Preparation of C60 High-Fluidity Concrete-Filled Steel Tube Concrete Using Steel Slag Sand

Literature Overview

This research by Ding Qingjun, Bao Jiacheng, Jin Hongcheng, and Zhou Peng from the School of Materials Science and Engineering at Wuhan University of Technology, published in the Journal of Wuhan University of Technology in 2019, addresses a critical practical problem in concrete-filled steel tube (CFST) construction: the shrinkage-induced debonding between the core concrete and the steel tube. The authors develop a C60 high-fluidity concrete using pre-treated steel slag sand as a partial replacement for natural river sand, leveraging the alkali activation effect and internal curing function of steel slag to compensate for core concrete shrinkage and form a dense interfacial transition zone.

Technical Approach and Material Design

The core innovation of this study is the use of pre-treated steel slag sand, which serves a dual function: it acts as a fine aggregate replacement and provides additional pozzolanic and alkali-activating reactions. The pre-treatment of steel slag is critical to activate its latent hydraulic properties and ensure proper dispersion in the concrete mix.

The concrete mix design incorporates several functional components:

The combination of fly ash and silica fume creates a synergistic pozzolanic effect that enhances the long-term strength and durability of the concrete. The superplasticizer and water-retaining agent work together to achieve high fluidity while maintaining adequate workability for pumping and placing into steel tubes.

Key Performance Results

The resulting C60 concrete achieved a 28-day compressive strength of 76.9 MPa, which exceeds the design strength requirement. More importantly, the concrete exhibited excellent volume stability, meaning that shrinkage was effectively controlled. This is the primary advantage of using steel slag sand: the internal curing effect of the slag particles releases water during hydration, compensating for the self-desiccation that occurs in high-performance concrete with low water-to-binder ratios.

The dense interfacial transition zone (ITZ) formed between the concrete and the steel tube is a critical quality indicator. In conventional CFST concrete, the ITZ is often a weak zone due to water accumulation at the steel surface and poor adhesion. The steel slag sand, through its alkali activation effect, promotes the formation of additional calcium silicate hydrate (C-S-H) gel at the ITZ, creating a stronger bond between the concrete and the steel tube.

Connection with Steel Pipe Manufacturing and Quality

From a steel pipe manufacturing perspective, the quality of the steel tube surface directly affects the concrete-steel bond in CFST applications. The internal surface of the tube should be free from mill scale, rust, oil, and other contaminants that could impair the bond. Surface preparation methods include shot blasting, acid pickling, or mechanical cleaning, depending on the required bond quality.

The high fluidity of the C60 concrete is essential for proper filling of the steel tube, particularly in large-diameter tubes where gravity flow alone may be insufficient. The pumpability and flowability of the concrete must be designed to ensure complete filling without voids or segregation. The superplasticizer dosage and water-to-binder ratio must be carefully balanced to achieve the required fluidity without compromising strength or durability.

Engineering Practice Considerations

The use of steel slag sand in CFST concrete has significant environmental benefits, as it diverts industrial by-products from landfill and reduces the demand for natural aggregates. However, the quality of steel slag varies depending on the steelmaking process, and pre-treatment is necessary to ensure consistent performance. The pre-treatment process should be carefully controlled and documented to ensure batch-to-batch consistency.

For CFST structures in aggressive environments, such as marine or industrial settings, the durability of the concrete is paramount. The high fluidity and dense microstructure of the C60 concrete provide good resistance to chloride ingress and carbonation, but additional durability measures such as corrosion inhibitors or epoxy-coated steel tubes may be necessary for severe exposure conditions.

The pumping and placing of high-fluidity concrete into steel tubes requires careful planning. The concrete must be placed continuously to avoid cold joints, and the pumping rate must be controlled to prevent segregation and bleeding. For tall CFST columns, the concrete placement may need to be performed in stages with appropriate construction joints.

Critical Reflections and Limitations

The study demonstrates the technical feasibility of using steel slag sand in CFST concrete, but several practical challenges remain. The long-term durability of the concrete, particularly under cyclic loading and environmental exposure, requires extended research. The alkali-activated reactions in steel slag may continue for extended periods, potentially affecting the long-term volume stability.

The high fluidity of the concrete, while beneficial for filling, may also lead to issues such as bleeding and segregation if not properly controlled. The water-retaining agent and shrinkage-reducing agent must be carefully selected and dosed to prevent these issues. Additionally, the high fluidity may make it difficult to control the concrete level during placement, requiring careful monitoring and control.

The study focuses on 28-day strength, but the long-term strength development of pozzolanic concrete is often more important for service life assessment. The alkali activation of steel slag and the pozzolanic reactions of fly ash and silica fume continue for months or years, potentially leading to strength gains beyond 28 days. Long-term testing data would be valuable for design purposes.

Study Insights and Practical Recommendations

This research represents a significant advancement in CFST concrete technology, addressing the critical issue of shrinkage-induced debonding through innovative use of industrial by-products. The key practical recommendation is that pre-treated steel slag sand, combined with appropriate admixtures and supplementary cementitious materials, can produce high-performance CFST concrete with excellent volume stability and steel-concrete bond.

For steel pipe manufacturers and CFST contractors, this work provides a technical pathway for improving the quality and durability of CFST structures. The emphasis on the interfacial transition zone highlights the importance of steel tube surface preparation and concrete mix design in achieving optimal bond performance. Engineers should consider incorporating these material innovations into their design and specification practices for CFST applications, particularly in projects where long-term durability and seismic performance are critical design considerations.