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ENGEO <br /> INCORPORATED <br /> <br /> <br />5150.100.101 <br />May 2, 2008 Page 22 <br />within a building. When water vapor migrating through the slab would be undesirable, we <br />recommend the following to reduce, but not stop, water vapor transmission upward through the <br />slab-on-grade: <br /> <br />1. Construct a moisture retarder system directly beneath the slab on-grade that consists of the <br />following: <br /> <br />a) Vapor retarder membrane sealed at all seams and pipe penetrations and connected to <br />all footings. Vapor retarders shall conform to Class A vapor retarder per <br />ASTM E 1745-97 “Standard Specification for Plastic Water Vapor Retarders used in <br />Contact with Soil or Granular Fill under Concrete Slabs.” The vapor retarder should <br />be underlain by <br /> <br />b) 4 inches of clean crushed rock. Crushed rock should have 100 percent passing the <br />¾-inch sieve and less than 5 percent passing the No. 4 Sieve. <br /> <br />2. To reduce porosity of the floor slab, use a concrete water-cement ratio for slabs-on-grade <br />of no more than 0.45 and a minimum compressive strength of 4,500 psi. <br /> <br />3. Provide inspection and testing during concrete placement to check that the proper concrete <br />and water cement ratio are used. <br /> <br />4. Moist cure slabs for a minimum of 3 days or use other equivalent curing specific by the <br />structural engineer. <br /> <br />The structural engineer should be consulted as to the use of a layer of clean sand or pea gravel <br />(less than 5 percent passing the U.S. Standard No. 200 Sieve) placed on top of the vapor retarder <br />membrane to assist in concrete curing. <br /> <br />8.3 TRENCH BACKFILL <br /> <br />Backfill and compact all trenches below building slabs-on-grade and to 5 feet laterally beyond <br />any edge in accordance with Section 4.5.2. <br />