Cast-in-place concrete basements provide durable, high quality living space. Some cracking is normal in concrete basement walls. Volume changes and other movements at an early age result in different types of cracks. These cracks can grow if the walls are not properly designed, due to the continued horizontal pressures applied by soils, water and temperature. Cracking can be minimized and problems prevented if the design and construction practices that follow are implemented.
Most builders or third-party providers offer limited warranties for basements. A typical warranty will require repair only when cracks leak, have measurable vertical displacement, or if the crack width exceeds 1/8-inch. The National Association of Home Builders (NAHB) requires repair or corrective action when cracks in basement walls cause leaks into the basement.
Soil type and conditions should be properly assessed for appropriate design and construction of foundations specific to the building site. The excavation should be at least to the level of the bottom of the basement slab and can be to the bottom of the footing. Soil or granular fill beneath the entire area of the basement should be well compacted by rolling, vibrating or tamping. Footings must bear on undisturbed soil or well compacted fill. Uniform soil bearing capacity should be ensured or the design should accommodate any variation.
Formwork must be installed and braced to withstand the pressure of the fresh and flowing concrete. Reinforcement is used to control crack width. Wall thickness and reinforcement should be provided in accordance with International Residential Code (IRC), ACI 332, or locally adopted Code.
Some cracks in basement walls can be controlled to occur in properly located formed joints.
Use concrete with adequate strength in accordance with the Code and project specifications. Excess water should not be added to concrete in the truck mixer. Water-reducing admixtures can be used to increase flow. Air-entrained concrete should be used for walls exposed to moisture and freezing temperatures.
Place concrete in a continuous operation to avoid cold joints and segregation. Adding excess water to concrete to facilitate placement will increase segregation, cause honeycombing or excessive cracking, and reduce strength. Consider placement points no greater 20 or 30 feet around the perimeter of the wall to minimize segregation. Properly designed higher slump concretes with admixtures will flow horizontally for long distances and placement points can be spread out. Curing should begin after placement. Forms should be left in place 5 to 7 days or at least until concrete attains adequate strength to support itself. Forms removed too early can result in premature drying and may cause cracking. In cold weather, forms should be insulated with blankets or other materials to retain heat. During hot dry weather, forms should be covered with wet burlap to retain moisture. Liquid membrane-forming curing compounds can be sprayed at the required coverage after forms are removed to prevent excessive drying.
Waterproof membranes are best applied to the exterior of foundation walls. These are spray-applied, painted, or mechanically fastened sheet systems. Positive side waterproofing (exterior) is generally better than negative side (inside) to keep water from leaking through cracks. Drainage systems should be designed to remove excessive soil moisture along the basement wall. Provide foundation drainage by installing drain tiles or plastic pipes around the exterior of the footing and properly connect them to a removal system or drain to daylight. Surface and roof drainage should direct water away from the residence. Water should be drained to lower elevations suitable to receive storm water runoff.