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Investigation on degradation of micromechanical properties of interfacial transition zone of ultra-high performance concrete under freeze-thaw cycles

In order to explore frost resistance mechanism of ultra-high performance concrete (UHPC) under freeze-thaw (F-T) action, the microstructures and micromechanical properties of two interfacial transition zones (ITZ) of steel fiber-paste and fine sand-paste are investigated using nanoindentation technology. The experimental results demonstrates that there exits an obvious weak zone in the ITZ of fiber-paste, of which the microstructure is largely affected by F-T action. The relative variation of the amplitude of modulus and hardness of calcium silicate and calcium hydroxide is less than 0.2 under F-T action. The volume fraction of micropores increases exponentially as the increase of F-T cycles, and the pore size gradually extends to tens of microns. The thickness of ITZ between steel fibers and paste gradually increased from 20 to 65 μm after 0–1500 F-T cycles. Thus, the volume fraction of micropores and the thickness of ITZ in steel fibers can be used to effectively characterize the degradation of micromechanical properties of UHPC under cold climates.