Measurement of the sand coating layer in CFRP tendons trough image processing

The sand coating layer on CFRP tendons will influence the bond strength. It is therefore important to quantify the level of sand coating, and the variability in the sand coverage, and to correlate these parameters with measured bond strengths. A test method was developed to assess the coverage of the sand coating layer on CFRP tendons. To investigate the variability in the sand coating both 'as supplied' sand coated and half sand coated tendons were compared. Images of the tendon surface were taken using a Nikon D80 DSLR camera with a microlens and then post-processed in Matlab. To assess the level of sand coating, one end of a CFRP tendon was held in a grip that could rotate. The specimen was then rotated through increments of 18° and a high resolution photo of a 40 mm length was taken with the DSLR camera supported on a tripod. This process continued until images covering the full 360° profile of the specimen were recorded. A Matlab script was written to process the photos according to morphological open and close methods. To save computational time the photos were edited as greyscale images. The level of the sand coating layer was measured based on the different intensity values between the pixels in the sand particles and the pixels on a bare surface. The method could roughly estimate the level of the sand coating layer but further developments are needed to overcome the certain limitations.