After a concrete floor has been constructed, the concrete is liable to undergo moisture and temperature changes which can affect its level and flatness over a period of time. As the concrete develops strength during the first seven days or so after laying, differential thermal contraction between the top and bottom surface of the slab can cause it to curl at the joints. Differential drying shrinkage of the concrete slab between its top and bottom surface over the first year can also cause curling, particularly around joints. For most slabs, any effects of thermal and moisture movement on the flatness of the slab surface measured over short distances are likely to be small. The effects on levels and levelness, particularly at joints, are likely to be greater, but again are not likely to be significant enough in most cases to cause concern.
Slab settlement can affect the levels and, to some degree, flatness of the slab surfaces in the long term. It is caused by compression of the sub-grade or fill beneath the slab as a result of imposed loads on the slab. Also, some clay sub-grades can shrink or swell, which can cause slab levels to change over time.
Where concrete ground slabs are supported, for example on piles, deflection of the slab spans under load and with time can cause changes to levels and, to a much smaller degree, flatness. All these factors must be taken into account by the building owner and the building designer both before a slab is designed and specified and also when assessing its flatness and levelness in accordance with the specification. For example, with some poor ground conditions, selection of a piled slab design might be considered essential to limit settlement, but even in these circumstances the effects of deflection under load on levelness in particular must be assessed.
The flatness achieved should be assessed by survey at the start of construction of the floor, and possibly at intervals during construction, to check that the construction method used is suitable for meeting the contract specification. The number and frequency of such checks, both at the initial stages and throughout the contract, will usually be determined by the floor contractor as part of his control procedures; more checks will reduce the risk of finding errors in flatness when the final overall contractual survey of the floor is made, but will increase his costs as the survey specialist will have to attend site more frequently. The first sections of the floor constructed should be checked within 48 hours of laying. In the contract, the building owner may require all individual sections of a floor to be checked immediately after construction, for the whole period of the contract, for compliance with the specification.
Survey assessment of the flatness of the whole floor should be made within one month of completing the whole floor or major sections of it to check that it complies with the specification. Beyond this time limit, the effects of the various movement influences on the flatness and levelness of the floor surface noted above could change the acceptable flatness and levelness initially achieved to the point in some cases where compliance with the specification is no longer achieved. The effect of these influences is largely out of the control of the floor contractor provided that the floor has been constructed with a good standard of workmanship.