Category Stone Matrix Asphalt. Theory and Practice

The notion of compactability has been repeatedly used in this text, especially in Chapter 10. Compactability (i. e., susceptibility to compaction) can be defined as the ability of an asphalt mixture to change density under the influence of compactive effort; or to put it another way, compactability is the material feature determined by the amount of energy necessary for the compaction of a given mass into the smallest volume (Schabow, 2005). Resistance to compaction is the reverse of compactability. Generally, it means that compactable mixtures (with a low resistance to compaction) do not need a lot of compactive effort.

Resistance to compaction is connected with features of a mixture, such as the gradation of the aggregate mix and the aggregate properties, which include the following:

• Content of crushed stone

• Particle microtexture

• Particle shape

• Hardness (resistance to crushing and wearing)

Additionally, resistance to compaction is also affected by the content and type of binder and its viscosity at the compaction temperature.

Compactability is tested in Europe in accordance with the European standard EN 12697-10, which provides for the following two methods for testing and assessing the compactability asphalt mixtures: [70]

The following equipment can be used for testing compaction resistance and com­parability according to EN 12697-10:

• Impact compaction (Marshall hammer) according to EN 12697-30

• Method I—results as compaction resistance C (units [42 Nm])

• Method II—results as compaction resistance T (units [21 Nm])

• Gyratory compaction according to EN 12697-31

• Method I—not used

• Method II—results as compactability K (dimensionless)

• Vibratory compaction according to EN 12697-32 (not applicable to SMA)

• Method I—results as compactability k (dimensionless)

• Method II—not used

A fairly comprehensive range of methods and test parameters can be found in publi­cations on SMA fatigue properties. For example, in Australian research (Stephenson and Bullen, 2002) the strain-controlled mode was used to conduct a four-point bend­ing beam test at 20°C, with continuous haversine loading at a frequency of 10 Hz and a range of strain levels from 100 to 1000 pe. On the grounds of the test results, it can be stated that the fatigue limit of an SMA mixture is higher than a comparative specimen of AC.

12.1 WORKABILITY

The concept of workability has been used for determining a series of mixture prop­erties significant at the time of placement of a pavement. Workability is the property that determines the ability of a mixture to be placed mechanically or spread manu­ally and finally compacted (Asphalt Institute Handbook MS-4, 1989; Gudimettla et al., 2003). Naturally, compactability is a feature of less extensive significance, so it is a reflection of workability.

Obviously, workability is affected by the content and type of binder and the mix­ture temperature. It has been stated in U. S. research (Gudimettla et al., 2003) that workability is also influenced by the properties of the aggregate mix and the maxi­mum particle size.

12.2 COMPARABILITY

Fatigue is an effect consistent with the formation of cracks in material caused by a series of repetitive tensile stress cycles that do not exceed the tensile strength of the material. (For more information on fatigue, refer to the many publications with descriptions of this phenomena [e. g., SHRP Reports A-312 or A-404].)

12.3.1 Test Methods

There are many methods for testing fatigue; for example, the European standard on testing fatigue EN 12697-24 has quoted the following ones:

• Two-point bending test on trapezoidal-shaped specimens (2PB-TZ)

• Two-point bending test on prismatic-shaped specimens (2PB-PR)

• Three-point bending test on prismatic-shaped specimens (3PB-PR)

• Four-point bending test on prismatic-shaped specimens (4PB-PR)

• Indirect tensile test on cylindrical-shaped specimens (IT-CY)

Note: the standard EN 12697-24 has clearly stipulated that results obtained with various methods are not comparable; also, the standard EN 13108-20 has limited fatigue tests exclusive to AC mixtures (as a part of initial type testing).

Extended comparisons of fatigue test methods can be found in the literature (di Benedetto et al., 1997; Said and Wahlstrom, 2000).

Fatigue tests are carried out under one of the following modes of loading:

• Stress controlled

• Stress is induced in a specimen and is held throughout the test; strain steadily increases with the loading cycles until failure of the specimen occurs, which signals the end of testing.

• The fatigue limit is proportional to the mixture’s stiffness.

• Strain controlled

• Strain is induced in a specimen and is held throughout the test; stress steadily decreases with the application of loading cycles until the speci­men’s stiffness reaches 50% of its initial level.

• The fatigue limit is inversely proportional to the mixture’s stiffness.

A significant impact of mixture type on cracking temperature for newly made mixtures or after a short time of aging has not been observed with TSRST test­ing mixtures of AC, SMA, and porous asphalt (Isacsson et al., 1997). Not until a longer aging period (i. e., more than 25 days) has elapsed has a difference in favor of AC and SMA been observed. Porous asphalt is more susceptible to aging, and the recorded difference of crack temperatures has reached 6°C and 25°C after 25 and 100 days of aging, respectively. Test results (Judycki and Pszczola, 2002) con­cerning a comparison of low temperature properties of various mixtures have not revealed essential differences between AC and SMA. In a U. S. site investigation (Schmiedlin and Bischoff, 2002), the comparison of capabilities to slow down the advancement of reflective cracking in various mixtures concluded that SMA is superior to AC. The impact of aggregate size and its resistance to crushing has also been noted. Those conclusions are based on a 5-year observation of test sec­tions. Similar results have been achieved on a test section in Australia (Pashula, 2005) where, among other things, the abilities of various mixtures to slow down the advancement of reflective cracking were compared. It was stated after a 10-year observation that SMA’s distinctive feature is that it possesses the greatest potential of slowing the occurrence of reflective cracking.

The TSRST method consists of attaching the ends of a specimen (250 mm long and 60 mm in diameter or 50 x 50 mm cross section) to the frame of a device situ­ated in a cooling chamber. The frame is rigid to keep the length of the specimen unchanged. As the temperature falls (at a rate of -10°C/hr), the specimen contracts and the tensile stress in the specimen rises because it is being held by the frame. The temperature at which the specimen cracks is the test result. This method has been standardized in AASHTO TP 10 and prEN 12697-46.

12.2.3.1 Semicircular Bending Test

The semicircular bend test method is described in prEN 12697-44. A half cylinder sample of compacted asphalt mixture is loaded using a three-point bending scheme. As a result, tensile stress is created at the bottom of sample. This test can be used either for testing fracture toughness (when the half cylinder has a crack sawed at the bottom center with a notch width of 0.35 ± 0.10 mm and a depth of 10 ± 0.2 mm) or for tensile strength (unnotched cylinder).

Reflective cracking is a well-known weak point of semirigid pavements. It advances upward from a rigid base through the asphalt layers. Various techniques for coun­tering reflective cracking (e. g., Stress Absorbing Membrane Interlayer (SAMI) and Stress Absorbing Membranes (SAM) membranes, geogrids) have been used. But their effect usually comes down to more or less effectively slowing the growth of cracking, not preventing it from occurring. Asphalt courses with increased crack resistance are characterized by considerable shear and tensile strength. These properties may be achieved through the appropriate selection of the gradation of an aggregate mix, the type of binder and possibly the mastic strengthening additives.

12.2.3 Test Methods for Crack Resistance

Different methods for testing crack resistance have been used, and so far there has not been one commonly regarded as the dominant standard. There are some popular methods such as the thermal stress restrained specimen testing (TSRST; discussed later), local procedures used by specific research centers (e. g., Judycki, 1990), and methods under standardization, such as the semicircular bending test (Krans et al., 1996; Molenaar and Molenaar, 2000). Test methods for asphalt binders (e. g., the BBR method) have also opened up some new possibilities. In addition, the oldest engineering method—namely, observation of test road sections—is still in use.

Low temperature cracking and reflective cracking will be discussed next. More infor­mation on the theory and origin of cracking can be found in the literature (Arand, 1996; Jacobs et al., 1996; Rigo, 1993).

12.2.1 Low Temperature Cracking

Low temperature cracking induced by a drop in temperature has been well-documented (e. g., Fabb, 1973; Isacsson et al., 1997; Marasteanu et al., 2004; Tuckett et al., 1969). Cracking of an asphalt course appears when the thermal stress, which increases with a drop in the temperature, exceeds the mixture’s tensile strength. It originates at the surface of the wearing course and advances downward.

An overview of factors influencing the development of low temperature cracking has been presented in the literature (Isacsson et al., 1997). Although binder proper­ties have commonly been regarded as responsible for a pavement’s susceptibility to this type of cracking, there are actually many other, though less common, contribut­ing factors (e. g., the content of voids and mastic).

Rutting resistance is one of the most widely tested properties of asphalt mixes, including SMA, and various methods of testing are conducted all over the world.

The impact of side support during creep tests of SMA and AC have been com­pared in Swedish research (Said et al., 2000). Results have explicitly shown the great significance of that feature to SMA, while the results from testing AC with and without side support have not changed considerably (Figure 12.3). Other research (Ulmgren, 1996) on the comparison of the dynamic creep test (RLAT 100/100 mm) and the modified one (IRLAT 100/150 mm) with the results of a wheel-tracking test have demonstrated a very good relationship between IRLAT and the wheel-tracking test, with R = 0.91, while the relationship between RLAT and the wheel-tracking test was much worse, with R = 0.36.

U. S. research (Cooley and Brown, 2003) on SMA mixtures used for thin wearing courses has proved the advantages of conforming with requirements for resistance

to deformation (based upon APA testing), despite the maximum gradation of the SMA not exceeding 9.75 mm. British results (Obert, 2000) concerning the com­parison of SMA with hot rolled asphalt (HRA) have confirmed a higher resistance to deformation more for the former than the latter. Other research undertaken in Finland on test sections (Kelkka and Valtonen, 2000) compared SMA with AC, graded 0/18 mm with various binders. This time SMA turned out to be the win­ner again. Polish research (Sybilski and Horodecka, 1998) has acknowledged that creep tests are not appropriate for determining SMA resistance to deformation and also that better SMA rutting resistance has been proved in comparison with AC on road sections and in wheel-tracking testing (large device—French LCPC type).

Briefly, to sum up this short review of test results with regard to rutting resistance, it may be concluded that they reflect SMA’s superior properties over those of AC.

The APA is the second generation of Georgia loaded-wheel tester (GLWT) used in the United States for testing resistance to deformation of asphalt mixtures. The test temperature depends on the climatic data for the region where the mixture will be placed, and it is usually close to the highest expected temperature of the pavement. The test conditions include the wheel load and contact pressure, which are individu­ally determined (usually 445 N and 690 kPa, respectively), and the number of load­ing cycles, 8000.

A full report about the APA can be found in the U. S. publication National Cooperative Highway Research Program Report No. 508 (Kandhal and Cooley, 2003).

A concept similar to side supported creep tests applies to the triaxial dynamic com­pression test. In this method, a specimen is subjected to compression with lateral support. This method has been widely regarded as one of the most accurate, reflect­ing the state of stress in a loaded pavement. Moreover, it enables the measurement of parameters of an asphalt mixture used for the analysis of pavement viscoelasticity (Huurman, 2000). Undoubtedly, it is a recommended method for testing rut resis­tance. It is described in EN 12697-25 as method B.

12.1.3 Wheel-Tracking Test

The best known direct tests for resistance to deformation are wheel-tracking tests, conducted with special devices. There are many types of them, such as European devices (according to EN 12697-22), the Hamburg Wheel-Tracking Device, and the APA.

The European standard EN 12697-22 classifies the rutting equipment into small devices and large devices.

According to Table B.5 in EN 13108-20 (the standard that regulates the methods for asphalt mixture type testing), the small device method (testing in air) has application in testing SMA. It is meant for pavement courses subjected to loads less than 13 tons per truck axle. Pavements subjected to loads heavier than 13 tons per axle are tested in a large device. More information on that subject can be found in Chapter 14.