Use of geothermal bridge deck deicing systems to mitigate concrete deterioration in Montana

Abstract

Reinforced concrete bridge decks face deterioration from thermal stresses, frost action, and early-age cracking. This thesis presents experimental testing and numerical simulations on a bridge deck deicing system's ability to mitigate concrete deterioration. Two experimental bridge deck models were constructed with embedded heat exchanger tubing and instrumented with thermocouples and strain gauges. The models were tested in a cold chamber laboratory under conditions representative of Montana winter weather. The experimental results suggested that a bridge deck deicing system with an inlet temperature of 8 °C shows promise in deicing, reducing thermal movements, and mitigating early-age cracking through thermal shrinkage. The temperature and strain results of the experiment were used to validate a numerical model constructed in COMSOL Multiphysics. Inlet fluid temperatures of 10 °C and 50 °C, chosen from common ground temperatures in Montana, were tested to evaluate the system's effect on frost action and thermal stresses. With a 10 °C inlet fluid temperature, the system showed promise in deicing and mitigating concrete deterioration. While the system did not always raise the surface temperature above 0 °C, the consistent increase in temperature suggested that under certain weather conditions, the system could keep the top surface temperature above 0 °C for a longer period than with no system. The system was also successful in reducing the range of strain due to thermal movements. The system was not able to mitigate the effects of frost action or temperature gradients. The temperature gradients induced by the system were at times worse than without the system, but the difference was insignificant. With a 50 °C fluid temperature, the system was more effective in deicing and mitigating frost action. The range of strain from thermal movements was also reduced more than with a 10 °C inlet fluid temperature. The thermal gradients, however, were at times slightly greater than design gradients provided by design specifications. The excessive gradients, however, only occurred during extreme weather conditions that are less common in Montana. While not perfect, geothermal bridge deck deicing systems show promise for mitigating some mechanisms of concrete deterioration, while keeping other mechanisms within allowable limits.