Over the past few decades, there has been an increasing level of awareness toward using recycled materials as an alternative to virgin materials in flexible pavements and, thus, recycling has become an integral part of road maintenance and rehabilitation activities.
Among the most commonly used recycling techniques are hot in-place recycling (HIR), cold in-place recycling (CIR), and full-depth reclamation (FDR). In each of these methods, existing pavement materials are removed and reused, allowing for surface distresses to be corrected in place. The ultimate objectives are to conserve virgin materials; reduce energy use, environmental impacts, construction time, traffic flow disruptions, and number of hauling trucks; and improve pavement conditions.
HIR is based on softening the existing pavement surface using heat, removing the pavement surface and mixing it with asphalt binder—and possible virgin aggregate—and re-placing the recycled material on the pavement. In the CIR technique, the distressed pavement surface is pulverized and the recycled material is then mixed with new materials and compacted back in place; thus, providing an improved base layer with the addition of a wearing hot-mix asphalt overlay. The only difference between the CIR and FDR methods is that in the latter the whole thickness of the existing asphalt concrete layer—plus a predetermined thickness of at least two inches of the underlying unbound granular layer—are pulverized and mixed together into a homogenous mixture before compaction, producing a stabilized layer.
Recently, researchers from the University of Illinois at Urbana-Champaign’s Illinois Center for Transportation (ICT), University of California-Davis, and Rutgers University teamed up to develop a user-friendly life-cycle assessment tool to assess the environmental impacts and energy use of transportation projects that involve maintenance and rehabilitation treatments using the above in-place recycling and conventional paving methods. The study, sponsored by the Federal Highway Administration, was conducted by a team of researchers led by ICT Director and Bliss Professor of Engineering Imad Al-Qadi as Principal Investigator.
The developed tool uses data, simulation, and models throughout the in-place recycling stages for pavement life-cycle assessment, including materials, construction, maintenance/rehabilitation, use, and end-of-life stages. “The developed tool is expected to help pavement industry practitioners, consultants, and agencies complement their projects’ economic and social assessment with environmental impacts quantification and allow them to select an optimum rehabilitation alternative technique based on service life performance and environmental impact,” Al-Qadi said.
The study also emphasized the main factors that impact emissions arising and energy consumed at every stage of the pavement life-cycle as a result of in-place recycling techniques. Therefore, the research team gathered detailed information about fuel usage associated with such techniques based on field data. They found that fuel usage is affected by pavement aggregate hardness, pavement width, air temperature, and horsepower of the equipment used.
Mouna Krami Senhaji, one of the graduate students who have assisted with the project, says that it is important that agencies understanding of maintenance and rehabilitation alternatives be enhanced for sound decision making when selecting and prioritizing projects. “The newly developed tool provides the opportunity to analyze the environmental impacts and performance of a wide range of treatments applicable to flexible pavements, so stakeholders around the nation will be able to accurately evaluate the benefits of in-place recycling versus conventional methods.”
Rutgers Civil and Environmental Engineering associate professor Hao Wang considers the study particularly significant for the east coast where high volumes of freight traffic on highways is a daily occurrence. “In-place recycling techniques significantly reduce traffic delays because they result in shorter periods of lane closures.”