Life cycle behaviour of concrete bridges laboratory test and probabilistic evaluation

Sammanfattning: The structural life for a concrete structure located in an environment where corrosion is promoted by humidity or chlorides from sea or de-icing salt could in general be described in the following manner. The structure is manufactured and is at that point considered to be intact. Corrosion is assumed to attack the steel reinforcement, and at a certain corrosion level the structure has to be repaired. The cover concrete is removed and the corroded steel reinforcement is cleaned from corrosion products. A repair system consisting of primer and repair mortar is used to refill the cavity left after the removed concrete. The structure is now considered repaired in the sense that the degradation rate is decreased and the signs of corrosion are taken away. The corrosion attack and repair procedure could affect the load carrying capacity of the repaired beam in terms of decreased steel content and changed interface conditions between the steel and repair mortar. Strengthening could be applied to fulfil a possible lack of load carrying capacity. The life cycle described above has been simulated in a laboratory environment. The test program and the results provided from the monitoring of beam specimens are presented in the thesis. A probabilistic approach is employed in this thesis to calculate the change in probability of failure for the different stages of the life cycle. First, all relevant parameters were considered as stochastic and given appropriate statistical properties. With this information the increase in probability of failure is estimated for the corroded, repaired and strengthened beams compared to the intact beam. It has been found that the accelerated corrosion setup provided a steel mass content loss of 12% in the corroded region, corresponding to an average decrease in steel bar diameter by 6%. This corrosion damage was obtained after 75 days of accelerated corrosion at a corrosion current density of 0,10 mA/cm2. Both evenly distributed corrosion as well as pitting corrosion attack was observed. The concrete beam stiffness was recorded to 2980 kNm2 before the corrosion process and decreased by 15% to 2530 kNm2 after corrosion of tensile steel reinforcement. This is verified both by measuring stiffness globally using displacement gauges and locally using strain sensors. The result indicates that there is a strong relation between the deterioration process and the change in curvature and stiffness, suggesting that this is a method to measure the status of the structure. The status could for instance be defined by a performance factor, which equals 1 for the intact structure and then decrease to represent the relation between the stiffness of the deteriorated and the intact structure. If the structure is strengthened, the performance factor could be larger than 1. The ductility of the corroded steel reinforcing bars decreased with 55% due to corrosion compared to the undamaged steel reinforcing bars. The ultimate strain for the corroded bars was recorded to 10%, while the ultimate strain for the undamaged bars was 22%. This reduction is caused by pitting corrosion, which produces local stress concentrations along the bar. The failure occurs when the ultimate strain capacity is exceeded in one cross section, leading to an early failure of the steel bar specimen. The global extension of the steel specimen remains small as the failure strain acts on a small region of the total length. For the structural element this will lead to a failure at a particular corrosion level, since the local pits will dramatically decrease the load carrying capacity in one section. A failure of a structural member which is attacked by pitting corrosion could be unnoticed in terms of visual evidence, since the elongation of the steel reinforcement is be kept at a moderate level at failure of steel reinforcement because of the local damages that pitting may create. The strain at yielding is recorded to 0,39% for the intact steel bar and 0,43% for the corroded. Failure was defined as yielding of steel reinforcement for unstrengthened beams, and as debonding of CFRP plate for the strengthened beam. The load carrying capacity for the intact beam was 79,8 kN. The load carrying capacity was decreased by 15% after corrosion of steel reinforcement to 69 kN. For the beam where the cover concrete was removed the load carrying capacity was decreased another 18% down to 60 kN in comparison to the intact beam. Yielding of steel reinforcement for the repaired beam occurred at 64,8 kN, and debonding of CFRP plate for the repaired & strengthened beam occurred at 82,7 kN. These results show that a 12% reduction of steel content in the cross section occurred during the corrosion phase, at the same time as the stiffness was reduced by 15%. An analytical model indicates that the 12% reduction of steel content should decrease the stiffness by 9%. The remaining stiffness decrease may be coupled to creep. Another important fact is that the particular strengthening design upgraded the repaired & strengthened beam to reach a load carrying capacity which exceeds the intact beam. The life cycle behaviour for the concrete beams used in the study shows the same general results in comparison to an FE-analysis. It should be mentioned that the FE-analysis performed has not been done on the tested beams in this study. An analysis of these will however be conducted at a later stage. The probabilistic approach of the studied life cycle shows that the probability of failure increased two times for the corroded beam compared to the intact beam, and further up to seven times for the repaired beam. The increase in probability of failure for the corroded beam is related to steel mass loss. The repaired beam has an even higher probability of failure than the corroded beam since the effective height is reduced during removal of cover concrete of the loaded beam. By strengthening the repaired beam by bonding a CFRP plate, the probability of failure is decreased beyond the intact beam for the particular strengthening operation performed in the study.