Band 20, Heft 6

We discuss the major role played by both the wetting properties of the porous material and the boundary conditions on the drying process. For hydrophobic materials, it will be shown how under some circumstances the evaporation rate becomes lower in the presence of an air flow over the surface than without an air flow, which at first sight seems counterintuitive. Furthermore, as hydrophobic treatment can facilitate the formation of liquid pockets, we discuss how entrapped sodium chloride solution can reach a high supersaturation before crystallization, contrary to what is commonly assumed for this salt. A consequence of such a high concentration is the very rapid growth of a single skeletal (Hopper) crystal, which can lead to damage.

The protection of historical concrete structures is a challenge for the coming years. Hydrophobic treatment may reduce the deterioration process while providing protection against water penetration. Existing commercial products - mainly silane or siloxane in either water or solvent based formulations - were submitted to different ageing processes including UV radiation, dry-wet cycles, thermal shocks, freeze-thaw cycles, carbonation and their effectiveness has been evaluated on the basis of contact angle, vapour and chloride permeability and, capillarity water absorption. The results obtained clearly show that hydrophobic treatments display a good behaviour under the main ageing conditions.

The paper focuses on how alkaline media, UV radiation, and carbonation as well as on-going cement hydration affects hydrophobic treatments of concrete and influences the properties of these water-repellent layers. Single-sided nuclear magnetic resonance measurements show that layers formed by impregnating samples with alkyl trialkoxysilanes are stable even under long-term exposure to alkaline solution and UV radiation, with the damage of the latter being limited to the topmost surface layers. Microstructural changes during accelerated carbonation of blast furnace slag cement based concrete have a major impact on the hydrophobic layer properties, while the carbonation of Portland cement concrete has no influence. On-going hydration additionally influences the hydrophobic layer properties.

The evaluation of nano-particle based products to protect granite surfaces from water absorption is presented. Wettability, water absorption, drying behaviour and water vapour transfer were considered as the most relevant parameters for this evaluation. The effect of the application methods on the final performance was also tested. The results allow to consider that nanostructured products may present some advantages when compared with conventional water repellent products, namely on some common relevant harmful effects, such as colour changes, water vapour or liquid transfer during drying, but their effectiveness as barriers against water absorption upon longer contact time may not be equally satisfactory.

The study reports the results of a new treatment based on water and ethylene glycol nano-TiO 2 dispersions showing photocatalytic properties that was tested on both Noto stone (calcarenite) and Carrara marble. To verify that the proposed treatments did not affect the intrinsic properties of the stone materials, colour measurement, static contact angle and water absorption by capillarity tests were carried out before and after the treatments. The efficacy of the photocatalytic activity of TiO 2 nanoparticles applied to the stone substrates was evaluated by decomposition tests of organic colorants (Rhodamine B), both after indoor natural light exposition and after ageing in a Xenon lamp solar box. The specimens were also exposed to outdoors environment in order to verify their properties as self-cleaning agents to prevent soiling. The results showed that nano-TiO 2 dispersions were effective and suitable for the use in the field of architectural heritage.

In general, old buildings consume substantially more energy than new ones. Many historical buildings have brick-, natural stone and lavishly decorated facades that limit installation of outer insulation hence this is done on the inner side during energetic refurbishment. As a result, heat hardly gets into these walls during winter. Therefore, when driving rain penetrates into the facade in winter months, moisture cannot properly dry out and consequently, such facades remain wet for a very long time cooling down extremely, which increases the potential for frost damages. A solution is to match the degree of water repellency of the existing building materials in the specific construction with its driving rain load as well as to the properties of the chosen interior insulation. This should be done under the premise of a minimum intervention, i.e., do as much as necessary but as little as possible. One example is thoroughly discussed to illustrate this approach.

The aim of this study is to perform a life cycle cost analysis (LCC), where the economic cost of extending service life by the impregnation of bridge edge beams is compared to the reparation of an edge beam. Previous economic analyses on bridge edge beams had shown that there was no clear economic benefit in impregnating the edge beams. However, results from this study pointed out that in most cases there is a clear economic benefit to impregnate the bridge edge beams even if it has to be repeated every 15 years.