TM 5-822-5/AFM 88-7, Chap. 1
APPENDIX D
USE OF INSULATION MATERIALS IN PAVEMENTS
material is predicated on the placement of the latter
D-1. Insulating Materials and Insulated Pave-
beneath the insulation.
ment Systems.
The only acceptable insulating material for use in
roads and airfields is extruded polystyrene board-
D-2. Determination of Thickness of Cover
stock. Results from laboratory and field tests have
Above Insulation.
shown that extruded polystyrene does not absorb a
On a number of insulated pavements in the civilian
significant volume of moisture and that it retains its
sector, the thickness of material above the insulation
thermal and mechanical properties for several years,
has been established to limit the vertical stress on
at least. The material is manufactured in board
the insulation caused by dead loads and wheel loads
stock ranging from 1 to 4 inches thick. Approval
to not more than one-third of the compressive
from HQUSACE (CEMP-ET) or the appropriate
strength of the insulating material. The Boussinesq
Air Force Major Command is required for use of
equation should be used for this determination. If a
insulating materials other than extruded polystyrene.
major project incorporating insulation is planned,
a. Synthetic insulating material. The use of a
advice and assistance in regard to the structural
synthetic insulating material within a pavement
analysis should be sought from HQUSACE
cross section is permissible with the written ap-
(CEMP-ET) or the appropriate Air Force Major
proval of HQUSACE (CEMP-ET) or the appropri-
Command.
ate Air Force Major Command. Experience has
shown that surface icing may occur on insulated
D-3. Design of Insulated Pavement to Prevent
pavements at times when uninsulated pavements
Subgrade Freezing.
nearby are ice-free and vice versa. Surface icing
Once the thickness of pavement and base above the
creates possible hazards to fast-moving motor vehi-
insulation has been determined, it should be
cles. Accordingly, in evaluating alternative pave-
ascertained whether a reasonable thickness of in-
ment sections, the designer should select an insu-
sulation will keep subfreezing temperatures from
lated pavement only in special cases not sensitive to
penetrating through the insulation. Calculations for
differential surface icing. Special attention should be
this purpose make use of the design air and surface
given to the need for adequate transitions to
freezing indexes and the mean annual soil
pavements having greater or lesser protection
temperature at the site. If the latter is unknown, it
against subgrade freezing.
may be approximated by adding 7 degrees Fahr-
b. Insulated pavement system. An insulated
enheit to the mean annual air temperature. If the
pavement system comprises conventional surfacing
design surface freezing index cannot be calculated
and base above an insulating material of suitable
from air temperature measurements at the site, or
thickness to restrict or prevent the advance of sub-
cannot be estimated using data from nearby sites, it
freezing temperatures into a frost-susceptible sub-
may be estimated by multiplying the design air
grade. Unless the thickness of insulation and over-
freezing index by the appropriate n-factor from TM
lying layers is sufficient to stop subgrade freezing,
5-852-6. For paved surfaces kept free from snow
additional layers of granular materials are placed
and ice, an n-factor of 0.75 should be used. For
between the insulation and the subgrade to contain
calculating the required thickness of insulation, the
a portion of the frost zone that extends below the
design surface freezing index and the mean annual
insulation. In consideration of only the thermal
soil temperature are used with figure D-1 to
efficiency of the insulated pavement system, 1 inch
determine the surface temperature amplitude A. The
of granular material placed below the insulating
initial temperature differential Vo is obtained by
layer is much more effective than 1 inch of the same
subtracting 32 degrees Fahrenheit from the mean
material placed above the insulation. Hence, under
annual soil temperature, or it also may be read
the design procedure outlined below, the thickness
directly from figures D-1. The ratio Vo/A is then
of the pavement and base above the insulation is
determined. Figure D-2 is then entered with the
determined as the minimum that will meet structural
adopted thickness of pavement and base to obtain
requirements for adequate cover over the relatively
the thickness of extruded polystyrene insulation
weak insulating material. The determination of the
needed to prevent subgrade freezing beneath the
thickness of insulation and of additional granular
insulation. If the required thickness is less than
D-1
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