Category Stone Matrix Asphalt. Theory and Practice

These methods are normally based on similar assumptions—namely, in saturating a mixture with water (with or without negative pressure, that is a vacuum) and holding it there at a fixed temperature for a given time. Afterward, a strength test is con­ducted, most often using one of the following methods:

• Marshall stability

• Resilient modulus at different temperatures

• Indirect tensile strength

The comparison of results for specimens conditioned in water with those untreated in water determines the water resistance of an asphalt mixture.

In an extended variant, specimens saturated with water are subjected to many cycles of freezing and thawing to find the mixture’s susceptibility to water and frost. Another variant that involves freezing specimens previously saturated with an aqueous solution of NaCl (e. g., 2%) has also been used. This is a much more effective test due to the aggressive action of the aqueous solution of salt on binder adhesion to the aggregate. This kind of test is often conducted in countries with colder climates.

This EN 12697-12 method test is conducted on cylindrical specimens prepared in a laboratory (in a gyratory compactor, using a Marshall hammer) or cored from a slab cut out of a pavement. Specimens of 100 ± 3 mm, 150 ± 3 mm, or 160 ± 3 mm in diam­eter may be tested. When testing specimens 100 mm in diameter (compacted using a Marshall hammer, for instance), only mixtures with a gradation not larger than 0/22 mm can be tested. The set of specimens (minimum of six) of an asphalt mixture is divided into two groups. Both groups should be prepared at approximately the same time (within 1 week or less of each other). Half of the specimens are stored without conditioning, while the other half are subjected to conditioning in water. Specimens are compressed in an indirect tensile test using EN 12697-23. The compression test may be conducted at a selected temperature within a range from 5-25°C.

The standard introduced an index called the indirect tensile strength ratio (ITSR) as a measure of the water resistance of an asphalt mixture, expressed in percent. Apart from the ITSR index, the type of failure, the degree of coating of an aggregate with a binder in a brittle fracture, and the type of aggregate breakage should be given in a test report.

The AASHTO T 283 method involves conducting tests on a comparable set of speci­mens in an original (unconditioned) state and after conditioning and then comparing the results. In some literature this test is also called the modified Lottman test.

Appropriately prepared specimens are divided into two sets. One set is designed for testing without conditioning, while the other is subjected to conditioning in water and freezing. Both the original and conditioned specimens are tested with an indi­rect tension apparatus. The ratio of the conditioned tensile strength to the strength of the original specimens is called tensile strength ratio (TSR). TSR is usually required to be greater than 70% or 80% (most often 80%).

Pavement durability is a broad term. Water and frost resistance of asphalt mix­tures have a disadvantageous effect on the mechanical performance of a course. Undoubtedly, the composition of a mixture—the type of aggregate, gradation of the mix, the type and quantity of binder, the presence of additives and the content of air voids—has an impact on this resistance. More information on this issue may be found in different publications (e. g., Kanitpong and Bahia [2003]; Santucci [2002])

The most common assessment methods for water and frost resistance of asphalt mixtures can be divided into the following two groups:

• Standardized methods

• Method AASHTO T 283

• Method EN 12697-12

• Non-standardized methods (devised in research centers for particular or local use)

A description of other methods for testing durability of asphalt mixtures and more details of this subject can be found in various papers (Chen et al., 2004; Hicks et al., 2003; Judycki and Jaskula, 1999; Martin et al., 2003; Ulmgren, 2004).

A comparison of macrotexture depths of courses executed with various asphalt mix­tures has been presented in an Australian paper (Oliver, 2001):

• Surface dressing (greater than 10 mm)—macrotexture less than 1.5 mm

• AC (greater than 10 mm)—macrotexture 0.4 to 0.8 mm

• Porous asphalt—macrotexture less than 1.2 mm

• SMA—macrotexture less than 0.7 mm

• Slurry seal—macrotexture 0.4-0.8 mm

• Cement concrete (brushed)—macrotexture 0.2-0.7 mm

Changes in some SMA characteristics occurring over time have been indicated in various investigations. In the United Kingdom, Richardson (1997) described changes in the SMA macrotexture during the months following the construction of a course. The SMA macrotexture, just after placement, was initially at a level of 1.5-1.6 mm. Then it gradually reduced to 1.1 mm after 21 months. It should be added that the reported results applied to an SMA with a gradation that was used in the United Kingdom to achieve a required high macrotexture after placement (see Chapter 10). The applied gradation was characterized by a gradation curve below the typical gra­dation limits according to German guidelines (see comparison of curves in Chapter 14, Figures 14.2 through 14.5).

It should be noted that gritting used to enhance SMA antiskidding properties has additional consequences, especially in the first period after execution, namely a slight decrease in the surface macrotexture. Grit particles (and the remains of them crushed by rollers) gather in spaces among SMA coarse aggregates and only some time after the opening of a road to traffic are they pulled up (sucked out) by vehicle tires and scattered across the shoulder or median (Richardson, 1997). So just after gritting, macrotexture may be low (see Figure 12.6b), it but will eventually improve.

Argentinean investigations (Bolzan, 2002) conducted just after a highway modern­ization project, including the placement of a new SMA 0/19 course, showed it provided high macrotexture 2.2 mm deep (measured by the sand patch method). At another section of SMA 0/12 mm, the macrotexture depth amounted to only 1.4-1.7 mm.

In Germany Behle et al. (2005) conducted research into the relationship between PSV and final skid-resistance of an SMA layer. The authors presented the results of measurements with a sideways coefficient routine investigation machine (SCRIM) over a period of 4.5 years and a concept for calculating the PSV of mixed aggregates with different individual PSVs by weighted average.

By and large, based on straightforward experiments and a series of results, the following general technological recommendations to improve the antiskidding prop­erties of SMA wearing courses may be put forward:

• Apply mixtures with a lower maximum aggregate size (more contact points between the SMA and tire).

• Consider microtexture—prefer aggregates with a high PSV index (low pol­ishing susceptibility) and aggregate mixtures of various rocks with various wear rates; if possible, also use artificial aggregates (slags).

• Consider macrotexture—avoid factors that increase the risk of squeez­ing mastic out on a surface such as mixtures with an insufficient con­tent of voids and or those susceptible to compaction under traffic; for the same reason do not use pneumatic rollers and use vibratory rolling with caution.

• Apply grit to make the surface rough by spreading the grit evenly, followed by rolling while the surface is hot enough.

• Open the lane to traffic only after the SMA has finally cooled off.

Antiskid features of wearing courses of asphalt surfacing are described in terms of the friction coefficient, which depends on the micro-texture of the aggregate and the macro-texture of the placed mixture (Gardziejczyk and Wasilewska, 2003). As this property is very important, the range of available publications on the topic at issue is quite broad (Gardziejczyk, 2002; German DAV Report, 2001; Huschek, 2004; Jordens et al., 1999).

Different pavement qualities, in relation to the speed of vehicles, have a deci­sive impact on antiskid properties (Hunter, 1994). At low speeds, polishing resis­tance has a decisive impact, hence the durable microtexture of the aggregate surfaces is significant; in this instance the polished stone value (PSV) can be used as a selective index for evaluating an aggregate’s resistance to polishing. At high speeds, the macrotexture depth of the wearing course has a decisive impact; the presence of small channels around aggregate particles on the surface enables water discharge, preventing the formation of a hydroplaning effect (skid­ding layer).

The SMA macrotexture depth depends on the maximum aggregate size in the mixture and the design of the mix (e. g., the level of filling voids among coarse aggre­gates on the surface of a course with mastic); see Figure 12.6.

There are a number of impressive publications in the technical literature that describe the results of testing antinoise properties of various pavements. (Refer to the Bibliography at the end of this book.)

Olszacki (2005) tested the sound-absorbing power of different asphalt surfacing types with diversified void contents. Figure 12.5 shows the relationship between the noise absorption coefficient and sound frequency. It is evident that SMA is character­ized by better properties than mixtures of AC, but not as good as porous asphalt with a much higher content of voids, from 10-22% (v/v).

Other research has also stated that pavement noise increases along with an increase in the maximum particle size of the wearing course mixture. Therefore, when antinoise properties are at issue, mixtures SMA 0/5 and 0/8 instead of 0/11 and 0/16 mm are preferred. Generally, the macrotexture of SMA makes it quieter

than AC by about 1-2 dB(A). When coarse SMA 0/16 is used, an increase in the noise level of about 1 dB(A) is reported (Sandberg, 2001). SMA’s distinctive fea­ture—namely, the grit of 2/4 or 2/5 mm aggregates—makes the pavement noisier, which is why gritting with finer aggregate is recommended. After some time, when the grit been removed by traffic, the SMA noise level will naturally reduce. So the conclusion may be drawn that the SMA antinoise properties change over the ser­vice life of the pavement. A site investigation (Schmiedlin and Bischoff, 2002) has proved that a classic SMA pavement is only slightly quieter than classical AC.

Finnish tests of SMA antinoise properties (Valtonen et al., 2002) have pointed out another problem when studded tires are permitted. Testing using the close-prox­imity (CPX) method was conducted at a speed of 50 km/hr. In spite of the SMA 0/5 mm having the best properties just after laydown (in comparison with SMA 0/8, 0/11, and 0/16), a deterioration in properties due to wear caused by studded tires was found after the first winter of operation (only 1 year of service life). Under these circumstances, the SMA mixture’s resistance to wear by studded tires should have been taken into account. The coarsest SMA gradation is more resistant to studded tire wear, in contradiction to its antinoise properties. Consequently, it has been con­cluded that SMA 0/8 and SMA 0/11 might be more appropriate than finer SMA 0/5 (in this research, SMA 0/5 has worn 10 times as much as SMA 0/16). In testing done in the United States, SMA 0/12.5 and SMA 0/9.5 mm were compared using the CPX method (Bennert et al., 2004). The results proved the increase in noise at the contact between tires and pavement along with the increase of SMA maximum aggregate size.

Table 12.2 shows the collective comparison of SMA properties with a reference mixture of AC. The new concept of a silent SMA characterized by better properties than classical SMA is presented in Chapter 13.

Source: From EAPA, Heavy duty surfaces. The arguments for SMA. European Asphalt Pavements Association (EAPA), 1998, With permission. a Negative values indicate an increase in noise level. b Calculated value.

c When the surface is treated with uncoated chippings smaller than 2 mm.

Noise had been defined as “audible sounds of any acoustic kind undesired in par­ticular circumstances, which irrespective of their frequency and level, are harmful, bothersome, and possibly induce a disorder in the listener’s hearing organ and other parts of their organism” (Kucharski, 1979). Many research centers around the world have been dealing with the problem of noise, and numerous publications have dealt with the subject. A comprehensive review of publications addressing the subject of noise can be found in Sandberg and Ejsmont (1999).

A source of noise emits an acoustic wave, which is subject to reflection and par­tial absorption by the pavement. “Silent” pavements are those with reasonably high sound absorption capabilities. Absorption depends on the characteristics of the pave­ment surface and the shape of available air voids.

Agents enhancing compactability by means of changing the temperature susceptibil­ity of a binder have been used in many countries. They enable the placement of a mix­ture at a lower temperature and make its compaction easier through the reduction of binder viscosity. Reduced binder viscosity enhances the compactability of a mixture, resulting in a decrease in the content of voids and an increase in the bulk density. In fact, it is this effect that can be seen in Figure 12.4, which shows example test results of a binder containing a Fischer-Tropsch (FT) wax. The application of an agent of this kind causes an increase in the bulk density of a mixture by about 15% in comparison with mixtures without this agent compacted at the same temperature. The presented mixtures contained the polymer modified binder PmB 45A (German designation).

SMA is a hard-to-compact mixture, and as a gap-graded mixture, it is characterized by a higher compaction resistance than materials with continuous gradation. The conclusions of tests carried out in Germany (Renken, 2004) on an SMA mixture revealed the following:

• An increase of the filler content brings about a drop in compaction resistance.

• An increase of the coarse aggregate content (particles larger than 2 mm) causes a sharp rise in compaction resistance.

• An increase of the binder content slightly reduces the compaction resistance.

Additionally, practice has proved that an increase in the manufactured sand content results in an increase in compaction resistance and that the opposite is true for the quantity of binder (those are commonly known relations). SMA mixtures with a low content of voids (1.5-2.5% v/v) are compacted more easily than those with a content greater than 4% v/v (Schroeder and Kluge, 1992).

When producing an SMA mixture, substantial deviations in batching the filler, binder, and coarse aggregate fraction also promote changes in compaction resis­tance, in addition to alterations in other properties.