Influence of hybrid fibers on the fresh and hardened properties of silica fume blended concrete

This research explores impact of hybrid steel (ST) and polypropylene (PP) fibers on both fresh and hardened behavior of 10% silica fume (SF) blended M30 concrete. Hybrid fibers are introduced at dosages of 0%, 0.5%, 1.0%, 1.5%, and 2.0% total volume (equal proportions of ST and PP). As the fiber dosage increased, the slump declined from 132 mm (control) to 34 mm (2.0% hybrid fiber), reflecting decreased workability. The initial and final setting times increased from 3 h 15 min and 5 h 20 min (control) to 5 h 30 min and 7 h 50 min (2.0% hybrid fiber). The 28-day compressive strength augmented to a maximum of 49.5 MPa at 1.0% hybrid fiber compared to 39 MPa for the control sample. However, the flexural strength increased from 4.3 MPa to a highest of 6.5 MPa at 1.0% hybrid fiber. Increasing the fiber dosage increased fiber balling, contributing to less strength due to poor packing. Therefore, the optimal hybrid fiber dosage is determined to be 1.0% by volume (0.5% ST + 0.5% PP) because this dosage provides the best mechanical performance, crack resistance, and acceptable fresh properties. This configuration provides good post-crack performance as well as acceptable workability and setting times for structural applications.