Lateral Wall Pressure on Square Steel Tube Concrete Columns During Wet Concrete Placement

Literature Overview

The paper by Chen Zhiyi, Qin Xiaoqi, Shen Zuyuan, and Chen Guojin from Tongji University and Zhejiang Hangxiao Steel Structure Co., Ltd., published in Building Structure (2003, Vol. 33, No. 7, pp. 27-28), presents experimental results on the lateral wall pressure and hoop strain of square steel tube concrete (CFST) columns during wet concrete placement. Three square CFST column specimens were tested, with varying concrete slump values and with or without vibration. The study investigates how these placement parameters influence the lateral pressure exerted on the steel tube walls and the resulting hoop strain in the steel tube. This research is directly relevant to the construction phase of CFST columns, where the temporary lateral pressure from wet concrete can affect the structural integrity and long-term performance of the column.

Core Technical Findings

The experimental results reveal the following key findings:

1. Slump effect: Higher concrete slump values result in higher lateral wall pressures. This is because more fluid concrete exerts greater hydrostatic pressure on the steel tube walls.
2. Vibration effect: Vibration during concrete placement increases the lateral wall pressure compared to non-vibrated placement. Vibration causes the concrete to settle and flow, increasing the effective pressure on the walls.
3. Hoop strain correlation: The hoop strain in the steel tube is directly correlated with the lateral wall pressure, with higher pressures producing greater hoop strains.
4. Square section behaviour: The square geometry of the steel tube introduces corner effects, where stress concentrations may occur at the corners due to the change in curvature from flat walls to corners.

Test Configuration and Parameters

The following table summarises the test parameters and conditions:

The experimental setup required careful instrumentation to measure both the lateral wall pressure and the hoop strain simultaneously. Lateral pressure was likely measured using pressure cells or load cells attached to the inner surface of the steel tube, while hoop strain was measured using strain gauges bonded to the outer surface of the steel tube. The accuracy of these measurements is critical, as the lateral pressures during concrete placement can be substantial and may vary with time and height.

Lateral Pressure Mechanism and Design Implications

The lateral pressure exerted by wet concrete on the steel tube walls is primarily hydrostatic in nature, governed by the unit weight of the concrete and the height of the concrete column. However, several factors modify this basic hydrostatic pressure:

• Concrete rheology: The slump and workability of the concrete affect its ability to flow and exert pressure. More fluid concrete exerts pressure closer to the theoretical hydrostatic value, while stiffer concrete may not fully develop hydrostatic pressure.
• Placement rate: The rate of concrete placement affects the pressure build-up. Rapid placement can result in higher pressures due to the dynamic component and reduced time for consolidation.
• Vibration: Vibration reduces the internal friction of the concrete, allowing it to flow more freely and exert higher pressures. It also causes the concrete to settle, which can lead to transient pressure spikes.
• Steel tube stiffness: The stiffness of the steel tube affects the equilibrium between the lateral pressure and the hoop stress. A stiffer tube deforms less, potentially allowing higher pressures to develop before equilibrium is reached.

For design purposes, the lateral pressure during concrete placement should be considered as a temporary load case that the steel tube must resist without permanent deformation or damage. The design should ensure that:

1. The hoop stress induced by the lateral pressure does not exceed the yield strength of the steel tube.
2. The hoop strain remains within elastic limits to avoid permanent deformation.
3. The steel tube does not buckle under the combined effects of axial load (from the weight of the concrete) and lateral pressure.
4. The concrete-steel interface maintains adequate bond after placement, without separation or damage.

Construction Practice Recommendations

Based on the findings of this study, the following recommendations can be made for the construction of square CFST columns:

1. Slump control: The concrete slump should be carefully controlled to balance workability and lateral pressure. A slump that is too high may cause excessive lateral pressure, while a slump that is too low may result in poor concrete placement and compaction.
2. Vibration strategy: Vibration should be used judiciously, with controlled vibration parameters (amplitude, frequency, duration) to ensure adequate compaction without excessive lateral pressure. Layered placement with controlled layer thickness can also help manage the lateral pressure.
3. Placement rate management: The concrete placement rate should be controlled to allow adequate time for pressure dissipation between layers. This is particularly important for tall columns where the cumulative pressure can be significant.
4. Instrumentation and monitoring: For critical structures, real-time monitoring of lateral pressure and hoop strain during concrete placement should be considered. This allows for immediate corrective action if pressures exceed design limits.
5. Steel tube design for construction loads: The steel tube should be designed to resist not only the service loads but also the temporary construction loads from wet concrete placement. This may require thicker steel tubes or additional temporary bracing.

Key Questions and Reflections

Several aspects of this study warrant further consideration:

1. The study is limited to three specimens, which may not provide a statistically robust dataset. Further testing with a wider range of parameters would strengthen the conclusions.
2. The long-term effect of the lateral pressure on the concrete-steel bond is not addressed. High lateral pressures during placement could potentially damage the interface, affecting the long-term composite action of the column.
3. The effect of concrete temperature on lateral pressure is not considered. Hot concrete (such as mass concrete or concrete placed in hot weather) may have different rheological properties and exert different lateral pressures.
4. The study focuses on square steel tubes, but the findings may not directly apply to circular steel tubes, which have different stress distribution characteristics under lateral pressure.

Study Insights and Implications

This research provides valuable experimental data on the construction-phase behaviour of square CFST columns, filling an important gap in the understanding of these composite systems. The demonstration that slump and vibration significantly affect lateral wall pressure highlights the importance of construction process control in achieving the intended structural performance. For engineers designing and constructing CFST columns, this study underscores the need to consider construction loads in the design phase and to implement appropriate construction procedures to manage lateral pressures. The integration of these findings into construction specifications and quality control procedures would contribute to the reliability and durability of CFST columns in practice. Future research should extend to larger-scale columns, different concrete types, and different steel tube geometries to provide a more comprehensive understanding of the construction-phase behaviour of CFST columns.