Blindside waterproofing membranes
For waterproofing applications on sites where there are limits to excavation or where standard waterproofing techniques are not an option, blindside waterproofing may be the only way to go. Blindside membranes are particularly effective in zero-lot-line construction as they allow for the membrane to be applied to the positive side—the preferred placement for any waterproofing membrane. Blindside waterproofing is becoming more common as major centres become more congested.
These materials can be applied horizontally, or installed vertically on the soil-retention system, and the concrete is then poured against the membrane. Two main types of these systems are used: bentonite waterproofing and composite sheet membranes.
Materials containing sodium bentonite consist mainly of expansive clay that, in the presence of water, expands and provides waterproofing of the substrate. The material is available in sheets or panels, and installed dry. Once in contact with water, it forms a monolithic membrane controlling the movement of moisture.
It is important to prevent bentonite membranes from becoming damp or wet on the jobsite before the concrete pour. This can sometimes be difficult on construction sites, but premature exposure to moisture causes the clay to swell prior to the concrete pour, and the effectiveness of the in-place membrane may be compromised.
A number of blindside sheet membranes are mechanically fastened to the soil-retention system. Concrete is poured against the membrane, which then forms a continuous mechanical bond to the concrete.
All these systems have been proven over time, but due to the complexities of the various types of substrates to which these systems are being applied, the critical factor in the performance of these systems are the details. Many also offer a number of accessory products to address the details and to ensure continuity.
Two main types of cementitious systems are available: crystalline and cementitious coatings, which consist of non-flexible and flexible formulations (the former being more cost-effective and the latter seeing more extensive use and offering better performance characteristics). Both these systems are breathable and can be applied on the positive or negative side of the structure. They are very commonly used in water-containing structures.
Crystalline technology works by forming crystals in the presence of water, plugging the pores, capillaries, microcracks, and other voids in the concrete. The crystalline materials consist of portland cement, silica sand, and proprietary chemicals. When combined with water, a catalytic reaction occurs that forms non-soluble crystalline fibres in the pores and capillaries of the concrete.
Crystalline waterproofing becomes integral with the concrete and can be used via various methods; it can be surface-applied, come in dry-shake form, or be added directly to the concrete during mixing.
Although an integral crystalline waterproofing system offers certain strengths that would appear to make it a preferred approach, numerous additional steps are required to provide a complete system. Integral waterproofing does not address construction or expansion joints, larger dynamic cracks, failing waterstops, penetrations, or poorly consolidated concrete. Crystalline products may be best-suited as part of a complete, combined system that would also incorporate a proper membrane on the positive or ‘wet’ side of the wall.
Flexible cementitious coatings are polymer-modified materials applied as a surface coating and, when compared to crystalline waterproofing, possess a significant number of advantages. They can be:
- highly flexible (low-temperature flexibility, in fact, sets these materials apart);
- possess good crack-bridging capabilities; and
- offer significantly better chemical resistance over exposed concrete systems.
Additionally, in poorly consolidated concrete, these coatings can still provide the waterproofing performance required, whereas a crystalline material would not provide the same performance.
For specific applications requiring certain levels of chemical resistance, some cementitious coatings possess comparable performance characteristics to conventional cementitious coatings while also providing resistance to chemical attack. It is important to check with the manufacturer on each specific application and provide all chemical information to ensure the material is suitable for the given chemical exposure.
As is evident, selection and installation of the correct waterproofing system is not a simple decision—there are many factors that need to be considered to ensure the system will not only perform in the short term, but also over the life of the structure. It is important to assess the conditions, select a system that will perform under them, and ensure the system can be installed to ensure continuity and long-term protection of the structure. Moisture infiltration into a structure is pivotal to mitigating the risk of any immediate or longer-range problems, and no one solution is right for every situation.
Russell Snow, CSP, CTR, BSSO, LEED AP, is currently the building envelope specialist for W.R. Meadows. His main responsibility is the development of specifications at the architectural/engineering level, along with providing technical support to all members of the sales team, continent-wide. Snow has been with W.R. Meadows for 14 years; his experience includes building envelope, concrete restoration, and products used in the treatment of concrete. He is currently involved with a number of associations including the Air Barrier Association of America (ABAA) Technical Committee, National Air Barrier Association (NABA) board of directors, and ASTM and American Concrete Institute (ACI) committees. Snow is a Certified Specification Practitioner (CSP) under CSC. He can be reached via e-mail at firstname.lastname@example.org.