Integral Water-repellent Mortar

Even the strongest masonry assemblage cannot resist the ingress of water. Although we cannot change the aggressive and sometimes persistent nature of water, we can change the way it interacts with masonry assemblages.

One way is to prevent water or moisture from entering the masonry system. This can be accomplished by sealing the masonry surface using either a membrane-forming sealer or one that is breathable. Each has its advantages and disadvantages. For instance a membrane-forming sealer can trap efflorescence salts behind it, increasing the potential for spalling of the block surface.

Another more economical and very effective way to mitigate moisture intrusion is to use a masonry assemblage that incorporates an integral water-repelling (IWR) admixture. Integral water-repelling admixtures have been used in masonry construction since the 1980s. IWR admixtures are available in both liquid and powder form and are composed primarily of stearates or polymers. Liquid admixtures are typically added to mortar mixtures at the job site, while powdered IWRs are blended into dry, pre-blended masonry products.

Block manufacturers also incorporate IWRs into concrete masonry units. When IWR-treated concrete masonry units are used in unison with an IWR mortar, the result is a water-resistant masonry assemblage. Treated concrete masonry units should not be used without IWR mortar due to a reduction in bond between the CMU and mortar1 not to mention the fact that the untreated mortar beds would act as pathways for moisture penetration.

The industry has gravitated largely to water-repellent treated CMUs with an IWR mortar, using either liquid or powdered admixtures. So is there any difference between liquid and powdered IWR admixtures? Ask a mason, and they'll tell you which they prefer.

Wayne Stephenson, Superintendent for DH Johnson in Chicago, says the difference is in the consistency. "We used to use the liquid admixture until we were able to get a pre-blended IWR mortar. It's just more convenient, more consistent. It's just easier."

Convenience and efficiency are also cited by Sean Sessa, Owner of Canyon State Masonry in Phoenix, Ariz. "My guys don't have to do any measuring to put it in a batch of mud," he says. "One of the reasons we use SPEC MIX [pre-blended integral water-repellent mortar] is that it's a no-thinking thing. It's also more cost-effective than a liquid water-repellent admixture, and it's more convenient."

In regard to performance, Robert L. Nelson & Associates, Inc., a nationally recognized testing agency in Schaumburg, Ill., conducted a comparative study in 2002 between mortars incorporating a liquid IWR and a powder IWR admixture. The study included both Portland lime and masonry cement mortars. A summary of the results is presented in the table (at left). Both admixtures were effective in making the masonry assemblages water-repellent, but the study also revealed an increase in entrained air and a decrease in flexural bond strength by as much as 46% when the liquid IWR was used.

When it comes to liquid or dry IWR admixtures, it's the contractor's choice. However, when adding a liquid admixture on site, you rely on the laborer for mortar quality and admixture addition rates, if it's added at all. Whereas, a dry admixture is pre-blended into the mortar, assuring the contractor that the mix is consistent, as well as having the benefits of a higher flexural bond strength and increased workability.

Water is everywhere ? even in Arizona, as Sessa says, "We do get our monsoons down here." So the issue of water penetration into a masonry assemblage is universal, and integral water-repellent admixture in mortar and CMUs is a proven and cost-effective solution.