UFC 3-260-03
15 Apr 01
melting of segregated ice leads to excess water in the base/subbase and/or subgrade and cannot drain
through the still-frozen underlying soil. Drainage could also be restricted laterally at this time of the year;
thus the period of severe weakening may last several weeks. Presence of drainage layers in the pave-
ment structure should decrease this period of severe thaw weakening.
b. Soils, such as clays, which often show no frost heave may significantly lose supporting capacity
during thawing periods. Frost-susceptible granular unbound base materials may also weaken significantly
during frost-melting periods because of increased saturation and associated decrease of moisture ten-
sion, combined with reduced density that is derived from expansion in the previously frozen state. As the
percent of fines in granular material increases, so does its potential for thaw weakening during frost-
melting periods due to reduction of its permeability.
c. Traffic loads may cause excess hydrostatic pressures within the pores of the frost-affected soil
during thaw-weakening periods, resulting in further reduction in strength or even failure. The degree to
which a soil loses strength during a frost-melting period and the duration of the period of thaw weakening
depend on the soil type, temperature conditions during freezing and thawing, the amount and type of
traffic during frost melting, the availability of water during freezing and thawing, and drainage conditions.
4. CRITICAL WEAKENING PERIOD. The critical weakening period comes during the early stages of
frost-melting and may occur intermittently during the winter, when the segregated ice in the base, subbase
and subgrade is melting. This critical period can last from 1 week to several months, depending on the
soil type. As the soil drains and reconsolidates, the pavement regains much of its lost strength. With the
subsequent gradual desaturation and the corresponding buildup of moisture tension in the affected soils,
the pavement gradually regains full normal-period bearing capacity. The length of the recovery period
varies from a few weeks to several months, depending on the intensity of ice segregation, the depth of
frost penetration, the rate of thawing, the permeability of the soil, the drainage conditions, precipitation,
and atmospheric humidity. The performance of highways with a comparable subgrade in the vicinity of the
airfield installation may be an indicator of the likely duration of the critical period, however since airfield
pavements are wider and drainage paths longer, the thaw-weakened period is also likely to be longer.
5. EFFECT OF FROST ACTION ON PAVEMENT SURFACE. The most obvious structural effect of
frost action on the pavement surface is random cracking and roughness as the result of differential frost
heave. Studies of rigid pavements have shown that cracks may develop more rapidly during and immedi-
ately following the spring frost-melting period as a result of differential thaw than during the period of active
heave. Deterioration and spalling of the edges of open cracks is a source of debris that is a potential
cause of Foreign Object Damage (FOD) to aircraft engines. Cracks in flexible pavements may also be the
result of contraction of the pavement during periods of extremely low temperatures. The effect of thaw
weakening of subgrades and base courses may be more severe than cracks caused by frost heave or
low-temperature contraction because it leads to destruction of the pavement, requiring reconstruction. Its
effect is felt through a process of greatly accelerated cumulative damage to the pavement under succes-
sive traffic loads. Eventually, the accumulation of damage leads to visible surface cracking. This cracking
may not become visible during frost melting. As a result, thaw weakening may not always be recognized
as the dominant factor causing accelerated failure.
6. MAGNITUDE OF SUBGRADE WEAKENING. The load-bearing capacity of both flexible and rigid
pavements can be severely reduced during critical weakening periods; however the reduction is less criti-
cal for rigid than for flexible pavements. Rigid pavements experience a smaller reduction because the
subgrade has less influence on the supporting capacity of rigid pavements than on that of flexible pave-
ments. Subgrade soils under rigid pavements are subjected to less shearing deformation and remolding
during critical weakening periods.
7-3
Previous Page
