Insulation materials
Several materials have been used in experimental road embankments. The most successful road insulation material has been foamed plastic board. Both polyurethane foam and polystyrene foam have been tried experimentally. The experiment showed urethane foam to be undesirable due to its high moisture absorption (as much as 72% by volume) and compression under soil pressures.
In a long term experiment (a roadway over permafrost) located in south central Alaska near Chitna the results have shown that the degradation of the permafrost can be slowed significantly by using foam insulation in the embankment. Thermally, the best location for insulation is at or near the top of the embankment. Insulation results in lower maintenance and longer life for the road embankment that must be constructed on permafrost.
Early experiments in road insulation used foamed sulfur, but researchers
found it absorbed too much water. McHattie, Esch and Reckard experimented with the insulating effect of artificially placing a peat layer beneath a section of road 50 miles southeast of Fairbanks, Alaska. They reported decreased thaw depths in the sections where the peat materials had been placed as compared to the control sections where the embankment had been placed directly on the mineral soil. However, in their final report they concluded that the advantage of peat as an insulation was not sufficient to warrant its general use as a standard design feature. The use of peat should be considered only in areas in which differential heave or settlement is expected to be severe. When peat is used, it should be placed well in advance of paving so that the peat layer can consolidate before paving. This may require paving to be delayed for a construction season.
The most durable foam insulation material at this time for direct soil burial is polystyrene insulation. Two forms of expanded polystyrene are currently manufactured. The first is extruded and expanded in a single-step process (XEPS) and has a homogeneous interior appearance with somewhat more dense surface skins. The second is molded polystyrene (MEPS) "beadboard" formed by first pre-expanding the polystyrene foam in bead form and then molding and sintering the beads together to form boards or billets that are cut into boards.
Urethane insulation absorbs large amounts of water into its open cell structure. Molded polystyrene beadboard (MEPS) may absorb considerable water in the pores between the expanded beads of polystyrene depending on the molded density and the process control in the molding stage. The absorption of water increases the thermal conductivity and reduces the effectiveness of the insulation. XEPS polystyrene appears to resist the absorption of ground water best, even when submerged below the water table for long periods at a time. After 20 years of exposure and at least 5 years of submergence in the Chitna Test Road (Esch 1990), XEPS insulation had moisture contents ranging from 1.1 to 2.2% by volume. Studies of the XEPS polystyrene insulation used in the Trans Alaska Pipeline workpad were made in 1981, approximately 6 years after
installation. These studies showed that moisture content in the buried insulation at 11 sites in the Brooks Mountain range and the north coastal plain were below-0.5% by volume, except for one submerged sample that had a moisture content of 0.8%.
Words and expressions:
Foam Пена
Undesirable Нежелательный, неподходящий.
Moisture absorption Поглощение влаги
Artificially Искусственно
Peat Торф
To consolidate Затвердевать
Cell Ячейка, отсек
Installation Установка
Content Содержание, объем
Site Участок
Thermal effects and properties of selected materials
It is not feasible to directly compare the thermal effects of a layer of insulation and a layer of embankment material. The thermal effect of a layer of insulation in a gravel embankment is a function of the thermal conductivity, the density, and the specific heat. Although the insulation will reduce the flow of heat through the layer, it will not store any significant amount of heat as compared to a similar layer of soil. The thermal diffusivity of a material gives a measure of how well heat diffuses throughout the material in the absence of a phase change in the moisture. Thermal diffusivities of the insulation and the embankment material it replaces would seem to provide one way to assess the effect of the insulation. However, during periods of equilibrium when the
temperature is not changing very much, heat storage in the layer is also not changing significantly. During these periods thermal conductivity (k) will provide a better picture of the heat flow process. The thermal diffusivity (a) is:
к a=
pC (6.1)
where:
к = the thermal conductivity (BTU/hr ft °F or W/m °K)
p = the density (lbm/ft3 or kg/m3)
С = the specific heat (BTU/lbm °F or cal/kg °C)
Thermal diffusivity, thermal conductivity, and moisture contents need to be used together in assessing the relative value of the insulation in protecting permafrost. Comparing the two shows that the effect of the insulation during transient temperature periods is not nearly as significant as the effect of insulation during the stable steady state condition. During steady state conditions the insulation has a decided advantage over gravel.
TABLE 1 Thermal Properties of Selected Material.
|
|
Specific |
Thermal |
|
Material |
Density |
Heat |
Conduct |
Diffusivity |
(Unfrozen) |
Obm/ft3 |
(BTU/lbmmm -F) |
(BTU/ft hr °F) |
(ftJ/hr) |
Gravel
5% moisture 130 0.44 18 0.32
Sand
10%moisture 120 0.32 17 0.44
Silt
20% moisture 90 0.59 8 0.15
Extruded polystyrene (dry) 3.5 0.29 0.18 0.18 Table 1 shows the thermal properties of selected soils and insulation.
It should be emphasized that neither type of polystyrene insulation is
compatible with hydrocarbon solvents such as gasoline, benzene, and diesel fuel. Contact with these solvents causes polystyrene foam insulation to dissolve. Gravel pads under fuel storage tanks or other areas in which a spill is likely are not good candidates for polystyrene insulation. Foamed glass or urethane, although inferior performers otherwise, are much better choices in these cases. The lower insulating characteristics should be compensated for by using thicker layers of insulation. The designer should increase insulation thickness by two to one for urethane, and by three to one for foamed glass when they are used to replace extruded polystyrene.
Thermal storage is an important property of a road embankment. The ability of the embankment to store heat and to release it later when the temperature drops, slows down the transient effects of changing temperature. This can have the effect of preventing the surface from becoming frost covered during sudden, short below-freezing temperature excursions.
Words and expressions:
Density Плотность, удельный вес
Transient Скоротечный
Steady Постоянный
Gravel Гравий
Sand Песок
Slit Ил
Hydrocarbon Углеводород
Property Качество
To store Хранить
To release Высвобождать