08-08-2007 Council Agenda - Laserfiche WebLink

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FLOOR SLAB MOISTURE/VAPOR PROTECTION Floor slab design relative to moisture /vapor protection should consider the type and location of two elements, a granular layer and a vapor membrane (vapor retarder, water resistant barrier or vapor barrier). in the following sections; the pros and cons of the possible options regarding these elements will be presented, such that you and your specifier can make an engineering decision based on the benefits and costs of the choices. GRANULAR LAYER In American Concrete Institute (ACI) 302.1 -96, a' base material" is recommended, rather than the conventional cleaner "sand cushion" material. The manual maintains that clean sand (common "cushion" sand) is difficult to compact and maintain until concrete placement is complete. ACI recommends a clean, fine graded material (with at least 10% to 30% of particles passing a # 100 sieve) which is not contaminated with clay, silt or organic material. We refer you to ACI 302.1 -96 for additional details regarding the requirements for the base material. In cases where potential static water levels or significant perched water sources appear near or above the floor slab, an underfloor drainage system may be needed wherein a draintile system is placed within a thicker clean sand or gravel layer. Such a system should be properly engineered depending on subgrade soil types an rate/head or water inflow. VAPOR MEMBRANE The need for a vapor membrane depends on whether the floor slab will have a vapor sensitive covering, will have vapor sensitive items stored on the slab, or if the space above the slab will be a humidity controlled area. If the project does not have this vapor sensitivity or moisture control need, placement of a vapor membrane may not be necessary. Your decision will then relate to whether to use the ACI base material or a conventional sand cushion layer. However, if any of the above sensitivity issues apply, placement of a vapor membrane is recommended. Some floor covering systems (adhesives and flooring materials) require a vapor membrane to maintain a specified maximum slab moisture content as a condition of their warranty. VAPOR MEMBRANE /GRANULAR LAYER PLACEMENT A nutnberr of issues should be considered when deciding whether to place the vapor membrane above or below the granular layer. The benefits of placing the slab on a granular layer, with the vapor membrane placed below the granular layer, include reduction of the following: • Slab curling during the curing and drying process. • Time of bleeding, which allows for quicker finishing. • Vapor membrane puncturing. • Surface blistering or delamination caused by an extended bleeding period. • Cracking caused by plastic or drying shrinkage. The benefits of placing the vapor membrane over the granular layer include the following: • The moisture emission rate is achieved faster. • Eliminates a potential water reservoir within the granular layer above the membrane. • Provides a "slip surface ", thereby reducing slab restraint and the associated random cracking. If a membrane is to be used in conjunction with a granular layer, the approach recommended depends on slab usage and the construction schedule. The vapor membrane should be placed above the granular layer when: • Vapor sensitive floor covering systems are used to vapor sensitive items will be directly placed on the slab. • The area will be humidity controlled, but the slab will be placed before the building is enclosed and sealed from rail. • Required by a floor covering manufacturer's system warranty. The vapor membrane should be placed below the granular layer when: • Used in humidity controlled area (without vapor sensitive coverings /stored items), with the roof membrane in place, and the building enclosed to the point where the precipitation will not intrude into the slab area. Considerations should be given to slight sloping of the membrane to edges where draintile or other disposal methods can alleviate potential water sources, such as pipe or roof leaks, foundation wall damp proofing failure, fire sprinkler system activation, etc. There may be cases where membrane placement may have a detrimental effect on the subgrade support system (e.g., expansive soils). In these cases, you decision will need to weigh the cost of subgrade options and the performance risks. - 3 6 -