Abrasion of concrete by ice in arctic sea structures: Dissertation

In arctic sea regions a concrete sea structure is subjected to heavy mechanical loads near the water level due to the moving ice sheets.Moving ice sheets load protruding aggregate stones, and the loads are considerably greater than the compressive strength of ice as determined in uniaxial compressive tests.This is due to the triaxial compression stress in the ice surrounding the stone surface.Also, recurrent freeze thaw cycles in the concrete wetted by waves and the tide expose the concrete to damage if it has not been designed to resist recurrent freezing in marine conditions. Temperature changes that exceed the approximate value ~T = 40 °C also deteriorate the bond between the stone cement and the stones and increase cracking in the cement stone between the aggregate stones.The determination of the abrasion depth is based on laboratory tests for measuring ice pressures against protruding aggregate stones, laboratory tests for determining the strength values of concrete after freeze thaw cycles in sea water, abrasion tests with an abrasion machine, abrasion tests with an icebreaker at sea, abrasion studies on Finnish lighthouses, and computer calculations.The abrasion depth and resistance of concrete in arctic sea conditions can in practice be determined by calculations and laboratory tests.When determining the abrasion depth by calculation the various strength values of concrete must be known under freeze thaw conditions.Both the compressive and tensile strength of concrete in addition to the bond strength between aggregate stones and cement stone must be known.The bond strength of aggregate stones and its resistance to repeated freeze-thaw cycles are especially crucial.The abrasion resistance can also be tested using two laboratory tests; a cyclic freeze thaw test of 50 cycles in sea water and an abrasion test of 10 minutes with an abrasion machine.The compressive strength of concrete with a good resistance to abrasion should be at least fc = 70 MPa.In addition, the concrete must of course be frost resistant.In the tests performed the compressive and tensile strengths of concrete retained their values during the repeated freeze-thaw cycles when the water cement ratio of the concretes did not exceed w/c = 0.30--0.35.However, the bond strength of the aggregate stones was reduced more rapidly than the compressive and tensile strengths in these concretes.The best results in both the strength and abrasion tests were achieved with concretes containing silica or blast furnace slag, and the poorest results with light weight aggregate concretes.The best resistance to abrasion of concrete is achieved by preventing frost damage, by keeping the concrete either so warm or so frozen at the water level that it is not exposed to freeze thaw cycles.Also the use of hard homogeneous concrete in the ice abrasion zone reduces abrasion because the surface is subjected to uniform abrasion and there are no detaching stones.Also increasing the maximum size of the aggregate reduces the abrasion because large stones protruding from the concrete surface break the ice before it affects the finer concrete substances.