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VTl notat 30-1997

The wear resistance of

bituminous mixes to studded

tyres - the Swedish

experience

Paper at the Fifth International Symposium on Cold

Region Development, Anchorage, Alaska, USA

Författare Torbjörn Jacobson

Programområde Konstruktion & Byggande

Projektnummer 12012

Projektnamn Konferenser

Uppdragsgivare VTI

Distribution Fri

Väg- och transport-

forskningsinstitutetä

CONTENTS

INTRODUCTION............................................................................... .. 2

METHODS FOR MEASURING THE WEAR RESISTANCE OF

AGGREGATES .................................................................................. .. 3

METHODS/EQUIPMENT FOR MEASURING THE WEAR

RESISTANCE OF ASPHALT SURFACING MIXES ...................... .. 4

FACTORS INFLUENCING STUD WEAR ...................................... .. 7

FORECAST MODEL FOR WEAR FROM STUDDED TYRES ...... .. 9

EXPERIENCE FROM TEST ROADS ............................................... .. 9

CONCLUSIONS ............................................................................... .. 10

LITERATURE .................................................................................. .. 11

VTI NOtat 30- 1997

INTRODUCTION

In Sweden, studded tyres are used in the winter, causing considerable surface wear

on roads with high traffic volumes thus resulting in rutting. For a long period, the

VTI (Swedish National Road and Transport Research Institute) has been working

on the development of wear-resistant asphalt pavements and equipment/methods

for measuring the wear resistance of aggregates and asphalt surfacings. The choice

of wear-resistant aggregates, introduction of stone-matrix asphalt and a change

towards studded tyres, more favourable to the pavement, resulted in a considerable

reduction of stud wear (Figure 1), which is now on an acceptable level. VTI

activities have been carried out as follows:

development of test equipment and measurement methods for wear studies

a survey of influencing factors

a survey and estimation of various pavement (surfacing) types

development of a forecast model for rutting, cross-profile and annual costs

based on the wear caused by studded tyres

150

140

130

120

110 -9 _A abrasmn value 10100.3.0: 90 _? abrasmn value 2a) 80n.g 70 _L matrix asphaltm 60m .

g 50 ball mlll test

40 light studs

30

20

10 H

0 l I

1975 1985 1990 1995 2000

Figure 1 Wear development on roads with high traffic volumes, in principle,

and development of test methods and mix types. (SPS-value = wear ingram per vehicle and kilometre).

VTI Notat 30- 1997

METHODS FOR MEASURING THE WEAR RESISTANCE OFAGGREGATES

At an early stage, the interest was focused on the quality of the aggregate and testmethods were designed. The abrasion value (related to the British test) and laterthe "Nordic" wet ball mill value (Figure 2) are such examples. The correlationbetween the "Nordic" wet ball mill value and road wear is shown in Figure 3. Inrecent years, the wet ball mill test has also become a Nordic method and has beenproposed as a European test method in the harmonisation of standards in Europe.

Figure 2 One type ofequipment for determining the "Nordic" wet ball mill

value.

middle gralned granlte

Ball

Mill

Value

0,5 1 1,5

Road Wear (mm)

Figure 3 Correlation between the "Nordic" ball mill test and road wear.

VTI Notat 30- 1997

METHODS/EQUIPMENT FOR MEASURING THE WEAR RESISTANCEOF ASPHALT SURFACING MIXES

The design of laboratory methods measuring the wear resistance of asphalt

specimens proved to be difficult, which resulted in the activities being

concentrated on road tests. At the beginning of the 903, there was however a

breakthrough thanks to new test methods where small slabs of asphalt mixes were

tested thoroughly and in realistic conditions. The slabs can be tested either in the

field (Figure 4) by placing them in existing pavements and measuring the wear

from traffic or by performing accelerated tests in the VTI's road simulator (a

circular machine with a diameter of five metres) Figure 5. The method of using

test slabs has facilitated a systematic survey of the factors influencing stud wear.

Since the start at the end of the 1980's, approximately 600 slabs have been tested.

Initially, the slabs were tested in the road, but in the course oftime, most of the

tests have been moved to the road simulator, which is now the most important

method for wear studies. This has been possible because of the very high

correlation between the road simulator and road wear (Figure 6).

Wear measurements in the field and in the road simulator were carried out with a

laser profilometer (Figure 7a) consisting of an approximately one metre long

monitoring beam with support. The profilometer is based on non-contact distance

measurements between the measurement beam and the road surface using laser

technique. The crossprofile is registered with a reading precision of approximately

0.01 mm and with a sampling density of 400 measurement points per metre. The

measurement data are stored and processed in a PC. Wear is calculated from the

difference between the zero measurement and the final measurement which is

carried out in the autumn and spring, respectively, just before and after the period

with studded tyres (Figure 7b).

In recent years, a laboratory method, Prall test (Figure 8), has been modified for

wear tests of specimens and drill cores. The method means placing a specimen in

a small chamber with a number of small steel balls. The chamber is then shaken

up and down at a high frequency and the steel balls hit the lid and the specimen

and material from the specimen is worn off. The method originates from

California (for durability resistance testing) and was later introduced to Germany.

VTI Notat 30- 1997

Figure 4 Wear measurements 0f test slabs in the field.

Figure 5 VTI's road simulator, PVM.

2,5

y :1,0481x - 0,0736

N

Road

Simulator(PVM)

0,5

0,5

Road

Figure 6 Correlation between slabs in the PVM and the road. Type ofpavement ABT12/B85 and ABT16/B85

VTI Notat 30- 1997

Figure 7a Laserprofilometerfar wear measurements (crossprofiles).

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 sia 4.0Profile length (m)

Figure 7b Wear profile (worn material).

Screw

Lid

Steel spheres <---- Cooling water

O-ring

Test ChamberStroke

43:1 mm

Balance wheel

Figure 8 Principal sketch of the Prall test equipment in Sweden.

VTI Notat 30- 1997

FACTORS INFLUENClNG STUD WEAR

Of all the studied parameters, the wear resistance of the coarse aggregate(exceeding 4 or 8 mm size) has proved to have a considerable influence on thewear resistance of the pavement. Figure 9 shows the importance of the aggregatein asphalt concrete with a maximum size of 12 and 16 mm. The differencebetween the poorest and the best pavement is approximately a factor 3. Figure 10gives asurvey of how stone-matrix asphalt is influenced by the coarse aggregate.The "Nordic" ball mill value of the pavements is between 4-13.

The proportion of coarse aggregate in the pavement influences stud wear

considerably. In principle, pavement resistance to studded tyres increases withincreased aggregate content, implying that stone-matrix asphalt has better wearresistance than dense asphalt concrete (Figure 11). The Swedish stone-matrix

asphalts contain about 70% material exceeding 4 mm.

The maximum aggregate size is another factor of great importance to stud wear. InFigure 12, stone-matrix and dense asphalt concrete with varying aggregate sizeare described. The aggregates are phorphyry and quartzite.

Polymer modified binders (PMB) can reduce wear from studs on specific types ofpavements (Figure 13). Dense asphalt concrete with PmB obtains reduced wearcompared with conventional bitumen while stone-matrix asphalt is not influencedby PmB.

Other factors influencing pavement wear are: degree of compaction, traffic

volume and frequency of studs, speed, lateral distribution of traffic (road width),

distribution wet/dry wear and the type of studs, stud protrusion and stud force.

Wet wear is often greater than dry wear but the difference depends on the type of

aggregate.

AC12 mm AC16 mm

Realtivewear

N

"

Porphyry Quartzite Fine Medium Gneiss Porphyry Quartzite Fine Medium Gneissgranite granite granite gran'

' :

:-

CD

Figure 9 Influence ofuggregate quality., Tests in the road simulator.

VTI Notat 30- 1997

Figure 12

VTI Notat 30- 1997

Influence ofmaximum aggregate size. Tests in PVM.

Relativewear

SPS-value(s

peci

ficwear)

Relativewear

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Figure 10 Influence ofmaximum aggregate size. Tests in the road simulator.

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Figure 13 Influence ofmodified binders (PMB). Aggregate: quartzite.Road tests.

FORECAST MODEL FOR WEAR FROM STUDDED TYRES

The systematic survey of influencing factors facilitated the development of aforecast model for rutting caused by wear from studded tyres. The medel mainlyconsists of three parts:

0 a model calculating the amount of wear per number of studded vehicles

0 a model calculating the distribution of wear on the width of the lane (wearprofile)

0 a model calculating the annual cost from used material and calculated term oflife

The inputs in the model are: the "Nordic" ball mill value, aggregate (>4 mm),maximum aggregate size, traffic volume, stud frequency, wear period, speed, roadwidth, proportion of lightweight studs, salted/unsalted roads and pavement costs(aggregate, bitumen, etc.).

EXPERIENCE FROM TEST ROADS

Continuous studies from test roads have shown that wear has been reducedconsiderably thanks to the actions taken. The relative wear (SPS value, gram wornmaterial per studded vehicle and kilometre) on a four-lane motorway with a totaltraffic volume of 60,000 vehicles per day is shown in Figure 14. The speed is 90km/h and the pavement is stone-matrix asphalt with quartzite and porphyry (thebest materials in Sweden). The reduction of the relative wear in recent yearsmainly depends on the lightweight studs which are more favourable to thepavement. Today, only tyres with lightweight studs are sold on the Nordic market.According to tests in the VTI's road simulator, lightweight studs (<l.4 g) wearapproximately half the amount of the earlier steel studs (1.8 g).

VTI Notat 30-1997

10

In the winter 1995/96, rutting was only 1.2-1.4 mm for the road mentioned,although 1,4 million studded vehicles trafficked the test sections. The pavement iscalculated to have a life time of 10 years.

Section 1

Section 3

[1]]Section 5

äSection 7

Section 8

ESection 10

SPS-value(s

peci

ficwe

ar)

90/91 91/92 92/93 93/94 94/95 95/96

Figure 14 Development of relative wear (SPS value) 1990-96. Road tests.

CONCLUSIONS

Aggregate quality is without doubt the specific parameter, together with theamount of coarse aggregate and maximum aggregate size, which is of mostimportance to the wear-resistance of asphalt pavements to studded tyres. Today,the best pavements in Sweden are probably five times more wear-resistant than thepavements in the mid-80s and possibly up to ten times better than those at the endof the 60s when studded tyres were introduced on the market. The development

towards more wear-resistant pavements has been considerable above all in recentyears, as a result of increased understanding of the importance of aggregates inbituminous surfacing mixes.

High quality stone-matrix asphalt together with the more lenient lightweight studshas drastically reduced stud wear in Sweden and nowadays wearing coarse layershave acceptable life time terms. Earlier (1989) the cost of pavement wear wascalculated to be millions of Swedish Crowns 250-300 annually because of wearfrom studded vehicles on the Swedish road network.

VTI Notat 30- 1997

11

LITERATURE

Gustafson K (in Swedish, Abstract and Summary in English). VTI Rapport 377,Tests with light studs in the VTI s road simulator, 1992.

Jacobson T (in Swedish). VTI Notat V197, Wear Characteristics of asphaltpavements. Tests in the VTI s road simulator and laboratory tests according toTrÖger test and PWR test, 1992.

Jacobson T (in Swedish). VTI Notat V173, Wearing course with polymer

modified binders. Test road E3/E18, Orebro-Arboga. Progress report 92.02, 1992.

Jacobson T (in English). VTI Särtryck 193, Wear resistance of bituminous mixesto studded tyres - A novel approach to field measurements and correlation withVTPS traffic simulator, 1993.

Jacobson T (in Swedish, Abstract and Summary in English). VTI Meddelande732, Study of the resistance of wearing course pavements to studded tyres. Testsin the VTPS road simulator. Final report. 1994.

Jacobson T and Wågberg L-G (in Swedish). VTI Notat 46-1994, Local pavements.Study of the resistance of bituminous mixes to studded tyres in VTPS roadsimulator, 1994.

Jacobson T (in English). VTI Särtryck 245, Study of the resistance of bituminousmixes to studded tyres - Test with slabs of bituminous mixes inserted in roads andin the VTIs toad simulator.d tyres - A novel approach to field measurements andcorrelation with VTPs traffic simulator, 1995.

Jacobson T and Viman L (in Swedish). VTI Notat 16-1996, Funktionsinriktad

metod för bestämning av nÖtningsresistensen hos asfaltbeläggningar, Prall. 1996.

Jacobson T (in Swedish). VTI Notat 64-1996, Study of the resistance of wearingcourse pavements to studded tyres. Test roads. 1996.

VTI Notat 30- 1997