Behavior And Mechanism Of Damage Induced By Freeze-thaw Cycles With Salt Solution Of Restrained Concrete

Concrete damage induced by Freeze-Thaw cycles with salt solution is a sever durability problem of concrete structures in cold areas.Deformation occurs is a symbol of concrete deterioration.In mostly used F/T tests or experiments concrete samples are free to deform but they are inconsistent with real concrete serving conditions under which concrete is restrained by surrounding structures.Test methods,behaviors and mechanism of concrete damage induced by F/T cycles were studied making the research much closer to real damage process.Main works and results are as follows:A restraining device was designed to restrain deformation of concrete in F/T cycles which consisted of a stainless ring and stainless anchors for expansion and contraction restraining;Special strain sensors were designed for instant concrete strain data collection;Surface scaling mass,relative dynamic modulus loss and residual strain were used as concrete surface and internal damage evaluation parameters.An integrated test method of F/T damage with salt solution for restrained concrete was studied and established.Behaviors of damage induced by F/T cycles with salt solution were studied and acquired.As F/T cycles increased,surface and internal damage of restrained concrete gradually deepened;Increasing air content(from 1.3% to 7.1%)and dropping w/c ratio(from 0.65 to 0.45)were able to reduce surface and internal damage of restrained concrete;Under the same conditions,surface damage of restrained concrete was severer than unrestrained concrete and internal damage of restrained concrete was lighter than unrestrained concrete.Surface maximum stress of restrained concrete in the process of F/T damage with salt solution was calculated to study the mechanism of surface damage.When temperature were decreased to-8°C,surface maximum stress exceeded tensile strength of concrete surface and surface scaling started.When temperature were decreased to-12°C subsequently which is the freezing point of concrete internal pores,the maximum stress of restrained concrete began to exceed that of unrestrained concrete,and as a result,surface damage of restrained concrete was severer.Pore structure parameters were analyzed and strain induced by pore cracks was calculated to study the mechanism of internal damage.Pore expansion degree,crack initiation numbers and crack growth rate of restrained concrete is less than unrestrained concrete.When temperature were decreased to-15.8°C ~ 20°C,irreversible strain occurred which meant the emerging of concrete internal damage.Irreversible strain of restrained concrete was less than that of unrestrained concrete thus internal damage of restrained concrete was lighter.