Application to the Danish test of a bond law deduced from a short RC tie

The analysis of the variations in the steel and concrete stresses obtained from a test on an RC specimen named as short RC tie, as its length represents the distance between two primary cracks, allows the corresponding bond law to be deduced. This avoids passing through the experimental data based on pull-out tests, where bond is substantially influenced by the state of stress, deformation and cracking of the concrete. The deduced bond stress - slip diagram clearly shows two distinct branches: a first approximately linear branch where, starting from the zero slip section, the bond stress and the slip both increase; a second branch where the bond stress decreases when the slip continues to increase. What happens is not just a local phenomenon and has to be considered as preponderant in service conditions in case of stresses close to the maximum values. Here, the proposed bond law is applied to the case of a Danish test, where two aligned bars of equal length, embedded into a concrete prismatic specimen, are put in tension. The separation of the two bars permits to eliminate any interaction between them. If one applies the fib bond law, the ascending distributions of the slip, occurring from the embedded end to the free end of the bars, lead to ascending distributions of the bond stress. This theoretical result is not confirmed if one applies the proposed bond law as the bond stress is null at both free ends of the bars.