Quantifying Energy Consumption and Carbon Dioxide Equivalent Generation in Typical Roadway Construction Projects
All roadway agencies monitor and maintain their infrastructure as it deteriorates over time. Agencies allocate the money that they have for maintenance, rehabilitation and reconstruction operations across their entire network. Regular and timely maintenance and rehabilitation treatments can postpone the need for reconstruction on a roadway. The need for infrastructure sustainability has been brought to the forefront of society and has become an important part of any public agency’s decision making processes. To achieve sustainable roadways social, economic and environmental benefits must be achieved while maintaining technically sound solutions. By considering the amount of energy that is consumed and the amount of greenhouse gas (GHG) emissions generated through various roadway treatments, sustainability can be brought into the decision making process. The objective of this research was to develop a probabilistic model that quantifies the amount of energy that is consumed and carbon dioxide equivalents (CO2e) generated for typical roadway construction, maintenance, rehabilitation and reconstruction projects in Saskatchewan and Alberta. The model constructed within this work was divided into three sub-models: 1) material production, 2) equipment usage and 3) material transport. For every variable that was required to be entered into each sub-model, a low, average or most likely and high value was determined. By using a range of input values the uncertainty of the values entered was incorporated and sensitive parameters were identified. A base case study of a one lane-kilometer (lane-km), 3,700 m2, section of rural roadway was analyzed. For the initial construction of a lane-km of traditional flexible pavement roadway it was determined that 1,870 GJ (giga joules) of energy is required. Based on an annual average amount of energy used per home in Saskatchewan, 126 GJ/year, 1,870 GJ would power approximately 15 homes for one year. Similarly it was determined that 152.4 tonnes (t) CO2e are emitted for the construction of a lane-km of traditional flexible pavement roadway. Based on an average CO2e generation value of 5.1 t per passenger vehicle per year the GHG emissions generated from the construction of a lane-km of roadway is equivalent to the GHG emissions released by approximately 30 passenger vehicles over one year. It was also determined that the volume of CO2e generated for initial construction compared to the volume of material in the roadway was a ratio of 30 to 1. The base case study also reviewed various maintenance, rehabilitation and reconstruction treatments for the amount of energy consumed and GHG emissions generated for one lane-km. From the modeled values it was found that the order of energy consumed and CO2e generated from least to greatest for maintenance treatments is: fog seal, slurry seal, micro surfacing, single, double and triple chip seal and ultra thin overlay. For rehabilitation and reconstruction treatments the order of energy consumed and CO2e generation from least to greatest is: cold in-place recycling, mill and fill, full depth reclamation, remove and replace with recycled materials and remove and replace with virgin materials. Through a sensitivity analysis of the input parameters, it was observed that for maintenance treatments the sensitive parameters were the equipment efficiency (EFE) value, the placement rate of the treatment, the aggregate application rate and the amount of asphalt binder included in the treatment. For rehabilitation and reconstruction treatments, the two most sensitive parameters were the asphalt concrete plant energy and the application rate of the Portland cement. Further investigation into how each sub-model contributed to the overall amount of energy consumed and CO2e generated found the production of materials contributed the greatest to the overall values. When examining the production of each layer in a traditional flexible pavement roadway structure, the asphalt layers contributed the greatest to the energy consumed at 72.1 percent of all materials produced. The asphalt layers also contributed the greatest to the GHG emissions generated from the production of materials at 42.7 percent. Further breaking down the production of the asphalt layers, the energy requirements at the hot mix asphalt concrete plant account for 75.9 percent of the energy consumed and 52.0 percent of the CO2e generated for the production of the materials of the asphalt layers. The cost of each treatment was reviewed based on the cost of diesel at $1.21/litre and the amount of energy consumed. The costs of energy for the maintenance treatments ranged from $174/lane-km for fog seal to $5,488/lane-km of the ultra thin overlay. The cold in-place recycling and mill and fill rehabilitation treatments had energy costs of $13,545 and $21,440/lane-km respectively. The costs of the energy consumed for the reconstruction treatments ranged from $21,710/lane-km for full depth reclamation and $71,164/lane-km for remove and replace with virgin materials. Based on a review of the City of Saskatoon’s 2012 proposed treatment plan for its roadway network the cost of energy was estimated at $1,232,000 for work on 93 lane-km of roadway. The costs of GHG emissions were also determined based on the amount of CO2e generated and the value of one tonne of carbon on the voluntary carbon credit market at $6/tonne. The costs of carbon for the maintenance treatments ranged from $3/lane-km for fog seal to $64/lane-km for the ultra thin overlay. For the rehabilitation treatments the cost of carbon for the cold in-place recycling was $224/lane-km and $266/lane-km for the mill and fill treatment. The reconstruction treatments ranged from $524/lane-km for full depth reclamation and $1,062 for remove and replace with virgin materials. Finally four field case studies were reviewed to determine the amount of energy consumed and GHG emissions generated through construction. The first was the reconstruction of Range Road 232, a rural roadway with virgin materials. The second was the reconstruction of Kenderdine Road with recycled materials. The energy consumed and GHG emissions generated for these construction projects are 1,917 and 1,146 GJ/lane-km, and 150.3 and 92.6 t CO2e/lane-km, respectively. The third case study further reviewed the use of warm mix asphalt concrete (WMAC) and the use of recycled asphalt pavement (RAP) in the Kenderdine Road pavement structure. This research determined that with the incorporation of WMAC and 10 percent RAP in the asphalt layers and with the use of recycled materials in the base layers the amount of energy consumed would be reduced by 31.8 percent and the GHG emissions reduced by 34.8 percent compared to a traditional virgin pavement structure. The final case study reviewed the City of Saskatoon’s 2012 proposed roadway restoration and reconstruction plan. From the model it was found that 38,281 GJ of energy was consumed and 2,617 t CO2e was generated. This work shows that the probabilistic model developed in this research may be applied to a variety of roadway treatments from maintenance to reconstruction in urban and rural applications. With the use of the model, roadway project managers can make informed decisions for roadway treatments based on energy consumption and GHG emission generation values. By incorporating the amount of energy that is consumed and GHG emissions generated into the decision making process of roadway infrastructure management, more sustainable infrastructure management can be achieved.
DegreeMaster of Science (M.Sc.)
DepartmentCivil and Geological Engineering
CommitteePark, Peter; Sparks, Gordon; Putz, Gordon
Copyright DateAugust 2013