Bond in a reinforced concrete (RC) structure is the interaction force that transfers force between the steel and concrete. It influences the structural performance and serviceability of a structure under both static and cyclic loading. Corrosion of reinforcing steel in RC structures is the primary reason behind bond loss in RC elements. A loss of bond in concrete results in a decrease in the serviceability strength and eventually causes a brittle and sudden failure. Structures, such as bridges, are vulnerable to corrosion and at the same time are subjected to repeated loading rather than static loading. Nevertheless, little experimental or analytical studies that address the problem of corroded steel-concrete bond under repeated loading exist. This study was aimed at increasing the understanding of the behaviour of bond between corroded reinforcing steel bars and concrete for structures subjected to repeated loading. In addition, the effect of fibre reinforced polymers (FRP) as a rehabilitation method was assessed. Fibre reinforced polymers is considered to be a state-of-the art rehabilitation material due to its advantages, such as high strength, light weight and ease of handling and application. Forty-seven anchorage-beam specimens were cast and tested. The specimens’ dimensions were 152 x 254 x 2000 mm reinforced with two 20M bars. The steel reinforcement in a specimen was unbonded except for 250 mm from each end. This bonded length was selected to ensure a bond failure. The corrosion was induced using an accelerated corrosion process. The parameters investigated were the corrosion level (0, 5 and 9% measured mass loss), whether the specimen was wrapped in the anchorage zone with a U-shaped carbon fibre reinforced polymer (CFRP) sheets or not, and the load range applied. The minimum load applied was 10% of the static bond capacity of the specimen. The maximum load was varied to give the desired range of fatigue lives (103 to 106 cycles). The test frequency for all repeated tests was 1.5 Hz. Results showed that the repeated loading either pushed the bottom concrete cover away from the steel bar by wedge action for unwrapped beams or cracked and crushed the CFRP confined bottom concrete cover for wrapped beams. The concrete damage caused the bond stress to undergo a gradual redistribution, moving the peak bond stress from the loaded end towards the free end, resulting in failure of the specimens by fatigue of bond. Corrosion levels of 5% and 9% decreased the fatigue bond strength on average by 19%. The rate of slip of the steel bar increased as the corrosion level increased. CFRP sheets changed the mechanism by which the concrete resist the bond forces by engaging the bottom cover. This in turn increased the fatigue bond strength at all corrosion levels on average by 31% compared to unwrapped specimens. Based on the test results and observations, a hypothesis of the mechanics of bond under repeated loading was postulated and a fatigue slip-growth analysis (similar to the fracture mechanics crack growth approach) was proposed to calculate the fatigue life of a specimen that fail in bond. The proposed analysis was in reasonable agreement with the experimental results.

Bond in a reinforced concrete (RC) structure is the interaction force that transfers force between the steel and concrete. It influences the structural performance and serviceability of a structure under both static and cyclic loading. Corrosion of reinforcing steel in RC structures is the primary reason behind bond loss in RC elements. A loss of bond in concrete results in a decrease in the serviceability strength and eventually causes a brittle and sudden failure. Structures, such as bridges, are vulnerable to corrosion and at the same time are subjected to repeated loading rather than static loading. Nevertheless, little experimental or analytical studies that address the problem of corroded steel-concrete bond under repeated loading exist. This study was aimed at increasing the understanding of the behaviour of bond between corroded reinforcing steel bars and concrete for structures subjected to repeated loading. In addition, the effect of fibre reinforced polymers (FRP) as a rehabilitation method was assessed. Fibre reinforced polymers is considered to be a state-of-the art rehabilitation material due to its advantages, such as high strength, light weight and ease of handling and application. Forty-seven anchorage-beam specimens were cast and tested. The specimens’ dimensions were 152 x 254 x 2000 mm reinforced with two 20M bars. The steel reinforcement in a specimen was unbonded except for 250 mm from each end. This bonded length was selected to ensure a bond failure. The corrosion was induced using an accelerated corrosion process. The parameters investigated were the corrosion level (0, 5 and 9% measured mass loss), whether the specimen was wrapped in the anchorage zone with a U-shaped carbon fibre reinforced polymer (CFRP) sheets or not, and the load range applied. The minimum load applied was 10% of the static bond capacity of the specimen. The maximum load was varied to give the desired range of fatigue lives (103 to 106 cycles). The test frequency for all repeated tests was 1.5 Hz. Results showed that the repeated loading either pushed the bottom concrete cover away from the steel bar by wedge action for unwrapped beams or cracked and crushed the CFRP confined bottom concrete cover for wrapped beams. The concrete damage caused the bond stress to undergo a gradual redistribution, moving the peak bond stress from the loaded end towards the free end, resulting in failure of the specimens by fatigue of bond. Corrosion levels of 5% and 9% decreased the fatigue bond strength on average by 19%. The rate of slip of the steel bar increased as the corrosion level increased. CFRP sheets changed the mechanism by which the concrete resist the bond forces by engaging the bottom cover. This in turn increased the fatigue bond strength at all corrosion levels on average by 31% compared to unwrapped specimens. Based on the test results and observations, a hypothesis of the mechanics of bond under repeated loading was postulated and a fatigue slip-growth analysis (similar to the fracture mechanics crack growth approach) was proposed to calculate the fatigue life of a specimen that fail in bond. The proposed analysis was in reasonable agreement with the experimental results.

Four beam-bond test specimens, two in accordance with RILEM TC-RC5 recommendation, labelled as RILEM and two based on a modified form of the ACI 208 beam-bond test method, labelled as Notched, were tested in four-point bending to investigate the bond stress distribution and values along the bar embedment length of a 15 𝑚𝑚 nominal diameter GFRP rebar. The beams experienced failure through the rupturing of the longitudinal GFRP tensile reinforcement. In addition, two Modified and ten Standard pullout specimens were tested using the same bar. The beam-bond and the Modified pullout specimens had embedment length of 600 𝑚𝑚 while the Standard pullout specimens had, in accordance with CSA S806, 60 𝑚𝑚 embedment, or four times the bar nominal diameter. The first Modified pullout specimen experienced concrete splitting failure and as a result, the second was lightly confined and failed by GFRP bar rupture. All ten Standard pullout specimens failed due to bar pullout. It was determined that the actual bond stress distribution as a function of the embedment length is practically parabolic and can be described by the derivative of a modified form of the logistic growth function used to approximate the strain distribution along the embedment length. Furthermore, the maximum bond stress location progressively moves from the loaded-end towards the unloaded-end as the bond continues to deteriorate with increasing GFRP stress levels. The development length recommendations by ACI 440.1 and to a lesser degree, CSA S806 and CSA S6 are quite conservative compared to that which is required. It is observed that pullout tests alone cannot provide sufficient knowledge regarding the bond behaviour of FRP reinforcement; consequently, the results of beam-bond testing are more appropriate. Standard pullout tests may be incorporated into quality assurance programs with the understanding that they cannot provide valuable information regarding bond stress distribution and required development length in real structural elements with large embedment lengths. In terms of the beam-bond test method, the RILEM TC-RC5 design recommendation appears to be superior since it eschews severe stress perturbation caused by incidence of flexural cracks at beam midspan. As a result, it produces stability in the terms of the data gathered from the strain gauges placed on the GFRP bar. This benefit outweighs the ease of constructability of the Notched beams as well as their resemblance to real beams. / Thesis / Master of Applied Science (MASc) / The force that bonds a reinforcing rod to concrete is determined using three test methods. Each method is recommended by some design standards, but it is unclear how the results of these tests compare to each other. To shed light on the issue, a 15 𝑚𝑚 fibre glass rod was tested using three well-known test methods. It was discovered that two of the methods give results that are reasonably close while the third gives variable results that generally do not agree with the results of the other two. It was also discovered that the required embedment length recommended for such a bar by design codes and standards are relatively excessive because they underestimate the actual bond strength of the rod. Since sometimes it may be difficult to provide such long length in practice, it is recommended that the code requirements be revisited.

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