Identification of Stress-Slip Law for Bar or Fiber Pullout by Size Effect Tests

Abstract

Test results on the size effect in the pullout strength of reinforcing bars embedded in concrete are presented. Attention is focused on failures due solely to interface slip, with no cracking in the surrounding concrete. This type of failure is achieved by using smooth round bars and a sufficiently large ratio of bar diameter to embedment length. Elimination of cracking in the surrounding concrete makes it possible to study the characteristics of the interfacial shear fracture between steel and concrete. The results of tests of geometrically similar specimens show that interfacial shear fracture causes a size effect on the nominal strength in pullout. The size effect is found to be transitional between plastic failure (the current approach of concrete design codes, for which there is no size effect) and linear elastic fracture mechanics (for which the size effect is the maximum possible). This transitional size effect can be approximately described by the size effect law proposed by Bažant in 1984 for quasibrittle failures in general. By fitting a theoretical formula obtained in a previous study (by Bažant and Desmorat in 1994) to the size effect data, the basic characteristics of the stress-slip law for interface fracture are determined. These include the interfacial fracture energy, the shear bond strength (debonding shear stress), and the residual frictional shear stress. The same method could be used for identifying the interfacial fracture characteristics of other materials, e.g., fibers in cementitious composites.