International Journal of Scientific & Engineering Research, Volume 1, Issue 1, January-2014 ISSN 2229-5518 Effect of Intersecting Beam on Bond Behaviour at the Joint Kafeel Ahmed, Ahmed Al- Raji, Uzma Kausar, Muhammad Ilyas, Abstract—Adequate and effective bond is necessary for composite action of reinforced concrete structural elements. At the joints of flexural members, bending stress of one beam magnifies the tangential bond stress around the main steel of the intersecting beam. This reduces the bond strength of intersecting beams and longitudinal splitting bond cracks initiate at lower loads, reducing the load carrying capacity of the structural member. Experimental study was carried out to determine this decrease in bond strength at the joint of intersecting beams. Beams, designed to fail in bond, were casted for this purpose. Each set consisted of three beams, two intersecting and one control. In all the beams steel and concrete strain gauges were used to measure the strain developed in steel and concrete. The results of the experimentation showed that the bond strength of primary beam of the joint of intersecting beams, reduced 15 to 30% as compared to bond strength of control beam. These test results may have an implication of on development length and splice length provisions at the joints of intersecting beams, in the building codes. This reduction in bond strength, necessitate the provision of bond improving measures like extra confinement through the stirrups or increased development length, may offer a solution to this problem. Key words. Steel slip, Bond splitting cracks, Fracture process zone, tangential bond stress, Cover to bar diameter ratio. ——————————  —————————— 1 Introduction The bond behaviour of concrete and embedded reinforcing steel is essential for composite action in reinforced concrete construction [1,2,3,6,7]. Though the pull out test to determine the bond strength, is easy to perform, however, the results do not directly represent the actual stress state. This is due to the fact that stress distribution that results in pull out test is different from that present in flexural members. Moreover the transverse confinement provided is also different. This transverse confinement affects the normal pressure on the pull out samples. The initiation and propagation of the bond splitting cracks are a function of confinement [1,8,9]. In reinforced concrete flexural members, the joints are critical as there are more chances of bond failure and subsequent slip of steel relative the concrete. Therefore it is necessary to study their bond performance. Heavy reinforcement at the joints, leaves very little space for concrete to be placed. Poorly compacted honey combed concrete results in low strength concrete keys. These are present between the ribs of the steel bar. The tensile strength of this concrete is reduced as compared to the concrete, in other parts of the beam. Due to the slip of the steel against the steel bar ribs, radial and tangential stresses develop around the steel bar. These tangential bond stresses are a function of tensile strength of the concrete. When the beams are loaded under service conditions, these concrete keys fail due to tangential bond stresses. The longitudinal splitting cracks initiate and propagate[15]. Hence reduction in bond stiffness at the joint may result in excessive joint rotation and mid span deflection. Keeping in view the importance of bond behaviour at the joints, bond beam tests with models of intersecting beams were planned to study the bond stress and slip relations of steel and concrete. In bond beam and models of intersecting beams, development length was kept constant. Bond beam acted as control beams and beams of intersecting model were named as primary and secondary beams. All these beams and models were tested and data was recoded. This data was processed and relationships were developed. This study was done for different strengths of concrete. In all the samples, bond strength of primary beam reduced as compared to control beams. The flexural stress, present at the mid span of the secondary beam, was acting in the same direction as tangential stress (circumferential tensile bond stress) developed around the steel reinforcing bars. This circumferential tensile bond stress developed due to the slip of the concrete key present between the two ribs of the reinforcing steel bars. Therefore this stress is magnified due to the flexural action of secondary beam. When this stress exceeded the tensile strength of the concrete, bond cracks initiated along the circumference of the steel reinforcing bar along the length. These longitudinal splitting cracks propagated rapidly, diminishing the bond strength of primary beam [15]. 2. Bond Fracture M echanics In normal strength concrete, bond strain softening and bond stress redistribution adjoining the reinforcing steel bar take place. The fracture process zone in front of primary and longitudinal splitting bond cracks is large as shown in Fig.1. and zone of perfect plasticity is well defined, the bond fracture energy consists of energy consumed in zone of perfect plasticity and surface energy[15,16,18]. This results in gradual crack propagation. The bond stress and slip relationship exhibited by normal strength concrete samples showed a non linear response [18]. Cracks in normal strength concrete initiate at lower load level of the ultimate load [10,17]. Therefore in bond beam tests and model tests, interface de bonding cracks and longitudinal splitting cracks initiate at lower bond stress. The bond stress and slip relationship exhibited by high strength concrete samples showed an initial stiff linear response [15]. Cracks in high strength concrete initiate at much higher load level, typically 70 to 80% of the ultimate load [15,17]. Therefore in bond beam tests and model tests, interface de bonding cracks and longitudinal splitting cracks initiate at much higher bond stress. This results in accumulation of bond strain energy in high strength materials. The bond fracture energy consists of surface energy. Once a crack forms at the interface due to slip between steel and concrete, all the accumulated bond strain energy is poured in to this crack where it is dissipated in 942 IJSER