Browsing by Author "Salem, Hamdi Abdulrahman Saif"

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    Bond strength and tension stiffening of fly ash geopolymer concrete
    (Universiti Teknologi Malaysia, 2022) Salem, Hamdi Abdulrahman Saif
    Fly ash geopolymer concrete has gained attention from concrete industries and researchers in view of its environmental sustainability in construction. This emerging concrete protects the environment by utilizing fly ash wastes and significantly reducing the demand for Ordinary Portland Cement (OPC) concrete that exhibits higher carbon footprints. A proper understanding of mechanical properties and structural behaviour of fly ash geopolymer is crucial at the service and ultimate limit state conditions. Therefore, this study aims to investigate the mechanical properties of high calcium fly ash (HCFA) geopolymer concrete and evaluate the effects of compressive strength and concrete cover-to-bar diameter (Cc/db) ratio on bond strength, tension stiffening, crack spacing, and crack width. The initial step of the experimental program was to develop a concrete design mix based on trial-and-error by varying alkaline activator to binder (AA: B) ratios between 0.30 to 0.40 to achieve concrete compressive strength between 20 MPa and 45 MPa using ambient curing regime. Pullout specimens were cast on 100 mm dimensional cube molds due to their accuracy in obtaining bond strength. Tension stiffening prisms were cast on molds with a square cross-section of 75 mm ×75 mm and a length of 650 mm, in which matrix cracking was allowed to occur. The Cc/db ratio for pullout specimens and tension stiffening prisms varied from 2.63 to 4.5 and 1.84 to 3.25, respectively. The pullout results showed that HCFA geopolymer concrete has a high bond strength and is affected by both compressive strength and Cc/db ratio. Most of the pullout specimens exhibited splitting bond failure characterized by a gradual decay of bond strength. A comparison between the experimental bond strength and that estimated using OPC concrete formulas revealed that the bond strength of HCFA geopolymer concrete is slightly underestimated due to a better interfacial transition zone in the concrete matrix. Significantly, the results of tension stiffening, crack spacing, and crack width showed that HCFA geopolymer concrete exhibited better tension stiffening with smaller crack width and spacing than OPC control specimens. These results were influenced by both compressive strength and the Cc/db ratio. Increasing the reinforcement ratio resulted in a higher crack number due to the smaller concrete cross-sectional area, which required a lower force to generate more cracks. This consequently reduced the average crack spacing and produced a narrower crack width. The results also showed that fly ash geopolymer concrete exhibited a ductile cracking compared to the brittle fracture mechanism observed in OPC concrete. In addition, the experimental crack spacing and crack width were validated against those values estimated using the standard codes of practice, suggesting that the existing provisions developed for OPC concrete underestimate crack spacing and crack width by 15% and 30% respectively. In conclusion, this study developed the empirical models based on the experimental results to estimate the bond-slip performance, crack width and crack spacing of HCFA geopolymer concrete.