ABSTRACT
It is well recognized that the strength degradation of cementitious materials and fibre cements is dependent on the loading rate as well as the time under which a sustained or cyclic load is applied (Mindess and Nadeau, 1977; Mindess, 1985; Hu et al., 1989). There are two main factors which control these time-dependent strength characteristics. One factor is the flaw statistics in terms of flaw density and flaw size distribution; the other factor is the slow crack growth process determined by the chemical reactive species at the flaw tip (Beaudon, 1986; Tait and Garrett, 1986). For slow loading rates both factors determine the strength characteristics of these materials. To avoid dealing with the statistics of pre-existing flaws the conventional approach is to consider the growth of a single crack only. This is acceptable for a uniform stress field since the largest flaw is the one that determines the strength according to the Weibull theory (1951). However, many cementitious structural applications have a non-uniform stress field and the single crack approach is not appropriate. A timedependent statistical fracture mechanics theory is necessary and is given in the next section.
