Cracks in CMU Wall/Stucco Due to Shrinkage and Thermal Movement
2011-10-31
Cracks in concrete masonry unit (CMU) walls are very common, and usually reflect through exterior stucco finish. Typical shrinkage and restrained thermal movement cracks in CMUwalls often manifest as vertical or step pattern cracks. While normal shrinkage cracks in CMU walls are typically not a cause for structural concern, they may present a source of moisture intrusion. These types of cracks usually occur at fairly regular intervals along the length of a long wall, or at wall openings such as windows or doors.
In general, cracks in CMU walls occur as the result of excessive structural loading, inadequate or adversely affected support, shrinkage and thermal movement, or a combination of the above. Excessive structural loading may be the least common cause of cracks in residential CMU walls, and is not considered further here.
Inadequate support may occur due to significant deflection of a support member or settlement of the affected wall's foundation. Shrinkage occurs due to a reduction in moisture content and carbonation. The focus of this discussion is cracks which occur due to restrained movement brought about by drying shrinkage and thermal effects. These cracks can be limited by the use of appropriate design measures.
Similar to shrinkage cracks in concrete slabs on grade, shrinkage and thermal movement cracks in CMU walls primarily occur due to restrained movement. One portion of a wall is restrained with respect to another, and this induces tensile forces within the wall. As with plain concrete, CMU is strong in compression but weak in tension. Therefore, restrained tensile forces within the CMU wall often lead to cracking. The restraint occurs as the result of friction and mortar bond between individual blocks and other structural elements such as the foundation, columns, etc.
Drying shrinkage occurs in all three cement products used within a CMU wall: block, mortar and grout. However, as the block itself makes up the majority of wall area, it is block shrinkage properties that are the primary consideration in drying shrinkage. The individual CMU drying shrinkage potential depends upon several factors including curing and moisture content, cement content, and aggregates used in the block. Typical CMU drying shrinkage varies, but a range of approximately one-fourth to one-half inch in 100 linear feet of CMU wall is typical.
CMU walls also crack due to restrained thermal movement. As with most materials, CMU expands when heated and contracts when cooled. A fifty degree Farenheit temperature change can result in more than one-fourth inch length change in 100 linear feet of a CMU wall. Obviously, the temperature change that a wall experiences depends upon the local climate, but it is also affected by exposure direction, wall color, insulation, etc.
Shrinkage and thermal effects should be accounted for in masonry design. This may be accomplished through the use of well-cured and dry block, horizontal joint reinforcement, and control joints. Control joints effectively break a long wall into smaller segments. In this way, the individual wall segments contract and move individually. The movement is accommodated at the joint, where a flexible and weatherproof sealant is utilized as part of the control joint.
The following is a photograph of a crack in exterior stucco typical of CMU wall shrinkage and restrained thermal movement:
