We built a concrete parking lot in Chicago 6 years ago. It's in excellent condition except for one area that appears to be progressively deteriorating. There are a couple of shrinkage cracks in this area and these are surrounded by a system of smaller cracks that seem to be filled up with a gray substance. The cracked area gets a little larger each year. The concrete was air entrained and we placed it with a low slump and vibratory screed. There's no scaling at all. What could have caused the cracks?
It might be D-cracking. D-cracks are a series of fine cracks that form at close intervals. In slabs on grade they are usually parallel to edges, joints, or other cracks. The cause is usually porous coarse aggregate particles that become saturated and then freeze during cold weather. Initially the particles crack internally, but as time goes on cracks extend into the surrounding mortar and to other aggregate particles. Sometimes the cracking spreads several feet beyond the joint or crack, and damage is severe enough to require replacing the affected concrete. The affected pavement section may still last 15 to 20 years before replacement is needed. Because it appears in only one section of the lot, aggregate in the other concrete may be of better quality and not susceptible to damage. We'd advise watching the section for a few more years to see if the cracked area continues to get larger. As long as the concrete still carries traffic loads there's no need to replace it unless appearance is objectionable. D-cracks frequently occur parallel to pavement edges, joints, or other cracks.