2019 CSCE Annual Conference - Laval (Greater Montreal) Conference
Dr. Dan Palermo, York University
Dr. Stavroula Pantazopoulou, York University
This paper presents results from an experimental study conducted to understand the bond-stress slip behaviour of steel reinforcing bars embedded in Ultra-High-Performance Steel Fiber Reinforced Concrete (UHPSFRC). This emerging concrete is currently considered an optimal, durable material, which can substitute conventional concrete owing to its distinct fresh and hard properties. The compressive strength can reach 120 MPa, while the tensile strength is approximately 20 MPa defined by a strain-hardening behaviour. Thus, it is essential to understand the mechanism of stress transfer between this concrete and conventional reinforcement that permits the composite action of both materials. A four-point bending test program was arranged and conducted on 7 beams designed for the bond development to occur in the constant moment region along a short embedment length (5Db) in order to achieve a uniform distribution of bond stresses, enabling measurement of bond strength through reverse engineering of beam strength and deformation. Confinement was only provided by the concrete cover (equal to one bar diameter), whereas three design mixes were assessed and compared: two commercial and one developed in-house. The main difference between the mixes was the binder paste and the type of steel fibers, resulting in different flowability and tensile strength. Additional material testing was conducted on prisms under 4-point loading in order to extract the mechanical properties for all material mixes considered. The bond-specimens failed either by pullout or by cone formation with minimal deterioration of the concrete cover, illustrating the high confinement provided by the novel concrete surrounding the bar in tension. The bond strength was determined to be directly proportional to the tensile strength capacity of the design mix, where for the strongest material the bond strength was approximately 30 MPa. Moreover, the test results indicated a very ductile flexural beam response accompanied by significant mid-span deflection (more than 5% rotation capacity) and substantial bar-slip values. The test results demonstrated that an embedment length of five times the bar diameter is sufficient enough to yield the reinforcing bar. Thus, the high bond strength provided by the concrete cover enables significant reduction in the design development length as compared to what is used today for conventional concrete.