CONCRETE

1
CONCRETE

• by
• Ikmalzatul Abdullah

2
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.

3
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.

4
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.

5

• 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

6
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.

7
Contd

• 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.

8
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.

9
Contd

• 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.

10
Contd

• 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).

11
Contd

• 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.

12
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.

13
Contd

• 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.

14
Contd

• 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
  machines efficiency and they should be cleaned
  out when the type of cement is changed.

15
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.

16
Contd

• 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.

17
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.

18
Formworks
19
Reinforcement

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

20
Contd

• Reinforcement should comply with the following
  standards
• 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

21
Reinforcement
22
Reinforcement
23
Contd

• 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.

24
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.

25
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.

26

• 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

27
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.

28
Contd

• 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.

29
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.

30
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.

31
Contd

• 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.

32
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.

33
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.

34
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.

35
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.

36
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

37
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.

38
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

39
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.

40
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.

41
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.

42
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.

43
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.

44
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.

45
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.

46
The End