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FLOOR SLAB MOLSTURE/YAPOR PROTECTION Floor slab desien rela�ive to moisture/vapor pro�ec:ion should consider the rype and locacion of nuo elemenu, a granular layer and a vapor membrane(vapor retarder,wa[er resistant barrier or vapar barrier). In the following secuons, the pros and cons oi the possible opdons reeardine these eiements will be presented, such that you and your specifier can make an engineering decision based on the benefirs and cosu of the choices. GRANUI.AR LAYER In?�merican Concrece Instim[e(ACI}302.1-96,a"base materiai"is recommended, racher than the convenaoaal cleaner "sand custuon" material. The ma.nual mainrains [hat clean sand (eommo❑ "cushion" sand) is difficult to compact and maintain until concrete placement is comptete. ACI recommends a clean, fine�aded material(with at leas[ IO% to 30 0 of parcicles passing a/tI00 sieve)which is not contamina[ed with clay,silt or organic material. We refer you to ACI 302.1- 96 for addidonal deraiLs regarding the requiremenrs for the base material. In cases where potential staric wa[er Ieveis or si�ificant perched water sources appear near or abave rhe tloor slab, an underfloor drainage syscem may be needed wherein a drainale system is placed within a�hicker ciean sand or cravet layer. Such a system should be properly engineered depending on subgrade soil rypes and race/head of water inflow. vAPOR MEMBRANE The need for a vapor membrane depends on whecher the IIoor slab will have a vapor sensidve covering, wili have vapor sensiuve items stored on the slab, or if the space above the slab will be a humidity controlled area. If the project does no[ have this vapor sensiuviry or moisture conaol need,placement of a vapor membrane may noc be necessary. Your decision will chen relace to whether co use the ACI base material or a convendonal sand cushion layer_However, if any of the above sensiaviry issues apply, placeme� of a vapor membrane is recommended. Some floor covering syscems (adhesives and flooring materiaLs)require a vapar membrane ro maintain a specified maximum slab moisture conrent as a condiuon of their warcan[y. VApOR MED�RANE/GRAMILAR LAYER PL�+iCEMENT A number of issues should be considered when deciding whether to place the vapor membrane above or below the granuIar Iaye:.The benefirs of placing the slab on a�anular Iayer,with the vapor membrane placed beIow the oranular Iayer, include reducrion of the following: • Slab curling�"n�the curin�and drying process. • Time of bleeding, which allows for quicker finishing. • Vapor membrane puncturing. • Surface blistering or delaminauon caused by an extended bleeding period. • Craclang caused by plasuc or drying shrinkage. The benefics of placing the vapor membrane over the gramilar layer include the followine: • The moisture emission rate is achieved faster. • Eliminates a potential war.er reservoir within the gramilar layer above [he membrane. • Provides a "slip surface", thereby reducing slab restraint and the associaced random craclang. If a membrane is to be used in conjunetion with a;ranutaz layer,[he approach recommended depends on slab usage and the construcuon schedule.The vapor membrane shouid be placed above the granular layer when: • Vapor sensiave floar covezing systems are used or vapor sensiuve items will be direcdy placed on the slab. • The area will be bumidiry conrrolled,but the slab will be placed before the building is enciosed and sealed from rain. • Required by a floor covezing manufacturer's system warranry. The vapor membrane should be pIaced below the o anular layer when: • Used in humidiry controlled areas(wirhout vapor sensiave coverings/stored icems), with the roof inembrane in place, and the buiIciing enclosed co the poin[ where precipiTafion wiil not intrude into the slab area. Considerarion should be given to slight sloping of the membrane to ed�es where drainule or other disposal methods can alleviare potendal water sources, such as pipe or roof leaks, foundarion wall damp proofing failure, fue sgrinklez system acavarion, eu. There may be cases where membrane ptacement may have a deuimental effect on the subgrade suppon system (e.g., expansive soils). In these cases,your decision will need ro weigh the cost of subgrade opuons and rhe performance risks. O1REP013(2/Ol) AMERICAN ENGIlVEERING TESTING, INC.