When concrete masonry accent bands are used in clay masonry walls, there is often cracking in the concrete masonry's head joints. How can this be avoided?
The cracking in the head joints in the concrete masonry is caused by differential movement between the clay masonry and the concrete masonry. Over time, concrete masonry shrinks; brick masonry, on the other hand, expands. This differential movement between the clay masonry and the concrete masonry places the concrete masonry in tension, which can cause vertical cracks in the concrete masonry. These cracks tend to form through the head joints because they are the weakest points in the masonry. It is recommended that whenever possible clay masonry accent bands be used with clay masonry and that concrete masonry accent bands be used with concrete masonry. If, however, concrete masonry accent bands are specified for clay masonry, or vice versa, two steps should be taken to improve the performance of the system. First, reinforce the concrete masonry with bed-joint reinforcement to help control the frequency and size of vertical cracks in the concrete masonry. Although this will not eliminate cracking, it will reduce the problem's magnitude. Second, place watertight slip planes above and below the accent bands to permit differential movement between the accent band and the masonry above and below. Whenever possible, place through-wall flashing beneath the accent band to provide one of these slip planes. This flashing should extend completely through the wall and should be bent down to form a drip. The bed joint above the accent band should be raked back and sealed using a backer rod and sealant. This could create a weak joint that would then crack and allow some differential movement. Filling this joint with sealant will prevent additional leakage. The slip plane above the accent masonry can be better defined by installing a plastic bond breaker within this joint. Please note that all slip joints that allow differential movement above and below the accent band also break the bond perpendicular to the bed joints in the outer wythe of the wall at the accent band's level. Because of this, the backup wall should be designed to carry all the horizontal wind loads. The accent band, as well as the masonry above and below it, should be adequately tied to the backup wall.