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CONCRETE by Ikmalzatul Abdullah Strength Strength sufficient for structural work are obtainable but the modulus of elasticity of aerated concrete is about one tenth ... – PowerPoint PPT presentation

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  • by
  • Ikmalzatul Abdullah

  • 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
  • Energy efficient - requires less energy to
    produce than steel.
  • On-site fabrication.
  • Aesthetic properties.

  • Low tensile strength - very brittle, must be
    reinforced with steel to carry the tensile
  • 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
  • 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.

  • 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
  • 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.

  • 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.

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

  • 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 watercement 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
    watercement ratio x weight of cement (kg).

  • 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.

  • 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.

  • When the concrete has been mixed the complete
    contents of the drum should be discharged in one
    operation to avoid segregation of the larger
  • Mixer should be thoroughly washed out and cleaned
    daily and even after short stoppages, to prevent
    caking with hardened concrete which reduces the
    machines 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
  • 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
  • The materials intended to be used are mixed in
    the laboratory in the proportions to be used in
    the work.

  • 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.

  • Benefits
  • Higher load capacity
  • More controlled failure
  • Reinforcing bar is placed in region of tensile

  • Reinforcement should comply with the following
  • BS 44491978 hot rolled steel bars for
    reinforcement of concrete
  • BS 448219821969 hard drawn mild steel wire for
    the reinforcement of concrete
  • BS 44861980 hot rolled, and hot rolled and
    processed high tensile alloys steel bars for
    prestressing of concrete
  • BS 47571971 nineteen wire steel strand for
    prestressed concrete
  • BS 44831969 steel fabric for the reinforcement
    of concrete
  • BS 58961980 high tensile strength steel wire
    strand for the prestressing of concrete

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

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.

  • 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
  • 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.

  • 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
  • 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
  • Usually easy to cut, chase and nail into.

  • Compressive strength and the modulus of
    elasticity are reduced (although the latter
    reduction may improve resistance to mechanical
  • 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
  • Sound insulation reduces as density of concrete

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
  • 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 sufficient for structural work are
    obtainable but the modulus of elasticity of
    aerated concrete is about one tenth of dense
  • Creep at working loads is not thought to be
  • 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

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

  • Aerated concrete is much softer than dense
  • 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
  • Cementaggregate ratio
  • Watercement 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.

  • 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.

  • 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
  • Reinforcement
  • Light reinforcement is advisable across the
    angles at openings.
  • A coating of cement grout reduces the likelihood
    of corrosion.

  • 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
  • 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