Question: We are considering bidding a project that would involve applying a clear water repellent on a recently constructed concrete block building. How should we approach the selection process for the proper material?

Answer: Most experts agree that there is no specific correct approach to the selection of appropriate clear water repellent treatments for concrete masonry. That being said, there has been a great deal of research on the subject to help contractors make an informed decision.

A practical source of information is Clear Water Repellent Treatments for Concrete Masonry, published by the Masonry Institute of America and the Concrete Masonry Association of California and Nevada.

In the 62-page book, author John Raeber provides some practical advice to your question. He advises that experience is typically the easiest means of finding an appropriate material for most projects. That's because most architectural firms only allow sufficient time on major projects for contractors or specifiers to thoroughly review product information and conduct weathering tests on system mock-ups.

While past practice may be a good guide, there can be problems associated to a selection based on performance. Changes in chemical technology of the sealant, often in response to environmental regulation changes, especially regarding limitations on volatile organic compounds (VOC), can cause manufacturers to change formulations. Then, variations in atmospheric conditions can also cause previous experience to be less applicable. In such cases it would be appropriate to research potential options.

For most projects, Raeber suggests that sealant product literature and local manufacturer representatives are the key sources of information regarding the product application. Certainly, it is in the manufacturers' interest to sell their products. Hopefully, manufacturers are the first to realize the danger to their reputation where their products prove unsuitable for a specific application.

Even so, Raeber offers 10 questions that when answered, can provide the contractor a uniform approach to compare various products.

1. What are the manufacturer's recommended applications?

Different resins have different appropriate applications. The natural porosity of concrete masonry may be inappropriate for some forms of water repellent treatments. Manufacturers will typically note whether or not concrete masonry is an appropriate substrate, if not, ask.

2. Is the product intended to be a surface film or penetrating sealer?

Most of the products are clearly formulated as either a surface film or penetrating type sealer, although some might be classified as a little of both. Where not clearly identified, the product is probably a surface sealer. Again, it would be best to check with the manufacturer.

3. How many coats are recommended?

Formulation differences make it important to understand the manufacturer's recommendations regarding application, especially the appropriate number of coats and the coverage rate in square feet per gallon. Timing between coats is also important to the application and inspection of the system. Some materials require sufficient time for the masonry to absorb the initial coat prior to reapplication. Otherwise chemicals might end up in the sewers or landscaping.

4. Can the treatment be reapplied?

Some forms of water repellent treatments create a water barrier, which even additional coats of the same material can not penetrate. It should be ascertained as to whether or not an additional coat or coats can be applied immediately, within a short period (a few days to a few months), and over the long term (after a few years).

5. Can the treatment be coated?

Whether or not the treatment is compatible with other treatments and coatings might also prove important in the long term. A new building owner might want a new color and want the concrete masonry painted. Repeated vandalism might require addition of antigraffiti coatings. Or, the treatment might not prove successful for some unforeseen reason. Knowing whether or not the water repellent treatment can be retreated, painted, or coated can be useful in making early decisions.

Where the water repellent treatment is classified as an antigraffiti coating as well, determine if there are specific types of graffiti for which the treatment will not allow easy eradication.

6. What are the effects of weather during and immediately after application?

The manufacturer can tell you what to do if it rains during or right after application. Recommendations usually list conditions required prior to application, such as requiring a clean, dry substrate. And they typically give limitations on weather conditions including minimum and maximum temperatures.

But, what should the construction team do if an unexpected shower begins in the middle of an application? Should they stop application and return after the rain ends? Should they (can they?) reapply the water repellent treatment to areas which were not yet dry when the rain began? Should the surface be protected from rain for a specific time period after application?

7. Are there any potential adverse effects such as chemical reactions with adjacent materials?

Water repellent treatments on concrete masonry will be in contact with the same materials in contact with the concrete masonry: joint sealers (silicones, polyurethanes, polysulfides, acrylics, etc.); windows and doors (aluminum, wood, steel, bronze, etc.); metal flashings (copper, galvanized steel, stainless steel, aluminum, lead coated, etc.); and other siding materials (concrete, brick, metals, wood, plastics, stucco, etc.).

Other materials such as glass and landscaping materials may not be immediately adjacent, but could also be affected by water repellent treatments due to wind, overspray, and run-off. And finally, paints on any of the adjacent and nearby materials could also be affected.

Does the treatment chemically react with these materials, damaging one or both? Does the treatment prevent adhesion, such as the bond between the joint sealer and concrete masonry?

8. Are there any special limitations on application?

Limitations should include what materials should not be coated. Maximum porosity and minimum permeability requirements might be important. Temperature limitations and moisture content at time of application might be critical to the water repellent treatment. Types of aggregate in the concrete masonry may also have some bearing on success.

9. Are there any known local limitations on use?

State and local limitations on volatile organic compounds might limit the availability of some otherwise excellent water repellent treatments in certain areas.

Such limitations may only be a concern in heavily populated areas. In other cases, such as in California, the limitations may extend to the manufacturing of the product as well as its sale and use.

10. What are the long-term expectations?

Since water repellent treatments are going through a period of change, both in chemistry and in limitations on VOC, information on what happens over time is only assumption and speculation. Where treatments have existed without change, it might be appropriate to ask for the location of some of the older applications along with recommendations of owners, architects, and contractors who have used the products.

What is the anticipated life expectancy of the product? What happens to the treatment over time: does it yellow, peel, embrittle, degrade?

What, if any, maintenance is required: periodic reapplication, total replacement? And if it doesn't last forever, what needs to be done in the future: must the existing material be removed; can it be removed; can it be reapplied; can it be coated?

Finally, remember that warranties can be beneficial in comparing the relative quality of one manufacturer. But, too often they become a sales tool with little true relationship to the durability of the product.

Clear Water Repellent Treatments for Concrete Masonry can be purchased from the Masonry Institute of America at 800-221-4000 or

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