Problem statement: Rate dependence of the newly developed Very-High-StrengthConcrete (VHSC) composites has received little or no attention so far. Approach: In this research, the bond-slip mechanisms of four types of steel fibers embedded in VHSC matrices were investigated through single-fiber pullout tests with the loading rates and matrix strengths are the primary variables. This study presented the experimental results of steel fiber-matrix bond characteristics and discussed the influence of loading rates on the pullout behavior. Results: The results were measured in terms of peak loads and total fiber pullout work or dissipated bond energy. Results indicated that the increase in pullout rate increases both peak load and total pullout work for all deformed fibers but had no effect on smooth, unreformed fibers. Conclusion/Recommendations: De formed and smooth fibers exhibit different rate sensitivities. The variation in response was attributed to the fiber end conditions. It is recommended that (1) additional experimental tests should be performed at other loading rates and (2) an analytical model should also be developed to analyze the rate effect on the interfacial deboning process of VHSC composites.

Problem statement:There is a need to better understand the rate dependence behavior of reinforced concrete structures in order to improve their response to impact and blast loads. Analysis and design of reinforced concrete structures subjected to seismic loadings has been recommended in many FEMA guidelines. However, reevaluation of design becomes extremely important in cases where large deformations are expected such as blast and impact resistant. Approach: This study presents a numerical model to evaluate reinforced concrete columns submitted to high strain rates expected for seismic, impact and blast loadings. The model utilizes dynamic stress-strain response and considers the effect of strain rate on concrete strength; strain at peak stress; yield and ultimate strength of steel; and slope of the softening portion of the stress-strain curve. Results: Results are presented in the form of interaction diagrams and compared with the available analytical and experimental results. Comparison with available data shows that the proposed model can give consistent prediction of the dynamic behavior of reinforced concrete columns. Conclusion/Recommendations: The established interaction diagrams may be used to design columns to withstand high velocity impact loads. Also, knowledge gained can be used to improve dynamic behavioral models and computer-aided analysis and design of reinforced concrete columns subjected to severe blast loadings.