A Comprehensive Analysis of the Influence of Fly Ash on the Bond Properties between Reinforcing Steel and Concrete

A significant contributor to CO₂ emissions is cement production, thus raising interest in fly ash being accepted as a sustainable alternative. The study attempts to quantify the variations in bond strength for 0–30% fly ash replacement, interrelate the properties of fly ash to the microstructure of the interface transition zone (ITZ), and establish the optimum replacement ratios for structural applications. The experiments were carried out on the Grade 53 OPC concrete (20 MPa target strength, 0.5 water-binder ratio) using measurements for compressive (ASTM C39), tensile (ASTM C496), and bond strengths (ASTM C900). The results found the 10% fly ash replacement optimal, enhancing compressive strength by 7.3% (23.6 MPa), tensile strength by 5% (4.2 MPa), and bond strength by 3% (13.9 MPa), attributed to pozzolanic densification of the ITZ. With greater fly ash substitutions (20–30%), compressive strength dropped 14–25% (to 19 MPa) and bond strength by 28–39% (to 9.7–11 MPa) due to unreacted fly ash diluting the binder along with very poor workability loss (slump: 0–0.9 in.). The study shows that fly ash acted as an ITZ microstructural enhancer at ≤ 10% and a performance-deteriorating agent beyond. The findings propose a 10% level of fly ash as the sustainable limit, representing an optimal balance between structural integrity and environmental efficiency. Super-plasticizers with silica fume/slag are recommended to address the workability issues associated with larger substitutions. Future work must tackle durability against exposure to chloride, carbonation, and seismic loads. This study thus far has furthered eco-considered construction by allowing fly ash use to meet engineering demands.