The interior surfaces of the concrete masonry walls in a school building in the Chicago area developed regularly spaced vertical cracks occurring at approximately 4 to 5 feet on center. The wall system is a nominal 4-inch clay brick veneer over an 8-inch concrete masonry backup wall, for an overall wall thickness of 12 inches. What could cause the cracks to be spaced so close together? I have seen shrinkage cracks on concrete masonry walls in the past; however, these cracks are usually spaced at 15 feet or more on center.

A 12-inch masonry wall with a clay brick exterior and concrete masonry interior wythe has a space approximately 3/4 inch between the wythes. In many cases, this joint is partially filled with mortar. A combination of moisture expansion of the clay masonry and shrinkage of the concrete masonry likely caused the cracking. Based on the frequency of the cracking, the wall probably does not contain reinforcement. If the building is more than 5 years old, most of the differential expansion has likely already occurred. Provided that the walls are spanning vertically, a structural problem is unlikely. Problems may develop at the bearing locations of joists or beams, or in walls that contain horizontal cracks. In these cases or when there are any displacements associated with the cracks, I recommend hiring a structural engineer to examine the cracking in greater detail.

This problem can be avoided by building a cavity wall. Using ladder-type bed-joint reinforcement or eye-and-pintle-type ties allows the clay masonry and the concrete masonry to move independently. Providing expansion joints at regular intervals in the clay masonry to accommodate moisture and thermal expansion can help prevent cracking and other movement-related problems. The concrete masonry walls should contain joint reinforcement to control shrinkage, and control joints should be added at regular intervals.