Constitutive Model of Lightweight Aggregate Concrete Confined by Steel Tubes

Literature Overview

The paper by Wu Dongyang, Fu Zhongqiu, Ji Bohai, and Wang Zhanfei, published in the Journal of Yangzhou University (Natural Science Edition) in 2019 (Vol. 22, No. 1, pp. 67-73), addresses the development of a constitutive model for lightweight aggregate concrete (LWAC) confined by steel tubes. Supported by the National Natural Science Foundation of China (Youth Fund Project No. 51208176), the research was conducted at Hohai University and Shenyang Jianzhu University. The authors compared five commonly used core concrete constitutive models, evaluated their applicability to steel tube-concrete members with high-strength, ordinary, and lightweight aggregate concrete, and proposed a modified model specifically for steel tube-confined lightweight aggregate concrete.

Background and Motivation

Lightweight aggregate concrete offers significant advantages in reducing structural self-weight, which is particularly beneficial for long-span structures, offshore platforms, and structures in seismic zones. However, the use of LWAC in steel tube-concrete (SRC) members introduces challenges in terms of material behavior characterization and structural analysis. The confined concrete behavior in SRC members differs substantially from unconfined concrete, and existing constitutive models developed for ordinary concrete may not accurately represent the response of LWAC under confinement.

The five constitutive models compared in the study include:

Comparative Analysis of Constitutive Models

The authors systematically compared the five models by computing the axial compression behavior of SRC members and comparing the predicted load-carrying capacity and stress-strain curves with experimental data. The comparison revealed several important insights:

1. The plastic damage model produced curves that better matched the experimental trends compared to the other models.
2. Models developed for ordinary concrete tend to overestimate the strength and stiffness of LWAC under confinement.
3. The confinement effectiveness parameter, which relates the confining pressure to the unconfined compressive strength, requires modification for LWAC due to its different microstructure and pore characteristics.
4. The post-peak behavior of LWAC under confinement is more ductile than that of ordinary concrete, which is not adequately captured by existing models.

Proposed Constitutive Model

Based on the comparative analysis, the authors proposed a modified constitutive model for steel tube-confined lightweight aggregate concrete. The model incorporates the following key features:

• A modified confinement effectiveness coefficient that accounts for the lower density and higher porosity of LWAC.
• A revised relationship between the confining pressure and the unconfined compressive strength, calibrated against experimental data.
• A plastic damage framework that better captures the nonlinear stress-strain response, particularly in the post-peak region.
• Parameters that can be calibrated for different LWAC grades and steel tube geometries.

The proposed model demonstrates improved accuracy in predicting both the load-carrying capacity and the stress-strain curve of steel tube-confined LWAC members. The finite element analysis using the proposed model showed good agreement with experimental results, validating the model's applicability for structural analysis.

Technical Discussion

The development of constitutive models for confined concrete is a challenging task that requires careful consideration of several factors:

1. Microstructural differences: LWAC contains lightweight aggregates with higher porosity and lower density than natural aggregates, leading to different crack propagation patterns and confinement effectiveness.
2. Steel tube geometry: The confinement pressure distribution depends on the steel tube's cross-sectional shape, wall thickness, and slenderness ratio.
3. Strain rate effects: Although not addressed in this study, the strain rate sensitivity of LWAC may differ from ordinary concrete, particularly at elevated temperatures.
4. Long-term behavior: The creep and shrinkage characteristics of LWAC are generally more pronounced than those of ordinary concrete, which may affect the long-term performance of SRC members.

The plastic damage model framework used in this study is well-suited for capturing the nonlinear behavior of confined concrete because it separates the damage (cracking) and plastic deformation components, allowing for more accurate representation of the complex material response under multiaxial stress states.

Engineering Practice Implications

For engineers designing SRC members using LWAC, the following considerations are important:

• The proposed constitutive model should be used in finite element analyses to ensure accurate prediction of structural behavior.
• Design codes should be consulted for any specific provisions regarding the use of LWAC in composite members, as existing codes may not adequately address this combination.
• Quality control during construction is critical, as the properties of LWAC can vary significantly depending on the aggregate source, mix design, and curing conditions.
• The confinement effectiveness of the steel tube should be verified through detailed analysis, particularly for members with high slenderness ratios or thin steel tubes.

Study Insights and Reflections

This research makes a valuable contribution to the understanding of confined LWAC behavior and provides a practical constitutive model for engineering applications. The systematic comparison of five existing models is particularly useful, as it helps engineers understand the limitations of current approaches and select the most appropriate model for their specific applications. However, I note that the study could be strengthened by including additional experimental data from different laboratories, as the variability in LWAC properties can be significant. Furthermore, the model's applicability to other types of composite members, such as rectangular steel tube-concrete columns or steel-reinforced concrete members with partial encasement, should be investigated. The proposed model represents a step forward in the rational design of SRC members using LWAC, but further validation through full-scale testing and long-term monitoring of real structures is essential for building confidence in its practical application.