Experimental investigation to determine factors affecting bond behavior of CFRP rebars in high strength concrete

Basic behavior of reinforced concrete (RC) relies on resisting compression forces with concrete and carrying tension forces with steel rebar. In accordance with new requirements and advanced material technologies in structural engineering, high performance concretes and rebars can be produced with different properties for addressing new demands. Recently, the use of carbon fiber reinforced polymer (CFRP) materials has become widespread in increasing the capacities of existing structures. It is also possible to design carbon fiber polymers, which are mostly preferred as fabric strips, as the main reinforcing element, which can be produced in rebar shapes and resist tensile stresses in reinforced concrete sections. In this direction, countries, like America, Canada and Japan have established and developed their design specifications. Due to their high tensile strength, carbon fiber polymer rebars even with small diameters which can carry much bigger forces when compared to steel rebars provide less space in reinforced concrete sections and allow easier placement during pouring concrete. Corrosion problems caused by rain, snow and icing in steel RC elements is not the case with carbon fiber polymer rebars. High chemical resistances in salty and acidic environments of road, sea and port structures, lightweight, not affected by possible magnetic devices in structures are other remarkable features of composite rebars. In this way, it is ensured that the stiffness and durability of RC sections are protected. Effectiveness of RC being a composite building material can only be realized by providing an appropriate bond mechanism to transfer forces between concrete and rebars. This force transfer is provided by developed bonding forces, which resist sliding/slipping parallel to the axis of concrete, surrounded the rebar. Although there are many experimental studies and models on the bond strength of steel rebars, studies on the bond strength of carbon fiber rebars are somewhat limited and bond strengths calculated by the existing specifications are not realistic according to the test results. Additionally, considering repeated/cyclic loads are rare. One of the original aspects of this study is to compare the obtained test results with current specifications and to observe the possible differences in the bond strength of monotonic or cyclic loading. In order to observe bond behavior, a total of 36 specimens was prepared with different properties to be used in the pull-out tests. Twelve of the samples were sandblasted (or sand-coated), two of them with smooth surface, and twenty-two of them were arranged with ribbed carbon fiber rebars. The specimens were tested at Bogazici University, Structural Engineering Laboratory with specially designed and modified test setup for this thesis. Unlike the known experimental setups in literature, the developed testing device is believed to be the first for pull-out experiments since it brings a complete pull-out area without creating a pressure and confinement effect on the surface of rebar connected to concrete. Carbon fiber rebars are pulled from the concrete surface under force and displacement control by a hydraulic actuator having 200kN tension and compression capacities and with ±100mm dispacement capacity. Twenty-five samples were tested monotonically and eleven of them were tested under cyclic loads. Note that cyclic loading was not fully cyclic (i.p. not reversed cyclic loading). High strength concrete mix was specially designed for this study and all specimens were poured at the concrete plant in five times and transported to the laboratory. In order to observe distribution of the load between concrete and the rebar during the tests, tree strain gauges were attached on the surface of each rebar before pouring the concrete. Strain gauge measurements were taken at the beginning, in the middle, and at the end of bonded area. According to experimental data obtained from the tested specimens, measured bond strengths are (10~18)MPa and (10~14)MPa for monotonic and cyclically loaded specimens, respectively. This indicates that the bond strength obtained in monotonic loading is considerably higher and therefore it is necessary to take into account the change in the bond strength of CFRP RC elements when repeated load effect is possible (bridge moving loads, earthquake, wind etc.). Measured bond stress values are generally higher than those calculated according to ACI 440. Specimens with anchorage lengths of 15db, 20db, and 25db have reached bond strengths that are approximately twice the value calculated from the specifications. However, it is seen that measured bond strengths are almost the same with the code values for specimens having 5db and 10db anchorage lengths. As can be seen from the strain gauges data, which are taken from rebar surfaces, force transfer between concrete and rebar is not distributed uniformly. There was a gradual increase in stress on surface of the rebar and a change in the distribution of the stresses until the ultimate bond strength capacity. It has been observed that, strain gauges which are located at the end of bonding region, started to measure strain just before the ultimate bonding load. This can be explained by progressive plastification along the bonding region. When the average bond stresses are examined, it is seen that the mechanical interlocking level provided by the ribbed fixtures gives 40% higher values than the sand-coated surface resistance provided by the friction. When the obtained results are compared with current specifications, it is seen that the relations and limitations proposed for the design are far from the real situation. The evaluated specifications have shown that the Canada CAN.CSA.S806 and Japanese JSCE-E131 codes remain on the safe side, while the American ACI 440 code does not produce any reliable results. In addition, for a better understanding of the stress distribution of forces transferred from rebar to concrete on the bond surface, monotonically loaded sand-coated rebar specimens were modeled and analyzed using ABAQUS 6.14 software. This numerical study gave bond-slip curves of the tested specimens. Comparing experimental and numerical results suggests good estimation for bond strength values although the overall load-slip graphs have some discrimination. Other than these, improved bond models for steel and carbon fiber rebars in literature have been investigated and a bond formula has been established which gives values close to the test results using the principal parameters. Contents of the main chapters of this thesis are given below: In the first chapter, aim and scope of the study are explained and information on the methods followed are elucidated. An overall evaluation of the previous theoretical and experimental studies on bond mechanism is given in the Second Chapter. In addition, bond strength design rules for CFRP bars in the current codes are summarized. Design steps of specially designed and constructed loading frame and its details are given in Chapter 3. Moreover, producing of high strength concrete, rebar material tests, steel cover for rebars, and instrumentation of the test setup are explained in detail. Further information about the loading protocols followed and results of pull-out tests are given in Chapter 4. In addition, experimental observations and failure mechanisms for each specimen are presented herein. Main findings derived from the test results from this experimental work are given in Chapter 5. Chapter 6 includes details of the performed finite element model (FEM) and assumptions used in these analyses. Moreover, bond-slip curves from tests and finite element analyses are compared and interpreted. Correlations/formulas developed for the determination of bond capacity of CFRP bars in high strength concrete in the direction of data obtained from the experiments are given in Chapter 7. In Chapter 8, general results reached are evaluated and suggestions for potential future studies are made.