Effect of hybrid polypropylene fibers on mechanical and shrinkage behavior of alkali-activated slag concrete

The growing global demand for cement in construction led to an increase in environmental concerns from the carbon dioxide gas release associated with its production. To foster the acceptance of other alternatives, such as alkali-activated slag (AAS) systems, mitigation of their main drawbacks, notably high shrinkage, is imperative. This study explores the influence of macro and micro polypropylene (PP) fiber hybridization in AAS concrete. Seven mixes with different fiber volume fractions (VF) of macro and hybrid fibers, in addition to a control mix, were investigated. Three macro-PP fiber mixes had fiber VFs of 0.3 %, 0.6 %, and 0.9 %. The other three hybrid PP fiber mixes were like macro fiber mixes with the addition of 0.1 % microfibers. The influence of PP fiber hybridization was explored concerning flowability, static elastic modulus, compressive, splitting tensile, and 3-point flexural strength and compared to control and macro-PP fiber mixes. Additionally, Shrinkage was measured for control and optimum fiber mixes. The results showed that the hybridization of PP fibers led to reductions in shrinkage up to 15 % compared to the control mix, while macro-PP fibers reached a maximum reduction of 6 %. Furthermore, the study evaluates the applicability of various literature and current code guidelines for predicting elastic modulus and tensile strength with the experimental results. ACI 318 could provide relatively accurate predictions in terms of splitting tensile and flexural strengths. Apart from that, with some exceptions, other codes and proposed equations in the literature were inadequate in providing satisfactory predictions.