Pavements are structures built to support the loads induced by vehicle traffic and to distribute them safely to the natural soils, typically referred to as subgrade soils.
There are three main types of pavement structures
- composite pavements.
Flexible pavement is the most common type used in the US. A conventional flexible pavement structure consists of a surface layer of asphalt concrete mixture and a base course layer of granular materials built on top of the subgrade soil.
The significant rise in prices of raw asphalt materials and the increasing demands for environment-friendly asphalt mixtures are two of the main challenges facing the asphalt paving industry and transportation agencies. Several technologies and methods have been used to address these challenges.
One of these technologies is the Warm Mix Asphalt (WMA). WMA is a generic term for an asphalt mixture that are produced and placed at lower than conventional temperatures. While heat is used to reduce the asphalt viscosity and to dry aggregate during mixing of conventional asphalt mixtures, WMA reduces asphalt viscosity by including water or special organic or chemical additives in the mixture. The reduction in viscosity still allows the asphalt binder to adequately coat the aggregates during mixing. The reduction in mixture viscosity also improves its workability and allows for mix compaction at lower temperatures.
During the past few years, the amount of asphalt mixture produced using WMA technology has increased tremendously. For example, in Ohio, more than 60% of the total amount of asphalt mixtures are currently produced using WMA technology.
Another way that the asphalt paving industry and transportation agencies have utilized to reduce the costs and adverse environmental impacts of flexible pavement is to increase the use of readily available recycled materials.
Reclaimed asphalt pavement (RAP), recycled asphalt shingles (RAS), and ground tire rubber (GTR) are the three main recycled materials that have been incorporated in asphalt mixtures used to pave roadways. Because of its’ valuable re-usable aggregates and asphalt binder, RAP has already become one of the most widely used recycled materials in the United States, while RAS is emerging as a material of interest to the paving community due to its high asphalt binder content. For example, it is estimated that using 5% shingles in an asphalt mixture can reduce its price by up to $2.8 per ton. GTR, also known as crumb rubber, has been incorporated in asphalt mixtures since the 1960s to enhance the performance and service life of pavements.
However, in more recent years, GTR has also gained interest as some studies showed that GTR can help improve the performance of asphalt pavements while reducing their costs and traffic noise. Thus, GTR has been viewed as a viable alternative to polymer additives typically used in asphalt mixtures.
Currently, research in the field of pavements is focused on the successful implementation of WMA technology as well as on maximizing the effective use of recycled materials in asphalt mixtures to develop sustainable roadways.
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Written By: Dr. Munir Nazzal, Ohio University online Master of Civil Engineering program