- Question: I have heard that there are problems associated with using molded brick in new masonry. Are you aware of any problems? Is there any advantage to using molded brick?
Answer: Molded brick units are selected often for their appearance. They are common in older buildings in the northeastern United States and are used in many new buildings to provide a similar appearance. There are two types of molded brick: water-struck units and sand-struck units. Water struck units use water as a release agent to facilitate the removal of brick from the mold. Sand-struck units, in contrast, use sand for this purpose.
Most of the molded bricks available today are sand-struck units. There have been some problems with sand-struck units not developing adequate bond between the mortar and the brick because some of these units have loose sand on the bonding surfaces. In these units mortar bonds to the loose sand instead of to the clay body. This creates separations between mortar and the brick, which reduces bond strength and increases water-penetration rates.
I recommend using sand-struck brick only in properly designed cavity walls. When the walls are constructed, any loose sand on the face of the units should be brushed off in order to prevent mortar from bonding to this loose sand. In existing wall systems that have developed excessive separations as a result of this problem, repointing the outer edge of the joint can be an effective repair. In this procedure all mortar will be removed from the bond surfaces for the depth of repointing. This will eliminate the loose sand, allowing the new pointing mortar to bond directly to the clay body. Repointing, however, can be very expensive and disruptive to the building operation.