Question: What should the concrete contractor have done differently to avoid the reentrant corner cracks?
Answer: There are a number of ways to reduce the chance that a random crack will develop off a re-entrant corner.
Along foundation walls, the most reliable method is to isolate pilasters with a diamond-shaped box-out, such that the point on the diamond is aligned with the proposed sawcut (see Diagram 4).
Foundation corner pilasters should be boxed out (see Diagram 5). While the box-out detail almost assures 100% success, you could have improved your chances of success by using a double thickness isolation material on all faces of the pilasters and using cut pieces taped together at the corners to maintain the full thickness of the material at the re-entrant corners (see Diagram 6).
At dock levelers, the most reliable approach is to form a doweled construction joint along the end face of the leveler units (see Diagram 7). This requires a separate concrete placement between the leveler. But usually, the placement can be combined with another portion of the slab placement.
To help keep a random crack from developing off an inside corner of a foundation wall, you can use one of two approaches. On larger projects, the slab placements can be divided such that concrete is not placed around an inside foundation corner monolithically (see Diagram 8).
On smaller projects where it is not cost-effective to make multiple placements, you can use a crack inducer. One such approach is to use a pre-formed metal keyway as an extension of the wall lines. By running 5 feet of the 10-foot keyway sections into the body of the slab, the re-entrant corner is removed and cracking will occur along the sawcut lines (see Diagram 9).
For this detail, the top of the keyway is purposely kept 1½ inches below the top of the slab, such that the sawcut can pass directly over the top of the depressed keyway. It is important to carefully deposit concrete on both sides of the keyway equally to maintain the proper alignment of the keyway section.
One additional measure that can be used at re-entrant corner locations is using a hand-held saw to finish off any sawcut that would otherwise remain inches short of the re-entrant corner or face of a pilaster.
Peter Craig is a regular contributing editor for Problem Clinic. He is an independent concrete floor consultant with the firm, Concrete Constructives. He has more than 34 years of experience with concrete slabs and specializes in diagnosing and correcting moisture-related flooring problems. Telephone 207-946-5697, or firstname.lastname@example.org.