What hydrophobic agents do modern water repellents contain and how long do they last?

Decades of research and development in the field of water repellents have led to modern silane/siloxane-based systems. Their special material properties enable a high water-repellent effect and thus effective protection of the concrete. However, water repellents are not permanently resistant and must be renewed every two to five years, depending on the substrate.  

Originally, silicones and silicone resins dissolved in solvents were used. However, these did not catch on because they had low penetration properties and insufficient alkali resistance. To counteract these weaknesses, alternative raw materials were developed based on low-molecular alkyl silanes and oligomeric siloxanes. Modern hydrophobic treatment systems are therefore usually silane/siloxane-based, but are only available as mixtures, for example, solvent-based or aqueous. Only after these substances have been applied and cured on the concrete substrate does the end product form – a pore-lining silicone resin film that can be regarded as chemically bonded in the concrete. This layer does not form a film, but varies in depth depending on the penetration depth and – since it is bonded in cross-section – cannot be easily removed. At the very least, it cannot be mechanically removed without removing the layer close to the surface.    

Both enable a reduction in water ingress into the component surface, which can have varying degrees of effectiveness. Over time, this effect wears off because hydrophobic coatings are relatively chemically inert, meaning they no longer react with potential reaction partners such as air and water over time. After application, the silicone resin film reacts completely within its specific reaction time. Over time, this film loses its effectiveness due to weathering and is therefore only considered temporary protection. In principle, hydrophobic concrete surfaces therefore exhibit minimized penetration of water and thus also of transported chlorides. This in turn leads to increased protection of the entire structure against frost and deicing salt stress. In addition, there are visual advantages: surfaces remain dry longer in rain and are largely resistant to biological infestation. 

Due to their chemical structure, hydrophobic coatings are UV-stable, but they are degraded by rain or reactive components in the atmosphere. This initially leads to uneven water absorption and can later cause visible color changes. As a result, the overall appearance can be impaired. Due to these aging processes, hydrophobic coatings generally have a significantly shorter service life than the concrete itself and must therefore be renewed several times during the entire service life of the concrete. In practice, it has been shown that these systems must be renewed approximately every two to five years, depending on the absorbency of the substrate. The manufacturer's instructions must always be followed.  

Modern silane/siloxane-based hydrophobic coatings have established themselves as an efficient temporary protection for concrete surfaces. They significantly reduce water and chloride ingress and thus contribute significantly to protection against frost and de-icing salt damage. Despite their chemical stability, however, they are susceptible to aging and must be renewed at regular intervals. Both the penetration depth and correct application to the respective substrate are crucial for their effectiveness.