Microcrack characterization of loaded Engineered Cementitious Composites via optical scans and photogrammetric analyses

Abstract

Concrete cracks accelerate the ingress of harmful species and must be gauged accurately for durability evaluation and prediction. As a novel class of ultra-ductile concrete, Engineered Cementitious Composites (ECC) develops multiple sub-hairline cracks that are difficult to visualize using conventional techniques. In this study, a semi in-situ laboratory approach is proposed based on optical microscopy and photogrammetric analysis to enhance the accuracy of crack width measurement in ECC. Microscopic images scanned from the material surface were reconstructed and analyzed to extract the statistics of crack width, location, number, and spacing. Results suggested that general portable microscopes can effectively identify cracks smaller than 2 µm in loaded condition and provide magnified high-resolution crack images that are hardly achieved by regular digital cameras. Crack openings can be isolated via 2D FFT filtering and binarized to attain the datasets of crack spacing and crack width distribution. In contrast to microscopy, conventional digital image correlation (DIC) as an indirect method was found to overestimate sub-pixel crack widths particularly when the crack width and spacing are substantially smaller than the image resolution. In these scenarios, it is recommended to combine optical microscopy with DIC to ensure simultaneously accurate and holistic crack width characterization. The proposed technique can be used for general materials with multiple fine cracks.