Life Cycle Assessment of Asphalt Using Crumb Rubber Pavement
Production of a crumb rubber pavement comprises a series of activities, such as materials production, construction, maintenance etc., all of which have environmental impacts. Life Cycle Assessment (LCA) can track and quantify the environmental impacts of both individual and total activities of products.
Crumb Rubber Pavement Introduction
Production of a crumb rubber pavement comprises a series of activities, including materials production, construction, maintenance, and end-of-life handling, all of which have environmental impacts. Life Cycle Assessment (LCA) is used to track and quantify the environmental impacts of both individual activities and the overall pavement system.
LCA helps identify the activities with the highest environmental burdens within the pavement life cycle and highlights the most environmentally effective strategies among available options. These advantages make LCA a widely used methodology for evaluating asphalt pavement systems.
As the number of vehicles continues to increase, more tires are being discarded. Current solutions for waste tires include energy recovery, chemical recovery, and granulate recovery. Rubberized asphalt pavement is an increasingly popular option because it uses crumb rubber from waste tires in asphalt mixtures. Crumb rubber can be added using either the wet process or the dry process.
Although crumb rubber pavement has received positive feedback in other countries, its ecological benefits and tradeoffs are still being studied in different regions. This article reviews a Swiss case study that uses Life Cycle Assessment to examine the environmental performance of crumb rubber pavement in semi-dense asphalt wearing courses.
This crumb rubber pavement overview includes:
- Asphalt mixture design for wearing courses
- Mechanical performance testing
- Life Cycle Assessment methodology
- Goal and scope definition
- Materials production, mixing, construction, and transport impacts
- Energy demand and global warming potential findings
Asphalt Mixtures
Three types of asphalt mixtures for the wearing courses were prepared in plant. The mix designs were based on Swiss standard SNR 640436 for SDA 4-12, with a maximum particle size of 4 mm and an air void content of 10 to 14 percent.
These mixtures were designed to compare a reference pavement system with crumb rubber pavement alternatives. The purpose was to understand how crumb rubber affects pavement performance and environmental outcomes while maintaining practical plant production methods.
Mechanical Performance of Asphalt Mixtures
To compare the mechanical performance of the investigated asphalt mixtures, the study included water sensitivity testing and wheel tracking testing. The results showed that the indirect tensile strength ratios of all specimens were comparable, ranging from 76 to 81 percent.
The rutting ratio of the reference mix and CR1.0 were both measured at 6.0 percent after 30,000 cycles, while CR0.7 showed a lower rutting ratio of 4.4 percent. Based on the available test data, the initial study assumed that the crumb rubber pavement wearing courses would have the same service life as the reference pavement.
The researchers also noted that additional testing would be needed in future studies to estimate service life more accurately and expand the understanding of long-term performance.
Life Cycle Assessment of Crumb Rubber Pavement
The international standard ISO 14040 divides Life Cycle Assessment into four major steps: goal and scope definition, inventory analysis, impact assessment, and interpretation. For crumb rubber pavement wearing courses, each of these steps helps quantify and compare the environmental impacts of reference and crumb-rubber-modified systems.
In this case study, LCA was used to investigate the environmental strengths and weaknesses of crumb rubber pavement and compare it with a reference pavement using conventional materials.
Goal and Scope Definition of Crumb Rubber Pavement
The purpose of the LCA was to quantify and compare the environmental impacts of reference and crumb rubber pavement wearing courses. The functional unit consisted of a wearing course with dimensions of 1 kilometer in length, 7 meters in width, and 30 centimeters in depth.
The analysis assumed that dimensions, service life, traffic volume, weather, and other pavement layers were identical for all the wearing courses studied. The system boundary included crumb rubber pavement processes as well as rubber and coal incineration.
Maintenance was treated the same as end-of-life for the wearing courses because both processes involved demolition and transportation of the used pavement to a recycling plant. Due to limited reliable emissions data, the use phase was not included in the analysis.
To create a fair comparison between current reference conditions and crumb rubber use scenarios, the study applied system expansion as an allocation approach. Three different scenarios were then evaluated to compare the effects of using crumb rubber pavement versus conventional alternatives.
Materials Production, Mixing, Construction, and Transport
Based on the functional unit, the study evaluated the raw materials needed to prepare each asphalt mixture. Data for base binder and polymer were obtained from industry sources, while aggregates and electricity data came from the ecoinvent database.
According to the industrial partner, grinding one tonne of waste tires produced 0.73 tonnes of crumb rubber and consumed 308 kWh of electricity. For asphalt mixing, the incorporation of crumb rubber pavement by dry process did not increase the energy consumption of mixing.
The production of one tonne of asphalt mixtures consumed 8.6 kWh of electricity and 216.3 MJ of natural gas. Qualitative measurements showed that crumb rubber played only a limited role in increasing total emissions during mixing, so those emissions were assumed to be the same across all asphalt mixtures.
Construction energy requirements were also quantified for each scenario, including energy consumed by pavers, transfer vehicles, rollers, and generators. Maintenance energy for milling and related operations was included, while overlaying a new wearing course was excluded because it was identical across scenarios.
Transportation distances among the rubber manufacturer, mixing plant, and construction site were based on actual locations in Switzerland and abroad. A 25-ton lorry was assumed as the transport mode for all relevant movements.
In this study, dry process crumb rubber pavement did not increase asphalt mixing energy consumption, which is an important finding in the overall environmental analysis.
Impact Assessment
The environmental impacts investigated in the article included cumulative energy demand and global warming potential. The collected data were integrated using Simapro software to quantify these target impacts across all scenarios.
The quantitative results showed that cumulative energy demand values between the reference pavement and crumb rubber pavement wearing courses were quite comparable. This suggests that the use of crumb rubber in wearing courses and cement kilns leads to relatively small differences in total energy consumption.
However, the greenhouse gas emissions from crumb rubber pavement wearing courses were lower than those of the reference pavement.
Environmental Results
The largest difference in greenhouse gas emissions between the reference pavement and the crumb rubber pavement systems occurred during binder preparation. The reference wearing courses used polymer modified binder, and the production of polymer released a large amount of greenhouse gases.
By comparison, the crumb rubber pavement wearing courses used base binder, which eliminated the greenhouse gas emissions associated with polymer production. This was the main reason the crumb-rubber-modified wearing courses showed lower climate change impacts.
From an environmental point of view, the results suggest strong potential for crumb rubber pavement as an alternative performance-enhancing material for wearing courses and as an alternative method of managing waste tires.
The study found that crumb rubber pavement had comparable energy demand to the reference pavement, but lower greenhouse gas emissions.
Conclusion
This study used Life Cycle Assessment to compare the environmental impacts of reference and crumb rubber pavement wearing courses in Swiss SDA pavements. Asphalt mixtures were prepared in plant, and mechanical tests were conducted to evaluate performance.
The results showed that cumulative energy demand was comparable between the reference and crumb rubber pavement wearing courses, while the climate change impacts of crumb-rubber-modified wearing courses were lower. The main reason was the greenhouse gas emissions caused by polymer production in the reference pavement system.
Overall, the results present a strong case for crumb rubber pavement as an environmentally promising alternative for improving wearing course performance while also providing a productive use for waste tires.