Category Stone Matrix Asphalt. Theory and Practice

An SMA formula has to be documented and demonstrated (declared). Any SMA mixture made according to the recipe has to meet the standard requirements deter­mined by a given country.

14.5.1 Gradation

The fundamental rules regarding the mix design include the following:

• The gradation should be expressed in mass percentages of the total aggre­gate mix; the accuracy of percentages passing

• all sieves (with the exception of the 0.063 mm sieve) should be expressed to 1%.

• the 0.063 mm sieve should be expressed to 0.1%.

• The content of binder and additives should be expressed in mass percent­ages of the asphalt mixture, with an accuracy of 0.1%.

• The type of fine aggregate used and the adopted ratios in the case of a mix may be given in a recipe or specification.

• The gradation may be described with either “basic sieve set plus set 1[75]” or “basic sieve set plus set 2t”; a combination of sieves from set 1 to set 2 is not permissible.*

The gradation of an SMA mixture should be established with a minimum of five sieves: 0.063, 2.0, D, 1.4D, and the characteristic coarse sieve (a selected sieve between 2.0 mm and D). Basically, the gradation limits, which are given in the stan­dard, must adhere to the rules for preparing NADs to the standard EN 13108-5. Each country, by its NAD, may determine an SMA mix’s gradation envelopes, guided by the following:

1. Overall limits on the target composition displayed in Tables 1 and 2 of the standard

2. Permissible ranges between maximum and minimum values on selected sieves (in Table 3)

The standard allows for the use of additional control sieves, called optional (char­acteristic) sieves, to enable a more precise description of the gradation as follows:

• A characteristic sieve for fine aggregate may be selected between the

2.0 mm and the 0.063 mm sieves; in addition, the standard stipulates the set of sieves to be chosen from 0.125, 0.25, 0.5, and 1.0 mm;

• An optional characteristic sieve for the coarse aggregate may be selected to provide one more additional sieve with a size between 2.0 mm and D.

Finally, to describe the gradation envelope, one can use the following set of sieves:

• 0.063 mm sieve (obligatory)

• Characteristic sieve for the fine aggregate (optional)—sieve between 0.063 and 2.0 mm,

• 2.0 mm sieve (required)

• Characteristic coarse sieve (required)—a selected sieve between 2.0 mm and D

• Additional characteristic coarse sieve (optional)—a selected sieve between

2.0 mm and D

• Sieve D (required)

• Sieve 1.4D (required)

It is worth noting that the freedom to select characteristic sieves gives a chance to choose those sieves that will provide the best possible control of an SMA mixture (e. g., breakpoint sieves). Figures 14.2 through 14.5 depict the position of boundary points for example mixtures SMA 8 and SMA 11 for sieve set + 1 and SMA 10 and SMA 14 for sieve set + 2. As can be seen, the scope of available solutions (positions of overall limits to a target composition) for any of the mixtures is quite broad. Additionally, in the same figures, the German (for SMA 8 and 11) and British (for SMA 10 and 14) gradation envelopes are presented as examples.

According to the standard, the use of RAP for SMA is permissible. The types, quan­tities, and requirements for RAP to be used in SMA mixes should be specified in an NAD appropriate to the intended use.

RAP should be classified according to EN 13108-8 and should conform to the relevant requirements for a particular application.

The maximum size of a particle in RAP cannot be larger than size D of the SMA mix. The quality of aggregate in RAP cannot deviate from the requirements for a new aggregate to be used in a given SMA.

When the used RAP contains road binder (unmodified) and when the binder added to the mixture is road binder then additionally one of two values should be determined: either the penetration at 25°C or the softening point (R&B) of a mixture created by combining the recovered binder from the RAP with the new binder added during SMA production. The test result (Pen25 or SP) of this binder mixture should meet the requirements for the target (design) road binder selected for a given SMA. Formulae for calculating properties of binder mixes can be found in the standard EN 13108-5, Annex A. The method of recovering binder from recy­cled asphalt paving mixture should be in accordance with EN 12697-3 (binder recovery—rotary evaporator) or EN 12697-4 (binder recovery—fractionating column). The penetration should be determined according to EN 1426 and the softening point according to EN 1427. Such an additional requirement is used in following cases:

• In a wearing course when more than 10% (m/m) RAP is used

• In a regulating (leveling) and intermediate course when more than 20% (m/m) RAP is used

When used RAP or the new SMA contains a modified binder or a modifier addi­tive, according to Item 5.1 of the standard the amount of RAP cannot exceed the following:

• 10% by mass of the total mixture if the SMA is intended for a wearing course

• 20% by mass of the total mixture if SMA is meant for a regulating* or intermediate course or a base course

Both the client and the producer of the SMA mix may arrange otherwise, pro­vided that other local (national) regulations are not infringed upon.

14.4.4 Additives

Additives should conform to the requirements for constituent materials; namely, they should have the determined suitability—marked by complying with an appropriate EN standard, ETA, or a demonstrable history of satisfactory use.

Natural asphalt may be employed for SMA as an additive to the road binder or modi­fied binder under the following conditions:

• If it conforms to the requirements of EN 13108-4, Annex B, Tables B.1 and B.2, for natural asphalts with high or low-ash contents, respectively

• When natural asphalt is being incorporated in the road or modified binder by means of the following:

• Intermixing with heated binder in a liquid state in a tank

• Direct batching into a pugmill in the case of natural asphalt in the form of a powder or granulate with particles not exceeding 10 mm

14.4.2 Aggregates

All types of applied aggregate (coarse, fine, all-in,[74] added filler) should comply with the requirements of EN 13043 selected for a specific use. The appropriate NAD with requirements for SMA aggregates corresponding to the standard EN 13043 should be selected (examples of such requirements are detailed in Chapter 5).

The amount of added filler should be fixed. Hydrated lime and cement may also be used as fillers.

Polymer modified binders (PMBs)—which, according to EN 14023, is divided on the basis of penetration at 25°C and softening point (ring and ball)—have also been used.

When a modified binder is being used to enhance a particular property of an SMA mixture, with no reference to binder features (e. g., resistance to fatigue), some additional tests should be performed to confirm the desired effectiveness of a given binder. These tests should be conducted using methods described in the standard EN 12697. Using the results of previous tests is permissible. The origins of this clause of the SMA standard (EN 13108-5) are in the structure of the standard concerning PMB (EN 14023), which has a combination of classes enabling the description of basic requirements for PMB. There is no direct correlation between these require­ments and the functional properties of asphalt mixes. Consequently, the effective­ness of a selected PMB in the asphalt mixture should be checked.

Only constituents with an established suitability are allowed to be used for SMA mixtures. The established suitability means meeting the material requirements of the following:

• A European standard

• A European technical approval (ETA)

• A documented, positive experience with a specific kind of material in the past in which the reference documents confirm the suitability of the material (e. g., test results combined with observations in places of performance)

14.4.1 Binders

Road (paving grade) binder according to EN 12591 and polymer modified binder according to EN 14023 have been used in SMAs. SMA mixtures containing chemi­cally modified binders, which are not classified by the standard EN 14023, are not covered by the clauses of the standard EN 13108-5. Natural asphalts according to EN 13108-4:2005, Annex B, may be added.

14.4.1.1 Road Binder

Road binders after EN 12591 are categorized on the basis of penetration at 25°C and have been incorporated in SMAs. When using them, a binder from the series 30/45 to 330/430 should be selected. This is a very wide range of bitumens (from hard to very soft ones) intended to allow for a wide variety of possible applications depend­ing on local conditions (e. g., climate, traffic loads). In reality, it is for the most part a choice between 50/70 and 70/100.

The standard stipulates that each delivery ticket should furnish at least the following information:

• Name of the manufacturer and mixing plant

• Mix identification code

• Designation of the mixture in the format

SMA D binder

where D is the maximum aggregate size in millimeters and binder is the binder type. For example, SMA 11 50/70 denotes an SMA mixture with a gradation up to D = 11.2 mm, with the road binder 50/70 (according to EN 12591).

Apart from the aforementioned information, a manufacturer should also provide the following: [73]

• Details regarding the compliance of a mixture with requirements concern­ing resistance to fuel and resistance to deicing fluids (if the delivery applies to an airfield)

• Details of additives used

Stone mastic asphalt is defined as a gap-graded asphalt mixture that has bitumen as a binder, and is composed of a coarse crushed aggregate skeleton bound with a mastic mortar.

The standard defines two types of recipes (job mix formulae [JMF]):

• Input target composition—this is the determined composition of the mix­ture given through listing its constituent materials, the gradation curve, and the percentage content of binder added to the mixture; this formula is the result of laboratory validation of the mixture.

• Output target composition—this is the determined composition of the mixture given through listing its constituent materials, the midpoint grada­tion, and the percentage of soluble binder content in the mixture, which are obtained as results of the composition analysis (extraction) of a produced mixture; usually this formula is the result of production validation of the mixture.

An additive is defined as a constituent material supplemented to the mixture in small amounts, (e. g., organic or nonorganic fibers and polymers added to enhance mechan­ical properties, workability, or the color of the mixture).

EN 13108-5 defines the requirements for SMA mixtures for use on roads, airfields, and other trafficked areas. This standard should be read along with other standards as follows:

• EN 13108-20:2006, Bituminous Mixtures—Material Specifications— Part 20: Type Testing

• EN 13108-21:2006, Bituminous Mixtures— Material Specifications—Part 21: Factory Production Control

• EN 12697-x, Bituminous Mixtures. Test Methods

The mutual relationships among those standards are displayed in Figure 14.1.

Sets of properties of asphalt mixtures (Figure 14.1) are listed in the standard EN 13108-x (any from 1 to 7). Each of these properties corresponds to an appropriate test method described in a standard from the series EN 12697-x. However, it should be noted that sometimes the standard provides for more than one test procedure for

Example of relationships between European standards concerning asphalt mixtures.

FIGURE 14.1

determining properties (Figure 14.1 indicates the procedures A, B, …). The standard EN 13108-x (1-7) does not specify which procedure to select but indicates that the right method can be found in the standard EN 13108-20, with directly recommended test methods and procedures provided in tables.

A series of European standards—designated EN 13108 and containing requirements for the design, testing, and production of asphalt mixtures—has been implemented in all member states of the European Committee for Standardization (CEN). It consists of 10 standards:

EN 13108-1:2006, Bituminous Mixtures— Asphalt Concrete

EN 13108-2:2006, Bituminous Mixtures— Asphalt Concrete for Very Thin Layers EN 13108-3:2006, Bituminous Mixtures – Soft Asphalt

EN 13108-4:2006, Bituminous Mixtures – Hot Rolled Asphalt

EN 13108-5:2006, Bituminous Mixtures – Stone Mastic Asphalt EN 13108-6:2006, Bituminous Mixtures – Mastic Asphalt

EN 13108-7:2006, Bituminous Mixtures – Porous Asphalt

EN 13108-8:2005, Bituminous Mixtures – Reclaimed Asphalt

EN 13108-20:2006, Bituminous Mixtures – Type Testing

EN 13108-21:2006, Bituminous Mixtures – Factory Production Control

The standards numbered from 1 to 7 are intended for constructing products harmo­nized with the Construction Product Directive No. 89/106, which is a classification type of standard. This type of standard does not include ready-to-meet sets of require­ments, but only a list of properties with a set of categories-levels of requirements. Basically, they are universal standards to be adopted by any CEN-member state.

It is impossible to establish the same requirements for all of Europe due to its substantial climatic differences and diversified road technology experiences. As a result, the common standard has to reflect these differences. The crucial point is that all countries use the same category symbols for SMA properties and the same testing methods for those mixtures. Owing to this classification standard, each country may individually specify its essential requirements for asphalt mixtures by placing them
in a national application document (NAD). The NAD is understood to be a document introducing an EN standard in national technical regulations, containing a combina­tion of properties and levels of requirements suitable for a given country. An NAD may appear in one of the following form of:

• Technical guidelines (e. g., British PD 6691)

• A national standard that does not contradict the EN standard (e. g., Austrian standard ONORM B 3584:2006)

• A National Annex to a national standard implementing the EN standard (e. g., Slovenian Standard SIST 13108-5)

Apart from the standards concerning separate asphalt mixtures (numbered from 1 to 7), the system contains three additional standards (numbered 8, 20, and 21). The standard EN 13108-8 applies to reclaimed asphalt (RAP), therefore it does not refer to a construction product directive. The standard EN 13108-20, entitled Type Testing, concerns the required range of tests necessary for marking a mixture with the CE symbol (for so-called conformity assessment). The last standard, EN 13108-21 Factory Production Control, includes the description of requirements for asphalt plants and quality control. All parts form a complete system of require­ments for design, testing, production, and the conformity assessment of asphalt mixtures.

The standard EN 13108-5, which describes the classification system of require­ments for SMA mixtures, will be further discussed. Subsequently, the clauses of EN 13108-20 concerning SMA mixtures will be outlined. The present chapter con­cludes with a description of factory production control according to the standard EN 13108-21.

Any reader interested in details of the presented standards should look at the original texts and clauses.

Colored asphalt surfacing is an aesthetically pleasing option. Colored wearing courses can also be executed with fine-graded SMAs (Blazejowski and Styk, 2000) using special synthetic (colorless) binders. Besides their aesthetic appeal, colored surfacing can also be used to mark pedestrian crossings or other safety-related features.

When making colored SMA, it is important to remember to carefully clean the asphalt plant, mixer, and silos, removing “black” mix remains. Loose fibers, specifi­cally those containing no binder additives and bright aggregates, should be applied.

The strong mineral skeleton applied in SMA has attracted some followers. After all, nothing stands in the way of using similar mixes in intermediate courses or base layers. Kjellbase makes such a mix. The concept, drawn on the “real SMA” idea, came into being in the beginning of 1997, when it was created by Kjell Sardal and S. Gouw. The first trial section was placed in 1997 (Sluer, 2001, 2002; Sluer et al., undated). In fact, Kjellbase is a 0/25 mm gap-graded mix. Now let us have a look at

a

a

’K "e3

CD

a

Figure 13.7, which depicts the grading curve of the Kjellbase mix. The gap grading between the 2 mm and 8 mm sieves is clearly visible.

Kjellbase consists of the following (Sluer et al., undated):

• 79% chippings of 8/11 mm, 11/16 mm, and 16/22 mm

• 15% crushed sand

• 6% filler

• 5% modified binder

A small content of fine aggregates (made of only about 5-6% filler and about 15-17% sand fraction) leads to a high void content. Relatively high binder contents close the mix structure, leaving up to 5% (v/v) air voids in the compacted pavement. Since the mix has a large quantity of binder, a mastic stabilizer (e. g., fibers) is required.

Some problems may occur in the laboratory when selecting the right method for the evaluation of mixes of that type. The inventors of the Kjellbase mix concept have estimated that the triaxial compression test may be the proper method.

To summarize the information on Kjellbase, the mineral skeleton of the mix (shown in Figure 2.4.b) is similar to the vision of Figure 2.5. The increased quantity of binder and lower contents of voids in the Kjellbase layer, compared with those of conventional base layers, improves fatigue durability. Thus the mineral skeleton gives that course a higher resistance to permanent deformation.