Waterproofing Products for Below-Grade Concrete Slabs and Structures

Adequate moisture and water control are critical for the success of any below-grade enclosure system. While concrete is a highly-durable and cost-effective building material, easy to make, and requires very little maintenance over its lifespan, it is also porous and absorbs water easily.

If untreated, below-grade concrete slabs, foundation walls, underground parking, plaza decks, or basements are vulnerable to moisture infiltration from below or outside the building. Concrete masonry units (CMUs) and pre-cast or poured concrete wall panels or floor slabs require appropriate waterproofing to block any moisture from moving up or laterally through the concrete that could lead to the deterioration of below-grade structures.

This article looks at how moisture impacts below-grade concrete structures and the Polyglass waterproofing systems available to seal them.

How Do Below-Grade Concrete Structures Get Wet?

There are several ways that moisture can enter a building through its below-grade components.

  • Concrete foundation slabs and walls can come into direct contact with groundwater, surface water, rainwater, plumbing water, or any water leaks from drainage issues on a construction site.
  • Any surface defects, cracks in concrete before or after hardening, design faults, or the improper installation of control or expansion joints can allow moisture into the building envelope.
  • Wet soil or material used to backfill a foundation or structural area can also impact below-grade slabs and foundation walls.
  • Concrete that hasn’t been allowed to dry correctly is more permeable to liquids and gasses, so wet materials installed during construction leave building enclosures vulnerable to water penetration.

Water Damage from Freeze-Thaw Cycles

Enclosed materials that haven’t been allowed to dry adequately raise indoor humidity levels, leading to condensation-related issues such as mold, poor air quality, health issues for occupants, and the structural decay of floors, walls, and metals.

In addition, as wet concrete dries, water vapor creates small cracks (capillary networks) in the material, leaving it vulnerable to moisture infiltration. Buildings in cold and wet climates are exposed to freezing temperatures, causing water in cracks and pores to freeze and expand, increasing internal pressures. As temperatures rise, the frozen water thaws, leaving behind enlarged cracks. These repeated freeze-thaw cycles weaken the concrete and cause slabs and foundation walls to scale and crumble.

Chemical and Salt Damage to Concrete

Concrete can also disintegrate when exposed to harmful chemicals, such as acid waters, ammonium chloride, or aluminum chloride, or when alkaline or acidic-based solutions from the surrounding soil or water penetrate the concrete and cause chemical reactions. These chemicals and chemical reactions can also damage the waterproofing products and barriers applied to protect concrete slabs and walls from water migration.

Any dissolved salt present in water and soil at building sites will have a degrading effect on concrete. As the calcium chloride in salt reacts with the calcium hydroxide in concrete, it forms calcium oxychloride, which is corrosive to reinforcing steel and causes concrete to crack and crumble.

Below-Grade Waterproofing Products and Solutions

Water control measures at building sites include draining surface water, rain, and stormwater away from buildings and keeping moisture-sensitive products dry during construction. In addition, waterproofing membranes, concrete surface primers, and drainage mats are all used as part of comprehensive waterproofing systems to protect below-grade building components from water damage.

Read our blog for more insights on choosing the right waterproofing system for below-grade structures.

Pre-Applied and Post-Applied Waterproofing Membranes

Water-impermeable materials like waterproofing membranes act as a barrier against moisture, water, and gas to help protect below-grade materials from water intrusion and lateral water migration.

Waterproofing systems can be:

  • Installed before (pre-applied) or after (post-applied) the concrete is placed
  • Permanently attached (bonded) or non-attached to the concrete works
  • Applied to the outside (positive-side) or inside (negative-side) of concrete structures

Positive-side waterproofing membranes are generally pre-applied, while negative-side systems are post-applied to concrete surfaces. Concrete slabs serving as mud mats below foundations typically require fully-adhered waterproofing membranes.

The primary advantage of a positive side waterproofing system is that water is prevented from entering the substrate surface entirely, protecting it from freeze-thaw cycles and any corrosive chemicals in the surface water or groundwater.

Polyglass’ Pre-applied Bonded Waterproofing Membranes

Mapeproof™ AL PRO

Polyglass’ Mapeproof™ AL PRO is a pre-applied, high-density polyethylene (HDPE) waterproofing membrane that bonds integrally to poured concrete. HDPE is known for its high strength-to-density ratio, resistance to naturally-occurring chemicals found in soil, and ability to withstand higher temperatures. It is also impervious to water, acids, solvents, and common chemicals, making it an ideal waterproofing material for below-grade structures.

Fully-bonded waterproofing membranes like Mapeproof AL PRO help prevents lateral water migration between the mud slab (or substrate) and barrier, which stops water from migrating to cracks in concrete that can later freeze, expand, and cause scaling or crumbling. In addition, Mapeproof AL PRO can be applied to vertical or horizontal surfaces and is designed as a kick-out roll to be walked on in horizontal installations, which is time- and labor-saving.

Mapeproof™ HW/ SW

Polyglass’ Mapeproof™ HW and Mapeproof™ SW waterproofing membranes are woven and nonwoven polypropylene (PP) geotextile fabrics encapsulating sodium bentonite clay. The nonwoven PP fabric is needle-punched through to the woven fabric, mechanically locking the sodium bentonite clay with thousands of fibers and forming a mechanic bond.

Sodium bentonite is a natural sealant, which expands when wet to form a single waterproofing membrane, while PP is known for its chemical resistance, elasticity, toughness, and fatigue resistance. Both Mapeproof HW and Mapeproof SW are ideal for horizontal slabs, vertical blindside, and backfilled wall applications. While Mapeproof HW is suitable for installing over damp or green concrete to prevent water migration, Mapeproof SW includes a special additive for mitigating the effects of salt and contaminated water on the swell index.

Polyglass’ Post-Applied Self-Adhered Waterproofing Membranes

Mapethene™ LT60/HT60


Mapethene™ LT60 and Mapethene™ HT60 provide an excellent barrier against moisture, water, and gas. Both are post-applied, self-adhered waterproofing membranes for below-grade foundations, horizontal decks, tunnels, and balconies where structural waterproofing is needed.

Mapethene LT60 and MapetheneHT60 membranes are cold-applied and comprised of a rubberized adhesive with an HDPE layer, making them highly durable and resistant to chemicals and gas vapor intrusion.  Mapethene LT60 is suitable for dry-weather, low-temperature applications, where the ambient and substrate temperatures are between 25°F and 60°F (-4°C and 16°C). On the other hand, Mapethene HT60 has been developed for dry-weather, high-temperature applications where the ambient and substrate temperatures are above 40°F (5°C).

Mapeseal™ GC

Polyglass’ Mapeseal™ GC is a single-component, liquid-applied, structural waterproofing membrane that can be post-applied to damp and green concrete and on vertical and horizontal surfaces.

Mapeseal GC does not require a primer, is very low in odor, guaranteed not to shrink, and can be applied in a variety of thicknesses or with reinforcing Mapeseal™ Fabric, making it customizable to any job.

Mapedrain™ Drainage Mats

Drainage mats, or drainage boards, protect above- and below-grade waterproofing membranes by channeling water away from installed waterproofing systems and relieving backfill and hydrostatic pressure.

Polyglass’ Mapedrain™ drainage boards or mats serve as a protection course for Polyglass above-grade or below-grade waterproofing membranes. Mapedrain mats are made from a woven geotextile filter fabric that prevents intrusion of soil, concrete, or construction grouts into the flow channels.

Mapedrain drainage boards are lightweight and easy to install, provide cost savings and eliminate the need for aggregate backfill. The Mapedrain core is 40% post-industrial recycled PP material, and each mat includes a polymeric protective film on the back side to provide additional protection to waterproofing membranes. Polymeric films are impermeable to gases and liquids and offer surface protection from corrosive substances. Mapedrain drainage mats can be permanently immersed in water without deteriorating or being affected by chemicals, bacteria, dilute acids, and alkalis.

Mapeprime™ 720W

Water-based Maprime™ 720W is an adhesive primer suitable for vertical and horizontal applications and can be used to attach Mapedrain™ drainage composites to Mapethene LT60/HT60 and Mapeseal GC.

Seal the Building Envelope with Polyglass

Polyglass has a team of industry experts ready to assist you with selecting the right below-grade waterproofing system to meet your project’s design, schedule, and budget needs. Contact us today to discuss your project requirements – we are happy to help!