UFC 3-260-03
15 Apr 01
h. Recovery Period. Interval from the end of the critical weakening period to the beginning of the
normal period. During this time the base, subbase, and/or subgrade strength is recovering to normal
strength from lowest strength, figure 7-1.
I. Normal Period. Interval during the year when the base, subbase, and/or subgrade strength is at
its nonfrost strength, figure 7-1.
j. Average Daily Temperature. The average of the maximum and minimum temperatures for one
day, or the average of several temperature readings taken at equal time intervals, generally hourly, during
a day.
k. Mean Daily Temperature. The mean of the average daily temperatures for a given day, usually
calculated over a period of several years.
l. Degree-Days. The Fahrenheit degree days for any given day equal the difference between the
average daily air temperatures and 0 EC (32 EF). The Centigrade degree hours for any given day equal
the average daily temperatures (EC ) multiplied by 24 hours. The degree-days or degree-hours are
negative when the average daily temperature is below 0 EC (32 EF) (freezing degree-days or hours) and
positive when above (thawing degree-days or hours). Usually, the degree-days or hours are reported in
terms of their absolute values and the distinction is made between freezing and thawing.
m. Freezing Index. The number of degree-days between the highest and lowest points on a curve
of cumulative degree-days versus time for one freezing season. It is used as a measure of the combined
duration and magnitude of below-freezing temperatures occurring during any given freezing season. The
index is determined from air temperatures measured approximately 1.37 meters (4.5 feet) above the
ground and is commonly designated as the air freezing index.
n. Design Freezing Index. The average air freezing index of the three coldest winters in the latest
30 years of record. If 30 years of record are not available, the air freezing index for the coldest winter in
the latest 10-year period may be used. The design freezing index at a site need not be changed more
than once in 5 years unless the more recent temperature records indicate a significant change in thick-
ness requirements for frost protection. The design freezing indexes for North American locations are
presented in figure 7-2.
o. Mean Freezing Index. The freezing index determined on the basis of mean daily temperatures.
The period of record over which average daily temperatures are averaged is usually a minimum of the
latest 10 years, preferably 30. Mean freezing indexes for northern Eurasia are presented in figure 7-3.
Design freezing indices are not available for Eurasia.
p. Combined Base Thickness. The combined thickness of base, subbase, drainage layer, and
separation layer.
3. FROST EFFECTS. The detrimental effects of frost action are frost heave and thaw weakening.
Frost heave, manifested by the raising of the pavement surface, is directly associated with ice segregation
and is visible evidence on the surface that ice lenses have formed in the subgrade, subbase, and/ or
base- course materials. Depending on variations in exposure to solar radiation or in the character of the
soil and ground-water conditions underlying the pavement, heave can be uniform or nonuniform.
Nonuniform heave results in unevenness or abrupt changes in grade at the pavement surface. If such
conditions are noted by the evaluation team, or are reported by flight or other personnel, the location and
description of the objectionable roughness will be included in the evaluation report.
a. When ice segregation has taken place in a frost susceptible soil, the soil is subsequently weak-
ened during prolonged frost-melting periods, as during winter partial thaws and early in the spring. The
7-2
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