Survey on the application of recycled pavement materials and WMA mixtures in the U.S.
As of 20:00 on November 22, 2015, the cumulative sunshine duration of Beijing since the beginning of November was only 31.8 hours, being the lowest in the same period since 1951, while the average value of the same period was 136.4 hours in the past. In addition, many places of northern China hit historical lows in sunshine duration. Except rains and snows, heavy haze that lasts for days is a major “sunshine killer”, and removing haze becomes an urgent “gray topic”.
All industries are making efforts in environmental protection, without exception for highway construction. A number of new maintenance processes, such as the warm mix asphalt (WMA) and pavement recycling technologies, have become effective haze-removing tools for the highway industry. These new processes and materials have made energy conservation, environmental protection and low carbon practically possible, where the WMA technology can reduce paving energy consumption and waste emissions effectively, while the recycling technology can turn waste asphalt into a recyclable “treasure”.
In the future, in order to expel haze, and regain the blue sky and sunshine, more new technologies and instruments will be applied to highway construction and maintenance, thereby further reducing energy consumption, and truly realizing China’s commitment to emission reduction.
The shortage of resources, energy crisis and environmental deterioration in the 21st century have made people vigilant about the changes brought by the development of human society. Along with extensive modernization in the context of rapid economic and social development, civil engineering has become an important source of resource and energy consumption, and the “three wastes” of the world, so people are increasingly concerned about the industry’s resource and energy sustainability, and environmental protection.
The stacking of asphalt pavement maintenance and repair milling materials not only occupies much land and wastes resources, but also pollutes soil, vegetation, waters and air. However, most milling materials are still recyclable. Utilizing renewable resources in such materials scientifically and rationally can realize the recycling, reduction and green treatment of milling materials, prevent the excessive consumption of natural resources, protect the ecological environment, beautify cities, promote benign local economic and social development, and generate great economic and social benefits. This will be an important way of developing a low-carbon society and a circular economy. The recycling of recycled pavement materials and WMA mixtures has become a task of global concern.
Extensive research on and application of WMA mixtures has been conducted in the U.S. From 2009 to 2013, the scope of application, consumption and proportion of recycled pavement materials in the U.S. kept rising, and the technical classification of WMA mixtures was gradually clear, with consumption and proportion rising gradually. Surveys and analyses reveal that there are numerous applications of recycled pavement materials in the U.S., such as asphalt mixtures, aggregates, cold recycling and backfilling, but hot mix asphalt mixtures or WMA mixtures are the best use of recycled pavement materials. Currently, the states are still cautious about the extensive use of recycled pavement materials, and always insist that their performance be analyzed strictly. In 2013, the share of WMA mixtures in the U.S. was 30%, in which mechanically foamed WMA mixtures accounted for 87%. As an economical and green warm mix technology, mechanical foaming has just started in China but has an extensive prospect. Therefore, it is necessary to further improve the foamed WMA mixture technology through in-depth research.
WMA mixture technologies increase the workability of mixtures by physical or chemical means, with production temperatures between those of hot mix and normal temperature asphalt mixtures, and higher than those of hot mix asphalt mixtures by 20~30℃ without affecting pavement performance negatively. Such technologies can reduce smoke and dust emissions, improve air quality and the working environment, protect the physical health of construction workers, reduce energy consumption effectively, save production costs, improve the compacting property of pavements, prolong construction seasons, reduce aging during construction, make pavements longer-lasting, and increase the consumption of recycled materials.
Extensive research on and application of recycled pavement materials and WMA mixtures has been conducted in the U.S. An analysis and review of the current situation of application in the U.S. will provide a reference for the research on and application of pavement recycling technologies and WMA mixtures in China.
The statistics of application of recycled pavement materials to asphalt mixtures, aggregates, cold recycling, backfilling, etc. in the U.S. are shown in Figure 1. It is estimated that in 2009 and 2010, less than 0.1% of recycled pavement materials were used for recycling; in 2011, less than 0.4% used; in 2012, less than 0.3% used; and in 2013, less than 0.2% used.
The proportion of mixing plants using recycled pavement materials was 96% in 2009-2010, 98% in 2011-2012, and 100% in 2013. From 2009 to 2013, the proportion of recycled pavement materials used for asphalt mixtures rose significantly, averaging 16.2% in 2009 and 20% in 2013.
The consumption of recycled pavement materials rose 22% from 2009 to 2012, and dropped 0.73% in 2013, mainly because the consumption of asphalt mixtures dropped slightly. From 2012 to 2013, the consumption of recycled pavement materials used for hot mix asphalt mixtures or WMA mixtures in the U.S. dropped from 68.3 million tons to 67.8 million tons by 0.73%, but the proportion of recycled pavement materials used for asphalt mixtures rose slightly from 19.6% in 2012 to 20% in 2013. The survey shows that the application of recycled pavement materials and WMA mixtures has promoted the sustainable development of the asphalt pavement industry.
WMA technologies are divided mainly into chemical additive, organic additive and foamed asphalt technologies. Active ingredients in chemical additives can improve the coating, working and bonding properties of mixtures at low temperatures. When added to asphalt cementing materials, organic additives can reduce asphalt viscosity and improve its lubricity. The foamed asphalt warm mix technology improves the coating and working properties of asphalt at low production temperatures. Currently, tens of WMA mixture technologies in three major categories are available.
The WMA mixture consumption of the U.S. grew from 86.7 million tons in 2012 to 106 million tons in 2013 by 23%, and mechanical foaming has become the most commonly used WMA mixture technology. The consumption of chemical additive WMA mixtures grew from 11.6% in 2012 to 13.1% in 2013.
The WMA mixture consumptions of the U.S. from 2009 to 2013 are shown in Figure 3, and the proportions hereof are shown in Figure 4. The WMA mixture consumption accounted for 5.9% of the gross consumption of asphalt mixtures of the U.S. in 2009, and 30.3% in 2013, an increase of 4.1 times.
The application of warm mix technologies in the U.S. from 2009 to 2013 is shown in Figure 5. The mechanical foaming warm mix technology accounted for 82.7% in 2009 and 86.9% in 2013 in the U.S., while other warm mix technologies accounted for about 10% in 2013.
From 2012 to 2013, the WMA mixture consumptions of 24 states of the U.S. rose at least 5%, while those of 10 states dropped at least 5%. The WMA mixture consumptions of Connecticut, Idaho, Kentucky, Louisiana, Montana and Oklahoma rose 25% or more, while those of Alaska and Tennessee dropped 25% or more.
The WMA mixture output of the U.S. was 106 million tons in 2013 and 86.7 million tons in 2012, an increase of 23%. From 2009 to 2013, the WMA mixture output of the U.S. grew 533%.
The survey on recycled pavement materials and WMA mixtures in the U.S. shows that recycled pavement materials are applied extensively, and as an economical and green warm mix technology, mechanical foaming has just started in China but has an extensive prospect, and will play a significant role in promoting energy conservation, emission reduction and sustainable development. However, there is still much work to do, such as the design and long-term performance of foamed WMA mixtures, combination with the pavement recycling and rubber asphalt technologies, etc.
From: www.chinahighway.com