Cracking plays an important role in concrete’s response toload in both tension and compression. The earliest studies ofthe microscopic behavior of concrete involved the responseof concrete to compressive stress. That early work showedthat the stress-strain response of concrete is closely associatedwith the formation of microcracks, that is, cracks that form atcoarse-aggregate boundaries (bond cracks) and propagatethrough the surrounding mortar (mortar cracks) (Hsu, Slate,Sturman, and Winter 1963; Shah and Winter 1966; Slate andMatheus 1967; Shah and Chandra 1970; Shah and Slate1968; Meyers, Slate, and Winter 1969; Darwin and Slate1970), as shown in Fig. 2.1.During early microcracking studies, concrete was consideredto be made up of two linear, elastic brittle materials; cementpaste and aggregate; and microcracks were considered to bethe major cause of concrete’s nonlinear stress-strain behaviorin compression (Hsu, Slate, Sturman, and Winter 1963; Shahand Winter 1966).
This picture began to change in the1970s. Cement paste is a nonlinear softening material, asis the mortar constituent of concrete. The compressivenonlinearity of concrete is highly dependent upon theresponse of these two materials (Spooner 1972; Spooner andDougill 1975; Spooner, Pomeroy, and Dougill 1976; Maherand Darwin 1977; Cook and Chindaprasirt 1980; Maher andDarwin 1982) and less dependent upon bond and mortar micro-cracking than originally thought. Research indicates, however,that a significant portion of the nonlinear deformation of cementpaste and mortar results from the formation of microcracksthat are several orders of magnitude smaller than thoseobserved in the original studies (Attiogbe and Darwin 1987,1988).
These smaller microcracks have a surface density thatis two to three orders of magnitude higher than the densityof bond and mortar microcracks in concrete at the samecompressive strain, and their discovery represents a signif-icant step towards understanding the behavior of concreteand its constituent materials in compression.
The effect of macroscopic cracks on the performance andfailure characteristics of concrete has also received considerableattention. For many years, concrete has been considered a brittlematerial in tension. Many attempts have been made to useprinciples of fracture mechanics to model the fracture ofconcrete containing macroscopic cracks.