Introduction to concrete building products
Practical Action
Normal concrete consists of a mixture of sand, gravel or stones, and cement in various proportions,
with added water. The larger the amount of water used in the mix the more workable it becomes.
However, this comes at the expense of the final strength of the concrete that generally decreases as
the amount of water is increased. It is usual to specify the water to cement ratio, by weight, for
concrete mixes. Most concrete is made with water to cement ratios of 0.4 to 0.7, although in some
specialist applications ratios outside this range can be used. It is normal to set the water to cement
ratio as the minimum required to make the mix sufficiently workable, but no higher to avoid
weakening the concrete and the risk that it could become less durable.
Concrete hardens by the chemical reaction of OPC with water. This is quite a slow reaction, but
importantly concrete starts to set and stiffen after only about half an hour. It is therefore poor
practice to make too much concrete that cannot be used up within half an hour or, at the latest, 40
minutes, and then to try to make the mix more workable again by adding more water.
Even when freshly mixed, concrete can still feel quite stiff, especially if a lean (low cement and
high aggregate) mix is used. It won’t then fill the mould completely if simply poured in and edges
and corners of the mould can be left void. Concrete needs to be assisted to fill the mould
completely, either by tamping it manually to force it into the corners and edges, or still better by
mechanical vibration.
Once concrete begins to set it still takes considerable time to reach sufficient strength for the
products made from it to be handled. This needs to be at least 10 hours, but preferably a whole
day. Then most types of products can be safely taken out of their moulds to continue curing for a
minimum of a further seven days in normal curing conditions, but preferably 10 days before they
can be put to use in the building.
The chemical reaction between OPC and water in the concrete while it cures generates heat. The
heating can cause drying out of the concrete and for the chemical to stop or become very feeble.
This can cause low strength of the concrete. It is therefore very important to ensure that concrete
products remain damp during the first 7 to 10 days of curing. This is best done by curing products
underwater or by spraying them regularly and frequently.
Concrete products may be used in building after 10 days, although at least double this time might
be required for some high strength and specialised products. The bonus is that concrete continues
to gain strength weeks, months and even years after casting, so surpassing its design strength and
giving an additional margin of safety.
Hardened concrete is a relatively strong but brittle material. It is strong in compression. Most
loading in buildings is compressive, so concrete is suitable for taking this loading. However,
concrete is relatively weak in resisting bending, twisting, shearing or powerful impacts. If it is
expected to sustain these types of forces concrete needs to be reinforced with steel rods, bars or
mesh or, to withstand these forces at lower levels, the incorporation of fibres. Reinforcement of
concrete with steel or fibres is a detailed subject area that it is not possible to cover in this brief.
Concrete is a highly durable material that if well made can last 100 years or longer. However
special precautions need to be taken if it needed to carry intermittent heavy loads such as truck or
heavy machinery movements, exposed to regular freezing and thawing due to the climate, used in
seawater or exposed to certain chemicals, mainly some types of oils and acids.
Common Types of Concrete Products
Blocks
Concrete blocks are generally popular for structural walls of up to two storey heights. They may
also be used for infill in taller buildings built on the structural frame principle. Special higher
strength and density blocks are required if the blocks are used in foundations or in other
applications where they are subject to a high level of wear and stress. Blocks specifically used for
insulation contain lightweight aggregate such as exfoliated perlite, vermiculite or pumice.
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