Concrete is an artificial construction material with characteristics similar to natural stone. Once used primarily for building foundations and outdoor structures such as patios and walkways, concrete is also a popular material for indoor applications such as countertops and flooring. Despite its strength, both old and freshly poured concrete will crack.
Concrete is a paste prepared by mixing aggregates, such as sand and gravel or crushed stone, with water and a cement, such as crushed limestone, marl, iron ore, shale, clay or fly ash. Freshly mixed concrete is soft; the chemical reaction between the water and cement causes the concrete to harden with time. It takes five to seven days for freshly poured concrete to cure, or harden.
Soft Concrete Cracks
New concrete that has not had the chance to fully cure is susceptible to cracks from two causes. Settlement cracks may develop in areas of the concrete under which dense structures lie, such as steel reinforcements. The soft concrete does not settle as much over dense areas as it does over loosely packed surrounding areas, resulting in settlement cracks. Plastic-shrinkage cracks also occur before the concrete has fully hardened. They result when the surface of the concrete dries faster than the sub-surface concrete. The water from the wet cement below rises, or bleeds, and breaks through the dried surface, causing cracks.
Hard Concrete Cracks
New concrete that has fully cured is also vulnerable to cracking. Although the concrete is hard, it continues to dry; as it dries, it shrinks. According to the Portland Cement Association, concrete shrinks approximately one-sixteenth of an inch for every 10 feet. Placing joints, or manmade cracks, in the concrete at regular intervals not only accommodates the shrinkage but also allows the contractor to control the appearance and location of the cracks in the cement.