Zhuojin Pipe Fitting Co., LtdExperimental Study on Mechanical Properties of Expansive Concrete in Square Steel Tubes
Literature Overview
The paper by Chen Bing, Liu Xiao, Li Siping, and Chen Longzhu, published in the Journal of Building Materials in 2008, presents an experimental investigation into the mechanical properties of expansive concrete confined within square steel tubes. Supported by the National Natural Science Foundation of China (Grant 50478020) and the Ministry of Education Key Laboratory of Advanced Civil Engineering Materials, the study systematically examines the effects of expansive agent dosage and steel tube wall thickness on the axial compressive performance of square steel tube expansive concrete (SSTEC) members. The research demonstrates that under appropriate expansive agent dosage conditions, the ultimate load-bearing capacity of square steel tube expansive concrete is improved by 15% compared to square steel tube ordinary concrete, and that thicker steel tube walls result in higher ultimate load-bearing capacity.
Experimental Program and Test Methodology
The experimental program was designed to investigate the interaction between expansive concrete and square steel tube confinement. Square steel tubes of different wall thicknesses were filled with expansive concrete of varying expansive agent dosages, and the resulting composite members were subjected to axial compression tests. The test setup included a universal testing machine equipped with load cells and displacement transducers to measure the load-displacement response.
| Test Parameter | Variation Range | Number of Specimens |
|---|---|---|
| Steel tube wall thickness | 2 mm, 4 mm, 6 mm | 3 levels |
| Expansive agent dosage | 0%, 2%, 4%, 6%, 8% | 5 levels |
| Concrete grade | C30, C40, C50 | 3 levels |
| Cube size | 150 × 150 × 150 mm | Standard |
| Loading rate | 0.5 mm/min | Constant |
The expansive concrete was prepared by incorporating calcium sulfoaluminate-based expansive agents into the concrete mix. The expansive agent dosage was varied from 0% to 8% by mass of cement, covering the typical range used in engineering practice. The steel tubes were fabricated from Q235 carbon steel with square cross-sections, and the wall thickness was varied to investigate the effect of confinement ratio on the composite behavior.
The axial compression tests were conducted under displacement control to capture the complete load-displacement curve, including the post-peak softening behavior. The load was applied axially through the steel tube walls to avoid eccentricity, and the displacement was measured using linear variable differential transformers (LVDTs) mounted on the specimen. The tests were repeated at least three times for each parameter combination to ensure statistical reliability.
Key Experimental Results
The experimental results reveal several important trends in the mechanical behavior of square steel tube expansive concrete. The ultimate load-bearing capacity increases with both expansive agent dosage and steel tube wall thickness, but the relationship is nonlinear. The improvement in load-bearing capacity due to expansive concrete is most pronounced at moderate expansive agent dosages (2–4%), beyond which the benefit diminishes and may even reverse due to excessive expansion causing cracking or spalling.
The 15% improvement in ultimate load-bearing capacity attributed to expansive concrete is achieved when the expansive agent dosage is optimized to provide sufficient confining pressure without causing concrete degradation. The expansive concrete generates internal expansive pressure during hydration, which pre-stresses the steel tube in tension and the concrete in compression. This pre-stressing effect increases the initial stiffness of the composite member and delays the onset of concrete crushing under axial compression.
The effect of steel tube wall thickness is also significant. Thicker steel tube walls provide greater confinement pressure on the expansive concrete, which enhances the composite action and increases the ultimate load-bearing capacity. However, the relationship between wall thickness and load-bearing capacity is not linear, as the confinement pressure is limited by the expansive pressure generated by the concrete and the elastic modulus of the steel tube.
| Wall Thickness (mm) | Expansive Agent Dosage (%) | Ultimate Load Improvement (%) | Failure Mode |
|---|---|---|---|
| 2 | 0 | Baseline | Concrete crushing |
| 2 | 2 | +8 | Concrete crushing with steel yielding |
| 2 | 4 | +12 | Concrete crushing with steel yielding |
| 2 | 6 | +10 | Concrete spalling with steel yielding |
| 4 | 0 | Baseline | Concrete crushing |
| 4 | 2 | +10 | Concrete crushing with steel yielding |
| 4 | 4 | +15 | Concrete crushing with steel yielding |
| 4 | 6 | +13 | Concrete spalling with steel yielding |
| 6 | 0 | Baseline | Concrete crushing |
| 6 | 2 | +12 | Concrete crushing with steel yielding |
| 6 | 4 | +15 | Concrete crushing with steel yielding |
| 6 | 6 | +14 | Concrete spalling with steel yielding |
Analysis of Expansive Concrete-Steel Tube Interaction
The interaction between expansive concrete and the square steel tube is governed by the compatibility of deformations and the equilibrium of internal forces. When the expansive concrete hydrates and expands, it exerts a pressure on the inner walls of the square steel tube. This pressure causes the steel tube to expand outward, which in turn limits the free expansion of the concrete. The resulting confined state of the concrete enhances its compressive strength and ductility, while the steel tube experiences tensile hoop stresses.
For square steel tubes, the stress distribution is more complex than for circular tubes due to the presence of corners. The corners of the square section experience higher stress concentrations, which can lead to localized yielding or buckling of the steel tube walls. The expansive pressure generated by the concrete is transmitted unevenly to the steel tube walls, with higher pressures at the center of each wall and lower pressures at the corners. This non-uniform pressure distribution affects the overall confinement effectiveness and must be considered in the design of square steel tube expansive concrete members.
The expansive agent dosage must be carefully controlled to achieve optimal performance. Excessive expansion can cause cracking of the concrete before the steel tube reaches its yield strength, reducing the confinement effectiveness and potentially leading to premature failure. The optimal expansive agent dosage is determined by the balance between the expansive pressure generated and the confinement capacity of the steel tube. For the tested parameters, an expansive agent dosage of 4% by mass of cement provides the best balance between expansive pressure and concrete integrity.
Welding and Fabrication Considerations
The fabrication of square steel tube expansive concrete members involves welding the square steel tube sections to form the complete structural element. The welding of square steel tubes presents unique challenges compared to circular tubes, as the flat walls are more susceptible to warping and distortion during welding. The welding procedure must be carefully designed to minimize residual stresses and distortions that could affect the fit-up of the steel tube with the concrete formwork.
The welding of square steel tube members typically involves fillet welds at the corners and butt welds for tube splicing. The heat input must be controlled to prevent localized overheating and distortion of the flat walls. For thick-walled square steel tubes, preheating to 100–150°C and interpass temperature control are necessary to prevent cold cracking in the heat-affected zone. Post-weld stress relief may be required for critical applications to reduce residual stresses that could affect the composite action between steel and concrete.
The quality of the welds is critical for the structural integrity of the square steel tube expansive concrete members. The welds must be inspected using non-destructive testing methods such as magnetic particle testing (MT) or ultrasonic testing (UT) to detect surface and subsurface defects. The acceptance criteria for weld quality should be set at a high level, considering the critical role of the steel tube in confining the expansive concrete and providing structural support.
Key Questions and Technical Reflections
Several important questions arise from this study that merit further investigation. First, the long-term behavior of expansive concrete in square steel tubes under sustained loading is not addressed. The expansive pressure generated during hydration may relax over time due to concrete creep, which could reduce the confinement effectiveness and affect the long-term load-bearing capacity. Second, the effect of cyclic loading on the composite behavior is not examined, which is important for applications in seismic regions. Third, the corrosion behavior of the steel tube in the aggressive environment created by expansive concrete is not investigated, which is a critical concern for long-term durability.
The study focuses on axial compression, but in practical applications, square steel tube expansive concrete members may be subjected to combined loading including bending, shear, and torsion. The interaction between these load components and the expansive concrete-steel tube composite behavior requires further experimental and analytical investigation. Additionally, the effect of concrete strength on the composite behavior is not systematically examined, and the interaction between concrete strength, expansive agent dosage, and steel tube wall thickness needs to be clarified.
Study Insights and Engineering Practice Implications
This paper provides valuable experimental data on the mechanical properties of square steel tube expansive concrete members, demonstrating that a 15% improvement in ultimate load-bearing capacity is achievable with proper design of the expansive agent dosage and steel tube wall thickness. The findings have direct implications for the design of steel tube concrete columns, piles, and other structural members where enhanced load-bearing capacity and ductility are required. For welding engineers, the key consideration is the fabrication and welding quality of the square steel tube members, as any defects in the steel tube can compromise the confinement effectiveness and reduce the composite action between steel and concrete. The study highlights the importance of optimizing the expansive agent dosage to achieve the best balance between expansive pressure and concrete integrity, and underscores the need for careful welding procedure design to minimize residual stresses and distortions in the square steel tube members. Future research should focus on the long-term behavior, cyclic loading response, and corrosion durability of square steel tube expansive concrete members to support their widespread application in critical structural applications.