Comparison of measured fracture toughness and size-independent fracture toughness for concrete

Laboratory measured results from a broad-based experimental program have been combined with cohesive cracking simulations to determine the size-independent fracture toughness of two batches of concrete. The two batches of concrete used aggregate with a nominal maximum size of 22 mm. The batches had average compressive strengths of 36 and 52 MPa. The laboratory experimental program consisted of three sizes each of single edge (75 – 305 mm high) and round double beam specimens (305 – 1220 mm high). The two-parameter, size effect, Barker and inverse analysis data reduction methods were used to obtain measured values of fracture toughness from the test data. Each of the data reduction methods makes different assumptions about the effects of the process zone. Therefore, differences in measured fracture toughness values from the various data reduction methods are possible. The comparison shows that the single edge and round double beam specimens, up to 305 mm high, with the two-parameter, size effect and Barker data reduction methods do not produce fracture toughness values within 10% of the size-independent value. As expected, the accuracy of the various combinations of test specimen geometry, size, and data reduction method improved with larger test specimen sizes. Only the inverse analysis data reduction method produces accurate values in the range of specimen sizes that can be lifted by a single person.