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Experiences with Stone Matrix Asphalt (SMA)

Sant Kumar Sehgal and Bernd Abele

Review

In the sixties and early seventies, studded tires were used in Germany during wintertime. The wear of asphalt surface courses under the spikes was extreme. Research and investigations on the performance of surface courses of asphalt concrete (AC) and gußasphalt verified that wear and rutting by studded tires was increased by:

high air void content of the surface course, •

low bitumen content, •

inadequate quality and quantity of aggregates. •

In the end of the sixties, these results were the starting point for several large companies of the asphalt paving industry to develop specific mixes which were published under different proprietary names. Later, all these mixes were standardized and called StoneMastic Asphalt or Stone Matrix Asphalt like in India (see IRC:SP:79:2008). In 1975, the use of studded tires was banned. In the following time surface courses with asphalt concrete and prototype SMA mixes showed a different performance of heavily trafficked higways: asphalt concrete with a high bitumen content was disposed to permanent deformation, whereas asphalt concrete with a low bitumen content was disposed to aging and preliminary cracking. However under equal conditions both types of distress were not observed with SMA - surface courses. Practical experiences and observations proved that SMA is a better mix than asphalt concrete. Since high performance highways, especially the "Autobahn," have been an asset of the transportation-system, there has been constant strive for better pavements and asphalt mixes. Consequently, SMA has gradually improved by investigations, tests and feedbacks from highway projects. Finally in 1984, SMA passed the stage of a contractor`s proprietary product. It was standardized and became a part of the German ZTV Asphalt (TechnicalSpecifications and Guidelines for the Construction of Asphalt Pavements), issued by the Federal Department of Transportation. Years after the successful application of the Stone Mastic Asphalt principle in Germany, other European countries are using this philosophy also for heavy traffic roads. Now, manycountries worldwide have taken over the philosophy of the Stone Matrix Asphalt.

The Philosophy of The SMA - Concept

The philosophy of the SMA - concept can be summarized briefly as follows:

High stability to permanent deformation and high wear resistance by an excellent particle interlock and a high content of crushed premium aggregates.

•

Longevity and durability to premature cracking and raveling by a very high content of bitumen and a void less mastic mortar which fills the voids of the stone skeleton and binds it together: high quality and quantity of bitumen are prerequisite for a long useful service life.

•

Stabilizing additives assurances the homogeneity (no binder drainage) of the mix during the manufacturing, transportation and laying. As a side effect, they improve the stability.

•

Summary: High quality and high quantity of bitumen and coarse gap-graded aggregates are the essentials for a long service life of SMA.

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29-02-2012http://www.nbmcw.com/articles/roads/21916-experiences-with-stone-matrix-asphalt-...

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Copyright © 2010 NBM Media Pvt. Ltd. All Rights Reserved.

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Figure 1: Gradation curve SMA 11

Figure 1 shows the gradation curve for SMA 11 according to German ZTV Asphalt specification. Advantages of Stone Matrix Asphalt The SMA-Concept has shown significant advantages for heavy traffic roads worldwide:

High stability against permanent deformation •

High wear resistance •

Slow aging and durability to premature cracking of the asphalt •

Good low temperature performance •

Broad range of application •

Longer service-life •

Higher asphalt content •

Premium materials and additives •

Better long-life behavior •

Countries Using SMA

Specifications

Technical specifications and guidelines for the construction of asphalt pavements are always the conclusion of experience. In the following, some requirements of SMA and its ingredients are explained. The European and German specifications for Stone Matrix Asphalt can be found at Table 1 and Table 2.

Table 1: European Specification SMA 13108-5



Stone Mastic Asphalt SMA 11 SMA 8 SMA 5

1. Aggregate mixture

Passing sieve 16 mm

% by weight 100 (1,4 D)

Passing sieve 11.2 mm

% by weight

90 -100 (D) 100 (1,4 D)

Passing sieve 8 mm

% by weight

50 – 65 90 -100 (D) 100


% by weight

35 – 45 35 – 55 90 – 100

Passing sieve 2 mm

% by weight 20 - 35 20 - 40 20 - 40


% by weight 5 – 13 5 – 14 5 – 14

2. BinderMinimum binder

content BminCategory according to 13108-5 - Table 4,

rectified to factor α

Stabilizing AdditiveAdditives like organic Fibers according to EN

13108-5 - Chapter 4.1

Table 2: German Specifications SMA - TL Asphalt - StB 07

Stone Mastic Asphalt SMA 11 S

SMA 8 S

SMA 5 S

SMA 8 N

SMA 5 N

1. Aggregate mixture C100/0; C95/1; C90/1 C90/1

Passing sieve 16 mm% by

weight 100

Passing sieve 11.2 mm% by

weight 90 -100 100 100

Passing sieve 8 mm % by weight

50 – 65 90 -100 100 90 – 100

100

Passing sieve 5.6 mm % by weight

35 – 45 35 – 55 90 – 100

35 – 60 90 – 100

Passing sieve 2 mm% by

weight 20 – 30 20 – 30 30 – 40 20 – 30 30 – 40

Passing sieve 0.063 mm % by weight

8 - 12 8 – 12 7 – 12 7 - 12 7 – 12

2. Binder

Binder, type and grade25/55-

55 50/70

25/55-55

50/70

45/80-50

50/70 25/55-

55

50/70 70/100 45/80-

50

50/70 70/100

Minimum binder contentBmin 6.6

Bmin 7.2

Bmin 7.4

Bmin 7.2

Bmin 7.4

3. Stabilizing AdditiveMixture content Vol.-% 0.3

4. Mixture/Marshall Spec.

Compaction temperature*

135 ± 5 / 145 ±5

Void contentVmin 2.5

Vmin 2.5

Vmin 2.0

Vmin 1.5

Vmin 1.5

Vmax 3.0

Vmax 3.0

Vmax 3.0

Vmax 3.0

Vmax 3.0

5. Layer

Thickness cm 3.5 – 4.0

3.0 – 4.0

2.0 – 3.0

2.0 – 3.5

2.0 – 3.0

Weight kg/m2 85 - 10075 - 100 30 - 50 50 – 85 50 - 75

Compaction degree % ≥ 97 ≥ 96 ≥ 97Void content Vol.-% ≤ 5.0 - ≤ 5.0

Aggregates Double crushed tough premium aggregates of definite size, soundness, shape etc., e.g. max. 20% of flaky and elongated aggregates; all aggregates (aggregates, sand, filler) need to be under a permanent quality assurance, which is independent an from a third party. Mineral type The mineral type is specified in the TL Gestein (technical deliver specifications of mineral aggregates). The TL-Gestein is based on the European Specification EN 13043. As minerals should be used granite, diabas, basalt etc. Limestone, sandstone and "soft" minerals shouldn't be used. Because of the longevity of the SMA and the low content of crushed sand, the minerals/aggregates must have a high resistance to polishing (high polished stone value; PSV 48) and they also need to have a good aggregate abrasion value (LA 20). Furthermore, if available, the use of two different types of minerals can improve the performance and skid resistance. Filler Usually mineral filler (grounded limestone) is used.



Sand 100% of the sand fraction must be crushed. Gradation Gap-graded: very high content of crushed coarse aggregates, see Figure 2.

Figure 2: Good coarse aggregate skeleton

If you have no problems with the aggregates used in asphalt concrete, you will also have no problems in the SMA-mixture. If you have problems with the aggregates used in asphalt concrete you will have more problems using these aggregates in SMA. Bitumen The use of the particular bitumen quality predominantly depends on climate and traffic conditions. Paving grade bitumen and polymer modified bitumen is specified by European standard EN 12591 and EN 14023 (PmB). Additives According to the specifications the following additives are permitted:

Cellulose fibers •

Polymers •

Artificial silicious material. •

The selection of the additive and its quantity is a part of the duties and responsibilities of the contractor. Reclaimed asphalt isn't being used for SMA; the feasibility has not yet been evaluated and experienced. More requirements are given in Table two. Range of Application SMA is adequate and it is recommended for any surface course. It is especially used for all heavy traffic roads: SMA is much more economical (Long-life Asphalt pavements 2007, EAPA) than asphalt concrete. Since the specifications were introduced in 1984, the use of SMA has significantly increased. The Stone Matrix Asphalt is appropriate for:

Highways •

Federal roads •

Rural roads •

Urban roads •

Airports •

Industrial areas •

There are no special regulations in the European Specifications 13108-5 for the design of SMA surface courses. The choice of SMA mix design is at the discretion of the client when preparing the RFP (requirements for pavement). For the highway projects, SMA is the most favourite and acceptable asphalt mix and therefore - by experience – it is used intensively for these highway pavements. By the selection of SMA, there is a strong tendency to a smaller top size, i.e. from 11 mm to 8 or 5 mm top size aggregates, because of the following reasons:

Lower unit price per sq. m because of smaller thickness is necessary •

Less noise under traffic •

Better skid resistance •

By the same reasons a SMA 16 mm surface course is not standardized in 16 mm SMA mixes of 16 mm and even larger top size are not used:

a larger top size requires a greater thickness which means a higher unit price per sq. m,

•

larger top sizes generate more traffic noise; but there is a strong demand of public to reduce traffic noise.

•



Mix Design

There is no specific mix design method for SMA, but there are information sheets for initial suitability tests on hot mix asphalt. The steps of the evaluation of an appropriate job mix formula (JMF) according to the information sheet mentioned above are as follows: a) In accordance with the RFP (requirements for pavement) and with respect to its experiences with former JMFs, mixing, paving, performance during the time of warranty, and - last but not least - the price, the contractor selects the aggregates and the filler. with the selected material (aggregates, sand, filler, additives) and on the basis of the feedback from other sites and JMFs, a tentative gradation is chosen. But there are requirements for abrasion value, polish stone value, freezing-thawing-test, etc. b) Mixes with the required minimum asphalt content and with three adjacent asphalt contents are prepared.

c) Marshall specimen are prepared at 135/145 ± 5°C and by 50 blows on each side the Marshall tests are running for the evaluation of the air void content which must range from 3 to 4 % by volume (i.e depending on German climate conditions). If the required air void content is not achieved, the following alterations of the tentative mix within the enforceable limits of the specifications are recommended:

Change total content or content of single sizes of crushed aggregates •

Change filler content •

Change mortar content •

It has to be emphasized that the Marshall-stability and -flow are not an adequate basis for the right choice and evaluation of the SMA job mix formula.

Figure 3: Schellenberg Binder Drainage Test

d) Drain-down test: Additives are necessary to avoid drainage of binder from the coarse aggregates during mixing, transportation and paving. Therefore, a drain-down test must be performed for the evaluation of the appropriate and necessary content of the additive. The Schellenberg Binder Drainage Test (Figure 3) is standardized in Germany and in the European Union (EN 12697-18: Bituminous mixtures – Test methods for hot mix asphalt – Part 18: Binder drainage). On the basis of the mix design results, the contractor decides on the JMF and submits it to the client for his approval.

Additives

Although fibers, polymers and silicious materials are also permitted, cellulose fibers are used very extensively. By tests, trials and by experience, it was discovered that only the use of polymer is not adequate to avoid segregation of the gap-graded coarse aggregates and the high bitumen content. The additive has to be a bitumen carrier; polymer as a bitumen improver is not sufficient. All SMA-JMFs with polymers also had a lower bitumen content than required or they had additionally fibers mixed in to achieve the requirements of each country. Cellulose fiber shows no chemical reaction with the bitumen and it is inert to mixing temperatures and it works excellently. Because of the performance of the cellulose fibers, the technical assistance of the supplier and - last but not least - the relatively low price of the fibers, the usage and the market share of the different types of additives for SMA e.g. in Germany is as follows: Cellulose fibers 95%, the rest mineral fibers and other additives.

Mixing

Mixing and paving is neither specified in the specifications nor in the RFP. Method and equipment are within the assigned responsibility of the contractor. His technical expertise and accountability are essential.

Table 3: Minimum and maximum temperature of SMA-mixture in °CBinder quality Stone Mastic Asphalt (SMA)

50/70 150 – 19070/100 140 – 180



25/55-55 150 – 19045/80-50 140 – 180

Drying of the unbound coarse material needs less heat than it is necessary for standard asphalt concrete (AC) and therefore the heat of the dryer must be reduced. Short term aging of the asphalt is very likely when it is mixed with too hot coarse aggregates. Drying of the minerals needs to be executed at such a temperature that the heat of the mix in the

pugmill does not exceed 190°C. The lower limit values are valid for the discharged asphalt mix at the construction location; The upper limit values are valid when the asphalt mix leaving the pugmill or silo at the mixing plant. Mixing times depend on the type of plant.

Figure 4: SMA mixing times with bituminous coated fiber pellets

In Europe, there are mostly batch mixing pugmills. Drum mixing is possible by using bitumen coated cellulose fiber pellets. As an example, the time and sequence of the mixing process is demonstrated in Figure 4. To enhance and improve the quality, the mixing time must be extended. Accordingly, the production rate of SMA is lower than of asphalt concrete: Quality has its price. Exact proportioning of the fibers is important. A variation of the fiber content ± 0.1% can result in a variation of the bitumen content ± 1.0% without being recognized visually. There are different possibilities to dose the fibers:

Dosing by big bag with automated dosing system (see Figure 5). •

Small packages – dosing by hand; this procedure needs much manual labor; it is obsolete, but still practiced (see Figure 6).

•

Figure 5: Automatic dosing system Figure 6: Manual dosing system

According to the specifications, quality control by the contractor is always necessary and required. Samples for quality control are to be taken, i.e. every 500 tons. Additionally and as a general rule, a quality assurance of the mixing plant by an independent third party is required.

Paving



Figure 7: Paving of SMA

Paving and compaction procedure is similar to every standard asphalt Figure 7.

Trucks must be covered until the SMA is discharged into the material hopper at the front of the road paver.

•

Although the asphalt content of the SMA is very high, a tack coat is recommended to ensure and improve the bond between the layers.

•

The temperature of the SMA in the road paver should be at least 150 (140)°C. •

An adapted pre-compaction of the asphalt paver is useful and should be achieved. •

Compaction should be started as soon as possible and the roller should move as close to the paver as possible. Steel wheel rollers of approximately eight to ten tons for compaction are necessary. Vibrating should be executed mostly two or three times. There is a hazard of mortar enrichment on the surface, if using too much vibrating roller passes. The consequence is a very low initial skid resistance. Vibrating is not advisable on thin layers of approximately less than 25 mm thickness

and at layer temperatures under 100°C, see Figure 8. Since sucking up of the mortar is very likely compacting with a pneumatic roller can be a hazard.

•

Figure 8: Compaction temperature of SMA

Because of the high asphalt content, the initial skid resistance can be low and it can cause safety problems on high traffic highways. Therefore, a surface treatment is recommended by spreading 0.5 to 1.0 kg of fine crushed aggregates per sqm. After the first or second roller pass aggregates which are free of fines and 1/3 mm should be used.

•

During, the execution of the work there is a quality assurance by the client at every 6,000 sqm: SMA - samples are taken from the paver and tested for:

•

Drain-Down Test •

Asphalt content •

Fiber content •

Gradation •

Marshall air void content •

Softening point ring and ball tested at the recovered bitumen •

Cores are taken from the layer and tested on: •

Air void content of the layer •

Compaction ratio •

Bond between the layers •

Solutions for Future Demands

Figure 9: SMA-Binder; Autobahn A73 - Germany



Due to the advantages of SMA, a variety of new developments, with gap-graded principle, has been the result. Solutions which will be able to serve future demands.

Stone Matrix Asphalt Binder Course (SMA-Binder, see Figure 9) •

Noise Reducing Stone Matrix Asphalt •

Porous Asphalt (PA) •

Twin Layer Porous Asphalt (TLPA) •

Authors

Mr. Sant Kumar Sehgal, Rettenmaier India, Mr. Bernd Abele, J. Rettenmaier & Söhne GmbH + Co KG Germany,

NBMCW March 2011

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