Experimental Research on Local Height Reinforcement of Square Frame Columns Using Steel Tube Confined Concrete

Literature Overview

This paper by Quan Xueyou, Zhuang Lengkai, Wu Dekuan, Hu Ying, and Liu Bao from Chongqing University, published in Building Structure (2020, Vol. 50, Issue 13, pp. 64-70), addresses a critical practical problem in structural engineering: the reinforcement of reinforced concrete columns that exhibit localized concrete strength defects. The authors propose a novel reinforcement method based on the steel tube confined concrete concept, in which a steel tube is installed around the defective zone of the column, and cement slurry is pressure-injected to ensure intimate contact between the concrete core and the steel tube.

Problem Statement and Proposed Solution

In construction practice, localized concrete strength defects in reinforced concrete columns can arise from various causes, including improper compaction, segregation during placement, contamination of the concrete mix, or damage during subsequent construction activities. These defects compromise the structural integrity of the column, particularly in the affected zone, and necessitate effective and reliable reinforcement measures.

The proposed reinforcement method has the following key characteristics:

The requirement that the steel tube outer surface be flush with the undamaged column surface is a significant practical constraint. This ensures that the reinforced column maintains its original architectural appearance and does not interfere with adjacent structural elements or finishes.

Experimental Program and Results

Three locally reinforced column specimens were tested under axial compression. The experimental results provide the following key observations:

1. The reinforcement method effectively improves the compressive bearing capacity of the defective zone. The steel tube provides lateral confinement to the defective concrete, enhancing its triaxial strength and ductility.
2. Prior to yielding, the compressive stiffness of the reinforced defective zone is comparable to that of the undamaged zone. This indicates that the reinforcement does not introduce significant stiffness discontinuity under service loads.
3. After yielding, compressive deformation is concentrated almost entirely in the defective zone. This observation is critical for understanding the post-ultimate behavior and has implications for the ductility assessment of reinforced columns.
4. The Mander model for confined concrete provides good agreement with the experimental results, confirming that the confinement mechanism operates as expected even in the context of localized reinforcement.

Analysis of Confinement Mechanism

The effectiveness of the reinforcement method relies on the confinement mechanism of the steel tube. When the defective concrete is subjected to axial compression, it tends to expand laterally. The steel tube resists this lateral expansion, generating a confining pressure on the concrete core. This confining pressure increases the triaxial compressive strength of the concrete according to the well-established relationship between confining pressure and concrete strength.

The pressure-injected cement slurry plays a dual role: it ensures intimate contact between the concrete and the steel tube, preventing slip and debonding at the interface; and it fills any voids between the irregular concrete surface and the smooth steel tube inner surface, thereby maximizing the effective confinement area.

From a welding and fabrication perspective, the steel tube reinforcement involves circumferential and longitudinal welds to join the tube segments. The quality of these welds is critical, as they must withstand the full confining pressure without cracking or separation. The weld design should follow applicable standards such as GB/T 985.1 for welding preparation and GB/T 986 for weld dimensions, with particular attention to the root pass quality and the absence of incomplete fusion defects.

Engineering Practice Considerations

The proposed method has several advantages for field implementation. The steel tube can be fabricated and transported to the site as a prefabricated component, reducing on-site construction time. The pressure injection of cement slurry can be performed using standard grouting equipment, and the flush surface finish eliminates the need for additional finishing work.

However, several practical challenges must be addressed. First, the access for installing the steel tube around the column must be ensured, which may require temporary removal of adjacent structural elements or formwork. Second, the quality of the cement slurry injection must be verified through non-destructive testing, such as ultrasonic testing, to ensure complete filling of the annular space. Third, the long-term durability of the cement slurry interface under cyclic loading or environmental exposure should be evaluated, particularly in corrosive environments where chloride ingress could compromise the steel tube.

The finding that post-yield deformation concentrates in the defective zone raises concerns about the overall ductility of the reinforced column. In seismic design, the concentration of deformation in a localized zone may lead to premature failure of the reinforced section, potentially compromising the energy dissipation capacity of the column. Future research should investigate the seismic performance of locally reinforced columns and develop appropriate design guidelines that account for this deformation concentration.

Study Insights

This research demonstrates that the steel tube confined concrete concept can be effectively applied to localized reinforcement problems, providing a practical solution to a common construction quality issue. The agreement between the Mander model and experimental results validates the underlying confinement mechanism and provides confidence in extending this method to other structural configurations.