Evaluation of aging and repaired reinforced concrete structures

This dissertation critically examines the issues of bridges subjected to spalling because of exposure to aggressive environmental conditions, truck impacts and use of deicing salts in cold weather countries. The aging of reinforced concrete structures is one of the biggest concerns in civil engineering today since billions of dollars are spent annually on deck repairs and replacements. The first section of the thesis investigates the reliability of bridge beams with exposed reinforcement to determine when reinforced concrete continuous bridges with T-cross sections and exposure of the reinforcement should be repaired with patches and in which case the use of temporary supports is necessary to keep the bridge in service. The random nature of both resistance and load parameters is considered for an increased understanding of the reliability when reinforcement exposure occurs. Tests on beams with T-cross sections and different length of exposed reinforcement are carried out to assess the strength and stiffness deterioration. A probabilistic analysis is then performed on a continuous bridge, designed in accordance with the Canadian Code, considering different scenarios of exposure. The probability of failure and the corresponding reliability index were calculated for each scenario with Monte Carlo simulations. Over the range of steel reinforcement exposures, the reliability index changed from 3.14 to 2.81 and the probability of failure changed from 1.6X10^-3 to 4.4X10^-3. The second section of the dissertation examines reinforced concrete box girder bridges rehabilitated with concrete overlays. The restraining effect of the substrate on the shrinkage of the new layer of concrete leads to the development of stresses, which may cause cracking and unbonding of the overlay. This study carries out a two-dimension finite element analysis of a reinforced concrete box girder bridge to evaluate humidity and free shrinkage strain profiles at different times, starting from the day of casting until 50 years later. The rate of shrinkage change is taken as a function of the humidity change. The humidity gradient between the overlay and the substrate generates differential volume changes between substrate and overlay: the substrate deformations are negligible while the overlay is subjected to high shrinkage; 78% of the ultimate shrinkage strain is reached after 3 years indicating a high susceptibility to cracking. Investigating the behaviour of aging and repaired bridges is a continuing concern in civil engineering, because the service life of damaged bridges is reduced and maintenance costs are increased, generating issues in the management of a country infrastructure network. The first section of this dissertation provides the first probabilistic assessment of a continuous reinforced concrete bridge with exposed reinforcement: considering a target value for the reliability index equal to 3 for an existing bridge with exposed reinforcement, results of the probabilistic analysis suggest that rehabilitation works should be prioritized for exposure lengths larger than or equal to 43%. The second section of the thesis explores with a 2D FEA the behaviour of a reinforced concrete box girder bridge rehabilitated with a concrete overlay: the high susceptibility to cracking that emerges from the results suggests that the adopted thickness of 250 mm and the selected concrete compressive strength of 30 MPa used for the overlay are not suitable for the bridge of the case study. Moreover, humidity and free shrinkage strain equilibrium is almost reached after 10 years, therefore an analysis along this time range seems to be appropriate for practical applications or standard designs in the study of the probability of cracking of a bridge repaired with a concrete overlay. The adoption of this limited time range will reduce computational work for the prediction of stresses, probability of cracking or displacements due to restrained shrinkage of new concrete overlay.

https://orcid.org/0000-0001-6997-3415