Deduction of the bond low from test on short RC tie in service conditions

Bond between ordinary steel bars and concrete plays a fundamental role in the behavior of the reinforced concrete. It is associated to the slip between steel and surrounding concrete and determines the variation in the steel and concrete stresses of cracked RC members. The need to formulate simple constitutive bond laws has led the international standards, such as fib Model Code 2010, to adopt an average bond stress versus slip relationship obtained from an extensive experimental campaign based on pull-out tests. However, in pull-out tests bond is substantially influenced by the state of stress, deformation and cracking of the concrete. Moreover, in-service conditions of RC members, in order to study the cracking behavior or the tension stiffening, an ascending distribution of the bond stress is usually considered as it corresponds to an ascending distribution of the slip from the zero slip section to the cracked section and in the literature the bond law is applied as it is. In particular, at the cracked section, the bond stress is maximum. But, this distribution of the bond stress is not confirmed by the experimental results available in the literature concerning the behavior of an RC tie with a length equal to the crack spacing, where close to the cracked sections a descending distribution of the bond stress is observed while the distribution of the slip remains ascending, with null bond stress at the cracked section. The modeling proposed here does not claim to be exhaustive due to the limited albeit accurate experimental data. However, it does indicate the opportunity to abandon bond laws based on experimental works that do not represent the real situation or are far from it.