There should be 2 primary considerations when choosing a floor depth, the first is the load the floor will take, this is usually used for designing structural floors for commercial and industrial premises.
Polished concrete screeds either with or without underfloor heating are usually used as a top layer for foot traffic over a predesigned and already installed structural base making the screed not structural in its design. After considering several factors we at Midland Flooring believe the minimum depth of this polished concrete screed can be 75mm. The biggest issue facing what are considered thin section concrete floors are cracking, a major contributor of this issue is curling of slab.
For indoor concrete slabs, curling is almost always caused by a differential moisture gradient in the slabs. As the surface of the concrete slab dries faster than its body and bottom, a differential moisture profile is developed along the thickness of the slab. The surface of the slab shrinks more and quicker than its bottom. This differential drying shrinkage gradient is what causes the concrete slab to curl. The greater the gradient is, the greater the curling. The results are gaps between the slab and sub-base resulting in movement resulting in cracking.
Thin slabs and long joint spacing will potentially increase the risk of curling and for this reason applications such as thin unbonded toppings need to have close joint spacings. This issue of close joint spacings is sometimes undesirable, however it is necessary. The rule of thumb for these spacings are 40 x slab depth e.g., 75mm x 40 = 3000mm this should be the gap between joints.
At 75mm the differential in the moisture profile is greatly reduced as the distance between the bottom and surface are so small that the potential of drying rates is reduced by adding other factors such as those listed below, we believe 75mm a better depth than 100mm in terms of curling.
Other factors considered
We take precautions to avoid excess bleed in the concrete mix
We use the largest practical permissible aggregate size
We avoid a higher than necessary cement content in the mix
We cure the concrete thoroughly
We consider joint spacings and joint arrangement
We consider the use of shrinkage reducing admixtures
Other slab depths
At 125mm the concretes self-weight massively reduces the risk of curling and as a result the risk of cracking. The mass of the concrete prevents the upwards movement dramatically thus restricting the gap size beneath the slab, this is not so saying curling does not happen at all this depth. At 100mm we believe there is an increased risk of curling as its self-weight is not enough to prevent the movement. Its body is deep enough as to allow a differential moisture gradient resulting the surface drying faster than its bottom. In many cases the result of this is curling and cracking.
Concrete slabs with a differential moisture gradient tend to curl at joints and around the perimeter, causing problems in projects. As a result, there is a risk that floors may deteriorate over time and in service, causing safety problems and requiring repairs. Although it may be possible to repair most slabs, curling can be minimised by careful control of the concrete mix design. The repair options available depend on the service conditions and the severity of the problem. Which ever repair is used the results are never aesthetically pleasing and leave a visible trace.