BACMIBritish Aggregate Construction Materials Industries(BACMI) is the national trade federation for theaggregate, coated materials and ready-mixed concreteindustries. It was founded in 1982 and includes in itsmembership the cornpanies producing roadstone, sand,gravel, asphalt and coated macadam, as well as theflexible paving contractors. BACMI membership alsoincludes the Plant and Equipment Group (companiesmanufacturing road construction and quarrying plantand equipment) and the Refined Bitumen Association(made up of the UK producers of petroleum bitumen).

ACMA PRODUCT GROUP

The ACMA Product Group comprises those membercompanies of BACMI who supply bituminous roadmaterials and,/or are involved in road surfacing.The name of the group and the retention of itsdistinctive emblem reflect the continuation withinBACMI of the activities of the former Asphalt andCoated Macadam Association in the technical,research, commercial and promotional fields.

Whilst every effort is taken to ensure the accuracy ofthe general information and advice offered in ttrispublication or given by staff of BACMI, no liability orresponsibility of any kind can be accepted by BACMIor its staff.

Designed by Paul SharpPdnted by Design &Print Partnerehip Ltd,Claygate, Surrey

This booklet presents a general review of the nature and uses of the different materials used in theconstruction and maintenance of bituminous roads. It is not intended for the experts who know their wayaround the labyrinth of detailed specifications. Its principal purpose is to assist that wider group ofengineers, architects and planners who are often concerned with roads: it should also be of value tostudents who are looking for a useful primer on the subject.

@ 1985Published by the ACMA Product Group ofBritish Aggregate Construction Materials Industries(BACMI), 156 Buckingham Palace Road,London SW1W 9TR Tel0l-730 8194

IntroductionGreat Britain is one of the few countries in the world inwhich the whole of the public road network has apermanent all-weather surface. Less than one-fifth of ourroad network has been built as new road in the last fiftyyears. Although this does include our motorway systemand many other important new roads, the majorproportion of our road network consists of old roads,some with foundations which go back to Roman times.These roads have been widened, strengthened and theiralignments improved to meet the enormous increases inroad traffic since the 1920s. It is one of the outstandingvirtues of bituminous mixes that they are so convenient touse in improving and strengthening old roads and inproviding them with surfaces which are safe andcomfortable to ride on, durable and strong enough tocarry the heaviest commercial traffic.

These virtues also make bituminous mixes particularlyeffective in building all types of new road, from privatedrives, farm roads and estate roads to the most heavilytrafficked motorways and city streets. They are equallyeffective on other paved areas, such as airfields, car andlorry parks, footways, cycle paths and playgrounds.

Contents

Introduction 2The make-up of a road 3Pavement design 4

New roadsExisting roads

What's in a road? 5AggregatesBituminous binden

Coated bituminous mixes 7Rolled asphaltCoated macadamAsphaltic concrete

Laying and compaction 10The flexible road 11

References and further reading- inside back cover

FiguresFig 1 The structure of a flexible road 3Fig2 A typical quarry producing aggregate 5Fig 3 A bitumen refinery 6Fig 4 A coating plant 7Fig 5 Rolled asphalt texture 9Fig 6 Coated macadam texture 9Fig 7 A modern paver in action 10

Acknowledgments:F ig2andT ARCFig 3 Esso Petroleum Co Ltd

toThis booklet presents a general review of the nature and uses of the different materials used in theconstruction and maintenance of bituminous roads. It is not intended for the experts who know their wayaround the labyrinth of detailed specifications. Its principal purpose is to assist that wider group ofengineers, architects and planners who are often concerned with roads: it should also be oi value tostudents who are looking for a useful primer on the subjept.

@ 1985Published by the ACMA product Group ofBritish Aggregate Construction Materials Industries(BACMI), 156 Buckingham palace Road,London SWIW 9TR Tel0l-730 8194

The make-up of a roadAll road pavements* consist of several layers of differentmixes which in combination make the road strong anddurable.

Bituminous pavements are frequently described as flexiblepavements, implying their ability to absorb the stressesimposed by traffic and weather without cracking. Thecomponent layers of such pavements are illustrated in fig l.

Fig I The structure of a flexible road

The function of each of the layers is as follows:The subgrade is the natural soil, on old roads usually wellcompacted by traffic, on new roads carefully shaped andcompacted to the appropriate level and profile. Subgradeimprovement may be possible by treatment of soils withlime or cement.

The sub-base is the lowest layer, put down to help buildup the strength of the pavement. It also provides aworking platform for the machinery used in laying andcompacting the layers above. It is usually made fromcrushed stone or gravel.

The roadbase is the main component which provides thestrength and load distributing properties of the pavement.For lightly trafficked roads it is usually made from gradedcrushed stone or hardcore, or it may be crushed stonebound with a small proportion of cement (cement-boundgranular base or lean-mix concrete). On more heavilytrafficked roads a fully bituminous roadbase may beemployed to provide a pavement of high strength anddurability.

The basecourse further contributes to the strength ofthepavement, and at the same time provides an even,well-regulated surface to carry the uppermost layer of thepavement.

* The wordpauement is used in this booklet in its correct engineeringsense to describe the whole structure of a road, from foundation tosurface, and NOT to indicate a pedestrian footwav.

The wearing course provides an even and weather-resistant running surface which can withstand the abrasiveforces of traffic and provide appropriate skid resistance.

The bituminous mixes used for the roadbase, basecourseand wearing course are mixtures of mineral aggregate(crushed rock, slag, gravel and sand) and a bituminousbinder, normally petroleum bitumen but occasionally taror a specially modified bitumen to provide particularproperties. These components are carefully proportionedand mixed to an appropriate specification. The mixes arethen transported to site and laid and compacted while hot.

All the bituminous mixes and the sub-base and roadbasematerials mentioned here are supplied by membercompanies of BACMI.

In Great Britain there are two types of bituminous mixes-asphalts and coated macadams. The asphalts are usuallydenser and more finely textured than the coatedmacadams.

The difference between the two types of mix is bestindicated by the following definitions:

Coated macadam - a road material consisting of gradedaggregate that has been coated with tar or bitumen (or amixture of the two) and in which the intimate interlockingof the aggregate particles is a major factor in the strengthof the compacted material.

Rolled asphalt* - a road material consisting of a densemixture of mineral matter and bituminous binder and inwhich the mortar of fine aggregate, filler and highviscosity binder is a major factor in the strength andperformance of the laid material.

The terms 'tarmacadam' or'tarmac'are often used by thepublic at large but are incorrect: many are asphalts. Thosethat are coated macadams are nowadays usually madewith petroleum bitumen rather than coal tar.

x Two types ofasphalt are used on roads in the UK: rolled asphalt andpaving grade mastic asphalt. However, the latter is used only forspecialised applications, such as bridge decks, city footways, bus lay-bys,whilst rolled asphalt is the principal wearing course used on major roads.References to asphalt in this booklet will therefore generally be to rolledasDhalt.

SURFACINGi

SURFACE\

Another semantic confusion arises from the difference inmeaning assigned to the word'asphalf in Europe andNorth America. In Europe it always means a mixture ofbitumen binder with mineral aggregate-as in rolled .asphalt, mastic asphalt, gussaphalt, fine cold asphalt etc.When North Americans talk about asphalt they usuallymean the petroleum bitumen binder.

The performance ofthe pavement depends agreat deal onthe care with which the bituminous mix is prepared, laidand compacted.

The new BACMI Guide, available later in 1985, will listall members of the federation who produce aggregate andbituminous mixes and also incl.udes names and addressesof experienced specialist surfacing contractors. Your localBACMI member will always be pleased to advise you onthe choice of appropriate specifications for bituminousmixes in any given circumstances.

Pavement designNew roadsThese must be designed to make them strong and stiffenough to distribute the traffic loads so that neither theunderlying soil nor any of the pavement layers areover-stressed. Thus, in designing new pavements theimportant parameters are the.strength of the soil subgrade,the weight and intensity of thdrxpected traffic and thestiffness of the materials available to make the differentpavement layers. Pavement design methods must reflect theenvironment for which they are intende{. Thus with ourwet climate and heavy clay soils, British methods layconsiderable emphasis on the importance of a correctevaluation of the load-supporting characteristics of the soilsubgrade, and on a proper evaluation of estimated trafficloading. Heavy wheel loads are particularly damaging. TheAASHO Road Test* concluded that pavement damage isapproximately proportional to the fourth power of theapplied axle load. This means that a l2-tonne axle, forexample, does about five times as much damage as an8-tonne axle.

Since 1970 the basis for designing pavements for new roadshad been the third edition of the Transport and RoadResearch Laboratory's Road Note 29 A guide to thestructural design of pavements for new roads. This wassupplemented in 1978 by the Department of Transport'sTechnical Memorandum 6/78 Road pavement design Atthe time of going to press the whole subject of pavementdesign is being radically reconsidered. As a start, a newdesign procedure for flexible roads has been published asTRRL Laboratory Report LR 1132 The structural designaf bituminous roads and designs based on this wereintroduced by the Department of Transport for majorroad construction from 1 April 1985.

* The AASHO Road Test was a large-scale US investigation on aspeiially built track carried out in the United States in the late 1950s. Itprovided a great deal of insight into the behaviour of road materials undervarious conditions.

4

Existing roadsBituminous mixes have ideal properties for use instrengthening pavements which are showing signs ofdistress under traffic loading. They can be superihposed onexisting roads as overlays, so that these roads can continueto carry heavier traffic than that for which they hadoriginally been designed. Alternatively, increased strengthin a pavement can be obtained by replacing one or moreold surfacing layers with new bituminous material.Many existing road pavements consist of layers of differentmaterials laid at different periods and it is often difficult toevaluate the residual strength ofsuch an old pavement andthus estimate the further strengthening required. Methodshave been developed over the past twenty years, however,to evaluate pavement strength in situ, by measuring thevertical deflection of the pavement under a slowly movingload. The deflection beam (Benkelman beam to those whorecall its designer) and its highly automated derivative, theLacroix deflectograph, are often seen in use on roads inGreat Britain. They help in the assessment of the currentcondition of our road pavements and provide data to helpin the design ofoverlays when needed. These techniquesare now highly sophisticated and in any situation where thethickness of overlay required is not immediately obvious itis prudent to employ experts using these techniques toprovide appropriate designs.

What's in a road?AggregatesMost civil engineers accept without any curiosity their ownspecialised use of the term'aggregate', but others may bepuzzled.In this context it means'an aggregation of hardnon-metallic minerals available in particulate form andprocessed for use in civil engineering'ie, crushed rock,gravel, sand and slag. Within these broad groups there is anenormous variety of aggregates available for use inroad-making, corresponding to the complicated geologicalformations of the country. There are some broadgeneralisations. Gravels and sands are found in old rivervalleys such as the Thames, the Trent and in the lowlandsof Scotland; also in marine deposits. Crushed rockgenerally comes from hilly areas. There are variations inthe nature and properties ofgravels and sand according tothe nature of the parent rock from which they derive. Thereare even larger variations in thb nature and properties ofcrushed rocks, deriving from their mineralogical origins.There are also artificial rocks such as the slag produced bythe iron and steel making industry. Building bricks do notcome under the definition of aggregate. But if they arecrushed, for use in making sub-bases or bases for morelightly trafficked roads, they might then be so designated.

For use in road making, indeed for most uses in civilengineering, the quality ofaggregates must be closely con-trolled. For use in roads the most significant properties are

- size: the'grading'of an aggregate mix signifies theproportion of standard sizes and largely determines itsmain properties;

- shape: this is important for the mechanical interlock ofaggregate layers; 'flakiness'is the extent to whichindividual chippings are not equi-dimensional,

- strengtnL the resistance to crushing;

- surface characteristics: good resistance to'polishing'and abrasion under traffic are important requirementsfor aggregates to be used in weaing cot,,ses to ensureadequate skid resistance and long life. These propertiesare characterised by the polished stone value (PSV) andaggregate abrasion value (AAV) respectively.

The tests for these properties are laid down in BS 812Methodsfor sampling and testing mineral aggregates,sands andfillers. Appropriate limits for quality on thesetests are normally contained in the British Standard orother specification for the particular road making productconcerned.

Fig 2 A typical quarr! producing aggregate

Bituminous bindersBituminous road materials consist of aggregate boundtogether by bituminous binders, the amount of bindernormally varying between 3 and 9 per cent by weight ofthe mix, according to the type of mix and the use to whichit is to be put.

There are three types of bituminous binder used in roadmaking:

- Petroleum bitumen, derived from the refinins of certaintypes of crude oil;

- Road tar, derived from coal, as a consequence ofindustrial processes for other purposes, eg in theproduction of coke for metallurgical purposes, in theproduction of coal gas (a process which for the timebeing at least has become obsolescent in the UnitedKingdom) and in the production of smokeless fuels;

- Natural bitumens, ie, the bitumens in natural asphaltssuch as the well-known asphalt lake in Trinidad. Thesenatural asphalts are soils or rocks which were onceimpregnated with bituminous crude oil and fromwhich, over geological periods of time, the morevolatile fractions have evaporaed.

By far the most commonly used bituminous binder in theUK is petroleum bitumen and its derivatives. Smallquantities of road tar are still produced but are used mainlyfor surface dressing. Some coal tar pitches are used to makespecial surfacings resistant to oil spillage. SometimesTrinidad Lake Asphalt (TLA) is also used in makingspecial types of bituminous surfacing.

All bituminous binders are characterised by being black,sticky and thermoplastic, ie, they become softer and morefluid when they are heated, and harden again when theycool.

F ig3 A

Petroleum bitumens are marketed in three different forms:

- penetration bitumens, ie, hard bitumens, graded by theirpenetration, which is the depth to which a standardneedle penetrates into the bitumen in 5 seconds undera load of 100 g at a temperature of 25"C;

- cutback bitumens, in which the viscosity ofpenetration bitumen is reduced to make it convenientto use at lower temperatures than for penetrationbitumen, by blending it with volatile oils;

- bitumen emulsions, in which penetration bitumen isdispersed in water in the form of very small globules,again to make it convenient to use cold or, withsome emulsions, at lower temperatures than forpenetration bitumen; the bitumen is held insuspension by the use of special emulsifying agents.

Cutbacks and emulsions find their main use on roads insurface dressing. Emulsions are also used as tack coatsunder bituminous surfacings. The more viscous cutbacksare used in making bitumen macadams, generally for themore lightly trafficked roads. Cutbacks and emulsions arealso useful in making patching materials which are to bestored fbr a period before use. By far the most commonlyused bitumens in bituminous road mixes, however, are thepenetration bitumens, the actual penetration grade beingchosen to suit the use to which the surfacing is to be put.

The various grades of bitumen are specified in BS 3690Bitumens for building and civil engineering. Appropriategrades for different uses are laid down in British Standardsor other specifications.

The various grades of bitumen emulsion are specified in BS434 Bitumen road emulsions and their different uses aretherein suggested.

Road tars are specified in BS 76 Tars for road purposes.Only the more viscous grades of road tar are used inmaking bituminous surfacings.

bitumen refinery

Fig4 Acoatingplant

Coated bituminousmrxes

Rolled asphalt

Rolled asphalt had its beginnings in the latter half of thel9th century when road engineers and paving contractorswere seeking to produce man-made imitations of naturalasphalts for the paving of city streets. A bright andenergetic American, Clifford Richardson, conceived andcarried through a plan to inspect and analyse successfulasphalt pavements all over the USA and Europe. Thesemixes were mostly made with natural sand, with refinedbitumen and road tar, both of which were then appearingon the market, to supplement natural asphalts as the binder.His book, Modern asphalt technologt, appeared in the early1900s and rapidly ran to three editions.

British asphalt paving contractors became highly skilled ininterpreting the tenets of Richardson's recipes for asphaltsurfacings under British conditions, offering first sandasphalt sheet and later mixes containing crushed rock orgravel, which provided extremely durable surfacings forthe traffic of that era.

Improvements have been continuous. Specifications forthe composition, mixing and laying of rolled asphalt arecontained in BS 594 Specificationfor rolled asphaltforroads and other paved areas, the first edition of whichappeared in 1935.

Rolled asphalt is a high-quality bituminous surfacing whichfinds its main use on more heavily trafficked roads and citystreets. Traditionally the specifications for composition arein the form of recipes specifying the quality andproportions of aggregate and bitumen to be used andindicating how the proportions should be varied to suittraffic and climate. There are separate specifications forwearing course and base course,mixes. An alternativemethod of designing rolled asphalt surfacings uses amechanical testing regime to assist in deriving optimumbinder contents. This alternative method is intendedparticularly for use in designing mixes for very heavilytrafficked roads or in refining specifications when usingaggregates, particularly sands, from unfamiliar sources. Forother purposes the recipe specifications provide a simplerand quite adequate guide to the most appropriate mixes touse in given circumstances.

An important characteristic of rolled asphalt surfacing isthat the aggregate is'gap-graded'that is to say, it consists ofhardly any medium-sized aggregate but of a matrix of sand,fine mineral powder and bitumen into which coarseaggregate is incorporated like raisins in a pudding. It is thisgap-grading which gives rolled asphalt surfacings theirparticular advantages in providing a durableweather-resistant surface which can accommodate heavyloads without cracking. The British Standard containsspecifications for mixes with a range of proportions ofcoarse aggregate between zero and 65 per cent ofthe totalaggregate. Mixes with lower stone content have a smoothsurface texture, and on roads where is it important to havea good skid-resistance it is normal to provide this byapplying precoated chippings onto the hot asphalt as it islaid. Mixes with higher stone contents (557o and 657o) arenormally used for roadbases or basecounes.

Coated macadamJohn Loudon McAdam would have been surprised to seethe wide range of materials which have been evolved usinghis name. But they do derive from one of his importanttechnical tenets-that road pavements should be madefrom broken stone, no particle ofwhich should be largerthan one inch in size. Opportunist as he was, he wouldhave been quick to claim proprietorship over the usefulmaterials which have evolved from this notion. McAdam'sl-inch size particles of stone, broken by hand, were laid inlayers up to 6 inches thick and his concept was that theselayers would remain smooth and stable under the highcontact pressures of iron-shod coach wheels. But his roadswere dusty in summer and, even before the beginning ofthe motor vehicle era, engineers had begun to apply crudetar and heavy oil to lay the dust. Out of these endeavoursemerged the modern technique of bituminous surfacedressing (a useful technique in the armoury of the roadmaintenance engineer, but outside the scope of thisbooklet).

Even stronger pavements could be made by applying thetar or bitumen to the broken stone before it was traffickedso that the binder penetrated for some distance down intothe layer ofbroken stone. Thus emerged grouted orpenetration macadam, a process still effectively used insome developing countries where there are pressures to usemethods employing hand labour.

It was an obvious further development to coat the stonewith tar or bitumen in a central plant before it was laid, andit was at this stage that the term'tarmacadam'moved intothdvernaculaljo describe bituminous road mixtures.Cohtraetoffiolved their own mixtures, many of whichwere marketed under trade names. At this stage they wereall essentially open-textured coated macadams, ie, mixescontaining stone of predominantly one size with aproportion, generally less than 20%, of finer aggregates toprovide extra stability. It was with these mixes that thecoated macadam industry grew to prominence in themiddle of the twentieth century providing large quantitiesof coated macadam for the strengthening and wideningof roads to carry the rapidly increasing motor traffic.

The first national specifications were issued in the early1940s as War Time Road Notes by the then RoadResearch Laboratory. There followed a period whenfull-scale experiments with different aggregates werecarried out on roads all over the country by the RRL inco-operation with highway authorities and industry. As aresult of these and of a synthesis of general experience, thefirst series of British Standards for coated macadams, withdifferent aggregates and road tar or bitumen, waspublished in the late 1940s and early 1950s. Now all thesedifferent specifications have been consolidated in oneBritish Standard, BS 4987 Specifuation for coatedmacadam for roads and other paved areas. This standardcovers open and medium textured macadams, indicatingthe variations in specification necessary with differentaggregates and to suit differing traffic conditions. It alsoincludes close textured macadams (something of acontradiction in terms of McAdam's tenets), a develop-

8

ment by industry to supply dense mixes particularlysuitable for use in building up the pavements on heavilytrafficked roads.

Coated macadam is available in an immense variety ofmixes suitable for different uses, all the way fromsurfacings for private driveways to the wearing and basecourses and roadbases for heavily trafficked roads. Thespecifications in BS 4987 are in the form ofrecipes,indicating appropriate gradings of aggregate, and typesand quantities of bituminous binder. Variations in thesespecifications cover different types of aggregate to producethe most suitable material for the use intended.

One particular type of open-textured macadam is somanufactured as to remain porous to water throughout itsuseful life. The material, known as pervious macadam inhighway engineering terms but as friction course inairfield use, has the ability to act as a self-draining layer,thereby reducing spray thrown up by vehicle wheels andalso reducing the risk of vehicles (or aircraft) aquaplaning.Whilst this type of macadam has been used extensively onairfield runways throughout the world for many years,only limited use has been made of it to date on highways.However, contiiuing development of the material is likelyto result in its more extensive highway use in the nearfuture. BACMI has been associated with recent researchand development of this type of material.

Asphaltic concreteThis material is a forryr of bituminous mix originallydeveloped in the USA and particularly intended to meetthe need for stiff and strong pavements to carry the heavyloads and high tyre pressures ofaircraft. Specifications arederived from a mechanical testing regime and the mixesare characterised by having aggregate of a continuousgrading and a bitumen content which must be verycarefully controlled. It is not normally used on roads inGreat Britain. It requires a very strong stiff structurebelow it, otherwise it is liable to crack under thepavement deflections caused by heavy vehicles. Inaddition it provides very little surface texture and wouldrequire a surface dressing treatment to provide anadequate level ofskid-resistance on high speed roads. Anappropriate specification for the use ofasphaltic concreteon airfields is contained in Specification for airfield work,issued by the Property Services Agency of the Departmentof the Environment. The mechanical testing regime usedto design asphaltic concrete has been adapted for use indesigning rolled asphalt mixes and is particularly usefulwhen a high stability mix is required for very heavilytrafficked roads.

Rolled asphalt texture

Coated macadam texture

-

Laying and compactionThe processes of laying and compacting bituminousmaterials are of prime importance in the provision of asatisfactory and durable job. The laying of these materials,and indeed of all of the various materials in all of thelayers of flexible roads, can be carried out quickly andaccurately by machines which are readily available andwhich should be used wherever practicable. The followingbrief notes outlining the laying process will be helpful:

Sub-strate conditionPrior to laying, it is important to ensure that theunderlying construction is strong and stable (or made so).Proper compaction cannot be achieved if the surface onwhich the material is being laid moves about under theroller: the criterion is that the area to be covered shall becompacted by, and be stable under, a roller of not lessweight or effectiveness than that which will be used onthe bituminous coated materials. To achieve this withsome types of subgrade it may be necessary to considerthe provision of sub-soil drainage.

Excessive variations in thickness due to the materialsbeing laid on a poorly shaped substrate can causevariations in initial compaction with subsequentconsolidation under traffic resulting in a reduction in theriding quality of the road. Where necessary, therefore,appropriate shaping or 'regulating' of the underlyingconstruction should be carried out prior to laying asubsequent layer.

LayingModern paving machines are of the floating screed typeand automatically take out minor and short irregularities.Depressions of from 3 to 6 m long can be taken out byadjustment of the thickness controls, but if over 6 m longthey should be made up by hand before the mainresurfacing starts. However, provided the machine isworking on a reasonably shaped base, once it has been setto lay the required thickness, the less interference there iswith the controls, the better the finished job.

There are several sizes of these types of machines.including a relatively small one suitable for use onfootways, and a selection can be made according to theoutput required and the width to be paved.

In addition to spreading the materials these machines also,by means of a tamper, compact the material to someextent, estimated at 60-80% of the density obtained afterrolling, with the result that there is less distortion of thesurface when the roller is applied.

Fig 7

Although a much greater proportion of coated macadamand rolled asphalt is spread by machine, there is still alarge tonnage spread by hand on such areas as school play-groun{i and footways, besides regulating existing surfaces.

For these conditions, where the specification permits,lower viscosity binders are normally used in coatedmacadams giving more latitude in the handling andcompaction of the materials. It is also possible to laystandard rolled asphalt and dense coated macadam byhand but a specialist contractor or an experienced gangshould be emoloved.

CompactionAfter the materials have been spread, even when this hasbeen done by machine, compaction is required, in the caseof a macadam to ensure adequate interlock between theaggregate particles and in an asphalt to achieve itsnecessary low void content and consequent high strengthand durability. A major factor in the workability of themixed material is the viscosity of the binder which isgoverned by the temperature of the material. The moreworkable the material, up to a limit of being too soft andpushing under the compacting equipment, the better thecompaction obtained. It is essential that material madewith high viscosity binders be laid and thoroughlycompacted while hot. In general rolling should commenceas soon as the material can be compacted without unduedisplacement and continue until all roller marks havedisappeared.

The weather and the site conditions and thickness governthe rate at which the material cools. On an exposed,windy site, material cools quickly and there should be nodelay in rolling. Showers of rain also cause material tocool quickly. At the other extreme on a hot day, the rateof cooling is slow and some delay in compaction may benecessary.

paver in actton

l 0

Three types of roller are available for the compaction ofbituminous materials, namely:

dead weight three-wheel and tandem rollersvibratory rollerspneumatic-tyred rollers.

Until recently the three-wheel type was the most commonand is available in weights ranging from 2 to 12 tonnes.British Standards give an indication of the weight of rollerto be used but an overall guide is 2.5 tonnes for footwaywork and 8-10 tonnes on carriageway construction work.The weight of roller used on any layer should not besignificantly greater than that used on any lower layers inthe construction.

In the past vibratory rollers were of relatively smallsize and weight, factors which made them suitable andconvenient only for footway work and small surfacingjobs. Their compacting capabilities, however, are in excessof their dead weight. Larger tandem-type sizes are nowavailable and are being used regularly on large-scaleworks. Pneumatic tyred rollers are seldom used in the UKfor compacting bituminous mixes, except in airfield work.

Application of coated chippingsWith the fine-textured types of wearing course, eg, rolledasphalt containing 407a or less of coarse aggregate, finecold asphalt and 357o stone content Dense Tar Surfacing(see BS 5273) it is usual to roughen the finished surfaceon carriageways by rolling in coated chippings of 20 mmor l4 mm nominal size while the material is still workable.

If the material is handlaid it should be rolled once beforethe chippings are spread, otherwise they may be buried.Machinelaid material receives initial compaction fromthe machine and the chippings should be appliedimmediately behind it.

When mastic asphalt paving is laid on carriagewayscoated chippings are also normally applied (see BS 1446l1447).

The flexible roadWe have seen how bituminous mixes are made uo andhow they fit in with modern road construction teihniques.They are by far the most predominant road surfacingmaterials. Indeed over the last fifteen years or so, overthree quarters of all new contracts for motorways andtrunk roads let by the Department of Transport havebebn in flexible construction, the remainder being in rigid(concrete) construction. Actual proportions vary fromyear to year. Overall, about 95 per cent of the UK totalroad network of 343,685 km (and indeed approximately98 per cent of the estimated 16 million km of the wholeworld's paved roads) may be said to be flexible, ie, builtusing bituminous materials-such as those discussed inthis booklet.

In addition to roads, bituminous mixes are used in manyother situations such as:

Vehicle parking areasHouse-drivesFootwaysPlaying areas-tennis courts, playgrounds etcAirfields-runways and access roadsAgricultural uses-farm roads, animal cubicles,

paved areasHard standings and storage areasHydraulic applications: dam construction, coastal

protection etc.

To understand why flexible construction predominates inroad construction we must go back to the fundamentalrequirements of a modern road pavement. It must beweatherproof, preventing the destructive ingress of water(even in the case of pervious macadam surfaces water isnot allowed to penetrate beyond the wearing course); itmust be durable, resisting the constant pounding of heavytraffic and structured in such a way as to distribute theload evenly through successive layers; it must haveeven-riding properties; and be skid-resistant.

The various bituminous mixes described have evolved tomeet precisely those requirements. In this sense theirproperties are also'flexible': specifically and purposelydeveloped for a wide variety of circumstances in roadconstruction and maintenance and aiming to make thebest use of available local road-making materials.

The inherent advantages of flexible pavement constructionthus closely match the requirements outlined, and with thecontinuous development over almost a century are able tocarry anticipated future traffic loads safely andeconomically.

Some of the principal characteristics of modern flexibleroads are:

Ease of constructionThe method of constructing in layers enables work toproceed at a number of points progressively along theroad as required. The materials required are easilyobtained and can be laid accurately and swiftly withmobile relatively inexpensive and readily available plant.However, the construction need not necessarily becontinuous along the road, but can be done in separatedlengths as found convenient for running ofthejob, to bejoined up later, without detriment to the finished road.

Even riding qualttyThe surfacing on flexible roads can be laid to a highdegree of accuracy, by modern paving machinery,providing a smooth-running finish for vehicles.

l l

Rapid opening to traffic In summary,As soon as the construction of a flexible road made withbituminous materials, or any stage of it above thesub-base, is completed. it can be opened to traffic.

The bituminous materials can be used by traffic as soon asthey have been laid and have cooled to atmospherictemperature. This feature can be particularly important inreconstruction on congested sites such as roundabouts andcity streets where the least hindrance to traffic flow cancause considerable dislocation and delay. By the use inparticular of bituminous roadbases, delays are kept to aminimum as traffic can run on the material the same dayas it has been laid.

FlexibilityThe normal slight movement under traffic on a flexibleroad is a feature which prevents any harshness in theriding quality of the road.

In addition, however, there is sufficient flexibility in theconstruction to accommodate settlement due to generalsubsidence or earthwork settlement eg, embankments orjunctions with bridge abutments, without completebreak-up. The riding quality of the road can be easily andinexpensively restored by the superimposition ofappropriate bituminous material.

Simple and economicmatntenanceThe surfaces of all types of roads are subject to wear andtear. The life of a flexible road wearing course can beextended easily, quickly and economically by superim-posing a new wearing course or by surface dressing.When it is not permissible to raise the level of the finishedsurface the existing wearing course can be removed easilyby heating and planing or by cold planing and a newone substituted to maintain the orieinal levels.

Skid resistanceBituminous wearing courses are available which canprovide the high resistance to skidding needed on highspeed roads and indeed in all situations where publicsafety is involved.

bituminous road mixes- provide weather-resistant surfocing

- are resistant to man)r dilute chemicals includingde-icing salt

durable

- give universally-acknowledged even-riding quahtl

- provide good ski.d-resistance

- allow early use ofa new surfacing by traffrc withoutthe needfor lenqthl curing periods

- combine strenglh with flexibility giving safe distributionof heavy traffic loads

- are easily and economically maintained andstrengthened

- present a neat, aesthetically pleasing and low-glareappearance

- provide good contrast to road markings thus contribu-ting to safety

- do not require joints to be incorporated in the con-struction, with their associated problems andmaintenance needs

There is one basic pre-condition for most of theadvantages listed: materials and constructionmust be of a high standard. This high standard isone of the characteristics that members ofBACMI have in common-from the suppliers ofroadstone right through to the contractors whocarry out the surfacing work.

I2

References and further reading

British Standards-available from BSI, Linford Wood,Milton Keynes, MK14 6LE

BS 63 Single-sized roadstone and chippings

BS 76 Tars for road purposes

BS 434 Bitumen road emulsions

BS 594 Rolled asphalt for roads and orher paved areas

BS 598 Sampling and examination of bituminousmixtures for roads and other paved areas

BS 812 Methods for sampling and testing of mineralaggregates, sands and fillers

BS 1446 Mastic asphalt (natural rock asphalt fineaggregate) for roads and footways

BS 1447 Mastic asphalt (limestone fine aggregare) forroads and footways

BS 3690 Bitumens for building and civil engineering

BS 4987 Coated macadams for roads and other pavedareas

BS 5273 Dense tar surfacing for roads and other pavedareas

Government publications- availab le from HM SOSpecification for road and bridgeworks -Department of

TransportNotes for guidance on the specification for road and

bridgeworks-Department of TransportMethod of measurement for road and bridse works

-Department of TransportNotes for guidance and library of standard item

descriptions for the preparation of bills of quantitiesfor road and bridge works-Department of Transport

Airfield pavement design and specificationpublications-Department of the Environment,Property Services Agency (Airfields Branch)

The design and performance of road pavements, by D Croney-Transport and Road Research Laborutory. 1977

(Alqo: Bituminous materials in road construction-RoadResearch Laboratory, 1962-now out of print butpossibly available in some specialised libraries)

Government publications- no t available from HM SOVarious reporls of the Transport and Road Research

Laboratory-available from the Library of TRRL,Crowthorne, Berkshire RGI I 6AU

Various Department of Transport Technical Memoranda,Departmental Standards and Advice Notes (includingTechnical Memorandum H6 / 7 8)-av ailable from DTpPublications Sales Unit, Building l, Victoria Road,South Ruislip HA4 ONZ

Other publicationsAsphaltic road materials-L W Hatherley and P C Leaver

-Edward Arnold Ltd, London, 1967Developments in highway pavement engineering Parts I

& 2-Applied Science Publishers Ltd, London 1978The performance of rolled asphalt road surfacings

(proceedings of a conference held in London on16.10.79)-Institution of Civil Engineers, London, 1980

Blastfurnace and steel slag-A R Lee- Edward ArnoldPublishers Ltd 1974 (available from BACMI)

Black top: a history of the British flexible roads industry-J B F Earle,1974 (available from BACMI)

A century of road materials J B F Earle, t 971 (availablethrough Tarmac Roadstone Holdings Ltd, PO Box 44,50 Waterloo Road, Wolverhampton WVI 4RU)

Mastic Asphalt Council and Employers' Federationtechnical publications (from MACEF, ConstructionHouse, Paddockhall Road, Haywards Heath RH l6 l HE)

Refined Bitumen Association technical information sheets(from RBA, Construction House, Paddockhall Road,Haywards Heath RHl6 IHE)

IP Standards for the testing of petroleum and its products-Institute of Petroleum, London (annual publication)

Institute of Petroleum-Bitumen safety code, June 1979-Institute of Petroleum, London

Standard methods for testing tar and its products-BritishCarbonisation Research Association, Chesterfield,Derbyshire

Asphalt Institute (USA)-various publications availabledirect from Asphalt Institute tsuilding, College Park,Maryland 20740, USA

BACMI publicationsCurrent list available on request (from the address given

on the title page of this publication). (Thesepublications include pamphlets giving guidance on theconstruction and surfacing of parking areas, housingestate roads and footways.)