VTIsätryck

130 1.989

Use of unbound pavement materials inSweden and other Scandinavian countriesPeet Höbeda, Swedish Road and Traffic Research Institute ( VTI),Linköping

Håkan Thorén, National Road Administration, Borlänge

Reprint from: International Symposium on Unbound Aggregatesin Roads (UNBAR 3), 77 73 April 7989 in Nottingham, Eng/and

%,Väg 'De/l Wii./(- Statens väg- och trafikinstitut ( VTI) 581 01 Linköping

Ill-91710181 Swedish Road and Traffic Research institute- s581 01 Linköping Sweden

ISSN 0347-6049

? VTIsärtryck

130 7.98.9

Use 0f unbound pavement materials in

Sweden and other Scandinavian countriesPeet Höbeda, Swedish Road and Traffic Research Institute ( VTI),Linköping

Håkan Thorén, National Road Administration, Borlänge

Reprint from: International Symposium on Unbound Aggregatesin Roads (UNBAR 3), 77 73 April 7989 in Nottingham, Eng/and

?, Väg-UCI) Hf/7!- Statens väg- och trafikinstitut (VT/) - 587 07 Linköping

[ St/tUtet Swedish Road and Traffic Research Institute ' S-581 0 7 Linköping Sweden

USE OF UNBOUND PAVEMENT MATERIALS IN SWEDEN ANDOTHER SCANDINAVIAN COUNTRIES

P. Höbeda, Swedish Road and Traffic Research Institute (VTI),Linköping and H. Thorén, National Road Administration, Borlänge

By tradition, unbound pavement materials have been widely used inSweden (S) and other Scandinavian countries. Increased trafficloadings have made it necessary to improve the material qualitiesand for this reason new specifications are currently being producedin S. The existing specifications are discussed and compared with arecent proposal by the National Road Administration. Commentsare made on the use of blast furnace slag and testing of unboundaggregates. Brief comments are made on the situation in Finlandand Norway, while the Danish situation is different and is thereforenot included.

INTRODUCTION

Apart from its southernmost parts, Sweden (S.) differs from Great Britain andcontinental Europe in several respects. Frost penetrates the subgrade and the soiland moisture conditions often promote uneven frost heaving. The risk of freeze thawweakening must always be considered. Also, soft clays and deep organic deposits arefrequently encountered. Building of roads in winter is considered necessary from thelabour market point of view. In such cases a flexible pavement, built with unboundlayers under the asphaltic ones, has been the preferred choice. Also, 5. is notdensely populated and therefore the money available for each km of road is notgreat. There still exist a certain number of unsurfaced low-volume gravel roads. Thesurfaced secondary road network has thin asphaltic layers. A special problem is wearby studded tyres in winter and the most heavily used roads are frequently resurfacedbecause of this special form of rutting. Problems concerning road surfacingaggregates have been treated by Hobeda (1989). The conditions in Finland andNorway are similar, while from the road conditions viewpoint Denmark is more apart of the European continent. The road networks and the conditions in Scandinavia

are presented in the PIARC magazine, Roads No. 265, 1988.

During the 80s, the traffic loading has increased, especially the proportion of heavylorries, which also tend to an increased degree to use single mounted wheels withhigher tyre pressures. Therefore their "aggressiveness" on the unbound layers hasincreased. Harmonization with EG rules in the near future also for S. means anincreased bogie load (from 16 to 18 tons) and total load (from 51AL to 60 tons). Allbridges will be strengthened and improved road material specifications will beproduced.

From the aggregate resources vieWpoint, the situation is favourable in S., sinceunweathered hard rocks, mainly of granitic composition, dominate, even if theirproperties show variations. Altered material is found only in disturbed areas, e.g.

faulted rock. Sedimentary bedrock areas are not extensive and such materials areseldom used in pavements. Of great importance are the glaciofluvial gravelformations, often found as eskers, that have provided a source of cheap aggregate,mostly of good quality, that reflects the composition of the bedrock formationsbroken up by the inland ice. In areas with sedimentary rocks, the gravel will beenriched in these components. However, gravel is an exhaustible resource andproduction is now increasingly restricted. At the same time more quarries producingcrushed rock are starting up production, especially near the major cities.

PAVEMENT TYPES IN USE

In S. The National Road Administration (NRA) is reSponsible for the materialsspecifications for road construction. There are two main types of pavements in useat present. The first is a fairly conventional type, in S. called the "gravel bitumen"type, which is built when glaciofluvial material is available. The other one is therock-bitumen type, constructed when a sufficient amount of rock is found in the

road cuttings. In this case, it is often economical to produce aggregate for the wholepavement, possible with the exception of surfacing aggregate, using mobile crushers.Depending on the specifications, it has also been possible to build this pavementtype with thinner asphalt layers mostly because of a lower water susceptibility.Most major roads have in fact been built with this construction using localmaterials. From the design point of view, the asphaltic layers are about the samethickness as in most European countries for the "gravel bitumen pavement, but areconsiderably thinner for the rock-bitumen pavement.

In Finland and Norway, such special designs for "rock-bitumen" type pavements havenot been developed.

Many newly build roads in S. have shown unexpectedly short service lives, probablybecause of the increased traffic loading, as well as construction deficiencies, andfor this reason the specifications are now being changed. More emphasis is nowbeing put on quality aspects instead of maximum capacity in a fully mechanizedconstruction, as in the 60s and 705.

(i) "Gravel bitumen" pavement.

This conventional pavement consists of a subbase, base and asphalt bound layers.The subbase can be made of crushed rock but is mostly constructed from unprocessed glaciofluvial material and is then classified as an A type (continuously graded)or a B type (single sized, sandy material). Often, there is a sand surplus in the eskerformations. Problems have occurred with both types. According to the presentspecifications, the max. fines content (<0.075mm) in the A-type material may be upto 16% of max. size 16mm, far too much for a material used in a climate with a riskof spring thaw weakening, when the permeability should be high. No gradingenvelope is specified. Also, the content of clay and organics may be up to 5 and 2%respectively. Such "dirty" materials have not been common, but occur in some areaswith sedimentary bedrock and therefore these excessive contents appearently havenot caused immediate concern. A few cases of road failures are known, however.

A material is classified as B-type if it has a content of less that 16% fines, S% clayand 2% organics, but any materials that have a capillary rise less that Im (test

performed on materials <2.0mm) are classified as B type. Such sandy materials havedemonstrated poor performance and therefore in 1986 the NRA specified that theuniformity coefficient (d60/d10) should be greater than 5, if the material is to beused less than 500mm below the surface.

The rather odd Specifications for A and B type materials seem to originate fromearly work by Beskow (1935), who developed a soil frost susceptibility classification,although he paid no Special attention to the stability properties.

A new grading envelope has recently (Nov. 1988) been proposed by the NRA. Thereare in fact two gradings (Figure 1) the inner and more narrow one considers thematerial when produced and the outer one the as laid material that has undergonesegregation and degradation. The material must be continuously graded and have alow fines content in comparision with the existing specification. The clay contentmust only be determined if the fines (<0.075mm) content exceeds 7% and shall thenbe <20% of the fines content. Also, the organics content is reduced to 2%. Thesechanges greatly improve the stability and water susceptibility characteristics. Thesubbase can be manufactured both from uncrushed and crushed material. Probablythe use of crushed rock will increase in the future as the grading proposed canhardly be produced from pit-run, glaciofluvial materials. Also, a material with verylow fines content should be crushed in order to ensure stability. The voids in thesurface must be filled with a finer sized aggregate before the roadbase can be laid.

The unbound roadbase has been made of crushed gravel or rock. The material hasbeen mostly of the crusher-run type, often crushed only once (Figure 2). The gradingenvelope is similar to specifications in many countries. The particle size distributioncurve must not cut the subordinate lines inside the grading envelope more that twiceleft from the 8mm sieve. The max. fines content has been lO%, quite high for gooddrainability and the max. clay content has been as high as 3%. This is valid for thecompacted material on the road. For high permeability, the fines content should beas low as possible, but such a material will be difficult to handle and compact andwill be less stable than a denser one so that a compromise must be sought. A highmax. particle sixe promotes stability, but increases segregation (Höbeda 1986).

A problem with crushed gravel has been that the content of crushed, angularmaterial has been too low, which results in reduced stability, and in 1986 the SRAspecified that the content of crushed material should be at least 50%. Also, becauseof the often poor stability of the subbase layer, the base has had the task offunctioning as the platform for construction traffic, with degradation problems as aresult.

A new grading envelope has been proposed by the NRA in Nov. 1988. As for thesubbase, there are two grading envelOpeS (Figure 3). The same values for max. clayand organic contents are valid as for subbase material. Because of the narrow rangefor the produced material, simply crusher-run material will probably be difficult toproduce and mixing of sizes will be necessary. Laid on an uncrushed subbasematerial, the base thickness will be 150mm. On a crushed material subbase orembankment of good quality rock, the thickness is only 70mm and in fact it becomesmore of a levelling layer. The min. thickness of subbase and base will be 500mm inorder to provide a sufficient bearing capacity also during the construction stage. Acapping layer of sandy material is used when the subgrade conditions are poor.

In Finland and Norway, the Specifications for both the subbase and base materialshave been more stringent than in S., especially considering the fines content. lnNorway, the Casagrande frost susceptibility classification has been used, whichgives quite a low fines content in well~graded materials. No grading envelope isspecified for subbase materials. Single-sized graded sand has not been permitted inthe upper part of the subbase. Also, in Finland the grading envelOpes for base andsubbase materials are similar to the S. proposal, but the fines contents are at alower level, max. S% for the inner and max. 796 for the outer envelope.

(ii) "Bitumen rock" avementP

If only crushing economy is considered, this means a size reduction as low aspossible. Until 1976, such pavements were constructed of sizes such as 75-200,25-75 and 0-25mm. The finest material was merely intended to give a smoothsurface for the asphalt paver, but he layers were often laid too thick. The surfacewas aslo easily degraded by construction traffic and a water susceptible layer thusformed under the asphalt bound layers that could result in alligator cracking.

The 75 200mm material in particular was difficult to handle, and a change wasmade in the NRA specifications of 1976. However, these were rather vague anddifferent types of constructions were built. A typical example is a pavement builtwith size 25 80mm, often laid with a considerable thickness, and levelled off with alO-Z5mm material. The surface was then penetrated with approx. 2 kg/m2 ofasphalt before construction traffic was allowed onto it. This proportion of binder ismuch too low for good binding action to take place. Such constructions have oftenbeen unstable because of the single sized macadam, especially one with a cubical"particle shape. Sometimes, the rock material has also been of poor quality and hasdegraded. In the 1984 NRA specifications, three grading envelopes were specified inorder to avoid the most single sized materials.

An optimum granular material, crushed from rock, should in fact be of the"drybound" macadam type, were the coarse aggregate particles are in contact witheach other and the voids are filled with angular fines. When properly constructed,such pavement layers can be superior to ones with well graded materials. However,this type on pavement layer would be very difficult to construct in a fullymechanized way and is not realistic.

The single sized macadam gradings have not been used any more in recent years inS. A continously coarse crushed rock grading has recently (1987) been tentativelyspecified by the NRA but the material costs would increase considerably. The mosteconomical construction method in rocky terrain seems to be the use of blasted rockas high up in the pavement as possible and the use of crushed rock only as a ratherthin layer. Such a pavement also will have an especially low deflection according tofalling weight measurements. Two major roads have recently been constructed inthis way. The coarsely graded (O~70mm) crushed material, which tends to segregate,was surfaced with a thin layer of penetration macadam in order to prepare a smoothsurface for the aSphalt paver.

In Finland and Norway, similar specially designed crushed rock pavement usingsingle sized macadam high up in the pavement, have not been specified. In Norway,e.g. blasted or crushed rock, the latter up to l§O~ZOOmm in size, is used as a

subbase.

THE SPECIAL CASE OF BLAST FURNACE SLAG

Because of the availability of hard rock in S., air cooled blast-furnace slag wasnot specified as a base material until 1984. This by-product has been appreciated asa high quality material with low moisture sensitivity and certain frost insulatingproperties. However, as the heat outflow also in hindered winter time, the roadsurface will be colder that one built with a natural aggregate and the risk of icingincreases in certain cases with high relative air humidity, especially in the earlywinter (Gustavsson 1988). Test sections, built in Central S. with a slag base, haveshown up as traffic safety hazards since an abrupt change of skid resistance maytake place. Slag roads therefore need extra careful winter maintenance. Important

in this respect is that in S. the use of deicing salt for winter maintenance is

considerably less than in many other countries.

TESTING OF UNBOUND MATERIALS

For testing the quality of unbound materials, only the grading and crushed materialcontent (for base only) are determinded at present. Rock material is classified inthree classes, 1 3, after petrographical composition. According to the new NRAspecification proposal, the base material must consist to 90% of class 1 or 2, in factigneous or metamorphic rock of fairly good quality. There is no strength or abrasiontest specified, as is the case for road surfacing aggregate, at present. However, atest is needed, especially as construction traffic is often run over base materials. Awet ball mill test seems to be promising in that respect. In Finland the Los Angelestest is specified and in Norway an impact test similar to that used in S. for road

surfacing aggregate (Hobeda 1989).

The stability and stiffness properties have not been studied yet as it is very difficultto test the coarse gradings necessary for high stability unbound layers. Repeatedload triaxial test are considered, but no apparatus, suitable for coarse gradings, yetexists. Because of the need for rapid drainage, especially during the spring period,permeability tests seem relevant, and experiments are in progress with a method,which is suitable for coarse gradings, described by Bioczysko (1985). The CBR test,not specified in Scandinavia, has shown promise with sandy, B type materials. Thereis a certain relationship with the uniformity coefficient and specifying thisparticular grading prOperty, as is the case at present, seems to be appropriate.

CONCLUSIONS

Increased traffic loadings make it necessary to improve the quality of the unbound

materials. Beacuse of the uniformity coefficient restriction, the use in certain partsof S. of sandy material in the subbase has already changed to the use of crushedrock, e.g. O lOOmm, a quite substantial bearing capacity improvement, if the

material is properly handled.

The use of unbound bases is likely to continue in a hard rock" country such as S.However, such materials must be manufactured in a more sophisticated way than atpresent, in order to improve their prOperties. From the pavement design viewpoint,the thickness of the aSphalt layers often has to be increased, especially for thelower volume roads, many of which are underdesigned or constructed with poormaterials. In Finland, the excellent performance of asphalt penetrated macadam

bases, compared with the unbound, more water-sensitive bases, has been noted(Orama 1988, pers. comm.). Also cement bound bases have been used with success insuch cases where the subgrade conditions have been favourable. In S., the often poorbehaviour of cement-stabilized bases is mainly due to use on too poor subgrades.Continuously graded bases seen to be out of favour in Norway because of their watersusceptibility and penetrated macadam, asphalt bound or cement bound bases willbe used more often (Refsdal 1988, pers. comm.). However, the quality of gravelaggregate is often less favourable in Norway than in the two other Scandinaviancountries.

REFERENCES

Beskow, G. (1935). Soil freezing and frost heaving with special application to roadsand railroads. (In Swedish). The Swedish Geological Survey, Yearbook 26, No. 3.

Bioczysko, S, J. (1985). Permeability of subbase materials. Unbound Aggregates inRoads (UNBAR 2), Nottingham University.

Gustavsson, K. (1988). Icing risks on roads built with slag. (In Swedish with anEnglish summary), VTI Meddelande 570.

Finnish Road Administration. (1985). Road Specifications. Pavements (In Finnish).

Höbeda, P. (1985, 1987). Literature studies concerning the properties of road baseand macadam type materials. (In Swedish with English summaries), VTIMeddelande 4112 and 469.

Höbeda, P. (1989). Influence of aggregates on the durability of road surfacing andtesting of aggregates Swedish experience. Conference on the Durability andPerformance of Bituminous Highway Materials. Hatfield Polytechnic.

Norwegian Road Administration. (1980). Specifications (Vegnormaler. In Norwegian).

National Road Administration, Sweden. (1981+). Construction Specifications andAdvice, (BYA 8# and later additions. In Swedish).

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