Initial Microcrack Characteristics of Concrete Interfacial Transition Zone and Cement Paste

Abstract

Three types of initial microcracks have been clearly observed in concrete: those that developed around aggregate/coarse sand, those that developed perpendicular to the aggregate, and those that developed in the cement paste. However, the evolution of each of these microcrack types has not yet been separately studied. The variations among these microcrack types and their effect on the microstructures of the interfacial transition zone (ITZ) and cement paste have not been further investigated. Accordingly, this study presents an experimental and statistical study of the systematic and quantitative analysis of the initial microcrack characteristics of the concrete ITZ and cement paste, focusing on the aforementioned core scientific deficiencies. First, the initial microcracks within concrete are quantitatively and statistically evaluated. This is achieved by developing an image recognition algorithm and interactive algorithm for boundary recognition based on the experimental results of the fluorescent epoxy impregnation method (FEIM). Then, the different behaviors of each microcrack type are identified based on the statistical analysis of the number, area, length, width, and axis orientation of the microcracks. Accordingly, the effect of each type of microcrack on the microstructure and macroproperties of concrete can also be deduced. Finally, the initial microcrack characteristics within the ITZ is examined. The proportion of the total length of microcracks to the corresponding aggregate/coarse sand perimeter (α) and the parameter RL, which can reflect the location distributions of microcracks, is used to evaluate the characteristics of the initial microcracks within the ITZ statistically and quantitatively. The statistical approach employed in this study establishes the experimental and theoretical foundation for evaluating the initial characteristics of microcracks in concrete. Concrete is a three-phase composite material consisting of coarse/fine aggregates, cement paste, and interfacial transition zone. The concept of interfacial transition zone has been widely accepted of concrete technology for many years. An interfacial transition zone is often considered a weak link because its increased microcracks may serve as a path for the ingress of aggressive species and provide preferential and easier pathways for crack growth. Because the interfacial transition zone plays a significant role in the macroproperties of concrete, further investigations focusing on the microcrack evolutions distributed in the interfacial transition zone are urgently needed. In this paper, the fluorescent epoxy impregnation method was used to obtain the real microcrack distributions within concrete. An image recognition algorithm and interactive algorithm for boundary recognition have been developed to evaluate the microcrack characteristics quantitatively. Thus, statistical analysis of microcracks distributed in the interfacial transition zone is conducted. Microcracks developed perpendicular to the aggregate and in cement paste are also analyzed for comparison. The statistical approach establishes the experimental and theoretical foundation for evaluating the microcrack characteristics and its effect on the strength, durability, and service safety of concrete engineering structures.