Category Stone Matrix Asphalt. Theory and Practice

Earlier, common practice has designated porous mixtures as the most effective way of reducing traffic noise. One – or two-layer pavements have been used, while the latter ones generally are a more effective option. However, that is an expensive solu­tion. On the other hand, it has been found (von Bochove and Hamzah, 2008) that gap-graded mixes composed in accordance with the SMA concept—with an air void content of 9-16% (v/v)—provide a worthwhile alternative to porous asphalt in urban areas. They are marked by a higher resistance to the loads occurring in urban traf­fic conditions, a longer service life, and good noise reduction properties (up to -5 dB[A]). At the same time, the authors have added that such a mixture cannot be a conventional SMA, but it has to distinguish itself by a significant gap grading and a strong skeleton of coarse particles.

The concept of a “silent” SMA (SMA LA), which is being developed in Germany, is an example of such a solution. Some test sections on roads in Bavaria (Germany) made of SMA LA mixtures 0/5 and 0/8 have been described (Gartner et al., 2006). The following are the expected values of SMA LA:

• Content of voids above 10% (v/v)

• Gap-graded aggregate mix

• Grading 0/5 mm for layers 15-25 mm thick

• Grading 0/8 mm for layers 20-30 mm thick

The SMA LA course is not gritted since noise reduction has been given high priority. Very positive results of skid resistance with the SKM (SKM – Seitenkraft- Messverfahren – Griffigkeit) (results greater than 0.58) method have been achieved, and noise tests with close-proximity (CPX) method have yielded reductions both at 80 km/h and 120 km/h. Finally the SMA 0/8 LA mixture has turned out to be more effective in noise reduction than the SMA 0/5 LA. Figures 13.5 and 13.6 depict the grading curves of SMA 0/5 LA and SMA 0/8 LA.

Also, in Denmark the road administration, together with industry and consul­tants, has created a system of classifying the noise reduction effects of various types of asphalt surfacings (Andersen and Thau, 2008). Assessment of the surfacing is carried out according to the CPX method at two speeds, 50 km/h (reference noise level 94.0 dB[A]) and 80 km/h (reference noise level 102 dB[A]). It is worth noting that in Denmark two types of SMA—6 + SRS and SMA 8 SRS—are used for noise – reducing asphalt surfaces as follows:

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SMA 6 + SRS—maximum aggregate size of 8 mm, air voids between 4 and 10% (v/v), ratio of binder volume to aggregate volume of at least 0.18, minimum thickness of 45 kg/m2

• SMA 6 + SRS—maximum aggregate size of 11 mm, air voids between 3 and 10% (v/v), ratio of binder volume to aggregate volume of at least 0.17, minimum thickness of 50 kg/m2

• SMA 8 SRS—maximum aggregate size of 8 mm, air voids between 4 and 12% (v/v), ratio of binder volume to aggregate volume of at least 0.18, mini­mum thickness of 55 kg/m2

These requirements are part of the first generation specifications used in tenders (contracts) in Denmark.

In some countries, SMA mixes have been applied in lower layers of the pavement structure. They are usually coarse-graded SMA mixes from 0/16 to 0/22 mm. Such solutions have been tested in the United States and recently in Germany, where they are called Splittmastixbinder (SMB) (Gartner et al., 2009; Schunemann, 2006). Because of the high binder contents, commonly a polymer modified binder with a stabilizer as well, the fatigue properties of the pavement are definitely better than those of conventional asphalt concrete. The application of a strong aggregate skel­eton increases the resistance to rutting.

In many cases, SMB 0/16 with a hard modified binder may be better than a conventional asphalt concrete layer. Experimental roadway sections in Bavaria (Germany) on the highway A73 are good examples of such an application (Gartner et al., 2009). The SMB mix has been designed with target air voids in Marshall specimens between 3.5 and 4.0% (v/v) and a minimum binder content of 5.0%. A hard modified binder (pen@25°C = 10/40, SP > 65°C) has been used. Figure 13.4

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FIGURE 13.4 Grading curves (limits) of the SMB mix used on Highway A73 in Germany. (Data from Gartner, K., Graf, K., and Schunemann M., Asphaltbinderschichten nach den Splittmastixprinzip. Strasse und Autobahn, July 2009 With permission.)

shows the grading curves of the designed SMB mix. The thickness of the com­pacted layer has been defined as 7.5 cm, the required compaction factor greater than or equal to 98%, and the content of air voids in the finished course from 2.0 to 6.0% (v/v).

The test results on SMB mixes with hard grade polymer modified binder have confirmed that an intermediate course of this type can be a better solution than the conventional asphalt concrete; with a similar resistance to rutting, the fatigue life of SMB is remarkably better (Schunemann M., 2006).

SMA mixes have performed well in wearing courses, so it was a natural process to test them in the remaining layers of a pavement. As everybody already knows, SMA has proved to be a good material for these places. Therefore, SMA has found its way

to intermediate layers. Also research on a special type of SMA for low-noise pave­ments is in progress in Germany.

Besides classic SMA mixes, the method of designing a strong mineral skeleton has encouraged many road engineers to carry out their own trials on new mixes. One of them is Kjellbase, though it is not a true SMA mix.

The latest atypical SMA applications are colorful mixes.

Considering the technology of thin SMA layers, but not only gradation of the aggre­gate blend, but the quantity and type of binder as well, must provide a mix that can be placed by mechanical spreading of a layer up to approximately 4 cm (usually less than 3 cm) thick using a standard paver. Furthermore, the component materials and the final mix itself have to produce a layer that has the following qualities (Sybilski and Styk, 1996):

• Is impervious to water and deicers (excluding porous asphalt)

• Has a suitably high coefficient of friction

• Is resistant to permanent deformation

• Is resistant to low-temperature cracking

• Is resistant to fatigue

• Has the potential to reduce traffic noise

It is also worth remembering that, despite their many strengths, thin courses neither reinforce the pavement substantially nor solve the problem of fatigue (net) and reflected (transverse) cracking (Pandyra et al., 1994). Many countries have their own original technological solutions for courses less than approximately 4 cm thick, which usually consist of various gap-graded mixes, including SMA.

Finer SMA (e. g., 0/5, 0/8) mixes are used for thin layers rather than the 0/11 mm and 0/12.5 mm SMAs. These finer SMAs are noted for many good points, such as similar or only slightly worse resistance to permanent deformation compared with 0/11 mm and 0/12.5 mm mixes. The finer SMAs are also characterized by lower water permeability at the same void contents as coarser SMAs (Cooley and Brown, 2003). Additionally, higher contents of binder in the finer mixes lead to an increase of durability and improved mix workability. If properly designed, finer SMAs tend to reduce the appearance of fat spots, so there is a possibility to reduce the stabilizer content.

The following remarks about SMA technology deserve mention: [72]

• Excessively heavy rollers should not be used because they may crush aggre­gate grains; vibration can only be used occasionally and with great care; on ultra-thin courses the vibrations should be turned off.

• Attention should be paid to temperature drops in the mix during its spread­ing because thin layers are very susceptible to fast cooling caused by cool crosswinds or a cold sublayer.

• Almost always, a thin SMA layer can be opened to traffic sooner than can a conventional thickness layer; in the case of ultra-thin layers, owing to their rapid cooling, opening to traffic can be done in as little as 30 minutes after the end of compaction (Carswell, 2002).

Thin SMA layers have been used all over the world. Descriptions from Argentina, the United States, the United Kingdom, Sweden, Poland, and other countries are available in literature (Bolzan, 2002; Carswell, 2002; Carswell, 2004; Cooley and Brown, 2003; Richardson, 1997). An interesting review of the performance (e. g., macrotexture, skid resistance connected to aggregates’ PSV, visual condition) of thin layer sections after 15 years can also be found (Nicholls et al., 2008).

Examples of thin SMA layers evaluated after a minimum of 10 years in opera­tion can be the most interesting. One case in point may be the wearing course on the DK3 route in Poland. The 2.5- to 3-cm thin SMA layer of 0/6.3 mm grading was laid in 1993. It was still in very good condition after 13 years in operation (Figure 13.3).

The bridge in Wroclaw, Poland, is a steel construction with short spans but sub­stantial deflections and vibrations. A completely new pavement was laid in 1997. It consisted of an asphalt mastic protection layer (2 cm) and two 0/8 mm SMA layers—the first one as an intermediate layer and the second one as a wearing course. The mastic layer was spread manually, whereas both SMA courses were

laid mechanically. An SBS modified binder—with 50/80 Pen@25°C, SP > 53°C, and ER > 50%—was used in all the asphalt layers. The condition of the pavement after 13 years in operation was still good, with no cracks or potholes (Figure 13.2.b). Slight rutting was observed (the bridge is located at the approach to a crossing with traffic lights), but repair was needed only in the area of the joints.

The bridge in Plock, Poland, is a steel, highway and railway, multispan bridge with a significant longitudinal slope. During reconstruction in 1998, a new sprayed pro­tection layer was applied and one layer of 0/16 mm SMA surfacing was laid down. An SBS modified binder—with 50/80 Pen@25°C, SP > 63°C, and ER > 80%—was used. It was an experimental SMA application of 0/16 mm grading on a steel bridge. Its surface integrity, after 2 years in operation, was at least a warning. Large areas of cracks and slight rutting could be seen here and there (Figure 13.2.a). The pavement will have to be reconstructed soon. This rapid pavement failure was probably caused by poor adhesion of the course to the protection layer and high permeability resulting from the use of the coarse 0/16 mm grading.

Another example of an SMA bridge application in Europe is found on one of the longest bridges in Europe—the Great Belt Link connecting Denmark with Sweden.

FIGuRE 13.1 Grading of the SMA aggregate mix, Roosteren design. (Data from von Brochove, G. G., Voskuilen, J., and Visser, A. F.H. M., Proceedings of the 4th Eurasphalt & Eurobitume Conference, Copenhagen, paper 402-102, 2008.)

It is a 6600-m long, prestressed concrete structure with an asphalt pavement consist­ing of the following courses (Wegan, 2000):

• 15 mm open graded asphalt concrete (drainage layer)

• 40 mm thick asphalt concrete (protective course)

• 40 mm thick SMA (wearing course)

The expected lifespan of the SMA layer on this bridge is 25 years.

What follows are descriptions of a few applications of SMA-type asphalt mixes for pavements on steel bridge decks. It has been noted before that this type of bridge construction poses the greatest challenge for the asphalt pavement.

13.2.1.1 Bridge in Roosteren, the Netherlands

In 2005, in Roosteren, the Netherlands, an experimental SMA pavement was made with mastic containing a binder that was highly modified with elastomer (Pen@25°C = 50/70, SP > 90°C) instead of typical mastic asphalt (von Brochove et al., 2008). Demanding requirements, as follows, were required of the SMA mix to compare it with mastic asphalt:

• Resistance to permanent deformation measured by the triaxial compression method after EN 12697-25

• Cracking resistance by the method based on the semicircular bending test at 0°C and 5°C after EN 12697-44

• Fatigue limit, four-point bending test, prismatic sample (4PB-PR) at 5°C after EN 12697-24

• Stiffness, four-point bending test, prismatic sample (4PB-PR) at 5°C after EN 12697-26

The test results proved that the designed mix had a very high fatigue limit. It was laid down in one layer with gritting performed. The grading of the SMA aggregate mix is shown in Figure 13.1.

Surfacing on bridge structures is not, and should not be, like that of a standard pavement on a soil subgrade. The essential difference lies in a different mode of operation. There are special circumstances that must be considered, including the following:

• The cooling and warming effect developing from underneath the bridge deck pavement caused by changes in air temperature under the steel struc­ture and faster changes of the pavement temperature due to wind action, which occurs faster and more intensely than in case of a pavement on grade

• Structural deflections of a bridge’s deck caused by passing vehicles

• The amplitude of bridge deck vibration, which is much higher than that of conventional road pavement

• Much more intensive applications of deicers, leading to the quick degrada­tion of asphalt mixes applied on bridges

For all these reasons, asphalt pavements on bridge decks are subjected to faster deterioration than their soil subgrade equivalents. Therefore, when designing a com­bination of bridge pavement courses, some additional points have to be observed as follows:

• The critical element influencing the pavement service life is the durable bonding of all the layers together (asphalt courses with a protection layer and the deck).

• The more flexible the structure, the more elastic the asphalt layers should be.

• Good compaction of the layers should be taken into consideration because it results in low-water permeability, although rolling on a low-stiffness bridge is challenging.

The deflections of orthotropic plate structures are usually higher than structures with cement concrete deck slabs. Consequently, when asphalt mixes are constructed on steel orthotropic structures, the most frequently applied asphalt mixes are those with the highest fatigue strengths (e. g., mastic asphalt with a highly modified binder) (Damm and Harders, 2000). In some countries, fine-graded SMA has been also used (see Section 13.2.1). Some interesting concepts and analyses can be found in several papers dealing with this subject (Huurman et al., 2003; Medani, 2001a; Medani, 2001b).

During the 1990s, a few airfield pavements paved with SMA mixtures were constructed in Norway. Various binders and additives were used, depending on the climatic zones in which they were placed. The biggest of these airfields with an SMA pavement is Gardermoen near Oslo. On Gardermoen’s runways, 4-cm thick SMA mix of 0/11 mm were placed on the western runway) and 4-cm thick SMA mix 0/16 mm was placed on the eastern runway. Two SMA runways, 3300 m and 2950 m long, were constructed there with styrene-butadiene-styrene (SBS) modified binders (Larsen, 2002).

13.1.3 Johannesburg Airport

In 1999 the following comparative trial sections of different mixes applied in new asphalt wearing courses were laid down at the international airport in Johannesburg, South Africa (Joubert et al., 2004):

• 0/19 mm asphalt concrete (continuous graded coarse mix)

• 0/9 mm SMA with tested parameters: binder content 7.1% (40/50 Pen type), voids in mix 5.7%, stability 6.7 kN, flow 4.0 mm, passing by sieve 2.36 mm 17%, density in place 92%

• 0/13 mm porous asphalt concrete

These mixes were also compared with the existing old wearing course of continu­ously graded mix.

All sections were located in the landing area, a zone of heavy dynamic loads. Tests were aimed at determining the practicality of various mixes, assuming ungrooved pavements. The surface integrity of the section of pavement and its surface proper­ties were inspected periodically. Special attention was paid to antiskid properties, lifespan, and the buildup of rubber with time (worn-off the airplane tires). The results for the SMA section were as follows:

• Grip number—initially 0.64, after 5 months 0.71

• Surface texture—initially 1.33 mm, after 5 months 0.9 mm

The summary of SMA’s performance on the trial section pavement proved that the surface properties of the tested SMA layer were better than the other mixes tested in the trial sections. The porous mix was also recognized for its good characteristics, with the exception of its durability, which was the lowest of those tested here. The conventional asphalt concrete pavement demonstrated poor antiskid properties and therefore needed grooving (Joubert et al., 2004).