Cracks in walls and building materials are due to several factors. The overall cause of cracks is having a constrained structure that is unable to move. Wall movement is inevitable and can be caused by structure settlement and drift, wind, moisture, heat and cold, and other conditions. Although these other factors are important, temperature and moisture content generally are the main conditions for movement in masonry materials, usually in the form of shrinkage.
Brick, as we all know, has a life of its own when introduced to moisture, heat and cold, leading to expansion and shrinkage.
"Concrete masonry, just like all concrete products, it's the largest volume that it will ever be at the time that it is constructed," says Dennis Gruber, Director of Technical Publications for the National Concrete Masonry Association. "It has moisture in it and as it dries out, it cures, and it shrinks. When it shrinks, it tries to pull the ends in and make the panel smaller. That's alright if it's not restrained, but it's restrained, as the wall is tied in to your foundation, roof and floor members, keeping it from pulling the ends in and from shrinking. This results in cracks, because masonry is much weaker in tension than it is in compression."
According to Carolinas Concrete Masonry Association, "A change of 50 degrees Fahrenheit will affect a 100-foot concrete wall with forces 410 PSI and movement of as much as 1/4 inch. Since the tensile strength of the wall is generally less than that, cracking generally occurs."
So how do you create a solid wall that allows for movement? Joints.
Concrete and Control Joints
Control joints, used effectively, will divide a large, solid wall into panels that facilitate longitudinal movement, thereby reducing the chances for cracks or limiting how much cracking actually occurs on the wall face.
"Control joints are for aesthetic and water penetration reasons," says Graber. "If it's an exterior wall and it's covered up with furring or something like that, they're just in there for aesthetic reasons. The shrinkage cracks don't inhibit the strength of the wall.
"The idea of control joints is to give a place so those joints can open up to reduce the restraint and, in essence, it's putting in pre-formed cracks. It's just a weakened plane joint so that when it dries out and it cracks, it cracks in a nice straight line. The way you do that is you rake out the mortar joints, so that they're weaker there, then you put a backer rod and sealant in so that you don't get water penetration."
The elastomeric sealant should be one that remains flexible, like a silicone sealant, and doesn't become hard or rigid. It should meet ASTM C920 standards and be installed using ASTM C962. In exterior walls it is especially important to keep the seal weather-tight, so the sealant should have good contact with the edges. By keeping the sealant thicker on the edges and thinner in the middle, around the backer rod, the sealant will remain more flexible and allow for movement without tearing.
In addition to the backer rod and sealant, a shear key with a gasket can be used in the joint for added load transfer. Joint reinforcement or bond beams can be used in conjunction to increase the tensile strength of the wall, whether laid in stacked bond or running bond patterns. Dowelled joints, although rarely used, can also be a way to transfer even higher loads at the joint.