Introduction to concrete building products
Practical Action
Three types of concrete block are generally made:
• Hollow block
• Solid block
• Ventilation of louvred block
Hollow blocks are by far the most common type of block produced and some producers may make
only this type of block and nothing else. The empty space in the block helps to save on concrete
and makes the blocks easier to handle by builders.
Hollow blocks can be used as a single layer to build satisfactory walls for low-rise housing and other
types of buildings. They are therefore not made to a particularly high compressive strength. Five to
10 N/mm2 (MPa) is typical. In contrast cast concrete for structural applications is usually specified
to reach a particular minimum compressive strength within the range 20 to 50 MPa after 28 days
curing.
Hollow blocks for non-structural use for use in freestanding walls or infill panels in which the
blocks are required to support only their own weight and blocks above them can be made to a
target compressive strength of only around 3 MPa. It is not usually practicable to make concrete
blocks to strengths lower than 2.5 MPa. Even though they might only be lightly loaded they can be
vulnerable in other ways, e.g. prone to breakage during transport and handling or damaged by
repeated heavy rains or abrasion by contact with people, animals and objects.
Concrete hollow blocks that are used in certain demanding situations such as in foundation levels
below the water table, for large water storage tanks and for buttresses and retaining walls to
stabilise very unstable slopes may need to be produced to higher strength specifications of 15
MPa, or higher. However, in such conditions it is more common to use solid rather than hollow
blocks.
Concrete blocks can be economical to produce due to their lower cement requirement than
conventional concrete. This is because:
• Concrete blocks are usually made to a lower strength specification than conventional
concrete, as noted above, so the mix can be leaner in cement.
• Concrete blocks are relatively small objects with the concrete tamped or vibrated vigorously
to fill the mould. The concrete mix can then be relatively stiff rather than flowing with a
lower water to cement ratio than conventional cast-in-situ concrete. This further reduces
the cement requirement.
• Large visible holes or voids are completely acceptable for concrete blocks in most
situations, whereas they would not be in concrete cast at the building site. Then mixing
and matching the aggregate size fractions to fill nearly completely all the voids becomes
less important. In particular an excess of the largest size of aggregate can then be used,
and this is usually the cheapest fraction. Also a further reduction in cement can be
considered as the cement and sand paste is only needed to coat the larger size aggregates
rather than to fill the spaces between them. Note, however, that with hollow blocks, the
largest aggregate pieces have to have a diameter of at the most one three quarters the
width of the thinnest section of the block, otherwise they become difficult to place in the
mould and lead to weakness in the block. This can place a limit on the size and amount of
large stones that it is possible to include in a mix for hollow blocks.
Solid concrete blocks are little produced except for special applications such as for below ground
foundations or basements prone to flooding risks. The main drawback of solid blocks for builders is
their heavy weight, making them difficult to handle. For construction in demanding situations many
builders may prefer to use smaller concrete bricks that are made with dense and strong concrete to
resemble fired clay engineering bricks.
Ventilation or louvred blocks can be incorporated in walls to provide airflow in and out of buildings
in hot climates. The moulds for these have a more intricate shape than for conventional blocks and
incorporate thin and curved sections to maximise the size of the openings in the block. This means
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