Crack propagation in flexural fatigue of concrete

In this paper the behavior of concrete subjected to flexural fatigue loadin g is studied. Notched concrete beams were tested in a three-point bending c onfiguration. Specimens were subjected to quasi-static cyclic and constant amplitude fatigue loading. The cyclic tests were performed by unloading the specimen at different points in the postpeak part of the quasi-static load ing response. Low cycle, high amplitude fatigue tests were performed to fai lure using four different load ranges. The crack mouth opening displacement was continuously monitored throughout the loading process. Crack propagati on caused by quasi-static and fatigue loads is described in terms of fractu re mechanics. It is shown that the crack propagation in the postpeak part o f the quasi-static load response is predicted using the critical value of t he mode I stress intensity factor (K-1C). The ultimate deformation of the s pecimen during the fatigue test is compared with that from the quasi-static test; it is demonstrated that the quasi-static deformation is insufficient as a fatigue failure criterion. It is observed that crack growth owing to constant-amplitude fatigue loading comprises two phases: a deceleration sta ge when there is a decrease in crack growth rate with increasing crack leng th, followed by an acceleration stage where the rate of crack growth increa ses at a steady rate. The crack length where the rate of crack growth chang es from deceleration to acceleration is shown to be equal to the crack leng th at the peak load of the quasi-static response. Analytical expressions fo r crack growth in the deceleration and acceleration stages are developed, w herein the expressions for crack growth rate in the deceleration stage are developed using the R-curve concept, and the acceleration stage is shown to follow the Paris law. It is observed that the crack length at failure for constant amplitude fatigue loading is comparable to that of the correspondi ng load in the postpeak part of the quasi-static response. Finally, a fract ure-based fatigue failure criterion is proposed.