Controlling Moisture in Masonry

Resolving the number one cause of structural deterioration in buildings
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Sponsored by Oldcastle® Architectural

Water-Repellent Admixture Advances in Block and Mortar

An effective method in controlling moisture in the building envelope is through water-repellent admixtures. Advances in technology have enabled manufacturers to offer water-repellent admixtures in both block and mortar which, if used together, offer superior moisture control and protection against all types of moisture infiltration.

Because untreated masonry units typically absorb water through capillary suction or wicking, the anti-wicking action of an integral water repellent will minimize the amount of water absorbed, and enable any water that has breached the surface to drain toward flashing and weep holes. No film forms on the surface of the masonry as a result of the integral water repellent, nor is the admixture impervious to moisture, which means the masonry remains breathable.

Historically, three types of admixtures have been used. Calcium Stearate has demonstrated an increase in air and a poor bond, as have the second type of admixture, tall oils. The third type, polymetric, has shown a better bond between units and mortar, though the air component has varied by manufacturer. High air content in mortar lowers the compressive strength and can reduce the bond between units. Low air content will reduce the freeze-thaw resistance of mortar. Polymeric admixtures do not alter the finished appearance of the block, nor do they affect its “paintability,” but instead provide a denser, more uniform unit as well as moisture control and limited risk of efflorescence. The admixture is incorporated at the plant, ensuring even distribution throughout the concrete mix.

Especially effective in lightweight concrete masonry units and single wythe construction as added protection against water infiltration and wind-driven rain, an integrated water repellent agent assures design professionals that the product will be executed as specified and in the proper proportion for maximum protection.

While an integral water repellent in a masonry unit will go a long way to controlling moisture in a building, the joints are still susceptible to water infiltration. Consequently, use of both masonry units and mortar with an integral water repellent is necessary to provide full protection and ensure the surface is not breached by moisture in any form. Recently developed, specially engineered granulated formulas can be added to the mortar/sand during the blending process of premixed mortars, which can also improve bond strength over the service life of the building.

Introduced in the early 1980s, integral water-repellent mortars are available in liquid and powder form, with the liquid admixture applied on site at the mixer. Powders are typically found in preblended mortars. Each type of mortar is available in Type M, S, and N compressive strengths in compliance with ASTM C270 and is engineered as a preblended mortar for consistency, workability, and yield—providing factory-controlled mixes, batch to batch, compared to field-mixed mortars. In addition to creating a superior bond, these mortars can be formulated for extended board life, eliminating retempering in hot or windy climates.

Board life is a job-site-related term used by masons to define the working time of the mortar, and for them to assure a proper application. Job site conditions such as high ambient air temperatures, sunlit walls versus walls in the shade, windy conditions, etc., may affect the board life, depending on the overall performance or water retentivity of the mortar. In the event that the board life is reduced by job site conditions, retempering, which is the addition of water to the remaining mortar in the mixer or mortar pans, is permitted once during the plastic stage. Under no circumstances should colored mortar be retempered, as the addition of water will affect the final color of the mortar.

This chart shows that a water repellent can be traced in harden mortar, a unique feature of certain products that proves the right amount of admix is actually in the system.

Photo courtesy of Oldcastle® Architectural

This chart shows that a water repellent can be traced in harden mortar, a unique feature of certain products that proves the right amount of admix is actually in the system.

Mortar mixes with integral water repellents have been formulated for special applications, including high-moisture environments, coastal climates, historic renovation projects, or where cement-based mortar is the preferred material.

Masonry with integral water repellent mortar after 72 hours.

Masonry with integral water repellent mortar after 72 hours.

Photo courtesy of Oldcastle® Architectural

Masonry with integral water repellent mortar after 72 hours.

Integral water repellents in mortars have several advantages over those that are site applied. An integral admixture assures accurate blending at the manufacturing facility—in short, that the water repellent will be precisely metered, properly proportioned, subject to quality control measures, and delivered as specified. Its presence is detectable and can be verified after the mortar hardens. Job site applications, on the other hand, are known to pose challenges in achieving consistency, particularly during difficult weather conditions. Site-applied water repellents have a limited surface life, too, ranging from two to seven years, and most manufacturers recommend a two-coat system, which adds to the project schedule and budget. Untreated units may become wet prior to sealing, raising the potential for moisture problems down the line. By contrast, integral water repellents enable masons to save time and costs as no measuring, mixing, or additional steps are required, eliminating the potential for job site mistakes and the need to stock and dispose of liquid admixture supplies.

Integral water repellents function well in high moisture areas.

Photo courtesy of Oldcastle® Architectural

Integral water repellents function well in high moisture areas.

Based on C1384 testing by the National Concrete Masonry Association, essential to demonstrate water repellency standards for mortars, the most advanced preblended mortar products have demonstrated up to an 8 percent increase in flexural strength, significantly decreased resistance to water absorption, and a nearly 50 percent longer board life than untreated mortars. Other specifications for masonry products with integral water repellents include the following:

  • ASTM 91 Standard Specification for Masonry Cement
  • ASTM C 150 Standard Specification for Portland Cement
  • ASTM 207 Standard Specification for Hydrated Lime for Masonry Purposes
  • ASTM C1714 Standard Specification for Preblended Dry Mortar Mix for Unit Masonry
  • ASTM E514 Standard Test Method for Water Penetration and Leakage Through Masonry
  • Meets ACI 530.1 Specification for Masonry Structures

Colored Mortar: To meet aesthetic goals, manufacturers have made colored mortar available. These products are sold as a masonry cement-based or Portland/lime-based product in more than 36 standard colors and can be color matched to other project components. Design professionals should consider incorporating architectural blocks and corresponding colored mortars as part of this integral system. Some manufacturers have their own array of colored blocks sold in a kit format—these can be used in conjunction with water-repellent masonry units to provide joint warranty wall system approaches to water repellency. Architects should note whether these products meet, or preferably exceed, ASTM C-270, and ASTM C-1384.

Colored mortar is available in a variety of colors to complement masonry units.

Photo courtesy of Oldcastle® Architectural

Colored mortar is available in a variety of colors to complement masonry units.

Delivery:It is important to note that in some cases these mortars are available in a silo delivery system. Bulk bags are often used on large, high-volume jobs where production is important, and bags weighing as much as 3,000 pounds can be loaded into a steel silo. Silo systems, capable of handling up to 30,0000 pounds of material, are widely considered to offer efficient, high-output product mixing at the job site, thereby improving productivity. Gravity silo systems do not require a power source—only a pull of the handle—and have been reported to reduce mixing times by up to 50 percent. In addition, automated batching systems and production quality controlhelp eliminate testing issues on fast-track projects. “These bulk silo systems also help reduce the amount of unused material and help reduce paper bag waste for a more environmentally friendly jobsite,” says Eric Peterson, senior vice president for Concrete Bagged Materials at Oldcastle, noting that bulk products and silo systems help maximize product usage, thereby eliminating the piles of wasted sand associated with field-mixed product. Bulk products can dramatically reduce the amount of paper waste associated with bagged products, diverting waste that would otherwise end up in landfills.

Design professionals should confirm the compatibility of water-repellent masonry products with any system that includes the products of other manufactures—all elements should be compatible, and their collective performance and intended use verified. Consideration must be given to the means and methods of application, products used, project-specific conditions being addressed, and standardized tests performed for each proposed system or variation. It is also advisable to use approved mock-ups or sample panels throughout the project process through to completion.

 

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Originally published in Architectural Record.

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