Pavement engineers are always looking for better materials and techniques for constructing hot-mix asphalt (HMA) pavements. One of the most durable asphalt mixture types is stone-matrix asphalt (SMA). A premium mix for surfaces, SMA has been used in Europe for over 40 years to provide improved pavement performance. SMA was originally developed in Europe to provide better resistance to studded tire wear but with time also was shown to provide better overall performance than conventional HMA. SMA mixtures have been shown to provide improved resistance to rutting and fatigue cracking.
Use of SMA mixtures on highways continues to grow, and although there have been many studies that have shown these mixtures to provide very good performance, there has been limited use of SMA on airfield pavements to date.
While SMA has not been widely used in aviation to date, a few countries have begun to use it successfully as a surfacing on airfield pavements. For example, China has begun to use a large amount of SMA on many of its civilian airports. In fact, China has constructed SMA surface mixtures on 11 runways to date. The oldest one, which is in Beijing, was constructed 13 years ago and is still providing good performance today. The overall performance in China has been very good and plans are to use SMA on all asphalt runways when an overlay is required. China has not used SMA in other areas of the airfield such as taxiways and parking aprons. As a result of China’s good experience with SMA, the U.S. recently sent a group of airfield pavement experts to inspect the performance of the SMA mixtures and to learn more about the standard practices for using SMA in China.
SMA mixtures are gap-graded and most that have been used on airfields use polymer-modified asphalts and fibers to minimize potential for drain-down of the asphalt binder. Producing and constructing SMA mixtures can be somewhat challenging to contractors when they first begin to use these mixtures, but with a little experience these mixtures are no more difficult to produce than conventional HMA. Today, many contractors have been involved in construction of SMA, so lack of experience should not be a major concern in most cases when considering SMA mixtures for airfields.
SMA mixtures have a significant amount of surface texture due to the high coarse aggregate content, which is good for improved friction on runway pavements, especially in wet conditions. Typically, SMA mixtures have approximately 70% coarse aggregate particles; by contrast, dense-graded mixtures have approximately 30 to 50% coarse aggregate particles.
Getting off the ground
There have been many SMA mixtures placed on airfields in a number of countries in Europe. These SMAs have generally provided good performance, and it is expected that they will continue to be used at increased amounts. It is important to ensure that good compaction of the mixtures is obtained during construction to ensure that the in-place air void level is sufficiently low; otherwise, excessive permeability can occur. These coarse-graded mixtures tend to be more permeable than dense-graded mixtures when the in-place density is a little lower than desired.
Due to the overall positive experience with SMA on airfields at various locations, the Airfield Asphalt Pavement Technology Program (AAPTP) located at Auburn University has funded a research project to look at the potential for using SMA mixtures on airfields. The project was funded in 2006 and is expected to be completed in 2008. An interim report that describes the experience of using SMA on airfields has been published. The project, AAPTP 04-04, is being performed by the National Center for Asphalt Technology.
There has been limited use of SMA on airfields in the U.S. to date. There was an SMA project in 2005 on Taxiway H at the Indianapolis Airport in Indiana. The SMA was placed 1.75 in. thick. It is too early to estimate how well this mixture will perform on this project.
The U.S. Air Force constructed an SMA overlay in 1999 in Aviano, Italy. Apparently, this is the first SMA airfield pavement with which the U.S. has been involved. This project has provided good performance to date, and as a result the U.S. Air Force is building another SMA project in Spangdahlem, Germany. All of these projects are being monitored so that optimum procedures can be determined and can be used to design and construct SMA mixtures in the future.
Because of the good texture and the good frictional properties of SMA mixtures, some rubber buildup develops. This buildup also occurs with conventional HMA, but it generally occurs quicker with SMA. If the rubber is not removed on a regular basis, the friction of the pavement surface can be reduced to an unacceptable value.
Losing the groove
Airfields constructed in the U.S. are typically grooved to provide better friction in wet weather. The primary reason for the grooves is to allow surface water to drain into the grooves and off the side of the pavement, thus removing the water from the pavement surface. However, channels in the surface texture of a well-compacted SMA pavement allow the water to drain below the pavement surface. Since the water will drop down into the texture of the SMA mixture and off the surface, it is believed that good friction in wet weather is provided even without grooves. The Italy project was not grooved, and there are no plans to groove the SMA runway in Germany unless the friction drops below some acceptable level during the service life of the mixture.
Many engineers believe that grooving the pavement surface is not necessary when using SMA. If this is supported by research, the additional cost for SMA may be at least partially offset by the savings in not having to groove the surface. The Federal Aviation Administration (FAA) currently requires that all runway pavements be grooved to help provide good friction in wet weather.
The only SMA specification for airfields currently available in the U.S. is Section 32 13 17 of the Unified Facilities Guide Specifications. This specification was developed by a group of experts that included representatives from the Army Corps of Engineers, the U.S. Air Force, the U.S. Navy, the FAA and the HMA industry.
The specification requirements for SMA on runways are not greatly different from those for highways. Generally, it is recommended that a polymer-modified asphalt be used, as well as some minimum percentage of fibers to prevent drain-down of the asphalt binder. It is essential that SMA mixtures be compacted to a relatively high density to make the pavement watertight and thus prevent moisture from seeping into the pavement. Generally it is desirable to compact the SMA mixture to a sufficiently high density to provide less than 6% in-place air voids. Experience has shown that it is easier to compact SMA mixtures than conventional HMA mixtures as long as the rollers stay close behind the paver.
In Europe, aggregate chips are often applied and rolled into the SMA surface in a process sometimes called gritting. The chips have been shown to improve the initial friction of the SMA. However, with time the asphalt binder is worn off of the SMA surface under traffic and the friction is increased to an acceptable level even when chips are not used. Generally in the U.S. the chips are not added to the surface and the friction is acceptable just after construction and even better after a short period of time. If the chips are used, then they must be rolled into the surface or removed from the surface to ensure that foreign object damage does not occur once the pavement is opened to air traffic.
It is standard practice to groove the runway surface on civilian airfields in the U.S. after completion of construction to provide for positive drainage of the water from the surface, but it is not always done outside of the U.S. or on military airfields within the U.S. As stated earlier, the SMA pavement in Italy was not grooved, and the one in Germany will not be grooved when completed. Many have constructed SMA mixtures assuming that the increased texture will be sufficient to provide good friction and that grooving will not be required. Because of the texture of SMA, it is not certain it can be effectively grooved.
These SMA mixtures should improve performance and reduce the frequency of maintenance and construction of overlays. It is expected that the amount of SMA used on airfield pavements will increase significantly in the near future after the research on AAPTP 04-04 is complete.