Axial Compression Performance of Steel Tube Sea Sand Recycled Concrete

Literature Overview

This paper by Huang Yijie and colleagues from Shandong University of Science and Technology and Tongji University, published in the Journal of Building Materials (2018, Vol. 21, No. 1), presents an experimental investigation into the axial compression performance of steel tube sea sand recycled concrete (SSRC) members. The research was supported by multiple funding sources including the National Natural Science Foundation of China (Grant 51408346), the China Postdoctoral Science Foundation Special Funding (Grant 2016T0041), and the Taishan Scholars Fund (Grant 10231202000). The study conducted 18 axial compression tests on SSRC specimens, with recycled coarse aggregate replacement ratio and chloride ion content in sea sand as the primary variables.

Experimental Design and Material Parameters

The experimental program systematically investigated the effects of two key material parameters on the axial compression behavior of SSRC members. The recycled coarse aggregate replacement ratio varied across multiple levels, reflecting the proportion of recycled concrete aggregate used in place of natural coarse aggregate. The chloride ion content in sea sand was also varied, representing the natural variability of sea sand sources and the potential for chloride-induced corrosion.

Core Technical Findings

The loading behavior of SSRC specimens follows the same three-stage pattern observed in conventional steel tube concrete members: elastic stage, elastoplastic stage, and failure stage. The primary failure mode is diagonal shear failure, consistent with the established behavior of CFST members under axial compression. The steel tube provides lateral confinement to the concrete core, enhancing both the compressive strength and the ductility of the composite member.

The peak load of SSRC specimens decreases slightly with increasing recycled coarse aggregate replacement ratio and with increasing chloride ion content in sea sand. This reduction is attributed to the lower quality of recycled aggregate (higher porosity, weaker interfacial transition zone) and the potential for chloride-induced corrosion of the steel tube. However, the peak strain exhibits a more complex trend: it increases with increasing recycled aggregate replacement ratio, likely due to the enhanced deformation capacity of the recycled aggregate-concrete interface, but shows a non-monotonic trend with chloride content, first increasing and then decreasing.

Corrosion and Welding Integrity Concerns

From a welding and steel pipe integrity perspective, the chloride ion content in sea sand represents a significant long-term durability concern. Chloride ions can penetrate through the concrete cover and reach the steel tube surface, initiating pitting corrosion. For welded steel tubes, the HAZ regions are particularly susceptible to chloride-induced corrosion due to the microstructural variations and residual stresses introduced during welding. The combination of HAZ softening and chloride attack can significantly reduce the local corrosion resistance and fracture toughness of the steel tube.

The welding of steel tubes for SSRC members requires special attention to corrosion protection. The internal surface of the steel tube, which is in direct contact with the chloride-containing concrete, must be protected against corrosion. Options include the use of corrosion-resistant steel grades (such as duplex stainless steel or weathering steel), internal coating systems, or the incorporation of corrosion inhibitors in the concrete mix. The welding procedure must account for the potential need for post-weld corrosion protection treatments, which may include coating application, heat treatment, or stress relief.

Stress-Strain Model Development

The paper compares the peak load calculation formulas from various design codes and standards against the experimental results, identifying significant discrepancies. Based on the experimental data, the authors developed a fitted stress-strain full curve model for SSRC axial compression. This model accounts for the effects of recycled aggregate replacement ratio and chloride content on both the elastic modulus, peak strength, and post-peak behavior of the composite member.

The fitted model provides a more accurate representation of the SSRC material behavior compared to conventional CFST models, which do not account for the degradation effects of recycled aggregate and chloride exposure. The model parameters were calibrated to minimize the deviation between predicted and experimental stress-strain curves, ensuring reliable predictions for design applications.

Engineering Practice Implications

The use of sea sand and recycled aggregate in CFST construction represents a sustainable approach to construction material utilization, but it introduces material variability and durability concerns that must be addressed through rigorous quality control. From a fabrication and welding perspective, the key recommendations are as follows:

First, the steel tube material specification should account for the chloride exposure environment. For applications involving high chloride content concrete, the steel tube should be specified with enhanced corrosion resistance, either through material selection or through protective coatings. The welding procedure must be compatible with the selected corrosion protection system, ensuring that the weld metal and HAZ maintain adequate corrosion resistance after welding and any subsequent heat treatment.

Second, the concrete mix design for SSRC members should incorporate corrosion inhibitors and optimize the water-cement ratio to minimize chloride penetration. The quality of the concrete cover and the density of the concrete matrix are critical factors in determining the long-term corrosion resistance of the steel tube. Non-destructive testing methods such as ultrasonic pulse velocity and half-cell potential measurements should be incorporated into the quality assurance program to monitor the corrosion status of the steel tube over the service life of the structure.

Study Insights and Conclusions

This research provides valuable experimental data on the axial compression behavior of steel tube sea sand recycled concrete members, addressing an important sustainability challenge in construction engineering. The findings confirm that SSRC members retain the fundamental mechanical behavior of conventional CFST members, with only slight reductions in peak load and modified ductility characteristics. From a fabrication and welding standpoint, the primary concern is the long-term corrosion integrity of the steel tube under chloride exposure, particularly at the weld heat-affected zones. Engineers should adopt a comprehensive durability design approach that integrates material selection, welding procedure qualification, corrosion protection, and long-term monitoring to ensure the structural reliability of SSRC members in chloride-containing environments. The developed stress-strain model provides a practical tool for design calculations, and the experimental database serves as a foundation for future research on the long-term performance and durability of SSRC structures.