Page 239 - Masonry and Concrete

Footings, Foundation Walls, Basements, and Slabs
238 CHAPTER SIX

sive shrinkage cracking and slab curling, and permits earlier concrete
finishing. Others feel that the vapor retarder can be better protected by
a geotextile fabric rather than sand and that the blotter effect of the
sand is not necessary to proper curing and finishing of the slab.
Reinforced polyethylene vapor retarders are more resistant to dam-
age than unreinforced polyethylene and are manufactured in multiple
plies for greater strength. If a sand cushion is not used, concrete mix
designs should take into consideration the effect of a low-permeance
vapor retarder on concrete curing, shrinkage, and drying time.
Depending on the type of finish floor materials specified and the ambi-
ent conditions, concrete drying to acceptable moisture levels can take
anywhere from 3 to 6 months. If scheduling is a potential problem,
consider using a low-slump concrete so that there is a minimum
amount of residual mixing water to evaporate after cement hydration
has taken place.

6.7 Insulation

Soil is not a good insulating material, but it does have thermal mass
which minimizes fluctuations in temperature. Daily temperature fluc-
1
tuations affect only the top 1- 2 to 2 ft. of soil. Annual temperature
fluctuations affect the first 20–30 ft. of soil. Below this depth, the soil
temperature is constant. Since average ground temperatures for most
of the United States are below comfortable room temperatures, base-
ments continuously lose some heat to the soil.
The thermal resistance of soil is generally estimated at R-1 to R-2
per foot of thickness. At an average of R-1.25, it takes 4 ft. of soil to
equal the insulating value of 1 in. of extruded polystyrene insulation.
Because heat flow from floor slabs and below-grade walls follows a
radial path (Figure 6-43), however, the effective insulating value of soil
is greater than would be initially apparent because the soil thickness is
measured along the radial lines. This radial path of heat flow means
that the perimeter of a slab-on-grade is subject to much greater heat
loss than the interior floor. Figure 6-44 shows the heat flow from the
perimeter of a floor slab to a cold exterior ground surface as a series of
nearly concentric radial lines. As the length of the heat flow path
increases, the effective insulating value of the soil increases, so ther-
mal insulation is generally required only at the perimeter of the slab

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.