When concrete is loaded, the structure undergoes elastic and inelastic deformations. Elastic deformations occur immediately after the concrete is subjected to a given load, according to Hooke’s Law. Inelastic deformations increase with time as the concrete experiences a sustained load. This inelastic deformation, also known as creep, increases at a decreasing rate during the loading period. During the first month of sustained loading, approximately one-fourth to one-third of the ultimate creep takes place. As time proceeds, usually one-half to three-fourths of the ultimate creep occurs during the first half year.
The amount of creep that the concrete undergoes is dependent upon 1) the magnitude of the sustained loading, 2) the age and strength of the concrete when the stress is applied, and 3) the total amount of time that the concrete is stressed. When the concrete is loaded, the specimen undergoes internal properties such as closure of voids in the concrete, viscous flow of the cement-water paste, crystalline flow in aggregates, and water flowing out of the cement "gel" due to drying and loading. Aggregates play an important role in both creep and shrinkage. A well graded, coarser aggregate with a low voids content decreases the effects of creep and shrinkage. Also, hard, dense aggregates that are not absorptive and have a high modulus of elasticity are desirable for low shrinkage and creep rates. Another important aspect to reducing creep is the type of curing procedure performed prior to loading. Very little creep occurs when a high-pressure steamed curing procedure is used. Atmospheric and high-pressure steam curing produces little creep when compared to the seven-day moist curing method. These two types of curing reduce the drying shrinkage by half as much as they reduce creep. Other factors affecting creep include type of cement, amount of cement paste, size and shape of concrete, amount of reinforcement (rebar), volume-to-surface ratio, temperature, and humidity.