block

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CONCRETE BLOCK
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General

• Concrete block construction is gaining
  importance in developing countries, even in
  low-cost housing, and has become a valid
  alternative to fired clay bricks and other common
  construction materials.

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Lightweight concrete

• Lightweight concrete is generally used for
  concrete blocks
• Lightweight concrete comprise lightweight
  aggregates (pumice, scoria..)and/or a high
  particles of coarse proportion of single-sized
  aggregate in a lean mix, which is not fully
  compacted.

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Types of Concrete Blocks

• Concrete blocks are produced in a large variety
  of shapes and sizes either solid, cellular or
  hollow, dense or lightweight, air-cured or
  steam-cured, load bearing or non-load bearing,
  and can be produced manually or with the help of
  machines.

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Stretcher blocks

• The most commonly used concrete blocks are the
  stretcher blocks with a nominal dimensions of
• 40 cm x 20 cm x 10cm
• 40 cm x 20 cm x 15cm
• 40 cm x 20 cm x 20cm

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Types of Concrete Blocks
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Solid blocks

• Solid blocks have no cavities, or have voids
  amounting to not more than 25 of the gross
  cross-sectional area.

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Cellular blocks

• Cellular blocks have one or more voids with one
  bed blind on the other face, and are laid with
  this 'blind end' upwards, preventing wastage of
  bedding mortar, which would otherwise drop into
  the cavities.

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Hollow blocks

• Hollow blocks are the most common types of
  concrete blocks, having one or more holes that
  are open at both sides. The total void area can
  amount to 50 of the gross cross-sectional area.

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Advantages of hollow concrete blocks

• The use of hollow concrete blocks has several
  advantages
• if lightweight aggregate is used, can be very
  light
• they require far less mortar than solid blocks
  (because of the cavities and less proportion of
  joints, due to large size), and construction of
  walls is easier and quicker

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Cont..

• the voids can be filled with steel bars and
  concrete, achieving high seismic resistance
• the air-space provides good thermal insulation,
  which is of advantage in most climatic regions,
  except warm-humid zones if desirable, the
  cavities can also be filled with thermal
  insulation material
• the cavities can be used as ducts for electrical
  installation and plumbing.

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Materials For Concrete Blocks

• Cement
• Aggregate
• Water
• Cement
• OPC, portland blast furnace slag, sulfate
  resisting,
• RHPC(Rapid hardening Portland cement)

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Aggregates

• The maximum particle size of coarse aggregate is
  13mm.
• Rounded stones produce a concrete that flows more
  easily than angular particles but the latter give
  high strength.

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Production Process

1. Batching and Mixing
2. Molding
3. Curing

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Batching and Mixing

• Aggregates can be batched by volume or by
  weight, but the latter is more accurate. For this
  reason, cement should only be batched by weight,
  or preferably by using only whole bags of 50 kg.

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Molding

• Concrete blocks can be molded by several
  methods, ranging from manually tamping the
  concrete in wooden or steel mould boxes to
  large-scale production with 'egg-laying' mobile
  machines and fully automatic stationary machines.

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Block making Machines
Automatic stationary machines
'egg-laying machine
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Curing

• The blocks are either left to set and harden
  where they were molded, or carried away on
  pallets to the curing place. In all cases it is
  important to keep the concrete moist, for
  example, by regularly spraying with water, until
  the concrete has obtained sufficient strength.
• water curing is the standard procedure for the
  strength development of concrete, by which the
  concrete is kept wet for 7 days or more.

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Contd

• Steam curing, by which the concrete is
  exposed to low or high pressure steam (in
  autoclaves), high early strengths can be achieved
  (with autoclaving the 28 day strength of
  air-cured concrete can be obtained in 24 hours).
• However, in developing countries, steam curing is
  unlikely to be implemented, because of its high
  cost and sophistication.

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Minimum compressive strength of HCB
Class Average of 6 units ( Mpa ) Individual Units ( Mpa )
A 4.2 3.8
B 4.0 3.2
C 2.0 1.8
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