Bond-Slip Performance of Recycled Concrete in Square Steel Tubes

Literature Overview

This paper by Li Weining and colleagues from Guangxi University investigates the interfacial bond-slip behavior between square steel tubes and recycled aggregate concrete through push-out tests on 10 short column specimens. The study is significant because the construction industry faces mounting pressure to utilize recycled concrete aggregates, yet the structural performance of composite systems incorporating recycled materials remains insufficiently characterized. The research was supported by the National Natural Science Foundation of China (Grant No. 50908057) and published in the Journal of Guangxi University (Natural Science Edition) in 2012.

Experimental Configuration and Test Parameters

The test matrix was designed around three independent variables, each selected to address a distinct engineering concern. The concrete strength grade affects the inherent bond mechanism at the steel-concrete interface; the embedment length determines the effective transfer zone over which shear stresses develop; and the recycled coarse aggregate replacement ratio quantifies the degree to which natural aggregate is substituted with recycled material.

The push-out test methodology, which is the standard approach for characterizing bond-slip in composite columns, was employed. This test isolates the interface between the steel tube and the concrete core, applying a shear force that induces relative displacement at the interface while constraining other deformation modes.

Load-Slip Curve Stages

The paper identifies four distinct stages in the load-slip response of the square steel tube recycled concrete system. Understanding these stages is critical for engineers who need to predict serviceability limits and ultimate capacity.

1. No-slip stage: The steel tube and concrete core deform together without relative displacement. The interface remains fully bonded, and the load increases linearly with slip. This stage corresponds to the elastic behavior of the composite system.
2. Stress-rise stage: Micro-cracking begins at the interface, typically at the corners of the square tube where stress concentrations are highest. The load continues to increase, but the stiffness decreases as the bond degrades. This stage represents the transition from elastic to inelastic behavior.
3. Stress-mutation stage: A sudden drop in load occurs as the bond breaks catastrophically at a critical location. This stage is characterized by a rapid release of stored elastic energy and represents the peak bond capacity.
4. Stress-decrease stage: After the peak, the load decreases as the interface separation propagates. Residual friction and mechanical interlock provide limited post-peak resistance, but the system has lost its composite action.

Key Findings and Technical Interpretation

Effect of Recycled Aggregate Replacement Ratio

The study finds that the replacement ratio has a relatively minor influence on both the initial slip bond strength and the ultimate bond strength. This is an encouraging finding for engineers considering recycled materials in composite columns. The reason is that the bond mechanism at the steel-concrete interface is dominated by friction and chemical adhesion, both of which are more sensitive to the concrete surface properties and the steel tube surface condition than to the internal aggregate composition. However, engineers should note that while the bond strength itself may not degrade significantly, the recycled aggregate can introduce additional porosity and micro-cracking within the concrete matrix, which may affect long-term durability and fatigue performance.

Effect of Embedment Length

The paper reveals that specimens with shorter embedment lengths exhibit higher initial slip bond strength and ultimate bond strength compared to those with longer embedment lengths. This counterintuitive result can be explained by the stress distribution along the interface. In shorter specimens, the shear stress is more uniformly distributed, and the full interface area is mobilized. In longer specimens, the shear stress is concentrated near the loaded end, leaving the far end underutilized. This phenomenon is well-documented in conventional steel-concrete composite columns and is consistent with the bond stress distribution models proposed by Popovics and others.

Effect of Concrete Strength

Concrete strength has a significant influence on both the initial slip bond strength and the ultimate bond strength. Higher strength concrete provides greater frictional resistance at the interface due to increased normal stress from Poisson effects under axial compression. It also provides a denser matrix with fewer voids, enhancing chemical adhesion. This finding reinforces the engineering practice of specifying higher strength concrete for composite columns where bond performance is critical.

Engineering Practice Implications

From a steel pipe manufacturing perspective, the surface condition of the square steel tube is a critical factor in bond performance. Engineers should ensure that the inner surface of the tube is clean, free of mill scale, rust, and contaminants before concrete placement. Surface roughness can be intentionally introduced through shot-blasting or mechanical profiling to enhance mechanical interlock. The square cross-section introduces corner effects that can either enhance or degrade bond performance depending on the corner radius and concrete placement method.

When specifying recycled concrete for composite columns, engineers should conduct additional testing beyond the push-out test, including pull-out tests, split-cylinder tests, and long-term creep tests, to fully characterize the bond performance under realistic service conditions. The relatively minor effect of replacement ratio on bond strength does not eliminate the need for quality control of recycled aggregates, as other performance parameters such as permeability, shrinkage, and freeze-thaw resistance may be more sensitive to the recycled content.

Study Insights and Outlook

This research provides valuable baseline data for the design of composite columns using recycled concrete, which aligns with global sustainability goals in the construction industry. However, the study is limited to short columns under controlled laboratory conditions. Future research should extend to full-scale columns, consider cyclic loading for seismic applications, and incorporate long-term durability testing including carbonation, chloride ingress, and freeze-thaw cycling. The square cross-section studied here is less common in practice than circular sections, and the bond behavior may differ due to the different stress distribution and concrete confinement geometry. Engineers should exercise caution when extrapolating these results to other cross-sectional shapes and loading conditions without additional validation testing.