Crazing – power trowelled floors
Surface crazing of power trowelled slabs would appear to be a very common phenomenon. Very many, possible nearly all, well-finished power trowelled floor slabs exhibit some crazing after a few weeks or months. Basically crazing is a result of differential contraction between the surface layer and the underlying concrete mainly brought about by drying shrinkage. Differential drying shrinkage must be common to all surfaces exposed to internal environments and yet most do not exhibit crazing to anything like the same extent as power trowelled floors. Probably the most significant additional factor is the composition and properties of the surface layer. With power trowelling a dense hard surface layer of mortar is formed, which is abrasion resistant but brittle, and hence more likely to crack. Thus it would appear that crazing is almost inevitable in this form of construction.
Further information is given in Concrete Advice No. 08, Crazing: power trowelled concrete for floor slabs.