Damage evaluation of geopolymer under three freezing-thawing treatments with different freezing temperatures based on ultrasonic technique

The damage monitoring of geopolymers in cold regions is important to ensure their durability. This paper presents an investigation on the damage evaluation of geopolymer under three freezing-thawing treatments with different freezing temperatures based on an ultrasonic technique. Firstly, 95 metakaolin-based geopolymer specimens were prepared and subjected to freezing-thawing (F-T) cycles under five freezing temperatures (-1.0 degrees C, -10.0 degrees C, -20.0 degrees C, -30.0 degrees C, -40.0 degrees C) by three F-T treatments, which are freezing in air and thawing in water (AW), freezing in water and thawing in water (W-W) and freezing and thawing in air (A-A), respectively. Secondly, the nondestructive ultrasonic tests were used to monitor the degradation of the geopolymer during F-T cycles and uniaxial compressive strength tests were conducted to study the residual strength of geopolymer after F-T cycles. Thirdly, the effects of freezing temperature and F-T treatment on the longitudinal wave velocity and ultimate compressive strength were analyzed. Finally, a nondestructive method was proposed to predict the compressive strength of geopolymer after F-T cycles. The results showed that the longitudinal wave velocity, dynamic elastic modulus and uniaxial compressive strength of geopolymer all decrease as the freezing temperature decreases. The damage difference of geopolymer under three F-T treatment methods gradually increases as the freezing temperature decreases. Compared with W-W method and A-A method, the geopolymer under A-W treatment method showed the most serious deterioration. The results also showed that the compressive strength damage and longitudinal wave velocity damage exhibited exponential relations. The nondestructive method based on longitudinal wave velocity can efficiently predict the damage evaluation of geopolymer under different F-T treatments and freezing temperatures.