Category Stone Matrix Asphalt. Theory and Practice

Throughout this book the term fine aggregate has been conventionally used as a term for the passive aggregate. Its upper limit depends on us—or more specifically, on

the type of coarse grain previously in process of SMA design accepted as a skeleton maker. The task of the fine aggregate is to fill voids among the coarse aggregate particles and facilitate their interlocking, though it is likely to be put the other way round; the fine aggregate cannot disturb interlocking of the coarse aggregates. Such a disturbance can best be illustrated by an example of rounded, uncrushed aggregate (e. g., natural sand or uncrushed gravel) with smooth surfaces that allow the coarse aggregates to glide easily. Introducing such “hard balls” into an SMA mixture causes problems with the stabilization (interlocking) of the aggregate skeleton. That is why in many countries the incorporation of uncrushed aggregates in SMA has been lim­ited to only low volume roads, or its use has been generally banned.

Angularity is a feature that describes the properties of fine aggregates and is typically defined in terms of a flow test, which is an indirect method of angularity measurement. In Europe, the flow coefficient (method EN 933-6) is labeled with the Ecs symbol according to European Standard EN 13043 and describes the time necessary for a standard amount of aggregate to flow out of a suitably shaped vessel through an opening. Obviously the more crushed aggregate with a better microtex­ture, the longer the flow-out time; that is, rough or angular aggregates tend to lock up and not flow as quickly as smoother particles. Aggregate with an Ecs greater than or equal to 30 or 35 seconds (i. e., Ecs30 or Ecs35 category) is regarded as appropriate for SMA. There are also requirements concerning the angularity of fine aggregate applied in the United States.[10] Tests are carried out in accordance with the American Association of State Highway and Transportation Officials (AASHTO) T 304 Method A (ASTM C1252), and the required fine aggregate angularity (FAA) values are greater than or equal to 45% (NAPA QIS 122). Angularity can also be

measured according to ASTM D3398 and a National Aggregate Association (NAA) test method as well.

Remembering the positive influence of angularity on properties such as defor­mation resistance, we still have to take into account some negative factors like compaction resistance that accompany an increase in aggregate angularity. Moreover, research conducted by Stakston and Bahia (2003) showed that the effect of FAA depends on the source aggregate and its gradation and that angu­larity could have an adverse effect on a mixture’s resistance to shearing. Further reading on the influence of FAA on asphalt mixtures can be found in the Stakson and Bahia study or in other papers (Johnson et al., 2004; White et al., 2006).

The content of free voids in a compacted fine aggregate is undoubtedly an essen­tial parameter in SMA volumetric design. This characteristic may be tested through various methods, (e. g., the AASHTO T19 standard). The packing of consecutive SMA ingredients cannot be determined without prior knowledge of this characteris­tic. That approach to SMA design is discussed in Chapter 7 in the section devoted to U. S. and Dutch design methods.

Mastic is the second largest component of stone matrix asphalt (SMA); it is approxi­mately 20-25% by weight of the mixture and 30-35% by volume. About 35-40% (v/v) of the compacted coarse aggregates is made up of voids, and after filling the aggregate with mastic, 3% to 5% (v/v) of empty space will be left.

Mastic[9] consists of the following:

• Fine aggregate

• Filler

• Stabilizer (drainage inhibitor) in the form of fibers or other additives

• Bituminous binder

Filler and binder make up mortar. Blends of the fine fraction of the filler with binder act like binders and can be tested as a binder, but blends of the total filler and binder act more like a mixture and can be tested in that manner (e. g., BBR1- stiffness, resil­ient modulus, and tensile strength) (Brown and Cooley, 1999).

In the preceding chapter we discussed how coarse aggregate particles make up a skeleton. The task of the coarse aggregate skeleton is different from that of the mas­tic. The functions of mastic are as follows:

• Binding (sticking together) the coarse aggregate skeleton

• Lubricating the coarse grains during compaction and enabling a proper aggregate structure in a compacted surface course

• Sealing the layer, or filling the voids in the compacted aggregate structure to provide it with high durability and resistance to other external factors such as water or deicers

• Withstanding stresses caused by load and temperature

Figure 3.1 shows the close packing of fine (passive) aggregate among coarse (active) grains.

Now let us deal with the mastic components.

A downward trend that has been recently evident in Germany involves reducing the aggregate blend gradation discontinuity and establishing an upper limit on the coarse aggregate (bigger than 2 mm) content. These changes have been justified as ways to secure a more durable wearing course and to improve its compaction.

Although in some respects very appealing, another viewpoint on building a very strong SMA skeleton with coarse grains only is not without controversy. From a technological point of view and also from Dr. Zichner’s original principles, SMA is almost a gap-graded mixture. But are all gap-graded mixtures SMAs? Certainly not. If something is going to be called SMA, namely Splittmastixasphalt, its dis­tinctive feature should be a gradation with ratios at least similar to those listed in Table 2.1.

The evolution of requirements for SMA gradation is also apparent in the United States. Some U. S. guidelines require SMA mixtures marked by very distinct gap gradation while other guidelines do not differ much from the German regulations. This trend may be well illustrated by comparing the SMA 0/12.5 mm gradation curves according to the American Association of State Highway and Transportation Officials (AASHTO)[6] [7] M 325-08 standard with those from the National Asphalt Pavement Association Quality Improvement Series No. 122 (NAPA QIS-122) guide­lines, as shown in Figure 2.8. It may be easily noticed that SMA gradations based on NAPA QIS-122 may be marked by substantially more single-sized aggregate and a stronger skeleton of 9.5- to 12.5-mm grains (up to 73% of grains retained on a 9.5-mm sieve), while gradation curves according to AASHTO M 325-08 look more continuous.

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2.2 AMONG THE SKELETON GRAINS

Having formed an adequate skeleton of coarse grains, we have to remember to place mastic between the active grains. Achieving the proper volume of mastic is critical; there must be the right amount of mastic to coat the coarse grains but at the same time to leave some free, unoccupied space. Figure 2.9 illustrates one way to look at the packing of SMA by reflecting the volume contents of coarse aggregates, mas­tic, and voids.

It may be concluded from Figure 2.9 that the volume of voids among the coarse aggregates has to be properly determined at the aggregate skeleton design stage. The ideal laboratory design method is one that accurately defines the volume of the free space between the coarse aggregates as they would exist after field compaction.

The design method may not accurately reflect the mixture volumetrics after field compaction, especially if the field compaction is a more effective compaction than assumed at the SMA design stage. This would result in a mastic volume that is too big in relation to the free space among the coarse aggregates, and as a result, mastic may be squeezed up to the surface,[8] causing fat spots to appear. It is also interesting that in such circumstances an unexpected decrease of free space for the mastic may result. But that is the subject of the following chapters.

Thus we have reached the moment when we have to deal with the mastic.

Voids in stones

Mastic

Voids in final SMA mixture

____________ Total volume of SMA mixture after compaction___________________

FIGURE 2.9 Volumes of SMA coarse aggregate skeleton and mastic. (Modified from Voskuilen, J. L.M., Ideas for a volumetric mix design method for Stone Mastic Asphalt. Proceedings of the 6th International Conference Durable and Safe Road Pavements, Kielce (Poland), 2000. With permission.)

2.3 SUMMARY

• SMA mixtures belong to a group of coarse aggregate-sand mixtures with a continuous coarse grain matrix—that is, their skeletons are formed by interlocked coarse aggregate particles that transmit loads.

• The term coarse aggregate skeleton is a conventional notion meaning a structure made up of grains with a specified lower limit of size. The most frequently accepted limit is the 2-mm (or 2.36-mm) sieve, though in many countries, such as the United States, it depends on maximum grain size.

• The SMA aggregate grains may be divided in two types: those forming a skeleton and carrying loads (so-called active grains) and those filling voids and carrying virtually no loads (so-called passive grains). The basis for developing an active grain skeleton in an SMA is to ensure contact exists among the active grains.

• A discontinuity in the SMA aggregate blend gradation (gap gradation) is produced by the lack of in-between fractions of aggregate bigger than 2 mm.

• A coarse aggregate skeleton as defined by the German method (the original method, after Dr. Zichner’s idea) consists of an adequate proportion of all aggregate fractions greater than 2 mm.

• A coarse aggregate skeleton defined by some other methods, such as the U. S. method, consists of an aggregate blend with a decisive predominance of coarse aggregate and a distinct gap gradation, which results in establish­ing stone-to-stone contact.

In some countries (e. g., the United States and the Netherlands) a method of con­structing an SMA skeleton has been developed based on the control of stone-to-stone contact or a real gradation discontinuity. Based on these methods, the definition of SMA is expanded to mean an asphalt mixture containing mastic stabilizer (drainage inhibitor) with a gap-graded aggregate blend and a very high content of coarse aggre­gates in which smaller grains are seated among the bigger ones, filling voids among them but not shoving them aside. Based on this definition of SMA, it is necessary to determine the level of gradation discontinuity at which active grains are not shoved aside by passive ones.

Let us look at Figure 2.6a to d, which shows an idealized arrangement of grains (represented by smooth spheres) in an aggregate blend and illustrates the relation­ships between the radii of the active coarse grains (marked R) that form the skeleton and the radii of the passive fine grains (marked r). The proportions of the different grain sizes have been selected so that the smaller grains do not shove the bigger ones aside.

Thus we have two sets of spheres, here being examined two-dimensionally. Now we may theorize a bit on their significance for the sought-after discontinuity. [4]

• Such an arrangement of grains as shown in Figure 2.6a and b is unnatural and unlikely. The one presented in Figure 2.6c and d is more likely. But it is easy to see that one consequence of a better (closer) distribution of coarse grains is the reduction of free space available for filling (passive) aggregate. Simple geometric analysis enables the calculation of the maximum dimen­sion of fine grains, equal to 0.16 R, to avoid shoving coarse grains aside in this scenario. If we look again at the example of the SMA 0/12 mm, apart from the active 8/12 mm fraction, the next material will be just the passive 0/2 mm (sand) fraction.* In the arrangement illustrated in Figure 2.6c and d, the necessary gradation discontinuity would consist of the absence of the 2/8 mm fraction.

These deliberations have been carried out on the assumption that the only active fraction is the 8/12 mm one, but this is by no means obvious. The SMA skeleton may also involve active grains smaller than the 8/12 mm fraction, such as grains bigger than 5 mm. Active grains of the 8/12 mm and 5/8 mm fractions would have a consid­erable influence on the formed skeleton owing to the following: [5]

• Better packing of coarse grains (here, larger than 5 mm)

• Reduction of the size of voids and therefore the size of the filling grains

If the 5/8 mm fraction is also active, the desired grain discontinuity may be secured by the absence of the 2/5 mm fraction. The Bailey method, described in detail in Chapter 7, is based on a similar geometric calculation; that approach assumes that each successive aggregate fraction should be equal to about 1/5 (0.22) of the larger fraction. This requirement is intended to prevent smaller particles from shoving the coarse aggregate skeleton aside. As we can see, the path from geometry to design method is not a long one.

These considerations are only theoretical because in a real mixture we do not deal with balls and the layout of grains is different. Yet those examples point out a signifi­cant issue; interfering with the gradation discontinuity (by adding aggregates of that size fraction) causes the loss of stone-to-stone contact of the coarse grains that form the SMA skeleton (Figure 2.7). This conclusion forms the basis of the U. S. method of SMA design and the so-called Bailey method (both presented in Chapter 7). The same conclusion has also been used in some Scandinavian solutions (e. g., in the real SMA concept).

A final remark—looking at Figure 2.4, we may be inclined to make a mixture of only one size fraction of coarse aggregate (8/12 mm) with mastic (0/2 mm). Obviously we would get a very strong aggregate skeleton, but we cannot forget the following technological consequences:

• Placement would be difficult; the coarse grains might be pulled by the screed plate.

• Compaction would be complicated; such a mixture is difficult to compact.

• We may have difficulty keeping the void content below 6% (v/v)* in the compacted surface course.

These reasons demonstrate the need for smaller, but still coarse, grains to participate in forming the skeleton. Further discussion on the features of such a mixture and anal­ysis of the influence of the coarse aggregate gradation will be found in Chapter 6.

to the original german method

The original German approach (by Dr. Zichner) to designing SMA aggregate blends is based on having adequate ratios of various aggregate fractions. In that context, the stone-to-stone contact is neither specifically analyzed nor controlled.

Since the beginning of the SMA concept, the ratios established by Dr. Zichner have been only slightly changed. Thus we may emphasize clearly that Dr. Zichner’s mixture is a really genuine SMA. The contemporary weight ratios of coarse aggre­gate fractions preferred in Germany (Druschner and Schafer, 2000) are shown in Table 2.1.

In the German design of the SMA coarse aggregate skeleton, all fractions of aggregate bigger than 2 mm are used. For example, for an SMA 0/11 we take not only aggregate sizes 8/11 and 5/8 but 2/5 mm as well. Manipulation of the ratios of these different sizes is required (see Table 2.1) to provide the desired skeleton by minimizing the share of 2/5 mm aggregate down to one part in seven (15%) of coarse aggregates’ mass fraction. Consequently the German SMA gradation curves have no sharp breaks related to the absence of successive aggregate fractions and do not exclude any fractions larger than 2 mm (i. e., all fractions of coarse aggregates are used). In a way, most of the original German gradations are дийш’-continuous grad­ings (all fractions are present in the mix), with a minimal share of specific coarse aggregate fractions. It should be emphasized that in most German guidelines the amount of coarse aggregates (bigger than 2 mm) are not very high. For example, for SMA 0/11 the lower limit of this fraction has been changed from 75% (in 1984) to 73% (in 2001). There were many reasons for this change, including better com – pactability, lower permeability, and improved rutting resistance.

The success of SMAs designed according to the ratios of aggregate sizes presented in Table 2.1 has been proven through long-term pavement evaluations, although these ratios have been refined now and then (within a limited scope). So the strength of the

German method is a designed SMA skeleton composition based on long observation and experimentation. (See Chapter 7 for a detailed description of the method used in Germany.)

Our aim in designing an SMA’s aggregate structure has already been identified—a strong skeleton of coarse grains. Let us now consider what requirements an aggre­gate mix has to meet to create such a desirable skeleton. There is no room in it for too many fine or weak grains. The key solution for that question is gap gradation—that is, the right proportion of grains of defined sizes.

Let us start by examining a continuous gradation. If we want to design an aggregate mix with a maximum density (or otherwise, with a minimum void con­tent), we should create it from an aggregate with a roughly equal share of grains from consecutive fractions. In other words, such a mix should contain a propor­tionally even quantity of all fractions. We would call this type of gradation a continuous gradation. The appearance of grains of different sizes makes closer packing in a volume unit possible. This also minimizes the volume of voids among grains. Asphalt concrete is an obvious example of a continuously graded mixture (Figure 2.5; solid line).

So what is a gap gradation? Gap gradation is a disruption in the occurrence of consecutive aggregate fractions in an aggregate blend; that disruption results from a lack or minimal amount of one or more aggregate fractions. Looking at Figure 2.4, we can see the formation process of a skeleton with coarse grains and some of the finest grains but without the sizes in between. Gap gradation means a lack or minimal share of specified fractions of intermediate aggregates. The role of the gap gradation is so essential that the lack of definite size grains must be evident. But which fraction or sizes of grains or fractions? Here we have a couple of definitions and methods.

What is the reason for developing mixes with stronger mineral skeletons? Surely it is because of heavier and heavier traffic. Not only have axle loads and traffic volumes grown steadily, but the structures of vehicle tires have also changed. The increased popularity of super single tires, for example, has changed the level of stresses applied to pavements. Obviously all those factors magnify the requirements for asphalt mixtures.

A well-known example can be used to present the SMA mineral skeleton struc­ture. If we put some coarse grains in a pot (Figure 2.3), compact and then load them, we shall obtain a structure with high compressive strength, depending on the aggre­gate’s fragmentation (crushing) resistance. A distinctive feature of such a compacted collection of coarse grains is the full and uninterrupted contact between them. This type of skeleton might be desirable for an asphalt surface mixture to provide a strong structure.

Now let us look at Figure 2.4, which presents a schematic showing how a load is carried by a mineral skeleton (of a surface course), assuming full grain contact. The transfer of load by adjacent grains through contact points between the coarse particles may be seen. If these contact points between coarse aggregate particles are not present, the finer particles will have to help carry the load, which results in the

(b)

development of a discontinuous load transfer by coarse, active grains and potentially weakens the whole structure.

Looking at Figures 2.3 and 2.4, we may notice one of the SMA’s characteristic features. During the compaction process on a construction site, the aggregate grains in the SMA skeleton are forced into direct contact. The coarse grains are brought into contact with each other, making the desired skeleton. Once that contact occurs, further compaction may be harmful. Why? Because it will crush grains. Let us look once again at Figure 2.3. Since the skeleton has already come into existence, further compacting will only lead to crushing grains. In other words, an SMA mixture has
to be skillfully compacted in such a way that the coarse grains are properly placed, securing stone-to-stone contact. This principle applies to compacting energy on a construction site, as well as in a laboratory.

To conclude our examination of the formation of an SMA skeleton, it is worth noting an idea put forth by Van de Ven et al. (2003), who said that there is probably no real, 100% stone-to-stone contact in a newly compacted SMA and that the coarse grains are separated from each other by the finest grains of filler, sand, and thin binder film. This means that there is an enlarging effect[3] of the volume of voids in the stone skeleton. The final arrangement of grains comes after some time under the influence of traffic and temperature of the layer. Small grains (sand and filler) can be crushed or moved, and the void content in SMA decreases.

Let us also bear in mind that the more stable a mineral skeleton is, the less sus­ceptible it will be to deformation. Even when the binder softens due to the increased temperature of the pavement, the layer will not necessarily be deformed if there is an adequate blend of aggregates. The weaker the skeleton and the higher the tempera­ture, the greater is the role of the mastic’s shear strength, and the more reason for reinforcing the mastic with polymers or special fibers.

In this part of the book we shall deal with the grains within the mix structure that are active in forming a coarse aggregate skeleton. The following significant questions will be answered, too:

• How is a stone matrix asphalt (SMA) mix skeleton formed?

• What does a gap gradation mean?

2.1 DEFINITION OF AN SMA AGGREGATE SKELETON

The notion of an aggregate skeleton of a mixture has a pretty broad meaning. Figure 2.1 shows various types of mineral mixes with different interactions of grains. These range from a sand mix, through mixes where the coarse aggregate particles occupy a more and more substantial share of the volume, and up to a mix consisting entirely of coarse aggregates. The types of aggregates that make up a continuous matrix and form a load-carrying component determine the specific group ranking.

While mixes consisting mainly of an aggregate with sizes up to 2 mm (or 2.36 mm in the United States) (e. g., sand asphalt) are rarely applied, sand-coarse aggregate mixes are within a continuum from mastic asphalt to asphalt concrete. SMA belongs in a coarse aggregates-sand group with porous asphalt, which is the type closest to the straight coarse aggregate type. Mineral mixes with exclusively coarse aggregates make so-called coated macadams, which are seldom used at present.

Now we deal with the first component of an SMA—the skeleton formed of coarse aggregates. Then in Chapter 3 we shall discuss the mastic made of a filler, sand, and binder.

First let us define the term coarse aggregate skeleton. A coarse aggregate skeleton is a structure of grains of suitable size that rest against each other and are mutually interlocked. In Europe, the coarse aggregates are generally taken to be those larger than 2 mm. Following the U. S. option, let us adopt an assumption that a fixed size of 2 mm (2.36 mm in the United States) is not necessarily the lower limit of the coarse aggregate fraction. Why? Well, to meet the structural requirements of a particular grain arrangement (layout), the mechanical resistance of the aggregate blend must be high enough to withstand loads. And this is related to the size of grains, among other factors. So let us agree that a skeleton is made up of adequate size grains but their lower limit is not necessarily equal to 2 mm (2.36 mm). Since the skeleton consists of various coarse aggregate grains, we often use the more universal term coarse aggregate skeleton.

Stone skeleton

FIGURE 2.2 Division of SMA into basic components: coarse aggregate (skeleton) and mastic.

To classify all grains of the mineral mix, they may be divided as follows (Figure 2.2):

• Those forming a skeleton (skeleton makers) and carrying loads

• Those filling in the voids in a skeleton and not carrying loads

That division also coincides with the following frequently used terms:

• Active grains (i. e., those forming a skeleton)

• Passive grains (i. e., those filling in voids)

The selection of a sieve to determine a skeleton maker is of great importance for the properties of a newly designed mix. That matter will be treated in detail in Chapter 6.

The content of SMA will be divided into the following parts (Figure 1.4):

• Coarse aggregate skeleton

• Mastic (i. e., binder, filler, fine aggregate, and stabilizer)

•

Voids in the asphalt mix

This division of SMA into parts (with the predominance of the main two—skele­ton and mastic) has been adopted to better explain the roles of each component of the SMA mix. A similar division may be found as a directly applied approach in some designing methods (e. g., the U. S. and Dutch methods).

The SMA components of the aggrate skeleton and mastic will be discussed in Chapters 2 and 3, respectively, and the significance of voids in the SMA mix will be discussed in Chapter 6, when we look at designing SMA, and in Chapter 9, when we examine the laydown (placement) of SMAs.

Now it is time to tackle the subject on hand!

To summarize the status of SMAs around the world in regard to the design of mix­tures and their aggregate gradations, two general trends consist of the following:

• German SMAs and others made by those more-or-less faithful to German guidelines have evolved somewhat based on long-standing systematic observations and experiences in SMA technology

• Research and development continues on new ways of designing SMAs; U. S. and Dutch techniques may be representative of that trend (see Chapter 7).

Various mixtures worldwide are referred to as SMA although they may differ greatly from Dr. Zichner’s invention. Some of these variants should, in principle, actually be called something besides SMA. Undoubtedly they are still gap-graded mineral mixes and SMA-like, but they are not identical with Dr. Zichner’s

Splittmastixasphalt.

In subsequent chapters, SMA composition will be discussed according to a scheme drawing on both trends. The existence of so many different attitudes toward the SMA mix has been problematic in selecting the right way to express the variety of opinions in a methodical and comprehensive way. That is why some specific refer­ences to particular trends may appear occasionally.