Category Stone Matrix Asphalt. Theory and Practice

One of the best-known applications of an SMA mixtures for airfields is the northern runway of the airport in Frankfurt on the Main (Fraport). It has become famous not only because of the SMA application but also because of the atypical arrange­ment of the reconstruction of the runway pavement. The 2-year long removal of the worn-out concrete pavement and the laying of a new asphalt structure was completed in 2005.

Fraport is one of the largest airports in the world, with a colossal traffic capac­ity and an impressive number of takeoffs and landings—more than 200,000 per year. The replacement of the northern runway pavement was carried out exclu­sively at night to reduce reconstruction-related air traffic problems. This necessi­tated organizational and technological arrangements to complete each stage of the reconstruction work early in the morning. The intended entrance to the work site was at 10:30 p. m. on a given night, followed by the first landing at 6:00 a. m. next day. The scope of work was divided into independent day lots (actually night lots), which had to be executed within a 7.5-hour break in takeoffs and landings.

The runway intended for rebuilding was 4000 m long and 60 m wide, for a total of 240,000 m2. The working lot reconstructed over one night was 15 m long. It took 300 nights to accomplish the pavement replacement. Work was started near the end of April 2003 and finished in June 2005. The old structure of the runway pave­ment comprised a set of cement concrete layers. All the new asphalt layers contained polymer modified bitumen (PmB) 25 or PmB 45, according to the German guide­lines for modified binders TL PmB 01. They were additionally supplemented with Fisher-Tropsch wax (Sasobit) to lower the compaction temperature of the asphalt. The Sasobit additive made possible the compaction of the asphalt mix at a lower tem­perature (usually at approximately 130°C) and faster cooling of the wearing course (the temperature after laying and compaction was about 100°C); this was critical since the mat had to cool within 1.5 hours prior to the first landing or takeoff at 6 a. m. The SMA 0/11 wearing course was 40 mm thick. Limestone with a hydrated lime additive was used as a filler. The SMA wearing course was spread at a speed of 5 m/min. Seven rollers were engaged in compacting the mat. At the moment of the first landing, the temperature of the pavement ranged between 52 and 79°C (Sasol GmbH, undated).

The replacement of Fraport’s northern runway pavement was successfully completed. At the same time it became one of the best-known runway reconstruction projects carried out without the disruption of transport services.

Despite the fast growth of applications on road pavements, SMA does not enjoy great success on airfields. This is because of the unique requirements for airfield pave­ments and special problems with SMA, which include the following:

• Combining high macrotexture with low permeability

• Low-initial antiskid properties and the impossibility of using gritting (FOD risk)

• Risk of segregation (e. g., fat spots, porosity) locally changing the surface properties

Regardless of the cited weak points of SMA, airport managers sometimes decide to apply it, taking into consideration the strong points known from the road industry. A comprehensive survey of SMA applications on airports may be found in Prowell et. al. (2009).

Requirements concerning wearing courses of airfield pavements are defined as fol­lows (namely, in the regulations of the U. S. Federal Aviation Agency (FAA) Advisory Circular AC No. 150/5370-10B):

• Be impermeable to water and suitably protect intermediate course.

• Provide surface free of foreign object damage (FOD) (i. e., loose particles).

• Resist loads from aircrafts.

• Be smooth, with uniform surface.

• Maintain required antiskid properties.

• In specific areas should be resistant to spillage of fuel, hydraulic and deic­ing fluids, or other solvents.

Guidelines on specifications for pavements on civil installations are contained in

International Standards and Recommended Practices, AERODROMES. Annex 14 to the Convention on International Civil Aviation, Volume I—Aerodrome Design and Operations by the International Civil Aviation Organization (ICAO). The ICAO regulations regarding civil aerodromes mainly define the features of pavements affecting air traffic safety—minimum level of friction, sufficient smoothness, lack of presence of any loose particles (i. e., FOD) or grains bigger than 3 mm resting on the pavement.

The rules of the FAA and ICAO govern the methods and frequencies of testing the friction and macrotexture. Methods of permanent measurement of the friction factor described in both documents will not be referred to here. Measurements of macro­texture are usually carried out either with calibrated sand (sand patch test) or with the application of special greases (NASA grease smear method). The minimum sur­face texture depth recommended by the ICAO is 1.0 mm. It is worth noting here that the essential task of macrotexture is to enable water discharge from the pavement so that a layer of water does not build up between a wet runway and an aircraft’s tire (hydroplaning).

Bearing in mind that a newly paved, ungritted SMA layer is marked by a high degree of slipperiness, some problems with achieving the intended level of friction may develop. On the other hand, gritting, even when followed by sweeping with mechanical brooms to remove unbonded grains, gives rise to a real threat to air­planes due to FOD; thus gritting has generally been ruled out. So the proper level of friction to be obtained by an adequate grain size distribution and the application of coarse aggregate with a high polished stone value (PSV) index.

Requirements for asphalt mixes for airfields mainly comply with specifications for highway engineering in some countries; however in other countries, special tech­nical requirements have been drawn up specifically for airfields. They differ from those for highway applications. But, in each case the ICAO specifications for the finished pavement should be satisfied.

One should also remember that binders applied to specified parts of airfield pave­ments should be checked for resistance to fuel and deicers because of the destructive actions of these substances. Some manufacturers of road binders offer special prod­ucts for such applications. Also some research has been done in this area (Steernberg et al., 2000). The following two documents in a series of European Standards estab­lished adequate methods of testing:

• EN 12697-41, Asphalt Mixtures—Test Methods for Hot Mix Asphalt—Part 41: Resistance to Deicing Fluids

• EN 12697-43, Asphalt Mixtures—Test Methods for Hot Mix Asphalt—Part 43: Resistance to Fuel

According to EN 13108-20, the aforementioned tests of resistance to deicing fluids and fuels are specifically applicable to SMA used on airfields.

This chapter describes some special applications of stone matrix asphalt (SMA). Several brief case studies illustrate some of the less common, though advantageous, ways SMA mixtures can be used and also suggest some areas where they should be used with caution.

13.1 AIRFIELD PAVEMENTS

According to a report by the European Asphalt Pavement Association (EAPA), asphalt surfacing covers the majority of runways (EAPA, 2003). The high perfor­mance of SMA pavements has been attracting the attention of airport management, creating the chance to apply SMA technology in wearing courses of airfield pave­ments. Numerous trial sections have sprung up on various airfields (e. g., Sydney, Australia, and Johannesburg, South Africa). Some important, larger applications of SMA may be seen in Frankfurt on the Main, Germany, and Gardermoen near Oslo, Norway. The airfield in Frankfurt will be described in detail later on, not only for the SMA technology itself but for its application.

In most countries, SMA mixtures are more expensive to construct than comparable mixtures of AC. Higher initial prices result from the application of the following:

• Larger amounts of binder (or a PMB)

• Greater amounts of added filler

• Large quantities of high quality coarse aggregates

• Stabilizers (most often fibers)

• Higher production temperatures

• Lower outputs of asphalt plants

The approximate price difference amounts to + 20% to + 30%, depending on the country and specificity of the placement. However, such a difference in price is accepted by road administrations owing to the better durability of SMA pavements. It is widely assumed that their average lifetime amounts to at least 20 years. In many countries it is difficult to verify this service life due to the small number of SMA sections that have been in place longer than 15 years.

It can be assumed that the higher initial costs of SMA mixtures have been com­pensated for by their longer durability and lower maintenance costs. Taking into account the lower costs of operation due to the absence of repair needs and hence fewer traffic disruptions for road users, the economical efficiency of SMA is higher than that of the classic AC.

The European Asphalt Pavements Association and Eurobitume report of 2004 entitled Environmental Impacts and Fuel Efficiency of Road Pavements (Beuving et al., 2004) is one of the main sources forming the basis for this section of the present chapter.

Rolling resistance is one of the numerous factors of intense interest when con­sidering the problem of fuel consumption while driving, especially in the context of SMA. Rolling resistance may be defined as the force necessary to move a vehicle along a pavement.

At a constant speed of 80 km/h, approximately 12% of the energy loss (fuel consumption) of a heavy truck is consumed in overcoming the roll­ing resistance; the energy spent on overcoming this resistance equals about 30% of the potential available mechanical power at the engine crankshaft (Sandberg, 2001).

Briefly, the rolling resistance is affected by microtexture, macrotexture, megatex­ture, and the unevenness of a wearing course.

According to Dutch research (Roovers et al, 2005), rolling resistance can be ranked by type of wearing course (results in cR[%]) as follows:

Cement concrete with burlap (smooth)—0.86 SMA 0/8—0.86

Double layer porous asphalt 2/6—0.97

Double layer porous asphalt 4/8—1.02

Cement concrete transversely brushed (rough)—1.04

Single layer porous asphalt 6/16—1.05

Dense AC 0/16—1.09

Mastic asphalt 0/11—1.18

Finally one should remember that rolling resistance measurements are strongly influenced by weather (e. g. sidewind velocity). Weather conditions could affect the test results.

Swedish investigations (Sandberg, 2001) have additionally pointed out that the pavement unevenness increases fuel consumption by as much as 12%, which seems to be a more significant factor than the type of asphalt surfacing.

Figure 12.10 shows the results from tests of water permeability of an SMA mix­tures with gradations 0/4.75, 0/9.5, and 0/12.5 mm (Cooley and Brown, 2003). All instances concern U. S. SMA, previously described in Chapters 6 and 7.

Figure 12.10 clearly shows the relationship between the maximum aggregate size of the SMA and the probability of its being permeable. The larger the maximum aggregate size of the SMA, the higher the probability of its being permeable. Thus the size of the maximum particle is a decisive factor for permeability of an SMA

mixture and so is the gradation within the coarse aggregate fraction (driven by the breakpoint sieve using the U. S. definition). An increase in this factor is followed by the growth of the size of internal pores, and consequently, the probability of their con­nection (Cooley and Brown, 2003). It has been stated in research (Cooley et al., 2002) that SMA mixtures are characterized by a higher potential for permeability than AC mixtures with the same content of air voids. Investigations of permeable pavements in Florida in the United States led to a definition of permeable mixtures as those mixtures with field permeabilities greater than 100 x 10-5 cm/sec[71] (Choubane et al., 1998). SMA mixtures may be prone to this excessive permeability, particularly those with a nominal maximum aggregate size greater than 10 mm.

For many people the surface appearance of an SMA course gives the impression of being excessively water permeable. Indeed, SMA differs from AC due to a substan­tially deeper macrotexture. SMA courses with an increased content of voids may occasionally be seen in practice, most frequently when the mixture was improperly compacted on a work site (such cases were elaborated on in Chapter 11). However, there is more to permeability than work site error.

Permeability is related to the content of air voids and the size, distribution, and existence of interconnections between internal pores. The permeability of a mixture depends on the maximum aggregate size, the thickness of the course, and the level of compaction (WsDOT, 2005). Research has shown that the larger the maximum aggregate size in a mixture, the higher its permeability (Cooley et al., 2002), which means an increase in maximum aggregate size results in the growth of the pore sizes and an increased probability of their connection (Cooley and Brown, 2003). The influence of VMA is also seen; permeability decreases as VMA increases for constant air voids (Brown et al., 2004).

To reduce the permeability of courses, one should appropriately match the mix­ture gradation to the course thickness so that the ratio of thickness to the maximum aggregate size in a mixture is not less than 3.0 (a ratio closer to 4.0 is preferred for SMA with a strong gap gradation). This should make the compaction of the course easier, reduce the content of voids, and limit connections among pores (Cooley et al., 2002). The following list summarizes the important points regarding permeability:

• The larger the maximum aggregate size in a mixture, the higher its permeability.

• The higher the content of air voids in a course, the higher its permeability; however, mixtures with the same content of voids can have different levels of permeability.

• The gradation of the SMA mixture (more or less gap graded) influences the permeability.

• The thicker the course, the lower its permeability.

The safety provided by wearing course mixtures is of primary importance. The anti­spray properties of some mixes enhance safety by preventing the buildup of water mist behind vehicles. After rainfall (or melting snow), drops of water are raised up from a pavement by vehicle tires, especially the tires of large, fast-moving vehicles. Consequently, a certain amount of water in the form of mist remains suspended in the air surrounding fast-moving vehicles (Figure 12.7). Then it settles on wind­screens, reducing visibility.

The generation of water mist and splash may be decreased by enabling quick water discharge after rainfall (through transverse cross-falls and having no ruts to hold water), a suitable depth of macrotexture, or a high content of air voids in a course. The last condition refers to open-graded friction course or porous asphalt. Water discharge through the whole thickness of a course is out of the question in the case of SMA. It is possible only through spaces among coarse aggregates sticking

up from the surface of the course; hence, the proper SMA macrotexture is all impor­tant (Figure 12.8). Investigations conducted in the United States have proved that SMA courses reduce water splash in comparison with AC pavements; nevertheless, water remains longer on the SMA course where it can be held in the surface voids (Schmiedlin and Bischoff, 2002). The issues of SMA macrotexture were discussed in the Section 2.7 devoted to antiskidding properties.

Furthermore, macrotexture that reduces the water film on the surface of a wear­ing course is also of significance to the improvement in visibility of road mark­ing after or during rainfall. Moreover, at night the light reflection (“glistening”) of vehicles travelling in the opposite direction is reduced. Figure 12.9 shows an image

of wearing course surfaces made of SMA and AC after rainfall and their light reflec­tions (during the daytime). The difference in the way water is discharged as a result of the surface macrotexture is clearly noticeable.

Durability tests of asphalt mixtures, especially those intended for wearing courses, have become commonly used around the world (Hobeda, 2000; Sybilski and Mechowski, 1996). The above-average durability of SMA has been emphasized in numerous publications so they will not be described here. This durability is the result of a high binder content and a thicker binder coating on an aggregate that has the same content of air voids as AC.