Ultra-High Performance Concrete. Surface Properties and Protection

Unlike more conventional concretes, ultra-high-performance concrete (UHPC) formulations are characterized by specific granular stacking, the presence of fine particles and a low water/cement ratio. In addition to mechanical properties unrivalled in the concrete sector, this type of formulation makes it possible to obtain highly aesthetic elements with potentially complex shapes, while providing fine control over surface texture. The field of application for this type of concrete is therefore expanding rapidly, enabling architects to think up ever more ambitious projects.

A UHPC formulation, in contact with a mold, results in a surface that is significantly smoother than that of other types of concrete, with relatively low permeability (to liquids and gases). This very low permeability ensures very long structural durability. Nevertheless, the very low porosity of UHPC is synonymous with very small pore diameters (close to ten nanometers). This open microporosity is mainly responsible for the degradation of surface aesthetics through soiling: dust, colorants (or tannins) can indeed get inside these micropores and become very difficult to dislodge or clean. This microporosity is also at the origin of condensation/drying of water at capillary level, which can lead, under certain conditions, to the appearance of whitish areas based on calcium salts. Finally, this open microporosity greatly influences adhesion between UHPC and protective coatings, which are usually applied a few days to a few weeks after the parts have been demolded.

From this information, it is easy to understand that formwork and storage conditions after demolding can strongly influence the surface properties of UHPC. For example, the adhesion of certain varnishes and the growth of certain biological agents (algae) can be influenced by the microtexture and topography of the concrete induced during forming. To avoid aesthetic damage, hydrophobic or hydrophilic functionalities can be sought by depositing protective coatings.

This article will first outline the intrinsic characteristics of ultra-high-performance concrete surfaces (morphology, roughness, etc.) linked to the coupling between their rather specific formulation and certain very specific formwork conditions (nature of molds and stripping agents, for example). Next, various aesthetic criteria will be discussed in order to highlight the key parameters and mechanisms governing the appearance and fouling of UHPC surfaces. Finally, several types of protective coatings will be presented, based on their formulation and their ability to protect UHP surfaces. Particular attention will then be paid to their level of adhesion with the concrete surface, depending on the latter's characteristics.