Concrete is porous and, if not waterproofed, absorbs water that can cause cracks, waterborne contaminants and chemicals that can cause deterioration. If you want to protect your concrete and ensure it has a long, serviceable life, waterproofing is essential.
But how? What’s the best method and the best material?
To make concrete really waterproof — which means both preventing water passage and resisting hydrostatic pressure — you can waterproof from the positive (exterior) side, negative (interior) side or from within the concrete itself (integral systems). Although the oldest and most widely used positive side technology is sheet membrane waterproofing, its failures and limitations are also common and costly. Since the 1980s, many construction projects around the globe have used integral crystalline admixtures to waterproof concrete. Integral systems block water passage from any direction by working from the inside out, making the concrete itself the water barrier.
It can be difficult to keep up with advancements in both membranes and crystalline admixtures — and there have been substantial advancements in both technologies. Here’s a summary that can help make the choice more clear.
Sheet membrane systems
Historically, hot-applied sheet systems — known as built-up bituminous membranes — were used for below-grade concrete waterproofing. These sheets were made from alternating layers of bitumen and felt. When heated, traditional bitumen — both coal tar pitch and asphalt — releases volatile organic compounds (VOCs) and potentially carcinogenic fumes.
Since the early 1990s, the bitumen system’s popularity has fallen due to an increasing number of bans on its use by governmental and regulatory agencies. Substantial steps have been taken by product manufacturers to replace these membranes.
Polymer-modified bitumens have evolved from the original bituminous sheet systems, offering a safer, cold-applied alternative. Cold-applied polymer-modified bitumen is a sheet membrane composed of polymer materials compounded with asphalt and attached to a polyethylene sheet. The polymer is integrated with the asphalt to create a more viscous and less temperature-sensitive elastic material compared to asphalt on its own. These sheets are self-adhering and eliminate the harmful toxins typically associated with asphalt adhesion. They also increase tensile strength, resistance to acidic soils, resilience, self-healing and bondability.
Despite such advancements, disadvantages persist. Their field fabrication requires intensive labour and carefully supervised installation.
Installation can be challenging as membranes require sealing, lapping, and finishing of seams at the corners, edges and between sheets. Additionally, sheet membranes must be applied to a smooth finish without voids, honeycombs or protrusions. Because the membrane can puncture and tear during backfilling, protection boards also need to be installed.
Sheet membranes also pose other limitations. They are challenging to use in vertical applications and difficult, if not impossible, for blind wall applications. They are often inaccessible for repairs after installation.
Performance and durability can also be issues. Performance depends on surface adhesion and proper seam lapping. Materials are strongest on the first day following installation, after which they gradually deteriorate.
Although polymer-modified bitumens are an improvement over their hot-applied predecessors, they still present challenges, including poor resistance to ultraviolet radiation, the need for solvent-based primer and adhesives and an air temperature warmer than –4° C (25° F) during installation.
Careful installation practices must also be followed. Sheets can debond if they are not promptly covered after installation, the top edges are not sealed, the primer is incorrectly applied or if tie holes are not flush with the concrete surface.
In spite of all these drawbacks, sheet membranes have been the industry norm in waterproofing for many years — they still hold the majority of the market share. Their continued use is due to impact resistance, toughness and overall durability compared to other membrane options.
Thermoplastic polymers have led to the creation of thermoplastic membranes. These membranes are composed of polyvinyl chloride (PVC), chlorinated polyurethane or chlorosulfonated polyethylene, with glass fibre-reinforced PVC being the most popular membrane type.
Thermoplastic materials soften when heated and harden when cooled, so sheets can be attached with solvent-based adhesives or by heat-welding at the seams — a significant advantage over field-fabricated seams. Thermoplastic membranes also effectively resist chemicals and hydrostatic pressure.
Despite these advantages, there are drawbacks. Thermoplastic membrane properties change depending on the temperature. The PVC deteriorates if it is in contact with hydrocarbons. Installation still demands the use of solvent-based primer and adhesives and any asphalt-based protection boards cannot be placed directly on the PVC membranes. In addition, the concrete must have a “floor quality” steel trowel finish to ensure good adhesion.
Thermosetting membranes (i.e. vulcanized rubber) are more resistant to heat, solvents, general chemical attack and creep than thermoplastic membranes are, due to the vulcanization of butyl, ethylene propylene diene monomer or neoprene rubber.
However, as thermosetting materials harden permanently when heated, these sheets can only be attached using solvent-based adhesives on the seams, and movement after application is very restricted. Also, because the seams between sheets are field-fabricated, they never attain the base material’s tensile strength. Thermosetting membrane sheets tend to stretch and are difficult to install on vertical surfaces. They can disband or blister if a negative vapor drive is present because they do not breathe. And they require the use of solvent-based primers and adhesives—another drawback.
Clay systems (bentonite)
This waterproofing method has been employed for more than 75 years, but its popularity has recently increased. Its effectiveness is based on the properties of impure clay, which swells to block water. Bentonite is versatile and comes in various forms, from prefabricated panels to trowelable mixtures.
Clay systems are excellent for waterproofing, but need sufficient hydration for success — and in some applications this can be difficult and unreliable. First, high hydrostatic pressure is required for complete hydration of the clay molecules. Hydration must occur immediately after installation and backfilling. It must also take place in an adequately confined area to avoid lifting or cracking the concrete slab.
Bentonite can self-heal, is non-toxic and is relatively easy to install, but it is rarely used in places where the risk of leaks must be minimal and humidity control is necessary. Bentonite materials are weather-sensitive and not resistant to soil chemicals (e.g., brines, acids or alkalines), which ultimately decreases their ability to thoroughly waterproof structures.
Bentonite systems cannot be installed during rainfall while groundwater level is fluctuating, or in areas with constant wetting and drying cycles because the clay will deteriorate. Installation is also not advised in places with free flowing water that would wash away clay. Once installed, bentonite is difficult to remove, so options for future repair or replacement are limited.
Bentonite sheets are most beneficial for blindside wall applications as they can be nailed directly to the foundation walls.
Liquid-applied membranes can be applied with a brush, spray, roller, trowel or squeegee, and usually contain urethane or polymeric asphalt (hot- or cold-applied) in a solvent base. These membranes are usually applied on the positive side of set concrete and have high elastomeric properties. More recent technologies have also made negative-side applications possible.
Successful waterproofing with liquid-applied membranes depends on proper thickness and uniform application. They call for skilled, experienced labour to apply them, a clean and dry substrate—which can often be a construction environment challenge—a protection layer before backfilling, properly cured concrete to avoid problems with adhesion and blistering and, on horizontal applications, a sub-slab. Liquid-applied membranes deteriorate when exposed to UV radiation and cannot withstand foot traffic. The liquids them-selves also contain toxic and hazardous VOCs.
Although liquid-applied membranes work well on projects with multiple plane transitions, intricate geometric shapes and protrusions, they are typically only used when prefabricated sheets do not work.
For the last three decades, a new type of waterproofing has been used around the globe. These integral admixture systems are added at the batching plant or onsite, and react chemically within the concrete. Instead of forming a barrier on the positive or negative side of concrete, they turn the concrete itself into a water barrier. Integral concrete waterproofing systems can be densifiers, water repellents or crystalline admixtures.
Densifiers react with the calcium hydroxide formed in hydration, creating another by-product that increases concrete density and slows water migration. They are typically not characterized as waterproofing materials or repellents because they have no ability to seal cracks and joints. Concrete under hydrostatic pressure requires additional waterproofing methods to protect it from damage and deterioration.
Water repellents are also known as “hydrophobic”. These products typically come in liquid form, and include oils, hydrocarbons, stearates or other long-chain fatty acid derivatives. Although hydrophobic systems may perform satisfactorily for dampproofing, they are less successful at resisting liquid under hydrostatic pressure. Pre-curing and post-curing stresses cause cracking in any concrete, which creates pathways for water passage. So the effectiveness of water repellents is highly dependent on the concrete itself.
Crystalline-based systems typically come in a dry, powdered form and are hydrophilic in nature. Unlike their hydrophobic counter-parts, crystalline systems actually use available water to grow crystals inside concrete, effectively closing off pathways for moisture that can damage concrete. They block water from any direction because the concrete itself becomes the water barrier.
In contrast to water repellents, crystalline technologies enable self-sealing. The admixture is a blend of cementitious and proprietary chemicals that actually work with the available water in concrete to form insoluble crystals. These needle-like crystals grow until all pores are blocked and no water can penetrate the concrete. The crystalline formula can allow concrete to self-seal hairline cracks up to 0.5 mm (0.02 in.), even years after the original construction.
Concrete treated with these admixtures contains chemicals that lie dormant within. If a crack forms, any water influx causes more crystals to grow, re-blocking and sealing the passage against water and waterborne contaminants. Whenever new water enters the concrete through changing water levels or new cracks, crystals continue to grow and seal the concrete. The crystals within the concrete are impervious to physical damage and deterioration; there is no danger of punctures, tears or seam leaks. As a result, a building’s durability increases when crystalline admixtures are used.
In addition to promoting and enhancing the natural hydration process of cement, these systems are highly versatile, useful and reliable for a wide range of applications. For example, concrete treated with crystalline admixtures are suitable for complex architectural designs. As architectural protrusions do not pose any waterproofing challenge, any type of concrete structure — vertical, horizontal or shaped — can be securely waterproofed.
Concrete waterproofed with crystalline admixtures affords other benefits, too. It contains no VOCs and can be completely recycled when demolition occurs. Membranes do not have to be separated from the concrete, waterborne contaminants are not present in the concrete, and petroleum-based materials are not left behind to leach into soil.
Additionally, crystalline admixtures offer installation advantages. Unlike traditional membrane waterproofing, which tends to be labour-intensive and expensive, crystalline technology decreases installation and maintenance costs and is easy to handle — admixtures can be shipped in dissolvable, pulpable bags that are thrown into the concrete batch during mixing. This speeds up the construction schedule and decreases labour costs by combining steps with concrete placing.
Integral crystalline waterproofing systems should not be used in applications under constant movement. During the crystallization process, crystals align in a three-dimensional array that breaks when subjected to excessive movement. Areas that require flexibility and face recurring movement would be better waterproofed another way.
Integral waterproofing admixtures tend to be less expensive for materials, and additional labour costs are almost nonexistent. They also allow for a larger building footprint and reduce maintenance and repairs over the long term.
About the Author:
Kevin Yuers is Vice-President of The Kryton Group of Companies and is responsible for Product Development, Technical Services and Operations. A life-long veteran of the construction and renovation industries, Yuers has spent most of his career in concrete waterproofing. Yuers has written several articles on the subjects of concrete waterproofing, Krystol technology and concrete coatings and repair systems. Kryton is a 37-year-old family owned business, based in Vancouver, BC, that manufactures concrete waterproofing and related products and distributes them to the construction industry worldwide. Yuers can be reached at firstname.lastname@example.org.