Structural behavior of RC jointed beams using recycled steel fiber and hybrid dowel bars

Using recycled steel fibers (RSF) from wastes of old car tires in combination with hybrid (steel and CFRP) dowel bars to improve the structural behavior of construction joints is the purpose of this study. The main objectives of this work were (i) to evaluate the effect of the shape and type of construction joints (vertical, inclined with 60°, key I, key II and fingers), (ii) to perform a comparison between joints with and without RSF (iii) to investigate the effect of adding hybrid deformed dowel (steel and/or CFRP) bars and the effect of their (number, diameter and length) on the performance of the jointed beam, and (iv) to study the effect of compressive strength class (normal, moderate and high) to highlight its influence on the structural behavior of RC beams. The results showed that each type of these joints has a different effect on the deflection-load response, cracking and ultimate capacity. The inclined joint has the most significant effect on the ultimate capacity (reduction is 20%), while the fingers and key joints have a slight effect (reduction is 8.4% and 9.58%), respectively. It was also found that the ultimate load of beams containing construction joints with RSF is greater than that of the monolithic beams without RSF by about 39.88–46.84%. Such beams also exhibited a higher stiffness and a change to a more ductile failure mode. Deformed steel one dowel bars with a length of eight diameter in RC jointed beams improves slightly all the load–deflection response with no significant increase in stiffness and ultimate load capacity. In using dowel bars in combination with RSF the enhancement becomes more obvious and may range between (1.6–1.9) times the failure load of the jointed beam specimens without RSF and dowel bar. It was also noted that the number, embedded length and the diameter of dowel bar have an effect on the structural behavior of jointed beams. Using CFRP dowel bars in construction joint, lead to a decrease in the ultimate load capacity by about 11.7% with a decrease in stiffness. Using hybrid (steel/CFRP) dowel bars gives better results of ultimate strength and stiffness compared with CFRP bars only. Concrete compressive strength also has an effect on the ultimate load and stiffness of beams containing construction joints, as it improves the load–deflection response.