Moisture Conditions and Frost Resistance of Concrete in Hydraulic Structures

Abstract: Owing to the winter conditions in Sweden, the effects of frost action may have a considerable impact on the deterioration of concrete. Both superficial and internal damage, which are suspected to have been caused by frost action, have been found in concrete in hydraulic structures. These observations have raised questions about the long-term behaviour of hydraulic structures in cold regions. Superficial damage, similar in appearance to salt scaling of concrete, can be seen at the waterline of hydraulic structures, such as hydro power structures. Progressive damage to the concrete surface results in exposure of coarse aggregate and also exposed reinforcing steel in the long-term perspective. A possible deterioration process of concrete at the waterline is leaching of the concrete surface, which takes place during the snowmelt runoff period. The concrete surface is thus more susceptible to frost action. During the following winter, the surface layer is damaged by frost action and later removed due to ice abrasion. During the next snowmelt runoff period, the process starts all over again. Experimental results obtained in this work confirm this deterioration process. Spalling of concrete has been observed below the water level in water retaining concrete structures subjected to long periods of freezing temperatures in winter. The hypothesis is that either poor quality concrete or the effects of aging make hardened concrete susceptible to macroscopic ice lens growth, i.e. ice segregation. Given constant thermal conditions, ice segregation occurs in undamaged concrete with water to cement-ratio (w/c-ratio) 0.9 or higher. In frost-damaged concrete, ice segregation occurs within a few days regardless of the w/c-ratio. Ice segregation may also occur in concrete with cavities or imperfections. The period of freezing required to facilitate ice segregation increases with decreasing w/c-ratio. Hence, the risk of concrete spalling in actual structures cannot be overlooked since unfavourable temperature and moisture conditions may exist in winter. Knowledge about concrete deterioration is important in order to improve the efficiency of maintenance of hydraulic structures. However, the overall purpose of gathering knowledge about damage mechanisms is to secure and to be able to safely extend the service life of hydraulic structures.