Crack-Parallel Stress Effect on Fracture of Fiber-Reinforced Concrete Revealed by Gap Tests

This paper presents an experimental study on how the crack-parallel stress affects the fracture properties of fiber-reinforced concrete (FRC) using the gap test—a new simple fracture test invented and used for concrete at Northwestern University in 2020. First, it was conducted for plain concrete and was successfully applied to cross-ply carbon-fiber composite and to aluminum. An advantage of this test is that it is unambiguous because the test setup changes from one statically determinate configuration to another. The gap test, combined with the standard notched three-point-bend test, is now applied to geometrically scaled FRC specimens to determine how the fracture energy, $G_f$, and the effective size, $c_f$, of the fracture process zone (FPZ), are changed by the crack-parallel stress, ${\sigma }_{xx}$. For ${\sigma }_{xx}$ equal to about $2/3$ of the standard uniaxial compression strength, the increase in $G_f$ is 64% and 78% for the two FRCs, respectively, which is large but not as large as the 126% increase observed in tests of plain concrete. This indicates that the fiber reinforcement mitigates the effect of ${\sigma }_{xx}$, while introducing some degree of ductility into the fracture process. The compressive ${\sigma }_{xx}$ also increases the effective size of the FPZ by about 81% and 64% while such increase is 134% in plain concrete. Because crack-parallel stresses are ubiquitous in practice, the implications for design are significant.