PAVEMENT WEAR CAUSED BY THE USE OF STUDDED TYRES - DIVA
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VTI notat 6A-2000
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Pavement wear caused by
the use of studded tyres
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Author Manuel Gonzalez Arrojo
Research division Road Maintenance and Operations,
Transport Systems
Project number 12501
Project name University co-operation
Sponsor VTI
Distribution Free
Swedish National Road and
Transport Research InstituteFOREWORD From 18 11 January to 24th June 1999 have been performing my final work at VTI (Swedish National Road and Transport Research Institute). The work concerns the use of studded tyres and its environmental and health effects. The work has been developed completely at VTI, where I had the opportunity of knowing the key opinions about the studded-tyre problem. My stay at the VTI has taken place within the framework of the Leonardo da Vinci Community programme of student s exchange between my Spainsh university (Oviedo University) and the University of Linko'ping, Sweden. My work has been supervised by Lennart Folkeson to whom I wish to express my sincere appreciation for support and help during the performance of the work. My thanks are also due to people at the lab of the VTI, especially to Fredrik Nilsson for his support during the laboratory work. Thanks are also due to Goran Blomqvist ,Peet Ho'beda, Torbjorn Jacobson, Olle Nordstro'm, Kent Gustafson, Gudrun Oberg, Mats Wiklund, personnel of the VTI library and in general to all the people at the VTI who helped me with whatever problem I had. It has been very easy to work at the VTI these last six months. I would also like to thank Per-Ame Melkerud for the performance of XRD analysis and Tony Barter and Timo Unhola for all the information they sent to me Via e mail. My thanks are also due to the coordinators, Janerik Lundquist at the University of Linkoping and Adenso Diaz, Oviedo University. I also want to thank Ruth Aguilar, teacher at the University of Linkoping, for her help whenever needed. Finally, thanks are also directed to all my personal friends in Linko'ping who contributed to making this period one of the best in my life. Linkoping in June 1999. Manuel Gonzalez Arrojo. VTI notat 6A 2000
VTI notat 6A 2000
CONTENTS PAGE
SUMMARY ........................................................................................................................................................... ..7
1. A LITERATURE REVIEW .............................................................................................................................. ..9
1.1. ASPHALT .................................................................................................................................................. ..9
1.1.1. Description ........................................................................................................................................... ..9
1.1.2. Types of asphalt in Sweden ................................................................................................................... .. 10
1.1.3. Aggregates ......................................................................................................................................... .. 12
1.1.4 Bitumen ............................................................................................................................................... ..15
1.2. STUDDED TYRES ...................................................................................................................................17
1.2.1. Introduction ........................................................................................................................................ .. 17
1.2.2. The tyre .............................................................................................................................................. .. 17
1.2.3. The stud ............................................................................................................................................. .. 18
1.2.4 Traffic safety and studded tyres ......................................................................................................... .. 19
1.2.5 Questionnaire on the use of studded tyres in Sweden, Finland and USA. .......................................... ..21
1.3 PAVEMENT WEAR CAUSED BY STUDDED TYRES ....................................................................... ..24
1.3.1 Introduction......................................................................................................................................... ..24
1.3.2 Factors. .............................................................................................................................................. ..26
1.3.3 Dust composition ................................................................................................................................ ..27
1.3.4 Properties of asphalt resistant to studded tyres .................................................................................. ..28
1.3.5. Means of dust transport...................................................................................................................... ..28
1.3.6. Methods for measuring wear resistance of asphalt pavement ............................................................ ..29
1.3.7. Predicting pavement wear with a mathematical model ...................................................................... ..30
1.4. ROAD DUST AND HUMAN HEALTH ................................................................................................. ..35
1.4.1. Introduction........................................................................................................................................ ..35
1.4.2. Studies about road dust and human health ......................................................................................... ..35
1.5. CONSEQUENCES OF A PROHIBITION OF STUDDED TYRES. ...................................................... ..39
1.5.1. Swedish studies. ...................................................................................................................................39
1.5.2. The Japanese study. .............................................................................................................................41
1.6. SOLUTIONS TO PROBLEMS RELATED TO THE USE OF STUDDED TYRES. ............................. ..43
2. FIELD WORK ................................................................................................................................................. ..45
2. 1 . EXPERIMENTAL .................................................................................................................................... ..45
2.2 RESULTS .................................................................................................................................................. ..46
2.2.1. Particle size and amount of dust collected. ........................................................................................ ..46
2.2.2 Analysis of pH .................................................................................................................................... ..47
2.2.3. Mineralogical analysis ....................................................................................................................... ..48
3. CONCLUSION ................................................................................................................................................ ..49
4. REFERENCES ............................................................................................................................................... ..50
APPENDICES:
Appendix 1: classes of rocks.
Appendix 2: particle size distribution.
Appendix 3: particle size distribution.
Appendix 4: results from the SPSS analysis.
Appendix 5: results from the mineralogical analysis.
VTI notat 6A 2000VTI notat 6A 2000
STUDDED TYRES SUMMARY The objective of this work is to make a literature review about problems due to the use of studded tyres and to characterise mineral dust particles sampled close to a highway in Sweden. Lots of studies have been made to find out if studded tyres are better than another types of tyres and if dust from pavement wear is a risk to human health. As a consequence of these studies, studded tyres are forbidden in Japan and Norwegian authorities are considering a tax on the use of studded tyres in major cities. In other countries, such as Sweden and Finland, authorities focus their efforts on improving the quality of the pavement and the properties of the studs. Due to these improvements, asphalt wear has been reduced in Sweden from 450,000 tonnes (in the winter 1988/89) to 300,000 tonnes (in the winter 1993/94). The forecast for 2000 is 130,000 tonnes. As to human health, studies have shown that studded tyres produce PM ) particles by wearing the asphalt. Upon inhalation, these particles can reach the lungs, but how particles harm human health is not well understood. Anyway, dust causes some problems in human health but the source of dust in cities is not only the studded tyres and sometimes it is difficult to separate sources. In the case of a ban of studded tyres, the consequences are analysed from two points of view using Swedish and Japanese studies, respectively. The Swedish studies focus on economics, the Japanese on technical solutions from a traffic safety point of view. Some solutions to reduce dust from studded tyres are analysed. Also included are the answers to a little questionnaire, which was sent to Finland, USA and Sweden asking key persons for some solutions to this problem. The review also presents some basic data on asphalt, aggregate and bitumen. Finally, the results of the fieldwork are shown. This fieldwork consisted of collecting dust in buckets placed at highway E 4 (outside Linkoping, Sweden) during studded tyre season in early 1999 from 11th February to 22nd April 1999. The content was analysed for pH, particle size and weight and mineralogical composition. VTI notat 6A 2000 7
STUDDED TYRES VTI notat 6A 2000
STUDDED TYRES 1. A LITERATURE REVIEW 1.1. ASPHALT 1.1.1. Description Asphalt1 is a mixture of mineral aggregates (stone material), filler and bitumen. The stone aggregate content is on average 95 %, and the bituminous binder 5%. Additives such as adhesives, polymers and different types of filler can modify the properties of asphalt. The complexity of bitumen makes it very difficult to chemically characterise. It is a hydrocarbon material containing components of many chemical forms. Groups of organic substances can be identified, as well as traces of metals such as vanadium, nickel, iron, magnesium, calcium, etc. The precise composition varies according to the source of the crude oil from which the bitumen originates. When tracing the source of metals in the roadside soil or runoff water, metals like vanadium and nickel are often assumed to emanate from bitumen. However, when considering the composition of asphalt, and comparing the amount of metals in the stone with the metal content in bitumen, it appears that the stone materials might be the major source of some metals. Lindgren has studied the chemical composition of asphalt. She analysed, among other materials, two types of rocks used in Swedish asphalt, gabbro and porphyry. Chemical analysis of the two stone materials confirms that there is a high content of some metals in asphalt aggregate. The gabbro has a higher content of most metals than the porphyry. Table 1, shows the result of the chemical analysis. When comparing the metal content in the two stone material with bitumen, note that asphalt contains approximately 95% stone and 5% bitumen. It is often assumed that the zinc, which is to be found in runoff water, originates from tyres. The zinc oxide content in tyres is l.5-2% of the rubber. Assuming a specific wear of 0.2 g of tyre rubber/ km per vehicle this results in a release rate of 2.4-3.2 mg/km Zn. By calculating with a specific studded tyre wear of 24 g of asphalt/km per vehicle, and a gabbro content of 95%, a release rate rate of 3.4 mg Zn/km is obtained. Similar comparisons can be made between stone and bitumen in asphalt. Nickel in asphalt wear might originate from gabbro or porphyry for example, at a release rate of 2.5 mg/km, compared with 0.03 mg from bitumen. 1 Lindgren (1996). VTI notat 6A 2000 9
STUDDED TYRES
Table 1. Chemical analysis of gabbro, porphyry and bitumen. (From Lindgren 1996)
11.2 2.52
0.127 0.136
238 63.9
70.5 21.9
110 16.2
2.75 19.8
215 24.9
149 36.3
1.1.2. Types of asphalt in Sweden
Swedish pavement consists of several layers: a wearing course of mix, a binder course of mix, a
roadbase of mix, a surface dressing and a roadbase of grouted macadamz. However, only the first layer
is of interest for this project, because this is where the pavement wear occurs. That is why only the
different types of wearing courses of mix are going to be described.
The thickness of the wearing course must be designed so that the roadbase should always be protected.
The following types of wearing courses of mix are used in Sweden:
DENSE GRADED ASPHALT CONCRETE (ABT): is hot mix, laid and compacted asphalt mix
consisting of mineral aggregate and bituminous binder. The binder type may be selected so that the
mix type can be adjusted to all types of climate. ABT can be used as a levelling course, base and
wearing course on all types of roads an in all climatic zones. Because of its moderate content of coarse
mineral aggregate its wear resistant is limited.
The standard types are ABT 4, 6,8,11,16 and 22.
STONE MASTIC ASPHALT (ABS): is a hot, plant-mixed, dense asphalt concrete that exhibits
excellent wear resistance. It also possesses very good stability and good ageing properties. To permit
a high bitumen content in relation to the gradation, fibres of various types are usually used as a
stabilising bitumen carrier, usually 03 15% by weight of the mass and a bitumen content of 60 75%
by weight, depending on the type of fibre. Alternatively, special fillers, rubber powder or polymers
can also be used.
ABS can be used as a levelling course and wearing course on all types of roads and in all climatic
zones. Thanks to the large percentage of coarse mineral aggregate, this type of bituminous pavement is
one of the leading ones both as regards wear resistance to studded tyres and resistance to plastic
deformation. This type ofbituminous pavement is designed for the part of the road network on which
2 Swedish National Road Administration (1996)
10 VTI notat 6A 2000STUDDED TYRES abrasion by studded tyres is great, and for junctions and other areas exposed to large amounts of abrasion. ABS is being used as the only wearing course on high volume roads and streets. It is primarily the resistance of this pavement to wear from studded tyres that justifies its extensive usage, even if its resistance to plastic deformation is also a positive characteristic. The largest grain size is normally 12 or 16 mm. ABS with a maximum grain size of 20 mm has also been used with good results. The standard types are ABS 4,8,11,16 and 22. POROUS ASPHALT (ABD): is a hot plant-mixed asphalt, with a small percentage of filler and a low bitumen content. ABD is porous, thereby permitting water to pass through relatively quickly. ABD may be used at locations where there may be a risk of aquaplaning and where it is particularly desirable to achieve good wet friction and where there are demands on low noise levels. The wear is faster than the corresponding dense bituminous pavement. The standard types are ABD 11 and 16. MASTIC ASPHALT (SGJA, GJA): is a mixture of mineral aggregate consisting of filler, sand and macadam, and a high binder content. The binder is adjusted so that the binder will completely fill the voids in the mineral aggregate. Wearing courses of mastic asphalt exhibit very good wear resistance and they are much used on roads with very high traffic density. They are also suitable as bituminous pavements on bridges where impermeability is demanded. The standard types are GJA 8,11 and SGJA. VERY SOFT ASPHALT (MJAB): is a mixture of mineral aggregate of continuous grading but containing relatively little filler and with a low binder content. The binder type is very soft bitumen (MB). MJAB can be used as wearing course on roads carrying little traffic that make great demands on flexibility. As the hardness of the binder can be changed relatively easily, MJAB is suitable for use where requirements change quickly along the road. For example, the product may be made harder at road junctions, on uphill lanes, southern exposures and through towns. The standard types are MJAB 11 and 16. OIL GRAVEL (OG): consists of mineral aggregate with continuous grading and a low filler content. Oil gravel may be used on roads carrying little traffic, where the demand on exibility is great and the possibility of heating the mineral aggregate is limited. Oil gravel is therefore well suited for continual maintenance work, pothole repairs, temporary wearing course works and minor reconstruction works. The standard types are OG 11, 16 and 22. COLD EMULSION MIX (AEB): consists of mineral aggregate with continuous grading and low filler content, and relatively low residual content of bitumen or soft bitumen. It is used on roads carrying little traffic, where the demand on exibility is great, such as where movements in the substrate can be expected. The standard types are AEB 8, 11 and 16. VTI notat 6A 2000 11
STUDDED TYRES 1.1.3. Aggregates 1.1.3.1. Introduction The amount of mineral aggregate in asphalt paving mixtures is generally 90 to 95 percent by weight or 75 to 85 percent by volume3. Mineral aggregate is primarily responsible for the load supporting capacity of pavement; accordingly, asphalt pavement performance is heavily in uenced by aggregate. Mineral aggregate has been defined as any hard, inert mineral material used for mixing in graduated particles or fragments. It includes sand, gravel, crushed stone, slag, rock dust or powder. The resistance4 of pavements to wear caused by studded tyres is determined to a large extent by the quality of the aggregate. It has been shown that wear resistance, rather than durability, is the most important property of the aggregate in this context. Tests have beencarried out in a road simulator in order to investigate the resistance of asphalt concrete to wear by studded tyres when the effects of different aggregate qualities are varied. Wear proved to be in linear correlation with resistance of the aggregate. Wear was also lower when aggregate content was increased, provided that the aggregate did not have an inferior resistance to wear. 1.1.3.2. Types of aggregates used in asphalt paving mixtures The origin of bedrock, from which all aggregate materials are derived, is important for understanding the performance of mineral aggregates. The mode of formation, composition and history of a rock have a bearing on its performance as a road making aggregate. Natural aggregates are obtained by the crushing and screening of quarried rock or by the excavation and screening of gravels and sands which may be crushed or uncrushed. All rocks are divided into three general classes: sedimentary, igneous and metamorphic (see appendix 1). The classes indicate the means by which the rocks were formed. Sedimentary rockssz as the result of the denudation of the rocks of the land surface through processes of weathering (mechanical and chemical) and of erosion during transport by moving water and ice, and by wind, large amounts of detrital and chemical, including biochemical, materials become available to form new sedimentary deposits. In addition, some sediments originate in ash, dust and rock debris from volcanism whereas others, like peat and coal, are principally organic residues and are not derived from pre existing rocks. 3 Asphalt institute (1989) 4 Hebeda (1978) 5 Smith & Collins (1993) 12 VTI notat 6A 2000
STUDDED TYRES Sedimentary rocks6 may be divided into two groups according to their mode of formation: clastic rocks and those formed in situ. Clastic rocks comprise the consolidated particles eroded from pre-existing rocks. They are classified in decreasing order of grain size as conglomerate, breccia, gritstone or sandstone, siltstone, and mudstone or shale. With few exceptions, only gritstone and sandstone are employed as roadstone aggregate. Of the sedimentary rocks formed in situ, only limestone and flint are used extensively for roadstone. A characteristic feature of sedimentary deposits is a layered structure. Igneous rocks: are formed by the cooling and solidification of hot molten materials erupted from or trapped beneath the earth s crust. They are divided into extrusive and intrusive rocks. Extrusive rocks are formed by material pouring out on the earth s surface, such as that produced by volcanic action, and are distinguished in general by their glass-like structure. Intrusive rocks are formed by cooling and solidification at great depths within the earth s crust. They are wholly crystalline in texture. Although rocks exhibit a very wide range of chemical compositions, their suitability for aggregates depends also on their mineral constituents, their crystalline fabric and texture and the degree of chemical alteration and of weathering. Basalt is often altered and weathered. From a chemical point of view, the vast majority of igneous rocks are made up by only 8 elements. Of these, oxygen is dominant, next is silicon and then aluminium, iron, calcium, sodium, potassium and magnesium. The range of composition (per cent) can be expressed roughly in the following terms: Silica (Si 02) : 35 75 Alumina (A12 03): 0 25 Iron oxides (Fe O and Fe2 03): 0 20 Magnesia (Mg 0): 0-45 Calcium oxide (Ca 0): 0 20 Sodium oxide (Na2 0): 0 16 Potassium oxide (K2 0): 0 12 (--) Metamorphic rocks: are generally formed by the modification of sedimentary and igneous rocks as a result of intense pressures set up by severe earth movements and by excessive heat and solutions. Factors causing such modification are complex, and the original form of the altered rock is often difficult to determine. In this process recrystallization occurs. The wide diversity of metamorphic rocks arises from the variety of metamorphic processes and pre existing rocks that are available for metamorphism and their usefulness for aggregate is equally variable. 6 Pike (1990) VTI notat 6A 2000 13
STUDDED TYRES
1.1.3.3. Gradation7
Aggregate gradation is the distribution of particles sizes expressed as a percentage of the total weight.
Gradation is determined by passing the material through a series of sieves stacked with progressively
smaller openings, and weighing the material retained in each sieve.
Aggregate grading specifications have been developed because of the need:
0 To control construction materials and in turn to obtain a desirable and quality pavement
0 To obtain optimum utilisation of locally available materials
0 To reduce cost through standardisation of sizes
The maximum size of an aggregate designates the smallest sieve through which 100 percent of the
material will pass. The nominal maximum size is the largest sieve size upon which any of the
aggregate material is (or is permitted to be) retained.
Maximum size and aggregate grading are invariably controlled by specifications which prescribe the
distribution of particle sizes to be used for a particular aggregate material.
The largest nominal aggregate particle size should be selected as large as possible in relation to the
design layer thickness, but should be within the following limits:
ABT/GJA: min 30%, max 45% of the layer thickness.
ABS/ABD: min 25%, max 45% of the layer thickness.
MJAB/lVIJOB/OG/AEB/AEOG: min 35%, max 50% of the layer thickness.
1.1.3.4. Desirable properties of aggregates for bituminous mixes8
It is generally recognised that the main qualities required from a roadstone to be used in pavement
surfacing materials are:
l. Toughness: resistance to slow crushng and resistance to rapid loading
2 Hardness: resistance to abrasior attrition
3. Resistance to polishing: for wearing course materials and surface dressings
4 Resistance to stripping ability to maintain adhesion to any bituminous binder in the presence of
water with which the aggregate is used
5. Resistance to weathering effects in the pavement
6. Ability to contribute to strength and stiffness of total mix intrinsic aggregate strength and shape
properties
7 Hobeda (1978)
8 Smith & Collins (1993)
14 VTI notat 6A 2000STUDDED TYRES
1.1.4 Bitumen
1.1.4.1. Description
Bitumen comprise viscous liquids or solids consisting essentially of hydrocarbons and their
derivatives, which are soluble in carbon disulphide; they are substantially non-volatile at ambient
temperature and soften gradually when heated. They are black or black-brown in colour and possess
waterproofing and adhesive propertiesg.
The chemical composition of bitumen (see table 2) depends on the composition of the original crude
oil and on the process used during refining ). Bitumen can generally be described as complex mixtures
containing predominantly cyclic hydrocarbons and a lesser quantity of saturated components. Also
found in bitumen are heteromolecules that contain sulphur, nitrogen, oxygen and trace amounts of
vanadium, nickel and iron.
Table 2. Chemical elements in bitumen. (From Barsdale et al. 1993)
" Chemical element I . Typical minimum ' 'p ' Typical maximum, j;_
Carbon (%) 75 f I I V 5 V 9'
Hydrogen (%) 8 15
Nitrogen (%) 0 3
Oxygen (%) 0 4
Sulphur (%) 2 10
Vanadium (ppm) 30 1200
Nickel (ppm) 0 150
Iron (ppm) 0 200
Bitumen should not be confused with coal-tar products such as coal tars or coal-tars pitches. The latter
are manufactured by the high temperature carbonisation of bituminous coals and differ from bitumens
substantially in composition and physical characteristics.
Similarly, bitumen should not be confused with petroleum pitches, which are often highly aromatic
residues produced by thermal cracking, coking or oxidation from selected petroleum fractions.
To overcome problems with the premature deterioration of the wearing course, chemical additives are
used . The use of these additives improves the properties of bituminous mixtures: adhesion agents
added to asphalt are generally based on amines, especially fatty polyamines, and amine derivatives
such as amides, substituted imidazoline, etc.
9 IARC monographs (1985)
1 Barsdale et al (1993)
11Cawsey & Wong (1991).
VTI notat 6A 2000 15STUDDED TYRES
Tertiary nitrogen heterocyclic material is also proposed to reduce moisture-induced damage in asphalt
aggregate mixtures.
Additives are added also to improve antistripping properties.
1.1.4.2. Broad chemical composition
The chemical characterisation of bitumen is based on its separation into four broad classes of
compounds: asphaltenes, resins, cyclics and saturates.
Asphaltanes are black amorphous solids containing, in addition to carbon and hydrogen, some
nitrogen, sulphur and oxygen. Trace elements such as nickel and vanadium are also present.
Asphaltanes are generally considered as highly polar aromatic materials of molecular weights of 2000-
5000 (number average) and constitute 5 25% of the weight of bitumen.
Saturates comprise predominantly the straight and branched chain aliphatic hydrocarbons present in
bitumen, together with alkyl naphthenes and some alkyl aromatics. The average molecular weight
range is approximately similar to that of the cyclics, and the components include both waxy and non-
waxy saturates. This fraction forms 5-20% of the weight of bitumen.
Cyclics (naphthene aromatics) comprise the compounds of lowest molecular weight in bitumen and
represent the major proportion of the dispersion medium for the peptized asphaltenes. They constitute
45 60% by weight of the total bitumen and are dark viscous liquids. They are compounds with
aromatic and naphthenic aromatic nuclei with side chain constituents and have a molecular weight of
500-900 (number average).
Resins (polar aromatics) are dark-coloured, solid or semi-solid, very adhesive fractions of relatively
high molecular weight present in the maltenes. They are dispersing agents or peptizers for the
asphaltenes, and the proportion of resins to asphaltenes governs to a degree the 301- or gel type
character of bitumen. Resins separated from bitumen are found to have molecular weights of 800 2000
(number average) but there is a wide molecular distribution. This component constitutes 15 25% of
the weight of bitumen.
1.1.4.3. Desired properties of bitumen
Some important properties for the bitumen are the following :
1. Consistency: bitumen are thermoplastic materials because they gradually liquefy when heated.
They are characterised by their consistency or ability to flow at different temperatures.
Consistency is the term used to describe the viscosity or degree of uidity of bitumen at any
particular temperature. The consistency of bitumen varies with temperature, therefore, it is
12 Asphalt institute (1989)
16 VTI notat 6A 2000STUDDED TYRES
necessary to use a standard temperature when comparing the consistency of one bitumen with
another.
2. Purity: normally, bitumen is free of water or moistures as it leaves the refinery. However,
transports loading bitumen may have some moisture present in their tanks. If any water is
inadvertently present, it will cause the asphalt to foam when it is heated above 1000 C.
3. Safety: bitumen foaming is a safety hazard, and specifications usually require that asphalt should
not foam at temperatures up to 1750 C.
1.2. S TUDDED TYRES
1.2.1. Introduction
The history of studded tyres started in the 1950 s, when they debuted in Scandinavia . In the winter of
1961 1962, studded tyres were introduced for general use in the Scandinavian countries and one year
later they were made commercially available in Japan. That year, a test market was established by the
tyre industry in Canada with an estimated 1 1/2 million tyre studs sold that year, 6 million the
following year and more than 25 million in 1964 65 winter season. In USA, 2-3 million studs were
sold in the winter 1963-64, 25-30 million in 1964 65 and 250-275 million in 1965-66.
In 1963 studded tyres were made commercially available in Japan. In 1970, the use of studded tyres
spread extensively in cold and snowy regions and in the second half of the 1970 s, the rate of studded
tyres approached 100%. Studless wintertyres started to be marketed in 1982 and one year later a
number of local governments throughout the country issued voluntary guidelines restricting the use of
studded tyres. Anyway, 8 million studded tyres were sold during 1985, the largest annual sales ever.
The following years were full of meetings, regulations, studies and tests to implement regulations on
studded tyres and finally the 318t of March 1991, the sale of studded tyres was halted by an act banning
the use of studded tyres in designated regions.
1.2.2. The tyre14
The use of studded tyres is the main source of pavement wear, but the tyres suffer wear too, and for
example 10,000 tonnes of rubber are worn off the tyres every year in Swedish roads.
The composition of a rubber tyre is: 85% rubbermix, 12% steel and 3 % textiles.
13 National cooperative highway research program synthesis of highway practice (1975)
14 Ahlbom & Duus (1994)
VTI notat 6A 2000 l7STUDDED TYRES
Rubbermix composition is showed in table 3; where rubber polymers used in tyres are: natural
rubber(NR), polyisoprene rubbers (IR), styrene-butadiene rubber (SBR), polybutadiene rubbers(BR)
and chlorinated isobutane-isoprene rubbers (CIIR).
Carbon black is the only reinforcing agent.
Very important are the softeners. The main one used in tyres is HA oils. These contain aromatic
components which are carcinogenic and toxic towards, e.g, aquatic organisms.
Among components used in smaller amounts, accelerators and activators are the most important from
the point of view of this work. Some accelerators are hazardous to aquatic organisms.
Zinc oxide is used as activator on rubber miX for tyres and due to tyre wear it is possible to find raised
levels of zinc close to roads. There is no alternative to zinc as activator but using zinc stearate it is
possible to lower the amount of it.
Table 3. Rubbermix composition. (From Ahlbom & Duus 1994)
Rubber polymer 40-60%
Reinforcing agents 25-30%
Softeners and extender oils 15-20%
Curing agents 1 2%
Accelerators O,5 2%
Activators 2-5%
RetardersSTUDDED TYRES controlled protrusion and spring studs, none of them as good as conventional studs, but better than unstudded snow tyres. Nowadays the typical studs are the following : with tungsten carbide pin: steel, plastic, aluminium and combined: steel/plastic, plastic/steel, steeUaluminium and composites: aluminium/aluminium oxide. 1.2.4 Traffic safety and studded tyres Since studded tyres started to be used, a lot of studies about them have been done. The main purpose of these studies is to investigate if studded tyres are better than the other types of tyres from a traffic safety point of view. The results are not too clear and the conclusions have been difficult to draw because the results vary substantially between studies. Anyway a study carried out at VTI17 showed that studded tyres increase friction in braking and cornering on ice (105 123 studs protruding 1.1 1.8 mm). For example, the braking distance from 70 km/h was 193 m with a friction of 0.1 is utilised by the non studded tyre. When a studded tyre with a friction of 0.2 was used; the stopping distance was 96 m. In this experiment, the studded winter tyres had up to twice the friction and in no case lower friction than the friction tyres (studless tyres). If studded tyres were compared with summer tyres the results were even better. To study properties of studded and non-studded tyres under different conditions a new study18 was carried out at VTI. It was done using passenger cars and trucks on ice and on wet asphalt pavement. The investigations concerning friction on ice were carried out at the VTI tyre test facility on polished black ice at temperature of -3/ 1O C. The facility that has a high speed at bed machine consists of a stationary but routable wheel suspension with transducers for simultaneous measurement of forces between tyre and road in longitudinal, lateral and vertical direction. The passenger car tyre test on wet asphalt was carried out with the VTI friction test vehicle BV12. This vehicle is a specially equipped truck with a test wheel suspension for passenger car wheels. Speed for the ice test was 30 km/h and for the wet asphalt tests 70 km/h. The best results for passenger cars on ice were found for the studded tyres, the worst had however the same performance as the best non studded tyres. About truck tyres, the difference between the best and worst tyre is about 100%. On wet asphalt a significant difference was found between winter tyres and summer tyres where the summer tyres had 10 to 20 percent higher friction. In Iceland, another study , carried out between 1989 and 1995, showed that drivers using studded tyres drove more safely than the others do. The study took into account accidents reported by the 16 Unhola, Personal communication. 17 N ordstrom (1991) VTI notat 6A 2000 19
STUDDED TYRES
police during that period in Reykjavik, and it classified road conditions in two groups: dry or wet and
ice or snow. Drivers were classified into two groups, "perpetrators" (those who make a mistake
leading to an accident) and "victims" (those who are involved in an accident without causing it).
For both conditions and groups, drivers with studded tyres drove more safely than drivers using
studless winter tyres, but under ice or snow conditions the difference is bigger.
On the other hand a SINTEF report20 shows that, in terms of accessibility and accident rates, there is
little difference between driving with or without studded
tyres. The study points out that studless tyres
performed best on snow, while those with studs were best on icy roads.
Rune Elvik, in a paper prepared for presentation at the Transportation Research Board s Annual
Meeting (1998), concluded that studded tyres provide only a slight safety advantage in winter driving
conditions. He compared the crash rate of cars equipped with studded tyres to identical cars with non-
studded tyres. In his analysis the evaluation studies were selected through a literature search. The
studies evaluated are listed chronologically in table 4.
Table 4. Results of studies that have evaluated the effects on automobile accident rates of using
(studded tires. (From Elvik, 1998)
Stud Lower Best Upper Lower Best Upper Lower Best _ Upper
y 95% estimate 5 95% 95 % estimate I 95% 95% estimate 95%
Normand 1971 ~49 ~43 Q ~36 ~52 ~47 ~41 ~49 ~45 ~41
Steen and Bolstad 1972 ~25 ~18 ; ~9 ~38 ~29 ~19 ~28 ~22 ~16
Ernst and Hippchen 5
1974 : ~42 ~39 ~36
Roosmark et al 1976 ~28 ~25 ; ~21 ~11 ~7 ~3 ~18 ~16 ~13
Perchonok 1978 ~73 ~72 ~71 ~69 ~68 ~67 ~71 ~70 ~69
Ingeb gtsfgngal nd FOSS ~48 5 ? +72 -47 4 +75 37 5 : +46
Junghard 1992 ~54 ~49 ; ~43 ~26 ~17 ~6 ~42 ~37 ~32
Konagai et al 1993 p I ~61 ~57 ~53
Fosser 1994 ~49 ~25 E +12 +40 +151 +351 ~21 +10 +53
FOSS igdgga ermo 21 4 : +16 15 +3 +25 ~16 4 +10
Roine 1996 49 14 3 +44 ~68 41 +8 ; ~48 ~28 3 +0
As it can be noticed, there are significant variations between the results and for example the crash rate
reduction on snow covered or ice-covered roads ranged from a high of 72 percent to a low of 4
percent. On the bare roads, the range was more dramatic, from a 68 percent decrease to a 151 percent
increase in the crash rate. This is the reason why Elvik did not propose any number as the best
18 Nordstrom & Gustavsson (1998)
19 Sigthorsson (1998)
20 VTI notat 6A-2000STUDDED TYRES estimation of the safety effect produced by studded tyres. Instead of that, he suggested reasons for these variations: the size of the crash sample used, whether the study specified road surface conditions, whether the study specified the types of tyres that were compared with studded tyres and the number and type of confounding variables accounted for. Despite of this, Carlsson21 studied the economic effect of a prohibition of studded tyres. The study shows that the positive and negative consequences of a studded tyre prohibition would be of the same order of magnitude. 1.2.5 Questionnaire on the use of studded tyres in Sweden, Finland and USA. In order to get a picture of the current state of studded tyre use, the present author sent a brief questionnaire to some key persons in Sweden, Finland and USA, countries Where studded tyres are used. First of all, the questionnaire asked about some regulations about studded tyres and so on and finally there were the following questions: Question lere you thinking about a ban of studded tyres in your country/region? Question 2: Is dust from pavement wear a problem in your country/region? Question 3: Do you have any preventive measure about dust from pavement wear? (For example vacuum tracks or something like that). 20 Taken from Internet. 21 Carlsson (1981) VTI notat 6A 2000 21
STUDDED TYRES
Sweden.
The regulations about studded tyres in Sweden are given in the table below:
Table 5. Regulations about studded tyres in Sweden. (From Carlsson et al. 1995)
Light vehicles (total weight3.5 t)
cars lorries
Permitted period 1 November first Sunday after Easter, or if slippery
ProtrusionSTUDDED TYRES
problems with retention and disputes with the stud supplierthey modified the weight to 1.5 grams to
allow for lightweight steel. Alaska and Washington have tried to regulate stud weight at 1.3 grams.
The reason for 1.3 is that they have larger vehicles in the US. and this will accommodate the light
trucks while the specification sheets will suggest a 0.9 to 1.1 for the passenger vehicle.
Question 1 on banning. As with any controversial issue thereare those who support and those who are
against. There have been discussions of banning, but it is not believed that it will happen in the near
future. One possible solution to this problem is to buy the new studless winter tires. Their performance
has greatly improved in the last couple of years and this seems to be a viable alternative. Surprising
enough they seem to perform like a low quality studded tire. The key features are softer rubber,
directional tread, sipping.
Question 2. There are three states in the US, which have acknowledged the principles of pavement
wear and introduce legislation to deal with the weight of studs: Alaska, Oregon and Washington. In
Anchorage (Alaska) studded tyres contribute to the air pollution but they have not been able to isolate
what portion. In the other two states, during the winter they receive a significant amount of rain, and
dust is not a problem. Studded tire dust contributes but may not be significant when one considers the
other sources.
Question 3. We use vacuum trucks and liquid de icers to minimize dust in the spring.
Finland.
In Finland , the regulations about the use of studded tyres and characteristics of the studs are the same
as in Sweden and the rest of the Nordic countries.
In winter, 90 % of light vehicles and 20 % of heavy vehicles use studded tyres in Finland.
Question 1: "No".
Question 2: "Yes, but notblamed on studded tyres but sand used to prevent skidding".
Question 3: "No, perhaps you can see brushing and washing the roadsides in spring as that".
25 . .
Unhola. Personal communication
VTI notat 6A 2000 23STUDDED TYRES
1.3 PAVEMENT WEAR CAUSED BY S TUDDED TYRES
1.3.1 Introduction
Pavement wear has been a problem since studded tyres were introduced. Until then, wear of surfacing
on Swedish roads (and all countries where studded tyres are used) was a minor problem.
The wearing of pavements due to studded tyres takes place in the following ways:
\/ The aggregate works loose from the pavement surface as a result of being scraped by studs.
\/ A stone grain is smashed by the impact of a stud and the broken pieces are detached by the
scraping of studs.
\/ The scraping of studs produces marks of wear on the mastic, made of binder and aggregate.
/ Scraping by studs produces marks of wear in stones. This causes the stone grains to wear away
completely.
At the beginning pavement wear was high, because of the asphalt and studs characteristics. But soon, a
lot of changes have been done to reduce the pavement wear: lighter studs, lower speed, shorter studded
tyre periods and prohibition of studs types most aggressive to the pavement. At the same time the
development of more durable pavements has contributed to the reduction of pavement wear.
As example, table 6 shows the evolution of pavement wear in Sweden26. SPS is a Swedish
abbreviation, which means:
"Abraded quantity of asphalt mix (g) per kilometre of road and studded vehicle".
The SPS index is calculated as follows:
average wear >< lane Width X road length (km) X [9qu density
SPS =
AADTIane >< wear period >< stud frequency
The magnitude of the asphalt wear from studded tyres and thereby the SPS index depends on several
factors. Therefore, the SPS cannot be regarded as a material constant for a specific object or pavement
type, but indicates the actual wear from a certain amount of traffic with studded tyres during a
particular measuring period, usually one winter.
The SPS index does not reveal the exact situation of pavement wear but it is enough to give an idea
about the total wear.
26 Carlsson et al. (1995)
24 VTI notat 6A 2000STUDDED TYRES
In 1989, the SPS index varied between 6 and 37 g/km for different types of pavements where the
lowest figure was measured on very good pavements (stone mastic asphalt type) while the highest was
measured onordinary dense asphalt concrete with local aggregate.
The reduction of pavement wear in 1994 depends on the new pavements with a lower SPS index and
to some extent on the 20% use of lightweight studs.
With the lightweight studs an SPS index of only 10g/km can be expected at the turn of the century.
1989 6 37 (meanz30) 450,000 250-300
1994/95 24 300,000 150 200
Forecast (2000) 10 130,000 65-90
The concentration of dust in the air is to a great extent dependent on weather conditions. In a dry road
and when studded tyres are used the dust concentration in air is high. On the other hand, in a wet road
the dust concentration is lower.
Usually, studded tyres are used in Sweden between November and April, and the wearing of pavement
follows a clear seasonal variation. In November, in the beginning the studded-tyre period the
pavement wear is high, because the roads are bare and wet (Table 5). In winter, the roads are often
covered with snow and ice, and then the wear is lower. Finally, in spring, the fine grained components
of pavements have worn off and at the same time they are ground to still smaller particles by the
passing cars. ("Car traffic and urban pollution", Steinar Larssen, Internet). Where necessary the roads
are repaved during the summer at places as frequently as each year.
In Norway studded tyres wear around 250,000 tonnes of asphalt per year. Most of this consists of large
particles which are deposited near the road (10-20 metres). Less than 10% of the dust becomes
airborne (PM1027), contributing to air pollution at a larger scale. On an annual basis calculations of
annual mean concentrations of PM10 in the largest cities in Norway show that 20% 30% of the
airborne dust comes from studded tyre wear.
27 PM10 : respirable particles. ¢< 10 p. m.
VTI notat 6A 2000 25STUDDED TYRES 1.3.2. Factors.28 29 30 Pavement wear depends on several factors, such as: Aggregate quality: this is the most important factor in pavement wear. Depending on the aggregate, the wear can vary a lot. For example, the difference between the worst and the best dense bituminous asphalt concrete (mixture of bitumen and mineral aggregate, laid and compacted) is a factor of three, it means that the best one is worn one-third of the worst. If we talk about porous asphalt concrete there is a factor of 5. Pavement type-aggregate content: tests have shown that the proportion of coarse material (aggregate content) in the pavement has a considerable in uence on stud wear. Besides, wearing resistance to studded tyres increases with increased aggregate content. Maximum aggregate size: the in uence of this factor in pavement wearis also high. For example a stone mastic asphalt with an aggregate size of 16 mm is worn half as much as stone mastic asphalt with 8 mm aggregate maximum size. Binder: it is possible to improve the properties of asphalt using modified binders and it may have a positive in uence on the capacity of the wearing course to resist wear from studs. Tra ic conditions: it includes traffic volume (more traffic, more pavement wear), proportion of studs, and speed. Since the dynamic stud force towards the pavement grows with speed, the wearing of pavement increases with speed. Geometry of the road: the wear of pavement due to studded tyres is often unevenly distributed, so it produces that the wear can vary at different points of the road, even on the same stretch of road or on the same pavement section. In such a case, the geometry of the road is often responsible, because wearing on a straight road stretch is smaller than in bends and in stretches where braking or accelerating takes place. Weather conditions: temperature and water in the surface. The in uence of temperature on pavement wear mainly concerns the binder. Bitumen products are viscoelastic substances and the effect of temperature on their hardness is high. The wear resistance of pavement improves with decreasing the temperature until 0 degrees is reached. Below this point the plastic properties of the binder disappear and the binder becomes susceptible to breaking. Water has a detrimental effect on the wear resistance of asphalt. The adhesion between binder and aggregate is weakened, the strength of aggregate is lowered and the mastic component of asphalt is weakened due to absorption of water. 28 Jacobson (1994) 29 Hobeda (1989) 3" Niemi (1985) 26 VTI notat 6A 2000
STUDDED TYRES
Studs and tyres: number of studs, design of studs and quality of tyres. For example, a lightweight stud
of the same tyre wears only half as much as a heavier steel stud.
De icing salt: it is important if the road is salted or not. An unsalted road may also be ice covered and
this reduces wear on the road. The salt retains water on the road surface, which increases pavement
wear.
1.3.3. Dust composition
The composition of dust from pavement wear can not be defined in an exact way. It depends on where
and when the samples are taken, the characteristics of asphalt, types of tyres, etc.
In general, dust from pavement wear contains: asphalt pavement material, paint (white), stud tip and
tyre tread.
But these components31 are mixed with materials from other sources such as soil, long distance
transported particles and motor exhaust soot.
The main elements in the white paint are Ca, Zn, Ti and Mg. The tyre rubber contains Zn, Cl, Ca, Na,
Al and K, with a slightly higher concentration of Zn. Stud tips contain W and Co.
Table 7 is an example of the differences between dust from different places, where the city of
Sapporo had an average usage of studded tyres 100%, Sendai: 50%, Niigata: 70%, Nagoya: 0%,
Stockholm and Oslo: 50% and Zurich 0%32
The composition varies greatly between the cities. Air borne dust in cities has many sources in
addition to pavement wear.
Table 7. The elemental composition of airborne particles in various cities.
(From Amemiya et al. 1984)
: Componient; _ .
Sapporo 15 1 50 4 13 6
Sendai 23 2,5 55 2,5 19 8
Niigata 20 2 50 8 16 5
Nagoya 10 8 31 15 18 5
Stockholm 8 9 37 10 20 4
Oslo 13 6 42 5 18 6
Zurich 12 8 15 23 15 8
31 Fukuzaki et a1. (1986)
32 Amemiya et al. (1984)
VTI notat 6A 2000 27STUDDED TYRES
But even the results vary in the same city. In table 7, it is possible to see the composition of airborne
dust in Sendai City. Another study carried out by Hiraga33 et al. showed that the main components of
suspended dust were: Mg, Fe , Al and Co followed by Mn, Cu, Ti, Pb and Zn.
1.3.4. Properties of asphalt resistant to studded tyres
There are some general properties that are important in asphalt to resist pavement wear caused by
studded tyres. For example, aggregates at the surface should be the largest possible stones and be of a
maximum strength to resist the impacts of studs without breaking. The amount of binder has to be
enough to ensure the binding of the aggregate to the pavement and to resist being scraped, and the
binder has to be of maximum hardness. Finally, the pavement should have such a composition that the
construction machinery and another factors do not cause too many faults in the new road.
1.3.5. Means of dust transport34
Dust from pavement wear is spread by the following mechanism: splash, spray, ploughing and particle
drift.
Splash, is usually defined as the water thrown away in the forward and side directions from the tyre
road interface. It consists of relatively large droplets that are not caught by the air streams around the
vehicle to any larger extent.
Spray, is thrown out by centrifugal action tangentially from the tyre tread and a great portion will
break down into small droplets with a low sinking speed when hitting others parts of the vehicle. The
spray is easily caught by the air steams and may be persistent in the air wake behind the vehicle for a
long time.
The importance of each mechanism (splash and spray) is then depending on a set of factors such as:
Traffic characteristics
\\\\\
Road surface characteristics
Maintenance and operation
Meteorological factors
Topography
\\\
Vegetation
Hydrogeology
Road drainage patterns
33 Hiraga et al (1983)
34 Blomqvist & J ohansson (1998)
28 VTI notat 6A 2000STUDDED TYRES
Wind, plays a important role which is bigger under dry conditions of the road.
1.3.6. Methods for measuring wear resistance of asphalt pavement
Nowadays, in Sweden, the techniques35 used to measure pavement wear consist of small slabs of
asphalt mixes that can either be tested in the field by placing them in existing pavements and
measuring the wear from traffic or by performing accelerated tests in a road simulator.
The first one consists of placing the slabs in a pavement that has been given a new wearing course, in
the right side of the wheel rut, and usually in high volume roads or motorways. Although initially the
slabs were tested in the road, most of the tests have beenmoved to the road simulator, which is now
the most important method for wear studies.
The road simulator (see photo 1) or circular test track consists of four or six wheels (it depends on the
model), mounted at the end of two or three axles, which rotate around a circular track built on a
horizontal base. The wheels of the machine can be placed at different distances from the centre of the
machine and the wheels can be displaced small distances in both directions when the simulator is
working. It is possible to change the speed, too.
Photo 1. VTI road Slmulator.
The advantage of this technique is that different types of asphalt can be tested at the same time, and it
is easier to compare the results. Different types of studs can be tested too, but not possible at the same
time. It is possible to adapt the conditions in the simulator to the conditions in the field as to for
example temperature, humidity, rain, snow and ice.
Another advantage is that in the simulator, the results are obtained faster than with the other technique.
35 Jacobson (1994).
VTI notat 6A 2000 29STUDDED TYRES
Finally, some studies show that there is a good correlation between wear in road simulator and wear
measured in the field.
1.3.7. Predicting pavement wear with a mathematical model
Pavement wear under varying conditions can be calculated using a mathematical model established by
Jacobson and Wagberg36.
With this model it is possible to calculate:
\/ The magnitude of wear: amount of wear per 100 metres driving line a year, amount of wear
during lifetime per 100 metres driving line and specific wear.
\/ How the wear is distributed across the width of the driving line: with this information it is
possible to predict when the wearing course needs maintenance and to calculate the service life of
a pavement.
/ Finally the model calculates the cost per year due to wear from studded tyres.
Some of the input data are limited, for example the wearing course are ABT (dense graded asphalt
concrete) or ABS (stone mastic asphalt), but these two types of wearing courses are the most common
in Sweden.
The model needs the following input data: road and traffic data, information about aggregates,
bitumen, additives and studded tyres. These data appear in the model in three blocks. The first one has
information about road, traffic and studded tyres (table 8), the second about aggregates (table 9) and
the last one on the economics data (table 10).
Table 8. Information about road, traf c and studded tyres. (From Jacobson & Wé lgberg 1998)
Standard cross section (1/2/3/4/5/6): (1:7 m; 2:9 m; 3:13 m; 4: wide lanes;
5=express way;6=tunnel)
Permitted speed: ' (50, 70, 90 or 110 km/h)
ATD traf c lane: vehicles/day
Wear period/year: Winter days/year
Studded tire share: % (average during the wear period)
Lightweight studded tire share: % (O,10,20,30....%)
Deiced (salt) road (Y/N):
Number of calculation years:
Permitted rut depth: Mm
Estimated other rut depth: Mm
Available rut depth wear: Mm
Stone content (HALT4) >4mm: % by weight
Largest stone size (MS): Mm
Ball mill value (KV):
36 Jacobson & Wagberg (1998)
30 VTI notat 6A 2000STUDDED TYRES
Table 9. Information about aggregates.
Stone content (HALT4) >4mm % by weight
Largest stone size (MS): mm
Ball mill value (KV):
Table 10. Economical data.
Aggregate: SEK/tonne asphalt mix
Bitumen: "
Additives: "
Production in asphalt plant: "
Set-up cost: "
Transportation (asphalt plant to road site): "
Paving: "
Other: "
Asphalt layer thickness: mm
Total kg/mz: kg/m2
Total SEK/tonne: SEIQtonne
Total SEK/mz: SEK/mz
Annual cost: SEK/m2
The method has the advantage that is possible to compare what happens if we change the conditions of
the road, permitted speed, percentage of studded tyres, type of aggregate and so on. In the following
examples it is possible to compare the effect of these factors on pavement wear, where rut is the wear
produced by studded tyres and because of it, car wheels are confined to the ruts created by studded
tyres.
Standard cross section 1 (1:7 m; 2:9 m; 3:13 m; 4: wide lanes;
(1/2/3/4/5/6): 5=express way;6=tunne|)
Permitted speed: 90 (50, 70, 90 or 110 km/h)
ATD traffic lane: 5500 vehicles/day
Wear period/year: 166 winterdays/year
Studded tire share: 52 "/0 (average during the wear period)
Lightweight studded tire share: 55 °/o (0,1 O,20,30....%)
Deiced (salt) road (YIN): Y
Number of calculation years: 5
Permitted rut depth: 17 mm
Estimated other rut depth: 4 mm
Available rut depth wear: 13 mm
Stone content (HALT4) >4mm: 75 °/o by weight
Largest stone size (MS): 16 mm
Ball mill value (KV): 9.1
VTI notat 6A 2000 31You can also read
