Constitutive modelling of transient thermal strain in high-strength concrete: Validation using a loaded 28-year-old column

This study integrates material modelling, numerical validation, and a full-scale fire test on a 28-year-old high-strength concrete (HSC) column to address gaps in predicting HSC fire behaviour. A new empirical transient strain equation was derived and implemented into OpenSees for Fire as the ConcreteHSThermal material model. Numerical validation showed that the model predicted peak axial displacements with less than 1 mm, whereas the default NSC-based model deviated by up to 10 mm. In the experimental phase, an 89 MPa HSC column was tested under a constant 5500 kN load (35 % of its capacity), a level selected to maintain near-zero net deformation. The column exhibited a stable plateau for the first hour as thermal expansion was balanced by transient strain and stiffness degradation, eventually contracting to 3.43 mm by the end of the heating phase. Only minor spalling without explosive failure was observed. In the post-fire test, the column retained a residual capacity of 6740 kN (42 % of pre-fire strength). • Developed an explicit transient thermal strain model for HSC in OpenSees for Fire. • Tested full-scale aged HSC column. • Validated the model against large-scale tests. • Offered practical guidance for fire resistance, cooling-phase, and post-fire residual assessment.