The influence of the parameters loading rate for reinforcement with concrete bond by numerical and experimental tests

Abstract

Bond between reinforcement and concrete is a fundamental factor ensuring the composite behavior of these materials, crucial for the load-bearing capacity, stiffness, crack resistance, and durability of reinforced concrete (RC) structures. Despite its importance, a universal theory of bond satisfying engineering requirements remains undeveloped. This difficulty stems from the influence of a large number of interrelated factors, systematically classified into seven major groups (including geometric and physical parameters of reinforcement/concrete, stress-strain state, deformation parameters, crack resistance, and environmental/corrosion effects). The study addresses the theoretical challenge stemming from classic bond laws (Rehm's step law, Guyon's elasto-plastic law, Kholmyanskiy's normal law, and Mirza and Houde's empirical law, BPE bond-slip model), which analytically describe the relationship between bond stresses and corresponding slip deformations. The necessity for this study is amplified by the critical nature of dynamic actions (impacts, explosions), categorized as complex stress-strain states, particularly relevant in the context of the full-scale war in Ukraine, where blast waves and missile strikes subject structures to severe dynamic loads.

The aim of this research was to establish and analyze the influence of the loading rate on key bond parameters and to evaluate the effiсiency of modern numerical and analytical models for predicting RC structural behavior under extreme dynamic excitations. The methodology involves synthesizing results from experimental pull-out tests conducted under quasi-static (0.1 mm/s) and dynamic (up to 100 mm/s, or higher for explosions) regimes, alongside advanced numerical simulations.

The results unequivocally demonstrate that increasing the loading rate is a significant factor that leads to an increase in bond strength. The predicted maximum bond stress (τ_bmax) at dynamic loading speeds (e.g., 5.00 mm/s) is markedly higher than under quasi-static conditions. It was established that the Dynamic Increase Factor (DIF) for can reach approximately 1.5 when compared to static loading. This strengthening effect is attributed to the strain rate effect in concrete and steel, as well as the rapid development of slip in the contact zone. Furthermore, at higher loading rates, the effective anchorage length is reduced, concentrating forces closer to the loaded end of the bar compared to static tests. The observed failure modes (pull-out, pull-out after yielding, and bar rupture) remain consistent with those observed during static tests. The research also highlighted the inherent high variability of test results due to concrete heterogeneity, noting coefficients of variation up to 18% for maximum bond stress and 23% for corresponding slip.

Numerical modeling of dynamic bond behavior requires specialized approaches to capture nonlinear effects. Effective modeling techniques include discrete methods (e.g., Slide Line Contact Model, CBISF in «LS-DYNA»), models utilizing the Concrete Damage-Plastic (CDP) concept, and analytical models enhanced with the DIF coefficient or the Layered Section Method (EI_eq) for blast analysis. The study concludes that numerical models calibrated with dynamic experimental data provide adequate prediction of RC behavior under extreme dynamic impacts, whereas simple assumptions of ideal bond often lead to significant inaccuracies.