Composite Elastic Modulus and Bearing Capacity of Concrete-Filled Steel Tubes - Experimental Study on the Qinglongchang Overpass

Literature Overview

This paper by Li Guohua and Ye Yuezhong, published in Concrete and Cement Products in 1998, reports on an experimental investigation of the composite elastic modulus, compressive strength, and Poisson's ratio of concrete-filled steel tubes (CFST) used in the Chengdu Qinglongchang overpass arch bridge, which was the largest overpass in the Southwest region of China at the time. The study compares experimental results with two prevailing national design codes and highlights the differences between them. A key practical finding is that improving construction quality, particularly ensuring dense and full concrete pouring inside the steel tube, significantly enhances the composite elastic modulus.

Core Technical Findings

The experimental program focused on three primary mechanical properties: composite elastic modulus, compressive strength, and Poisson's ratio. These properties are fundamental to the structural analysis and design of CFST members, as they determine the member's stiffness, load-carrying capacity, and lateral deformation behavior under axial loading.

The study found that the composite elastic modulus of CFST members is not simply the sum of the individual contributions of steel and concrete. Instead, it reflects the composite action between the two materials, which is influenced by the interface bonding quality, the relative stiffness of the two materials, and the degree of concrete confinement provided by the steel tube.

Interpretation of Technical Points

The experimental results revealed that the composite elastic modulus obtained from tests was consistently higher than the values predicted by the two national design codes. This discrepancy is attributed to several factors. First, the codes typically use simplified assumptions that do not fully capture the composite action between steel and concrete. Second, the codes may not adequately account for the beneficial effect of steel tube confinement on the concrete core, which increases the effective stiffness of the composite member. Third, the quality of concrete pouring and compaction in the actual construction significantly affects the measured elastic modulus, and the codes do not explicitly address this construction quality factor.

The study emphasizes that the composite elastic modulus is highly sensitive to the quality of concrete placement inside the steel tube. In the Qinglongchang overpass project, the steel tubes were filled with concrete through a process that required careful attention to vibration, pouring sequence, and air exclusion. Any voids or incomplete filling would reduce the effective composite action and lower the measured elastic modulus. This finding has direct implications for construction quality control in CFST applications.

Process and Standards Analysis

From a construction quality control perspective, this study underscores the importance of ensuring dense and full concrete placement within steel tubes. The following process considerations are critical:

1. Concrete Mix Design: The concrete should have a slump of 100-150 mm to ensure adequate flowability into the steel tube without segregation.
2. Vibration Method: Internal vibrators should be used to compact the concrete, with vibration time determined by the concrete grade and tube diameter.
3. Pouring Sequence: For vertical tubes, concrete should be poured from the bottom up, with the pouring rate controlled to prevent segregation and ensure uniform compaction.
4. Quality Verification: Post-pouring inspection methods, such as ultrasonic testing, should be employed to detect voids or incomplete filling.

The comparison with national design codes reveals that the codes may underestimate the composite elastic modulus, leading to conservative deflection predictions. This conservatism, while safe, can result in oversized structural members and increased material costs. The study suggests that future code revisions should incorporate more accurate models for the composite elastic modulus that account for construction quality and material interaction effects.

Integration with Engineering Practice

In the context of the Qinglongchang overpass arch bridge, the CFST members served as key structural components that required high stiffness and load-carrying capacity. The arch ribs of the bridge were fabricated as CFST members to take advantage of the composite action between the steel tube and the concrete core. The experimental data obtained from this project provided valuable input for the structural analysis and design verification of the bridge.

For engineers working on similar projects, the following practical recommendations emerge from this study:

• Conduct full-scale or large-scale model tests on representative CFST members before finalizing the design, particularly for bridges and long-span structures where the composite elastic modulus significantly affects the structural response.
• Implement rigorous construction quality control measures to ensure dense concrete filling, including pre-pouring inspections, in-process monitoring, and post-pouring verification.
• Use the experimentally determined composite elastic modulus in structural analysis models rather than relying solely on code-based values, especially when the project involves innovative or large-scale CFST applications.
• Document and report the construction quality parameters (slump, vibration time, pouring rate) alongside the test results to establish a correlation between construction quality and mechanical performance.

Key Questions and Reflections

A significant question raised by this study is whether the existing design codes adequately reflect the mechanical behavior of CFST members in real-world applications. The consistent overestimation of stiffness by the codes suggests that designers may be relying on overly conservative assumptions, which could lead to unnecessary material usage. However, the opposite concern is also valid: if construction quality is poor, the actual composite elastic modulus may be lower than even the code predictions, leading to unexpected deflections and potential serviceability issues.

The study also highlights the importance of considering construction quality as a design parameter. In traditional reinforced concrete design, construction quality is assumed to be adequate through quality control measures. For CFST members, however, the quality of concrete placement inside the steel tube is a critical factor that directly affects the structural performance, and it should be explicitly addressed in the design process.

Study Insights and Implications

This paper by Li and Ye provides valuable experimental data on the composite elastic modulus and bearing capacity of CFST members, with direct relevance to the design and construction of CFST bridges and buildings. The study demonstrates that the composite action between steel and concrete is more effective than predicted by current codes, but that this effectiveness is contingent upon good construction quality. The findings encourage engineers to adopt a more integrated approach to CFST design and construction, where material properties, structural analysis, and construction quality are considered as interconnected elements rather than isolated design inputs.

In summary, this experimental study on the Qinglongchang overpass CFST members contributes to the understanding of composite elastic modulus and bearing capacity, highlights the limitations of current design codes, and emphasizes the critical role of construction quality in achieving the expected structural performance of concrete-filled steel tubes.