Steel-to-concrete bond after concrete splitting: constitutive laws and interface deterioration

Abstract

In a previous paper, bond behaviour after concrete splitting was presented as a relevant aspect of the interaction between the reinforcement and the concrete, in r.c. elements characterized by large steel ratios, small concrete covers, small free bar interspaces and high strength steel. Within this framework, the general philosophy and the experimental set-up of an innovative kind of test were presented, and the results of a three-phase research project regarding pull-out, presplit r.c. specimens were shown and discussed. During each test the opening of the preformed splitting crack was kept constant. In the present paper some of the previous test results are briefly recalled in order to work out empirical constitutive laws regarding bond stiffness, bond strength and bond stress as a function of bar slip and of confinement stress, and tangent friction coefficient as a function of the crack opening. Taking advantage of the special bars used in Phase B tests (where the stresses close to the bar are roughly plane and the deformations of the concrete can be examined at the surface of the specimen by means of the moire technique), the contribution of microcracking and shear strains to bar slip is analysed, as well as the extension of the deteriorated concrete close to the bar. The formulation of suitable constitutive laws is a first step towards a rational design of the transverse reinforcement (i.e. stirrups in an r.c. beam) while the evaluation of the damaged zone within the concrete mass can help in the modellization by finite elements, since bond-related non-linearity may be limited to narrow bands close to each bar.