Abstract This paper investigates the stress-strain characteristics of Hybrid fiber reinforced concrete (HFRC) composites under dynamic compression using Split Hopkinson Pressure Bar (SHPB) for strain rates in the range of 25 to 125 s-1. Three types of fibers - hooked ended steel fibers, monofilament crimped polypropylene fibers and staple Kevlar fibers were used in the production of HFRC composites. The influence of different fibers in HFRC composites on the failure mode, dynamic increase factor (DIF) of strength, toughness and strain are also studied. Degree of fragmentation of HFRC composite specimens increases with increase in the strain rate. Although the use of high percentage of steel fibers leads to the best performance but among the hybrid fiber combinations studied, HFRC composites with relatively higher percentage of steel fibers and smaller percentage of polypropylene and Kevlar fibers seem to reflect the equally good synergistic effects of fibers under dynamic compression. A rate dependent analytical model is proposed for predicting complete stress-strain curves of HFRC composites. The model is based on a comprehensive fiber reinforcing index and complements well with the experimental results.

Abstract The paper presents experiments involving punching of RC slabs strengthened using externally bonded carbon fiber reinforced polymer (CFRP) sheet and textile reinforced mortar (TRM). Twelve RC slab specimens of two concrete grades (39.9 and 63.2 MPa) and employing two strengthening schemes (CFRP and TRM) were tested. Specimens were supported on two opposite edges. Experimental load-displacement variations show two peak loads in strengthened slabs and one peak followed by a plateau in control. Second peak or the plateau corresponds to the combined action of aggregate interlock and the dowel action of back face rebars and strengthening layers. The dowel action of back face rebars and strengthening layers had no role in ultimate punching load (i.e. first peak). Strengthened slabs showed 9-18% increase in ultimate punching load (i.e. first peak) whereas there was significant increase in the second peak load (190-276% for CFRP; 55-136% for TRM) and energy absorption (~66% for CFRP and 22-56% for TRM). An analytical model was also developed for predicting the punching shear strength (first and second peaks) of strengthened slabs showing good comparison with experiments.