Quantitative damage evaluation of concrete suffered freezing–thawing by DIP technique

•FT-caused microcracking in concrete w/& w/o air-entrainment is studied by DIP.•Matrix microcrack characteristics all increase with the increase of FT damage degree.•ITZ cracks instead of mortar cracks are the major part of FT-caused cracks.•FT-caused microcracks in concrete w/& w/o air-entrainment are compared.•Relationships between mechanical properties and microcrack densities are established.

Most of the hydraulic concrete structures in cold regions of China are suffering freezing–thawing (FT) deterioration and the first step to choose repairing measures is to appropriately and quantitatively evaluate the FT damage degree in concrete. The current evaluating parameters for FT damage in concrete, such as resonant frequency and ultrasonic wave velocity are all phenomenological descriptions and cannot reveal the essence of the FT damage, which is the initiation and propagation of microcracks. This paper proposes a method to evaluate FT-damage in concrete based on vacuum–epoxy-impregnation method and digital-image-processing (DIP) technique. Quantitative microcrack analysis and mechanical tests including compressive tests, flexural tests and axial tensile tests were performed for the concrete specimens with/without air-entrainment suffered different FT cycles. Evolutions of microcrack characteristics of the matrix cracks (including Interfacial Transition Zone (ITZ) cracks and mortar cracks) in the concrete specimens were thoroughly investigated. Relationships between the relative mechanical properties and the microcrack density were established. Results show that the length density, area density and crack width of microcracks in the matrix of concrete all increase with the increase of damage degree and the ITZ cracks instead of the mortar cracks are the major part of FT-caused microcracks. The high correlation coefficients between the mechanical properties and microcrack density indicate that quantitative microcrack analysis could be used as a powerful and promising tool of quantifying FT damage in concrete.