We've been hired to repair shrinkage cracks in 8-inch-thick, 8-foot-high basement walls of a 3-year-old office building. The cracks are not leaking. Yesterday we capped the cracks and were planning to inject the crack with epoxy today. But when we arrived at the jobsite, we found that the capping material had cracked. This leads us to believe that we are dealing with a moving crack. As a result, we are now having second thoughts about injecting the crack with a rigid epoxy. Should we inject the wall with a more flexible material to accomodate this movement?
Your capping material probably cracked because the wall underwent small thermal movement overnight. If the walls were designed properly, they should have vertical contraction joints about every 24 feet. These joints should be able to accomodate any expected movement in the walls due to shrinkage or temperature changes. Even if the crack in the wall was needed to accommodate movement, injecting the crack with a more flexible material, such as a polyurethane, wouldn't solve the problem. Because the thickness of the wall is far greater than the thickness of the crack, the slightest movement of the wall would have to be accompanied by an excessive amount of deformation of the injected material. The injected material would either fail, lose bond, or prevent movement of the wall.