Category Stone Matrix Asphalt. Theory and Practice

The United States is one of the few countries where requirements for cellulose fibers and mineral stabilizing agents have been standardized. Some other countries have adopted the U. S. requirements in their own specifications.

The requirements for cellulose and mineral fiber SMA stabilizers after AASHTO MP-8-05 include the following: [22]

• Shot (nonfibrous material) content

• Passing 0.25-mm sieve: 90% ± 5%

• Passing 0.063-mm sieve: 70% ± 10%

• Ash content of cellulose fibers: 18% ± 5% nonvolatiles

• pH of cellulose fibers: 7.5 ± 1.0

• Oil absorption of cellulose fibers: 5.0 ± 1.0 times fiber mass

• Moisture content of cellulose fibers: maximum 5% (m/m)

Specifying the desired properties of stabilizers is a troublesome task, which is why there are not very many examples of formalized requirements for them. In fact, detailed testing of a stabilizer’s properties is practically impossible in the asphalt plant laboratory. Therefore, in many countries, testing is limited to draindown check­ing (i. e., only an empirical assessment of stabilizer effectiveness).

A few examples of standardized regulations come from Germany and the United States. Some requirements were also in the Finnish specification PANK 2000.

5.3.1 Germany

The requirements adopted in Germany were cited in a 1997 document entitled

Testing and Marking Stabilizing Additives and Materials Applied for Bituminous

TABLE 5.7 POLAND SLOVAKIA GERMANY SWEDEN PN-EN 14023:2009/ Katalogove Listy TL-Bitumen 07 VV Publ 2008:113 AC2010 Asfaltov KLA 1/2009 Properties Test Method 25/55-55 40/100-75 45/80-55 45/80-55 Penetration at 25°C, 0.1 mm EN 1426 25-55 40-100 45-80 45-80 Softening point, °С EN 1427 >55 >75 >55 >55 Force ductility (low speed traction), J/cm2 EN 13589 EN 13703 >3 at 5°C TBR >3 at 5°C >2 at 5°C Change of mass, % EN 12607-1 <0.5 <0.5 <0.5 <0.5 Retained penetration after RTFOT, % EN 1426 >60 >60 >60 >60 Increase in softening point after RTFOT, °С EN 1427 <8 <10 <8 <12 Flash point, °С EN ISO 2592 >235 >220 >235 >250 Fraass breaking point, °С EN 12593 <-10 <-12 <-12 <-18 Elastic recovery at 25°C, % EN 13398 >50 — >50 >50 Elastic recovery at 10°C, % EN 13398 — >75 — — Plasticity range, °С p. 5.1.9 — TBR TBR — Storage stability EN 13399 <5 <5 <5 <5 Difference in softening point, °С EN 1427

Storage stability

Difference in penetration, 0.1 mm Drop in softening point after RTFOT, °С Elastic recovery at 25°C after RTFOT, %

Elastic recovery at 10°C after RTFOT, %

Note: TBR = To Be Reported.

TABLE 5.7 POLAND SLOVAKIA GERMANY SWEDEN PN-EN 14023:2009/ Katalogove Listy TL-Bitumen 07 VV Publ 2008:113 AC2010 Asfaltov KLA 1/2009 Properties Test Method 25/55-55 40/100-75 45/80-55 45/80-55 Penetration at 25°C, 0.1 mm EN 1426 25-55 40-100 45-80 45-80 Softening point, °С EN 1427 >55 >75 >55 >55 Force ductility (low speed traction), J/cm2 EN 13589 EN 13703 >3 at 5°C TBR >3 at 5°C >2 at 5°C Change of mass, % EN 12607-1 <0.5 <0.5 <0.5 <0.5 Retained penetration after RTFOT, % EN 1426 >60 >60 >60 >60 Increase in softening point after RTFOT, °С EN 1427 <8 <10 <8 <12 Flash point, °С EN ISO 2592 >235 >220 >235 >250 Fraass breaking point, °С EN 12593 <-10 <-12 <-12 <-18 Elastic recovery at 25°C, % EN 13398 >50 — >50 >50 Elastic recovery at 10°C, % EN 13398 — >75 — — Plasticity range, °С p. 5.1.9 — TBR TBR — Storage stability EN 13399 <5 <5 <5 <5 Difference in softening point, °С EN 1427   EN 13399
EN 1426
EN 12607-1, EN 1427	<2
EN 12607-1, EN 13398	>50

TBR <5

TBR >50

>50

№ vo

Surfaces (FGSV Arbeitpapier 42). It contains requirements for all kinds of stabiliz­ers used in aggregate mixtures with a high content of binder, which includes the following:

• Organic fibers

• Mineral fibers

• Powders and dusts

• Compounds of filler and all sorts of fibers

• Special fillers.

The range of testing depends on the type of stabilizer. The following is a set of recommended tests for fibers:

• Macroscopic assessment

• Homogeneity

• Color

• Odor

• Agglomeration (balling)

• Microscopic assessment

• Structure

• Other properties

• Gradation distribution

• Diameter—in specified cases

• Mass loss after drying

• Mass loss after ignition

• Specific gravity

• Water susceptibility

• Stiffening properties (an increase of the softening point after the Wilhelmi method)

The requirements for SMA binders used in the United States are related to the Superpave system prevailing in the country. The basis for the selection of binders for wearing courses, in principle their functional type (performance grade [PG]), is the climate of the given area of application. So all binders have to meet the same requirements for deformation resistance, fatigue, and low temperature cracking, but they meet those limits at different temperatures that relate to the climate in various parts of the United States. In specified situations, such as low-speed traffic or very high-traffic loads, the system additionally allows increasing the upper (high tem­perature) grade of the binder. Finally, a binder that meets the specific climatic and traffic requirements for the construction location is then specified or selected.

So there are no special nationwide requirements for SMA binders. The rules for binder selection from Superpave are generally followed for all mixtures. Readers interested in the PG system can find detailed information in SHRP or Asphalt Institute publications (e. g., SP-1 Superpave Performance Graded Asphalt Binder Specifications and Testing).

In Europe, road binders (paving grade) according to EN 12591 and modified binders according to EN 14023 have been used in SMA mixtures. Normally, road binders of 50/70 and 70/100 are used; however softer 160/220 types have also been cited in the regulations for roads with light traffic. The properties of each type of paving grade binder are fully specified in EN 12591; Table 5.6 shows selected properties of 50/70, 70/100, and 160/220.

European Standard EN 14023 for polymer modified binder (PMB) is similar to EN 13043 for aggregates. It comprises two tables with PMB properties to be chosen by any CEN country. In this way, PMB with the same designation could vary sig­nificantly from one country to another. Table 5.7 provides examples of some PMBs specified for SMA.

Recently, a technology utilizing the simultaneous application of modified binder with a viscosity-reducing additive at high temperatures (e. g., F-T wax) has achieved great popularity.

SMA mixtures are chiefly laid as wearing courses. Binders for them should therefore have suitable properties for asphalt mixtures applied to that layer.

The majority of SMAs are placed in moderate climates. Therefore the SMA binder is usually an unmodified binder, or sometimes a polymer-modified one, with a penetration between 50 and 100 (0.1 mm) at 25°C. In several countries, multigrade

TABLE 5.5

Requirements for SMA Aggregates (Coarse, Fine, and Filler) in the United States

properties requirement comments

coarse aggregates

<30 There is a suggestion to use additional test

methods like Micro-Deval or SGC degradation test. Despite experiences with aggregates of LA 30-45%, their use is not recommended due to possible grain crushing during compaction both in the laboratory and on the road.

<20 at 3:1, Aggregates that have a high percentage of

<5 at 5:1 flat and elongated particles:

• Tend to break down during compaction

• Have higher voids within the aggregate

Requirements apply to a whole coarse aggregates fraction in SMA but not to individual materials (fractions).

100% one face, Property is important for interlocking of >90% two faces aggregates skeleton.

<2

<15 in sodium Tests show that there is good correlation

sulfate, <20 in between magnesium sulfate soundness and

magnesium sulfate micro-Deval abrasion test.

Fine aggregates

<15 in sodium See Coarse Aggregates.

sulfate, <20 in magnesium sulfate

>45 Indication of interlocking potential of fine

aggregates

<25 The liquid limit is the water content at

which the material passes from a plastic Nonplastic to a liquid state. The plasticity index is

the numerical difference between the liquid limit and the plastic limit; it is the moisture content at which the material is in a plastic state. The goal is to eliminate aggregates with clay or silt particles.

(Continued)

binders are also used. Under special circumstances, especially in countries with a cold climate, soft binders (with penetration higher than 100 at 25°C) are used. A steady increase in the percentage of polymer-modified binders in SMA mixtures has been observed recently. Considerable research (see Chapter 12) has proved that a polymer-modified binder substantially improves the characteristics of a finished SMA layer.

U. S. requirements for aggregates constitute a compromise between high quality con­ditions and the necessity of taking into account the economics of manufacturing asphalt mixtures. The number of properties specified are limited, while the require­ments themselves are somewhat broad (see Table 5.5), compared with the European standards. The requirements also vary from state to state. Similar requirements are used in the United States for SMA airfield surfacing (ETL 04-8).

Additionally, the possibility of using reclaimed dusts (baghouse fines) from an asphalt mixing plant is a fine example of the pragmatic approach to the selection of aggregates for asphalt mixtures.

Dusts, mineral powders, hydrated lime, and pulverized fly ashes are allowed, while lumps and organic impurities are excluded as fillers in the United States.

In the Comite Europeen de Normalisation, or European Committee for Standardi­zation (CEN), the member states’ requirements for aggregates designed for asphalt mixtures have been unified in the EN 13043 standard entitled Aggregates for Bituminous Mixtures and Surface Treatments for Roads, Airfields, and Other Trafficked Areas. It provides a set of aggregate properties and a series of requirement levels (categories) for each property.

Each CEN member state adopting this standard has defined its own national requirements, considering such issues as local climatic conditions and experience of engineering, from among the alternative properties and categories provided in the standard.

The requirements for SMA aggregates according to the EN 13043 standard are displayed in Tables 5.1 and 5.2. The following is an explanation of records in the Tables 5.2 through 5.4: [20]

• No requirement category (NR), which means that in a given country’s national specification, the requirement for this property is not used, e. g., MBFNR

• Fractions of aggregates are described as d/D (e. g., 2/5 mm) where:

• D means nominal upper limit of gradation (oversized grains are allowed)

• d means nominal lower limit of gradation (undersized grains are allowed)

Some additional explanations are provided in Table 5.1, but details can be found in EN 13043. An example of a similar type of EN standard is described in Chapter 14.

Tables 5.2 through 5.4 present the requirements from selected European coun­tries. The substantial differences among these countries may be confusing. In the most important properties, the demanded level is more or less similar, e. g., LA index is from 20 to 25% or flat and elongated content is usually as high as 20-25%.

Although it is the same SMA mixture, the requirements for components or aggre­gates, depend on the following factors:

• Materials available in a specified place—countries specify their require­ments based on long-term experience with aggregates, test results, and research. Requirements also depend on accessible sources of aggregate— countries in which sufficient amounts of very good quality materials exist are able to limit the number of required properties. One example of such a situation are the Nordic (Scandinavian) countries. However, several coun­tries have quite a wide range of aggregates with very different qualities. There is a need to balance the technical requirements with a view toward economics.

• Technology and previous experiences with materials also have an impact on the set of requirements. For example, using hydrated lime (in mixed filler) is not very popular in Europe, therefore only a few countries put them into their specifications (filler category Ka).

• Test methods in EN 13043 come from different national practices, as described in Chapter 14, hence many countries do not use some of them in practice. For example, the LA method has not been commonly used in Germany, and the same can be said for Rigden’s method in Poland, Aggregate Abrasion Value (AAV) outside the United Kingdom, and the Nordic abrasion value outside Scandinavian countries. Many countries use declared categories because without past experiences it was hard to establish any reasonable requirement.

• In specified number of countries, production of aggregates for asphalt layers is regulated with precisely prescribed fractions and their individ­ual gradation limits. Such a situation, in which all producers of aggre­gates make the same fractions and with very similar gradation, is very comfortable for asphalt mix producers. At the same time any additional

TABLE 5.1

Comments on System of Requirements Based on EN 13043

comments

Gradation of aggregate fractions after EN 13043 are labeled as GXY category; the numbers describe allowable amounts of oversized (X) and undersized (Y) material, e. g. Gc90/15 means that only 10% of oversized and 15% of undersized material is allowed, similarly GC85/15 means that only 15% of oversized and 15% of undersized material is allowed; category Gc90/10 is the highest possible choice and GC85/35 is the lowest.

There is only one category for fine aggregate,

GF85, which means the limit for oversized grains (>2 mm) is 15% by mass.

When all-in aggregates are used one of two GaX categories, can be used Ga90 and Ga85, where maximum limits for oversized material are 10% and 15% (by mass), respectively.

The idea of this type control is based on a requirement that the producer will document and declare the typical gradation of any produced aggregate fraction; tolerances of typical gradation, labeled as GX/Y, are used for gradation control within the fraction; depending on the D/d coefficient, the control sieve is chosen as D/2 or D/1.4; the number x means x—overall limits of amount of material passing by control sieve (20 means 20-70% by mass; 25 means 25-80% by mass), and the number y means y – tolerances for typical gradation on control sieve declared by aggregate’s producer (15 means ±15%; 17.5 means ±17.5%); example category G25/15 means that on sieve D/2 or D/1.4 overall limits are 25-80% and the tolerance from the declared value is ±15%

This requirement applies to tolerance of percentage of grains passing by sieves D, D/2, and d compared with the gradation declared by producer;

category Gtc10 mean tolerances on sieves: D ±5%, D/2 ±10%, d ±3%,

category Gtc20 mean tolerances on sieves: D ±5%, D/2 ±20%, d ±3%,

Comments on System of Requirements Based on EN 13043

TABLE 5.1 (CONTINUED)

Comments on System of Requirements

Properties Label of category

Water absorption, Wcm0.5

according to EN 1097-6 WA24X

Resistance to freezing and Fx

thawing, according to EN 1367-1

Resistance to freezing and MSx

thawing, according to EN 1367-2

Resistance to thermal —

shock, according to EN 1367-5:

Affinity of coarse —

aggregates to bituminous binders, according to EN 12697-11

“Sonnenbrand” of basalt, SBSz or SBla

according to EN 1367-3 and EN 1097-2

Coarse lightweight ^lpcX

contaminators, according to EN 1744-1, p.14.2

based on EN 13043

comments

The method of testing is chosen depending upon the size of the aggregate:

• Using EN 1097-6 clause 7 refers to category WA24X, where X means maximum allowable percentage absorption by mass.

• Using EN 1097-6 Annex B refers to category Wcm^-5, where 0.5 means maximum allowable percentage absorption by mass (there is only one category <0.5%).

Additionally EN 13043 connects water absorption and resistance to freeze-thaw of aggregates; aggregates with small absorption are assumed to be freeze-thaw resistant.

Category Fx is used, where X means maximum allowable percentage loss of mass; test can be conducted in water, salt solution, or urea.

Category MSx is used, where X means maximum allowable percentage loss of mass; test is conducted with magnesium sulfate.

Test of aggregate resistance for high temperature; results are declared

Test of binder adhesion to aggregate; results are declared

This is to check for basalt rock decay, which results in lowering aggregate strength and in most cases in very low freeze-thaw resistance of basalt aggregate; categories SBsz and SBla mean that after test (boiling for 36 hrs) and crushing (in SZ or LA, respectively), the aggregate must meet required values:

• Loss of mass after boiling: max 1.0% (and)

• Increase of impact value: max 5.0% (SBsZ) (or)

• Increase of LA coefficient: max 8.0% (SBla) The content of coarse lightweight organic

contaminants larger than 2 mm should be maximum X% by mass.

Comments on System of Requirements Based on EN 13043

Note: AAV = Aggregate abrasion value; FI = flakiness index; LA = Los Angeles; PSV = polished stone value; SZ = Schlagzertrummerungswert; SI = shape index.

TABLE 5.2

Requirements for SMA Coarse Aggregate according to EN 13043 in Selected CEN-Member Countries, Aggregates for SMA at the Highest Traffic Level (Reference Mixture SMA 0/11)

List of Categories by Country
Austria

Germany ONORM В

(Continued)

List of Categories by Country

TABLE 5.2 (CONTINUED) Requirements for SMA Coarse Aggregate according to EN 13043 in Selected CEN-Member Countries, Aggregates for SMA at the Highest Traffic Level (Reference Mixture SMA 0/11) Austria Germany ONORM В TL Gestein StB 04 3584:2006 Switzerland Poland Anhang F Slovakia RVS 08.97.05:2007 SN 670130a: WT-1 Kruszywa and TL Asphalt StB 07 KLK 1/2009 (Class C1) 2005 2008 Properties3 1 2 3 4 5 Percentage of crushed and Qoo/o, C100/0 Qoo/o C95/1 Qoo/o broken surfaces in coarse C95/1, aggregates, according to EN 933-5 Q0/1 Resistance to fragmentation LA20 LA 25 LA 20 4/8 mm—LA 2 . LA2o or LA 25 (crushing), according to EN (max 20%) (max 25%) (max 20%) 8/11 mm— LA20 depending on 1097-2 clause 5 (LA 11/16 mm— LA25 petrographic type method) of aggregate Resistance to fragmentation SZI8 — — — — (crushing), according to EN 1097-2 clause 6 (German SchlagzertrUmmemngswert) (max 18%) Resistance to polishing, PSV51 psvx PSV50 PSV50 PSV50 according to EN 1097-8 (min 51) (min 56) (min 50) (min 50) (min 50) Resistance to surface — — — — — abrasion, according to EN 1097-8 Annex A

WA241 Declared WA241 Wcn,0.5 F, ^NaCl^ (max 1.0%) (max 7% in 1% NaCl solution)

Min 85% Declared — method В SBLA – SBLA — mlpcO.1 mLPC0.1 (max 0.1 %) (max 0.1 %)

(iContinued)

TABLE 5.2 (CONTINUED)

Requirements for SMA Coarse Aggregate according to EN 13043 in Selected CEN-Member Countries, Aggregates for SMA at the Highest Traffic Level (Reference Mixture SMA 0/11)

List of Categories by Country

Note: Cells with — mean no requirement (NR) category; FI = flakiness index; LA = Los Angeles; SI = shape index. a Names of properties after EN 13043

TABLE 5.3

Requirements for SMA Fine Aggregate according to EN 13043 in Selected CEN-Member Countries, Aggregates for SMA at the Highest Traffic Level

TABLE 5.4

Requirements for SMA Filler according to EN 13043 in Selected CEN-Member Countries, Aggregates for SMA at the Highest Traffic Level

List of Categories by Country

Germany TL Gestein StB 04 Slovakia Austria ONORM В Switzerland Poland WT-1 Anhang F KLK 3584:2006 SN 670130a: Kruszywa Properties and TL Asphalt StB 07 1/2009 RVS 08.97.05:2007 2005 2008 Grading, according to EN 933-10 Harmful fines (fines quality), Declared According to Table 24 of standard: Sieve 2.0 mm = 100% passing Sieve 0.125 mm = 85-100% passing Sieve 0.063 mm = 70-100% passing MBF10 according to EN 933-9 Water content (added filler), according <% <i% <1% <1% (max 10 g/kg) <1% to EN 1097-5, % Stiffening properties: voids of dry ^28/45 _ ^28/38 T28/45 T28/45 compacted filler (Rigden), according (min 28%, (min 28%, (min 28%, (min 28%, to EN 1097-4 max 45%) max 38%) max 45%) max 45%) Stiffening properties: “Delta ring and Ar&r8/25 Ar&b8/16 — Ar&b8/25 Ar&r8/25 ball,” according to EN 13179-1 (min 8°C, (min 8°C, (min 8°C, (min 8°C, max 25°C) max 16°C) max 25°C) max 25°C)

Water solubility, according to EN Н7>10

1744-1 (max 10%)

Water susceptibility, according to EN Declared

1744-4

Calcium carbonate content of CC70

limestone filler aggregate, according (min 70%)

to EN 196-21

Calcium hydroxide content of mixed Declared

filler, according to EN 459-2 Bitumen number of added filler, —

according to EN 13179-2

Declared

— Declared

CC80 Declared

(min 80%)

Ka20 To be established

(min 20%) in contract

ПМ _

Dn 28/39

(min 28 max 39)

requirement for tolerance of gradation is not necessary; this is the situa­tion in Germany, Switzerland, and a few other countries. In Poland, where limits for gradation during aggregate production do not exist, it was nec­essary to put such a requirement (categories G and GTC) in the national specifications.

• Legal systems and approaches to the requirements’ system also play roles.

In Europe, aggregates for asphalt mixtures are construction products and are produced and placed on the market according to Construction Product Directive[21] regulations. Internal regulations of each country have to be con­sistent with this directive. The product (aggregates) must fulfill specified requirements for intended use; countries are free to determine how they specify the system of requirements (only a few properties are indispens­able). When we see Tables 5.2 through 5.4, it is obvious that some countries built a very broad system and established more detailed specifications than others. The reason is most likely in the existing approach to the require­ments for components. In some countries only a few properties are speci­fied because the final mixture (e. g., SMA) features are treated as the most important and a large degree of freedom is left for asphalt mix producers as long as they meet the final desired properties. In other countries everything is specified—both components and final mixture properties as well, which can ultimately lead to overspecification.

A distinctive feature of stone matrix asphalt (SMA) components is their high quality. In this chapter we shall have a look at the requirements for those materials in various countries. Comparing them may be interesting because of the diversity of emphases different countries place on individual conditions.

Consequently, the following components will be subsequently discussed in detail:

• Aggregates

• Binders

• Stabilizers

• Reclaimed asphalt

• Other additives

5.1 REQUIREMENTS FOR AGGREGATES

The requirements for aggregates are quite diverse, and they chiefly concern fun­damental properties that influence the performance of SMA and are attributed to the size of the grains. This has given rise to a variety of requirements for particular fractions—some for coarse (active) aggregates and other for fine ones. The require­ments for fillers have been defined in Chapters 2 and 3.

One may find that almost everywhere these requirements center around the fol­lowing properties:

• Coarse aggregate: hardness (resistance to fragmentation or crushing), percentage crushed, shape of grains, polishing resistance, and resistance to external factors (e. g., water, frost, deicers)

• Fine aggregate: angularity, amount of dust, and absence of swelling frac­tions in dusts

• Fillers: stiffening properties—difference in softening points (SPs), absence of swelling fractions, and air voids in a compacted filler.

Taking into account the type of aggregate skeletons required for SMAs, the pri­mary property seems to be resistance to crushing. This resistance is often assessed using the very popular Los Angeles abrasion test (L. A. index). The range of the allowable LA index in national specifications typically varies from 20 to 30%; how­ever, many publications from North America reported that some successful SMAs have aggregates with LA index values that are greater than 30%.

In one report (Celaya and Haddock, 2006) the authors express their opinion that the LA index is not the only important property of coarse aggregates for SMA. Besides the LA index, one should take into account the Micro-Deval test, which is conducted using water[19] and the Strategic Highway Research Program (SHRP) Superpave Gyratory Compactor (SGC) compaction degradation. This difference between the LA index and Micro-Deval test can show the aggregates that are sensitive to crushing in the presence of water. Finally, the results from the LA index, Micro-Deval test, and SGC degradation test provide the best way to select suitable coarse aggregates for use in SMA mixtures. Comprehensive reviews of coarse aggregate test methods are avail­able in the published literature such as Fowler et al. (2006) and White et al. (2006).

In Germany, the Schlagzertrummerungswert (EN 1097-2, Part 6) method is used instead of or together with the LA index. In general, resistance to crushing is crucial and must be tested because the use of weak aggregates may result in poor perfor­mance of the whole layer (Figure 5.1).

For the fine aggregate portion, the most commonly specified property is angu­larity. This can be tested using different methods; common ones include the flow rate method as in European Standard EN 933-6 (results in seconds, shown as Ecs) or the measurement of voids in uncompacted aggregate as in AASHTO T 304 (results in % [v/v], known as fine aggregate angularity [FAA]). Obviously other methods are used and could also be good indicators of angularity. Johnson et al. (2004) have conducted interesting research to evaluate the influence of FAA and

other variables on asphalt mix performance. That study confirmed that FAA is a good tool to predict dynamic modulus and rut resistance.

The requirements for filler, besides gradation, are focused on analyses of clay and silt in the filler and on the stiffening properties represented by an increase of the SP and Rigden voids. The specific areas of filler are gradually being withdrawn from specifications in some countries (e. g., Poland and Finland). Applications of different raw materials as fillers were described in Chapter 3. Some waste materials, such as coal fly ash, waste ceramics, and steel slag, have also been tested, and the results indicate a good potential for use in SMA (Muniandy et al., 2009).

The requirements for aggregates in selected countries are described throughout this chapter. The data are divided into two parts—that for CEN countries (mainly European Union) and that for the United States. A more comprehensive record of data would surpass the scope of this book.

Various methods for testing the tendency for binder to draindown in a particular SMA are presented in Chapter 8. The original Schellenberg’s method and other techniques according to AASHTO and EN 12697-18 standards are also described there.

In most countries, Schellenberg’s or a similar method for determining the quantity of stabilizer in a mix has been applied. The maximum allowable mass of drained-off material is essentially the same almost everywhere and is equal to 0.3%. A recom­mended, safer limit is 0.2%.

4.2 SUMMARY

Factors influencing a mixture’s susceptibility to binder draindown are presented in Table 4.1.

From the point of view of an SMA mix producer, stabilization efficiency is a decisive factor. But there is one more, equally important point: the quality of a stabi­lizer must be constant over the whole season of use. What does it matter, exactly, if a product is best at the moment of testing? If the quality varies, we may find ourselves wondering, “What happened? This cellulose is not as good as it used to be.” This variable quality often results in fat spots on the compacted SMA surface.

Almost every working road laboratory has conducted many series of draindown tests. The results of companion tests on a wide variety of stabilizers are seldom published. However, the following interesting conclusions can be drawn from large – scale research conducted in the United States (Brown and Mallick, 1994):

• SMA binder draindown was seen to be a time-progressive process; the fast­est run-off occurred within the first 30 minutes of hot mix storage, then it continued over 2 hours during testing. Therefore one can conclude that the major part of the draindown process takes place during the 1st hour after SMA production.

• The most effective stabilizers tested were cellulose and mineral fibers.

• Differences between binder-absorbing stabilizers and viscosity-enhancing stablizers were observed.

• Lack of a stabilizer caused as much as 4% (m/m) mastic drain-off in a 0/19 mm coarse-graded SMA.

During the course of a season of roadwork, we decide, for various reasons, to change the supplier of stabilizers, but we do not always have enough time to check the SMA draindown with the new product (after all, it looks more or less the same). At that time, we have to take into account some unexpected problems and be prepared to react quickly—for example, to increase the quantity of stabilizer, if needed.

JMF – Job Mix Formula – recipe of the mixture.