Category Stone Matrix Asphalt. Theory and Practice

SMA owes its fast and wide-spreading growth to some unquestionable merits such as the following:

• Long working lifetime (service life)

• High resistance to deformation due to the high-coarse aggregate content and strong skeleton of interlocked aggregate particles

• Increased fatigue life as a result of the higher content of binder

• Increased in-service traffic wear resistance because of the presence of hard coarse aggregate grains,

• Good macrotexture of the layer surface and decreased water spray gener­ated by traffic on wet surfaces

• Good noise-reduction properties

However, despite its strong points, the following drawbacks also exist:

• Low initial skid resistance unless a fine aggregate gritting or a crushed sand finish is applied

• High cost of the mix compared with a conventional asphalt concrete (initial costs can be increased by 10-20% due to higher contents of binder, filler, and stabilizer, but the extended service life of the pavement can result in reduced life cycle costs)

• Risk of different types of fat spots appearing on the surface as a result of errors or variability during SMA design, production, or construction

To end this part of the discussion, let us recall the words of Dr. Zichner of his patent:

All in all, the method taught by the invention and the procedures made possible thereby

provide a wear surfacing characterized by high resistance to abrasion, long-lasting

roughness, and reliable adhesion to the road surface. (Zichner, 1971).

Until the beginning of the 1980s, SMA was essentially known only in Germany. Its application in other European countries was limited in scope. Scandinavian states where studded tires used were the quickest to adopt the SMA concept; for example, in Sweden a few sections of roads had been paved with SMA by 1974 European Asphalt Pavement Association (EAPA, 1998). In Poland, which at that time was behind the Iron Curtain, concepts in West German publications were officially dis­allowed. Despite that, the Polish road administration permitted the first road sec­tion of an SMA-like mixture to be placed within its borders in 1969 (Jablotiski, 2000). Very positive results of that trial made the Polish Central Authority of Public Roads inclined to draft a standard (ZN-71/MK-CZDP-3), which was put into prac­tice in 1971. After forschungsgesellschaft fur straPen-und verkehrswesen (Germany)
(FGSV) published the first German technical standard for SMA (ZTV bit-StB 84), the mix became more popular and several European countries started to test the SMA mix. Now, virtually all European countries use SMA or, like France, nation­ally standardized mixtures conceptually similar to SMA.

The significant growth of the SMA application started in the early 1990s outside of Europe. This growth was certainly boosted by its popularity in the United States, and research on developing an American method of designing an SMA mix com­menced (see Chapter 7). Popularization of SMA in North America led to the release of SMA guidelines in other countries such as Australia, New Zealand, and China. During the last 20 years, SMA has become a global mix, and thus it may be seen almost everywhere where mineral-asphalt layers are applied.

For more than two decades, stone matrix asphalt (SMA), called stone mastic asphalt in Europe, has been taking over the global asphalt paving market at a remarkably high speed. Its fast-growing popularity has been surprising to many people. Although asphalt concrete may have appeared to be the indisputable leading choice for an asphalt layer, increasing vehicle axle loads have forced the application of new and better solutions.

The SMA mix, or Splittmastixasphalt in German, has been known since the mid – 1960s. Dr. Zichner, a German engineer and manager of the Central Laboratory for Road Construction at the Strabag Bau AG, was its inventor. It was an attempt to solve the problem of the damage to wearing courses caused by studded tires. The trend in wearing course mixtures at that time in Germany was to use Gussasphalt (i. e., mastic asphalt) and an asphalt concrete with a low content of coarse aggregates. These types of surfaces were subject to fast wearing by vehicles equipped with stud­ded tires. Both components, the mastic and the fine aggregates, were too weak to provide the mixture with suitable durability. Due to the high cost of pavement reha­bilitation, strong demand for a new surface mix that could withstand studded tires was created. This was the impetus for Dr. Zichner’s work.

When we try to picture ourselves in Dr. Zichner’s situation, we would face—as he did—the task of designing an asphalt mixture that is resistant to wearing caused by studs and also is durable enough to have a long service life. He stated that coarse aggregate grains resistant to dynamic fragmentation, or crushing, were those that might guarantee the right wearing resistance. Thus they had to be the major com­ponents of prepared mixes to provide the needed wear resistance, whereas a high content of mastic and binder would produce a long service life. So the early idea for SMA consisted of creating a very strong aggregate skeleton of coarse aggregates and filling the spaces between them with mastic (i. e., a mix of binder, filler, and sand). That type of composition of an aggregate blend is typically called a gap-graded mineral mixture.

Initial trial attempts to construct the new mix consisted of spreading a hot mastic layer followed by spreading high-quality coarse aggregates over the mastic, then compacting the surface (with a road roller). The ratio of mastic to coarse aggregates
(by weight) was 30:70. The mastic was made up of 25% B80 or B65 binder,* 35% filler, and 40% crushed sand (RETTENMAIER, 2009a). Based on experiments with such a composition, Dr. Zichner drew up a recipe for production of the mixture in an asphalt plant. The approximate composition of the first large scale production mix was as follows (RETTENMAIER, 2009a):

5/8 mm coarse aggregates 0/2 mm crushed sand Filler

B80 (B65) binder

As we can see, there was none of the 2/5 mm aggregate in that mix that was typically used in other mix types; the absence of this size fraction produces a gap grading, as we shall see later (in Chapter 2).

Then the problem of draining-off the binder from the aggregate was encountered; with such a high binder content and few fines to hold the binder in the mix, the binder tended to flow off the coarse aggregate particles. It was acknowledged, after labora­tory tests, that an additive of asbestos fibers would be a good drainage inhibitor (so – called stabilizer). Such a designed mix could be produced, transported, and laid in a traditional way (RETTENMAIER, 2009a).

The mixes were named by Dr. Zichner in 1968 as follows (Zichner, 1972):

• MASTIMAC—the name referring to mixes for layers 2-3 cm thick,

• MASTIPHALT— the name referring to mixes for layers thicker than

3 cm.

Early road sections of MASTIMAC were used on internal roads of asphalt plants belonging to the Strabag/Deutag Consortium, enabling them to gain experience with the new mixture. Eventually a public road was paved with the MASTIMAC mix on July 30, 1968, in Wilhelmshaven, Germany, on Freiligrath Strabe. The result was so encouraging that some other sections were paved with MASTIMAC soon afterward (RETTENMAIER, 2009b). Gradation curves of the new mixtures (Figure 1.1) were presented in a German publication (Zichner, 1972).

The stone mastic composition and its laydown were patented by Dr. Zichner in Germany, the United States, Sweden, France, and Luxembourg.[1] [2]‘ It is interesting how the inventor described his ideas in the U. S. patent text (Zichner, 1971):

the gravel size mixtures are composed…so that the percentage of the coarser size is greater than that of the smaller size. In this manner a relatively great interstitial volume is achieved in the gravel mix on the one hand, and on the other hand a good interfitting

Mastic on grains

Pavement

FIGURE 1.2 First stage of the mix performance according to U. S. patent No. 3797951— after laying.

of the individual pieces [of] gravel is assured….The quantity and fluidity of the mas­tic is such that during and after the compacting, the mastic flows into the interstices between the stones forming the wearing course as aforesaid, and which the volume of the mastic is less than the interstitial volume of the stones..

In that patent, the approximate percentage of the mix composition was defined as 70% coarse aggregates, 12% filler, 8% binder, and 10% crushed sand. It was also indicated that stabilizing additives may be needed as well. As the reader can see, the mixture described above is similar to the contemporary understanding of an SMA mixture (Figures 1.2 and 1.3).

Today it is generally admitted that the idea of SMA has changed little since its inception. What is SMA today? We can agree on its definition here; SMA is an asphalt mixture containing a gap-graded aggregate mixture, with high contents of coarse aggregate fractions, filler, and binder. Most often a stabilizing additive (drainage inhibitor), which prevents the draindown of the binder from the aggregate, is needed.

Although SMA mix was originally intended only for wearing courses, in some countries it is also applied in binder (intermediate) courses (see Chapter 13). Polymer modified binders were not commonly available in the 1960s and 1970s, so only conventional binders were used. Although the binders were quite soft, it was soon noticed that SMA layers were very durable, and their rut resistance became espe­cially evident.

The SMA mix was not forgotten in 1975 when a ban on the use of studded tires was introduced in Germany (Bellin, 1997). The concept of SMA turned out to be good not only for that kind of damage but also for rutting resistance and durability. Today SMA is regarded as an ideal mixture for heavy-duty asphalt pavements that require the highest resistance to damage and a very long service life. It is a practi­cally synonymous with resistance to rutting.

Krzysztof Btazejowski, a graduate of the Civil Engineering Department of the Warsaw University of Technology, Poland, completed his PhD dissertation at the Kielce Technical University, Poland. Since 1992 he has been working as a research engineer for the Road and Bridge Research Institute in Warsaw, Poland, and then in research departments of various companies that manufacture such products as road binders, aggregates, and concrete. He is also the author of a series of publications on asphalt surfacing. In addition, Dr. Blazejowski remains active in standardization and train­ing. When he is not involved in writing or research, Dr. Blazejowski spends time as a mountain guide.

Due to differences in terminology, chiefly between the United States and European countries, some assumptions were made. The universal term binder was used in the book instead of the U. S. term asphalt cement or the European bitumen. That decision has carried with it a change in the name from a binder course to an inter­mediate course.

Labeling mixtures according to the SMA 0/D system were used throughout the book where D denotes the nominal maximum particle size in a mixture. In Europe, marking SMA D (without ‘0/’) is grounded in the standard EN 13108-5 and has been in use since 2006. Also, aggregate blends are labeled according to a similar system as d/D where d and D are the lower and upper limits of aggregate fraction, respec­tively; for example, a coarse aggregate 8/12.5 mm means aggregates with grains of size between sieves 8.0 and 12.5 mm with a permissible amount of oversizes and undersizes.

The abbreviations m/m and v/v refer to ratios by mass and volume, respectively. The abbreviation PMB means polymer modified binder.

Many years have passed since the beginning of stone matrix asphalt’s (SMA’s) world­wide success. Therefore now the right moment has come to review all accessible information.

Through this new book I have tried to assemble, in an organized manner, a cer­tain body of knowledge obtained from a vast number of publications the world over, thus providing a review of the achievements of numerous engineers from various countries working on bringing recognition to SMA and developing its design meth­ods. I did my best to explain that knowledge and to present it in an accessible way. Some useful hints resulting from my experiences encountered during the introduc­tion of SMA in the early 1990s, discussions with many process engineers, and later reflections and observations round out the theoretical deliberations.

Knowledge of SMA is steadily improving, and new test results are revealed every now and then. In light of this, the moment when we can say that we really “know everything” about it is still far away. Alas, there is not one foolproof method for obtaining a perfect SMA in this book. Moreover, I tend to think such a recipe does not exist at all. This is not necessarily a bad thing because there is nothing like the ability to think and imagine when designing a mixture. The information put into the text is intended to help with using SMA. The range of accessible literature is really broad, so its accumulation and explanation are almost like putting together a jigsaw puzzle. Only the combination of many pieces of information enables associating certain relationships or finding out cause-and-effect relations. It is the reader’s task to judge whether that jigsaw puzzle has been appropriately completed.

Undoubtedly, the diversified terminology and the methods of testing properties of constituent materials and mixtures were quite a challenge when compiling this book.

Summing up this foreword, I feel that it should be emphasized again that the knowledge of this technology constitutes the considerable sum of experiences of many people. Therefore there is no particular individual who knows “everything” about SMA. In other words, no matter how much you already know about SMA, it is always worth broadening your knowledge!

Krzysztof Btazejowski

Acknowledgments

If it were not for the support of the many people who played a part in the publica­tion of this book, it could not have been completed. I would like to express my deep gratitude to all who helped me, in particular:

Prof. Klaus-Werner Damm, Germany

Dr. Bohdan Doizycki, Poland

Lothar Druschner, Germany

Horst Erdlen, Germany

Klaus Graf, Germany

Jan P. Heczko, UK

Konrad Jablonski, Poland

Jiri Kaspar, the Czech Republic

Dr. Karol Kowalski, the United States and Poland

Janez Prosen, Slovenia

Gregor Rejewski, Germany

Halina Sarlinska, Poland

Marco Schunemann, Germany

Stanislaw Styk, Poland

Ewa Wilk, Poland

Kim Willoughby, the United States

Bartosz Wojczakowski, Poland

Jan M. Voskuilen, the Netherlands

Also, I wish to express my sincere appreciation to Dr. Rebecca McDaniel, Purdue University, United States, for the time she devoted to the review and for a number of valuable contributions and suggestions that have substantially enriched the book.

The undoubtedly difficult-to-translate text has been quite a challenge for Leszek Monko, Poland; and Murdo MacLeod, Scotland, who have been entrusted with its proofreading.