A MODEL FOR AGING VISCOELASTIC TENSION SOFTENING MATERIALS

An analytical constitutive model similar to the Kelvin chain theologic al model associated with solidification theory, is developed for time- dependent tension softening of ageing materials like concrete. The sti ffness of spring elements is allowed to vary with time via a function of load bearing volume fraction as in the solidification theory. The d evelopment of cracks reduces the load bearing volume fraction with tim e, so that the overall behaviour of springs is a softening type. A red uction in the load bearing volume with time ensures a gradual reductio n in the spring stiffness without violating prescribed retardation tim es. In some circumstances, depending on the boundary conditions, the s tress in a structure can remain unchanged over a period of time. Durin g this period, any cracks in the structure will continue to experience an increased opening due to creep. In other parts of the structure, a gain depending on the boundary conditions, the crack opening displacem ents may remain unchanged over a period of time, so that the stress wi ll relax over these parts of the structure. In a large concrete struct ure, creep and relaxation may be taking place simultaneously in differ ent parts or in the same part but at different times. There is thus a need for a visco-elastic tension softening model for ageing concrete t hat will cater for both creep and relaxation. (C) 1998 John Wiley & So ns, Ltd.