Influence of steel fibres on bond and development length of deformed bars in normal strength concrete

Transverse reinforcement (stirrups) plays an important role in improving bond and anchorage of deformed bars in reinforced concrete structures. Steel fibres or steel fibre reinforced concrete (SFRC) can be used in lieu of stirrups to provide a similar beneficial effect. The application of steel fibres in lieu of stirrups is not recognized in codes of practice for concrete structures because of limited research for this type of application.;The results of this study are based on 18 large scale test beams (250 mm wide by 300 mm high and 3.4 m long). Control cylinders and flexure prisms are used to obtain the required concrete material properties together with tension tests of the steel rebar. The focus of this research is to investigate the influence of steel fibres to enhance bond and development of deformed reinforcing bars in normal strength reinforced concrete beams. An attempt is also made to develop an understanding and rationale of the effect SFRC has on improving bond. Longitudinal reinforcement in most of the beams is lap spliced with different types of confinement in the spliced region (plain concrete, plain concrete with stirrups, SFRC, and SFRC with stirrups), and evaluated under third point loading to ensure the spliced bars are subjected to a constant tensile force in the region of constant moment. All of the beams with spliced reinforcement failed in bond before yielding of the longitudinal reinforcement. The SFRC mix uses steel fibres at an 80 kg/m3 dosage (1% by volume).;The plain concrete beams without any transverse reinforcement failed suddenly without any warning. The presence of steel fibres did not affect the flexural cracking load of the specimens, but did provide a consistent increase in the load capacity at bond failure and ensure a more controlled failure. The spliced beams with SFRC exhibited a 22.5% increase in the bond failure load capacity compared with the plain concrete beams. The combined effect of fibres and transverse reinforcement provided a 28.5% improvement in the bond failure load, whereas stirrups alone helped by about 8%. This demonstrates that steel fibres can be effective in helping to increase the bond failure capacity of lap spliced beams (depending of course on the amount and type of fibres used in the concrete mix).;Steel fibres help to improve the bond failure load capacity of beams by (1) reducing the bar stress in the spliced bars because of the tensile resistance provided by the SFRC at a cracked section, and (2) from confinement in the transverse direction. Once account is taken of the reduction in bar stress from the tensile resistance of the steel fibres at the cracked section, the SFRC beams show a 13% improvement in bond capacity while the SFRC beams with stirrups show a 20% improvement. Hence, the transverse confinement provided by SFRC can have a significant effect on bond and development of flexural reinforcement.;Results of the spliced beam tests are also used to evaluate existing bond prediction models. Bond performance was underestimated for the beams with plain concrete and over estimated for the plain concrete beams with stirrups. A confinement model is developed for SFRC that under predicts bond performance when compared to the test results.