Methods For Minimizing Seasonal Effects On Subgrade In Road Pavement
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, increasing matric suction in the freezing zone; water then 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, water content increases in the subgrade soil, and 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. However, the exact relation between the moisture transformation within the pavement system due to seasonal changes in properly developed mechanistic prediction models for unbound materials performance is still unknown.
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, enabling design engineers to moderate the base/subbase layers’ thicknesses; hence, this will limit the depletion of natural aggregate resources and result 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.
The concept of using insulation material in the road system is suggested by Oosterbaan and Leonards, (1965) and the first full-scale field test road using insulation material was built in Iowa, Michigan, and Minnesota. Then, frost penetration depths beneath small concrete slabs, which included an insulating layer of cellular glass, were measured at a test road in Winnipeg. Plastic foam boards are verified to be effective in both decreasing the frost depth and extenuating the induced heave. During one year data monitoring, elevation measurements conducted at Wolf Creek Pass in Colorado indicated that a 5-cm Styrofoam layer section experienced 14 cm less frost heave compared to the previous winter when no insulation layer was used.
In 1972, a roadway near Chitina, Alaska was built with 5 cm and 10 cm Styrofoam layers to prevent settlement. This road was monitored for approximately three years, and the results indicated that both insulated sections were effective in preventing frost penetration. The settlement in the adjacent conventional Control Section was eight times greater than the 5 cm Styrofoam section and 11 times greater than the 10 cm Styrofoam Section. Insulating a test road section in Alaska showed that 5-cm and 10-cm thick Polystyrene insulation layers could noticeably reduce the thaw depth from 61 cm to 20 and 10 cm, respectively. Additionally, settlement data collected during the Alaskan study from July 1971 through September 1972 presented up to 9 cm less settlement in the insulated sections when compared to the normal sections.
Gandahl investigated the frost resistance capacity of the Polystyrene insulated sections and showed that the water content of the underlying layers as well as the thickness and thermal conductivity of the Polystyrene play a significant role in the frost resistance capacity of the insulation layer. Longitudinal frost heave measurements collected from a test road in north Sweden indicated less heave in the insulated sections compared to the non-insulated sections. Results from a recent case study performed in Edmonton, Alberta showed the impact of a 5-cm thick Styrofoam Highload 40 extruded Polystyrene insulation board in reducing frost penetration into the subgrade by 40 percent compared to the non-insulated section. Field measurements of the frost line’s advancement over time agreed with geothermal modeling predictions.
Several researchers have evaluated and compared the performance of different recycled thermal resistant materials as insulation layers. Bottom ash, which is a byproduct of coal combustion when burnt in the boiler furnace of electric power plants, has recently been introduced as an insulation material in pavement applications. Bottom Ash is mainly composed of silica, alumina, and iron and is utilized in highway construction, primarily in cold-mixed asphalt, embankments, and base courses. In 2012, approximately 52 percent of Alberta’s power was generated via burning coal. In addition to this, approximately 650, 000 m3 of bottom ash and fly ash are produced annually through the province’s power generation process. Alberta’s rate of ash production is high enough to raise concerns that insufficient disposal space will be available in ash lagoons by 2015. The use of bottom ash in road embankments to mitigate frost damage is a new concept.
Few studies have investigated the ability of Bottom Ash mixed with the subgrade and base materials in limiting frost depth. A study conducted in the City of Helsinki, Finland over the course of three winters revealed that frost depth in Bottom Ash sections was 40 to 60 percent of that in gravel sections. This study showed that carefully compacted ash is not susceptible to frost due to its low permeability and hardening. Huang (1990) showed that mechanically- and chemically-stabilized Bottom Ash can be considered a high-quality base material for highway applications. Experimental results concluded that most Bottom Ashes met several performance criteria, such as physical appearance, gradation, and soundness, which make them suitable for pavement construction. The application of Bottom Ash on top of saturated silt could effectively mitigate frost heave and transverse cracking in western Canada.
In a study conducted by Nixon and Lewycky (2001) in Edmonton, Alberta pavement sections comprising a 500-mm gravel base course and a 1100-mm Bottom Ash insulation layer showed a maximum frost depth of 1. 5 m, with no frost reaching to the underlying silt layer, while it was predicted that the frost would reach up to 2. 4 m below the surface in the conventional section.
While using the insulation layers in pavement has a long history of application, incorporating their effects in pavement design is still unexplored. Over time, many different methods were developed to integrate the effect of environmental factors on pavement. Recently, the pavement design methods have evolved from AASHTO design guide, which is a pure empirical approach, to advanced mechanistic design procedures, Mechanistic-Empirical Design Guide (ME-PDG), which correlate the pavement structural responses from traffic loads under different environmental conditions to pavement distresses. Therefore, diurnal and seasonal fluctuations in the moisture and temperature profiles in the pavement structure brought by changes in ambient temperature, ground water table, precipitation/infiltration, freeze-thaw cycles, and other external factors are modelled in a very comprehensive manner by a climatic model called the Enhanced Integrated Climatic Model (EICM). The EICM is a one-dimensional coupled heat and moisture flow program that simulates changes in the behavior and characteristics of pavement and subgrade materials in conjunction with climate condition over several years of operation. It can generate patterns of rainfall, solar radiation, cloud cover, wind speed, and air temperature to simulate the upper boundary conditions of a pavement-soil system. The program calculates the temperature, suction and pore pressure without loading effects, moisture content, and resilient modulus for each node in the profile for the entire analysis period, as well as frost, infiltration and drainage behavior. The direct measurement of the moisture and temperature profiles in pavement via pavement instrumentation can be very beneficial to the development and verification of the EICM in the case of using different insulation materials and under different environmental conditions. However, it should be noted that there is no model defined until now to properly account for the existance of an insulation layer.
The comparison of the field data and EICM results indicates that the EICM provides acceptable predictions of temperature profiles in pavement systems. However, the studies performed in Ohio and New Jersey revealed that the EICM is not able to properly simulate the fluctuations in water content within depths of the pavement. The need for a trustworthy model for calculating the moisture and temperature change in the pavement is particularly critical, when the freeze/thaw cycle moisture level fluctuation can lead to sever structural performance drop, especially during spring.
