The dew point is the temperature at and below which moist air is saturated and water then condenses on available surfaces. When temperatures drop below freezing, the dew point becomes the frost point.
As atmospheric temperatures drop to freezing (0 C) and below, there will be a drop in the temperature of the pavement surface and, subsequently, of the sub-surface components of the pavement. Frost first occurs when the surface temperature drops to 0 C and the dew point is above 0 C. Frost and ice can produce tricky driving conditions due to the dramatic lowering of skid resistance levels.
The lowest depth at which freezing occurs during the course of the winter is called the frost line. Because soil and snow are effective insulators, the frost line will rarely be more than a metre below the surface. The amount that the frost line does descend will be related to the length of time during which sub-freezing temperatures occur and the extent of those temperatures. This is measured by the Freezing Index, which is the (negative) area of the time-temperature graph at a site, from the beginning of freezing to the beginning of thawing.
Section 9.2.4 discussed how soil suction can raise water through a pavement structure and thus saturate some of the pores above the water-table. Near-freezing temperatures also create a suction effect, raising the free water in the soil towards the frost line. As water in very small pores has a lower than normal freezing point, the cold-suction effect also causes this small pore-water to be drawn towards any already-frozen larger pores. The soil below the frost line from which the water has been drawn will now be dry and contain shrinkage cracks, giving it a characteristically rough appearance.
As a consequence of this migration of water to frozen sites, lenses of ice can form and these will subsequently draw even more water towards themselves. Water expands about 9 percent upon freezing and large lenses can cause the soil surface to heave by amounts of up to 300 mm. The process is called frost heave. By opening up surface cracks, cold air can then enter the basecourse and cause small subsidiary ice lenses to form, thus creating further expansion. Significant pavement damage in the form of differential frost-heave can occur as a result of these factors.
For dry soils and in less severe climatic conditions, ice lenses will not form and the soil may sink rather than heave as a result of normal thermal contraction.
Pavement thawing can occur either downwards, upwards, or in both directions. At the beginning of the thaw, the pavement surface will usually be clear of snow, whilst the verges will probably have a higher-than-average covering of snow. This will lead to more rapid thawing and settlement under the pavement, particularly with sudden thaws. The melt water will fill shrinkage cracks as well as the original interstices and will have little chance of lateral or vertical drainage. Pore pressures will develop and, following Sections 9.2.4–5, the material will exhibit low strength and stiffness. The effect will be exacerbated by the constraint of the surrounding frozen soil.
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Thus, although a frozen pavement will be quite strong, thawing will completely reverse the effect to the extent that traffic at the early stages of thawing will therefore cause a new cycle of major deformation, usually called spring break-up. It is therefore necessary to exercise some restraint in permitting traffic to use pavements after the spring thaw has left the upper courses saturated whilst the lower drainage provisions are still frozen and inoperative. The length of the freeze–thaw cycle is therefore of critical importance.
Not surprisingly, frost damage is usually the major cause of pavement damage in freezing areas. Susceptible soils for frost heave are clearly those that permit easy water movement, through permeability or suction. Thus, silts, seamed clays, and poorly-graded mixtures are susceptible to frost heave; whereas well-graded sand and heavy clays are not, unless drainage is poor — e.g. if there is a perched water-table able to feed water to the lenses. Susceptibility tests for frost heave have been developed (Roe and Webster, 1984). Typically, susceptible material to about 70 percent of the depth to the frost line is replaced. Given common limits for the penetration of the frost line, this implies soils within 700 mm of the surface in normal, and 1000 mm in extreme conditions. The thermal conductivity of the pavement and subgrade will also be a relevant property.
In summary, design should concentrate on: * avoiding frost-susceptible material,
* designing for saturated conditions and/or limiting traffic during spring break-up, * avoiding the entry of water into the pavement structure, and
* using insulation to manage the frost line.
Anti-icing operations are pro-active treatments conducted at the pavement surface. Commonly, a chemical such as salt brine (liquid NaCl) is applied to lower the freezing point of the surface water. This prevents any snow or frost from bonding to the pavement surface and thus improves driving conditions and simplifies snow removal. De-icing is a reactive treatment that breaks the existing bond between the snow and the pavement surface. Salt (NaCl) is commonly used for anti-icing and de-icing. It is usually used as a brine to reduce losses. However, it increases vehicle corrosion, damages concrete and steel structures, produces surface grime, and adversely affects the environment. One commonly used replacement for salt is calcium magnesium acetate.
The frequency with which snow is removed depends largely on the levels of traffic flow. Typically, if:
* AADT > 10 000, all lanes are kept clear;
* 10 000 > AADT > 2 000, wheel-paths are kept clear for two lanes; and * 2 000 > AADT > 800, intermittent wheel-paths are kept clear.
Dry snow generally needs to be over 30 mm in depth to begin to affect traffic performance in communities accustomed to snow conditions.