Size-Dependent Constitutive Model for Normal- and High-Strength Unconfined Concrete

Document Type : Original Article

Authors

Department of Civil Engineering, University of Minho, Azurém, Guimarães, Portugal

Abstract

This study presents a unified stress–strain model for unconfined concrete under axial compression that accounts for key size-dependent effects and varying material strength levels. The proposed formulation is designed to overcome limitations in existing constitutive models, particularly their restricted applicability across different specimen geometries, strength classes, and their limited ability to consistently represent both pre-peak and post-peak behavior within a single coherent framework. The model adopts a two-stage representation of the compressive response, allowing independent yet physically consistent characterization of the nonlinear ascending branch and the strain-softening post-peak regime. The formulation explicitly incorporates the influence of concrete strength and specimen geometry, enabling the model to capture transitions from more ductile behavior in lower-strength concrete to increasingly brittle responses in higher-strength materials. The proposed approach is calibrated and validated using an extensive experimental database covering a wide range of concrete strengths, specimen dimensions, and aspect ratios compiled from multiple independent research programs. Comparisons with experimental stress–strain curves demonstrate that the model reliably reproduces both the peak response and the subsequent softening behavior across diverse testing conditions, including cases with pronounced geometric and material variability. The results confirm that the proposed formulation provides a simplified but robust representation of the compressive behavior of unconfined concrete, making it suitable for nonlinear structural analysis and performance-based engineering applications without the need for case-specific recalibration.

Keywords

Main Subjects

References

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History

  • Receive Date: 02 July 2026
  • Revise Date: 22 July 2026
  • Accept Date: 08 September 2026
  • First Publish Date: 08 September 2026

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