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Basic Digital Image Color Analysis for Nondestructive Testing of Setting and Curing Process of Building Products: Application to Thin Render Layers Sprayed at Building Site

Many building products are put in place in liquid or plastic state. During the first hours and days, the products set and dry, changing their mechanical properties, and their visual aspect also changes in parallel because of the hardening process. In the case of sprayed products, the technical supervisor must know at any time the quality of every sprayed layer and finally determine when it has reached its point of maturity that allows considering it as ready or suitable to receive the next layer. Experimented professionals often use personal visual ability to recognize the evolution of visual colors and surface textures of thin render layers applied on site as capacity to determine this right moment, especially in the case of dry mix sprayed products that harden progressively during curing process. However, these visual assessments are not considered decisive as a technical procedure to assess the evolution of the sprayed product properties. As they are thin layers, it can be considered that the information acquired from their surface is sufficiently representative of their interior qualities. The aim of this study is to correlate information obtained in successive local measurements of humidity and penetration resistance taken on the early surface of a dry mix product just sprayed (portland cement mortar and gypsum plaster) during its setting phase and to compare it with its surface digital color evolution during this process. The goal was to explore whether it is possible to establish a relationship between evolution of resistance penetration and evolution of some digital image basic parameters during the setting and drying period. If this relationship were sufficient, conventional point-to-point contact tests (destructive tests) could be replaced in the future by nondestructive surface visual tests. This is very useful at the building site where there are big surfaces to assess and only a short time to make the right decisions.