Experimental Evaluation of HPFRCC effect on behavior of coupled beams without diagonal and spiral reinforcement

HPFRCC is the materials including cement mortar, aggregate, and fibers which represent strain hardening within tensile load. The HPFRCC can be used in numerous cases such as seismic rehabilitation of structural members. One of the structural members is the existed coupling beam in coupling shear walls which is applied as shear fuse. Using the materials in the members can enhance ductility and energy absorption and also delays failure. This paper investigates a study on the effect of existence of diagonal reinforcements and spirals of diagonal reinforcements of the coupling beam. For this purpose, three prototypes of coupling shear walls with coupling beam were designed by HPFRCC with length-to-depth ratio of 2 and 1/2 scales. The first prototype is considered as reference and we use concrete with reinforcement design based on ACI 318-08 code. The other prototypes are built by HPFRCC with PPS fibers. But in one of them the spiral of diagonal reinforcements and in the other both spirals and diagonal reinforcements are omitted. In order to simulate the test set-up with real behavior, two strong walls were considered and cast at both sides of coupling beams. The rotation of these walls should be prevented, so in the experimental set-up, vertical small steel column in addition two strong steel roller were considered during tests. The several strain gauges were installed on longitudinal and diagonal and vertical bars to measure the strains during tests and particularly showing the displacement and load of yielding points of reinforcement. LVDTs were installed to measure the maximum displacement of the tip of beam and also to measure probable rotation. The drift if the ratio of the tip displacement of the specimen to the beam length and the ductility is the ratio of ultimate displacement to the yielding displacement and finally the energy absorption is the area under load-displacement cure for each separate cycle.

Results are indicating of appropriate effect of HPFRCC concrete in enhancing ductility and energy absorption capabilities and it can also reduce diagonal reinforcements. In addition improved crack pattern and shrinkage of cracks represent an appropriate participation of fiber in increasing the shear capacity. Comparing these prototypes, it is found the one in which spirals were omitted load capacity، ductility factor، energy absorption and failure displacement capabilities have been increased 15%, 36%, 69%, 35%. And the prototype in which diagonal reinforcements were omitted, has decreased load capacity down to 36% and ductility factor and failure displacement have been increased up to 13%, 35% and finally energy absorption has no changes. The pinching loops of load-displacement hysteresis curves of specimens were compared and the results indicated that the pinching of HPFRCC specimen was reduce comparing to reference specimen even in case of omitting the spiral. The stiffness slope of each specimen was calculated and results showed that the HPFRCC specimen with diagonal bars had more 8 percentage but the stiffness of HPFRCC specimen without diagonal bars was reduced up to 60% comparing to reference regular concrete specimen. Elastic experimental shear capacity of specimens was about 5 times of elastic Design code (ACI) shear capacity because the shear capacity calculated by ACI is conservatively only based of diagonal bar shear capacity.