WILL MY CONCRETE CRACK?
Concrete does crack and it is its nature to do so.
Concrete sets after it’s poured in its fluid type form. As it sets it is hydrating, giving off water. It does this and its volume is getting less and less. As the volume gets less, one concrete mass will pull away from another. This causes cracks to appear. Hydrating concrete will continue for weeks. Perhaps after a week most of the water has hydrated, and the concrete has reached perhaps 90% of its strength. A month later it has strengthened much further. This is generally called curing.
During this curing process there is this loss of volume. The volume loss will show as cracking. The best pour is one that has lots of minor small hairline cracks that are hard to see. The problem pour is one that produces large cracks over an eighth of an inch wide. They are unsightly.
Under a ground floor, the earth beneath can move, depending on the nature of the soil. I just finished working some projects in Australia’s most earthquake prone areas, and to minimise damage, each concrete slab would have two feet of sand beneath the slab. This acts like a billion ball bearings so as to allow the ground to move while the building does not.
In hot seasons, concrete can shrink faster due to evaporation. This is a loss of volume that creates cracking due to heat, but not through hydration. Cracks can appear in these circumstances, but with no corresponding strength. Thus, there have to be ways to reduce this. One can pond the concrete, surround it with soil and keep water on it, so there is no evaporation from the concrete. One can also put polythene over the poured slab, as this stops evaporation. There are also spray on products that help. I have found polythene as good as any, and even when it is a very hot day. there will be water found under the polythene weeks later, indicating that the concrete has not had its water content evaporated and it is still hydrating okay.
As the concrete is losing its volume, it is best to try to direct where this volume is expected to reduce. If you have a large wide slab, it could be that one side pulls away from the other. So before you pour this, put in a control joint. This is a separation between the two masses. It is purely to get the concrete to shrink where you want it to, so the crack is along the joint, where you want it, and not elsewhere.
If you have a slab that is an odd shape, like an L, for example, you will have two different masses pulling against each other, so it is really good if you can induce cracking where you want it, unless you are okay will cracking in the elbow of the joint of the L.
Concrete cracking does not mean it is a bad thing, as it just happens. Most cracks on the ground level are aesthetically bad, but not a structural problem.
They can be a problem however, if the slab is still shrinking and you want to place ceramic tiles or stone work on the floor. If there is going to be carpets, there is generally no trouble. Personally, I would not put any natural stone finish on a concrete floor until three months after the pour.
Suspended floors are a slightly different subject. Always check with your engineer on the job too. Different places have different local phenomena, and your engineer will know what they are best.
Nick Broadhurst is a building inspector, Class A Builder, and the author of science fiction books, children’s picture books, and comics. He has worked in many countries.