Abstract: In the purpose of obtaining variation characteristics of pore distribution for material under freeze-thaw cycles, Nuclear magnetic resonance (NMR) technique was employed in this research to scan concrete specimens undergone 0, 50, 100, and 200 times of freeze-thaw cycles. The spectrum of relaxation time T₂ and NMR scanning images were acquired. The variation of T₂ distribution curve reflected the evolution and expansions of pores in the specimen undergone freeze-thaw cycles. With the proceeding of cyclic freezing and thawing, the T₂ spectral area which characterizes the porosity of the materials surged in the initial stage; but such increment attenuated later. The proportion of small pores reduced whereas the ratio of macropores increased. The NMR scanning images of concrete specimen with various damage degrees synchronized well with the variations of T₂ distribution spectrum, which implied the accuracy of scanning results. In conclusion, the joint failure due to the combined action of osmotic force and frost heave force in freeze-thaw cycles is the fundamental cause for the formation of connecting fracture.

Key words: concrete, NMR, freeze-thaw cycles, pore structure, damage characteristic