Concrete bridges in cold climates

Concrete bridges are susceptible to freeze/thaw cycles and the use of chloride-based de-icing salts where sub-zero temperatures are prevalent. Factors affecting the durability of unprotected reinforced concrete are examined, and the latest developments in methods of protection are discussed. Water must be present in the capillary pore structure of the cement paste for it to be damaged by frost action. When the water freezes its expansion is restricted and a pressure is built up further depressing the freezing point. Chloride ions present in de-icing salts break down the passive protection layer on steel reinforcement. Sulphates present in de-icing salts can also cause problems. Absorbed sulphates will react with the calcium aluminate hydrates to form ettringites and with free lime to form gypsum. The development of cementitious polymer modified coatings has provided the technology to protect concrete and steel reinforcement from the principal causes of deterioration. It is now possible to achieve a matrix density such that water cannot penetrate a 2mm thick film under a 100m pressure head. The same technology provides a high degree of resistance to the diffusion of carbon dioxide and oxygen. The application of a suitable chloride barrier will prolong the effective life of structures where chloride contamination is not too advanced. Polymer modified repair monitors are also available. Examples are given of its use on: (a) the Borgarfjadur bridge in Iceland; and (b) on bridges in Arendel, Norway where the surface had been damaged by studded tyres and snow chains. Where the concrete is protected on construction, evidence suggests that the structure performs reliably and effectively even in the harshest of climates.