Time-dependent properties of concrete influence the behavior and effectiveness of concrete structures. By developing inelastic strain creep and shrinkage cause significant changes in the deformations, stresses and affect the long-term performance of concrete structures.
The time dependent deformation of concrete is partly a result of the response of the various components of the gel to stimuli which alter the pressure in, and the hygral equilibrium of, the cement paste. The bulk of creep deformation is related to changes in the interaction of water and the cement hydrate, while shrinkage is mainly attributed to capillary pressures and changes in the surf ace energy of the solid gel particles.
Many structural materials such as concrete, timber, metals at elevated temperatures and plastics exhibit the characteristic of deforming with time under sustained loading. This type of behavior is termed creep and affects the strength and long-term performance of structures. In addition concrete also exhibits changes in strain with time when no external stress is acting, due to movement of water to or from the ambient medium. This change is referred to simply as shrinkage or swelling, respectively. It is believed that Hatt was probably the first to acknowledge the significance of creep on structural behavior and White recognized structural significance of shrinkage of concrete.
Though shrinkage adversely affects the structures causing stress redistribution in non-homogeneous and composite structures, sections and stress losses in prestressed structures by influencing the state of deformation and inducing stresses when they are restrained, the consequential effects of creep can be beneficial or detrimental. Creep exercises a beneficial effect by relieving undesirable stresses due to shrinkage and other imposed deformations like extreme initial temperatures, or settlement of supports and yielding of restraints. The detrimental effect is that creep adversely affect long-term reliability of structures.
