What causes concrete to crack within several hours of the time it is placed? We did a driveway recently and within three hours cracks began to appear over most of the surface. The cracks range from a few inches in length to several feet, and some appear to be as much as 1/8 inch in width and fairly deep.
The type of cracking described is probably due to plastic shrinkage caused by rapid evaporation of surface water. Under most conditions bleeding will bring water to the surface of a slab at a fast enough rate to replace the water removed by evaporation. But if the concrete temperature is too high, or the freshly placed surface is exposed to hot, dry wind before protective curing begins, the surface may dry out sufficiently to develop tension before the concrete has any tensile strength, and cracking is inevitable. There are fortunately a number of protective measures which can be adopted, but the best procedure is to place the concrete at as low temperature as possible, regardless of whether the work is being done in summer or winter. Other measures include erecting sun shades and wind breaks, dampening formwork and subgrade before placing concrete, and initiating curing procedures as quickly as possible. Plastic cracks may also appear in concrete because of movement of embedded items such as reinforcing steel or inserts, or when subgrades or forms move for any reason before sufficient strength develops.