What could account for the cracking in a waffle floor slab that occurred just before the concrete hardened? The cracks follow the web pattern and we find that they occurred where the section changes in thickness.
This sounds like plastic settlement cracking, which can occur in places where concrete is being differentially restrained in settlement during hardening. This phenomenon is sometimes seen over ties in narrow wall formwork, over reinforcement near the surface, or below the flared head of a column. Often nothing can be done to change the restraint, but something can be done to minimize the amount of bleeding. Anything that will improve the cohesion of the concrete until it has hardened will help, such as using an air-entraining agent or increasing the sand content of the mix. Prompt and efficient curing may minimize the problem. If plastic settlement cracking has already begun it can sometimes be overcome by revibrating the concrete after settlement, perhaps an hour or an hour and a half after placing. Usually plastic settlement cracks don't extend all the way through the section and destroy the structural integrity, but it is usually desirable to seal the cracks to prevent corrosion of the steel.