Q. What's the difference between hydrophilic and hydrophobic polyurethane grouts? Whenever I ask anyone, they say that hydrophilic grouts "love water" and hydrophobic grouts "hate water," but this means nothing to me when I'm trying to select a grout for a certain application. When should each be used?

A. According to William Jacques, president of Strata-Tech Inc., a Des Moines, Iowa, manufacturer of polyurethane grouts, the terms hydrophilic and hydrophobic evolved more as marketing terms than technical definitions.

In 1969, 3M Co. used the term hydrophilic to describe the industry's first polyurethane grout, the 3M Brand Elastomeric Sewer Grouting Compound. The grout was mixed with water at a 1:1 ratio and cured into a soft foam. Later, Japan-based Takanaka introduced the Takanaka Aqua-Reactive Chemical Soil Stabilizer (TACSS), a grout that was mixed with an accelerator and, upon contact with water, cured into a rigid foam. This type of grout was labeled hydrophobic.

Today, polyurethane-grout chemistry has evolved to the point where these terms are not as useful for separating grout types. For example, hydrophobic grouts that produce flexible foams are now available. To select the right grout, you must consider several factors.

According to Jacques, you should avoid rigid foams if the grout will need to expand and contract, such as when you inject moving cracks. However, rigidity is desirable when the grout is used for applications such as soil stabilization, where you don't want the grout to deflect excessively under loads.

Hydrophilic grouts retain water after curing. If the grout is mixed at a high water-resin ratio, it may shrink too much when exposed to wet-dry cycles. Some manufacturers and applicators recommend using only hydrophobic grouts in wet-dry conditions, but others insist that hydrophilic resins perform well if you limit the water-resin ratio.

The viscosity of the resin also is important. Thinner resins are needed at low temperatures or when you are injecting narrow cracks.

For more information on selecting polyurethane grouts, see "Choosing the Right Polyurethane Grout," Concrete Repair Digest, December 1996/January 1997, pp. 294-295.