Frost Damage in Concrete in the Waterline of Porsi Hydro Power Plant

The majority of the large hydro power plants in Sweden were built 1935 - 1975 and are located in a cold climate in the northern parts of Sweden. Since the power plants were designed for a service life of 50-100 years, some of them are soon to reach their estimated service life and some have already passed it. During a condition assessment to determine the status of the Porsi hydro power plant, concrete damage, such as scaling, was observed in and around the waterline. The power plant is exposed to long and cold winters where air temperatures down to -30ºC are common.

The scope of the study presented in this paper was to determine the dominating cause of the surface damage that could have been caused by frost action, leaching or abrasion. An ice coating on the concrete surfaces below the waterline can be seen during the winter time in spite of that the concrete structures are in contact with unfrozen fresh water of a temperature above freezing. This observation raised the question that freezing of concrete in contact with unfrozen water could be the dominating deterioration mechanism. In the laboratory two concrete mixes were tested - water/cement ratio 0.65 with no air entrainment and water/cement ratio 0.45 with air entrainment. The type of cement used was CEM II. The lower halves of the specimens were submerged in water during the frost test.

The goal was to show that ice could be formed on the submerged surfaces even though the surrounding water never was frozen. The climatic boundary conditions were created in such way that the temperature gradients of the specimens were similar to the temperature gradient of the real structures of Porsi hydro power plant. The specimens were subjected to 50 freeze/thaw cycles of each 13 hours for achieving desired temperatures in the specimens. The ambient air temperature was alternating between -36ºC and +10ºC. It has been shown that concrete structures could be subjected to about 50 freeze/thaw cycles during a normal winter in Sweden.

The specimens with water/cement ratio 0.65 were damaged by scaling below the waterline, similar to the surface damage on Porsi hydro power plant. The rate of scaling was calculated to be about 0.8 mm a year.No signs of damage were observed after the frost test on the specimens with water/cement ratio 0.45. The study showed that concrete in and around the waterline of Porsi hydro power plant could be damaged by scaling in fresh water, due to frost action. Probably, the damage could be worse when leaching.