Brief 

Kim Basham, an ACI fellow and president of KB Engineering, offers tips on how to minimize the root causes of vertical wall cracks in concrete. Basham recommends either installing contraction joints or reinforcing horizontal ways as the best methods to control cracks.

 

Insight

Concrete volume changes due to changes in moisture and temperature. As concrete dries, it shrinks. Length change related to drying shrinkage for unreinforced concrete can range up to 0.08% whereas normal reinforced concrete is about 0.02% to 0.03% due to the internal restraint associated with the reinforcing.[1]

For an unreinforced or lightly reinforced 100 ft. wall, drying shrinkage tries to shorten the wall up to about one inch. If the wall is normally reinforced, shortening is reduced to about 1/4 to 3/8 of an inch. With regards to thermal contraction, our 100 ft. wall subjected to a 50º F temperature drop tries to shorten an additional 5/8 of an inch.

Wall shortening from concrete drying shrinkage and/or thermal cooling combined with restraints including wall corners, intersecting walls and foundations with dowel bars causes vertical cracks to form (see figure 1). Due to shrinkage and restraints that resist wall shortening, tensile stresses form and when stresses exceed the tensile capacity of the concrete, cracking occurs.

Concrete Shrinkage + Restraints = Tensile Stresses = Wall Cracks

Another way of thinking about this shrinkage and restraint behavior is to place our 100 ft. wall in outer space. Floating in space without restraints, the wall could freely shrink or shorten to a new length without cracking.

Back on earth with wall restraints, cracks form and relieve the tensile stresses created by the concrete shrinkage. If wall stresses are not sufficiently relieved by cracking, new cracks occur between previously formed cracks. As illustrated in Figure 1, most wall cracks start at the bottom and grow upwards due to the rigid restraint created by the foundation.

 

READ MORE
Share
Top