Typical reentrant crack at window.
The most frequent call to the Concrete Foundations Association (CFA) is some version of “My foundation wall has cracks, should I be concerned?” I usually begin by quoting an old phrase—Concrete is guaranteed to do two things: get hard and crack.
What about cracks in concrete?
Is there positive drainage away from the wall? If not, water can accumulate around the foundation causing the lateral pressure to exceed the forces for which the wall is designed. All it takes is one plugged downspout to cause soil saturation, when there is improper drainage. Saturated soil greatly increases the lateral pressure on the wall. Is there a perimeter drainage system and is it functional? A good drainage system, which incorporates a porous material and drain tile, can reduce water saturation pressures.
Cracking in concrete is a natural phenomenon. Concrete is strong in compression but weak in tension. When stress is exerted on a concrete element, the portions that are placed in tension are susceptible to cracking. These forces include shrinkage, temperature change, lateral pressure from soil, settlement, and point loads from structural elements. Although the cracking cannot be prevented, there are ways of minimizing the cracks and problems that may ensue.
Vertical (or near vertical) cracks. Just because a wall has cracked doesn't mean that it has failed or that remedial action is required. If the crack is narrow (1/8 inch or less), is nearly vertical, has no lateral separation between the adjacent portions of the wall, and no water is leaking through the crack, no action generally is required. This is a shrinkage crack and occurs as moisture in the wall evaporates causing the wall to shrink into the voids created by the escaping water. This type of crack is controlled, or minimized but not eliminated by, using horizontal reinforcement called temperature or shrinkage steel, which helps distribute the stresses in the wall. If horizontal steel is present, you are more likely to get several very small cracks in lieu of one or two much wider cracks. A second method of limiting shrinkage cracks is to control the amount of water used in the mix. This can be done by using either a low water-cement ratio for the concrete mix or adding a water-reducing admixture. The latter approach is common since a flowable mix is desirable to reduce labor and improve consolidation.
Reentrant Cracks. Whenever a concrete element has a sharp angle, there is a stress concentration. This almost always results in a crack called a reentrant crack that emanates from the inside corner. It may be vertical, horizontal, or diagonal as it exits the corner. This phenomenon exists in nearly all materials. Round openings can dissipate the stress but this is impractical in concrete wall construction. The typical remedy to reduce this type of crack is the placement of steel reinforcement, either vertical and horizontal or diagonal rods spanning the corner. This will not eliminate the cracks but it will keep them tight.
Horizontal cracks. Horizontal cracks require greater scrutiny. Most residential foundation walls are designed to span from the footing or floor slab to the connection of the floor deck above. An 8-inch concrete wall in normal soil conditions usually is strong enough to withstand the forces exerted on the wall with no vertical reinforcement. Exceptions include areas with high ground water conditions or expansive soils. Tables in the International Building Code, the ACI-332 Standard, and the CFA Standard can be used to determine whether or not a wall will require vertical reinforcement. If there is vertical reinforcement in the wall, a horizontal crack is probably not a concern. An expert should be consulted when a horizontal crack appears to evaluate whether there is a risk.
The biggest concern is water leakage. If a waterproofing system was installed during construction, the basement won't leak even if there is a large crack. Keep in mind that waterproofing is different than dampproofing. Installing an exterior waterproofing system after the wall has been backfilled can be cost prohibitive. The best solution is the use of an epoxy injection system. It will adhere to the side of the cracks and actually may strengthen the wall. These systems should be applied by a professional.