Experimental investigation of the bond-slip behavior between steel section and concrete under different loading histories

To investigate the bond-slip behavior between steel sections and concrete under complex practical loading conditions, three loading protocols were adopted: symmetric cyclic loading, asymmetric cyclic loading, and monotonic loading. The steel reinforced concrete (SRC) specimens included plain concrete as a blank group, as well as mixtures with varying volume fractions of polypropylene fiber (PPF) and nano-silica (NS) to evaluate the effects of PPF and NS on bond strength and slip performance at the steel-concrete interface. The failure modes, hysteretic behavior, envelope curves, characteristic bond loads, and bond stress degradation were analyzed to provide insights into the bond-slip mechanisms influenced by PPF and NS. Based on the experimental results, empirical formulas for calculating bond stress were also proposed. Experimental results indicated that plain concrete specimens mainly experienced splitting failure, while PPF delayed crack propagation and NS improved bond through matrix densification. Under cyclic loading, plain concrete exhibited significant bond degradation: the peak bond stress ( τ u ) and residual bond stress ( τ ᵣ) were reduced by up to 30 % and 51 % under symmetric loading, and by 16 % and 43 % under asymmetric loading, compared to monotonic push-out loadings. The addition of 0.15 % PPF or 2 % NS enhanced the bond strength most effectively, increasing τ u by up to 37.8 % and 25.7 %, respectively, compared to plain concrete. However, higher dosages led to reduced effectiveness due to fiber agglomeration or particle clustering. Finally, empirical models were developed to predict normalized bond stress ( τ / f ₜ) as functions of PPF content, NS content, and loading type, and these models showed excellent correlation with the experimental results.