Long-term Deformation of Steel Tube Micro-expansive Concrete Axially Compressed Short Columns

Literature Overview

The study by Wang Yuyin, Geng Yue, and Zhang Sumei (2011), published in China Journal of Highway and Transport, investigates the long-term deformation behavior of circular steel tube micro-expansive concrete (CFST) axially compressed short columns. This research is directly relevant to CFST arch bridges, where micro-expansive high-performance concrete is commonly used to enhance the composite action between steel and concrete. The authors conducted experimental testing on short columns with varying axial compression ratios and loading ages, and applied the incremental integration method to analyze long-term deformation using five different concrete shrinkage and creep models. The study addresses a critical practical concern: the time-dependent behavior of CFST members under sustained axial loading, which directly affects the long-term serviceability and safety of bridge structures.

Core Technical Points

The research focuses on the interaction between concrete creep and shrinkage, steel tube elastic deformation, and the micro-expansion effect of the concrete on the long-term axial deformation of CFST short columns. The incremental integration method was employed to numerically integrate the time-dependent constitutive relationships, accounting for the aging effect of concrete.

The study reveals that the corrected EC2, MC90, and ACI 209 models provide the highest accuracy for analyzing CFST members with loading ages not exceeding 28 days. This finding is practically significant because most CFST bridge members are loaded within the first month after construction, when the concrete is still young and the creep rate is highest.

The micro-expansion effect of the concrete plays a crucial role in the long-term behavior. Micro-expansive concrete generates internal expansive stress that compresses the steel tube, creating a pre-compression state that partially offsets the tensile stress in the steel tube under axial loading. This pre-compression effect reduces the overall axial deformation and improves the long-term stability of the member.

The parametric analysis identified the following key influences on long-term deformation:

• Steel ratio: Higher steel ratio reduces long-term deformation by providing greater elastic stiffness and reducing the relative contribution of concrete creep.
• Loading age: Earlier loading (younger concrete) results in greater long-term deformation due to higher initial creep rate.
• Sustained load duration: Long-term deformation increases logarithmically with time, with the majority of creep deformation occurring within the first year.
• Concrete strength: Higher concrete strength reduces long-term deformation due to lower creep coefficient.

Process and Standards Analysis

From a welding and fabrication standpoint, the long-term deformation of CFST members has direct implications for weld design and quality control. The differential deformation between steel tube and concrete core under sustained loading generates interface stresses that can affect weld integrity over time.

For the fabrication of CFST short columns used in arch bridges, the following standards and practices are relevant:

The micro-expansive concrete used in these applications typically contains expansive agents such as calcium hydroxide or magnesium oxide, which generate internal pressure during hydration. This internal pressure must be controlled to prevent excessive radial expansion that could damage the steel tube or create unacceptable residual stresses in the welds. The expansive pressure should be limited to 0.3-0.5 MPa to ensure proper composite action without overstressing the steel tube.

Weld quality control for CFST members subjected to long-term axial loading should include:

• Visual inspection (VT) of all welds per ISO 17637
• Magnetic particle testing (MT) of longitudinal and transverse welds per ISO 17638
• Ultrasonic testing (UT) of transverse butt welds per ISO 17640
• Hydrostatic testing of assembled members per GB/T 24160
• Hardness testing of HAZ per GB/T 4340 to ensure no excessive hardening

Integration with Engineering Practice

CFST arch bridges are widely used in mountainous regions of China, where long spans and heavy loads require efficient structural systems. The long-term deformation of CFST members directly affects the arch shape, which is critical for the structural performance of arch bridges. Excessive long-term deformation can lead to:

1. Increased bending moments in the arch ribs due to deviation from the ideal arch shape.
2. Differential settlement of supports due to uneven long-term deformation.
3. Accumulated creep deformation that reduces the effective cross-sectional area over time.
4. Potential delamination at the steel-concrete interface due to differential shrinkage.

Engineers should consider the following practical measures to control long-term deformation:

• Use of high-strength concrete (C50 or higher) to reduce creep coefficient.
• Optimization of steel ratio to balance stiffness and cost.
• Delayed loading where possible to allow concrete to reach higher strength before sustained loading.
• Use of micro-expansive concrete to maintain interface pressure and prevent delamination.
• Regular monitoring of arch shape and deformation during the service life of the bridge.

Key Questions and Reflections

The study provides valuable insights into the long-term behavior of CFST members, but several aspects warrant further consideration. First, the micro-expansive concrete used in the study may not represent the full range of expansive agents available in practice. Different expansive agents have different expansion rates and magnitudes, which can significantly affect the long-term behavior.

Second, the study focuses on short columns under pure axial compression. In practice, CFST arch ribs are subjected to combined axial compression and bending, which creates a non-uniform stress distribution across the cross-section. The long-term deformation under combined loading is more complex and may differ significantly from pure axial compression.

Third, the environmental conditions in which CFST arch bridges operate can affect long-term deformation. Temperature variations, humidity, and exposure to corrosive agents (such as deicing salts) can accelerate concrete deterioration and affect the long-term performance. The study does not address these environmental factors, which is an important limitation for practical applications.

From a welding perspective, the long-term interface stresses between steel and concrete can affect the fatigue life of welds. The cyclic nature of traffic loading combined with sustained axial compression creates a complex stress state that may lead to fatigue cracking at weld toes. This interaction between long-term deformation and weld fatigue should be investigated further.

Study Insights and Implications

This research contributes significantly to the understanding of long-term behavior of CFST members with micro-expansive concrete. The comparison of different creep and shrinkage models provides practical guidance for engineers on model selection. The finding that EC2, MC90, and ACI 209 models (with appropriate corrections) provide the highest accuracy for loading ages up to 28 days is particularly useful for practical design. The study also highlights the importance of the concrete aging effect, which should be considered in all long-term deformation analyses of CFST members. Engineers working on CFST arch bridges should incorporate these findings into their design and monitoring practices to ensure long-term structural safety and serviceability.