Testing the Test: Water repellents

by David H. Nicastro, PE
Water repellents can be confusing, from the imprecise terminology (i.e. sealers versus coatings versus sealants) to the wide variety of chemistries available. There can also be a stigma associated with repellents—many consultants are concerned about negative side effects from applying a material to the outside of a porous material, potentially impeding vapor drive from the interior.

Caution is always prudent, especially with historic buildings that can be permanently damaged by wrong-headed remediation. With reasonable care, however, appropriate repellents can be selected to provide a range of benefits, including:

  • water repellency;
  • preventing mildew growth;
  • reducing efflorescence staining;
  • preventing graffiti from adhering;
  • protecting concrete from freeze-thaw damage and corrosion of embedded reinforcing steel; and
  • stopping compounds in concrete panels from leaching onto glass, where it converts into insoluble surface deposits.

Some repellents are catalyzed so they cure on any substrate (which requires masking unintended surfaces, like windows), while others require a reaction with a specific substrate to cure, such as the high pH of concrete. Some formulations deteriorate in sunlight faster than others, requiring reapplication after five to 10 years.

Several products use room-temperature-vulcanizing (RTV) silicone, diluted with a solvent; the resulting liquid with five to 15 percent solids can be considered ‘hybrids,’ falling between a typical penetrant and a film-forming coating. In addition to the benefits listed, these robust products have better durability and can be integrated with silicone sealant on challenging joint substrates, like rough aggregate precast concrete panels. Spray-applied after the joint sealant cures, testing indicates the chemically compatible silicone repellent fills micro-voids between the sealant and the aggregate that would otherwise leak.

However, these advantages come at costs—the products are more expensive than traditional repellents, they may darken substrates, and they have a strong solvent odor during application.

Although repellents are usually considered to be clear, the author’s firm has worked with manufacturers to add colored dye, including emerald green and obsidian black, to certain repellents so they can enhance the appearance of a faded façade.

David H. Nicastro, PE, is the founder of Building Diagnostics Inc., and leads the research being performed at The Durability Lab. He can be reached by e-mail at dnicastro@buildingdx.com.

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