Concrete is the least forgiving material on site because its performance is fixed within hours and cannot be corrected afterwards. Strength comes from the water-cement ratio and curing far more than from the cement content buyers usually argue about. This guide covers how cement, aggregate and ready-mix are specified and bought for South African projects, and where the avoidable money is lost. It sits alongside our building materials range.
What this guide covers
- Cement types and what the designation means
- Aggregate: the part nobody inspects
- Water-cement ratio and why site water ruins concrete
- Ready-mix versus site-batched
- Curing: the free step that is routinely skipped
- Buying well and comparing quotes properly
Cement types and what the designation means
Cement is classified by composition and strength class. Blended cements containing slag or fly ash generally gain strength more slowly but end up more durable and generate less heat — useful in thick pours.
A higher strength class is not automatically better. Rapid early strength suits precast and fast striking times; for a large foundation pour the heat it generates can cause thermal cracking.
Store cement dry and off the ground and use it in rotation. Cement that has absorbed moisture lumps and loses strength, and no amount of extra mixing recovers it.
Aggregate: the part nobody inspects
Aggregate is most of the volume, so its quality drives the result. Well-graded, clean aggregate produces workable concrete at a lower water content, which is the single largest factor in final strength.
Dirty aggregate is expensive. Clay and silt coatings prevent the cement paste bonding to the stone, and the loss shows up as low cube results nobody can explain.
Maximum aggregate size is limited by section thickness and reinforcement spacing. Stone that cannot pass between bars causes honeycombing at exactly the points where cover matters most.
Water-cement ratio and why site water ruins concrete
The water-cement ratio governs strength. Adding water on site to improve workability is the most common and most damaging change made to a specified mix, and it happens after the supplier responsibility ends.
Workability problems should be solved with admixtures or a revised mix, not with a hosepipe. Plasticisers achieve flow without adding water and without giving away strength.
Every extra litre reduces strength and increases shrinkage cracking. A mix that arrives correct and is watered down on site will not meet the design, whatever the delivery note says.
Ready-mix versus site-batched
Ready-mix gives consistency, documentation and speed, and suits anything structural or of meaningful volume. It requires access, a pour plan and someone ready to place it.
Site batching suits small or remote work but consistency depends entirely on control of batching and water. Where results must be proven, ready-mix with delivery tickets is the defensible route.
Order on placed volume plus a realistic allowance for over-excavation and spillage. Running short mid-pour creates a cold joint at a position the engineer did not choose.
Curing: the free step that is routinely skipped
Curing keeps water in the concrete while it gains strength. Concrete left to dry in the sun and wind can lose a large share of its potential strength, and nothing later recovers it.
South African conditions make this harder than the textbook assumes. High summer temperatures, dry highveld air and wind all accelerate surface drying within hours of placing.
Curing compounds, plastic sheeting and damp hessian all work. What does not work is placing on a Friday and returning on Monday.
Buying well and comparing quotes properly
Compare like for like: same strength class, same aggregate size, same slump, same delivery point. A cheaper cubic metre is usually a different specification.
Delivery, pump hire and waiting time are real costs that rarely appear in the headline rate. Ask what is included and what is charged by the hour.
For repeat pours, a supply arrangement protects both price and availability, and gives you consistent material across the structure rather than a different mix each week.
Frequently asked questions
Which cement should I use for foundations?
A blended cement is often preferable for larger pours because it generates less heat. Send us the element size and exposure and we will confirm.
Can I add water on site?
No. Adding water reduces strength and increases cracking. Use a plasticiser or revise the mix instead.
What strength class do I need?
It comes from the structural design, not from preference. Send us the drawing or the engineering specification.
Ready-mix or mix on site?
Ready-mix for anything structural or of volume; site batching only for small work with good control.
How long should concrete be cured?
Keep it moist for as long as practical after placing — the first days matter most, and hot dry conditions shorten the window.
What aggregate size should I specify?
It is limited by section thickness and bar spacing. Tell us the element and reinforcement and we will confirm.
Why did my cubes fail?
Most commonly added water, poor curing or dirty aggregate. All three are avoidable and none can be fixed afterwards.
Do you deliver bulk quantities?
Yes, nationwide. Send quantities and delivery address for a quote including delivery.
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Reference: SABS standards.
For specification support, current pricing or a delivered quote, contact Builders Store SA. Request a quote