Study on freeze-thaw deterioration model of new-to-old concrete based on pore surface fractal characteristics

Abstract

New-to-old concrete (NTOC) structures are widely used in engineering practice, the adhesive interface as a weak part is easily damaged under freeze-thaw environment. At present, most of NTOC deterioration models are grounded in the empirical summary of experiment data. In this paper, the bonding and durability of NTOC adhesive interface under different freeze-thaw cycles were investigated by splitting tensile test and ultrasonic nondestructive test, and the pore distribution characteristics of adhesive interface were obtained by mercury intrusion porosimetry. Based on the S-N equation and the constitutive relationship of damaged concrete, the phenomenological freeze-thaw damage model of NTOC was established. According to the fractal analysis of the effective bearing area of the pores, the freeze-thaw deterioration model of NTOC adhesive interface was further established, which was named PDM model. The splitting tensile strength (macroscopic mechanical properties) and ultrasonic wave velocity (microscopic defects) of the concrete repaired by four kinds of alkali activated slag-fly ash were used to verify PDM model. The results showed that PDM model could reflect the change of bonding properties and the defects of NTOC adhesive interface under freeze-thaw conditions. The microscopic pore evolution characteristics reflected the macroscopic mechanical properties. With a restricted number of monitoring sites, the PDM model could n forecast the changing trend of the whole freeze-thaw degradation process of NTOC.