D. Fakhri - Department of mining engineering, Imam Khomeini international university, Ghazvin, Iran
M. Hosseini - Department of mining engineering, Imam Khomeini international university, Ghazvin, Iran
M. Mahdikhani - Department of mining engineering, Imam Khomeini international university, Ghazvin, Iran

Fracture toughness is an important concrete property that controls crack extension and concrete fracture. Concrete is the most widely used material in civil engineering containing the most conventional and cheapest materials. Accordingly, cracks and fractures may cause irreparable damages. To this end, fibre-reinforced concretes have been recently constructed in order to overcome the aforementioned weaknesses. Crack propagation and fracture toughness of various concrete specimens are analyzed by the straight notched Brazilian disc (SNBD) test. The specimens are conventional concrete lacking micro-silica and limestone powder, and those containing various volume percentages of fibers including the concrete specimens containing ۰.۳۵% individual polypropylene (PP) fibers, ۰.۳۵% individual glass fibers, concrete specimens containing ۰.۱۷% PP and ۰.۱۸% glass fibers, and concrete fibers containing ۰.۱% PP and ۰.۲۵% glass fibers. Micro-silica has replaced ۱۰ wt% cement in all fiber-reinforced concrete specimens, and limestone has replaced ۵ wt% cement. Crack extension from the pre-existing cracks in the specimens and mode I, mode II, and mixed-mode fracture toughness are calculated. The BD test is performed on the specimens at the crack inclination angles of ۰°, ۱۵°, ۲۸.۸۳°, ۴۵°, ۶۰°, ۷۵°, and ۹۰°. The experimental results show the initiation of wing cracks at angles less than ۶۰° (۰ < α < ۶۰°) from the tip of the pre-existing cracks. The crack growth and propagation path approach the loading direction by continuing loading. However, the cracks are initiated at a distance of d from the crack tip at angles larger than ۶۰°. The observed distance is larger in the fiber-less specimens than in the fiber-reinforced specimens. The concrete specimens reinforced by ۰.۱۷% PP and ۰.۱۸% glass hybrid fibers containing micro-silica and limestone powder showed the highest mode I, mode II, and mixed-mode fracture toughness compared to the other concrete specimens.

Fracture toughness, Microsilica, Limestone powder, Glass and polypropylene hybrid fibres, Mixed Mode