Masonry Magazine December 2002 Page. 28
Control JOINTS
# Controlling the Cracking
Used wisely, control joints can reduce or eliminate cracking in concrete masonry unit walls.
By Jennie Farnsworth
Managing Editor, Masonry Magazine
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.