A major cracking problem that joint control will not solve, plastic cracking (also referred to as plastic shrinkage cracking) is cracking that occurs in the surface of fresh concrete soon after it is placed and while it is still plastic. I have long felt that the bottom of the concrete was the most important area in which to control cracking, and working from that base I concluded that: If we can take the extra workability water from the mix out the bottom just as fast or a little faster than it goes out the top we can densify the concrete while it is in the plastic state. This will allow very little tensile stress to build up during the early curing. Then if the concrete is cured properly and adequate joints are provided, the cracking will take place only in the control joints. This, of course, will not take care of structural cracking or the every-now-and-then crack that seems to happen for no good reason, but it will eliminate the majority of our slab-on-grade cracking problems.
This theory was tested by casting identical slabs on different bases including impervious polyethylene and pervious sand and sand-cement mixtures. The results were clear, graphic and dramatic. Why was there serious cracking on the impervious base and none at all on the sand? On the impervious base, all the extra bleed water must come out at the top. As the top starts to dry, the concrete wants to curl. Because it can't take any tensile stress at this point, the slab starts to tear open on the top surface, creating immediately visible plastic shrinkage cracks as well as weakened areas where future cracking will occur.
On the sand base, however, excess water leaves fairly evenly, top to bottom, enabling the concrete to densify without creating uneven stress within the slab. This prevents curling, virtually eliminates immediate plastic cracking and materially reduces the possiblity of later shrinkage cracking.