STONE MASTIC ASPHALT

ABSTRACT

            Since the 1960s, Stone Mastic Asphalt (SMA) pavement surfaces have been used successfully in Germany on heavily trafficked roads as a durable road surfacing to resist wear from studded tyres. Because of its excellent performance characteristics, road authorities in Germany as well as major European Countries quickly adopted SMA as a standard wearing course. During the last few years, SMA has become one of the most popular asphalt pavements.

The deformation resistant capacity of SMA stems from a coarse stone skeleton providing more stone-on-stone contact than with conventional Dense Graded Asphalt (DGA) mixes. Improved binder durability is a result of higher bitumen content, a thicker bitumen film, and lower air voids content. This high bitumen content also improves flexibility. Addition of a small quantity of cellulose or mineral fibre prevents drainage of bitumen during transport and placement. The essential features, which are the coarse aggregate skeleton and mastic composition, and the consequent surface texture and mixture stability, are largely determined by the selection of aggregate grading and the type and proportion of filler and binder. SMA has proved superior on heavily trafficked roads and industrial applications.

SMA has distinct advantages as a surfacing, due to its potential for high resistance to fatigue and rutting (wear and tear due to traffic loading).

Limitations of SMA are the increased material cost, increased mixing time, possible delays in openings (the road) etc.

This report gives an overview of the history of SMA, its performance characteristics, composition, advantages, disadvantages as well as its applications. 

CONTENTS 

                                                                                                           

1.      INTRODUCTION                                                                                                                                                                   

2.      HISTORY OF SMA                                                                                    

                                                           

3.      PERFORMANCE CHARACTERISTICS OF SMA                              

                                   

4.      COMPOSITION OF SMA                                                                         

                                                           

5.      ADVANTAGES                                                                                          

                                                                                               

6.      DISADVANTAGES                                                                                          

                                                           

7.      APPLICATIONS                                                                                        

                                                                       

8.      CASE STUDY                                                                                             

8.1 Fine-graded Stone Mastic Asphalt –Pavement Rehabilitation         

      of Bloomington Road (York Region Road 40)

8.2 Stone mastic asphalt (SMA) road trial: Ankara, Turkey                  

      September 1999

9.      CONCLUSION                                                                                            

                                                                                   

10.  REFERENCE                                                                                                                                                                                                                                                            

                                                                                                           

LIST OF FIGURES

                                          

Fig. 3.1 Stability in SMA mix.                                                                               

Fig. 4.1 Composition of SMA                                                                                

Fig. 6.1 Moisture seeping from surface after rain / white fines

1.     INTRODUCTION

            Stone Mastic asphalt (SMA), otherwise known as Stone Matrix Asphalt / Split Mastic Asphalt, was developed in Germany in the mid of 1960's and it has spread throughout Europe and across the world in 1980's and 1990's respectively. The excellent performances include resistant to mechanical and temperature deformation, cracking, and particularly rutting, resistant to weathering actions such as aging and low temperature cracking. Durability is excellent even under slow moving heavy traffic. The textured surface increases skid resistance and provides environmental and driving comfort by reduced noise level, and improved visibility in rainy days.

SMA provides a deformation resistant, durable, surfacing material, suitable for heavily trafficked roads. SMA has found use in Europe, Australia and the United States as a durable asphalt surfacing option for residential streets and highways. SMA has a high coarse aggregate content that interlocks to form a stone skeleton that resist permanent deformation. The stone skeleton is filled with mastic of bitumen and filler to which fibres are added to provide adequate stability of bitumen and to prevent drainage of binder during transport and placement. Typical SMA composition consists of 70−80% coarse aggregate, 8−12% filler, 6.0−7.0% binder, and 0.3 per cent fibre. The deformation resistant capacity of SMA stems from a coarse stone skeleton providing more stone-on-stone contact than with conventional dense graded asphalt (DGA) mixes. Improved binder durability is a result of higher bitumen content, a thicker bitumen film and, lower air voids content. This high bitumen content also improves of flexibility. Addition of a small quantity of cellulose or mineral fibre prevents drainage of bitumen during transport and placement. The essential features, which are the coarse aggregate skeleton and mastic composition, and the consequent surface texture and mixture stability, are largely determined by the selection of aggregate grading and the type and proportion of filler and binder.

SMA is characterized by a stone-on-stone structure. SMA uses a high proportion of larger stones or aggregate that contacts each other. This skeleton of larger stones resists heavy loads by transmitting them to the pavement below. If the under laying pavement is sufficiently strong then the SMA will resist the heavier loads effectively. (A surfacing cannot compensate for a weak pavement).

The bituminous mastic is intended to hold the aggregate in place and to inhibit the ingress of moisture into the pavement and to provide durability. The mastic consists of bitumen and fine aggregate particles; it may also include a polymer modified bitumen and filler material to increase the mastic’s strength. Fibres may also be added to stabilize the bitumen and to prevent the binder segregating from the aggregate during transport and placement.

It is important that the aggregate material consist of only the larger stones (in the structure) and fines to provide effective mastic. The intermediate size aggregates are not included, as these keep the larger aggregate apart and reduce the strength of the SMA. 

2.     HISTORY OF SMA

          Stone Mastic Asphalt (SMA), an asphalt paving mixture, was originated in Germany in the 1970s to provide maximum resistance to rutting caused by the studded tyres on European roads. Strabag, a large German construction company, led to the development of SMA. After the use of studded tyres was no longer allowed, it was found that SMA provided durable pavements which exhibited such high resistance to rutting by heavy truck traffic and proved to be extremely effective in combating wear. In recognition of its excellent performance a national standard was set in Germany in 1984. Since then SMA has spread throughout Europe, North America and Asia Pacific. Several individual Countries in Europe now have a national standard for Stone Mastic Asphalt (SMA), and CEN, the European standards body, is in the process of developing a European product standard. In the United States, Australia, New Zealand and in Asia, the use of SMA is increasing in popularity amongst road authorities and the asphalt industry.

3.     PERFORMANCE CHARACTERISTICS OF SMA

The development of modern pavement technology is needed to accelerate significant improvement of pavement quality of highways, airport runways and urban roads.

Fig. 3.1 Stability in SMA mix.

SMA meets the following demands upon an asphalt pavement:

·         Good stability at high temperatures

·         Good flexibility at low temperatures

·         High wearing resistance

·         High adhesive capacity between the stone granules and the bitumen

·         A mix with no tendency to separate

·         Good skid resistance

·         Reduced water spray

·         Lower traffic noise

Good stability at high temperatures

SMA mix has a self-supporting stone skeleton of crushed high quality coarse aggregate, which provides an increase in internal friction and shear resistance and hence its extremely high stability.

Good flexibility at low temperatures

SMA mix has a binder rich mastic mortar which has superior properties over dense graded asphalt in resisting thermal cracking.

High wearing resistance

SMA mix has low air voids, which make the mix practically impermeable, and provide satisfactory ageing resistance, moisture susceptibility and durability.

High adhesive capacity between the stone granules and the bitumen

With the increase of the amount of filler, cellulose fibres are added as stabiliser. The three dimensional structure of cellulose fibre assists the bitumen to maintain a high viscosity, thickens the bituminous film and improves the bitumen/aggregate adhesion.

A mix with no tendency to separate

            An efficient stabilisation of the mastic in order to prevent its segregation from the coarse particles.

Good skid resistance

Because of the macro-texture of the road surface and the use of coarse aggregates with a high Polished Stone Value, SMA pavement achieves a better level of skid resistance.

Reduced water spray

Because of its greater texture depth, there is less water spray, and at night there is fewer glares reflected from the road surface and better visibility of road markings.

Lower traffic noise

SMA road surfaces generally offer lower levels of noise due to the texture properties.

4.     COMPOSITION OF SMA

Stone Mastic Asphalt is characterised by its high stone content which forms a gap-graded skeleton-like stone structure. The voids of the structural matrix are filled with high viscosity bituminous mastic. The high stone content of at least 70% ensures stone-on-stone contact after compaction. The required degree of mastic stiffness is achieved through the addition of crushed sand.

SMA mixes have a bitumen content of minimum 6.5%. The bitumen in the gap-graded mix is stabilised during the mixing process, intermediate storage, transportation, surfacing and compaction through the addition of cellulose fibre stabilising additive.

Addition of cellulose fibre does not chemically modify bitumen, but rather enhances physical property of the finished product by allowing the use of higher bitumen contents. It tends to thicken or bulk the bitumen so that it does not run off the aggregate prior to compaction. The content of cellulose fibre is 0.3% by weight of mixture. If the technological requirements of SMA are fully met, good results can be obtained by just using standard bitumen and a cellulose fibre drainage inhibitor.

Fig. 4.1 Composition of SMA

4.1 Materials Used

1.      Coarse and fine aggregate

2.      Bitumen

3.      Fibre

4.      Filler

4.1.1 Coarse and fine aggregate

The aggregates are crushed by using jaw pressure to get different size of aggregates varying from 16 mm to 75 micron. The coarse aggregate must be hard, durable, and roughly cubical in shape when crushed. Qualities of aggregates were check through various tests like Impact Value Test, Crushing Value Test, Los Angel’s Abrasion Value Test, Flakiness and Elongation Index Test.

4.1.2 Bitumen

Bitumen  act as a binder in SMA mix. Different grade of bitumen are used in different mix like hot-mix or gap-graded mix or dense-graded mix. For preparation of SMA mix we used 60/70 bitumen.

4.1.3 Fibre

Fibres are used as stabiliser in SMA mix. Fibres help to increase the strength and stability and decrease the drain down in SMA mix. There are different types of fibres are used in SMA mix like cellulose fibre, polymer fibre, natural fibre and mineral fibre.

4.1.4 Filler

Filler is used in SMA mix for better binding of materials. Rock dust, slag dust, hydrated lime, hydraulic cement, fly ash, mineral filler and cement are used as filler in SMA mix, also we can use the fine aggregate below 75 micron as filler.

5.     ADVANTAGES

·         20-30% increase in pavement life over conventional pavements

·         Good aggregate interlock

·         Low permeability

·         Improve in skid resistance due to the high percentage of fractured aggregate to motoring public particularly on wet pavement.

·         Surface texture characteristic may reduce sound from the tyre and pavement contact as well as water spray and glare.

·         Strength and stiffness derived from binder and aggregate structure

·         Relatively high binder contents provide good Durability

·         Durability (longer in-service life) of SMA should be equal to, or greater than, DGA and significantly greater than OGA.

·         It provides a textured, durable and rut resistant wearing course.

·         Surface texture characteristics are similar to OGA, so noise generated is lower than DGA but slightly higher than OGA.

·         It can be produced and compacted with the same plant and equipment as for normal hot mix DGA using procedural modifications.

·         SMA can be used on heavily trafficked roads where good deformation resistance is required.

·         Surfacing may reduce reflective cracking from underlying cracked pavements due to its flexible mastic.

·         At the end of its service life it is 100% recyclable.

6.     DISADVANTAGES

·         SMA mix requires higher mixing temperature.

·         Potential construction problem with SMA mixtures are drainage and bleeding.

·         Storage and placement temperatures cannot be lowered to control drainage and bleeding problem due to the difficulty in obtaining the required compacted.

·         Increased material cost associated with high bitumen and filler content.

·         Increased mixing time and time taken to add extra filler may result in reduced productivity.

·         Possible delays in openings (the road) as SMA should be cooled to 40°c to prevent early flushing of the binder to the surface.

·         Needs more carefully monitoring the composition at the mixing plant.

·         Moisture seeping from the SMA surface for long periods after rain.

·         White fines on the surface of the pavements.

·         Premature rutting

·         Stripping of asphalt layers below the SMA surfacing.

·         Potholing

Fig. 6.1 Moisture seeping from surface after rain / white fines

7. APPLICATIONS

Stone Mastic Asphalt has proved superior on heavily trafficked roads and industrial applications:

·         with high lorry frequency

·         intense wheel tracking

·         at traffic lights

·         at intersections

·         on highways

·         on gradients

·         on bridges

·         in bus lanes

·         at bus-stops

·         in car parks

·         in harbours

·         on airport runways

·         on un/loading areas

Gap-graded Stone Mastic Asphalt reduces noise emissions considerably. The macro texture of this road surface absorbs traffic noise. Because of its noise absorptive property, this surface is very suitable for access roads in residential areas and on estates. Fine Stone Mastic Asphalt grades laid in thin layers are used extensively for preventive maintenance and road repair purposes. The stone skeleton matrix can accommodate unevenness of the underlying pavement to improve driving comfort.

8.     CASE STUDY

8.1 Fine-graded Stone Mastic Asphalt –Pavement Rehabilitation of Bloomington          Road (York Region Road 40)

Bloomington Road is a two-lane roadway that carries more than 10,000 AADT with approximately 10 % heavy commercial vehicle mostly hauling aggregate to the Greater Toronto Area. Prior to rehabilitation, the pavement was severely oxidized and thermal cracking was extensive. The pavement design included an in-place recycling technique to mitigate reflective cracking and 100 mm of new surfacing HMA. The partial depth recycling process was selected using a rapid curing recycling system to accelerate the buildup of cohesion of the recycled material. A heavy duty dense graded HMA was selected as a binder course, while a fine-graded SMA was selected as a thin surfacing course. Both HMA mixes were tested for rutting using a European rut testing device to ensure rut resistance performance. Both mixes were produced using polymer-modified bitumen to mitigate both thermal cracking and permanent deformation.

The current concept of Stone Mastic Asphalt (SMA) or “Split Mastic Asphalt” was developed in Germany in the mid-sixties and introduced in Canada in 1991. Stone Mastic Asphalt is a gap graded bituminous mixture with a high content of chippings which constitute an interlocking mineral skeleton to resist permanent deformation. The space within the chippings skeleton is filled to a large extent by a mortar rich in bitumen-filler mastic to provide durability. Fine-graded SMAs is a specific category of SMA produced using single-size chippings no greater than 6.7 mm. The resulting mixture provides an aggressive but fine macro-texture conducive to surface drainage and good frictional properties. The rolling noise reduction has been reported to be as much as 3dB (A) compared to dense graded surfacing mixes.

Bloomington Road is a two-lane rural roadway located in the heart of York Region. Bloomington Road is a major York Region arterial road that runs between the York-Durham region boundary line and Bathurst Street. The existing roadway was built to its present standard in 1969.

The 2005 rehabilitation project was located between Hwy 48 and Kennedy Road. The length of the project was the equivalent of two concessions, which equates to approximately 4.0 km. The total roadway surface area to be rehabilitated was 30,000 m2. The rehabilitation project did not include the roadway area at the intersection of Bloomington Road and McCowan Road located in the middle of the stretch of roadway between Hwy 48 and Kennedy Road. This section of Bloomington Road carries more than 10,000 AADT with approximately 10 % heavy commercial vehicle mostly hauling aggregate to the Greater Toronto Area. The volume of traffic is not only high but it is also considered very aggressive.

Prior to the 2005 roadway rehabilitation, the bituminous surface was severely oxidized and thermal cracking was extensive. Yet, the longitudinal & transverse profiles of the roadway were still in relatively good condition and there was no sign of major structural failures. The geotechnical consultant recommendation for the rehabilitation of the roadway included an in-place partial depth recycling technique to mitigate reflective cracking and 100 mm of new surfacing HMA. The Region elected to select roadway rehabilitation techniques that were compatible with the recommended rehabilitation strategy proposed by the consultant, but could also provide additional safe guards to the Region with respect to constructability and long term performance. The recommended in place cold recycling process was replaced with a rapid curing partial depth cold recycling process to accelerate the buildup of cohesion of the recycled material. The traditional binder course HMA was replaced with a heavy duty dense graded HMA specifically design to resist rutting. Finally, the recommended dense graded HMA surfacing was replaced with an SMA for durability and rut resistance reasons. Both HMA mixes were tested for rutting using a European rut testing device to ensure rut resistance performance and both mixes were produced using polymer-modified bitumen to mitigate thermal cracking and permanent deformation.

8.2 Stone mastic asphalt (SMA) road trial: Ankara, Turkey September 1999

Stone Mastic Asphalt (SMA) is a premium quality bituminous wearing course material having excellent durability and performance properties. It is routinely used in Germany, where it was originally developed, and is also increasingly being used in other European countries and further afield. The proven performance of SMA under high traffic and high load situations is primarily due to the material having a high stability which resists rutting. The binder content is also high which provides for its good durability. In Germany, and elsewhere, the binder content is further enhanced by the addition of fibres (usually cellulose) which serve as a ‘binder carrier’; this results in an extremely durable mixture.

BP Bitumen has been supplying binders (both paving grade and PMB) for SMA applications for many years. As the demands placed upon the highway increase, there is a greater use of PMBs in SMA mixtures. The additional benefits of the PMBs are particularly seen in better fatigue and low temperature crack resistance.

Recently, BP bitumen has promoted the use of PMB technology in Turkey. It is considered that the Turkish highways would greatly benefit from the use of bituminous mixtures containing PMBs’ given both the increasing traffic loading and the seasonally high summer temperatures. As part of BP Bitumen’s commitment to supplying and supporting high quality products, an initial road trial was proposed to introduce alternative designs based on proven PMB technology. For this, BP Bitumen, in collaboration with Enfalt, supplied a PMB for use in an SMA road trial near to Ankara, Turkey. The binder had to meet the rigorous requirements of the newly introduced Turkish PMB Specifications.

The mix design was developed by the Turkish Highway Authorities (TCK). A limited laboratory test programme was carried out to ascertain the suitability of the aggregates and filler for the SMA design. A feature of the design (based on a 0/12.5 grading) was that fibres could not be used because the resultant material would be prohibitively expensive for the Turkish market. The Olexobit ‘TS3’ binder (150 tonnes) was supplied direct to the contractor’s mixing plant prior to mixing.

A limited plant mixing trial was carried out to ensure compliance with the specified grading. All mixing was carried out by the asphalt contractor, Cemil Ozgur, at their asphalt plant about 30 km from the road site. The road trial site is about 80 km south of Ankara on the southbound lane of the Bala-Kulu section of the Ankara-Konya highway.

The Olexobit ‘TS3’ binder (150 tonnes) was supplied direct to the road contractor’s mixing plant (Cemil Ozgur) prior to mixing. The tank storage was approximately 60 tonnes. It is recognised that it is impossible to fully drain out a binder tank at the mixing plant and it is highly probable that the first delivery would have been contaminated with about 1-2 tonnes of normal paving grade bitumen (60/70). However, this is considered to be typical. Binder samples taken from the binder storage tank during the first day’s mixing showed slightly inferior properties to samples taken a number of days later.

BP Bitumen provided recommendations for the mixing and compacting temperatures based on the design mixture, also taking into account the requirements given in the Turkish SMA standard. It was also stressed that, for compaction, only steel-wheeled rollers should be used, without vibration if possible. A limited plant mixing trial was carried out to ensure compliance with the specified grading. Adjustments were made on a number of occasions (both before and during the trial) to cope with the higher than normal fine fraction (compared with AC production). No initial laying trial was carried out at the plant.

The asphalt mixing plant is about 30 km from the road site. The road trial site is about 80 km south of Ankara on the southbound lane of the Bala-Kulu section of the Ankara-Konya highway. The trial site, comprising about 2,500 tonnes of SMA material laid 40mm thick, formed part of a much larger contract to reconstruct the two-lane highway.

Apart from the SMA trial section, the remainder was surfaced with 40mm of an AC wearing course mixture using 60/70 pen. The wearing course was laid on a recently reconstructed AC base course (binder) layer. The weather conditions throughout the day (26th September 1999) were sunny with little, or no, breeze. The ambient temperature was about 35°C. The surface was lightly tack-coated before the application of the SMA material. The material arrived on site at temperatures in excess of 175°C. No sheeting was used to cover the asphalt trucks. Laying commenced at around 11:30 and it soon became clear that there were problems with the laying of the material; they were:

·         binder drainage from some of the asphalt trucks

·         binder rich areas in the middle 2m of the 6m paved width

The problems were effectively overcome by:

·         reducing the binder content by 0.1 per cent (5.5 per cent)

·         reducing the mixing temperature to 165°C maximum

·         increasing the speed of the paver screws to ensure adequate transverse distribution of material

·         more carefully monitoring the composition at the mixing plant

Later compositional results from the asphalt mixing plant showed the mixture to be also low on filler; this would have exacerbated the problems seen in the laid mat. Where there were binder rich areas, they were treated by the application of 12.5 to 3 mm grit with the hope of soaking up the excess binder.

Given that this was the first time the asphalt crews had experience in both an SMA type material and the use of PMBs, these initial problems were almost to be expected. In fact, one could argue that a lot of experience was gained in the first day of laying the PMB modified SMA. In particular, it highlighted the need for good temperature and material quality control. Additionally, it was noted that the paving operations stopped after each lorry load because of there not being another lorry present to continue with the laying. It is recommended that the paving should not commence until one can guarantee a continual paving operation. This should ensure a better laid product.

Once these initial problems were identified and resolved, the material laid well and the appearance of the surfacing was good. No testing was carried out on the finished surface, but the texture appeared good. No application of grit (nominal size of 3mm) was made to provide for an early life skidding resistance; it was assumed that, after a couple of weeks, the surface binder film would be removed by the traffic to expose the coarse aggregate.

The revised recommendations mentioned earlier (above) were adopted by the asphalt contractor for the remainder of the contract. Photographs 1 to 4 show the general paving and rolling operations.

The following key conclusions are made from the SMA road trial using Olexobit ‘TS3’ near to Ankara, Turkey:

·         A good working relationship, involving the Turkish Highway Authorities (TCK), Cemil Ozgur, Enfalt, and BP Bitumen, resulted in close collaboration at all stages during the development and placement of the SMA material.

·         The 0/12.5 SMA mix design was satisfactory but could have been improved through the use of better quality filler. For a higher PMB binder content SMA mixture using these aggregates, fibres would be required.

·         There were problems initially with the laying but these were easily overcome, once the causes had been identified. Many of the problems would have been solved prior to the actual road trial if a plant trial had been carried out first.

9. CONCLUSION

Stone Mastic Asphalt has proved superior on heavily trafficked highways all over the world during recent years. The use of SMA is increasing in popularity amongst road authorities and the asphalt industry.

SMA’s longer service life gives it a better return on investment than most alternative materials even though the initial costs may be higher. Given that a life span increase of at least 5–10 years can be obtained and that additional advantages covered earlier are gained, it is clear that the choice of SMA can be a good investment.

As a result of different climatic conditions in individual areas, there must be limited differences in mix specification relating to voids, binder content and binder stiffness. In wet and cold regions a lower void content and higher bitumen content is used whilst in drier and warmer regions the void content is generally higher and the binder content lower with a stiffer binder. However, aggregate grading should remain fairly consistent other than in exceptional cases such as wearing course for airport runways.

To gain the maximum benefit from SMA it is important to ensure that the mixture is well designed and a high standard of production and lying is maintained.

STONE MASTIC ASPHALT – The asphalt pavement for the New Millennium

10. REFERENCE

1.      AAPA (2000) Stone Mastic Asphalt Design and Application Guide, AAPA Implementation Guide IG-4

2.      Austroads (2002) Asphalt Guide AP-G666/02

3.      Austroads (2003) Selection and Design of Asphalt Mixes: Australian Provisional Guide. APRG Report 18, ARRB Transport Research

4.      Austroads (2003) Guide to the selection of road surfacing, AP-G63/03

5.      Troutbeck.R, Kennedy.C 2005, ‘Review of the use of Stone Mastic Asphalt (SMA) surfacing by the Queensland Department of Main Roads’, Queensland University of Technology

6.      Woodward, WDH., Woodside, A.R., Jellie, J.H. 2005 ‘Early and mid life SMA skid resistance’, International Conference Skid Resistance, Christchurch, New Zealand

7.      European Asphalt Pavement Association. 1998. Heavy Duty Surfaces. The Arguments for SMA

8.      National Asphalt Pavement Association, U.S.A. 1994. Designing & Construction SMA Mixtures-State-of-the-Practice

      Amazon.com links on Stone Mastic Asphalt

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