Influence of Fly Ash and Coarse Aggregate Proportions on Fracture Parameters of Alkali Activated Self-Compacting Concrete

Alkali-Activated Self-Compacting Concrete (AASCC) offers a sustainable alternative to conventional cementitious systems, with lower carbon emissions and improved durability. However, its fracture behaviour received limited attention. This study investigates the influence of Fly Ash (FA) content and Coarse Aggregate (CA) volume on the fracture performance of slagbased AASCC. Fifteen mix designs were developed by partially replacing Ground Granulated Blast Furnace Slag (GGBS) with FA (20%–50% by weight) and varying CA volume from 40% to 60% of total aggregate volume. An OPC-based SCC mix at each volume fraction of CA was developed as a control mix. Beam specimens were tested under three-point bending to evaluate fracture properties using the Work of Fracture Method (WFM) and Size Effect Method (SEM). Fresh and hardened strength properties were first assessed. Fracture energy (GF) was determined using WFM for all 15 mixes. A 20% FA replacement improved mechanical strength and GF across all CA volumes at 28 days, outperforming OPC-based SCC and FA-free alkaliactivated mixes. Fracture parameters such as fracture toughness (KIC), fracture energy (Gf), effective fracture process zone length (Cf), and critical crack tip opening displacement (CTODC) were calculated using SEM for five mixes with 40% CA. Results showed the sensitivity of fracture parameters to material variations. Peak values observed were Gf = 46.33 N/m, Cf = 42.33 mm, KIC = 34.44 MPa√mm, and CTODC = 0.028 mm. The average GF/Gf ratio was 3.43, indicating consistent scaling behaviour. The GF/Gf ratio helps to calibrate simplified testing methods for field applications, making fracture-based quality control and performance assessment more accessible. The findings suggest that incorporating 20% FA and 60% CA enhances the fracture performance of AASCC, making it more possible for structural applications requiring high energy absorption and crack resistance. Major Findings: Increasing the coarse aggregate (CA) volume from 40% to 60% significantly improves fracture energy (GF) and characteristic length (Lch) across all fly ash (FA) levels. Replacing GGBS with 20% FA leads to the most significant improvement in fracture energy (Gf), fracture toughness (KIC), and fracture process zone length (Cf), indicating enhanced ductility and toughness. The brittleness number (β) highlights the nonlinear fracture behaviour of AASCC, indicating the need for design based on nonlinear fracture mechanics instead of linear elastic fracture mechanics.