Reentrant corner cracks can ruin an otherwise impressive slab, whether it be decorative concrete (see Photos 1 and 2), an industrial slab (see Photo 3), a high-end exposed retail floor, or your backyard patio. Besides aesthetic concerns, such cracks can spall if they are wide enough and are exposed to wheeled traffic or freeze/thaw cycles. The basic causes, prevention, and mitigation of reentrant corner cracks have been known for years, but many misconceptions still persist. These basics and more in-depth concepts and analyses will be presented, along with practical suggestions to help minimize the cracking potential.
Common reentrant corner locations include inside corners of walls (see Photo 1), dock leveler pits (see Photo 3), equipment bases, foundation wall pilasters/piers, and significant slab penetrations (see Photo 2).
A reentrant corner creates a tensile stress concentration at the corner as a slab tries to linearly shrink and move in two directions at right angles to each other. Slabs also can do this as the result of temperature changes. Curling tensile stresses in the top of the slab that build up in the first few hours after slab placement are additive to the right angle linear shrinkage and temperature stresses. The combination of these tensile stresses in the top of the slab try to rip the slab apart, starting at the corner. When the tensile strength of the concrete is exceeded, concrete cracks diagonally at approximately 135 degrees from each edge of a 90-degree corner. As further shrinkage and curling occur, the crack widens and lengthens. Because linear shrinkage and curling continue at a decreasing rate for more than two years, the cracks can become quite wide and long in some cases. For example, the shrinkage of a 6-inch slab after one year is only 60% to 80% of its ultimate shrinkage, which means cracks that appear early will grow significantly over time unless they are kept tight by appropriately designed and installed reinforcement.
Joint installation issues
In some cases, the timely installation of slab contraction or contraction (control) joints at 90 degrees in both directions to or from an inside corner will work. However, many times what appears to be a logical contraction joint detail does not result in success. The simple explanation is the concrete will crack where the greatest tensile stress develops. A more complex explanation is that there are many factors that affect whether cracks occur below the joints as desired or a diagonal crack develops first. One example is if the slab base is not planar with low friction, there may not be enough slab movement for the joints at the corner to activate, or crack below the joint.
Another example is if contraction joint sawing is started too late, then cracking can still occur. Even if timed correctly, if the joint immediately adjacent to the reentrant corner is not deep enough, or stopped short of the corner, the crack may occur on the diagonal because the tensile stresses are greater there than below the joints. A good example of this is at the end of the sawcut where the bottom of the cut becomes shallower and may not reach the end of the joint because of an obstruction to the saw, such as a wall or column. In this case, a smaller diameter saw should be used at the end as soon as feasible, and a plunge sawcut made at the very end where appropriate. If there is a significant drop in temperature as a weather front moves in during concrete finishing or in the first few hours thereafter, it may be extremely difficult, if not impossible, to sawcut the contraction joints soon enough before a crack develops, due to the rapid thermal contraction of the slab found especially in the top and the slower strength gain to resist these tensile stresses.
To show how these tensile stresses can occur at the reentrant corners, several computer analyses were conducted and combined. As can be seen in Figure 1, there is a significant buildup of tensile stress at the reentrant corner due to only a small increase in the curling of the slab. This significant tensile stress is due to the geometry of the slab before the sawcuts are made and can occur with only a small decrease in temperature from the top of the slab to the bottom. Therefore, it is important the sawcuts indicated in Figure 1 be done first to minimize this tension stress and hopefully prevent a diagonal crack from developing.
The following are common situations and suggested strategies for minimizing the possibility of out-of-joint cracking. These strategies have been used successfully on a number of projects.
Wall and isolated foundation corners. When wall corners and isolated foundations are installed adjacent to monolithically placed slabs, reentrant corners occur with cracks possibly occurring and running off at about 135 degrees (see Photo 1). Several strategies can be used to minimize the chances of such a crack. The most common strategy is to make the sawcuts to the corner as soon as feasible before the adjacent cuts and then finish off the joint by doing a plunge cut as deep as possible at the corner. Another option is to make one of the joints a construction joint with appropriate load transfer devices that allow differential movement parallel to the joint, such as plate dowels. Then, a crack usually can be avoided. One of the authors has had success avoiding a crack by installing a crack inducer. One such approach is to use a 3-foot-long section of preformed metal keyway extending from the corner directly under each proposed sawcut. With this detail, the top of the keyway is purposely kept 1½ inches below the top of the slab so the sawcut can be made over the metal strip. It is important to carefully place concrete on both sides of the keyway equally to maintain alignment and to make the sawcut promptly and as closely aligned as possible.
Dock leveler pits. If concrete is placed monolithically around a dock leveler pit, two reentrant corners are formed in the slab. As with inside corners of a wall and other similar locations, sawcutting at right angles to the dock face often will not eliminate random cracks from forming off the corners of the leveler unit. At dock levelers, the most reliable approach is to form a doweled construction joint along the end face of the leveler units (see Figure 2), preferably with plate dowels to allow differential movement parallel to the joint. This approach does require a separate concrete placement between the levelers, but in most cases it can be combined with another portion of a slab placement. A critical consideration often overlooked is the steel armor angles on the three sides of the pit must be isolated from each other, not welded as a single unit, for the construction joint to function properly.
Wall pilasters/piers. Wall pilasters or piers are enlarged inside areas of the exterior foundation that provide support for structural columns around the perimeter of a building. Wall pilasters typically form two corners into the slab while corner pilasters form one. Some consider these pilaster edges reentrant corners and others do not. However, these corners often have cracking occurring from them due to an improper detail that does not isolate the pilaster from the shrinkage or thermal movement of the slab. It is typical to place isolation material along the face of foundation walls and around the perimeter of the pilasters to help keep the slab from bonding to the face of the wall. Asphalt-impregnated fiberboard and closed-cell polyethylene foam plank commonly are used as isolation materials with the foam plank allowing for more slab movement. Increasing the thickness of the foam plank to 1 inch around the pilaster, placing it at full depth of the slab, and ensuring there are no other slab restraint issues—such as column anchor bolts protruding into the bottom of the slab—has been successful in preventing cracks. However, the most reliable method to prevent cracking along foundation walls is to isolate pilasters with a diamond-shaped box-out at the point the diamond is aligned with the proposed sawcut. Of course, the diamond also forms a reentrant corner; thus, the same sawcut joint considerations should be implemented.
Reentrant corner reinforcement
Where properly installed corner joints are not feasible, #4 diagonal reinforcing bars often are used (see Figure 3). The rebars will not prevent a reentrant crack from forming, but when located correctly, they can keep the crack much tighter and shorter. When the slab is 6 inches or thicker, a layer of rebar in the top and bottom typically have helped to maintain any cracks as tight as possible. If there is only one layer of rebar, it should be placed in the top because the greatest tensile stress at a reentrant corner forms there and that also is where the crack can be seen. It is important that the reinforcement be installed similarly to the as location in Figure 3 because if more cover is provided, then there is a significant increase in the crack width. A good rule of thumb is that if the concrete cover is doubled, the crack width will be doubled. This can make the difference between a short 12- to 15-mil crack that does not cause problems and a long 25- to 30-mil crack that spalls under traffic.
Appropriate details, construction sequence, and timing are critical when reentrant corners are unavoidable. When these measures are practiced, there will be less of a chance that a reentrant corner crack will damage your work or your image.
Peter Craig is an independent concrete floor consultant with more than 35 years of construction and repair experience. He is a voting member of ACI Committee 302 and a past national president of the International Concrete Repair Institute. Rick Smith of Structural Services Inc., Richardson, Texas, has 21 years experience and conducts numerous floor surveys which include crack assessments and repair recommendations. Wayne W. Walker, P.E., is the director of engineering services at Structural Services Inc. He is on several ACI committees, is the current chairman of ACI 360, Design of Slabs on Ground, and has more than 30 years of experience worldwide with the design and construction of concrete slabs. Jerry A. Holland, P.E., FACI, is director of design services for Structural Services Inc. He is on several ACI committees, teaches seminars for ACI and the World of Concrete and has more than 40 years of experience worldwide with the design and construction of concrete slabs.