June 15, 2007
Waterproofing systems are required on all below-grade surfaces that are occupied and subjected to hydrostatic pressure. It is the architect or designer’s responsibility to specify the system that is best suited for the facility. There are five types of common positive side waterproofing systems used in commercial construction. The type of material used in the system determines the application methods. The types of systems are:
• Reinforced sheet membranes.
• Hot-applied bitumen systems.
• Liquid-applied systems.
• Single-ply membranes.
• Bentonite clay.
Reinforced Sheet Membranes
The most common type of system used in vertical applications is reinforced sheet membrane. This is due to the reliability of these systems over the years. There are manufacturers in the United States that have supplied these systems to the market for over 60 years.
The main advantage of these systems is that the built-in reinforcement through the use of felts and/or fabric has supplied additional forgiveness to protect against damage from structural movement through settlement, curing of the concrete and the normal accommodations of the building to temperature variations. This accommodation by the membrane helps to ensure the sought-after continuous barrier to the infiltration of water in both liquid and vapor forms.
Hot-Applied Bitumen Systems
Hot bituminous systems have been used since the beginning of waterproofing. These systems employ multiple plies of fabric, felt, or felts and fabrics combined. They are applied in alternating layers of bitumen (coal tar or asphalt) and felt (fiberglass or organic). The bitumen serves as the waterproofing agent and adhesive. The felts and/or fabrics provide reinforcement, which stabilizes the membrane and provides tensile strength.
These systems have provided life spans of over 30 years or more and were common in past decades, so their past track record can be seen as one advantage. Another advantage is redundancy; three to five plies of reinforcement provide exceptional durability.
Installation of hot-applied systems has declined in the past two decades due to safety risks and concerns centered on the odors from the fumes. Caution is required when these systems are applied on vertical surfaces because of the individual mopping of each ply with materials heated to 400 degrees or more. These systems, when installed near existing building intakes, can pose fume problems.
Although certain application techniques are similar to roofing, the asphalt bitumen used in waterproofing is a different type of material. The asphalt used in roofing is airblown to produce a soft, viscous state. Asphalt used in waterproofing is not air-blown. The result is a harder substance that achieves superior water repellence. Air-blown asphalts are also susceptible to deterioration with soil contaminants, which is another important criterion of waterproofing systems.
Liquid Applied Systems
Liquid applied membranes (LAMs) are field applied using a variety of liquid components that are installed directly to the substrate. These liquid systems usually require a primer and then a multiple coat application of the liquid materials to overcome any surface tension presented by the substrate surface, such as pinholing. The number of coats necessary can vary from manufacturer to manufacturer depending on the material type and its viscosity. Some manufacturers require two layers of liquid material with a sheet of fibrous matting in between. This creates a stronger waterproofing barrier.
LAMs can be applied with hot or cold polymer modified asphalts. Hot-applied systems are heated to 400 degrees in specialized equipment and applied in thicknesses up to 180 mils. The most important design criteria for improved material performance are the material’s solids content and coating thickness. The materials solids content should be a minimum of 80 percent to reduce the probability of pinholing. The minimum specified coating thickness should be 0.060 mils. This is required to reduce the stress concentration from irregular substrate conditions such as cracks, honeycombs, etc.
These systems allow for both abovegrade and below-grade application, including planters and split-slab construction. They are not resistant to ultraviolet weathering and cannot withstand foot traffic, so they should not be applied on exposed areas. When LAMs are applied on horizontal applications, a sub-slab must be in place for application of the membrane. A topping of concrete, tile or other hard finishes is required. LAMs are applicable over concrete, masonry, metal and wood surfaces.
Single-ply membranes are prefabricated reinforced sheets that are coated with a variety of films consisting of rubber modified bitumen, ethylene propylene diene (EPDM), butyl rubber, styrene butadiene styrene (SBS), and attactic polypropylene modifications of asphalt and catalytically blown asphalt.
The most prevalent single ply sheets used in waterproofing at this time include butyl rubber, which is 60 to 120 mils thick, specially formulated PVC sheets, which are 59 to 120 mils thick, and modified bitumen sheets. The systems are fully adhered to the substrate surfaces and - in most cases - the seams are heat welded. The critical components determining single ply use for waterproofing over roofing applications are the material’s water absorption capacity and its resistance to ground chemicals. The maximum water absorption rate for waterproofing materials is 3.2 percent by weight. Some modified bitumen sheets have absorption rates of nearly 10 percent. PVC and butyl rubber typically have the lowest absorption rates, which are typically about 2 percent. Butyl rubber and PVC also offer significant chemical resistance. The PVC used in waterproofing sheets includes additives to resist chemicals, alkaline materials and algae in lieu of the UV stabilizers and high-temperature inhibitors that are added to PVC roofing sheets. Thermoplastic sheets are available in three compositions: PVC, chlorinated polyurethane (CPE) and chlorosulfonated polyethylene (CSPE or Hypalon).
The membranes are available in varying thicknesses. PVC membranes are 30 to 60 mils thick, CPE membranes are 20 to 120 mils thick and CSPE are 30 to 35 mils thick.
Bentonite is a traditional waterproofing material that dates back nearly 80 years. Bentonite is granulated smectite clay that provides waterproofing capabilities by swelling to nearly 15 times its dry volume when it absorbs water. In recent times, advanced technology of polymer chemistry has increased the use of bentonite by making it a more versatile material. It is currently offered in four forms:
• Prefabricated panels.
• Prefabricated geo-textile sheets.
• High-density polyethylene sheets.
• Trowel-grade mixtures used for detailing.
These changes in technology have added to its ease of application. In the past, skilled workmen achieved uniform thicknesses with bentonite by spraying and troweling. Now bentonite panels can be placed on the substrate, properly lapped, and nailed into place.
Bentonite sheets typically contain 85 percent to 90 percent of montmorillonite clay and a maximum of 15 percent natural sediments such as volcanic ash.
The application of bentonite is typically successful for blindside waterproofing in deep excavations and structures without sensitive occupancies that have a minimal risk of leaks or humidity control. The main advantage of bentonite is that it can be placed over concrete surfaces almost immediately after the forms are removed. One potential concern is that the material must be covered directly after application. Any contact of the cardboard exterior with precipitation (rain, snow, dew, etc.) will contribute to material deterioration and odors.