Strength deterioration law and microstructural mechanism in concrete sprayed with inorganic coatings under the freeze–thaw cycle

Based on compressive strength analysis, ultrasonic velocity testing and microstructural damage of three groups of concrete sprayed with inorganic coatings with different mix ratios were carried out under the freeze and thaw cycles (F-T). The strength attenuations of three groups of concrete were investigated, and a linear regression model showing the relationship model between acoustic parameters of three groups of concrete and their physico-mechanical properties were constructed, and the micro-mechanism behind the strength decay of concrete was explained via scanning electron microscopy. The results show that in case of the same F-T cycles concrete sprayed inorganic coating adding a polypropylene fibre leads to a good anti-freezing performance. The trend in ultrasonic velocity decay in concrete under the F-T cycles is consistent with the trend in compression strength change. The ultrasonic velocity (UV) of the concrete shows a great correlation with compression strength: the greater the compression strength of concrete, the higher the UV. The losses in compressive strength of concrete in the three kinds (A, B and C, A is with silica fume, B is plain concrete, C is with polypropylene fibres) after 300 freeze–thaw cycles are 54.55%, 62.25% and 22.26%, respectively, which of ultrasonic compressive wave velocities are 13.81%, 16.65% and 3.77%, respectively. Concrete strength decreases during the freeze–thaw process; this is microscopically manifested as large pores, an increase in cracks, and the development of scattered primary pores affecting the centralised connectivity. The cracks of A group have a width of 5–10 μm, which of B group have a width of 5–20 μm), which of C group have a width of 1–2 μm. The whole process of F-T is the process of generating and enlarging cracks in the inner microstructure of the concrete, which results in a markedly reduction in the mechanical characteristics of concrete.