Question: I have been working on a masonry project with frequent vertical cracks (5 feet to 10 feet on center) on the interior concrete masonry wythe. The wall system consists of a 4-inch brick outer wythe with a 6-inch concrete masonry wythe behind it. The total wall system is 10 inches thick, and the collar joint between the two wythes is partially filled with mortar. These cracks appear to be too frequent to be the result of concrete masonry shrinkage. What could cause such cracking?

Answer: These cracks most likely are caused by differential movement between the concrete masonry and the clay masonry. The clay masonry will expand as a result of moisture expansion. This is a permanent expansion that begins from the time the units are removed from the kiln.

The concrete masonry, in contrast, will undergo drying shrinkage. Although shrinkage is reversible, the units typically have a higher moisture content when they are installed during construction than they have after they reach equilibrium with their surroundings. The differential movement between the clay brick masonry and the concrete masonry causes either outward bowing or regularly spaced cracks in the concrete masonry backup wall.

In many cases these cracks do not significantly affect the load-carrying capacity of the wall system. Such as when the walls are spanning vertically, most vertical cracks do not impact the bearing of joists on the wall.

In other cases horizontal cracking can develop from this differential movement. Here the ability of the walls to resist lateral loads such as wind may be compromised. More extensive repairs, including grouting at cracks or full-depth repointing of joints, may be required to reestablish the bending strength of the wall system across these cracks.

If the masonry walls span horizontally to resist lateral loads such as wind, it may be necessary to rebuild portions of the walls at cracks or to develop some alternative repair to reestablish the bending strength of the wall.