Best Practices: Waterproofing Sandwich Slabs
Waterproofing technology has advanced over the past several decades with the introduction of new materials and the promulgation of new standards. These advancements reflect the study of past failures and an increased awareness of common design errors. While most of the moisture intrusion occurrences in waterproofing systems remain at penetrations and vertical transitions, there have also been documented concerns with horizontal applications. A specific design/construction defect that has been eliminated within the past several decades is how sandwich slabs are constructed.
The practice of applying waterproofing between the structural slab and a wearing slab is commonly referred to as a sandwich slab. In the past, the waterproofing membrane was applied in isolation between the structural slab and the wear slab. This application method propagated shrinkage cracks from the structural slab below the membrane or movement from the topping slab above the membrane. To counteract this problem, the most common waterproofing system prior to the advancement of new materials in the 1980s was multi-ply bituminous built-up membranes without cover boards or thermal insulation, which were susceptible to splitting. (Splits largely occurred because these types of membranes have strain of less than 2 percent when they are exposed to low temperatures.) The movement and/or cracks created openings in the membrane that allowed for moisture infiltration.
In the mid-80s ASTM introduced three important design principles that have eliminated these problems in sandwich slab construction. The design principles were:
1. Sloping the deck for positive drainage.
2. Considering thermal insulation in colder climates.
3. Requiring a drainage course above the membrane.
The key attribute of these changes was the realization that the success of the system increased through the isolation of the membrane from the wear surface. This can be achieved through addition of a thermal insulation and/or a drainage course placed above the membrane.
Insulation applied in waterproofing systems serves two fundamental purposes: thermal resistance and membrane protection. Insulation in waterproofing systems should always be applied above the membrane. The installation of the insulation above the membrane reduces thermal contraction stresses by keeping the membrane in a narrow temperature range. In both vertical and horizontal applications, insulation also protects the membrane from backfill and construction traffic when it is applied over the membrane. Waterproofing systems are exposed to higher traffic loads than roof systems and the insulation serves as further protection with its high compressive strength. Insulations thermal resistance is much greater than aggregate or earth fill on heated and air- conditioned occupied spaces. Even in the coldest climates the insulation — when applied above the membrane — will maintain the membrane temperature above the dew point, eliminating condensation.
Extruded polystyrene (XEPS) board, or “bead board,” provides both a high compressive strength (60 psi) and moisture resistance. Moisture resistance is required because the insulation is not protected and it is exposed to continual moisture infiltration. Studies have indicated that XEPS retains approximately 80 percent of its dry thermal resistance in continually wet conditions. Insulation should be set in a fully adhered application on vertical surfaces. Insulation application on horizontal surfaces should be in accordance with the waterproofing system manufacturer’s requirements.
Protection boards are required to shield the membrane from susceptible damage created by other trades and ultraviolet radiation. Since the waterproofing membrane is the first component completed, it is not unlikely that foot traffic, equipment, scaffolding, or dropped tools could damage it. The protection board should be applied prior to exposure to members of other trades immediately after the flood testing of the waterproofing membrane is completed. Any repairs required after the flood testing is completed should be performed prior to the application of the protection board.
The most common type of protection board is an asphalt-core, laminated panel that comes in thicknesses of 1/16, 1/8, or ¼ inch. This panel is faced with polyethylene film on one side that is applied to prevent the panel from sticking during transport and storage. Some manufacturers also promote the use of a minimum 6-inch thick polyethylene film as a protection layer. The reasoning is that membrane deficiencies are easier to detect and repair with the nominal protection layer. A general word of caution is that the minimal protection layer is more susceptible to damage from equipment, machinery, and scaffolding.
Drainage for Horizontal Applications
Drainage components for a plaza deck with a wearing surface typically include (from the substrate up) the membrane with protection board; filter fabric; pea gravel or a geotextile mat; insulation; and the wearing surface.
Drainage systems on horizontal applications should be comprised of all components from the wearing surface down to the membrane. Horizontal drainage is required at two levels: the wear surface and the membrane level. At the wear surface, drainage is required to minimize saturation that may occur from disintegration during freeze-thaw cycling. At the membrane level, drainage is required to accommodate hydrostatic pressure from accumulated water and freeze-thaw cycling of trapped water.
Proper drainage can be achieved by adequately sloping the horizontal substrate a minimum of 1 percent to 2 percent to allow proper flow to the drains. The drainage course medium is either gravel or plastic drainage panels. Drainage is typically accomplished by employing a multi-level drain component. In this configuration, strainers are applied at the wear surface to accommodate flow from moisture that enters the composition. This design allows for the drainage course to be applied between the membrane and the insulation, reducing condensation above the membrane in cold weather climates (melting snow) without impairing drainage.
At the wear surface, drainage is typically accomplished by internal drains through one of two methods: an open-jointed system or a closed-joint system. An open-jointed system filters the water down to the substrate drains through openings in the wear surface. A closed-joint system is sloped to the surface drains through closures in the wear surface from mortar or sealant joints.