The Influence of 30 Years Outdoor Weathering on the Durability of Hydrophobic Agents Applied on Obernkirchener Sandstones

Abstract

The durability of eleven different water repellents applied on one sandstone type was studied after a long-term weathering at seven different locations in Germany. By measuring colour changes, it could be shown that the formation of black crusts, the deposition of particles and biogenic growth caused a gradual darkening as well as significant changes in total colour over time. Additionally, the water absorption behaviour was investigated with two different methods: applying a low pressure using the pipe method and capillary water absorption measurements from a wet underlay. Afterwards, the test results were analysed with four different evaluation methods: calculation of the protection degree from pipe method and capillary water absorption, determination of the velocity of water uptake during capillary water absorption and calculation of the damaged depth of the stone surface using single-sided NMR technique. The growing damaged depth leads to an increase of the water uptake velocity and to a decrease of the protection degree of the applied hydrophobing agents. Three protective agents based on isobutyltrimethoxysilane showed already after two years of outdoor weathering a clear loss of performance, which significantly increased after 30 years of exposure.

About the effectiveness of a hydrophobic surface treatment of Baumberger Sandstones

“…1). All locations are characterised by different climatic conditions and emission loads (Orlowsky et al 2020). In addition, specimens were stored in an archive in Bochum, Germany, over the entire period and served as reference.…”

“…Climatic data and pollutant values on the sites during the last 10 years of exposure can be found in Orlowsky et al (2020). After 2 years a 3 cm thick sample was cut off each prism (Fig.…”

“…The specific water content (kg/m 2 ) determined after 90 min as well as the remaining protection degree DIN EN (16581) were compared with the amplitude values to define a specific amplitude threshold for the Baumberger Sandstone samples. The intersection of the absorption tests and depth profiles with this limit value enables the calculation of the penetration and damaged depth as well as the effective zone of the respective hydrophobing agents (described in detail in Orlowsky et al (2020)).…”

Life Cycle Prediction and Maintenance of Buildings

“…After 30 years, all the agents show a decrease in performance, but some protective agents still provide an effective hydrophobic layer. Succinctly, the authors [7] concluded that: (i) the protective agents based on isobutyltrimethoxysilane show a clear loss of performance after 2 years of outdoor weathering; (ii) agents containing siloxane, the low-molecular methylethoxysiloxanes, show a good performance, which is similar to, partly better than, that of the oligomer methylethoxysiloxane; (iii) the agents with oligomer siloxane based on an isooctylmethoxy-structure have a higher performance loss than the agents containing low-molecular methylethoxysiloxanes; (iv) after 30 years of outdoor weathering, the effectiveness of the protective agents based on silicone resin is comparable to that of low-molecular siloxanes. Concerning the exposure conditions, the degradation of the treated stones is higher in southern Germany than in North Rhine-Westphalia, mainly due to a longer weathering time of 6 years as well as the rougher environment.…”

“…Orlowsky et al [7] analysed the durability of 11 different water repellents applied on Obernkirchener Sandstones. The performance of the hydrophobic agents applied is analysed after the samples have been subjected to long-term weathering (30 years of outdoor weathering) in seven different locations in Germany.…”