ABSTRACT
The deteriorating process of concrete suffered Freezing-Thawing (FT) is a process of initiation and development of microcracks. It’s very important to establish the correlation between the macroscopic properties and the microcrack characteristics to evaluate the damage extent in concrete. The first step is to quantify the microcrack characteristics such as length, width and density. Though CT or MRI is a promising tool of analyzing microcracks nondestructively, the spatial resolution of either of them is far from satisfactory to detect FT-induced microcracks. Traditional microcrack-analyzing techniques such as Scanning Electron Microscope (SEM), Optical Microscope (OM), Fluorescent Microscope (FM) are all based on a limited number of small microscopic samples 1 or 2 mm in dimension cut from concrete slices. However, the limited samples are not representative and the microscopic analysis results are not reliable because concrete is a highly heterogeneous materials composed of mortar, Interfacial Transitional Zones (ITZ) and aggregates. Accordingly, this paper presented a method of acquiring panoramic microscopic images of concrete slices (10 cm * 10 cm in size) impregnated with fluorescent epoxy by Automated Panoramic Fluorescent Microscope (APFM) which is designed and developed by our team. The APFM is characterized by four automatic modules, i.e., auto-scanning module, auto-focusing module, auto-mosaicing module and auto-analyzing module. Panoramic images of the concrete slices can be obtained by APFM in about two hours, which is efficient and accurate. In addition the area, length, density of microcracks in the slices can also be extracted and analyzed in the auto-analyzing module by digital-image-processing technique once the panoramic image is obtained. The evolution of microcrack characteristics in concrete during the FT damaging process is obtained and the results show that the length density and area density of the micro-cracks increase with the increase of the FT damage degrees. Relationships between the mechanical properties and the microcrack density are also established. Quantitative microcrack analysis based on APFM is a promising tool in evaluating FT damage and revealing the essence of damage in concrete.
