A comparison of the effects of pozzolanic binders on the hardened-state properties of high-strength cementitious composites reinforced with waste tire fibers

Experimental and statistical analyses were conducted to examine the effects of partially replacing ordinary Portland cement (OPC) with pozzolanic binders (silica fume and fly ash) on the hardened-state characteristics of high-strength cementitious concrete reinforced with steel fibers recovered from waste tires. The variables used in the analyses were the concentration of recycled steel fibers (volume fraction = 0.5% and 1%) and the proportion of silica fume or fly ash used to replace OPC (weight = 10%, 20%, and 40%). The effects of combining fibers at two volume fractions with pozzolanic binders in 14 mixtures were investigated through an experimental program carried out in two stages. First, the effects of replacing OPC with silica fume or fly ash on the matrix were determined by assessing the heat of hydration, the formation of the crystalline phases using X-ray diffraction analysis, the gel structure and mass loss at elevated temperatures with thermogravimetric, and differential thermogravimetry analyses, and porosity structures of pastes with using the mercury intrusion porosimetry test. Second, the effects of adding recycled steel fibers on the hardened-state properties of OPC-based concretes containing pozzolans were explored by evaluating the water absorption by immersion and capillary rise, ultrasonic pulse velocity, compressive strength, splitting tensile strength, flexural strength, and impact resistance of the reinforced mixtures. The experimental results were subjected to linear regression and statistical analysis to correlate the mechanical and impact properties of the mixtures and identify probability distributions, respectively. The findings showed that using pozzolanic binders enhanced the mechanical and impact properties of the mixtures reinforced with fibers from waste tires, thereby affecting the fibers' frictional pull-out behavior. Therefore, significant differences on the impacts of fiber content were found on the post-peak responses of the mixtures under flexural loading. In general, using silica fume had higher impact on enhancing the hardened-state characterizations than fly ash. The greatest enhancement in mechanical properties was observed when OPC was replaced with 40% silica fume. The analytical results confirmed that silica fume was better fitted to the normal distribution than fly ash, while fly ash produced a higher coefficient of correlation between mechanical and impact resistance than did silica fume.