Bond-Slip Behavior of Square Steel Tube Recycled Concrete Interface

Literature Overview

This paper by Li Weining and colleagues from Guangxi University (2012) investigates the interfacial bond-slip characteristics between square steel tubes and recycled concrete composite columns. The research was funded by the National Natural Science Foundation of China (Grant No. 50908057) and related provincial programs. Ten short-column specimens were fabricated using recycled coarse aggregate derived from demolished concrete waste, and push-out tests were conducted to obtain load-slip curves at the steel-concrete interface. Three key variables were systematically studied: concrete strength grade, embedment length, and recycled aggregate replacement ratio.

Core Technical Findings

The load-slip curves exhibit four distinct stages: (1) no-slip stage, (2) stress-rising stage, (3) stress-mutation stage, and (4) stress-descending stage. The following table summarizes the influence of each variable:

Interpretation of Technical Points

From a metallurgical and materials science perspective, the interface between steel and concrete is fundamentally governed by mechanical interlock, chemical adhesion, and frictional resistance. The use of recycled coarse aggregate introduces surface microcracks and residual mortar films that could potentially degrade the concrete's cohesive strength. However, the study's finding that replacement ratio has only minor influence on bond strength is encouraging. This suggests that the dominant bond mechanism at the steel-concrete interface is mechanical interlock through the roughness of the steel tube inner surface rather than concrete's intrinsic tensile strength alone.

The observation that shorter embedment lengths yield higher bond strength values is counterintuitive at first glance. This phenomenon can be attributed to stress concentration effects: in shorter specimens, the load transfer zone is more concentrated, leading to higher localized bond stresses before debonding initiates. In longer specimens, the load distributes over a greater area, resulting in lower average bond stress values even though total load capacity may be comparable.

The significant influence of concrete strength grade on both initial and ultimate bond strength aligns with established composite column theory. Higher strength concrete provides greater confinement pressure against the steel tube walls, enhancing frictional resistance and delaying interface debonding. This is consistent with the confinement model described in ACI 318 and Eurocode 4, where the lateral confining pressure is proportional to concrete compressive strength.

Connection with Steel Tube Manufacturing and Welding Practice

From the perspective of steel tube production, the internal surface quality of square steel tubes is critical for composite column performance. The manufacturing process—whether cold-bending, hot-rolling, or welding of plates—determines the internal surface roughness profile that governs mechanical interlock. For welded square tubes, the internal weld bead geometry, penetration profile, and heat-affected zone characteristics directly affect the bond interface.

Key manufacturing considerations include:

• Internal weld bead height should be minimized to avoid creating stress concentrations at the concrete interface
• Surface roughness in the range of 0.5-2.0 mm amplitude provides optimal mechanical interlock without creating stress risers
• Internal oxide scale from hot-rolled tubes should be removed to prevent weak bonding layers
• Residual stresses from welding should be relieved to prevent differential thermal expansion effects at the interface

The recycled concrete aspect introduces additional considerations for steel tube manufacturers. Recycled aggregate concrete typically exhibits higher permeability and lower modulus of elasticity compared to natural aggregate concrete. This means the steel tube will experience greater lateral deformation under load, potentially leading to earlier local buckling of the tube walls. Thicker-walled tubes or tubes with higher yield strength grades (e.g., Q355 or Q420 per GB/T 1591) may be warranted when using recycled concrete.

Quality Control Implications

For engineering practice, the following quality control measures are recommended based on this research:

1. Verify internal surface condition of square steel tubes through visual inspection and roughness measurement prior to concrete pouring
2. Ensure proper cleaning of internal surfaces to remove oils, rust, and manufacturing debris
3. Conduct bond strength verification tests on representative specimens before mass production
4. Monitor concrete mix design to ensure adequate workability for filling the interior of square tubes completely without voids
5. Implement vibration protocols that avoid damaging the steel tube walls while ensuring full concrete compaction

Study Insights and Reflections

This research contributes valuable data to the growing body of knowledge on sustainable composite construction using recycled materials. The finding that recycled aggregate replacement ratio has limited influence on bond-slip behavior is particularly significant for promoting circular economy principles in construction. However, I note that the study's scope is limited to short columns under push-out testing conditions, which do not fully replicate the complex stress states encountered in full-scale structural members under combined axial and bending loads. Future research should extend to long-column buckling behavior and fatigue performance under cyclic loading, which are critical for seismic design applications. The interface bond mechanism under sustained loading and long-term creep conditions also warrants investigation, as recycled concrete's higher permeability may accelerate carbonation and corrosion at the steel-concrete interface over the service life of the structure.