دانلود رایگان مقاله توزیع تنش باند و استحکام باند در UHPFRC تحت تنش مستقیم

abstract

Several studies on tension lap splices have shown the improvement of bond strength using Ultra-high performance Fibre Reinforced Concrete (UHPFRC). The bridging effect of fibres on cracks improves the bond splitting strength substantially in comparison to normal concrete. This paper investigates the in- fluence of fibre content on the strength of tension lap splice of reinforcing bars in UHPFRC without additional transverse reinforcement. Different splice lengths and UHPFRC mixes were tested. Internal strain measurements were used to capture the force transfer mechanism and the evolution of longitudinal strain distribution and associated bond stresses. The bond performance is clearly related to the preand post-cracking tensile capacity of UHPFRC. At a distance exceeding 2 db from bar extremities, bond stress distribution at failure displayed a quasi-constant value regardless of the lap splice length up 10 db. This reveals for short lap splices that the bearing action of all ribs along the splice length contributes equally in resisting the applied force. This experimental program provides experimental results for understanding the local force transfer mechanism in UHPFRC lap splice and contribute for further developments on bond in UHPFRC.

5. Conclusion

In this study, direct tension lap splice tests were carried out to assess the bond strength and bond stress distribution in specimens embedded in UHPFRC. Specimens were designed to fail by bond splitting before reaching the yield strength of the reinforcing bars. The investigation objectives were to determine the contribution of UHPFRC with different fibre contents and variable lap splice length on the performance of reinforced concrete member lap splices, and to evaluate the bond splitting behaviour with the only confinement provided by UHPFRC cover. Test results indicated that confinement with UHPFRC is an extremely promising solution with high potential for the enhancement of bond performance in splice regions. From the results and analysis of the seven tests with three different splice lengths and fibre contents, the following conclusions can be drawn. 1 Internal strain measurement, which was implemented in this study, was an extremely effective way of measuring longitudinal strain distribution along lap splice. Through this method, it has been possible to obtain refined information without disturbing the interface area between steel and concrete. It has proven that it was an effective procedure to obtain bond stress distribution in UHPFRC. Since no transverse cracks developed through the section up to failure, the bond stress was not disturbed by other cracks than longitudinal splitting crack. 2 Steel stress distribution can be characterised mainly by three sections. At free end, a steep gradient is being developed over a short distance, characteristics of force transfer between spliced bars. On the opposite, at loaded end as the load increased to splitting failure, steel stress amplitude reaches a maximum value and the distribution becomes progressively flattened due to progressive degradation of the interface. Between the 2 bar extremities, steel stress distribution displayed a quasi linear increase of stress regardless of the lap splice length used. 3 Bond stress distributions are associated to steel stress variation and consequently characterised by three stages, two local peaks at free and loaded ends, and a quasi-constant bond stress distribution between these two regions. Before splitting crack is visible (<80% Ft), peak bond stress was always noted close to the loaded end. As the load increase, first peak bond stress move inwardly, and the peak bond stress at loaded end reached a maximum value, which magnitude is related to the splitting crack propagation from loaded end. 4 For a given position inside the lap splice, the steel stress developed in the bar is lower with a greater fibre contents used in the UHPFRC. 5 The evolution of transverse strain measured on the concrete surface close to the end of splices with respect to the evolution of the bar force of lap splice specimens emphasise the contribution of UHPFC tensile properties in its strain hardening stage to counterbalance the radial bursting pressure. Maximum splice strength capacity is clearly related to the inelastic capacity and strain hardening performance of UHPFRC. Fibres play a key role in increasing the maximum strength of lap splice specimen.