b'YOUR TOOLKIT FOR BUILDING EXCELENCEsequence is that the slab provides a smooth, hard,material after the slab has been placed will be and level work surface for building completion. held and distributed under the slab by the vapor Forpost-framebuildingsusingcontinuousbarrier/retarder (picture a swimming pool under posts, it is generally best to complete the entireyour slab).Captured water will actually enhance building shell prior to floor slab casting.Thisinstead of ebb the movement of water vapor into ensures that posts are exactly positioned priorthe slab.Placing a vapor barrier/retarder under to slab casting (important when the slab is castthe subbase material also makes it difficult to against the posts), and in most cases, protectsdeal with rainwater and other precipitation that the casting operation from precipitation, directaccumulates in the subbase prior to slab placement. sun,wind,andtemperatureextremes.ThisWhenplacingconcretedirectlyonavapor construction sequence is often desired becausebarrier/retarder, an adjustment in the concrete it enables all or a large portion of both above- andmix design is generally required to reduce the below-slab utility work (i.e., electrical, plumbingamount of bleed water for routine finishing. and mechanical equipment installation) to beTo avoid damage to the barrier/retarder during completed at the same time (i.e., prior to slabconstruction, use a thicker barrier/retarder.A placement).The downside of this constructionminimum 10 mil thickness is common with 15 sequenceisthatitoftenlimitsthetypeofmil recommended when laser screeds or heavy equipment that can be used in slab placement.placing equipment will be on the barrier/retarder. More specifically, a lack of head clearance and/ A more complete discussion on vapor barrier/or unobstructed access to the entire floor arearetarder installation is found in ACI 302.1 R-15. may eliminate certain concrete pumping options and limit direct placement via concrete truck. Photo 1: Concrete being deposited for a reinforced Under-Slab Vapor Barriers/Retarders floor slab with two mats of steel reinforcement bars.Vaporbarriers/retarders placed in direct contact with the underside of a slab-on-grade will ebb the flow ofwatervapor from the subgrade up into the slab and can also help prevent soil gases from permeating intotheslab. Vaporbarriers/retarders should notbeplaced under the subbase material since anyPhoto 1: Concrete being deposited for a reinforced waterthatleaksfloor slab with two mats of steel reinforcement bars.into the subbase continued on page: 16 FRAME BUILDER - OCT2020 / 15'