Introduction to concrete building products
Practical Action
The quality of some types of concrete products, blocks and bricks for example, is likely to be
governed by national or international standards. Other products, such as fencing posts, are unlikely
to have any standards for them, in which case the producer may need to experiment with different
concrete mixes to achieve a product which is not costly to make but which satisfies market
demand.
There are two aspects of good quality control - good production practices and regular testing of the
materials and products. Good production practices can be achieved with training and experience
and by maintaining a commitment to good quality production. Some aspects of good production
practice have already been mentioned in this text including the need to keep tools and equipment
clean and well maintained, not keeping mixed already concrete so long that is starts to set before
use, adequate curing of the products and maintaining good production records.
Batching by weight, as already mentioned, is also much better for maintaining quality than
batching by volume. However, the small producer may find it easier to initially batch out by volume
using separate batching boxes for cement and aggregate. The boxes may then be taken to the
scales for weighing and adjustments made by adding or taking material away to ensure that the
proportions of cement, fine and coarse aggregate correspond to the mix proportions determined to
be used for the mix.
Quality Control Tests
The best way to test if concrete is sound is to perform compressive strength tests on hardened
cubes or cylinders of the concrete, usually at 7 and 28 days, but also optionally after 3 and 90
days after mixing. To perform the tests samples are taken from the concrete batch to fill several
cube or cylinder moulds. These are cured for the specified period under closely controlled
conditions before testing in a special compression machine.
It is unlikely that a small-scale producer would want to spend a lot of money to produce the
concrete samples and then to get them tested independently, although evidence from such tests
can be very useful in case of disputes with customers over product quality. Instead, two simple
qualitative tests can be used which would at least give an indication of a bad quality product.
In the ring test the product is lifted off the ground and struck with a coin or other small metal
object. A high sharp sound indicates satisfactory performance, while a dull lower-pitched noise
indicates that there could be a problem. A large product should be struck in several places in case
there are any areas of weakness in it. In the scratch test attempts are made to scratch the product
with a coin or similar metal object. If noticeable grooves are left in the product then it is probably
not of satisfactory quality.
Weighing a number of the same type of product is also a useful check for consistency. If the
weight varies by more than about 5% of the expected weight then that could indicate a problem
with production procedures or the raw materials used.
In some countries or districts building standards or local regulations give specifications for sizes of
some types of products. These apply particularly to bricks and blocks because these would tie in
with standard sizes for door and window openings. If possible, the standards on sizes should be
consulted or, at least, measurements made of products already available for sale and used for
building. When buying an imported machine it is important to ensure that it can fit a mould of the
standard size in the locality.
If no local sizes are available for guidance, fairly standard sizes are 40 x 20 x 20 centimetres for
hollow or solid blocks and 20 x 10 x 10 centimetres for bricks.
The length, width and height of products need to be measured periodically with a ruler or
measuring tape to check that they conform to the specifications. With repeated use moulds can
become deformed or misshapen and change in the dimensions of the products can indicate that
this is happening. Moulds which have changed significantly in dimensions should no longer be
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