Negative-side waterproofing 101

by sadia_badhon | November 22, 2019 1:01 pm

by Harry Lubitz, CSI, CDT

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While negative-side waterproofing is not the usual first choice in new construction of foundations, it clearly has its place. It is often selected as the last resort when other options (positive and blind sides) are impractical. Often, negative-side waterproofing is better than re-excavating the entire structure when the original waterproofing materials fail and water is actively leaking.

When positive-side and blindside waterproofing are applied, water under hydrostatic pressure would not permeate the foundation and the building is expected to remain dry for life. Unfortunately, this is not always the case, and the solution either involves excavating the foundation to conduct repairs or retrofitting the insides of the wall with a negative-side waterproofing application.

Negative-side waterproofing is the application of a coating material to the interior or the ‘dry’ side of a foundation or wall. Its primary function is to create a barrier coat to stop water from leaking through the wall and keeping the space dry.

Negative-side waterproofing can also help offer a secondary function of providing a decorative surface that offers a value above and beyond a coat of paint. It makes the foundation space more attractive and livable, while serving to keep water out.

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All foundation coatings, including negative-side applications, must be impermeable and withstand hydrostatic pressure, the force exerted in every direction on a foundation. Standing water exerts approximately 3 kPa (0.43 psi) for every foot of water. Consequently, the significant force created by the water will find its way through the foundation wall and into the (formerly) dry space.

Categories

Negative-side waterproofing materials typically fall into two categories:

• penetrating liquid acrylic waterproofing materials sealing voids and capillary openings where water seeps through; and
• cementitious surface coatings providing a secondary cement-based barrier.

Penetrating liquid acrylic waterproofing materials have expanding polymer or crystalline additives as part of their chemistry. On application, these additives expand or grow to block the water-invading pathways. Cementitious surface coatings rely on the applicator to ‘push’ the product into the pathways, and then the product hardens to create a barrier.

Negative-side waterproofing has clear advantages. It can be installed with full visibility and can be easily repaired. It can be applied any time after the substrate material has cured. It can be applied with minimal training and is usually cost effective. Unfortunately, since it is installed from the inside of the structure, the substrate material is not waterproofed, and therefore, unprotected from corrosive soil materials or freeze/thaw cycling.

Hydraulic cement

Negative-side waterproofing does have its limitations. It requires secondary assistance when the substrate has cracks larger than 1.5 mm (¹/₁₆ in.) or actively flowing water leaks. In these circumstances, hydraulic cement is the recommended course of action. Hydraulic cement is a more modern invention with unique properties and several advantages.

First, hydraulic cement is easy to mix with clean water and can be troweled into any shape. Second, it expands on curing and will seal static cracks up to 19 mm (¾ in.) wide and/or deep. Third, it will work to plug actively flowing water. Finally, it sets quickly (within three to four minutes) using cold mixing water—warm mixing water accelerates the set time and reduces working time. Using hydraulic cement to tool joints between floors and walls is also a recommended practice. It is important to note hydraulic cement will not stop moving (dynamic) cracks.

Care should be exercised when using hydraulic cement as it is Portland cement based. Proper personal safety equipment, such as eye protection, rubber gloves, and a National Institute for Occupational Safety and Health (NIOSH)-approved respirator, should be used. Breathing Portland cement dust is a health hazard. Some heat is also generated during the chemical reaction with water, therefore, it is best to avoid skin contact with the material.

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It is a good practice to dampen surfaces prior to applying hydraulic cement. This is also recommended before using cementitious surface coatings. Hydraulic cement is compatible with both penetrating liquid acrylic waterproofing materials and cementitious surface coatings that could be employed as topcoats after curing.

Providing a proper substrate for these materials is critical. Negative-side waterproofing coatings need an unobstructed access to the base substrate material to penetrate the voids and capillary pathways the water is using to intrude. Removing old surface coatings, paints, foreign matter, dirt, efflorescence, and other substances that would impede both penetration and proper bonding are critical for success. Commercial cleaning and etching materials are available to assist in this process.

Etching or abrading the substrate provide a proper surface profile to allow for better penetration and ‘grip’, especially on smooth troweled or precast substrates. The International Concrete Repair Institute (ICRI) has developed a concrete surface profile (CSP) classification to simplify the specification. It is advisable to refer to the waterproofing material manufacturer’s product data sheet to ascertain the recommended CSP.

There are circumstances other than foundation water intrusion where negative-side waterproofing is the logical choice from the outset. These fall under the category of waterproofing where access to the positive side of the substrate is impossible or financially impractical or the negative-side application adds to the entire waterproofing system.

Tunnels

Sophisticated passenger, traffic, and rail tunnels rely on negative-side waterproofing to keep them dry. High-tech geotextiles are incorporated into the structural layers underneath a final surface coating as part of a waterproofing and water management system. Often, as a ‘belt-and-suspenders’ approach, a final topcoat of penetrating liquid acrylic waterproofing is applied to give a final layer of protection, and is tinted to provide decoration.

Less sophisticated tunnels rely on more traditional negative waterproofing products like those previously mentioned to seal foundation walls. To maintain these types of tunnels over time, the occasional topcoat of tinted penetrating liquid acrylic waterproofing is recommended to keep the interior dry and attractive.

In-ground tanks and cisterns

A negative-side waterproof coating is often used to coat the inside of in-ground tanks to seal in its contents and prevent water intrusion. In-ground potable water storage tanks and cisterns utilize negative-side waterproof coatings. These should be specified to comply with NSF International/American National Standards Institute (NSF/ANSI) 61, Drinking Water System Components. Also assure any confined spaces like enclosed tanks are properly ventilated to protect workers and ensure proper curing of the coating.

In-ground water features

Similar to tanks, other in-ground, water-holding devices like decorative water features and swimming pools need to be kept clean and sanitary. Holding ponds for other wildlife need to be properly coated to keep ground contaminates out and clean water in. Negative-side applications of penetrating liquid acrylic waterproofing materials or cementitious coatings serve this purpose. Adding tints to liquid waterproofing offers multiple decoration options. Interior waterproof coated surfaces are easier to maintain and do not tend to leak as those treated with concrete paint films alone.

Retaining walls

A retaining wall is really an outdoor foundation designed to keep soil and water under control. Positive-side waterproofing placed on the soil side of the wall creates a primary barrier, while negative-side waterproofing develops a secondary barrier to water migration. The latter can also be tinted to add beauty to the visible (dry) side of the retaining wall.

Elevator pits

Elevator pits should be properly waterproofed from the positive side as they are a ‘foundation’ surrounding the elevator components. Unfortunately, the positive-side waterproofing tends to fail here more often than regular foundations due to the lack of proper selection and installation attention. Providing an additional barrier with a negative-side waterproofing application helps keep elevator pits dry. By adding a tint to a liquid acrylic waterproofing material, the elevator pit becomes a more attractive place for elevator service persons to work and easier to maintain. The negative-side coating will seal the pit from the inside and capture any leaking fluids from the elevator system and prevent them from penetrating the pit walls.

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Floors and slabs

Concrete floors and slabs should also be properly waterproofed from the positive (or under) side as they are a component of the foundation. Occasionally, the underside waterproofing will fail if improperly selected or installed, causing water to seep through the floor slab. The addition of a negative-side waterproofing application can forestall an expensive or potentially impossible repair. It is recommended to choose a negative-side waterproofing product specifically designed for use on horizontal surfaces that will block the hydrostatic pressure forcing water up through the floor. Depending on the specific location, the chosen waterproofing material may need to handle foot or vehicular traffic. Adding tints and non-skid additives to liquid acrylic waterproofing materials can also offer decorative and safety advantages.

Installing a negative-side waterproofing material to a concrete floor or slab also creates a moisture barrier underneath flooring materials such as tile, carpet, or wood. Using a liquid acrylic latex-based waterproofing material assures compatibility with other latex-based flooring adhesives.

Negative-side waterproofing retrofit: A case study

Moisture intrusion was detected at the Brevard County Courthouse in Viera, Florida, due to water damage and mold on the gypsum board. A fast, easy-to-use, cost-effective method was needed to provide a masonry patch and moisture mitigation solution on the interior and exterior of the courthouse without a major renovation. The moisture had intruded through gaps between concrete masonry unit (CMU) block columns that were poorly tied into face brick returns at the window openings. An existing negative internal pressurization of the structure, aggravated by strong coastal wind-driven rain, pushed moisture through the exterior wall gaps, thereby causing condensation on the interior steel gypsum board studs.

Prior to any patching and waterproofing activities, the negative pressurization issues in the building were solved.

Hydraulic cement was then used to fill the gaps between the returned face brick and the concrete columns on the interior (negative side) of the building exterior walls on the third and fourth floors. Backer rod was inserted into deeper gaps and filled with a concrete patch.

After completing patching, two coats of a latex-based masonry waterproofer were applied on the interior side of the walls, over the patches, and around the windows. In the author’s experience, the material provides a long-term barrier to both casual moisture and wind-driven rain intrusion along with sealing the recently applied patching materials.

A mockup area was first repaired and tested by Intertek PSI, the project’s consultants. A professional high-pressure spray rig was utilized to simulate wind-driven rain conditions. The mockup preformed as expected, and the complete project repair commenced.

All the repairs were completed within a few weeks, and the courtrooms and judges’ chambers were restored to their original interior finish condition.

Conclusion

Negative-side waterproofing is another tool designers and specifiers can use in order to keep spaces dry and protected in new construction. Choosing the correct waterproofing barrier position or a combination of settings can protect a building for life. Negative-side waterproofing can also be invaluable if the original waterproofing installations fail, or interior (dry side) protection is the only solution.

Designers and specifiers must consider the individual project circumstances and chose the best option for their needs. Manufacturers are always available to consult on what may be a selection of solution options, and what has worked for them in the past. They can also share job-specific instances to provide more confidence in the selection of the right solution for the specific need.

Harry J. Lubitz, CSI, CDT, is the architectural sales director for the United Gilsonite Laboratories, Inc. Lubitz has more than 30 years of experience in the building materials industry, is active in numerous architectural and professional organizations including CSI. He has served as an adjunct faculty member at Lord Fairfax Community College in Virginia. Lubitz can be reached at harry.lubitz@ugl.com[5].

Source URL: https://www.constructionspecifier.com/negative-side-waterproofing-101/

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