Characterization of the influence of the casting mould on the surface properties of concrete and on the adhesion of a protective coating

Protective coatings are deposited on concrete to improve aesthetics and to prevent ageing. However, their adhesion on concrete depends on several interlinked parameters. In this study, the surfaces of concrete are characterized according to the process of casting and post-treatment used (sandblasting) by using Scanning Electron Microscopy (SEM), Fourier Transformed-Infrared (FTIR) spectroscopy and profilometry. The surface properties are correlated to the adhesion force of a polyurea (PU) coating. The development of a specific peel test (a strengthened and porous membrane is introduced into the layer of liquid coating before its crosslinking) ensures a reproducible debonding of the coating/concrete system and allows measuring the fracture energy. Moreover, the interface after debonding is analyzed by FT-IR to highlight the presence of concrete/coating residues and to determine the locus of failure. Results underline that the nature of casting mould influences the concrete surface and modifies the adhesion of PU coating. The mould made of polyoxymethylene (POM) induces a micro-tearing of the extreme surface of concrete during demoulding. By increasing the roughness and the open porosity of the concrete surface, this tearing enhances the adhesion of the coating. On the contrary, the smooth concrete surface, induced by the use of a polyvinylchloride (PVC) mould, decreases the anchorage of the coating. Finally, the sandblasting of the surface, by increasing the roughness and the interface area, is an interesting treatment to promote the adhesion of PU coating, whatever the mould used for the casting.