Q.: We have been hired to place a 4-inch-thick concrete slab over an existing one. We plan to break up the old slab with an iron ball. However, one area of the old slab has heaved during the winter, probably because water has gotten underneath the slab and frozen there. Do you think we should break up the old slab as planned, and if so what precautions should we take in the area that is heaving?
A.: If built on a very rough base of broken concrete, the new concrete slab would undoubtedly crack. We suggest you don't break up the old concrete unless you plan to remove it. You need to do something to prevent the cracks in the base slab from being transmitted into the new slab. You could put down a thin layer of sand, 1/4 to 1/2 inch thick, or you could apply a layer of polyethylene sheeting to break the bond between the new and the old concrete. In either case, you should take care while placing the new slab to avoid tearing the polyethylene sheeting or disturbing the sand layer so that you don't allow fresh concrete to bond to the old slab. You should remember that concrete placed on top of polyethylene sheeting will usually produce more bleed water and this should be allowed to evaporate before doing any finishing or troweling.
The area that has heaved should be broken out, and the earth removed and replaced with crushed stone, gravel or some other material that will not undergo expansion during freezing weather. This subbase should then be properly compacted before concreting.