Finishers were allowed to add whatever water they felt they needed, according to current practice. Cracks were recorded after one year, and all but two occurred in the concrete cast directly on the polyethylene. Cracks in this surface stopped abruptly at the edge of the sand base area. Details of the test slab program, showing three different base conditions as well as four curing conditions on each of the bases, for each slab. Three different concrete mixes are described on the right A view across the width of Slab 3 shows cracks stopping at the edge of the sand-base area.
Black spots are oil from trucks which use the test slabs as a parking area.
The results were clear, graphic and dramatic. Figure 2 shows the reasons. On the impervious base, all the extra bleed water must come out 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 welt as weakened areas where future cracking will occur.