True fracture energy of concrete

If fracture energy of concrete is to be considered a material property, it must be a size- and geometry-independent parameter. At present, there are three different approaches for determination of fracture energy of concrete: Hillerborg's work of fracture test, Bazant's size effect model, and Shah's two-parameter fracture model. All three approaches yield varying values for fracture energy of concrete. In this study, comparison of fracture energies (G_F) obtained from tension tests and notched beam tests (Hillerborg's work of fracture test) reveals that fracture energy is size and structural geometry independent fracture parameter provided that the beam, tests are carried out accurately. In the beam tests, the effect of notch-depth and span-depth ratios on fracture energy were considered. It was found that the method for determination of fracture energy from beam tests can be significantly affected if the load line measurements are not accurate. This is identified as the single most important factor that may have caused researchers in the past to conclude that G_F obtained from notched beam tests are size-dependent and hence invalid. A proposal is made m this paper that uses the crack mouth opening displacements (CMOD) instead of deflection measurements to calculate fracture energy of concrete. Although in this study high-strength concrete (HSC) is studied, the conclusions formed can easily be extended to all strengths of concrete.