HOW TO CONTINUED Figure 1. Typical vertical wall cracking if no joints are provided. Also, reentry corner cracks can form due to stress concentrations at interior corners that can contribute to vertical wall cracking.
create tensile stresses within walls that create vertical cracks. Consider placing joints at abrupt wall changes, in line with openings to help control reentry corner cracks, and in locations to create an acceptable visual appearance. Each wall should be evaluated to determine the best joint locations and spacings. Exterior grooves can be sealed with a non-sag, elastomeric sealant (e.g., polyurethanes and silicones) to prevent penetration of cold air, moisture, water, and insects through cracks. Sealants can also be used in interior grooves to conceal cracks.
Reinforcement Horizontal wall reinforcing controls the width of shrinkage cracks—not the location. How tightly cracks are held together depends on the amount and spacing of the reinforcing. Increasing the amount or reducing the spacing of reinforcing passing through cracks, decreases crack widths. In many cases, this is the designer’s choice to control vertical wall cracks. With this design choice, don’t expect a crack-free wall. Expect random cracking to occur but crack widths should be limited by the amount of wall (horizontal) reinforcing. Using both load and building code requirements, designers establish the required reinforcing for walls. Horizontal reinforcing may be determined by load conditions, minimum code
requirements to ensure structural integrity, or minimum “temperature and shrinkage” reinforcing as specified by building codes or the local building officials. The amount of horizontal reinforcing can be quantified by computing the reinforcing percentage that is based on the reinforcing and wall thickness. For example, #4 at 12 in. on center located in the middle of an 8-in. thick wall yields a reinforcing percentage of 0.21%.[3] While 0.21% exceeds the minimum 0.18% temperature and shrinkage reinforcing specified by the International Building Code, it may not hold the vertical wall cracks sufficiently tight to satisfy the wall’s function or the owner’s expectations. If our example wall was 23 ft. long with 0.21% horizontal reinforcing, approximate crack widths exceeding about 0.010 inches should be expected. To limit cracks widths to about 0.010 in. and 0.004 in. (watertight), reinforcing percentages would need to be increased to approximately 0.30% and 0.55%, respectively.[4] To achieve a
0.30% value, the reinforcing would need to be #4@8 in. on center and for 0.55%, #5 at 7 in. or #6 at 10 in. on center, respectively. As explained with this simple wall example, significant amounts of horizontal reinforcing are required to keep vertical crack widths narrow. For watertight retaining walls, the minimum wall thickness is typically 10 in. with two mats of reinforcing with significant amounts of horizontal reinforcing to keep crack widths less than 0.004 in. Designers commonly increase the amount of horizontal reinforcing in the lower 3 to 5 ft. of walls to help control the width of cracks that start at the bottom and grow upwards.
Contraction Joints & Reinforcement Contraction joints and reinforcing can be combined to control vertical cracking but if too much reinforcing crosses joints, joints may not be sufficiently weakened to activate and control the cracking location. Recommendations for reinforcing at contraction joints range from stopping all reinforcing 2-3 in. from the joint, allowing 50% or another amount of the reinforcing to continue through the joint, and
A crack compactor card can help you determine the width of the crack in question. Place the card on the crack and align the line that matches the crack width. Kim Basham
16 CONCRETE CONTRACTOR
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