance-free, generally no protective

coatings.

•Fire resistant - can maintain structural integrity.

•Energy efficient - requires less energy to produce than

steel.

•On-site fabrication.

•Aesthetic properties.

DISADVANTAGES OF CONCRETE:

•Low tensile strength - very brittle, must be reinforced

with steel to carry the tensile stresses.

•Low ductility.

•Volume instability - shrinkage and creep.

•Low strength-to-weight ratio.

Manufacture of Concrete

Concrete can either be made wholly on the site or the potential advantaged of factory production can be partially secured by the use of ready mixed concrete or wholly secured by the use of precast products.

The process of manufacture are: Checking and storage of materials Batching Mixing Tests on mixed concrete Formwork and reinforcement Transport to formwork and placing Compaction Curing Removal of formwork Protection Construction joints

Storage of Materials

Storage of materials must prevent deterioration of cement and contamination and segregation of aggregates.

Cement must be kept dry. Paper bags cannot be relied upon to prevent air setting

and resulting lumpiness. Exceptionally, where it is not certain that cement can be

stored in dry conditions or it can be used soon after delivery it may be advantageous to use hydrophobic Portland cement.

Particular care should be taken in storing extra rapid hardening and ultra high early strength Portland cements and supersulphated cement.

Cont’d High alumina cement should be preferably be kept in a

store separate from Portland cement. Paper bags should not be stacked more than 4 or 5 feet

high to avoid warehouse set caused by compaction. Cement should be used in the order in which it was

received. Aggregates should be kept on clean hand

surfaces and not directly on the ground. The various sizes of aggregates should be kept

separately and where possible stock piles should be duplicated so that deliveries can drain for at least twelve hours before use.

Batching

Accurate batching of cement, aggregates and water make for saving in cost of designed mixes by enabling a lower control factor to be employed.

It used to be customary to specify and to batch cement and aggregates in proportions by volume, as so called nominal mixes, but volume batching tends to be inaccurate because both cement and sand are subject to bulking and coarse aggregate is difficult to measure accurately by volume.

Cement in batched by weight and normally and preferably the aggregate also.

Cont’d Cement

Varies in bulk density from about 1120-1600kg/m3 according to the way in which the container is filled.

Where a weighing device is not available, the bag can be used as a unit.

Sand Dry and wet sands have the same volume, but damps

sand has a greater volume and if sand is measured by volume and allowance is not made for bulking, concrete mixes may be seriously under sanded.

Cont’d

Coarse aggregate Deep and narrow gauging boxes reduce error in

volume batching but the method is laborious. Properly maintained weight batching machines

are very accurate and easy to use.

Water As the water:cement ratio determines the

strength and durability of concrete, the amount of water contained in each batch is critical.

The gross weight of water (kg) per batch is water:cement ratio x weight of cement (kg).

Cont’d

The tanks fitted to the larger mixers have gauges which enables a measured quantity of water to be added to each batch.

This must be adjusted from time to time to allow for the water contained in the aggregate.

During the progress of work if changes in the moisture contents of aggregates are small, provided the quantities of cement and aggregates and the type of aggregates remain the same, the quantity of added water can be adjusted so as to maintain the workability indicated by a slump test on the first batch.

Mixing

Concrete may be mixed on the site, or ‘at works’ for precast concrete or for delivery to the site a ready mixed concrete.

On site mixers The most commonly used type are batch mixers

of the single compartment drum type.

Truck mixers Some mixers incorporate weight batching equipment and

attachments for hand scrapers to assist in loading the hoppers and normally 200 liter and larger mixers can measure volumes of water.

So that water is evenly distributed, it should enter the mixer before or at the same time as the other materials.

The proportion of coarse aggregate should be reduced for the first batch or two each day to compensate for the loss of mortar which sticks to the blades and inside the drum.

The time required for thorough mixing varies according to the characteristics of the mix and of the mixer.

Cont’d

Cont’d

When the concrete has been mixed the complete contents of the drum should be discharged in one operation to avoid segregation of the larger stones.

Mixer should be thoroughly washed out and cleaned daily and even after short stoppages, to prevent ‘caking’ with hardened concrete which reduces the machine’s efficiency and they should be cleaned out when the type of cement is changed.

Tests On Mixed Concrete

Consistency of Manufacture The slump test, which is easy to carry out, indicates

variations in the shape of grading of aggregate, or in the proportion of water being used.

Workability The slump test gives an approximate indication of the

workability of Portland cement mixes which are neither too stiff nor too plastic.

The compaction factor test is more accurate, but neither test is suitable where the maximum size of aggregate exceeds 40mm.

Compression Tests Cubes made before and during the placing of concrete on

the site are tested in crushing machines to give some indication of the strength which would be acquired by the actual work.

Preliminary Cube Tests Preliminary compression tests require very accurate

control of materials and test conditions. The materials intended to be used are mixed in the

laboratory in the proportions to be used in the work.

Cont’d

Formwork

Formwork provides the shape and surface texture of concrete members and supports them during setting and hardening.

It must be grout-tight, true in line, level, face and profile and strong enough to accept all constructional loads including those resulting from mechanical compaction.

Formwork is the best constructed in units for easy erection, striking without damaging the concrete and so that it can be reused.

The faces of formwork should be treated with mould oil to give a clean release but avoiding excess oil which stains concrete and which may interfere with bond for plaster.

Formworks

Benefits Higher load capacity More controlled failure

Reinforcing bar is placed in region of tensile stress.

Reinforcement

Reinforcement should comply with the following standards:

BS 4449:1978 hot rolled steel bars for reinforcement of concrete

BS 4482:1982:1969 hard drawn mild steel wire for the reinforcement of concrete

BS 4486:1980 hot rolled, and hot rolled and processed high tensile alloys steel bars for prestressing of concrete

BS 4757:1971 nineteen wire steel strand for prestressed concrete

BS 4483:1969 steel fabric for the reinforcement of concrete

BS 5896:1980 high tensile strength steel wire strand for the prestressing of concrete

Cont’d

Reinforcement

Reinforcement

Cont’d

Reinforcement should be free from loose mill scale, loose rust, oil or grease.

Reinforcement should be placed in the exact positions shown on the drawings and the specified cover ensured, eg by spacers fixed to the reinforcement.

Great care should be taken to avoid damage or disturbance to formwork when positioning reinforcement.

Transport to Formwork and Placing Whether concrete is moved from the mixer by lorries,

barrows, dumpers, mechanical skips or pipeline it is important that the composition of the mix is not altered and that segregation does not take place.

All pant, chutes, etc should be thoroughly cleaned after use without allowing the waste water to enter formwork.

‘Wet’ mixes are particularly likely to segregate and where possible, these should not be dropped into position.

Chutes should be arranged so that a continuous flow is discharged at the lower end.

Immediately, before concrete is placed, formwork should be thoroughly cleaned out and formwork and reinforcement should be re-checked.

Compaction Trapped air which should not exceed about 2 %

when concrete is placed must be released if the maximum density associated resistance to chemicals, water vapor, frost and abrasion is to be be obtained.

Thorough compaction is also very important where concrete faces are to be exposed to view.

Air is very liable to be trapped against form faces and at joints between hardened and newly placed concrete.

Compaction should commence as soon as possible once water has been added to concrete although so long as it remains possible to fully compact concrete by the means available, delay in doing so may not be serious up to perhaps two hours even in cold weather.

Curing In order to obtain the desired strength,

compacted concrete must be free from physical disturbance, Water must be retained in the concrete Temperature must be controlled

Removal of Formwork

Formwork must be left in position, and the supports maintained, until concrete is sufficiently strong to safely support its own weight and any loads which may be put on it.

Concrete should have a cube strength at least twice the stress to which the concrete is likely to be subjected at the time of striking.

The times which should elapse before formwork is remove vary considerably according to the cement used, temperature of the concrete during curing and other factors.

Cont’d

Supports should be eased away uniformly and very slowly so that the load is not suddenly imposed on partly hardened concrete.

Formwork must be stripped carefully to avoid damage to arises and projections, especially where vertical surfaces are exposed within 12 hours of casting.

Protection After stripping formwork, it may be necessary to protect concrete

for damage by knocks, shocks and vibration; from drying in hot weather and from loss of heat in cold weather.

Construction Joints Whenever concreting is interrupted the construction

which are inevitable formed are potentially weak. They may allow water to enter and they are always

visible, particularly after a period of weathering. The positions and design of construction joints should

therefore be decided at an early stage. Joints should be straight, either vertical or horizontal, and

in walls in positions related to window openings and other features.

Generally, in columns, construction joints are made as near as possible to the beam haunching and in beams and slabs within the middle third of span.

Vertical joints should be formed against temporary but rigid stop boards which must be designed to allow reinforcement to pass through.

Lightweight Concrete Examples:

Aerated concretes Lightweight aggregate concretes No fines concretes

Weighing less than 1920kg/m3 Are made in densities down to about 160kg/m3. Advantages of using lightweight concrete than dense concrete:

Savings in costs of handling materials and of supporting structures

Superior thermal insulation and fire resistance Superior sound absorption of unplastered surfaces; some of

which offer better key for plaster Usually easy to cut, chase and nail into.

Cont’d Compressive strength and the modulus of elasticity are

reduced (although the latter reduction may improve resistance to mechanical damage)

The moisture movement of aerated and lightweight aggregate concretes is high.

Reversible moisture expansion is usually as great as the initial drying shrinkage.

Protection of reinforcement against corrosion may reduce

Sound insulation reduces as density of concrete decreases.

Three Main Ways Lightweight concretes are made in 3 main ways:

Aerated or cellular concrete Minute and non communicating cells are formed by

introducing air or gas into a matrix of cement with, in all but the lightest non structural concretes, ground sand, pulverized fuel ash or other fine siliceous material as fine aggregate.

Lightweight aggregate concrete Made by incorporating a cellular coarse aggregate

No fines concrete Made by omitting the fine aggregate and the smaller

particles of coarse aggregate so as to leave voids.

1. Aerated Concrete Have the lowest density, thermal conductivities and

strengths. Like timber, they can be sawn, screwed and nailed, but

they are non combustible. For work in situ, the usual methods of aeration are by

mixing in a stabilization foam or by whipping air in with the aid of an air entraining agent.

Full strength development depends upon the reaction of lime with the siliceous aggregate, and for equal densities the strength of high pressure steam cured concrete is about twice that of air cured concrete.

No further curing is required after autoclaving. Blocks are usually cut at works to the required size

from larger units.

Strength

Strength sufficient for structural work are obtainable but the modulus of elasticity of aerated concrete is about one tenth of dense concrete.

Creep at working loads is not thought to be greater.

Moisture Movement The moisture movement of cement not being

restrained by rigid aggregate, air cured aerated concrete has very high drying shrinkage and without frequent shrinkage joints, this concrete if placed in situ would crack.

Weather Resistance

Unprotected single leaf aerated concrete block walls have good resistance to rain penetration and to frost.

However, for densities of 825 and 497 kg/m3 water absorptions are about four times and eight times greater than that of dense concrete and external rendering is desirable wherever reinforcement is present.

Thermal Insulation Thermal conductivities of 0.084 W/m degree Celsius

and less are obtainable in dry concrete. External surfaces should be rendered or otherwise

protected to avoid serious loss of thermal insulation due to absorption of water.

Fire Resistance Fire resistance as defined by BS 476: Part 8: 1972

tests, is good, for example for walls without finishes: 102mm loadbearing wall 2 hours 102mm non loadbearing wall 4 hours 142 mm non loadbearing wall 6 hours

Hardness

Aerated concrete is much softer than dense concrete Requires protection from abrasion in the lower parts of walls

and in similar positions. It can be easily sawn, worked with simple tools and nailed into. Retention of nails is better cut nails than wire nails, and with

the denser concretes.

2.Lightweight Aggregate Concretes

Deal with structural applications. Foamed slag, expanded clay, expanded slate and sintered

pulverized fuel ash concretes are suitable for reinforces concrete structures with

strengths in compression up to 62 N/mm2 densities 30-40 % thermal conductivities 50% or more, less than those

gravel concretes. As with dense aggregate concretes, the strength properties

of lightweight aggregate concretes depend upon: Type of aggregate Grading of aggregate Cement:aggregate ratio Water:cement ratio The degree of compaction

Moisture Movement

Drying shrinkage is generally about twice that of dense concrete.

The poor workability of some lightweight aggregates should be compensated for by the addition of sand or an air entraining agent rather than by using a richer mix which would increase drying shrinkage.

Although the proneness of lightweight concrete to shrink and crack may be largely offset by its lower modulus of elasticity, the precautions advised for aerated concrete should be taken.

3. Non Fines Aggregates

Commonly applied in concretes which contain only a single size 19.0 to 9.5mm coarse aggregate (either a dense aggregates or lightweight aggregate) with sufficient cement to join the particles while leaving voids between them.

The density is about 2/3 to ¾ that of dense concretes made with the same aggregates.

No fines concrete is almost always cast in situ mainly as loadbearing and non loadbearing walls.

Walls

The surface of no fines concrete provide and excelled key for external rendering and internal plaster finishes, which are essential to prevent air movement through walls with loss of thermal and sound insulation.

Any rain which penetrates external renderings will travel inwards only 20 to 50mm or so, but damp courses and construction joints should be designed to throw such water outwards.

Dry Shrinkage

Aerated and lightweight aggregate concretes have high drying shrinkage but that of no fines concrete is usually less even than that of dense concrete made with the same aggregate.

Also because no fines concrete shrinks more rapidly than dense concrete, plasters and renderings are less likely to crack.

Thermal Insulation The thermal conductivity of no fines gravel aggregate

concrete is comparable to that of typical brickwork.

Sound Insulation The sound insulation of plastered no fines concrete

walls is slightly inferior to that of solid brick walls of comparable thickness.

Mixing Aggregate should be damped before being placed in

the mixer, cement and then sufficient water should be added so that particles of aggregate are coated with cement without it bridging between them.

Formwork Because no fines concrete exerts only about 1/3 of the

pressure exerted by ordinary concrete, formwork can be of light construction.

It does not require to be grout-tight and if expanded metal is used the mix can be seen as it placed.

Reinforcement Light reinforcement is advisable across the angles at

openings. A coating of cement grout reduces the likelihood of

corrosion.

Placing Mixes should pour freely. Some gentle rodding may be needed but vibration

should never be resorted to. The concrete should be placed evenly in horizontal

layers. As no fines concrete does not segregate horizontal

joints can be at three storey interval; Cement slurry should be brushed on immediately

before placing new concrete.

Fixing Lightweight aggregate concretes may accept nails

but plugs should be built into walls made with dense aggregates.

The End