Size effect in the bonding of smooth and deformed bars: a comparison between Normal-Strength and High-Performance Concrete

Size effect is unanimously recognized as a peculiar – and critical – aspect of the mechanical behavior of many structures, provided that either their material or the interfaces among their parts have a quasi-brittle behavior. In this paper, the results obtained during two experimental campaigns carried out at Politecnico di Milano on long smooth bars and on short deformed bars are recalled. In both cases size effect is shown to be very strong and in good agreement with the well-known size-effect power law proposed by Bažant and coworkers. Size effect is also modeled by numerically solving the 2nd-order bond equation in one dimension. In the case of smooth bars, a simple but effective approach is adopted, by assuming for the interface an elastic-frictional behavior. In the case of deformed bars, the test results make it possible to formulate a size-dependent local bond-stress/slip law, that fits fairly well also the results obtained by other authors.