CHAPTER 3 - Creep and Moisture Movements

Abstract

This chapter discusses the creep and moisture movements. Most building materials, when subjected to a sustained load, undergo an instantaneous (elastic) deformation followed by a time-dependent deformation, generally known as creep. The creep strain is made up of delayed elastic strain, which is eventually recoverable after the removal of the load, and viscous strain that appears as a permanent set when the load is removed. The various types of strain, or movements, become clearer when represented on a diagram. The chapter presents a typical strain—time curve for a viscoelastic material under a constant stress. When the load is applied certain deformation occurs instantaneously. This deformation is called the instantaneous strain or, more often, the elastic strain. In porous materials capable of absorbing or giving off moisture, the creep strains are closely linked with moisture movements that can lead to progressive shrinkage or expansion, depending on the nature of the material, its initial moisture content and the external environment. The magnitude of the moisture strains is not negligible in relation to the strains caused by load and they must be taken into account in the study of long-term movements. To compute the creep movements caused solely by loads, one must deduct algebraically the moisture strains from the total delayed strain.