Shrinkage Deformation and Shrinkage Stress of Concrete-Filled Steel Tube Arches

Literature Overview

This paper by Chen Baochun and Lai Xiuying (2016, Journal of the China Railway Society, Vol. 38, No. 2, pp. 112-123) investigates the shrinkage deformation and resulting shrinkage stresses in concrete-filled steel tube (CFST) arch bridges. Through systematic experimental programs involving 11 CFST members and 2 plain concrete sealed specimens, the authors examine the effects of concrete strength grade, fly ash content, steel tube diameter, and steel ratio on shrinkage behavior, evaluate existing shrinkage prediction models, and analyze shrinkage-induced stresses in CFST arch structures.

Core Technical Content

Concrete shrinkage represents a time-dependent deformation that develops in all concrete structures, but in CFST arches, the interaction between the shrinking concrete core and the constraining steel tube creates complex stress states that significantly influence structural behavior. Unlike conventional reinforced concrete where shrinkage is partially restrained by reinforcement, CFST members experience near-complete radial confinement from the steel tube, creating a fundamentally different shrinkage stress regime.

Experimental Program Design

Shrinkage Deformation Findings

The experimental results establish clear parametric trends:

1. Fly ash content effect: Shrinkage deformation decreases with increasing fly ash content. This is attributed to the pozzolanic reaction consuming calcium hydroxide and producing additional C-S-H gel, which reduces the free water available for shrinkage.
2. Concrete strength grade effect: Shrinkage deformation increases with higher concrete strength grades. Higher-strength concretes typically have lower water-cement ratios and higher cement content, both of which contribute to increased autogenous and drying shrinkage.
3. Steel ratio effect: Shrinkage deformation decreases with increasing steel ratio. The steel tube provides radial restraint that partially inhibits concrete shrinkage, and higher steel ratios provide greater restraint.
4. Steel tube diameter effect: The diameter of the steel tube has relatively minor influence on shrinkage deformation, suggesting that the confinement effect is more related to the ratio of steel area to concrete area than to absolute dimensions.

Shrinkage Prediction Model Evaluation

The ACI 209R-92 model provides the best prediction for CFST members with fly ash content not exceeding 20%. For higher fly ash contents, the model over-predicts shrinkage because it does not fully account for the reduced shrinkage potential of fly ash-modified concrete. The European models (CEB-FIP) systematically under-predict shrinkage for CFST members, likely because they were developed primarily for reinforced concrete and do not adequately represent the unique shrinkage-confinement interaction in steel tube confined concrete.

Shrinkage Stress Analysis in CFST Arches

The stress analysis reveals two distinct components:

The shrinkage self-stress arises from the differential shrinkage between the concrete core and the steel tube within the composite section. As the concrete shrinks, it is restrained by the steel tube, creating compressive stress in the steel and tensile stress in the concrete. This is the dominant stress component and must be included in structural design calculations.

The shrinkage secondary stress, which arises from the geometric incompatibility of shrinkage deformation in statically indeterminate arch structures, is comparatively small and can be neglected in preliminary design stress estimations.

Critique of the Equivalent Cooling Method

The commonly used equivalent cooling method for calculating shrinkage effects in concrete arches is criticized by the authors for CFST applications because:

1. The range of shrinkage deformation values for CFST arches is large and variable, making it difficult to determine a rational equivalent cooling value
2. The method cannot calculate the relatively large shrinkage self-stress component, which is the dominant stress in CFST members
3. The equivalent cooling approach assumes uniform temperature change, which does not accurately represent the non-uniform shrinkage strain distribution in a composite steel-concrete section

Engineering Practice Integration

For steel pipe and structural engineering practice, this research has several important implications:

1. Material specification: For CFST arch bridge applications, concrete mixes with fly ash content up to 20% are recommended to balance shrinkage control with workability and strength requirements. Beyond 20% fly ash, the ACI 209R-92 model requires modification for accurate shrinkage prediction.
2. Steel tube selection: The steel ratio should be optimized to provide adequate confinement while considering economic factors. The finding that steel tube diameter has minor influence on shrinkage suggests that the steel ratio (area proportion) is the more important design parameter.
3. Long-term structural behavior: The significant shrinkage self-stress must be incorporated into the design of CFST arch bridges to prevent cracking in the concrete core and excessive stress in the steel tube. This is particularly important for long-span arches where shrinkage-induced stresses can be comparable to live load stresses.
4. Construction sequencing: Time-dependent shrinkage effects should be considered in the construction sequencing of CFST arch bridges, particularly for the timing of load application and temporary support removal.

Common Issues and Countermeasures

Key Questions and Reflections

The research raises several important questions for future investigation. First, the long-term shrinkage behavior (beyond the experimental duration) and its interaction with creep in CFST arches under sustained loading requires further study. Second, the effect of environmental conditions — temperature cycling, humidity variation, and freeze-thaw exposure — on CFST shrinkage in different climate regions is not addressed. Third, the practical implementation of shrinkage stress calculations in routine design workflows needs standardization, as the current approach requires detailed material property data that may not always be available at the design stage. Finally, the interaction between shrinkage-induced stresses and fatigue from traffic or wind loading in CFST arch bridges represents an important combined loading scenario that warrants investigation.

Study Insights and Engineering Implications

This comprehensive study establishes that concrete shrinkage is a first-order design consideration for CFST arch bridges, not a secondary effect that can be neglected or approximated with simplified methods. The clear recommendation against using the equivalent cooling method for CFST arches represents an important methodological advancement that should be reflected in design codes and practice guidelines. For steel pipe manufacturers and structural engineers involved in CFST arch bridge projects, the key actionable findings are: select concrete mixes with fly ash content at or below 20% for reliable shrinkage prediction using ACI 209R-92; ensure adequate steel ratio to provide shrinkage restraint; and include shrinkage self-stress in all design calculations for CFST arch members. This work exemplifies the importance of time-dependent material behavior in composite steel-concrete structures and reinforces the need for integrated design approaches that consider both mechanical and time-dependent effects throughout the structure's service life.