Recycled Coarse Aggregate Replacement Ratio Effects on Stainless Steel Tube Recycled Concrete Bond Performance

Literature Overview and Research Significance

The study by Liu Wenyuan and Leng Jie (2020), published in Science Technology and Engineering, Volume 20, Issue 22, investigates the bond performance between circular stainless steel tubes and recycled concrete containing varying proportions of recycled coarse aggregate (RCA). This research was funded by the National Natural Science Foundation of China (51978166), the Jiangsu University Youth Blue Project (2017), and the Taizhou Science and Technology Support Project (TS201521). The work is significant because the increasing use of recycled aggregates in construction, driven by sustainability goals and waste management regulations, introduces uncertainties in the structural performance of composite members. Understanding the bond behavior at the steel-concrete interface is essential for the reliable design of stainless steel tube recycled concrete (SSRC) members.

Experimental Program and Test Methodology

Five circular stainless steel tube recycled concrete specimens were designed with different RCA replacement ratios and subjected to reciprocating push-out tests. The reciprocating loading protocol is particularly important because it simulates the cyclic loading conditions encountered in seismic applications and provides information on the degradation of bond strength under repeated loading. The test setup typically involves a steel tube specimen with a concrete core, where the concrete core is pushed out relative to the steel tube while measuring the relative slip at the interface.

Bond-Slip Curve Behavior and Failure Analysis

The study reveals that the bond strength-slip curves exhibit a distinctive three-stage distribution, which is consistent with the established understanding of steel-concrete bond mechanics but with notable modifications due to the recycled aggregate content. The first stage corresponds to the elastic bond behavior where the interface is intact and slip is minimal. The second stage represents the transition from chemical adhesion to mechanical interlock, where slip increases rapidly with relatively small load increments. The third stage corresponds to the residual bond strength maintained by mechanical interlock and friction after the interface has been damaged.

The reciprocating loading has a pronounced effect on the bond strength, causing progressive degradation with each loading cycle. This degradation is attributed to the cumulative damage at the steel-concrete interface, where the repeated slip causes abrasion of the concrete surface and removal of the chemical adhesive layer. The RCA replacement ratio further accelerates this degradation process because recycled concrete typically has a higher water-cement ratio and contains adhered old mortar on the aggregate surface, resulting in a weaker interfacial transition zone (ITZ).

Regression Analysis and Practical Formula

The authors developed a practical calculation formula for the ultimate bond strength that considers both the RCA replacement ratio and the concrete compressive strength as key parameters. This formula provides engineers with a tool to estimate the bond performance of SSRC members during the design phase. The regression analysis demonstrates that the bond strength decreases approximately linearly with increasing RCA replacement ratio, while the relationship with concrete strength is more complex and follows a nonlinear trend. The practical formula should be validated with additional experimental data before being adopted in design codes.

Engineering Practice Considerations

For engineers designing structures that incorporate recycled concrete in stainless steel tubes, several practical considerations emerge from this study. First, the use of stainless steel tubes offers excellent corrosion resistance, which is particularly advantageous in marine environments or structures exposed to deicing salts. However, the bond performance degradation due to RCA content must be accounted for in the structural analysis. Second, the stainless steel tube material, typically grades 304 or 316, has a lower elastic modulus than carbon steel, which affects the confinement pressure developed on the concrete core. Third, the fabrication of stainless steel tubes requires special welding procedures to avoid intergranular corrosion, and the welding process must be carefully controlled to maintain the corrosion resistance of the tube.

The push-out test results also highlight the importance of surface preparation of the steel tube interior. For recycled concrete, which may have a rougher aggregate surface, the mechanical interlock contribution to bond strength is enhanced, partially compensating for the reduced chemical adhesion. However, the overall bond strength still decreases with higher RCA replacement ratios, and design adjustments are necessary to ensure adequate structural safety.

Study Insights and Outlook

This research contributes valuable experimental data on the bond behavior of stainless steel tube recycled concrete members, filling an important gap in the literature. The findings clearly demonstrate that while recycled aggregates can be used in CFST applications, the bond performance must be carefully evaluated and accounted for in design. Future research should investigate the effect of RCA particle size distribution, the age of the recycled aggregate source, and the use of chemical admixtures to improve the bond interface. The development of design guidelines and code provisions for SSRC members using recycled concrete is an important next step to facilitate the widespread adoption of this sustainable construction technology.