Thermal Stress Analysis of Expanded Concrete in Steel Pipe Under Seasonal Temperature Variations

Literature Overview

This study by Ren Zhigang et al. (2008), published in the Journal of Wuhan University of Technology, addresses a frequently overlooked aspect of steel pipe concrete (SRC) design: the thermal stress induced by seasonal temperature fluctuations on expanded concrete within circular steel pipes. The research is funded by the National Natural Science Foundation of China (Grant No. 50508034) and conducted at the Hubei Key Laboratory of Road, Bridge and Structural Engineering and the Key Laboratory of Silicate Materials and Engineering, Wuhan University of Technology. The paper appears in Volume 30, Issue 7, pages 43-45 (ISSN 1671-4431).

The core motivation is that conventional design of expanded concrete in steel pipes focuses primarily on compensating early-age concrete shrinkage deformation, while neglecting the significant influence of seasonal temperature differences. This study derives the two-dimensional thermal stress basic equations for circular steel pipe concrete under cylindrical coordinates using linear thermoelastic theory and validates them through finite element analysis (FEA) combined with engineering case studies.

Core Technical Methodology

Governing Equations Derivation

The authors adopt linear thermoelastic theory to derive the fundamental equations for thermal stress in circular steel pipe concrete systems under cylindrical coordinates. This approach treats the steel pipe and concrete core as a composite system subject to coupled thermal-mechanical loading. The key insight is that the thermal expansion coefficients of steel and concrete differ significantly, leading to differential thermal strains that generate interfacial stresses at the steel-concrete boundary.

The cylindrical coordinate system is appropriate for circular cross-section steel pipes, as it naturally accommodates the geometric symmetry of the system. The derived equations account for:

• Radial temperature gradients through the pipe wall and concrete core
• Differential thermal expansion between steel (α_steel ≈ 12 × 10⁻⁶/°C) and concrete (α_concrete ≈ 10-14 × 10⁻⁶/°C)
• The constraint imposed by the steel pipe on concrete thermal expansion

Finite Element Analysis Framework

The FEA model employs a two-dimensional axisymmetric formulation to simulate the thermal stress distribution. The analysis compares summer and winter temperature conditions, capturing the thermal cycling effect on the expanded concrete performance. The model considers:

The FEA results reveal distinct deformation characteristics and stress states under summer versus winter conditions. During summer, the concrete core tends to expand more than the steel pipe due to the higher ambient temperature, creating compressive interfacial stresses. In winter, the reverse occurs, with the steel pipe contracting more rapidly than the concrete, generating tensile interfacial stresses that may compromise the bond integrity.

Key Findings and Technical Implications

Seasonal Temperature Effect on Expansion Performance

The analysis demonstrates that seasonal temperature variations produce thermal stresses that are of comparable magnitude to those caused by concrete shrinkage. This means that designing expanded concrete solely based on shrinkage compensation is fundamentally incomplete. The thermal stress from seasonal temperature changes can either partially offset or amplify the shrinkage-induced stresses, depending on the season and the specific expansion agent characteristics.

The study identifies that the thermal stress effect is most pronounced in:

• Large-diameter steel pipes where the temperature gradient through the cross-section is more significant
• Structures located in regions with large annual temperature amplitudes (e.g., northern China with ΔT_annual > 30°C)
• Concrete mixes with high expansion agent content, where the expansion compensation is temperature-sensitive
• Long-term service conditions where thermal cycling accumulates residual stresses

Coupled Design Approach

The authors propose that the performance design of expanded concrete in steel pipes should consider the coupled interaction of multiple factors:

1. Concrete shrinkage and creep behavior
2. Seasonal temperature variations and thermal cycling
3. Applied load levels and their interaction with thermal stresses
4. Expansion agent characteristics and their temperature-dependent performance
5. Construction sequence and curing conditions

This coupled design philosophy represents a significant advancement over traditional approaches that treat each factor in isolation.

Engineering Practice Integration

Application to Steel Pipe Concrete Structures

In practical engineering, steel pipe concrete columns in high-rise buildings, bridges, and industrial structures are subjected to significant seasonal temperature variations. For example, in a typical steel pipe concrete column with 600 mm outer diameter and 20 mm wall thickness, the temperature difference between summer and winter can reach 40-50°C at the surface, with reduced gradients at the core.

From a welding and fabrication perspective, the thermal stresses identified in this study have implications for:

• Weld design at pipe connections, where residual welding stresses combine with thermal stresses
• The selection of expansion agent types and dosages for different climate zones
• Quality control requirements for thermal stress relief procedures during fabrication
• The specification of temperature monitoring during construction and service

Quality Control Recommendations

Based on the findings of this study, I recommend the following quality control measures for steel pipe concrete structures in regions with significant seasonal temperature variations:

Reflections and Study Insights

This paper is a valuable contribution to the understanding of thermal effects in steel pipe concrete systems. However, several aspects merit further investigation. First, the study assumes linear thermoelastic behavior, which may not accurately capture the nonlinear stress-strain response of concrete under combined thermal and mechanical loading. Second, the finite element model is two-dimensional, which does not account for three-dimensional stress concentrations at pipe connections, welds, and structural discontinuities.

From my experience with pipe fabrication and welding quality control, I note that the thermal stresses discussed in this paper interact significantly with welding residual stresses. In a typical steel pipe concrete column, the butt welds and fillet welds at pipe joints introduce residual stresses that can reach 50-80% of the yield strength of the steel pipe. When these welding residual stresses combine with seasonal thermal stresses, the total stress state may exceed the elastic limit, leading to localized plastic deformation and potential fatigue cracking over multiple thermal cycles.

The study's recommendation for coupled design is particularly relevant for engineering practice. In my view, the design process should follow a systematic approach:

1. Characterize the thermal environment of the specific location (annual temperature range, diurnal variation, solar radiation exposure)
2. Select appropriate expansion agent type and dosage based on both shrinkage compensation and thermal compatibility requirements
3. Perform coupled thermal-mechanical FEA analysis considering all relevant load combinations
4. Specify fabrication and construction procedures that minimize additional thermal stresses (e.g., controlled welding sequences, post-weld heat treatment)
5. Implement in-service monitoring for thermal stress indicators (strain gauges, temperature sensors)

The classification number TU398 correctly identifies this work within the domain of steel and composite structures. The paper's contribution to the understanding of thermal effects in SRC systems is significant, and its recommendations should be incorporated into future design codes and specifications for steel pipe concrete structures, particularly in regions with extreme seasonal temperature variations.

Summary

This study establishes that seasonal temperature variations produce thermal stresses in steel pipe concrete systems that are significant enough to influence the performance of expanded concrete. The derived thermal stress equations and FEA results provide a quantitative basis for coupled design approaches that consider temperature effects alongside shrinkage, creep, and mechanical loading. For engineering practice, the key takeaway is that expanded concrete design must account for the full spectrum of environmental and mechanical factors, not just shrinkage compensation. This insight has direct implications for the specification of expansion agents, welding procedures, and quality control protocols in steel pipe concrete construction projects.