A life-cycle cost model for use in the optimization of concrete bridge deck repair and rehabilitation

This research shows that it is possible to develop a life-cycle cost model incorporating both deterministic and probabilistic components. The development of the cost model by assembling large amounts of data from many state highway agencies was critical to the success of the model. By correcting the data for time and location it was possible to develop a reasonably deep data base for the repair alternatives. The deterioration model combines the deterministic diffusion equation with the probabilistic nature of composite materials, like concrete. This was made easier by the use of a Monte Carlo simulation. Probability distributions for the diffusion coefficient, equilibrium chloride concentration, reinforcing bar cover and chloride threshold limit were developed and used as input for the simulation. The outcome of the simulation was the chloride content at the bar depth, the percentage of the deck which was corroding, and the corrosion due to subsidence cracking. These were combined to determine corroded area and then the quantity needed for repair. The life-cycle cost model was developed to optimized the time at which partial depth patching occurs and selection of patching and/or overlaying as repair strategies. The most significant factor in the optimization was the cover depth. This played a significant part because of time to corrosion effects. The present value of repairs for shallow cover was much greater than for deeper cover. It was determined to be economically feasible to increase the cover depth from 2.0 inches to 3.0 inches based on the savings from repairs compared to the additional cost of concrete in the deck. In a comparison between the costs for patching and overlaying the cover depth provided influence on the economics of repair choices. For the shallowest cover, it was found that the most economical strategy was not to patch the deck but provide overlays at 13 and 38 years. For 2.5 inch cover, the economic decision was to patch the deck from year 15 to 24 then overlay. This option was also the optimal strategy for 3 inch cover. At this cover depth, the gap between the most and least expensive options narrowed.