Proposal of a Mix Design Method for Low Cement Fiber Reinforced Concrete

Abstract

Concrete, the second most used material in the world, presents great performance and economic benefits. Yet, it is often characterized by a brittle behaviour, low tensile strength, and toughness. Fibers are usually added to concrete to counteract its brittle behaviour, increasing ductility and toughness, controlling crack propagation and delaying concrete failure. However, their addition significantly worsens the fresh state performance of the material. To improve fresh state of the so-called Fiber Reinforced Concrete (FRC), conventional mix-design methods recommend the use of high paste content, which results in a significant increase of Portland cement (PC) content and raises the carbon footprint of the material. The latter is responsible for 8% of the global annual carbon dioxide (CO2) anthropogenic emissions. Given the current worldwide concerns on global warming, the construction industry is in a need to lessen the demand, and thus production of PC. Recent studies have been focusing on the use of advanced mix-design techniques (i.e. particle packing models- PPMs) along with Inert Fillers (IF) as an alternative to reduce PC content in concrete. However, the latter was not applied to conventional FRC. In this work, advanced mix design techniques (i.e. PPMs) are used to overcome the aforementioned issues and mix-proportion eco-efficient FRC with low cement content (< 300 kg/m3). Fresh (i.e. VeBe time, slump, rheological behaviour) and hardened (i.e. compressive strength, and flexural behaviour) state tests were performed on the proposed mixtures and compared with control high PC content (375 kg/m3) FRC mixes. Results show that PPM designed mixes presented higher minimum torque (yield stress) but quite comparable apparent viscositiy when compared to conventionally designed mixtures. Moreover, the flowability (i.e. VeBe time, and slump) tends to decrease as fiber content, length, and/or as the amount of fillers increase in the mixtures. In addition, PPM mixes exhibited a shear thinning behaviour following the Herschel-Bulkley model, which enables the design of FRC PPM mix-proportioned mixtures for applications requiring high torque regimes such as vibrated and/or pumped concrete. Finally, results show that the use of PPMs to mix proportion eco-efficient low cement FRC mixtures produced improved hardened (i.e. compressive strength, and flexural performance) state behaviour with lower environmental impact than conventional ACI designed FRC mixtures.