Introduction to concrete building products
Practical Action
The main constituent materials for concrete are usually sand or fine aggregate, crushed rock or
gravel as the coarse aggregate, cement and water. When batching these materials it is
recommended to invest in a set of heavy duty weighing scales for the solid materials and a
measuring jug or cylinder for the water.
Even for the very small producer buying these items can be worthwhile. This is because it is
impossible to batch the solid materials accurately by volume as the materials settle differently
when they stockpiled and transported from site to site. The bulk density of OPC can vary from
1200 to 1800 kg/m3 depending on settling conditions so when batching by volume it is impossible
to know exactly how much cement to add to keep with the specified mix proportions. Just to be on
the safe side it is then necessary to put in a bit more cement and in the long run this turns out to
be more expensive than investing in a set of scales in the first place.
Similarly batching water by eye is also not a good idea as the risk is that too much water is added
making the mix to runny and the concrete weak when it sets. Adding more cement to compensate
for adding too much water also adds to the costs, and keeping production costs down is important
for the business to achieve profitability.
On the metric scale it is very easy to convert the weight of water to volume. The specific gravity of
water is almost exactly 1.0 at normal outdoor temperatures. This means that one litre of water
weighs one kilogramme.
Mix proportions are therefore only shown by weight in this brief. However, a comparison with
batching by volume is shown in the accompanying brief - Concrete Works: Information and
Guidelines for Concrete Products Makers and Builders Making and Using Concrete at the
Building Site.
With batching by weight suitable mix proportions for blocks are 1:3:9 OPC to sand to gravel.
With good quality control at the production site such a block may reach a compressive strength
of around 5 N/mm2 (MPa), so suitable for a range of applications in low-rise construction or for
non-structural use (e.g. for infill panels) in multi-storey construction. A dry stiff mix with a slump
of 20 to 40 mm can be used to blocks. This is likely to correspond to a water to cement ration of
0.5 to 0.6.
For higher strength blocks richer mixes may be used, e.g. 1:2:6 or even 1:1:3 cement: sand:
gravel for strong or super-strong blocks.
If the producer has no alternative than to batch by volume than a mix of 1:2:4 or 5 parts by
volume cement to sand to gravel may be trialled. This mix may be used for blocks with a target
compressive strength of 5 N/mm2. The producer may need to adjust mix proportions after the
trials to produce blocks with the required characteristics and be very careful to monitor any
changes in the bulk densities of the cement, sand and coarse aggregate that may call for the
need to readjust the volume mix proportions.
For most other applications a 1:2:6 mix, by weight, is likely to be suitable. Again a stiff mix
would be used with a water to cement ratio of 0.45 or 0.5. With steel reinforcement included in
the product a richer and wetter mix may be used to ensure that the concrete mix flows around
and bonds with the reinforcement when the product is vibrated in its mould. This corresponds to
a slump of 50 to 100mm when a slump cone test is carried out on the fresh concrete. So, a
1:2:4 cement: sand: gravel mix may be tried with a water to cement ratio of around 0.6.
An alternative way to increase the workability of concrete for reinforced concrete products is to
add superplasticizers to the mix and keeping the mix proportions and water to cement ratio
unchanged from the unreinforced mix.
For ordinary concrete blocks and most other concrete products up to one quarter of the weight of
OPC may be replaced by a pozzolanic additive. Pozzolanas are materials of active oxides of
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