Fly Ash Replacement in Conventional Concrete: ABAQUS-Based Assessment of Compressive Strength and Damage Evolution

Partial replacement of Portland cement with Class C fly ash can alter concrete compressive behaviour, but the response may differ by concrete grade and replacement level. This study assessed compressive strength and damage evolution in a defined set of numerical concrete-cube models. Fifteen 150 × 150 × 150 mm cube models representing M20, M30 and M50 concrete were analysed in ABAQUS at fly ash replacement levels of 0%, 10%, 20%, 30% and 40%. The models employed the Concrete Damaged Plasticity formulation, displacement-controlled compression and surface-to-surface contact with cylindrical loading components. A global mesh size of approximately 8 mm was selected after a mesh-sensitivity assessment of the M30 models. Peak load, calculated compressive strength, stress distributions and tensile and compressive damage variables were examined. The highest predicted compressive strengths within the investigated range occurred at 20% replacement: 32.9191 MPa for M20, 48.1627 MPa for M30 and 92.9013 MPa for M50. Relative to their respective control models, these values represented increases of 27.52%, 27.62% and 17.57%. At 40% replacement, predicted strengths were lower than the control values for all grades. The five M30 numerical predictions differed from published 28-day experimental strengths by a mean absolute percentage error of 3.27%, although specimen geometries differed. Within the adopted numerical framework, strength responses varied by grade and replacement level. The findings support comparative interpretation of these models but do not establish a universal optimum or independently validate local damage fields.