This study investigates the differences in shear behaviour between reinforced beams made of slag-based alkali-activated concrete (AAC) and Portland cement concrete (PCC), considering the effects of material properties, age, shrinkage, and strength class. A series of tests were conducted to analyze tensile strength, elastic modulus, autogenous and drying shrinkage, and their impact on shear performance over a period of 28 days to 6 months.

Results indicate that AAC mixtures exhibit lower tensile strength and elastic modulus compared to PCC mixtures, with the highest strength class (C50) of AAC showing an unexpected reduction in these properties over time. Additionally, AAC mixtures demonstrated significantly higher autogenous and drying shrinkage, with the S-AAC-C50 mixture exhibiting up to three times the shrinkage of its PCC counterpart. The increased shrinkage likely contributed to the development of micro-cracks, which may affect long-term performance.

Despite reductions in stiffness and tensile capacity, AAC mixtures showed a greater increase in crack stabilization stage, associated with aggregate interlock, dowel action, and residual tensile stress, leading to an improved ultimate shear failure capacity over time. The S-AAC-C30 and S-AAC-C50 mixtures exhibited increases of +83% and +38% in the crack stabilization stage, respectively, compared to +46% and −24% for PCC mixtures. This suggests that while AAC exhibits lower initial mechanical properties, its shear capacity benefits from an extended crack stabilization stage.

Overall, the study concludes that while AAC experiences higher shrinkage and a reduction in tensile strength and stiffness over time, its shear behavior is enhanced due to an extended crack stabilization mechanism. However, the exact role of shrinkage and its long-term implications remain unclear and require further investigation.