Permeability Reduction of Restrained Concrete in a Chloride-Rich Environment

Mississippi Department of Transportation (MDOT) is now requiring waterproofing (permeability reducing) admixtures in bridge deck overlays in an effort to improve the long-term performance of bridges. These admixtures reportedly reduce permeability of concrete by plugging pores and capillaries throughout the entire mass of concrete. These admixtures react with water and cement hydration by-products to either formulate coalescing polymer globules or crystalline structures that seal pores, capillary tracts and micro cracks in hardened concrete. This makes hardened concrete less permeable to water and chemicals that corrode the reinforcing steel and create costly repairs. While some manufacturers note that these chemicals exceed performance requirements of industry standards such as ASTM C494 Type S, limited data is provided to document their performance in a bridge deck. Bridge structures create a restrained system for concrete shrinkage and many Mississippi bridges are exposed to chloride ions when salt is broadcast on the decks in winter months. This laboratory study will use testing equipment and chloride ion exposure to simulate these field conditions. Data compiled from this study will provide MDOT Engineers with information needed to verify that ordinary portland cement (OPC) or portland limestone cement (PLC) combined with these admixtures will reduce the permeability of portland cement concrete (PCC) in bridges. In addition, a testing protocol will be developed for MDOT to use for evaluating and approving future permeability reducing admixtures submitted to MDOT. This study will use forty-one laboratory mixtures to develop data to evaluate waterproofing admixtures that are currently approved for use on MDOT projects. Hardened concrete properties that will be evaluated include: 1) compressive strength; 2) restrained shrinkage cracking tendency; 3) chloride ion profile; 4) rapid chloride permeability; and 5) surface resistivity. The experimental mixtures will include Mississippi gravel aggregates and fine lightweight aggregate. The maximum nominal sizes of coarse aggregate will be No. 57 or No. 67 for bridge deck mixtures and No. 7, No. 78, or No. 8 for overlay mixtures. Mixtures will be developed with a blend of either OPC or PLC and supplementary cementitious materials (SCM’s) meeting the requirements of concrete mixtures for bridge decks and bridge deck overlays provided in special provision 907-804-20. Only one source of OPC, PLC, and SCMs will be utilized. Eighteen of these mixtures will focus on bridge deck overlays. The overlay mixtures will include compressive strengths ranging from 2,500 psi to 3,000 psi at 24 or 72 hours. The remaining twenty-three mixtures will meet requirements for bridge decks and will include both normal weight and lightweight fine aggregates for the purpose of internal curing.