Simulated oxygen concentration gradients for encapsulation rehabilitation optionAbstract
The University Bridge in Saskatoon is a multi-span concrete arch bridge with a service life that began in 1916. Rehabilitation of the bridge deck is currently underway and expected to be completed during the summer of 2015 with rehabilitation of the bridge arches and piers planned to occur in 10 to 15 years.
This manuscript outlines an investigation taken to evaluate a number of different arch rehabilitation strategies using numerical models and experimental data. The rehabilitation strategies considered in this investigation include: do nothing, Fiber Reinforced Polymer wrap, Galvanic Cathodic Protection, Impressed Cathodic Protection, Re-alkalization, Electrochemical Chloride Extraction, and applying a Penetrating Sealer.
Experimental data was obtained concerning properties of the concrete, pore solution, rebar, and environmental conditions within the concrete. Cores were removed for compression testing and pore solution composition analysis. Rebar was also removed for electrochemical testing, including potentiodynamic scans and long-term exposure tests. Semi-permanent probes were inserted to measure environmental properties inside the concrete related to water saturation and oxygen content. Values from these experimental data were then used in the development of numerical models.
Simulations and measured data demonstrated low moisture levelsand high oxygen content within the concrete were coupled with low/insignificant corrosion rates on the reinforcing metal in most regions. However, measured data suggested high negative half-cell readings could be associated with regions of high concrete moisture (above 60% by volume) due to surface runoff and water pooling on the arches. Numerical modeling and experimental data suggest the relatively low moisture levels within the concrete could be utilized and enhanced to mitigate the corrosion of reinforcing steel. Therefore, building on these conditions, a synergistic mitigation strategy was recommended that incorporated two technologies: discrete Galvanic Cathodic Protection anodes and a Penetrating Sealer. Calculated savings due to this study revealed savings of the order-of-magnitude in the range of $2 to $4 million when compared with alternate rehabilitation strategies.
Additionally,the results of the assessment demonstrate the age of a structure has limited bearing on the remaining service life. Rather, if the underlying deterioration processes are controlled or minimized, the life of the element can be extended indefinitely so long as regular monitoring is in place to identify changes in either the environment or condition of the element that would affect the deterioration rates. As such, with the proposed rehabilitation method the City will have a system that with regular monitoring, which is already part of their City wide asset management system, will be easily capable of meeting the desired 50 year service life extension at a minimal cost impact to the overall project.
