Akira Yasojima

Shear-Peeling Bond Strength between Continuous Fiber Sheet and Concrete

Fiber-reinforced polymer (FRP) sheets can be externally bonded to reinforced concrete structural members to increase strength. However, the bond between the FRP sheet and concrete may adversely affect the performance of flexurally strengthened members when there are diagonal tension cracks in the members. This study investigates the bond strength between the FRP sheet and concrete interface for both shear bond and peeling conditions. In the experiment, 27 rectangular specimens with FRP sheets bonded on two sides were tested in uniaxial tensile loading. The specimens were designed for different step angles at the middle to ensure that the interface acts for both shear and peeling conditions. Three types of woven composite sheets made of aramid and carbon were used to allow the study of various sheet stiffnesses and strengths. The results showed that the bond strength decreases considerably due to the peeling effect. Step angle and fiber stiffness greatly influence the bond strength between the FRP sheet and concrete interface for combined shear and peeling conditions. The highest bond strength was observed for the lowest axial stiffness of the laminate. Based on the test results, a modification to an existing bond strength model is proposed that improves the prediction of the bond strength between FRP laminate and concrete for shear-peeling conditions.

Strength of Masonry Walls Retrofitted with Epoxy Resin Injection

This paper presents the results of the fundamental loading tests with brick elements strengthened by the epoxy resin injection method. Three types of loading tests were performed to evaluate the effects of epoxy resin injection on compressive, shear, and flexural characteristics. The variables considered in this experiment were the strength of the mortar and the applied axial force. To verify the effects of this strengthening method, seismic loading tests were performed on three brick walls. From the test results, an increase in the strength of the element was observed in each test. However, the strength of the mortar did not affect the shear and flexural strength of the brick elements. We proposed equations to estimate the strength of the elements strengthened by the epoxy resin injection method. Improvements to the seismic performance of the brick wall strengthened with epoxy resin were explicitly confirmed.

Evaluation of Shear and Tensile Bridging Characteristics of PVA Fibers Based on Bridging Law

Strain-Hardening Cementitious Composites (SHCC), in which short fibers are mixed in mortar, show improved tensile performance and ductility of the cementitious material because fibers bridging the crack transfer tensile forces after first cracking. It is considered that the stress field at the shear crack surface in the structural element under the shear force is a biaxial stress field in which tensile and shear stresses exist concurrently. In fiber-reinforced cementitious composites, both tensile and shear stresses are transmitted via fibers that bridge shear cracks. It is necessary that the effect of fibers bridging a shear crack under tensile and shear stresses is investigated. In this study, uniaxial tension tests were carried out for specimens which have a square cross-section and an inclined notch. The biaxial stress field can be expressed by the inclined crack surface produced by the tensile loading. From the test results, it was confirmed that the tensile stress decreased with increasing notch angle in the tension tests. A calculation method for the bridging law with an inclined crack was introduced and the calculation results were compared with the test results. Though the maximum tensile stress in the tests was smaller than that in the calculation results, the curves after the maximum stress show good agreements with the calculations. The maximum stress reached in the tests tends to decrease with increasing crack angle (notch angle) as in the calculation results.

Influence of Fiber Orientation on Structural Performance of Beam-Column Joints Using PVA FRCC

In recent years, many studies on fiber-reinforced cementitious composite (FRCC) has been conducted actively to improve brittle behavior of cementitious materials. FRCC shows the improved tensile performance and ductility because fibers across the crack can transfer tensile force after first cracking. When fibers tent to orient perpendicularly to crack surface, higher bridging effect of fibers is observed. When fibers tend to orient parallel to crack, however, bridging performance of fibers becomes poor. As one of the examples, the authors have already conducted that the casting direction of FRCC influences the tensile performance of FRCC. The objective of this study is to investigate the influence of fiber orientation on structural performance of PVA FRCC beam-column joint by using two types of casting method. Horizontal and vertical casting beam-column joint specimens are tested by reversed cyclic load simulating earthquake force. A vibrator rod is also applied during the casting. The applied load on beams and the story drift angle of two specimens are obtained directly from the experiment. According to the experimental results, specimens of horizontal casting and vertical casting show almost the same shear capacities. Using a vibrator rod during casting has an influence on shear capacity of PVA FRCC beam-column joint.