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The acceptable design of a pavement system in a layered format depends on a couple of complex tasks which normally should be performed iteratively. Two main parts of these iterative tasks are applying a computational model to incorporate the external environmental effects over the design life of the pavement system and, secondly, quantifying the effect of these changes on the performance of each pavement layer.
Environmental conditions such as precipitation, variation in ambient temperature, and depth to the water level cause seasonal variation in the moisture content of the unbound layers, which leads to fluctuation in resilient modulus of different layers of the pavement. Fluctuation in moisture content of unbound material and as a result, changes in effective stress, has a significant effect on the resilient modulus, stability and permanent deformation. Damage due to freezing occurs when frost penetrates the subgrade soil, increases matric suction in the freezing zone. Then, water moves toward the freezing front and ice lenses form. Ice lenses in fine soils continuously expand by attracting moisture from the underlying shallow water table, resulting in frost heave at the pavement surface.
Frost heave can negatively impact the performance and ride quality of the road. When thawing occurs in the following spring, ice lenses melt, causing an increase in water content of subgrade soil, which cannot be discharges out of the pavement system rapidly. Consequently, the strength of the subgrade soil decreases, leading to structural damage, differential settlements and damage to the pavement structure when exposed to heavy traffic loads. These issues become critical for roads in cold regions where increased freeze-thaw cycles are expected in the future as a result of climate change. Frost heave and associated thaw-weakening in subgrade soils and unbound pavement materials are complex engineering problems that have been studied for several years.
One strategy for minimizing seasonal effects on subgrade is using insulation layers to protect frost-susceptible subgrade from being influenced by frost. An insulation layer controls the heat transfer between the ambient air and the pavement layers and delays thawing and/or freezing. The insulation layers minimize the seasonal fluctuation in resilient modulus of subgrade by reducing the frost penetration. They also decrease the risk of thaw weakening during early spring. If the insulation layers provide an adequate load bearing capacity for the pavement and do not create an unfavorable moisture regime in the system, using them will result in reducing the depth of frost penetration into the pavement structure. Limiting the rot depth in pavements enables design engineers to moderate the base/subbase layers’ thicknesses. Decreasing the thickness of crushed structural layers, leads to limiting the depletion of natural aggregate resources and results in more economical and sustainable design strategies. The amount of insulation required will depend on the type of pavement structure, the thermal constants of the various layers, and the climate.
In recent years, in line with other sectors of society, the pavement community has increasingly used more sustainable and environmental friendly practices and materials. Consequently, road highway agencies in cold regions are urged to use waste and recycled materials as an economical insulation layer in an ongoing trial-and-error process. As there are some difficulties associated with the prediction of reliable thermal patterns in pavement structures, there are also concerns regarding long term structural performance of these material and field trials have been initiated. Since the 1990s, several materials were introduced and evaluated as insulation layers, including sawdust, tire chips, and plastic. Polystyrene boards are one of the most well-known insulation materials that have had a long history of application since 1965. Some recycled materials have recently been introduced as thermal resistive layers. These materials can be a sustainable and cost-effective option while still providing the same benefits as conventional insulation layers.
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