Cracking in concrete is simply where the internal strength of the concrete is overcome by applied forces and stresses. These applied forces and stresses may be present for a number of reasons. It must be noted that as the concrete hydrates and cures, the internal strength of the concrete begins to develop until a maximum is achieved.
Therefore the applied force and stress required to overcome the internal strength of the concrete at any point during the curing process increases as the concrete hardens.
The purpose of this technical note is to consider the mechanisms that occur which lead to cracking in concrete that is not due to poorly designed floor details or operational overloading problems.
The following are deemed as the main groups of non-structural cracks in concrete:
1- Plastic Shrinkage Cracking (before concrete hardening.
The primary cause of plastic shrinkage cracking is the rapid drying out of concrete, which can cause the shrinkage strain of concrete to exceed its tensile strain capacity. The rate of evaporation of water from concrete can also be increased greatly by even the lightest of wind, at least doubling the chances of seeing shrinkage cracks develop; these can create a series of parallel shrinkage cracks, predominantly in one direction. For this reason, openings in building elevations should be avoided during the floor construction process.
Curing is obviously crucial to stop shrinkage cracking, and theoretically, should take place when bleed water has evaporated, but before further drying causes large plastic shrinkage. In reality, when large area casting techniques are used, this can be extremely difficult to achieve, and the rate of evaporation of bleed water is awkward to constantly review and manage on-site. For this reason, the likelihood of some level of cracking should be expected and accepted by all parties.
The action of power floating effectively re-compacts concrete and closes plastic shrinkage cracks as they form. For this reason, floors which require only a basic concrete finish are more likely to exhibit shrinkage cracking.
However, cracks in non-power floated floors should not necessarily require heavy scrutiny, as the reason for opting for a skip-float finish in the first place may be down to a particular client preference (for example, retail stores may be covered with tiling). If this is the case, non-structural cracks may be deemed of little significance by the client due to the aesthetic limitations of non-power floated surfaces, unless the chances of cracking ‘reflecting’ through to the floor finishes are a genuine possible issue of concern.
2- Plastic Settlement Cracking (before concrete hardening)
3- Early Thermal Contraction Cracks (after hardening)
Thick slabs subject to excess temperature gradients, followed by rapid cooling, can exhibit early thermal contraction cracks after one day and up to several weeks.
4- Long Term Drying Shrinkage Cracks (after hardening)
Long term drying shrinkage cracks are particularly prevalent in thin slabs subject to high shrinkage and in slabs which have insufficient stress relief. Cracks of this type can occur up to several years after casting.