Q.: I have a 24- by 26-foot slab 3 inches thick cast over precast concrete planks on top of 8-foot walls. It has been in for 3 years. It has a full-length crack over the center seam of the planks. How can I fix it?

A.: Such a crack is caused by drying shrinkage, thermal contraction, differential movement or a combination of these. Probably the best way to repair it would be to create a joint by making two parallel saw cuts the length of the slab, one on each side of the crack, and then breaking out the concrete between them to a depth of about 1/2 inch. The depth should be about half the distance between the two saw cuts. The joint should then be sealed with an elastomeric joint sealant such as polyurethane. The polyurethane should be prevented from bonding to the bottom of the joint by putting in a strip of paper, polyethylene or roofing felt so that the joint sealant can stretch freely.