Early-age Shrinkage Measurement in 3D Printed Con-crete by Digital Image Correlation – Influence of Spray Paints

3D concrete printing is the most popular automation technique in the construction industry. One notable concern while using 3D-concrete printing is the early-age shrinkage. Early-age shrinkage can be assessed by digital image correlation (DIC) techniques, which work by creating a high-contrast grayscale pattern on the sample surface, usually obtained by using black and white spray paints. This study examines the effect of different spray paints used for performing early-age shrinkage measurements using DIC. Spray paint of black and white colors made of different materials (chalk and acrylic) was applied on the surface of the fresh 3D-printed concrete prism samples. The free shrinkage for the first 6 hours after casting was determined by tracking the movements of marker pins by using a subpixel-based MATLAB program. In addition, the mass loss of the sample was also monitored. The results indicate that chalk-based paints reduce evaporation at very early ages (within 120 minutes from the onset of casting), while acrylic-based paints significantly influence the latter-age shrinkage (beyond 45 minutes from the onset of casting). The spray paints applied on the sample surface may also modify the hydration rate of near-surface concrete. To assess the impact on cement hydration, isothermal calorimetry studies were performed by mixing spray paints at a dosage of 1% binder content. The results show that black acrylic-based paint delays and reduces the main hydration peak and, therefore, may influence the autogenous shrinkage behaviour. This study indicates that attention should be paid to the type of spray paint being used (both color and material) while performing early-age shrinkage measurements using DIC.