Size and Shape Effects on Mass Concrete Strength: Experimental Evaluation and Model Development

This study investigates the influence of specimen size and shape on the compressive strength of mass concrete, aiming to develop a robust predictive model for large-scale structural applications, particularly in hydraulic and foundation engineering. Ordinary Portland Cement (OPC) R42.5 and three coarse aggregate gradations (20–40 mm, 40–80 mm, and 80–120 mm) with a maximum aggregate size of 150 mm were used, with water-cement ratios ranging from 0.40 to 0.50. Specimens of different sizes (cubes: 150–450 mm; cylinders: Φ150×300 mm to Φ450×900 mm) were tested at 28, 90, and 180 days. The study finds that compressive strength consistently decreases with increasing specimen size, with a more pronounced size effect in cylinders than in cubes. A multi-factor model, extending Bažant's Size Effect Law (SEL), was developed to incorporate curing age, water-cement ratio, and maximum aggregate size. The model's predictive accuracy (R² = 0.84 for cubes, R² = 0.86 for cylinders) significantly outperforms classical models such as Bažant’s SEL (R² = 0.33) and Abrams’ water-cement ratio formula (R² = 0.31). This model offers improved applicability for mass concrete with large aggregates and provides valuable insights for safety evaluations, code calibration, and the design of large-scale structures.