The effect of specimen size on the fracture energy and softening function of concrete

This paper examines the effect of specimen size on the fracture energy of concrete, G(F), as measured using the three-point bending test on pre-notched beams prescribed by RILEM TC-50 [1]. The concept of partial fracture energy is introduced and used to explain the observed size effect. The opening displacement at the top of the notch in the test specimen at the end of the test, w(f), is affected by the size Of the specimen, which in turn affects the measured value of the concrete fracture energy. In theory when the specimen is large enough to allow the fracture process zone to develop fully, w(f) will reach its critical value, w(c), and the effect of specimen size on G(F) will be eliminated. The experimental results included here show that in reality the size of the specimen does affect the measurement of G(F), even when the size is such that the fracture process zone develops fully. This may be due to local plastic deformation in the area around the loading point, which is particularly significant in larger specimens. It may also be due to differences in the influence of the boundary conditions of the test for different specimen sizes. Tn addition, a procedure is outlined for the determination of the softening function for concrete based on the fracture energy measured ill RILEM tests, in which the specimens are small enough to ensure that the energy measured is actually due to fracture and not plastic deformation.