Category Stone Matrix Asphalt. Theory and Practice

DESIGNING AN AGGREGATE MIX BY. APPLYING GRADATION LIMITS

The application of gradation limits has become the most commonly used method for designing SMA mix composition. This method involves gradation analyses of all the constituent aggregates, including the filler, followed by balancing the proportions of all the aggregates in such a way that the ultimate gradation curve is situated between the adopted gradation limits. Used alone, it is a very simple method. Unfortunately it is characterized by some drawbacks that may result in a poor SMA mix design.

The advantages of the method of gradation limits include the following:

• The method is simple to use and quick to produce results. After the grada­tion analyses of the constituent materials are carried out, the proportions of the mix constituents can be quickly calculated simply by using computer software. It is also easy to relocate the gradation curve by manipulating the percentages of the constituents. However, for the very experienced engineer there are better ways to analyze and predict the behavior of the mix.

The disadvantages of this method are as follows:

• The position of a gradation curve inside gradation limits does not absolutely secure an appropriate design of an aggregate mix. For example, the grada­tion curve graph itself is not enough to predict and secure both a suitable aggregate skeleton and the impact of flat and elongated particles.

• Some major or minor errors occur in volume relations of constituent materi­als when using only mass units to show the gradation. If the method does not stipulate this, differences in densities of constituents are not taken into consideration. Significant differences in aggregate densities produce sub­stantial differences in the volume relations of a mix. This is not apparent on a gradation curve if it shows the weight distribution retained on sieves, though it can obviously be solved by presenting the gradation in terms of volume units.

• It is presupposed that gradation limits illustrate the area of an appropriate gradation. However, as practice proves, the limits of a suitable gradation should also be periodically verified and mistakes corrected.

In summary, it is safe to say that relying only on the method of gradation limits leads to designing SMA with a low degree of reliability of performance. In Chapter 7, some SMA design methods are discussed, with the application of some concepts extending beyond the use of gradation limits.

30-20-10 rule

The 30-20-10 rule suggests that proper stone-to-stone contact is created if the per­centages of aggregate passing sieves of 0.075 mm, 2.36 mm, and 4.75 mm equal 10%, 20%, and 30%, respectively, which should provide for the appropriate discontinuity in the gradation. After comparing this rule with the data of Table 6.4, one can see that the proportion of grains larger than 4.75 mm from this rule (70%, or 30% passing the

TABLE 6.4

Approximate Contents of Aggregate Fractions for Zichner’s Mastimac and Mastiphalt

Filler Fraction

sand Fraction

Aggregate

Aggregate

Aggregate

mixture

< 0.09 mm

0.09-2.0 mm

2/5.6 mm

5.6/8 mm

8/12.5 mm

MASTIMAC

12-13%

11-12%

15%

60%

(SMA 0/8) MASTIPHALT

12-13%

11-12%

10%

27%

38%

(SMA 0/12.5)

Source: Based on Zichner, G., MASTIMAC unad MASTIPHALT bituminose Gemische fur hochwer – tige Deckschichten. STRABAG Schriftenreihe 8, Folge 4, 1972.

30-20-10 rule Подпись: Coarse grains continuous matrix, no sand

FIGURE 6.1 Relationship between contents of voids in the aggregate mix and the coarse aggregate fraction. (Based on Ferguson, A., Fordyce, D., and Khweir, K., Proceeding of the Third European Symposium on Performance and durability of bituminous Material and Hydraulic Stabilised Composites, AEDIFCATIO publishers, D-79104 Freiburg i. Br. and CH-8103 Unterengstringen/Zurich, 1999; Francken, L. and Vanelstraete, A., Proceeding of Eurobitume Congress Stockholm, Sweden, 1993; Lees, G., Journal of the Association of Asphalt Paving Technologists, 39, 1969; van de Ven, M. F.C., Voskuilen, J. L.M., and Tolman, F., The Spatial approach of hot mix asphalt. Proceedings of the 6th RILEM Symposium PTEBM’03, Zurich 2003.

each other (a skeleton of coarse grains has been formed and is filled with fine grains); the replacement phase has come to an end.

• Then follows the reverse direction of changes in the contents of voids—the mixture becomes open by means of gradually removing the fine aggregate among coarse grains up to 100% (m/m) coarse aggregate when the highest content of voids is reached; that process could be named the filling phase.

The aforementioned relationship between the amount of air voids and gradation of the coarse aggregate fraction directly translates into the binder content in SMA (Druschner and Harders, 2000; Schroeder and Kluge, 1992). The difference in the binder content, which is dependent on the content of grains larger than 2 mm, has been proved in the previously mentioned German publications (and many others pub­lished in Germany). In these examples of tested SMA mixes, the optimum binder con­tent in an SMA mixture depended on the coarse fraction content. For example, with 73% (m/m) of coarse particles content and air voids at the level of 3% (v/v), the binder content amounts to 5.5% (m/m); after an adjustment of the aggregate mix and an increase in the content of coarse aggregate up to 80% (m/m), the same 3% (v/v) of air voids are achieved at a binder content of about 7% (m/m). These results were achieved with the use of a Marshall hammer with a compaction effort of 2 x 50 strokes.

This relationship among contents of binder, air voids, and the coarse aggregate fraction establishes a rule that the content of voids in a designed SMA mixture should not be adjusted by changing the binder content. There is a much higher potential for changing the air voids by adjusting the content and gradation of the coarse aggregate fraction or, generally, by altering the gradation curve.

According to the European Standard EN 13108-5

The requirements for gradation of SMA mixtures have been provided in the European standard PN-EN 13108-5 (see Chapter 14). This standard does not set out the crite­ria and conditions for selecting the particular gradation of a mixture. Establishing these criteria remains the responsibility of each CEN-member state. In Figures 14.2 through 14.5, examples of gradation limits are presented.

6.1 GENERAL RULES

6.2.1 Original Zichner’s Proportions

In his publications (Zichner, 1971; Zichner, 1972) Dr. Zichner described a recom­mended composition of an SMA mixture as follows:

• The stone content should be about 65-80% (m/m), preferably 70-75%, using only crushed stones.

• The main rule governing gradations is that the mixture is composed “so that the percentage of the coarser size is greater than that of the smaller size.”

• Mastic content is 20-35% (m/m).

• 23-28% of the mastic is a binder.

• For layers with different thicknesses, different types of stones should be used as follows:

• Thickness less than 3 cm—stone fractions 2/5.6 and 5.6/8 mm in pro­portions of 25 and 75%, respectively

• Thickness 3-4.5 cm—stone fractions 2/5.6, 5.6/8, and 8/12 mm, or only 5.6/8 and 8/12 mm

• Thickness greater than 4.5 cm—stone fractions 2/5.6, 5.6/8, 8/12, and 12/18 mm

• Only manufactured sand should be used in the mastic.

Looking at Zichner’s MASTIMAC and MASTIPHALT gradation curves presented in Chapter 1 (see Figure 1.1), we can find approximate contents of aggregate frac­tions (Table 6.4).

The proportions of different fractions of coarse aggregates used in current German SMA mixtures are presented also in Chapter 2 (see Table 2.1) after the German DAV Handbook (Druschner and Schafer, 2000).

Traffic Loading and Location

Coarse-graded mixtures make stronger skeletons. That is why the majority of requirements for SMA contain a noticeable tendency toward increasing the maxi­mum particle size in a mixture in conjunction with an increase in the traffic loading. When selecting a mixture, both the strengths and weaknesses of an accepted solution
should be considered; mixtures with larger maximum particle size (let us suppose greater than 10 mm) are characterized by higher rut resistance but lower noise reduc­tion ability and poorer skid resistance. Therefore, when at all possible, SMA 0/11 is being gradually abandoned for SMA 0/8.

In Germany, SMA 0/8S and 0/11S have been used on the most heavily trafficked roads while on lesser trafficked roads mixtures without an S marking are used. The guidelines from 2001 to 2007 (ZTV Asphalt-StB 01 and ZTV Asphalt-StB 07, respec­tively) have differentiated mixtures according to traffic, with the understanding that mixtures with the same maximum aggregate size but intended for various traffic loadings differ in the shape of the gradation limits. German SMA aggregate mixes of the S type differ from the lower traffic SMAs in that they have the following:

• Lower filler contents

• Lowered gradation limit curves

• No non manufactured (natural) sand

As a result, German SMA mixtures of the “S” type should be coarser and, at the same time, less closed. This is logical since there should be more voids in a heavily trafficked course (refer also to the Dutch method in Chapter 7).

Подпись: TABLE 6.3 SMA Division according to the Traffic Category Prevailing in Locations SMA mixes with Gradation canada germany united (ontario) ZTV States OPSS. Asphalt- NAPA MUNI application stb 07 IS 128 1151 Low and medium traffic roads 0/8N, 0/5N Principal 0/11S, 0/9.5, 0/9.5, roads and 0/8S 0/12.5, 0/12.5, highways (heavy traffic) 0/19 0/19
In many countries, as in Germany, different SMA gradation curves have been specified depending on the traffic category (traffic loading), as shown in Table 6.3.

slovakia

Poland

slovenia

sist

Australia

klaz

WT-2

1038-

NAs 2EcI

1/2008

2008

5:2008

2004

0/5,

0/4, 0/8,

0/7, 0/10

0/8

0/11

0/8,

0/8,

0/8, 0/11 0/10, 0/14

0/11,

0/11

0/16

What Thickness of a Course?

In bituminous mixtures, the adopted rule used to be that the thickness of a course should not be less than 3.5 times the nominal maximum aggregate size (NMAS) in a given mixture. However, due to problems with compaction, usually 3.5-4.0 times the NMAS in a mixture is normally suggested as the appropriate thickness of an SMA course. A course that is too thin in comparison with the maximum particle size causes the following:

• Tearing the mat during laydown and cracking during rolling

• Problems with compacting the course

• Breaking of weaker particles during rolling

• Problems with the appropriate arrangement of particles and weakening of the aggregate structure

Many SMA guidelines provide for a range of thicknesses for each particular mix­ture. Some regulations from the old German guidelines ZTV Asphalt-StB 01 and from the new ZTV Asphalt-StB 07 on the thickness of the SMA 0/11 course are noteworthy because of an exceptionally narrow range of placement thickness for this mixture, specifically 35-40 mm (Table 6.1).

In some countries, a mass criterion on 1 m2 of the surface area (spread rate) is specified instead of designating the recommended thickness of a course. Knowing the SMA bulk density, the thickness of a layer of that mixture may be calculated. The German ZTV Asphalt-StB 01 and ZTV Asphalt-StB 07 and Finnish regulations PANK 2008 are examples of such approaches (Table 6.2). Table 6.1 shows particle sizes and corresponding courses according to various guidelines.

Finally, it is worth noting that a thin course gets cold much faster than a thick one, which has a significant effect on compaction during the cooler periods of the year.

TABLE 6.1

Thicknesses of SMA Wearing Courses, depending on the Maximum Particle Size according to various Guidelines for smA

Thickness of a compacted course in Different countries, mm

Austria

slovakia

poland

germany

ztv

germany

ztv

united

Kingdom

Rvs

klaz

WT-2-

united states

Asphalt-

Asphalt-

Bs

Mixture

08.16.01

1/2008

2008

NApA is 128a

stB 07

stB 01

594987:2007

SMA

0/5

20-40

20-30 (N)

20-40

20-40 (SMA 0/6)

SMA

0/8

25-35

20-40

25-50

25-37.5 (SMA 0/9.5)

30-40 (S) 20-35 (N)

30-40 (S)b 20-40

SMA

0/11

30-40

30-50

35-50

37.5-50 (SMA 0/12.5)

35-40

(11S)

35-40

(11S)b

25-50 (SMA 0/10)

SMA

0/16

40-60

50-75 (SMA 0/19)

35-50 (SMA 0/14)

Note: N = low/medium traffic; S = heavy traffic.

a In NAPA IS 128 publication, only recommended minimum lift thicknesses ranges are presented. b In special circumstances, when the SMA bottom layer is not even, the following course thicknesses have been allowed: of SMA 0/11S from 25 to 50 mm, and of SMA 0/8S from 20 up to 40 mm.

table 6.2

The Mass of sMA Wearing courses, depending on the Maximum particle

sizea

Mass of 1 m2 of a surface Area, kg

sMA

sMA

sMA

sMA

sMA

sMA

guidelines

0/22

0/16

0/11 s

0/8s

0/8

0/5

German guidelines ZTV

85-100

70-100

45-100

45-75

Asphalt-StB 01

German guidelines ZTV

85-100

75-100

50-85

50-75

Asphalt-StB 07

Finnish guidelines PANK 2008

100-150

80-125

60-100

50-90

50-75

a According to the German ZTV Asphalt-StB 01 and ZTV Asphalt-StB 07 and Finnish regulations PANK 2008.

SELECTING SMA GRADATION AND SIZE

No matter the method applied when designing the SMA composition, the first step that must be taken is establishing the maximum particle size in the aggregate mix. The following factors should be taken into account when considering this issue: [23]

6.1.1 In Which Course?

Due to the outstanding performance of SMAs in surface courses, they have also been used in intermediate courses (e. g., in the United States, Australia, and recently Germany). Normally mixtures with larger maximum particle sizes (e. g., 0/19 or 0/22 mm) are selected for that purpose when they are permitted by local regulations. So far, aside from possible economic obstacles (high cost), it has not been stated that SMA is unfit to be used in intermediate courses. Consequently, if one can afford to and knows how to design and execute SMA intermediate courses, why not dare to?

When using SMA as a course on a bridge deck, the gradation should be selected rather conservatively (i. e., it should be finer). The composition of the asphalt mixture should also be designed with great care and mostly with the use of the type of poly­mer modified binders that increase the fatigue life of an asphalt mix. (See Chapter 13 for further discussion of SMA for bridge decks.)

Designing SMA Composition

Stone matrix asphalt (SMA) mixtures and courses made from them have many strengths. Naturally, these mixtures must be well-designed, but that attribute has various shades of meaning. A review of publications on this subject has not revealed a method that is clearly “the best.” There is as wide a variety of design methods as there are approaches to the roles of particular constituent materials.

Having decided to place such a mixture, a civil engineer faces a challenge. It is not an easy material to deal with, neither during design nor construction. The first essential task is to achieve the proper SMA composition. All the remaining aspects of SMA construction—namely, production, transportation, placement, and compac­tion—are affected by this first step. Most problems at subsequent stages of work with SMA can be avoided only by achieving a good mix design.

In the various sections of this chapter, the following issues will be presented and discussed:

• Selection criteria for an aggregate mix size (the maximum particle size) depending on the design thickness of the course

• Design method with gradation limits

• Designing the part of the aggregate mix greater than 2 mm

• Designing the part of the aggregate mix less than 2 mm

• Selection of the proper binder content

• Final assessment of the properties of the designed mixture

Chapter 7 presents an overview of selected SMA design methods developed in vari­ous countries.

Artificial Slag Aggregates

Slags of different sorts may be used in SMA mixtures, providing they meet suit­able requirements. For example, in Europe these can be determined in the National

Application Documents for the EN 13043 standard; examples were presented in Table 5.2 (for more information, see Chapter 14). When considering the possibility of using slag in an asphalt mixture, one should remember the following specific properties for these aggregates:

• The density could sometimes be substantially different from that of crushed rock aggregates and can be variable; when designing an SMA mixture using the weight method, differences can manifest themselves in differing volume relations between mixture components.

• The binder content should be determined carefully; a higher density of an aggregate mix usually brings about the necessity for reducing the quantity of binder, but the high porosity of external surfaces of slag grains should also be taken into account.

• The complex chemical composition of slag means that adhesion promoters should be carefully selected and confirmed with testing.

• One of the basic requirements imposed on slag aggregates is an invariabil­ity of properties determined during testing called chemical disintegration.

In addition to these properties, some research (Airey et al., 2004) has proven that mixtures with slag aggregates demonstrate very good interlocking characteristics and a relatively high resilient modulus. Conversely, mixtures with slag showed an increased susceptibility to age hardening (in long-term laboratory aging).

Other tests of slag aggregate show very high PSV values, which could be used for designing skid-resistant asphalt mixes.

RECLAIMED ASPHALT

Generally, most regulations do not recommend using reclaimed asphalt or simply do not allow recycling old asphalt layers into SMA. However, some good test results do exist for SMA mixtures with reclaimed asphalt (Perez et al., 2004).

The requirements for reclaimed asphalt to be used for SMA according to the EN 13108-5 standard are discussed in Chapter 14.

5.2 OTHER MATERIALS

5.5.1 Natural Asphalt

Some results of the use of natural asphalt as an additive to road binder manufac­tured in a refinery are cited in relevant literature (Hausler and Arand, undated; Radenberg, 1997). Natural asphalt is usually added to increase resistance to perma­nent deformation.

The requirements for natural asphalts have also been cited in the EN 13108-4 standard, Annex B.

Finland

The requirements for cellulose fibers according to PANK 2000 have been divided into obligatory and recommended ones as follows:

• Obligatory requirements

• Water content according to PANK 3103: < 8.0% (m/m),

• Instantaneous heat resistance (mass loss) according to PANK 3104: < 7.0% (m/m)

• Recommended values for bulk fibers

• Bulk density according to PANK 3105: 20-35 g/dm3

• Homogeneity according to PANK 3107: 2.0-2.8%

• Fibers’ length distribution:

– 80% value: 1.2-1.9 mm

– 50% value: 0.5-0.9 mm

• Specific surface area according to PANK 2401: 2.0-3.0 m2/g