Mind the Gap

Self-Sealing Concrete


When designing and building any concrete structure, waterproofing is an essential item that needs careful consideration.
Whether keeping water in, like with a reservoir, or out, as with a basement, the wrong choice can lead to liability and very costly repairs. On its own, concrete is not waterproof. Normal concrete is filled with interconnected capillary pores that allow the concrete to breathe, but also allow water to pass through. Concrete is also prone to cracking, which is unavoidable in a concrete structure of any complexity; once cracks form they provide a direct path for water and waterborne contaminants to enter or exit the structure.

Protecting concrete from water has traditionally been accomplished by applying an external membrane of some kind to the surface. Typical membranes have been either liquid coating or flexible sheets that are joined together and have two things in common: they are installed after the concrete has been placed; and they can be easily compromised by a puncture or separated seam.

A new type of waterproofing is gaining popularity. Integral waterproofing admixtures are added to the concrete during batching and become part of the hardened concrete. They make the concrete waterproof by blocking the passage of water through its capillary pores, cracks and joints, by turning the whole concrete mass into the waterproofing membrane.

External membranes
Coatings are applied to the outside of concrete foundations and the inside lining of reservoirs. Waterproof foundation coatings are typically applied using spray equipment and have the advantage of being able to conform completely to rough and irregular surfaces. Coatings are seamless and easily wrap around pipes and other penetrations. Application is fast and the finished product is very resilient.

In spite of these advantages, coatings suffer from some significant drawbacks that limit their appeal. Most obvious is their susceptibility to puncture. Surface preparation is another issue, as coatings need to be applied to a clean and dry surface. Timing and scheduling can also be an issue, as coatings may require that the concrete cure for a period of time prior to application. And, access to the application side of the concrete must be provided, which can be difficult and in some cases impossible. Installation labour is a critical variable as poor workmanship can lead to thin or missed spots, especially at corners and penetrations. As a result of these drawbacks, using a sheet membrane product for deep-hole foundations and other critical waterproofing is preferred.

Sheet membrane products are applied to the surface of the concrete with an adhesive or can also be loosely laid. They generally have greater tensile strength than coatings and are more likely to retain their integrity when the concrete cracks beneath them. Unlike coating systems, sheet membranes can be used on blind-wall applications.

Despite such advancements, sheet membranes can be compromised by a single puncture or tear. Water entering behind the membrane will lead to water invasion through cracks, construction joints, penetrations and the concrete itself. Poor workmanship is arguably greater in the case of sheet membranes than with coatings. As with coatings, membranes require a clean, dry surface and a specified length of time the concrete needs to cure. The membrane must be carefully wrapped around corners and protrusions. Laps, seams and edges are numerous and all must be perfectly sealed. The time required for installation of sheet membranes is normally greater than for spray applied coatings.

In spite of all these drawbacks, sheet membranes have been the industry norm in waterproofing for many years and they still hold the majority of the market share.

Integral membranes
Internal waterproofing membranes use the whole concrete mass and have several major advantages over surface-applied products. First and foremost is their invulnerability to damage. The “internal membrane” cannot be scraped, punctured or torn. It lasts as long as the concrete. There are no workmanship issues to be concerned about because there is no installation required. Since there is no installation, the general contractor eliminates requirements for space, access, scheduling, etc. Since no time is spent waiting for concrete curing, surface preparation and membrane installation, backfilling, and other construction steps can proceed without delay. Over all, integral waterproofing systems can save a significant amount of time and an enormous amount of money as compared to surface-applied systems.

The most effective integral waterproofing products are reactive and offer long-term protection that can self-seal any micro cracks that form over the structures life span.

Crystalline technology, also known as “smart concrete,” is the newest type of integral waterproofing admixture, although it is not necessarily all that new. The first crystalline waterproofing admixture was invented over 30 years ago, while crystalline technology itself has been used as a waterproofing treatment for existing concrete since well before then.

Considered an innovation in waterproofing and combating corrosion, crystalline technology reacts with water to form pore-blocking crystalline deposits. This method of integral waterproofing of concrete is considered the most effective long-term protection at stopping water ingress.

A crystalline waterproofing admixture, when added to concrete, reacts with water and un-hydrated cement particles to form insoluble needle-shaped crystals that fill capillary pores and micro-cracks in the concrete and block the pathways for water and waterborne contaminants. The most unique and effective feature of certain crystalline admixtures is the ability to “self-seal.” This critical performance property offers protection, so when micro cracks form due to curing shrinkage, settling, seismic activity, etc., any water that enters creates new crystals to form and grow, blocking the water by filling the crack. The crystal structure is comparatively rigid and strong and thus will withstand extreme hydrostatic pressure.

Probably the most unique feature of crystalline technology is its ability to lie dormant indefinitely within the concrete. If a new crack should form in the concrete and water begins to penetrate, the chemicals will react to form new crystals at the leaking location and grow to shut off the flow of water. This feature is incredibly valuable because it means that the concrete has the ability to repair itself and thus is many times more reliable than surface-applied systems over the long term. It is almost as if the structure is alive and can heal from a wound. However, it is important to note that the repair is not structural. The concrete does not regain its pre-cracked strength as a result of the crystal growth. Common use of the term “self-healing” is therefore a bit misleading. A better term for the process is “self-sealing” since this is what is actually happening. Crystalline technology’s ability to self-seal is perhaps its greatest strength.

Just as with the various surface-applied products, integral products have their own advantages and disadvantages. They have proven effectiveness at blocking the passage of water through concrete. For structures subjected to hydrostatic pressure and in critical applications, a crystalline technology will provide the performance necessary and its self-sealing ability will provide the most reliable long-term waterproofing solution available. On the downside, integral waterproofing products are not very tolerant of constant movement. Even though crystalline technology can self-seal cracks as they form, the crystals take time to grow and may not be acceptable for all waterproofing applications, such as above living space. For this reason, integral waterproofing admixtures are best suited to waterproofing rigid concrete structures such as below ground parking, foundations, tunnels, and reservoirs, as well as protecting against corrosion in infrastructure.

Although surface-applied membranes have been the industry norm, integral waterproofing admixtures are being considered and selected more and more often. Advancements in testing and proof of self-sealing performance have made these highly versatile products a preferred method for waterproofing. To prove efficacy, consistent test methods for self-sealing have been developed and patented (US Patent 9038477). The test method includes creating a consistent and reproducible crack in a concrete sample and a fluidic delivery system that provides a consistent flow of fluid for testing the self-sealing properties of the concrete sample. This test method has been licensed to a number of third party test labs to compare how dosage rate and different crystalline technologies affect self-sealing performance.

Results indicate that some technology has the ability to self-seal and stop a very high rate of water flow.

Essentially, the addition of certain crystalline admixtures lowers permeability of concrete and provides enhanced self-sealing, thereby offering reliable long-term waterproofing and corrosion resistance. This ultimately lowers maintenance costs and reduces risks associated with keeping water in, or out of, a structure.

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COVID-19 has created new concerns and complications for mass transit. Physical distancing is challenging – if not impossible – on subways, buses, and trains. For instance, when encountering just 30 percent capacity – an estimated total of 510,000 riders each day – passengers on Toronto’s public transit would be unable to maintain a safe distance to prevent the spread of COVID-19, according to the Toronto Transit Commission. This challenge has been seen throughout countless cities over the past year.

February 26, 2021, 5:24 PM EST