Concrete Slabs and Hardstanding in Nairobi

Concrete cracks. The question is where

A slab shrinks as it cures and moves with temperature. It will crack. Designed joints decide where, in a straight line you chose, and no joints means it cracks wherever it likes, usually diagonally across the middle.

Movement and control joints at sensible centres are not an optional refinement. They are the difference between a slab that looks deliberate at five years and one that looks broken.

Mesh in the middle, not on the ground

The commonest defect we see on Kenyan slabs: reinforcement mesh laid on the subgrade and concrete poured over it. Mesh sitting on the bottom does almost nothing: it needs to be in the tensile zone, held up on spacers.

Mesh dragged up with a hook as the pour proceeds ends up at a random depth. We use spacers and we check them before the truck arrives.

Curing is not drying

Concrete gains strength by hydration, not by drying out. In Nairobi sun a slab left uncovered loses water from the surface fast, which gives you a weak dusty top layer and shrinkage cracking.

Covered and kept damp for the first week, the same mix is substantially stronger. It costs almost nothing and it is skipped constantly.

Ready-mix or site-mixed

Ready-mix gives a consistent, specified class and arrives fast, which matters because a large slab should be poured in one continuous operation. KES 9,000 – 15,500 per m³ for typical residential grades, from about 8,500 for Class 15 up to around 20,000 for Class 30.

Site-mixed suits small pours where a truck cannot reach or the volume does not justify one. It is far harder to keep consistent between batches, and consistency is what a slab depends on.

For anything above a few cubic metres we recommend ready-mix, and we will say when a job genuinely does not need it.

Access

A ready-mix truck needs to reach the pour or the concrete needs pumping, and pumping is a real cost. On a Karen or Runda plot the truck usually gets there. On a tight Kilimani or Parklands site it does not, and that belongs in the quotation rather than in a conversation on the day.