RESUMO
The ductility of FRP-reinforced concrete structures is reduced by the brittleness of FRP bars. To address this issue, this study employs the hybrid reinforcement of stainless steel (SS) and GFRP bars to enhance the ductility of concrete columns. A total of 21 axially compressed seawater and sea sand concrete (SWSSC) circular columns are fabricated, including 15 hybrid GFRP and SS bar-reinforced SWSSC (GFRP-SS-SWSSC) columns, 3 GFRP bar-reinforced SWSSC (GFRP-SWSSC) columns, and 3 SS bar-reinforced SWSSC (SS-SWSSC) columns. The test results are analyzed in terms of failure mode, load-axial displacement curve, bearing capacity, and ductility. Results show that GFRP-SWSSC columns suffer brittle failure, while GFRP-SS-SWSSC columns and SS-SWSSC columns demonstrate ductile failure characteristics. Furthermore, the hybrid reinforcement contributes to an improvement in the bearing capacity of the columns. A calculation equation for the bearing capacity of axially compressed columns was established, providing reasonable predictions of bearing capacities, with a design compressive strain of 2000 µÎµ for GFRP bars. It was found that hybrid reinforcement enhanced the ductility of GFRP-SWSSC columns. In addition, when the percentage of the SS reinforcement ratio reaches 50%, the ductility indexes of the GFRP-SS-SWSSC columns closely approach those of the SS-SWSSC columns.
RESUMO
The objective of this study is to measure the crack propagation speed in three types of self-compacting concrete reinforced with steel fibers loaded under four different loading rates. Central-notched prismatic beams with two types of fibers (13 mm and 30 mm in length), three fiber volume ratios, 0.51%, 0.77% and 1.23%, were fabricated. Four strain gages were glued on one side of the specimen notch to measure the crack propagation velocity, a fifth one at the notch tip to estimate the strain rates upon the initiation of a cohesive crack and the stress-free crack. A servo-hydraulic testing machine and a drop-weight impact device were employed to conduct three-point bending tests at four loading-point displacement rates, the former to perform tests at 2.2 µm/s, 22 mm/s and the latter for those at 1.77 m/s, 2.66 m/s, respectively. With lower fiber contents, smooth mode-I cracks were formed, the crack speed reached the order of 1 mm/s and 20 m/s. However, crack velocities up to 1417 m/s were obtained for the concrete with high content of fibers under impact loading. This value is fairly close to the theoretically predicted terminal crack velocity of 1600-1700 m/s. Numerical simulations based on cohesive theories of fracture and preliminary results based on the technique of Digital Image Correlation are also presented to complement those obtained from the strain gages. In addition, the toughness indices are calculated under all four loading rates. Strain hardening (softening) behavior accounting from the initiation of the first crack is observed for all three types of concrete at low (high) loading rates. Significant enhancement in the energy absorption capacity is observed with increased fiber content.