Crack Growth and Lifetime of Concrete under Long Time Loading

Edge-notched eccentrically compressed fracture specimens made of aggregate of reduced size are loaded in standard creep test frames. Measurements of the time rate of crack mouth opening in notched concrete specimens subjected to constant load of almost one month duration are reported and analyzed. To reveal the size effect, geometrically similar specimens of four sizes in the ratio I :2:4:8 are tested. The results are successfully described by a previously proposed time-dependent generalization of the R-curve model, in which the rate of crack growth is a function of the ratio of the stress intensity factor to the R-curve, and linear aging viscoelastic creep in the bulk of the specimen is treated according to the operator method. Good predictions are also obtained with a simplified method in which the R-curve is replaced by a constant asymptotic value of the critical stress intensity factor and creep is handled in similarity to the effective modulus method, neglecting the history effect. The time curves of crack opening terminate with an infinite slope, indicating the lifetime. The finiteness of the lifetime is not caused by creep, but by time-dependent crack growth, which dominates the final stage of crack opening. The initial stage of crack opening, on the other hand, is dominated by creep. Tests are conducted both for concretes of normal strength of 33.4 MPa (4,847 psi) in compression and relatively high strength of 46.4 MPa (6,442 psi). For the stronger concrete, the lifetimes are found to be longer. An increase of specimen size is found to decrease the lifetime. Since the same type of model was previously shown capable of describing all other known time-dependent fracture phenomena in concrete, a rather general applicability may be expected.