UFC 3-260-03
15 Apr 01
CHAPTER 7
PAVEMENT EVALUATIONS FOR FROST CONDITIONS
1. GENERAL. This chapter presents criteria and procedures for the evaluation of airfield pavements in
seasonal frost areas. If the existing base, subbase, and/or subgrade soils under the pavement structure
are susceptible to detrimental frost action during part of the year, then the bearing capacity of the pave-
ment structure will be less than if the same soil conditions existed in a nonfreezing environment. The con-
ditions required for detrimental frost action are freezing temperatures, frost susceptible soils, and a source
of water near the freezing front. The emphasis of the evaluation is in the reduction of the bearing capacity
during thaw-weakening periods. The reduction in load-carrying capacity develops as the soil structure
changes and the melting of the ice releases an excess of water that does not readily drain or redistribute
itself, thus softening the soil. Recovery from the softened condition comes about initially as a process of
reconsolidation and dissipation of pore water pressure, followed by progressive desaturation and buildup
of moisture tension, which stabilizes the soil. If such conditions conducive to detrimental frost effects
exist, then the evaluation will be made up of two parts; normal period and period of weakening. The first
will be based on normal, nonfreezing conditions and will be applicable to that period of the year during
which the pavements are not affected by thawing of the base, subbase, or subgrade. The second,
applicable to the thaw-weakening period, will be based on subgrade strengths using FASSI and FAIR as
prescribed in this chapter. Evaluations of airfields during thaw-weakening periods will use pass intensity
levels identified in chapter 2.
2.
FROST CONDITION TERMINOLOGY. The following terms are used in this chapter.
a. Frost Action. A general term for freezing and thawing of moisture in materials and the resultant
effects on these materials and on structures of which they are a part, or with which they are in contact.
b. Frost Susceptible Soil. Soil in which significant detrimental ice segregation will occur when the
requisite moisture and freezing conditions are present. These soils will lose a substantial portion of their
strength upon thawing.
c. Nonfrost Susceptible Materials. Cohesionless materials such as crushed rock, gravel, sand,
slag, and cinders that do not experience significant detrimental ice segregation under normal freezing
conditions. Cemented or stabilized materials that do not experience significant detrimental ice segrega-
tion, loss of strength upon thawing, and freeze thaw degradation are also considered to be nonfrost sus-
ceptible materials.
d. Frost Heave. The raising of the pavement surface due to formation of ice lenses in the under-
lying soil.
e. Frost-melting (Thawing) Periods. Intervals of the year when the ice in the base, subbase, and/ or
subgrade returns to a liquid state. A period ends when all the ice in the ground has melted or when the
previously frozen material is refrozen. In general, there may be several significant frost melting periods
during the winter months prior to the spring thaw.
f. Periods of Weakening (Thaw-weakening Periods). Intervals of the year when the base, sub-
base, and/or subgrade strength is below its normal summer values. These intervals correspond to frost-
melting periods. The period ends when either the material is refrozen or when the subgrade strength has
returned to the normal summer value at the end of the spring thaw-weakening period, figure 7-1.
g. Critical Weakening Period. Interval during the period of thaw weakening when the base, sub-
base, and/or subgrade strength is at its lowest strength, figure 7-1.
7-1
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