Introduction to concrete building products
Practical Action
then be left to cure before handling. Since the mould can then be used to make another block,
there will be a saving in investment cost because fewer moulds will need to be bought.
(iii) Wet Mix
This type of mix will feel wet in the hand, will leave a cement stain on the palm and will have an
appreciable slump. The slump can be tested in a slump cone by measuring how much a cone of
concrete slumps when the cone mould is pulled away.
Some national standards for concrete specify the size of the slump cone to use for test, and the
producer should invest in this equipment, if possible. If a slump test cone cannot be obtained an
old bucket with the bottom cut out could be used as an alternative. The procedure for carrying out
the test is as follows:-
1. Put concrete into the slump cone mould in three layers, tamping after each layer.
2. Carefully remove the cone to leave the cast concrete standing in the shape of the mould or
slumped to varying degrees.
3. Put the mould against the concrete cone and measure the slump (the distance by which the
concrete cone has shortened).
If the cone slumps to about one third of its height, or less, the concrete may be too wet to use for
most concrete products because it will be too weak when set, unless it was made deliberately with
a high slump by the use of plasticizers and superplasticizers.
With wet-mix concrete, vibration is more effective than pressure to achieve good consolidation.
With concrete blocks, wet-mix concrete can still be used to deposit the blocks immediately after
casting, provided the slump test does not give a value of more than about 10 per cent slump.
Because the blocks are much squatter than the slump cone they are unlikely to slump on standing.
Hand-Moulding
This is the simplest method of making concrete products. Moulds can be of steel, wood or plastic.
Concrete will stick to wood and steel so it is advisable to apply special mould oil or, if this is not
available, old engine oil to the inside of the mould to facilitate demoulding, although the used
engine oil may give staining. The moulds need to be made in sections to facilitate demoulding.
These sections can be held together with removable pins, bolts or light nails in the case of wood.
More elaborate moulds have also been developed for concrete blocks to allow the mould to be
dismantled and removed from the block immediately after casting to leave the newly-cast block
standing on the ground.
Usually the mould is filled in several layers with each layer being tamped thoroughly. This ensures
that the concrete penetrates corners in the mould and that some of the air in the mix is expelled.
Wooden moulds would be good for up to 50 castings, plastic moulds probably for about a few
hundred, while good quality steel moulds if well cared for can be re-used several thousand times.
With a simple low-cost machine for producing blocks, concrete is poured into a mould and tamped
down. Then a hinged top plate is brought down on top of the block to press it down. After this, the
plate is swung back and a lever is used to either eject the block on a wooden bottom plate on
which it is left to cure, or the block is ejected still in the mould and the mould is then tipped over
and the block ejected on the ground.
With this type of machine, blocks of slightly better quality can be produced compared with purely
manual moulding, and working with such a machine is more convenient. A realistic upper limit to
the number of blocks which could be produced with such a machine is about 50 per hour.
Egg-laying Machine
This type of machine is generally used to produce concrete bricks and blocks. The Egg-laying
machine is so-called because it is mobile, depositing the blocks as it casts them then moving
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