Excerpts from “Japanese RAP Practices 2014 Scan Tour: Lessons Learned”
By Randy West, Ph.D., P.E., and Audrey Copeland, Ph.D.
On average, 47 percent reclaimed asphalt pavement (RAP) is used in Japan’s asphalt pavements. In the United States, the average amount of RAP used in asphalt mixtures is about 20 percent. Since 2007, the U.S. pavement industry has made a concerted effort to increase the amount of RAP used in asphalt pavements due to increased cost of virgin materials and a desire to emphasize sustainable, environmentally friendly practices. Based on annual industry and highway agency surveys, there appears to be an upper limit on the average amount of RAP in asphalt mixtures of about 20 percent; however, these same surveys have identified that the majority of state department of transportation (DOT) specifications and standards will allow up to 30 percent RAP on average.
The National Asphalt Pavement Association (NAPA) and Federal Highway Administration (FHWA) promote the increased use of RAP in asphalt pavements. They wanted to learn how Japan has achieved a national average of 47 percent RAP in asphalt mixtures, the quality achieved, and the best practices applied. An industry scanning tour of Japan was planned and took place in December 2014 to learn about Japan’s use of high RAP. The scan trip also focused on construction operations and practices used in Japan.
Japanese contractors and road owners follow a simple mix design and materials testing process with a focus on performance. One of the most striking differences between Japan and the U.S. is the United States’ use of the Superpave tests and materials specifications. Japanese contractors and officials stated that the Superpave system is complicated and testing is expensive. In Japan, there is well-established trust among the government and industry; however, on government projects contractors must get permission to use RAP. The quality of Japan’s pavements and roadways is due to attention to details and a focus on quality.
There are about 1,150 asphalt plants in Japan, serving more than 90 percent of the country and producing about 55 million tons of hot-mix asphalt (HMA) annually. Of those 55 million tons, about 41.9 million tons (nearly 38 million metric tonnes) are recycled hot-mix asphalt (HMA). In the United States, there are more than 3,000 plants producing a total of about 350 million tons of asphalt mixture annually. The vast majority of asphalt plants in Japan are batch-type plants that use and produce smaller quantities of materials and have slower production rates than the drum plants common in the U.S. Approximately 15 percent of asphalt mixtures in Japan use polymer-modified asphalt binder. The majority of asphalt mixture production is for surface courses. Recycling HMA is considered routine in Japan, and the main focus for innovation centers on porous asphalt pavements (drainage pavements), water-retaining pavement structures, and heat-insulating pavements.
The average RAP content in Japan asphalt mixtures increased from 33 percent on average in 2000 to 47 percent on average in 2013. About 99 percent of RAP in Japan, similar to in the U.S., is recycled into new asphalt pavement mixtures, but some RAP is reused as base course. The average RAP contents are similar in colder northern and warmer southern regions, and vary from 20 to 60 percent on average.
Summary of Key Innovations
The tour of asphalt pavement innovations in Japan was enlightening in many ways. The U.S. tour group took away an appreciation of the Japanese attention to quality, simplicity, and trust between highway agencies and contractors. There were, however, some aspects of Japanese practices that are unlikely to take hold in the U.S., such as the return to batch plants and low production rates.
The primary area of interest for the U.S. tour group was to understand how Japan was able to accomplish very high percentages of asphalt recycling. Japan’s average RAP content — approximately 47 percent — is more than double the current average percentage in the United States. The tour group learned that several factors have enabled Japanese contractors to achieve such high RAP contents. Some factors are technical, and some are political or cultural.
Political and cultural factors include a strong emphasis on the need to preserve raw materials, to avoid generating waste, and to recycle. Japan has a number of environmental laws that require the responsible reuse of waste pavement materials. Milled pavement material and rubble from small-scale pavement demolition must be tracked from the project site through processing and recycling back into new asphalt pavements. Therefore, the primary motivation for recycling appears to be for conservation reasons rather than economic ones. Japanese contractors are willing to invest heavily in state-of-the-art RAP processing plants and sophisticated asphalt mix plants to effectively utilize RAP at high percentages. However, the Japanese specifications for mixes containing RAP are not complex. Rather, their specifications seem designed to encourage the use of high RAP contents. Key aspects of the Japanese asphalt recycling specification include:
- RAP is processed from multiple sources. No restrictions are made as to the origin of the RAP.
- RAP quality is judged by three criteria:
- It must have a minimum asphalt content of 3.8 percent.
- The recovered RAP binder must have a penetration greater than 20 or samples of the compacted RAP must have an indirect tensile (IDT) modulus of less than 1.70 MPa/mm.
- The processed RAP material may not contain more than 5.0 percent P200 fines.
- Fractionation of RAP is a contractor’s choice, not a requirement. Most contractors choose to fractionate the RAP.
- Blending charts are used to determine ratios of virgin and recycled binders or dosage rates for recycling agents. Mix designers may use soft virgin asphalts or recycling agents to meet a target penetration value for the composite binder or a desired IDT modulus for the mixture.
- Mix designs use the Marshall Method and criteria with a simple supplemental performance test, the IDT modulus, which limits mixes with very high stiffness (and low cracking resistance).
In several regards, Japanese contractors follow best practices for RAP management that have been recognized for years in the U.S., but are not necessarily widely used by many U.S. asphalt mix producers. Following are some of the standard practices used in Japan:
- Stockpiles are covered and on a paved surface.
- Moisture and dust contents of the RAP are minimized during crushing.
- RAP binders are recovered and tested to evaluate their stiffness.
- RAP is fractionated, and the plants are equipped with multiple RAP feed bins.
The most obvious contrast in mix production between the United States and Japan is the prevalence of batch plants in Japan compared to continuous mix plants in the U.S. However, the most significant paradigm shifts were (1) the method of heating RAP in a separate dryer followed by mixing, and (2) conditioning the dry and hot RAP with a rejuvenator for several hours prior to mixing with hot virgin aggregate and asphalt. This is not to say that the production facilities (i.e. drum plants) in the U.S. are the limiting cause for lower RAP use; in most cases it may be specifications and local practices that have limited RAP use. The U.S. overall method for production –indirect heating in a steam (low oxygen) environment — has been proven not only on the NCAT test track but also by the millions of tons used in North America since the 1980s. If contractors use well-known best practices for running RAP, there is no technical reason the percent RAP in the U.S. can’t continue to grow.
Key aspects of the Japanese method for high RAP mix production include:
- Drying and heating of RAP in a separate parallel dryer.
- Use of thermal oxidizers to handle emissions from the RAP dryer.
- Mixing a rejuvenator with the hot RAP and giving the material time to “activate and condition” the aged RAP binder.
- Low production rates, typically 100–180 metric tons per hour. The conditioning step noted above is likely the controlling process.
Japan has reached its current high percentage of RAP utilization over several decades of research and field performance evaluations. Through analysis of pavement performance on hundreds of projects and experimentation in the lab and field, they have developed standards and practices that have proven to provide equal performance for high RAP content mixes and virgin mixes.
Another area of interest for the U.S. tour group was Japan’s innovations with porous friction course mixtures. Despite the growth in the use of open-graded friction courses, the use of porous friction courses (PFCs) remains more widespread in Japan than the United States. The Japanese asphalt industry and highway agencies have worked to improve the performance of PFCs and address many of the common issues with these layers. Key innovations for PFCs in Japan are:
- Thick PFC layers (4–5 cm) to better suppress tire-pavement noise and to improve drainage. The economic justification of thicker layers is aided by the fact that PFCs are given the same structural layer coefficient as dense-graded mixes.
- Use of highly modified asphalt binders (4 to 8 percent SBS polymer) to improve resistance to raveling and eliminate the need for fibers.
- Use of edge drains with PFCs placed in curb and gutter sections (i.e., urban areas).
- Use of spray-pavers to apply a thick asphalt emulsion layer just before the PFC mix. The residual asphalt from the emulsion creates an impermeable layer at the bottom of the PFC to protect underlying asphalt layers.
- Routine cleaning of the PFC with specialized equipment that flushes the PFC voids to help maintain permeability for a much longer period of time.
One aspect of Japan’s asphalt industry that has likely aided rapid implementation of new technologies is the small number of producers and contractors. These large companies have not only invested heavily in state-of-the-art plants and equipment, they also have built and staffed advanced central laboratories to explore innovations and development of proprietary products. The technical expertise of the Japanese companies is impressive, and the level of trust between the highway agencies and the contractors appears to be much better than is generally seen in the United States.
Safety, quality, and care for the environment are hallmarks of Japanese culture, and are clearly the top priorities in the road construction industry. Productivity and competition are much less of an emphasis compared to the U.S. road building industry. However, Japan’s focus on engineering production and construction creates an environment that results in quality pavements. Japan’s success with RAP can be summarized with simple mix design and testing requirements, a recycling specification for RAP use, and best practices for processing and handling RAP and production, which ensures the RAP binder is activated and blended in the asphalt mixture.
The tour group is finalizing a report that will document the findings from the Japan high RAP tour. The tour delegates (see sidebar) included contractors, equipment manufacturer, State DOT representatives, researchers and a State Asphalt Pavement Association representative. Visit www.AsphaltPavement.org; the report will be available soon.
Dr. Randy West is Director of the National Center for Asphalt Technology. Dr. Audrey Copeland is Vice President of Engineering, Research & Technology at NAPA.