Experimental Study on Comparison of Cyclic Bond Behavior of Ribbed and Sand-Coated CFRP Bars in High Strength Concrete

Although monotonic bond tests are available in the literature, limited experimental studies exist for cyclic loading conditions. To fill this gap, this work describes results from cyclic pull-out tests of eleven specimens with Carbon Fiber Reinforced Polymer (CFRP) rebars embedded in high strength concrete (f_ck > 50 MPa). Six of the samples were sand-coated while the remaining five were ribbed carbon fiber rebars to account for various bond conditions. CFRP rebars were concentrically embedded into U-shaped 350 mm × 350 mm × 300 mm concrete blocks that provided sufficient development lengths. Rebar surfaces and development lengths with multiples of rebar diameters of 5d_b, 10d_b, 15d_b, 20d_b, 25d_b, 30d_b and 40d_b were taken as the main parameters in this experimental work. Both tension load versus slip relationships and CFRP bar strain measurements were obtained and presented. In order to observe the distribution of the tensile stresses between the concrete and embedded part of the CFRP bar, three strain gauges were attached on the surface of each bar before pouring the concrete. During the tests, typical bond slip (i.e. debonding) and concrete splitting failures were observed. According to the experimental data obtained in this work, bond strengths were in the range of 7–11 MPa and 9–14 MPa, for the sand-coated and ribbed CFRP bars, respectively. These values indicate that the bond strength obtained for ribbed rebars is considerably higher, and therefore, it is necessary to take into account the change in the bond strength of CFRP rebars embedded in RC elements according to the rebar surface conditions. When the obtained results are compared with current specifications, it is seen that relationships and limitations proposed for the design are not in good agreement with the experimental data obtained in this work. The evaluation of specifications has shown that the Canadian CAN.CSA.S806 and Japanese JSCE-E131 codes remain on the safe side while the American ACI 440 code does not produce reliable results for the class of surface conditions considered here since the obtained design capacities using the ACI formula are very close to the experimental capacities without any safety factor. Bond stiffness’s of specimens has decreased as number of load cycles has increased. Cyclic loading reduced the bond capacity of the specimens by maximum 40%.