Abstract
Including insulation layers in pavement structures has become a common strategy to minimize frost penetration in cold regions. This study investigated the performance of two different insulation materials, extruded polystyrene board and bottom ash, in a test road in Edmonton, Alberta, Canada, eight years after construction. The two insulation materials were used in a fully instrumented test road, including three insulated sections 20 m in length. The insulated sections are as follows: the first section has 1 m of bottom ash (B. Ash), the second section has a 10 cm polystyrene layer (Poly-10), and the third section has a 5 cm polystyrene layer (Poly-5). Both B. Ash and polystyrene layers were placed on top of the subgrade layer, at a depth of 70 cm from the surface. A conventional section next to these three sections was used as the control section. Volumetric water content data and temperature variation were used to analyze the influence of the insulation materials on the subgrade. It was concluded that both B. Ash and Poly-10 layers protected the subgrade from freezing. The Poly-10 section showed the lowest rate of change in subgrade temperature during the monitoring period. B. Ash and Poly-10 reduced the frost depth by 23% and 70% compared with the control section, respectively. It was concluded that Poly-10 protected the subgrade soil from freezing and excessive moisture more effectively than B. Ash; however, the temperature in the layer above the insulation layers (pavement base layer) was significantly lower during winter for the Poly-10 section.
In cold regions, pavements are subject to harsh conditions, including long periods of extremely cold temperatures, as well as freeze–thaw cycles during spring. In the winter, as both the air and pavement temperatures drop, ice lenses form in the base and subgrade, which can result in differential frost heave on the pavement surface. In particular, a sudden change in moisture occurs when the temperature drops below 0°C ( 1 ), the freezing point of water. As the temperature increases in the late winter, ice lenses in the pavement begin to melt. While the sublayer is still frozen, its permeability remains low, and excess water accumulates, which generally leads to a higher moisture content in the sublayer during the thawing period. Since both unbound pavement materials and thin pavement structures are sensitive to moisture variation, the higher moisture content in the thawing period can potentially lead to lower load capacity ( 2 – 4 ). Thus, pavements in cold regions typically exhibit premature damage, and have shorter service lives and higher maintenance costs, among other issues ( 5 ).
The use of insulation layers in pavement structures has been a standard method to protect pavement structures from extremely cold conditions. Polystyrene has been used as an insulation layer to protect pavement structures in many cold regions since 1967 ( 6 ). Previous studies have shown that a 5 cm layer of polystyrene can significantly reduce both thaw depth and frost heave. Moreover, case studies on pavement settlement also showed that, for pavement sections with polystyrene insulation layers, settlement was less than for pavement sections with a standard design ( 7 – 9 ). Currently, a more environmentally friendly insulation material, bottom ash, is also being used in pavement construction. Bottom ash is a byproduct of electricity generation by combustion of coal, and is mainly composed of silica, alumina, and iron. However, research on the use of bottom ash as an insulation layer to protect the subgrade from frost heave in cold regions only began around the year 2000 ( 10 ).
Researchers at the Integrated Road Research Facility (IRRF) designed and constructed a test road located in Edmonton, Alberta, Canada in 2012, with a section of the road used to investigate the long-term performance of bottom ash as an insulation layer. Studies have been conducted on the thermal performance of the bottom ash section over a one-year period, the effect of different insulation materials on the load-bearing capacity of the pavement, pavement strength in different seasons, and evaluation of damage to pavement with different insulation layers. The results of this series of studies ( 11 ) indicate that insulation layers can block heat exchange between the base layer and subgrade layer and reduce the frost depth compared with the control section. However, it was found not only that the bottom ash protected the subgrade from frost heave in winter, but also that the section insulated using bottom ash had a higher load-bearing capacity in non-freeze–thaw conditions than the section insulated with a polystyrene layer ( 12 , 13 ). The pavement section insulated with bottom ash also had a lower risk of fatigue cracking than the control section ( 14 ).
Polystyrene board is a traditional insulation material, and much literature has already illustrated its use for various applications. With economic development and increased environmental awareness, however, more sustainable and environmentally friendly practices and materials are used in road construction. Bottom ash is a recycled material, so it may have less environmental impact than polystyrene. Although researchers have already investigated how bottom ash influences temperature distribution in the pavement structure ( 11 , 12 ), only one or two years’ worth of data (immediately after completion of the IRRF test road) was available, with no traffic loading data. The IRRF test road has now been open to public traffic for over eight years, with detailed information about traffic patterns since it was opened. In previous studies on the insulated test sections, moisture content variation was not studied, although it has a significant influence on pavement load capacity. The primary purpose of this study is to investigate the long-term thermal performance of each of the test road sections (containing either bottom ash or polystyrene as an insulation layer) under field conditions and thus determine how the insulation material influences the moisture content in the subgrade.
Studies by other researchers have indicated that slippery conditions formed on a surface with an insulation layer, while an adjacent conventional surface remained dry ( 15 , 16 ). A test road in Québec City was used to measure the rate of surface cooling of the pavement. In this case, the pavement had one layer of a granular material as a base course on top of a clayed silt subgrade soil. For the uninsulated section, a 7 cm layer of hot mix asphalt (HMA) was placed on a 48.5 cm layer of granular material (base course). In the insulated section, the thickness of the HMA layer and the base course were the same as for the uninsulated section, but a 5 cm polystyrene layer was placed at a depth of 21 cm. Compared with the adjacent uninsulated pavement, the surface cooling rate of the pavement with a 5 cm polystyrene layer was drastically increased when a freezing front reached and stayed in the polystyrene layer. The surface cooling rate was 0.20 to 0.37°C/h for the uninsulated pavement, whereas the surface cooling rate reached 0.46 to 0.59°C/h in the insulated pavement ( 17 ). In the case of the IRRF test road, no sensors were installed in the HMA layer of the polystyrene section and the bottom ash section, so similar data on cooling rates of insulated and non-insulated test sections are not available.
Background
The IRRF test road is an access road to the Edmonton Waste Management Center (EWMC) and is located 15 km from downtown Edmonton. The test road consists of two lanes and is about 500 m long, with a pavement width of 14.1 m and a pavement slope of 4:1. Construction of the test road took place in two stages (as shown in Figure 1). The first stage took place from May 2012 to August 2012, with a 16 cm layer of HMA completed. The second stage took place in October 2013, with the existing HMA layer topped with another 9 cm of HMA. The test road has been open to public traffic since 2015 (mid-October), with garbage trucks accounting for 10% to 20% of all traffic. Weigh-in motion (WIM) systems installed on the test road have been collecting data since June 2015. Based on WIM data, the average daily traffic was 745 in 2015, 2,312 in 2016, and 2,548 in 2019. Average daily traffic increased by 10% from 2016 to 2019.
Cross-section of pavement layers and as-built depth of thermistors and moisture probes.
As shown in Figure 1, there are three insulation sections and one control section in the test road. Each of the sections is 20 m long, and all sections are composed of 25 cm of HMA on top of a 45 cm granular base course (GBC) layer. The required pavement thickness was calculated based on traffic loading. Bottom ash was used as an insulation layer in one section of the test road. Closed-cell Styrofoam Highload 100 extruded polystyrene boards, provided by Dow Chemical Company, were used in the polystyrene test sections. Insulation layers were intended to protect the subgrade layer from freezing; thus, the insulation layer was placed under the base layer and on top of the subgrade layer (at a depth of 70 cm from the pavement surface).
Figure 1 shows the first section, from Station 130 + 240 to 130 + 260, which is the control section. The control section, which has no insulation layer, is used for comparing the variation in temperature and moisture content. In the three insulated sections, a 100 cm bottom ash layer (B. Ash), a 10 cm polystyrene layer (Poly-10), and a 5 cm polystyrene layer (Poly-5) are located between the GBC and subgrade layers. The purpose of the section containing the 5 cm polystyrene layer is to avoid a sudden temperature change between the control section and the Poly-10 section and thus this section is not instrumented (i.e., no data is collected).
Based on ASTM C 136-06 and the Unified Soil Classification System, the GBC layer is well-graded gravel, and the subgrade soil is clayey sand. The subgrade is moderately frost-susceptible. Approximately 27% and 21% of subgrade soil (by weight) passed through a 0.075 mm sieve and a 0.02 mm sieve, respectively. The liquid limit of the subgrade soil is 25%, and the plastic index is 9%. The bottom ash used for the insulation layer had a maximum particle size of about 5 mm, and did not contain large lumps and impurities. According to the weight of the material, the amount of unburned coal particles in the bottom ash was less than 5%.
The thermal resistivity, R, of the insulation materials is defined as in Equation 1:
where D is the layer thickness, and k is the thermal conductivity. R is generally used to compare the resistance of a material with conducting heat (i.e., a material with higher R value shows that a material is more effective as insulation). For bottom ash, R is equal to 1.43 m2·°C/W , while for the 10 cm polystyrene board, R is 16.67 m2·°C/W ( 18 ), as in Table 1.
Properties of Materials Used in Integrated Road Research Facility Test Road Construction
Note: Cp = specific heat at constant pressure; k = thermal conductivity; na = not applicable.
During construction of the IRRF test road, 109AM-L thermistors and CS650 time domain reflectometers (TDRs) were installed to monitor variation in temperature and moisture content. The thermistors only capture the temperature, whereas the TDRs record both temperature and volumetric water content (VWC) (i.e., water that is not frozen). As shown in Figure 1, one thermistor was placed at a depth of 0.18 m in the middle of the HMA layer in the control section. Two TDRs were installed in the GBC layer, and three TDRs were installed in the subgrade layer at a depth of 2.65 m from the top of the subgrade. In the B. Ash section, there are five thermistors: one in the GBC layer, one in the bottom ash layer, and three in the subgrade layer. There is a TDR installed in the B. Ash section at a depth of 1.85 m from the pavement surface. There are also five thermistors in the Poly-10 section: one at the bottom of the GBC layer, and four others in the subgrade layer, along with two TDRs. One TDR is located at a depth of 0.75 m from the pavement surface and just below the 10 cm polystyrene, and the other one was installed 1 m below this. All VWC data collected were adjusted using a formula determined from calibration of the TDRs in the laboratory. The temperature and VWC data collected by the thermistors and TDRs were stored using a CR1000 datalogger, which was programmed to collect data from each of the sensors at 15 min intervals. Data is retrieved from the IRRF test road sensors by remote access from an on-site computer, with an off-site computer at the University of Alberta used for analysis of the data.
Precipitation is one of the most critical factors influencing moisture content variation in warm wet regions, with its effect decreasing with increasing depth ( 20 ). However, in cold dry regions (such as Edmonton), the moisture content variation is dominated by temperature instead of precipitation. For example, in Figure 2, the moisture content variation (as measured using a TDR-CS-1 at a depth of 0.48 m, in the GBC layer) is shown for 2015, as well as precipitation. The observed trend is that moisture content dropped in fall, remained at a low level through winter, and increased in spring. Through summer, the moisture content was high.
Moisture content variation at a depth of 0.48 m (in granular base course layer) and precipitation over time (2015 data).
Data Collection and Analysis
Control Section
Ambient temperature data were collected from the Oliver AGDM weather station, which is the weather station that is closest to the Edmonton Waste Management Centre (there is a distance of about 6 km between the IRRF test road and the weather station, as determined using Google Maps). From 2012 to 2020, the average air freezing index was 1177°centigrade (C)·day, and the air average thawing index was 662°C·day, based on air temperature recordings collected at Oliver AGDM over this time period. The daily ambient temperatures over 42 months, along with the temperatures measured in the base and subgrade layers, are shown in Figure 2. From October 2016 to April 2020, the maximum average daily ambient temperature (24 h average) was 23°C and the minimum average daily ambient temperature (24 h average) was −38°C (January 14, 2020).
Figure 3 illustrates the temperatures of the base and subgrade layers of the control section from October 2016 to April 2020. As a result of technical issues, temperature data from February 2018 to June 2018 and from September 2018 to March 2020 are missing for all sections. The temperature data for the base and subgrade layers were collected using a TDR-CS-2 and a TDR-CS-4. The as-built depths for the TDR-CS-2 and TDR-CS-4 sensors are 0.67 m and 1.79 m from the pavement surface, respectively. Temperature data, which were collected from the sensors every 15 min, were used to calculate the average daily temperature. The temperature in the base layer was observed to change synchronously with ambient temperature, but the change in subgrade temperature lagged behind. The lowest temperature recorded in the base layer was −14°C (January 18, 2020). The temperature of base layer was below 0°C from November to April, however, the subgrade temperature remained between −1°C and 0°C from January to April.
Daily ambient temperature and average daily temperature from sensors installed in the base layer and the subgrade layer (control section).
Figure 4 indicates the variation in temperature with depth from February 2017 to January 2018 for the control section. As seen in Figure 4, the range in temperature values decreases with increasing depth. The temperature in the HMA layer (at a depth of 0.18 m) varies from −23°C to 32°C, whereas the temperature variation at a depth of 2.70 m is between 1°C and 16°C. The base layer stays frozen from November to March, while the subgrade layer is frozen from January to March. As seen in Figure 4, freezing can be observed in the subgrade in colder months (as early as October and as late as April). Although the range of temperature variation decreases with increasing depth, temperatures below 0°C are observed in the subgrade until a depth of 2.1 m is reached.
Temperature variation with depth in the control section of the Integrated Road Research Facility test road.
VWC in the various layers was determined using TDRs (excluding frozen water) and subgrade temperature. Once the temperature drops below 0°C, the form of water changes from liquid to solid, so when the subgrade temperature goes below 0°C in the winter, VWC decreases ( 1 ). Later, when the subgrade temperature increases above 0°C, VWC spikes, and, because of inadequate drainage of the lower frozen soil in spring, the moisture content reaches the highest value of the year. As the accumulated water slowly drains and evaporates, VWC falls to a more stable value.
Figure 5a shows the daily pavement temperature and VWC for the control section at a depth of 0.80 m (the as-built depth of the sensor TDR-CS-3 is 0.83 m). The subgrade temperature varied from −10°C to 28°C, and VWC ranged from 0.03 m3 H2O/m3 to 0.39 m3 H2O/m3. At the top of the subgrade layer, VWC started to drop in December, reached its minimum in about February and increased to its highest value in March.
Volumetric water content (VWC) and temperature in the control section at a depth of (a) 0.8 m and (b) 1.8 m.
Figure 5b shows the daily pavement temperature as well as VWC for the control section recorded by TDR-CS-4, which is installed 1 m below TDR-CS-3 in the subgrade layer (the as-built depth for TDR-CS-4 is 1.82 m). From 2014 to 2015, the temperature of the pavement varied between −2°C and 22°C and VWC ranged from 0.06 m3 H2O/m3 to 0.36 m3 H2O/m3. At a depth of 1.8 m from the pavement surface in the control section, VWC was observed to decrease in February, reach its minimum in February 2014 and March 2015, and reach its maximum in May 2014 and 2015.
Bottom Ash Section
Figure 6 shows the average daily temperature for the base and subgrade layers in the section of the IRRF test road insulated using bottom ash (for October 2016 to April 2020). The recordings were recorded by the sensors TH-B. Ash-1 and TH-B. Ash-3 (see Figure 1), with as-built depths of 0.72 m and 1.80 m. The maximum base temperature observed was 28°C in July, and the minimum base temperature was −17°C on January 17, 2020. Over the same time period, the temperature in the subgrade layer varied from 2°C to 15°C, never dropping below 0°C.
Daily ambient temperature and average temperature recorded at sensors installed in base layer (depth 0.72 m) and subgrade layer (depth 1.80 m) in Integrated Road Research Facility test road section insulated with bottom ash.
Figure 7 shows the temperature distribution in the test section insulated using 100 cm of bottom ash. Although the range of the temperature change also decreases with increasing depth, the subgrade temperature below the insulating bottom ash layer stays in a more stable range, because of the presence of the bottom ash layer. The temperature at the bottom of the GBC layer (at a depth of 0.60 m) varies from −10°C to 27°C (a range of 37°C), the temperature at a depth of 1.80 m varies from 2°C to 15°C (a range of 13°C), and the temperature at a depth of 3.2 m varies from 3°C to 13°C (a range of 10°C). The GBC layer remains frozen from November to March. At the same time, in the B. Ash section, the maximum frost depth was 1.60 m and the temperature of the subgrade remained above 0°C all year.
Temperature variation along depth across pavement structure in the bottom ash section.
Figure 8 presents VWC and daily average temperature recorded in the section of the test road insulated with bottom ash. The data was collected using the sensor TDR-B. Ash-1 (see Figure 1) which is located at a depth of 1.85 m. VWC varied from 0.21 m3 H2O/m3 to 0.31 m3 H2O/m3. The temperature of the subgrade below the bottom ash layer never dropped below 0°C, so water in this layer did not freeze. This may be the reason that VWC did not show a drop as observed for VWC in the control section at the same depth. However, VWC showed a small decline from January 2013 to February 2013 and December 2013 to May 2014. In Hoekstra’s laboratory measurements of moisture content in unconsolidated undrained soil ( 21 ), it was shown that moisture content migrates from thawed to frozen areas of soil when there is a moisture content gradient. According to Hazen’s equation, the coefficient of permeability can be expressed as in Equation 2:
where k is the coefficient of permeability (cm/s), C is a constant ranging from 0.4 to 1.2 (typically assumed to be 1.0), and D10 is the grain size corresponding to 10% by weight passing, also referred to as the effective size (mm) ( 22 ). In this work, C is assumed to be 1.0. The grain size corresponding to 10% by weight passing is 0.18 mm for the subgrade soil and 3.79 mm for bottom ash. Therefore, for the subgrade soil, k is 0.03 cm/s, and for bottom ash, 14.36 cm/s. Thus, the coefficient of permeability of the subgrade soil is much lower than for bottom ash, and it is possible for water to migrate from the subgrade to the bottom ash layer. In winter, the decline in moisture content observed at a depth of 1.8 m indicates that water migrated upwards to the bottom ash in the winter as a result of water suction.
Volumetric water content (VWC) and temperature in the bottom ash test section measured at a depth of 1.8 m.
Polystyrene Section
Figure 9 shows the base temperature and subgrade temperature in the IRRF test section insulated with 10 cm of polystyrene board from October 2016 to April 2020. The as-built depths of the sensors, as shown in Figure 1, were 0.64 m (GBC layer), 0.75 m (subgrade), and 1.75 m (subgrade). The temperatures observed at these three depths varied from −26°C to 31°C, 1°C to 13°C, and 3°C to 11°C, respectively. Below the Poly-10 layer, the subgrade temperature remained above freezing. The lowest base temperature, −26°C, was measured on January 16, 2020, and the highest temperature values were measured in July, August, and September for the base layer, the top of the subgrade, and 1 m below the top of the subgrade, respectively. The hysteresis between ambient temperature and the temperature within the pavement structure increases with depth.
Variation in daily ambient temperature and average temperature over time at various depths in the 10 cm polystyrene test section.
Figure 10 presents the variation in pavement temperature with depth in the test section containing 10 cm polystyrene from February 2017 to January 2018. The temperature was recorded by five thermistors (as in Figure 1): sensor TH-Poly-1 is located in the GBC layer, with an as-built depth of 0.64 m, and sensor TH-Poly-2 is just below the polystyrene layer, with an as-built depth of 0.75 m. The temperature measured within the GBC layer indicates that it is frozen from November to March. The temperature measured by the thermistor TH-Poly-1 (GBC layer) ranges from −14°C to 26°C, a difference of 40°C. In contrast, the temperature measured just below the polystyrene layer (by TH-Poly-2) ranges from 1°C to 12°C, a difference of 11°C. The 10 cm polystyrene layer blocks heat exchange between the GBC layer and the subgrade layer, which leads the temperature to decrease rapidly over a short distance (0.11 m). The yearly variation in temperature measured at depths from 2.25 m to 3.25 m in the Poly-10 test section is less than 5°C, as shown in Figure 10.
Temperature variation along depth across pavement structure in the 10 cm polystyrene section.
Figure 11 shows the temperature and VWC measured just below the polystyrene layer (using TDR-Poly-1, which has an as-built depth of 0.75 m; see Figure 1) from January 2013 to January 2017. The same data is shown for a depth 1 m below (measured using TDR-Poly-2, located at an as-built depth of 1.72 m) from January 2013 to January 2015.
Change of volumetric water content (VWC) and temperature in the 10 cm polystyrene section at depths of (a) 0.8 m and (b) 1.8 m.
As discussed previously, because of the phase change of water from liquid to solid at 0°C, the VWC of the subgrade layer will drop in winter. Figure 5a shows the change in VWC in the control section at the same depth. Unlike the control section, however, the subgrade in the test section is insulated by the 10 cm polystyrene layer. For the Poly-10 section, the subgrade temperature remains above freezing all year, and there is no significant drop in VWC observed during the winter. Also, the temperature of the GBC (above the polystyrene layer) changed from –14°C to 26°C, so the GBC layer in the Poly-10 section will have frost in winter. Unlike VWC measured at a depth of 1.80 m in the B. Ash section, VWC measured at a depth of 0.8 m in the Poly-10 section only varies between 0.24 m3 H2O/m3 and 0.27 m3 H2O/m3, a difference of 0.03 m3 water/m3. This means that no water suction occurs, and moisture does not migrate from the subgrade (where there is no freezing) to the frozen GBC layer. The deepest sensor in the Poly-10 section (TDR-Poly-2) is at 1.72 m, and VWC varies from 0.27 m3 H2O/m3 to 0.29 m3 H2O/m3 at a depth of 1.80 m, a difference of only 0.02 m3 H2O/m3. Thus, the VWC of the subgrade does not change as much in the test section insulated with 10 cm polystyrene.
Discussion
Insulation materials block heat exchange between the base layer and the subgrade layer of the pavement structure, resulting in different base temperatures. The thermal characteristics of the three test sections are presented in Table 2. From February 2017 to January 2018, the maximum base temperature in the control section and the B. Ash section was 28°C, whereas the maximum base temperature in the Poly-10 section was 31°C, 3°C higher than the control section. From April 2019 to April 2020, the maximum base temperature in the control section was 26°C, 3°C higher than the B. Ash section and 2°C lower than the Poly-10 section. In the summer, the maximum base temperature difference observed between the control and B. Ash sections was 3°C. However, in the winter, the maximum base temperature difference observed between the control and Poly-10 sections was 12°C. Furthermore, in winter, the maximum base temperature difference between the control section and the B. Ash section was 3°C. The pavement structure containing the 10 cm polystyrene layer results in a much lower base temperature in winter, which negatively influences the pavement performance. Lower pavement temperatures in winter make the pavement more prone to low-temperature cracking. This is in contrast to the B. Ash section, which performs closer to the control section; thus, the bottom ash layer prevents low-temperature pavement damage more effectively.
Thermal Characteristics of Integrated Road Research Facility Test Road
Note: CS = control section; B. Ash = bottom ash section; Poly-10 = 10 cm polystyrene section; na = not applicable.
R was used to justify how insulation materials influence the temperature trend. R for the bottom ash layer is 1.43 m2·°C/W, while R for the polystyrene layer is 16.67 m2·°C/W, ten times that of the bottom ash layer. The polystyrene layer, with a higher R, blocks heat exchange and results in a much lower base temperature in winter. However, when used in road construction, the polystyrene layer is 10 cm thick, while the bottom ash layer is 100 cm thick. Thus, the temperature in the section insulated with a 10 cm polystyrene layer changed more rapidly.
Because of the presence of insulation materials in the test sections, the sensors in the subgrade (depth 1.80 m) showed less temperature variation compared with the control section. During the monitoring period, the maximum difference observed in subgrade temperature was 20°C, 13°C, and 7°C for the control section, B. Ash section and Poly-10 section, respectively. In the same period, the bottom ash and 10 cm polystyrene layers had a lower difference in subgrade temperature, and in both cases the temperature of the subgrade remained above 0°C all year round.
To compare the bottom ash layer and 10 cm polystyrene layer as insulation materials, the temperature gradient and temperature change rate were calculated, with the results summarized in Tables 2 and 3. The temperature gradient was calculated using Equation 3,
Comparison of Rates of Temperature Change of Test Sections with Control Section for Different Layers
Note: B. Ash = bottom ash section; Poly-10 = 10 cm polystyrene section; na: not applicable.
where ΔT1 is the difference in temperature (as measured by the shallowest and deepest sensors in the subgrade), and ΔD is the distance between the sensors.
The rate of temperature change is calculated as in Equation 4,
where ΔT2 is the temperature difference between the maximum and minimum temperatures over the time period, and Δt1 is the change in time. A lower value of temperature gradient or rate of temperature change in the subgrade layer demonstrates that the material works more effectively as an insulation layer. The comparison ratio—that is, the ratio of difference in rate of temperature change between the test section and the control section to the rate of temperature change for the control section—is defined as in Equation 5.
Since there are no data available from sensor TDR-CS-5 between April 2019 and April 2020, as a result of technical difficulties, the temperature gradient for the control section and rate of temperature change at a depth of 2.65 m for the control section are not available for that period. The temperature gradient for the control section is −5.7 to 6.1°C/m, while the same value ranges from −2.6 to 1.41°C/m and −1.7 to 2.1°C/m for the B. Ash and Poly-10 sections, respectively. For the next year when data are available (April 2019 to April 2020), the Poly-10 section has a lower temperature gradient than the B. Ash section.
The annual average rate of temperature change in the base layer was 0.23°C/day, 0.24°C/day, and 0.32°C/day, for the control section, B. Ash section, and Poly-10 section, respectively, from February 2017 to January 2018. The results for April 2019 to April 2020 did not change significantly, so the test section insulated with bottom ash performed similarly to the control section and better than the test section with the 10 cm layer of polystyrene. In the subgrade layer, the rate of temperature change in the control section was 0.12°C/day from February 2017 to January 2018 and 0.10°C/day from April 2019 to April 2020. For the test section containing bottom ash, similar rates of temperature change were observed in the subgrade and the control sections. However, the rate of temperature change calculated for the polystyrene section was 0.03°C/day, 73% less than for the control section. Thus, the test section insulated with a 10 cm polystyrene layer resulted in relatively more stable temperatures within the subgrade layer.
Considering the data recorded at the deepest sensor, although the rate of temperature change rate in the control section declined to 0.07°C/day, the rate of temperature change was still the lowest in the Poly-10 test section. The rate of temperature change at the deepest sensor was similar for the B. Ash and Poly-10 sections. Therefore, the results for the test section with the 10 cm polystyrene layer had both the highest rate of temperature change in the base layer and lowest rate of subgrade temperature change. This indicates that the use of a 10 cm polystyrene layer as insulation within the road structure is beneficial for the subgrade but worse for the top layer. In contrast, the test section insulated with bottom ash not only resulted in protection of the subgrade layer, but also did not lead to lower temperatures in the top layers during winter. Overall, the bottom ash layer performed better as an insulation material than the 10 cm polystyrene layer.
Maximum frost depths were obtained based on temperature data collected from February 2017 to January 2018, as shown in Figure 12. The maximum frost depths were found to be 2.10 m, 1.60 m, and 0.71 m for the control section, B. Ash section, and Poly-10 section, respectively. Thus, both bottom ash and polystyrene layers decreased the maximum frost depth and protected the subgrade layer below this depth from freezing. The bottom ash layer reduced the frost depth by 23%, and the polystyrene layer decreased the frost depth by 70%, indicating that the polystyrene layer performed best at protecting the subgrade.
Maximum frost depths for test sections from February 2017 to January 2018.
The moisture content and moisture content change ratios at a depth of 1.80 m in the subgrade layer for the control, B. Ash, and Poly-10 sections are shown in Figure 13 and Table 4. Based on the temperature and VWC in the subgrade, the year can be divided into times in which the ground is frozen (frozen period), thawing (thawing period), and with no freezing or thawing (non-freeze–thaw period) ( 12 ). Late summer, when the ground is drained and recovered, belongs to the non-freeze–thaw period. It is clear that only the control section freezes at a depth of 1.80 m from the pavement surface in the subgrade during winter, and both solid and liquid water was present in the subgrade layer.
Temperature and volumetric water content (VWC) at a depth of 1.8 m (subgrade) for control section, bottom ash test section, and polystyrene test section.
Moisture Content and Change Ratio
Note: CS = control section; B. Ash = bottom ash section; Poly-10 = 10 cm polystyrene section.
During the two monitoring periods, VWC is the most stable in the Poly-10 section, with the lowest change ratio, between 0% and 7%, while VWC in the B. Ash section shows the most significant change in moisture content, with a change ratio of 48% and 50%. The maximum values of VWC observed for the control section were 0.36 m3 H2O/m3 in 2013 and 0.33 m3 H2O/m3 in 2014, which was higher than the maximum VWC observed in the B. Ash and Poly-10 sections in May. Salem ( 23 ) and Bayat ( 24 ) have reported that there is an inverse correlation between resilient modulus and soil moisture content. In Alberta, the lowest subgrade resilient modulus typically is observed in April and May ( 25 ). Thus, the higher subgrade VWC in May leads to a lower load capacity for the control section. The bottom ash and polystyrene layers did protect the subgrade and avoid the lower load capacities in May. VWC was more stable and had a lower value in the Poly-10 section, so, with respect to load capacity, the polystyrene layer functioned better as an insulation layer than the bottom ash layer.
Although extruded polystyrene board works effectively as an insulation material and is time-saving for construction, previous studies have shown that it is not a cost-effective solution as an insulation material ( 26 – 28 ). However, as a waste material, bottom ash may be an economical alternative as an insulation material for road construction, particularly in regions such as Alberta, where a large amount of bottom ash is produced as a byproduct of power generation. A previous study compared cost effectiveness and sustainability for a bottom ash layer and a 5 cm polystyrene layer in the IRRF test road. It was concluded that for frost depths between 1.7 and 3.5 m in the control section, using bottom ash would be more cost effective than polystyrene. Since the maximum frost depth in the IRRF test road was determined to be 2.10 m in that study, bottom ash was considered to be the more cost-effective material ( 11 , 29 ).
When choosing material for an insulation layer within pavement, there are also other factors to be considered, such as pavement load-bearing capacity. Various reports in the literature ( 13 , 28 , 29 ) discuss how bottom ash and polystyrene board influence structural capacity and pavement life. The use of 10 cm polystyrene board as an insulation layer in the IRRF test section could potentially result in a loss of 34% of the load-bearing capacity of the pavement structure compared with the control section, whereas the 100 cm bottom ash layer does not have a negative impact on the load-bearing capacity ( 28 ).
Since the IRRF test road was opened to traffic in 2015 (over five years ago), no visible pavement deflection has been observed for either the control section or the insulation sections. Long-term monitoring of the IRRF continues, including visual inspections. While surface frost heaving and differential icing are other important considerations in the appropriate selection of insulation material, this would require regular on-site observations during winter and is beyond the scope of the project.
Conclusions
The ambient temperature collected from the Oliver AGDM weather station and temperature data recorded using sensors embedded in the IRRF test road in Edmonton, Alberta, from October 2016 to January 2018, July to September 2018, and April 2019 to April 2020, were used to investigate the long-term performance of bottom ash (100 cm) and extruded polystyrene board (10 cm) as insulation layers for HMA pavement. The bottom ash and polystyrene insulation layers are 70 cm from the pavement surface (on top of the subgrade). VWC data from sensors installed in the test road sections was used to determine the impact of the bottom ash layer and 10 cm polystyrene layer on the moisture content in the subgrade. The following conclusions were made, based on a detailed analysis of temperature and VWC data for the different IRRF test sections.
Both insulation layers (10 cm extruded polystyrene board and 100 cm bottom ash) block heat exchange between the base layer and the subgrade layer. The Poly-10 test section, with a higher R (16.7 m2·°C/W), resulted in a larger temperature variation in the base layer than the B. Ash section, which has an R of 1.4 m2·°C/W. Furthermore, the difference in base temperature between the control section and the Poly-10 section in winter is 12°C, which is four times the temperature difference of 3°C observed in summer.
During the two monitoring periods, from October 2016 to January 2018 and April 2019 to April 2020, the rate of base temperature change is highest in the Poly-10 section. The base temperature change rates for the control section and the B. Ash section are the same, 0.23°C/day, while the temperature change rate of the base for the Poly-10 section is 0.31°C/day, which is much higher than the rate observed for the control section. This indicates that the pavement insulated with the 10 cm polystyrene layer is prone to low-temperature cracking in winter.
Although the rate of change of temperature for the control and B. Ash sections is similar for the subgrade, the temperature change ratio at the deepest monitoring location of the Poly-10 section and the B. Ash section are lower than for the control section. Thus, both the polystyrene and the bottom ash did protect the subgrade layer from freezing when used as insulation layers. However, the rate of temperature change for the test section insulated with a 10 cm polystyrene layer is lower than the section with bottom ash, so the subgrade temperature in the Poly-10 section is the most stable among these three sections.
The maximum frost depths observed are 2.10 m, 1.60 m, and 0.75 m for the control, B. Ash, and Poly-10 sections, respectively. Compared with the control section, the test sections insulated with bottom ash and polystyrene showed a reduction in frost depth of 23% and 70%, respectively.
Pavement temperature data indicated that subgrade temperature of the B. Ash section never dropped below 0°C and VWC data showed that the bottom ash layer protected the subgrade below it from freezing, however, the bottom ash layer itself was frozen. Although the subgrade under the bottom ash layer did not freeze, the effect of water suction from the frozen bottom ash layer caused VWC in the subgrade layer to decrease in winter. At the same time, the VWC of the subgrade in the Poly-10 section did not show any variation at a depth of 0.80 m.
The VWC change ratio (in the subgrade) for the Poly-10 section was found to be 0% and 7% for 2013 and 2014, respectively, while the B. Ash section has a VWC change ratio of 48% and 50% for 2013 and 2014. This indicates that the value of VWC is the most stable in the Poly-10 section. In spring, the control section, which has the highest VWC in the subgrade, will have a lower load capacity compared with the insulated sections, since there is an inverse correlation between resilient modulus and soil moisture content.
Based on the observations listed above, it can be concluded that both bottom ash and polystyrene protect the subgrade below from frost when used as insulation layers. In both cases, no frozen water was observed in the subgrade. However, the two insulation materials (bottom ash and polystyrene) show some differences. The temperature gradient and rate of temperature change for the B. Ash section is close to that of the control section, so the IRRF test section with bottom ash performed similarly to the control section. Although the test section insulated with a 10 cm polystyrene layer decreased the frost depth by 70% and resulted in the lowest variation in moisture content in the subgrade layer (depth 1.80 m), it also had a lower base temperature in winter, indicating the pavement would be more prone to low-temperature cracking. Thus, while the 10 cm polystyrene insulation layer protected the subgrade from freezing, it made the top layers more prone to low-temperature cracking in winter, compared with the control section and B. Ash section.
Footnotes
Acknowledgements
The authors would like to thank Alberta Transportation, the Edmonton Waste Management Centre, the City of Edmonton and Alberta Recycling for their financial and in-kind support of the IRRF test road. The authors would also like to thank to Lana Gutwin for her assistance in the preparation of this paper. Funding from NSERC is gratefully acknowledged.
Author Contributions
The authors confirm contribution to the paper as follows: study conception and design: A. Bayat, L. Hashemian; data collection: Y. Huang, M. Nojumi; analysis and interpretation of results: L. Hashemian, M. Nojumi, Y. Huang; draft manuscript preparation: L. Hashemian, Y. Huang. All authors reviewed the results and approved the final version of the manuscript.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received financial support for the research, authorship, and/or publication of this article from Alberta Transportation, the Edmonton Waste Management Centre, the City of Edmonton and Alberta Recycling and Natural Sciences and Engineering Research Council of Canada (NSERC).