I have just had a flagstone walkway installed. The flagstone consists of a combination of flagstone, installed over an existing concrete slab and "cap & pour" construction installed over an existing flower bed. The cap and pour section got wet and it looks like a veneer is blistering and a thin layer is flaking off. Also, the color wipes off of the "stones". I haven't completely paid for the project yet. Is this common and should I get the contractor to fix it (can it be fixed?). Thanks.