A microcapsule-type fluorescent probe for the detection of microcracks in cementitious materials

Fluorescence-based microcrack probing was accomplished by using a polymeric coating in which fluorescent fluid-loaded microcapsules are embedded. A fluorescent fluid, as a crack-indicating agent, was microencapsulated with urea–formaldehyde polymer. The formation of microcapsules was confirmed by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). The average diameter and size distribution of the microcapsules was controlled by agitation rate. A fluorescent microcrack probe was fabricated by dispersing the microcapsules in a commercial coating formulation followed by coating the resultant mixture on cellulose-fiber-reinforced-cement (CRC) board or mortar specimens. It was confirmed by optical microscopy that, when microcracks occur in surface of the coated specimens, the fluorescent fluid is released from ruptured microcapsules and fills the damaged region. The microcracks can be effectively detected under 365 or 450nm light through strong fluorescence emission, while it was difficult to detect the cracks under white light. The intensity of fluorescence emission through the cracks increases with increasing microcapsule density of the coating. The microcapsule-type fluorescent microcrack probe system can offer the advantages of easy fabrication and effective detection of the exact position of microcracks.