3-Point Bending Tests of Notched Beam: A Suitable Test to Investigate Size Effect of Plain Concrete

Presented By: Christian Carloni, Case Western Reserve University
This work presents the results of three-point bending (TPB) tests on the largest set of plain concrete notched beams ever tested. Five different sizes (depths), nominally 3 in., 6 in., 10 in., 20 in., and 40 in., were tested to evaluate size effect experimentally. The largest size was tested horizontally while floating on Plexiglas balls in order to remove the effect of the self-weight. In addition, for the second to largest size some specimens were tested horizontally and the others vertically in order to confirm that the different test set-up did not affect the results. Particular care was paid to obtain a set of specimens that were carefully cast from the same batch of concrete, cured under the same conditions, and tested at virtually the same age under the same environmental conditions. In quasibrittle materials like concrete, the fracture front is blunted by a zone of microcracking (that eventually coalesce into an actual crack) where nonlinear softening behavior occurs. This region is often called fracture process zone (FPZ). In a three-point bending (TPB) test of notched beams, the FPZ starts to develop before the peak load is attained. However, the FPZ continues to extend further after the peak is reached during the descending portion of the response. When the FPZ is fully developed, it may occupy a large portion or even the whole cross-section of the specimen. In small-size TPB specimens, the FPZ may not fully develop. In fact, as the FPZ continues to extend, it is contrasted by the presence of a compression zone on top due to the presence of bending moment in TPB tests. The size of the fully established FPZ is linked to the characteristics of the quasibrittle material investigated and can be used to predict to what extent notched beams and more in general structural elements (made of a given quasibrittle material) exhibit size effect that deviates from the one predicted by linear elastic fracture mechanics (LEFM). The load responses, the peak load Pmax.

[@Ignition1303]

Excellent work Dr. Carloni