Experiences with Stone Matrix Asphalt (SMA)

Sant Kumar Sehgal and Bernd Abele

Review

In the sixties and early seventies, studded tires were used in Germany during wintertime. The wear of asphalt surface courses under the spikes was extreme. Research and investigations on the performance of surface courses of asphalt concrete (AC) and gußasphalt verified that wear and rutting by studded tires was increased by:

• high air void content of the surface course,
• low bitumen content,
• inadequate quality and quantity of aggregates.

In the end of the sixties, these results were the starting point for several large companies of the asphalt paving industry to develop specific mixes which were published under different proprietary names. Later, all these mixes were standardized and called Stone Mastic Asphalt or Stone Matrix Asphalt like in India (see IRC:SP:79:2008).

In 1975, the use of studded tires was banned. In the following time surface courses with asphalt concrete and prototype SMA mixes showed a different performance of heavily trafficked higways: asphalt concrete with a high bitumen content was disposed to permanent deformation, whereas asphalt concrete with a low bitumen content was disposed to aging and preliminary cracking. However under equal conditions both types of distress were not observed with SMA - surface courses. Practical experiences and observations proved that SMA is a better mix than asphalt concrete. Since high performance highways, especially the "Autobahn," have been an asset of the transportation-system, there has been constant strive for better pavements and asphalt mixes. Consequently, SMA has gradually improved by investigations, tests and feedbacks from highway projects. Finally in 1984, SMA passed the stage of a contractor`s proprietary product. It was standardized and became a part of the German ZTV Asphalt (Technical Specifications and Guidelines for the Construction of Asphalt Pavements), issued by the Federal Department of Transportation.

Years after the successful application of the Stone Mastic Asphalt principle in Germany, other European countries are using this philosophy also for heavy traffic roads. Now, many countries worldwide have taken over the philosophy of the Stone Matrix Asphalt.