The properties of asphalt pavement allow it to expand and contract with fluctuations in the material temperature. However, extreme cold temperatures can cause asphalt pavements to contract to a point that exceed the material’s elastic capabilities, resulting in thermally cracked pavements. If these cracks are left untreated, the asphalt layer, granular base, and lower soil layers are susceptible to damage from water that infiltrates the crack. Water can “strip” the asphalt binder from the sides of the crack, weakening the pavement structure. Water can also increase the moisture content of the granular base and subgrade soils, thereby weakening the foundation layers. Both situations can lead to depressed cracks, decreased ride quality, and a shortened pavement life.
The effect of material factors of asphalt concrete (AC) mixtures on low temperature cracking is reasonably well understood. The temperature susceptibility of the asphalt cement has been found to be a deterministic factor for performance. Pavements in cold regions are subjected to thermal and traffic loading, where pavement temperatures can range between -50°C in the winter to +50°C in the summer. The pavement distresses related to temperature are thermal cracking and rutting. It is suggested that the thermal cracking in cold regions occur by two distress mechanisms. The first mechanism is related to the transverse cracks that may be caused by the overall contraction of the entire pavement structure and/or underlying subgrade. As a result, cracks extend through the entire pavement structure and into the subgrade. Cracks can also extend across the pavement surface into the shoulder and be several centimeters wide. The thermal contraction of soil (in the base, subbase, and/or subgrade) is primarily responsible for such types of transverse cracks rather than the AC surface layer. Such transverse cracks can even occur in unpaved roads at intervals of 39.4 to 295.3 feet (12 to 90m) and depths extending to 6.6 feet (2m). The second mechanism responsible for low temperature cracking in AC relates to the volumetric contraction that occurs as the material experiences a temperature drop. Provided that a material is unrestrained, it tends to shorten as the temperature falls. However, if such material is restrained, resembling the case of AC in a pavement structure, the development of thermal stress can produce cracking when the stress equals the tensile strength of the material. During warm weather, AC can be considered to act as a viscoelastic material. Thus, the developed thermal stresses due to temperature drop in a warm temperature range can be dissipated through stress relaxation. Unfortunately, this doesn’t apply to cold weather where the AC behaves as an elastic material. The thermal stresses are not dissipated and cracking can occur.
Low temperature cracking of AC pavements represents a serious problem in the northern United States, Canada, and other cold regions. Severe cracking has been reported in areas where the freezing index, a measure of the combined duration and magnitude of below freezing temperatures occurring during any given freezing season, is equal to or greater than 13,330°C-hours, based on air freezing indices. It has been reported that low temperature cracking can even occur in areas where the freezing index is as low as 8665°C-hours. Generally, low temperature cracking has a primarily pattern that is transverse to the direction of traffic and is fairly regularly spaced at intervals of 98.4 to 196.9 feet (30 to 60m) for new pavements and less than 16.4 feet (5m) for older pavements. Longitudinal cracking may also occur if the transverse crack spacing is less than the width of the pavement, leading to the development of a block pattern. However, thermal cracking occurring wholly in the AC surface layer represents a more serious problem than the transverse cracking caused by the overall contraction of the pavement structure and subgrade. The presence of cracks that are restricted to the AC surface layer enable ingress of water, which increases the rate of stripping and allows pumping of a fine granular base course. In addition, reduction of the bearing capacity of the pavement system may occur, thereby leading to premature failure. Upward lipping at the crack edge can also occur as a result of water entering in the crack during the winter that may lead to the formation of an ice lens below the crack. Also, localized thawing of the base can occur due to the entering of de-icing solutions into the crack that may cause a depression around the crack.
Factors Influencing Low Temperature Cracking in AC Pavements
Factors influencing low-temperature cracking in AC pavements can be categorized as (1) material, (2) environmental, and (3) pavement structure geometry.
Several material factors can affect the thermal behavior of asphalt-aggregate mixtures. These factors include asphalt cement, aggregate type and gradation, asphalt cement content, and air-void content:
- Asphalt cement
The single most important factor affecting the severity of low-temperature cracking in an AC mix is the temperature-stiffness relationship of the asphalt. The most important considerations are the stiffness or consistency (i.e., viscosity or penetration) at a cold temperatures and the temperature susceptibility (i.e., the range in consistency with temperature). Lower viscosity (or penetration) grades or lower temperature performance graded materials will have a reduced rate of increasing stiffness with decreasing temperature. This results in a lower potential for low-temperature cracking. It has been found that the addition of polymer to liquid or heated asphalt generally improves field performance because it imparts flexibility to the asphalt.
- Aggregate type and gradation
Aggregates that have high abrasion resistance, low freeze-thaw loss, and low absorption show improved resistance to transverse cracking. Little variation in low-temperature strength is associated with aggregates that possess these characteristics. The low-temperature strength is reduced through absorptive aggregates because the asphalt cement remaining in the mix for bonding is less than it would be in a mix with a non-absorptive aggregate. Little influence on low-temperature strength can be related to the gradation of the aggregate used in the mix, provided that the mix is designed to provide reasonable resistance to rutting.
- Asphalt cement content
No significant influence on a mix's low-temperature cracking performance has been reported when changes in asphalt cement content occur within a reasonable range of optimum. Increasing asphalt content increases the coefficient of thermal contraction and decreases the stiffness. This leads to equilibrium between the thermal stress developing and the stress that developed before the asphalt cement content was changed.
- Air-void content
The degree of compaction and related air void content and permeability do not significantly influence the low-temperature cracking characteristics of the mix.
Several environmental factors can affect low-temperature cracking. These factors include the following:
It was reported that, for a given mix, as the pavement surface temperature is reduced, the incidence of thermal cracking is increased. The ambient air temperature and wind speed both affect the pavement surface temperature. The majority of low-temperature cracking occurs when the temperature decreases to a level below the glass transition temperature and is maintained at this level.
- Rate of cooling
A faster rate of cooling will result in greater tendency for thermal cracking.
- Pavement age
The incidence of thermal cracking is associated with older pavement. This occurs as a result of the increasing stiffness of aging asphalt cements. The aging characteristics of a mix may be affected by the air void content. In addition, as the pavement's service life increases, the probability of more extreme low-temperatures occurring will increase.
Pavement Structure Geometry
Several pavement structure geometry factors can affect thermal cracking response. These factors include pavement width, pavement thickness, coefficient of friction between the AC layer and base course, subgrade types, and construction flaws:
- Pavement width
It has been suggested through field investigations that thermal cracks are more closely spaced in narrow pavements than in wide pavements. Initial crack spacing for secondary roads of 24 feet (7.3m) width is approximately 98.4 feet (30m). As the pavement ages, secondary and tertiary cracks develop and the differences in crack spacing are not apparent.
- Pavement thickness
In general, lower incidence of thermal cracking has been recorded for thicker AC layer pavements. In a study made Burgess et al., it was fond that increasing the thickness of the AC from 3.9 inch to 12 inch (10cm to 25cm) resulted in one-half the cracking frequency when all other variables were consistent.
- Coefficient of friction between the AC layer and base course
It was found that the existence of a prime coat on an untreated aggregate base course layer reduces the incidence of low-temperature cracking. This result was attributed to the fact that an AC layer is "perfectly" bonded to the underlying granular base with a reduced thermal contraction coefficient (because the granular base has a lower thermal contraction coefficient than the AC). The gradation of the base course, particularly the percentage of material finer than the No. 200 sieve, may have a minor influence on the incidence of low-temperature cracking.
- Subgrade type
The frequency of low-temperature cracking is usually greater for pavements on sand subgrades than on cohesive subgrades.
- Construction flaws
Steel roller compaction of asphalt layers at high temperatures and low mix stiffness creates transverse flaws. As the pavement cools, cracks may be initiated at these flaws, often spaced closer than the width of a lane.