Effect of kenaf fiber on mechanical properties of high-strength cement composites

•Different strength grades of HC reinforced by kenaf fiber were studied.•Kenaf fiber effect flexural properties of HC positively but compressive properties negatively.•The positive effect of kenaf fiber on HC increased as the increase of fiber content.•The negative effect of kenaf fiber increased with the increase of strength grade.•A model for the relationship between compressive strength and flexural strength was developed. Defined by European Standard EN 206-1, the concretes with a compressive strength greater than 50 MPa belong to high-strength concrete. Such the high-strength cement composites reinforced by synthetic fibers have been widely used in the construction industry. However, limited research focused on high-strength cement composite (HC) reinforced by natural plant fibers although numerous studies have been completed for that of low strength. This study aims at experimentally exploring the effect of kenaf fiber as reinforcement on mechanical properties of HC. Kenaf fiber reinforced high strength cement composites (KFRHC) with three different strength grades which controlled by different water-cement ratios (0.25, 0.3 and 0.35) were investigated. The influence of the strength grade and fiber content on the compressive strength, flexural strength, deformation behavior and toughness of KFRHC were analyzed. The results showed that the utilization of kenaf fiber as reinforcement in HC had a negative effect on compressive properties and a positive effect on flexural properties. The compressive strength of KFRHC was decreased by 12.2–46.2% while the flexural strength was increased by 30.7–66.9%. The negative effect of compressive strength and toughness was increased with the increase of the strength grade and fiber content. The reinforcing effect of kenaf fiber on flexural strength and toughness of the high-strength cement composite was more effective than that of the low strength grade one. The optional fiber content for flexural strength and deflection of KFRHC was 1%. The improvement for deformability of KFRHC increased as the strength grade increased. An appropriate model for the relationship between the compressive strength and flexural strength of KFRHC was proposed.