Stone mastic asphalt (SMA), also known as mastic asphalt in some European countries, has spread rapidly around the world for over two decades. Although asphalt is a popular and suitable choice for road pavement, the increasing axle load of vehicles has forced stakeholders to employ new and better solutions.
The History of Using SMA
SMA asphalt was introduced in Germany in the mid-1960s. Dr. Zichner invented this mixture to address the pavement polishing problem due to the movement of vehicles on snow tires. Back then, Germany was using gussasphalt (similar to mastic asphalt) or asphalt mixtures with coarse aggregate. Pavements with these mixtures were quickly exposed to abrasion due to the movement of vehicles on snow tires.
Both gussasphalt and normal asphalt were too weak to make a sufficiently durable mixture. Due to high pavement repair costs, creating a new mixture that could withstand snow tires incentivized Dr. Zichner.
Dr. Zichner tried to design a mixture that was resistant to the abrasion caused by snow tires and sufficiently durable for long-term service. He stated that the coarse aggregates that were mechanically resistant to impact or crushing were also abrasion-resistant. Although a great deal of binder was used in the asphalt mixture, it led to a long service life.
Hence, the first subject discussed in making SMA was to create an incredibly strong coarse aggregate skeleton and filling the voids with mastic. The preliminary laboratory efforts to create this new mixture involved creating a hot mastic layer, spreading a high-grade coarse aggregate on top, and compacting the surface (with a roller). The mastic to coarse aggregate (weight) ratio was 30 to 70.
The mastic consists of the following materials:
25% bituminous binder
35% filler
40% fine aggregate
Accordingly, the first asphalt was produced with the following ratios:
Compared to other mixtures, this mixture does not use a 2.5 mm aggregate and its absence creates an open or interrupted gradation. Due to the binder's separation from the aggregate and the large amount of binder and little fine-grained rock materials for holding the binder in the mixture, the binder tends to separate from the coarse aggregate. This problem was first addressed using fire-resistant fibers. In 1968, Dr. Zichner named these mixtures as follows (1972):
MATIMAC: This name refers to mixtures with 2-3 cm-thick layers.
MASTIPHALT: This refers to mixtures with layers that are more than 3 cm thick. The first MASTIMAC mixtures were used on the local roads of Strabag/Deutag and represented an experience in creating new mixtures. Finally, a public road in Wilhelmshaven, Germany was paved with the MASTIMAC mixture on July 30, 1968, and other roads followed. The grading curve of these new mixtures were presented in a German publication.
Dr. Zichner patented stone mastic mixture and the relevant subjects in Germany, the United States of America, Sudan, France, and Luxembourg. The inventor’s description in the patent registered in the United States includes the following:
Rock materials have a specific grading, and the coarser aggregate content is higher than finer aggregates. This state leads to a suitable rock material interlocking in these mixtures in which mastic’s quantity and fluidity is important to mixture compaction. The mastic matter flows between stones but has a lower volume than the empty space in the aggregate.
This patent defines the mixture's estimated contents as follows:
70% coarse aggregate
12% filler
8% binder
10% crushed gravel
The mixture may also require stabilization additives.
What is SMA?
SMA is an asphalt mixture comprised of open-graded aggregate and plenty of coarse aggregate, filler, and binder. This mixture usually needs a stabilization additive that prevents the binder's separation from the aggregate. Although the SMA mixture was initially used to prevent surface abrasion and polishing, some countries also use it for the wearing course. In the 1960s and 1970s, the modified polymer bitumen was not widely adopted and ordinary bitumen was used instead. However, the durability of SMA layers and their rut resistance received significant attention. The SMA mixture is resistant against abrasion, rutting, and extremely durable. Nowadays, SMA is an ideal mixture for asphalt pavement with heavy loading.
SMA's Application in Other Countries
Until the early 1980s, SMA was only known in Germany, and its application in European countries was restricted to specific fields. Scandinavian countries that use snow tires quickly adopted the SMA mixture, and the first German standard for StB (ZTV it SMA84) was published in 1971. This mixture gradually became more common and several European countries began to experiment on the SMA mixture. At present, all European countries are using SMA and some countries, like France, use similar mixtures. The rapid adoption of SMA outside Europe began since the early 1990s with its popularity in the United States and research on developing an American SMA mixture design method. The SMA mixture's adoption in North America led to the publication of SMA guidelines in other countries, including Australia, New Zealand, and China. During the past 20 years, SMA became a global asphalt mixture that was seen wherever there was a need to wearing course.
The Strengths and Weaknesses of the SMA Mixture
SMA developed rapidly due to the following reasons:
• Long service life (use)
• High deformation resistance (due to the high coarse aggregate content and the mixture's strong interlocked aggregate structure). Due to its high binder content, this mixture wears out late.
• Due to its hard, coarse aggregate, it has suitable abrasion resistance against traffic.
• Having a coarse tissue in the pavement surface reduces water spray by vehicles moving on wet surfaces.
• Reduces sound pollution of moving vehicles
Despite these advantages,
The SMA mixture has the following weaknesses:
• Low initial skid resistance (except when fine aggregate or crushed gravel are employed)
• High initial mixture costs compared to normal asphalt (The initial cost is higher due to the higher bitumen, filler, and mixture stabilizer content, which adds 10 to 20% to the asphalt price, but the cost over its lifespan is lower due to its longevity.)
• The pavement surface bleeding risk due to errors or changes caused by SMA design, production, and implementation.
Differences between SMA Mixtures in Various Countries
Countries have two approaches to explaining the differences between SMA mixtures in terms of design and rock material grading:
• German SMA mixtures or those that are built according to German guidelines and are somewhat based on regular observations and experiences in SMA technology.
• Researching and developing new SMA mixture design methods (including the standards of the Untied Stated and the Netherlands). There are various mixtures around the world referred to as SMA despite their potentially significant differences with Dr. Zichner’s invention (the first SMA mixture). Some of these differences are so stark that they should be referred to as secondary SMA. Although these mixtures are mineral with open-graded or SMA-like grading, they have significant differences with Dr. Zichner’s invention.
The Main Components of SMA
The main components of SMA are as follows:
• Coarse aggregate structure
• Mastic substance or mortar (including bitumen, filler, fine aggregates, and stabilization fibers)
• The empty voids in the asphalt mixture