Q.: We pour parking blocks with leftover concrete using a plastic rebar chair to hold the ½-inch rebar in place. We almost always observe a crack where the plastic chair is placed in the form. The chair has to rest on the bottom of the form. What causes the cracking? Is there a remedy?
A.: We are not sure what causes the cracking unless it is the weakening of the section by the plastic chair. If that is the cause you might try switching to the type of chair that consists of a small concrete cube with wires protruding from the top. The rebar is placed on the cube and the wires are then twisted to hold the rebar in place. The concrete cube, of course, becomes a permanent part of the parking block.
If you cannot buy such chairs in your area you could probably make your own.