Design Guide for Road Surface Dressing by j c Nicholls (4th Ed)

8/4/2019 Design Guide for Road Surface Dressing by j c Nicholls (4th Ed)

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'.. . by J CNichollswith the advice of a panel representing theIndustry and the clients under the

Chairmanship of D M Colwill,Research Fellow, TRL- :...

TRL ROAD NOTE 39(Fourth Edition)


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The Transport Research Laboratory is the largest and most comprehensive centre for the study of roadtransport in the United Kingdom. For more than 60 years it has provided information that has helpedframe transport policy, set standards and save liv~s.TRL provides research-based technical help which enables its Government Customers to set standardsfor highway and vehicle design, formulate policies on road safety, transport and the environment, andencourage good traffic engineering practice.As a national research laboratory TRL has developed close working links with many other internationaltransport centres. :.,It also sells its services to other customers in the UK and overseas, providing fundamental and appliedresearch, working as a contractor, consultant or providing facilities and staff. TRL's customers includelocal and regional authorities, major civil engineering contractors, transport consultants, industry, foreigngovernments and international aid agencies.TRL employs around 300 technical specialists - among them mathematicians, physicists, psychologists,engineers, geologists, computer experts, statisticians - most of whom are based at Crowthorne, Berkshire.Facilities include a state of the art driving simulator, a new indoor impact test facility, a 3.8km te t track,a separateself-contained road network, a structures hall an indoor facility that can dynamicalty testroads and advanced computer programs which are used to develop sophisticated traffic control systems,TRL also has a facility in Scotland, based in Livingston, near Edinburgh, that looks after the specialneeds of road transport in Scotland.The laboratory's primary objective is to carry out commissioned research. investigations. studies and.tests to the highest levels of quality, reliability and impartiality. TRL carriesout its work in such a wayas to ensure that customers recei ve results that not only meet the project specification or requirement butare also geared to rapid and effective implementation. In doing this, TRL recognises the need of thecustomer to be able to generate maximum value from the investment it bas placed with the laboratory,TRL covers all major aspects of road transport. and.is able to offer a wide range of expertise ranging fromdetailed specialist analysis to complex. multi-disciplinary programmes and from basic research to advancedconsultancy.TRL with its breadth of expertise and facilities can provide customers with a research and consultancycapability matched to the complex problems arising acrsssthe whole transport field. Areas such assafety, congestion, environment and the-infrastructure require a . multi-disciplinary approach and TRL isideally structured to deliver effective solutions.TRL prides itself on its record for delivering projects that meetcustomers quality, delivery and costtargets. The laboratory has, however, instigated a programme of continuous improvement-and continuallyreviews customers satisfaction to ensure that its performance stays in line with the increasing expectationsof its customers.

Transport Research Foundation Group of Companies-Transport Research Foundation (a company limited by guarantee) trading as Transport Research Laboratory.Registered in England, Number 3011746. TRL L_imited. Registered in England, Number 3142272.Registered Office: Old Wokingham Road. Crowthorne, Berkshire . RG45 6AU

TRL operates a quality management system which is certified as complying with BS EN 900[.


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TRANSPORT RESEARCH LABORATORY

TRL ROAD NOTE 39(Fourth Edition)

S M E C Lib,ra.tyDhaka Rtf-HB ook S I. No .;

DESIGN GUIDE FOR ROAD SURFACE DRESSING

This report describes work commissioned by the Road Engineering and EnvironmentalDi.vision oftbe Highways Agency under E151CIHR, R&D Advisory Service.This design guide was prepared by J C Nicbollswith the advice of a panel representingthe Industry and the Clients under the Chairmanship of D M Colwill, Research Fellow,Civil Engineering Resource Centre, TRL

Copyright Transport Research Laboratory 1996. All rights reserved.

Transport Researcb LaboratoryOld Wokingham RoadCrowtbome, Berkshire, RG45 6AU

Highways AgencySt Christopher House

Southwark Street, London SEI OTE

Transport Research Foundation Group o r CompaniesTransport Research Foundation (a company limited by guarantee) trading as Transport Research Laboratory. Registered in England, Number .30U.746.TRL LiIIlited. Registered in England, Number 3142272. Registered Offices: Old Wokingham Road. Crowthome,Berkshire, RG45 6AU.


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The views expressed in this design guide are the consensus of the revision panel andnot necessarily those of the organisations represented or the Transport ResearchLaboratory.

, I

The information contained herein is the property of the Transport Research Laboratory.This report has been produced by the Transport Research Laboratory, commissionedby the Highways Agency. Whilst every effort has been made to ensure that the matterpresented in this :report is relevant. accurate and up-to-date at the time of publication,the Transport Research Laboratory cannot accept any liability for any error or omission.

First Published 1996ISSN 0-9521860-6-3

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CONTENTSPage Page

Executive Summary 1 7.3 Rate of Spread of ModifiedBituminous Binder 30.Part I - INTRODUCTION 3 7.4 Rate of Spread of Cured-Resin Binder 301. FOREWORD 3 Part ill-APPLICATION OF THE DESIGN 311.1 Scope 3 8 . SPECIFICATION 311.2 Health and Safety 3 8.1 Recipe Specification with

Design by Client 31.3 Revision Panel 32. CONCEPTS 4 8.2 Recipe Specification withDesign by Contractor 312.1 Reasons for Surface Dressing 4 8.3 End-Product Specification 312.2 Types of Dressing 5 8.4 Records 322.3 General Principles 5 9. APPLICATION ON SITE 32Part n -DESIGN 8 9.1 Correction of Existing Site Defects 323. DESIGN METHODOLOGY 8 9.2 Plan of Work 323.1 Basic Approach 8 9.3 Traffic Control 323.2 Computer Design Program 10 9.4 Application, Aftercare and Remedials 324. iNPUT PARAMETERS 10

10.. ACKNOWLEDGEMENTS 32.1 Subdivision of Parameters 104.2 Parameters for Selecting the Type of 11. REFERENCES 34Dressing and Determining the Stage 1 Part IV - APPENDICES 36inder-Spread Category 104.3 Parameters for Selection of Materials 15 APPENDIX A: WORKED EXAMPLES OF

DESIGN METHOD 36.4 Parameters for Determining the Stage 2Binder-Spread Category 15 APPENDIXB: THE MEASUREMENT OF4.5 Parameters for Determining the ROAD SURFACE HARDNESS 44 IStage 3 Adjustment Factors 18 APPENDIX C: THE IMMERSION TRAY TEST5. SELECTION OF TYPE OF DRESSING 19 FOR DETERMINING THE CONCENTRATION5.1 Sections within a Site 19 OF ADHESION AGENT REQUIRED FORCUT-BACK BINDERS 455.2 Type of Dressing for a Section 19

5.3 Rationalisation of Dressing APPENDIX D: OTHER APPLICATIONS FORTypes for a Site 19 SURFACE DRESSING 46

6. CHIPPINGS 19 APPENDIX E: INDEX 486.1 Type of Clippings 19 APPENDIX F: APPLICATION FORMS FOR6.2 Size of Chippings 22 COMPUTER PROGRAM 516.3 Rate of Spread of Chippings 24 Proforma for Recording Designs 557. BINDER 257.1 Binder Types 257.2 Rate of Spread of Unmodified

Bituminous Binder 26


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EXECUTIVE SUMMARY

Road Note 39 is a guide for the design of surface dressingfor roads throughout the United Kingdom. The documentisdivided into four parts for ease of use, each part having adifferent purpose. The parts and their purposes are asfollows:

Part I - Introduction: General. information on thescope of the document, the reasons forsurface dressing, the types of dressingand surface dressing operations; the in-formation is provided primarily for gen.eral guidance.

Part II - Design: Details of the parameters usedand the method employed to design. asuitable surface dressing for specific CQn-ditions. Options, figures and tables nec-essary for the design process are high-lighted by a shaded background to aUowfor easy identification. Within thismethod. the design for the rate at: spreadof the binder is divided into three stages.dictated by the availability of informa-tion. These stages are: Stage l - Basic design based on infor-mation about the site;

Stage 2 - Adjustment when propertiesof the component materials becomeknown; and

Stage 3 - Local adjustments along thesite with changes in conditions.

All three stages need to be completed. tooptimise the design and hence maximisethe probability of achieving a . successfulsurface dressing.

Part TIl - Application of the design: Explanationof the types of specification that can beprepared with the designs together withbrief details on the application of thedesigns to the site; the information isprovided as general guidance.

Part IV - Appendices: Examples of the design proc-ess, supporting methods of test for thedesignmethod, information on some lesscommon situations and details of how toobtain the computer program and UserGuide developed to follow the proce-dures given in the document a re vari-ously set out.

Road Note 39 is a design guide; it is NOT prepared as, norshould it be used as, a specification. The results of thedesignprooess can be used toprepare specifications, but theadvice in this Design Guide has not been drafted in a formthat can be used directly as a specification, However, it isexpected tbat the advice contained will be indispensable tothose drawing up specifications,

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DESIGN GUIDE FOR ROAD SURFACE DRESSINGconsulting the code of practice for surface dressing pub-lished by the Road Surface Dressing Association (RSDA,1995a).

PART I - INTRODUCTION1. FOREWORD

Read Note 39 is a design guide, it is NOT prepared as,n.or should it be used as, a specificati.on. The results .ofthe design process can be used to prepare specificati.ons,but the advice inthis Design Guide has not been draftedina form that can be used directly as a specificati.on.However. it is expected that the advice contained wiI1beindiSpensable to those drawing up specificati.ons.

1.1 SCOPESurface dressing can be used successfully on aU types ofroads, from the country lane that carries only an occasionalvehicle to trunk roads and motorways carrying thousandsof vehicles a day. Itprovides a cost effective and simpleform of maintenance (Carswell, 1994). Unfortunately. theattention paid to design, control, supervision and aftercareis frequently less with surface dressing than with moreexpensive forms of construction. Lack of attention to detailshortens the useful life ofasurface dressing; the benefits ofadequate-control. particularly in the period when traffic isfirst allowed 00the new dressing. cannot be emphasised toostrongly.

The methodology for this Design Guide (Chapter 3) basbeen developed from experience with the traffic intensities,construction practices and weather conditions found in theUK.Therefore, the design method may not be appropriatefor use in other situations, particularly ifthey are markedlydifferent from those found in the UK.1.2 HEALTHANDSAFETYFor many years, surface dressing practice bas been carried

out according to Road Note 39. which is now inits fourthedition. The principal differences from the third edition are:

Health and safety aspects are nat within the scope of thisdesign guide, but all parties involved. in surface dressingshould take fullaccount of the requirements of the Healthand Safety at Work, etc, Act 1974 (House of Commons,1974), the Control of Substances Hazardous to Health(COSHH) Regulations /988 (House of Commons, 1988),and the Construction (Design & . Management)Regulations1994 (House of Commons, 1994).

(a) A revised layout. with allowance being made forwhen data becomes available. separating the calcu-lation of the rate of spread of binder into threestages;

(b) A complete review of the binder-spread rates;1.3 REVISIONPANEL(c) The traffic categories have been simplified;This Design Guide was prepared by aPanel onder thechairmanship of a Research Fellow of the GivU Engineer-ing Resource Centre at the Transport Research Laboratory(TRL). The Panel was drawn from organisations represent-ing all sides of the industry, including customers, producersand material suppliers (Chapter 10). The organisationsrepresented on the Panel were:

(d) The design criteria and data are highlighted for easeof identification;

(e) An allowance for latitude in the definition of theroad hardness;

(f) 20mm nominal size chippings are excluded otherthan in exceptional. circumstances; Association of Metropolitan District Engineers

British Aggregates Construction Materials IndustriesCounty Surveyors' SocietyDepartment of Transport, Highways AgencyRefined Bitumen AssociationRoad Emulsion Association LimitedRoad Surface Dressing AssociationScottish Office Development DepartmentTransport Research Laboratory

(g) Tar is excluded, but not tar-bitumen blends; and(h) The, terminology bas been revised to accord with

that used in the harmonised European standards forsurface dressing being developed by the ComiteEuropeen de Normalisation (CEN).

Methods and recommendations are presented to provideguidance on the choice and design ofan appropriate surfacedressing. Managers and Supervisors of surface dressingprogrammes can obtain comprehensive guidance on thepreparation, application and aftercare of the works by

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2. CONCEPTS2.1 REASONSFOR SURFACE

DRESSING2.1.1 PurposesSurface dressing as a maintenance process has three pur-poses:(a) to provide both texture and skid-resistance to the

surface;(b) to seal the road surface against ingress of water; and(c) to arrest disintegration.

In addition, it can be used as a treatment:(d) to provide distinctive colour to certain road sur-

faces, principally in urban areas; and(e) to provide arnore uniform appearance for apatched

road.2.1.2 Texture and skid-resistanceResistance to skidding, which is of importance on all roads.is dependent on the texture of the surface, both micro- andmacro-texture. Micro-texture is the texture of the aggregateparticles with a peak to trough height of 0 to O.2mm whilemacro-texture is the overall texture of the road, created bythe disposition of the aggregate particles, with a peak totrough beight ofO.2mm to 3rnrn. Micro- and macro-textureare illustrated in Figure 1.Micro-texture cannot be measured directly, while macro-texture can be measured by:

the sand patch test (which )gives the mean texture depth);the laser texture meter (whichgives the root mean square); or .other laser-based equipment monitoring road-condition. .

(both to BS598:Part 105(BSI, l~Oa

MICRO(texture of the stone)

Micro-texture is needed for skidding resistance at all speedsto provide grip, whilst macro-texture is more important athigher speeds in wet weather because it helps to removewater from the tyre/road interface. Surface dressing withchippings of appropriate resistance to polishing under theaction of traffic win provide both micro- and macro-texture.Concrete surfaces can be effectively re-textured by surfacedressing. The design options will depend upon the trafficintensity and the degree of skid-resistance required at thesite.2.1.3 SealingSurface dressing. with appropriate rates of spread ofbinder,can.be effective in.sealing binder-lean surfaces and hairlinecracks. Repairs to cracks or the road surface generallyshould be carried out before surface dressing takes place.2.1.4 Arresting disintegrationIfapplied to the surface of a road before major patching isrequired, surface dressing will protect the structure andextend the period before more major, and hence costly,repairs are required. If surface dressing is left until thesurface has begun to show signs of disintegration, theability of a dressing to arrest further disintegration rnay belimited.21..5 LimitationsSurface dressing will not restore the riding quality of adeformed road nor win it directly strengthen the madstructure. However, sealing the surface, and thereby reduc-ing theingress of water! will protect the structure fromdeterioration. Road surfaces where vehicles brake 'or tumsharply need to be considered separately as the designrequirements are more rigorous. J

MACRO(overal l texture of the road)

Flg.1 Micro- and Macro-Texture

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2 . 1 . 6 Performance parameters

,ITexture, skid-resistance and noise are three properties thatcould be considered toreflect aspects of the performance ofa surface dressing. Single surface dressing using largechippings can sometimes be considered noisy. However,the noise can be reduced by using types of dressing withmultiple layers of aggregate and by the use of smaller-sizeaggregate, while maintaining the requirement for surfacetexture. In seeking to optimise noise and texture, there iscurrently no method of accurately predicting from thedesign parameters the noise or the texture of surface dress-ings after embedment.2.2 TYPES OF DRESSINGThere are several types of surface dressing which varyaccording to the number of layers of chippings and binder.The fundamental types (excluding the high-friction sys-tems) are shown in Figure 2. Another factor to be consid-ered in selecting a system is the robustness of the binder, Inparticular the use of a modified or unmodified binder(Chapter 7).2 . 2 . 1 Single surface dressing

2.2.4 Inverted double surface dressingAn inverted double surface dressing (previously known aspad coat and single dressing) is a single dressing withstnallsize chippings which is applied to aroad which has unevensurface hardness, possibly due to extensive patching by theutilities, followed -later by a second single dressing withlarger size chippings. The first single dressing (the padcoat) produces a more uniform. surfacing whicb can besubsequently surface dressed, Inverted double dressingshave also been used on very hard or bard road surfacings,such as concrete, to reduce the effective hardness of thesurface, but a racked-in dressing is now the generallypreferred option.

The single surface dressing refers to the fundamental typeof a single film of binder followed by a single, layer ofchippings, The classification excludes cured-resin-binderdressings (Sub-Section 2.2.6). A single dressing has theleast number of operations, uses the least amount of mate-rial and is sufficiently robust for many situations. However,there is a limit to the stresses that this type of dressing willwithstand.2.2.2 Racked-in surface dressingIn a racked-in surface dressing, about 90 per cent of thechippings that would be used in a single dressing are laidbut with a thicker layer of binder. The gaps inthe matrix arethen filled by smaller chippings. These smaller chippingslock the larger chippings in position, producing a stablematrix. There should be a slight excess of the smallercbippings to ensure that each larger size chipping is locked-in by the adjacent smaller size drippings. Racked-in dress-ings are principally used where traffic is heavy and/or fast.2 . 2 . 3 Double surface dressing

2.2.5 Sandwich surface dressingA sandwich surface dressing is a dressing where a layer ofchippings is spread prior to a single surface dressing beingapplied. Sandwich dressings are principally used in situa-tions where the road surface condition is binder rich, andalso in hot weather on heavily trafficked single carriage-ways.2 . 2 . 6 High-friction systems

A double surface dressing has two layers of chippings andtwo applications of binder, the second being placed be-tween the layers of chippings, There are no gaps in the firstlayer of chippings, This t.ype of treatment is particularlysuitable for road surfaces which are binder lean, but it isalsoused as an alternative to a racked-in dressing. Generally,double dressings produce a marginally lower texture depththan racked-in dressings using the same size chippings;however, it is a quieter and more robust treatment,

For high-risk sites requiring high skid-resistance. such as atjunctions and adjacent to pedestrian crossings, the .PSV ofnatural aggregates is generally not sufficient and artificialaggregates, such as calcined bauxite, are often required ..High-friction systems employ either a . cured-resin binder ora premium .grade modified bituminous binder. High-frio-tion systems, particularly cured-resin-binder dressings, areexpensive compared with other types of surface dressingand, hence, they are only used in selected areas. High-friction bituminous-binder dressings are unsuitable for useon the most highly-stressed sites.2.3 GENERAL PRINCIP.LESMany operations a r e performed in surface dressing; themore important ones are listed inTable 1.The Table sets out.the operations in the order in which they are normallyperformed; they are not necessarily undertaken by the sameorganisation.Part 11of this Design Guide is primarily aimed at coveringitems 3 and 4 in Table 1. The guiding principles In theassessment of sites and the design of surface dressing arethat the type selected for a particular site should be suffi-ciently robust to retain the chippings, and th.at the size ofchipping should be selected to take account of embedmentby traffic. Part 1II gives information on where to findguidance on items 7 to 10 in Table L

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Fig.2.1 Single Surface D..... lng

Fig.2.2 Racked-in Surface Dr_sing

Fig.2.3 Double Surface Dreasing

Fig.2.4 Inverted Double Surface Dress:ing

Fig.2.5 Sandwich Surface Dressing

Flg.2 Schematic ReprC!Sentatlonof the Types of Surface Dressing Prior to Embedlment

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TABLEtSurface Dressing Operations

Number Operation Description1 IDENTIFY

2 REPAIR3 ASSESS

4 DESIGN5 MATERIALS

6 CONTRACTADMINISTRATION

7 METHOD OFWORKING

8 SITEPREPARATION

9 TRAFFIC CONTROL& EXECUTION

10 AFTERCARE

II RECORD

12 INSPECT

13 INVESTIGATE

Identify roads to be dressed. Advise Statutory Undertakers that anyworks they might foresee should be completed prior to the dressingbeing laidRepair potholes and damaged areas.Measure the hardness of the road surface and assess the number ofcommercial vehicles. Note areas of high stress - roundabouts,sharp bends. traffic lights, steep mils, altitude, etc.Using above data, select type of dressing, binder and chipping type and size.Decide who should be responsible for ordering chippings* and binderwith the appropriate properties.Contracts should be let wen in advance of the work to secure skilledcontractors and the best equipment,Select tenderers.Evaluate tenders. including discussions on binder types, aggregatesources methods. programmes of work and resources to be used.Agree method of working and traffic control between Client and Contractor.Where necessary, give advance warning to those likely to beinconvenienced by the work,Remove vegetation from the road edge, sweep .mask reflective studsand ironwork, note and record matkings.Implement warning signs and traffic control, sweep, apply the binderand chippings and-roll.Remove surplus chippings, control the speed and path of traffic untilthe dressing has stabilised, Replace stop, give way and other road markings.Dust the dressing during periods of hot weather.Keep a record of the materials used, including their application rates,and the weather conditions, including air and ground temperaturestogether with relative humidity at the time of dressing if an emulsionbinder is being used,Note any unusual occurrences during the work.Inspect. the work regularly during the early life and record any deficiencies.Progressively extend the period between inspections, but inspect afterthe first frost and after periods of sub-zero temperatures.Where defects have occurred, assess the reason and, in extreme cases,consider the need for remedial works.

* chippings need to be ordered well in advance.

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PART II - DESIGN3. DESIGN METHODOLOGY3.1 BASIC APPROACHThe design methodology follows the steps inTable 2whichprovides an index to the sections within this document

TABLE 2

concerning the various steps an engineer should take whendesigning a surface dressing. Itforms a useful aide-memoire,but an engineer will need to use judgement on the applica-bility of these steps to a specific scheme, particularly forroads carrying heavy traffic at high speeds.The designs should be properly documented, preferably 011a proforma specifically prepared for the purpose, an.exam-pIe of a suitable form being shown in Figure 3 (Figure 3

Operation TaskOperations in Designing Surface Dressing-

Selection SectionectionConceptSite

Decide to surface dressType selection and Stage 1 binderspread category parameters

Material selection

Site Divide. up siteSelect type of dressing

Rationalise types of dressingMaterial Select type of drippingsselection

Select size of chippingsSelect type of binder

Rate ofspread ofbinder

Unmodified bituminous bindersSTAGE I binder spread. categorySTAGE 2 binder spread category(from aggregate properties)STAGE 3 adjustment factors(from site conditions)

Target rate of spread of binder 7.2.3Modified bituminous binders 7.3Cured-resin binders 7.4

8

2.14 . 2 Latitude

AltitudeRoad hardnessTraffic categoryTotal traffic and traffic speedGeneral surface conditionHighway layoutSkid-resistance requirementsSeason and weather conditions

4.2.14.2.24.2.34 . 2 . 44 . 2 . 54.2.64.2,74.3 .14.3,2

4.3

5 " . 15 . 2 Single dressing

Racked-in dressingDouble dressingInverted double dressingSandwich dressingHigh-friction dressing

2.2,12 . 2 . 22.2.32.2A2 . 2 . 52.2.65 . 3

6.1

6.2

Uncoatoochipping$Light1y.~eoated chippingsHeated cllippingsArtificial aggregatechippings6 mm, 10mm, 14 rom,20mm or combinationsUnmodified bitumen emulsion, 7.1.1aut-back biramen or tar-bitumen blendMOdified binder 7.1.2

6.1.26.1.36.1.46.1.8

7. 1

Cured resin 7.1.4

7.2.17.2.2 ChlppjIig shape

Type of chippingSurface conditionGradientShadeLocal traffic

4A.l4 . 4 . 24 . 5 . 14 . 5 . 24.5.34.5.4

7.2.3


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Design of Road Surface Dressingsto Road Note 39 (Fourth Edition)Road number:

Len~h:

Hard SoftGeneral surface condition: * I Very binder rich Normal Very binder lean~========~====~==~======~.---~----~Radius of curvature: * over 250 mOverall gradient: * Expected Month on Site:Junction or crossing: *

Other:Racked-In High-Friction

Chipping size: * 20mm

Bitumen Emulsion Int. Grade MOdified Rremium Grade Modified1-70Cut-Back Bitumen

10 % to. 25 % c More than 25 %40 % to 60 % Less than 40 %

Int. Grade Modified Premium Grade ModifiedTar-Bitumen Stage 1 binder spread category:

Aggregate type: *Flakiness index: (10, 14 & 20 nun) *Retained on 5 rom sieve: (6 rom) * a Less than 10 %More than 60 %Stage 2 binder spread category: First layer - 1 - ' Second. layer * I

LocationSurfacecondition

Stage 3Factors Rate of spreadof binderradient

Vm.2

Designer: Initials: Date:I'- I__ I__ ~* Highlight or delete as appropriate Shaded box indicates data which Client should provide when seeking tenders

fig.3 Suitable Proforma for Recording' Designs 9


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may be copied for use in design, if required). Two workedexamples of the design method are given in Appendix A.The design method described in this Design Guide can beused for the appropriate type of surface dressing andconstituent materials for most situations. Ifthere are anyproblems. advice may be sought from the Civil Engineer-ing Resource Centre of the Transport Research Laboratory.Further, TRL is keen to hear about the successes and thefailures with surface dressing. particularly with the lesswidely used types; this information will highlight the needfor and assist in the revision of this document.3.2 COMPUTER DESIGN

PROGRAMFor those who expect to be regular users of the methodol-ogy set out in this Design Guide, a user-friendly computerprogram, based on this design approach, has been writtenand is available for purchase from TRL. The program usesinputs for the site and lane(s) and suggests a suitable typeof dressing for each lane. Default values for parameters areavailable. The types of surface dressing from which thechoice is made are single dressing. racked-in dressing,double dressing, inverted double dressing, sandwich dress-ing (all with modified or unmodified binders) and resin-based high-friction dressing. Ifrequired, the user can thenrationalise these choices within prescribed limits in orderto, say. minimise changes between lanes on a particularsite.The alternative materials available as binder are bitumenemulsion, cut-back bitumen (both modified or unmodi-fied). tar-bitumen blends and resins. whilst the aggregatescan be crushed rock, slag, gravel (all either lightly-coatedor uncoated and either heated or net) and calcined bauxite.Having completed a design. the user can review the inputdata, making any changes required, and obtain hard copiesof the design, which may be used in Quality Assurance.The computer program is intended to supplement and notreplace this Design Guide; the program carries out thedesign but the explanations and general advice on surfacedressing are contained inthis document. Details of how toobtain copies of the computer program are given in Appen-dixF.

4. INPUT PARAMETERS4.1 SUBDIVISION OF

PARAMETERSThe values for different parameters are needed for differentaspects of the design. These aspects are:

10

Selection of type of surface dressing;Selection of type of component materials (binderand ehippings);Calculation of the rate of spread of binder. which isfurther sub-divided (Section 7.2) into:

o Stagel Basic design based on informationabout the site;

o Stage 2 Adjustment when properties of thecomponent materials become known;and

o Stage 3 Local adjustments along the site withchanges in conditions.

Some parameters are needed for more than one aspect oftbedesign, in particular all those needed for determining theStage 1binder-spread category are also used in the selectionof the type of surface dressing. Therefore, the parametershave been grouped for discussion into:

Parameters for selecting the type of dressing anddetermining the Stage 1binder-spread category;Parameters for selecting the component materials;

Parameters for determining the Stage 2 binder-spread category;

Parameters for determining the Stage 3 adjustmentfactors.

4.2 PARAMETERS FORSELECTING THE TYPE OFDRESSING ANDDETERMINING THE STAGE 1BINDER-SPREAD CATEGORY

4.2.1 LatitudeThe, climate in the UK is not uniform,wLth the averagetemperature lower in the north. At lower temperatures.there is less opportunity for the chippings to be embeddedin the substrate and, hence. more binder is required to holdthe chippings during the winter. In this Design Guide, threecategories of l;ttitude are applied to allow for the ambienttemperature needed fo.rsimilar embedment characteristicsinbothselectingthe road hardness category (Hgure 5, Sub-Section 4.2.3) and designating the surface dressing season(Table 6, Sub-Section 4.2.3) The latitude categories (Fig-ure 4) are:

South: Sooth of a line approximately throughLiverpool, Manchester and Sheffield;

Central: Between a line approximately throughLiverpool, Manchester and Sheffield and onethrough Glasgow and Edinburgh; and


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Fig.4 Latitude Categories in the British Isles

North: North of a line approximately throughGlasgow and Edinburgh,

Ireland and Wales tend to have lower mean July tempera-tures than the equivalent. parts of England and Scotland atthe same latitude. Therefore, Wales is allocated to theCentral category and Northern Ireland to the North cat-egory.The categories are intended to be a general guide andconsideration should be given to the local climate. Forexample, variations in average temperatures can be foundin deep valley floors and around coastlines. In particular.the moderating influence of the Gulf Stream should beconsidered where appropriate. Most Highway Authoritieshave road sensor information which will enable them toidentify road temperatures at any time during the year; suchdata can be used to check whether the location is typical ofits geographic position.4.2.2 AltitudeThe altitude of the site influences the properties of thebinder required to retain the chippings because of the

change in temperature, affecting the degree of embedmentoccurring. Also, the deliberate use of larger chippings andmore binder at high altitude may be useful on minor roadsto assist traction in winter. Altitude is separated into twocategories to give-an allowance forthereduction in ambienttemperature with height when determining the road hard-ness category (Figure 5, Sub-Section 4.2.3) The altitudecategories are:

over 200m altitude above sea level; and 200m or less altitude above sea level.

4.2.3 Road hardnessRoad hardness is a property intended to represent theresistance ofan existing road surface ina particular locationto the embedment of drippings. As such, it is a propertyinfluenced by the local climate as well as the surfacingmaterial because the hardness of all asphalt surfacingmaterials are temperature dependent. The property is afundamental component of this design methodelogy, sothat an essential step at any site is to measure the hardness

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of the existing road surface in the specific location in orderto. select the correct size of chipping. The site should bedivided into areas with visual differences in the surfacing, ifappropriate, and a representative length ofthe nearside wheel-track in each lane of each area should be selected; no.lengthshould represent more than 1 lane kilometre. Measurementsare made on each representative length using the methoddescribed in Appendix B. The surface temperature, whichshould preferably be between 15 and 35C, is recorded and

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10 15 20 25 30 35Road surface temperature fC )

the hardness.category determined from the meanofa se t oftenpenetrationreadings using the appropriate graph inFigure 5;the appropriate latitude and altitude categories (Sub-Sec-tions 4.2 ..1 and 4.2.2) should be selected in making thisdecision. Interpolation between. graphs for areas close to achange in category may be necessary.Ideally, road hardness should be measured in the seasonprior to that in which the surface dressing is to. be carried

222018

E 16.. 14c:0. . . 12ic: 10~Z 8eIl. 6

42.0

222018

E 16.. 14c:oiii 1.2J::GIK 10~ 8Il. 6

///IL// //

/V //Very s~ //' Soft/V ///V / No~/' V> :./ .-: - - - - - - - - - - - - - - ~ Hard.J._---'I----- 1---- veryj hard II10

(a) South category, Ie.. than 200m attitude

o

//L /V //I Very s o t V V;-I / / // S o y v/-: V. -:" NormalV VV ,/. . . ,./ /' --/.- V:->~ >= - - - Hard_f- - ~_ - hard

15 20 25 30 35 40

10 15 20 35

Road surface temperature ( 'lC){b) South category, more than 200m altitude

Central category, less than 200m altitude

4

1I LiI / j /II Very soft / v// // V I /Soft V/ -: /'-: -:NormalV,.//" // Y ",,/~/:- - - - - >,....-/ IH ~,_.- l----- -Very hardI- --

25 30Road surface temperature C O e )

(c) North category, less than 200m altitudeCentral category, more than 200m altitude

2o

10 15 2025 30 35Road siJrface temperature C O G ) 40

(d) North category, more tha.n 200m attitude

Fig.S Hardness categories from depth of penetration and road surface temperature for different combinationsof latitude' and altitude categories

12


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TABLE 3Traffic Categories

COMMERCIAL VEHICLES 0 21 1m Z51 5 01 1,251 2,001 2.501 ,IPER LANE PER DAY to to to to to to to to Over20 1 0 0 250 ~oo 1,250 2,000 2,500 3.250 3,250

Free flowing traffic H G F E D C B B ARestricted. flow of traffic G F E D C B A A ANEW ROADS & STREET WORKS 4t 3t 3 2 1 S S ,ACT ROAD TYPE * 4

out. An alternative to in-situ assessment is the use of150mm diameter cores that have been extracted from theroad for some other purpose; these can be tested for hard-ness in the laboratory.Concrete road surfaces present extreme resistance to em-bedment of chippings under the action of traffic and areclassified as very hard, Conversely ,patched areas of as-phalt Isurfacings are usually softer than the rest of the roadand may need to be considered separately from the rest ofthe site.4 . 2 . 4 Traffic categories

other major roads, the traffic flowsshould be known andcan be used directly. As an alternative for other roads, theNew Roads and Street Works Act 1991 (House of Com-mons, ]991)3 road type can be used with an estimate ofwhether it is in th e upper or lower section of the classifica-tion. The conversion has been calculated on a 2 per- centgrowth rate. In the event of noinformation being available.a manual count can be made ..The count should be over atleast an hour and be at an. appropriate time for the localtraffic pattern; the conversion to daily rate will depend onth e time when the count is made and the local traffic pattern.

A major factor in selecting a type of surface dressing is theanticipated volume of traffic that each lane of the road isrequired to carry. Because commercial vehicles cause mostof the embedment of chippings, the principal measure oftraffic for design purposes is the number of commercialvehicles per day2currentJy travelling in the lane underconsideration.Eight traffic categories (A to H) are used in the designmethod. The relevant categoryfor a lane can be obtained fora particular site from Table 3. For dual carriageways and

On two-way roads with one lane in each direction . thetraffic on each lane is assumed to be half the sum in bothdirections. On single-track roads, the total traffic must beused. Narrow roads with the two directions sharing acommon offside wheel-track sbouldbe designed with balfthe total traffic for the nearside wheel-tracks and with allthe traffic for the central one.Lane 1 (i.e. the nearside or left-band lane) of both dualcarriagewayandtaree-Iane roads usually carries the major-ity of commercial vehicles, Conversely, in urban streets,parked vehicles may force moving traffic towards thecrown of the road Therefore, it is necessary to consider

* Converted from msa to cv!1ld based ana growth .rate of 2 per cent.t Division between type 3 and type 4 roads at 125 cv!1ld, not 100 cv/l/d,

The term "asphalt" is taken to be any mixture of aggregate and either bitumen. tar or a combination thereof (each either modified orunmodified) and therefore includes mixture types such as rolled asphalt, macadam. mastic asphalt, stone mastic asphalt and thinsurfacing materials.

2 A commercial vehicle is defined as avehicle of unladen weight greater than 1.5 tonnes(Mg) and the day is nominally a 24 hour period,but usually a manual count would be made between 06.00 and 22.00 hours and the value obtained multiplied by l.06 to estimate the24-hour figure.

3 The New Roads and Street Works Act 1991 (House of Commons, 1991) mandated Highway Authorities to categorise their roads bytraffic flows in order that they can inform the Statutory Undertakers of the standard to which repairs toexcavation have to be carriedout. Whilst the categories are in terms of million standard axles (msa), the Highway Authorit ies will have derived the data fromknowledge of the traffic flow, including commercial vehicles. Therefore,. the Highway Authorities should have access to arepresentative traffic flow (in commercial vehicles per day) which can be a guide to the designer.

13


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different specifications for each lane of multi-lane roads.The proportion of commercial traffic. using lane 1 of amulti-lane carriageway can be calculated according toFigure 3.3 ofHD 24 (DMRB 7.2.1). The remaining com-mercial traffic on a 3 lane motorway should be using lane2 as they are excluded from lane 3. For surface dressingpurposes, itis usually adequate to estimate that two-thirdsof the commercial vehicles are in Lane Iand one-third inLane 2 of a three-lane dual carriageway.4.2.5 Total traffic and traffic speedThe number of commercial vehicles is the most significantaspect of traffic but, where the proportion of commercialvehicles is low compared with the total volume, the totaltraffic needs to be taken into consideration. Examples arethe offside lane of a major motorway, in which the onlycommercial vehicles are usually coaches and vans, butwhich can still be severely stressed. and some urban com-muter rosses, The total.traffic is-measured as the number ofvehicles currently travelling in the lane per day. The fol-lowing categories are used inthe selection of the type ofsurface dressing (Figure 6. Section 5.2):

more than 10,000 vebiclesllane/day; 5,000 to 10.000 vehic1esllane/day; and less than 5 ,000 vehic1es1lane per day.

very binder rich; binder rich, normal or binder lean; very binder lean; and variable road hardness,

Also, roads. orlanes ofroads, on which the traffic ismovingat relatively high speeds will increase the possibility ofdamage from loose chippings and this increased risk needsto be taken into account in the selection of the type ofsurface dressing (Figure 6, Section 5.2). For this purpose,the speed of the traffic. which is taken as the 85 percentilespeed (that is the speed below which 85 per cent of thetraffic travel) is divided into:

greater than lOOkmlh (60 mileslh); or less than or equal to 100 km1h .

Allocation to a particular category is a subjective assess-ment which should be carried out by anexperienced person.4.2.7 Highway layout

4.2.6 General surface conditionThe overall condition of the existing surfacing is importantin determining the most appropriate type of surface dress-ing in order t o minimise the potential problems that canarise with certain surface conditions. The Design Guideuses surface condition categories for the purposes of select-ing the type of surface dressing (Figure 6. Section 5.2),which are:

14

The gradient, the tightness of bends and the extent of anysuperelevation will affect the stresses imposed by vehicleson a road surfacing ..Similarly. there are additi onal stressesdue to sharp deceleration a nd turning at junctions andcrossings. Therefore, when considering the appropriatetype. of surface dressing. the inclusion of some of thesefactors needs to be considered to ensure that a sufficientlyrobust surface dressing is designed and constructed Thefollowing categories are used in the selection of th e trw ofsurface dressing (Figure 6, Section 5.2): Gradient - up to ten per cent gradient;

and over ten per cent (1 in 10)gradient

Radius ofcurvarure- under 100mradius; 100 - 250mradius; and over 250m radius ..

Junction or c ro ssin g - approach; and non-approach.

The presence and extent of lengths for which differentcategories apply should be taken into account in decidingwhether to divide a site for thepurpcses of design (Section5.1).The gradient for representative lengths of a Site can beobtained as described in Sub-Section 4.5.2. The radius canbeobtained from surveys by the High-speed Road Monitor(FIRM) or on site using two tapes. Ifa 30m tape is stretchedwith both ends on the edge of the kerb. the radius can becategorised by the distance that the 15m mark is from thekerb as:under 100 m radius when the distance. is , over 1.13 m;100 m to 250mIaaius when the distance is between0.45 and 1.13 m;over 250 m radius when the distance is less than 0.45 m .


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4.3 PARAMETERSFORSELECTION OF MATE.RIALS

4.3.1 Skid-resistance requirementsOne reason for surface dressing a road may be inadequateskid-resistance in thecontext of the Highway Authority'sskidding policy. Skid-resistance is effected by both themacro-texture of the road surface and the micro-texture ofthe aggregate (Sub-Section 2.1.2). At present there is nodefiniti ve method to design for the macro-texture that willexist after embedment has taken place (Sub-Section 2.1.6)whilst. for motorways and trunkroads, the requirements formicro-texture are laid down in HD28/94 (DMRB7.3.1).Investigatory levels are given for various classifications ofsite in terms of the skid-resistance, as defined by the MeanSummer SCRIM Coefficient (MSSC), at either 50 or20kmlh, with the SCRIM values being measured by aSideway-force Coefficient Routine Investigation Machine.The Polished Stone Value (PSV) of the aggregate in theroad surface and the commercial vehicle traffic have beenfound to correlate with the skid-resistance of the road. Therelationships between skid-resistance. traffic and the re-quired PSV of the aggregate have been established and HD28194 (DMRB7 .3.1) sets out the required minimum PSV ofdrippings for new construction and maintenance works ontrunk:mads and motorways. The requirements for differentareas of a site can be found using the table in HD 28/94,reproduced as Table 4 (NOTE: Designers should ensurethat they are using the current version of this Table).However. although the skidding requirements may varyalong a . site, the use of different aggregates of varying PSVon the main lengths of a site is usually impractical. PSVlevels in excess of 70 are usually achieved with calcinedbauxite in cured-resin-binder dressings at relatively highcost. This type of dressing is the exception in that it.is onlylaid in localised areas where required.4.3.2 Seasons and weather conditionsSurface dressing is a seasonal activity. This is primarilybecause the long-term stability ofthe treatment is depend-ent upon the chippings becoming embedded inthe substratebefore the onset of cold weather. If embedment does notoccur, the chippings are liable to be removed by traffic. Thegeneral principle is that larger chipping sizes should beused as early in the season as possible because they aremore dependent on embedment for stability. Use of modi-fied binders may reduce the susceptibility of a surfacedressing to early failures. Ideally. surface dressing shouldonly be carried out when the pavement temperature isbetween those given in Table 5 for the particular bindertype.Based on Table 5, the seasons that apply in the UK for thedifferent binders and different traffic categories are givenin Table 6.

The seasons are only a guide because the weather in anyyear may differ from the mean so that the period whensurface dressing can be expected to be carried out success-fully may be reduced or extended to take account of: high humidity conditions where emulsion binders

are to be used; I Ilong-term weather forecasts;

the consequences that would result from a failure,usually determined by the class ofroad to bedressed;and

other local situations.Therefore, although Table 6 takes account of the effectwhich different ladnrdes have on the surface dressingseason, it should be understood that the season can also beaffected by other regional and climatic variations. In someyears, it ispractical. to extend the surface dressing season onroad categories D to .H and even for categories B and C.Table 6 should beregardedas a guide and.work undertakenon the basis of an end-product contract should not berestricted to these periods.Within these seasons. no binder is totally tolerant of ex-treme weather conditions. Emulsions win break. sl.owly incold or wet conditions or when humidity is high. Whenhumidity is over 80 per cent, emulsion break can be delayedand breaking agents may become necessary, Rain canaffect binder systems. principally the initial adhesion ofchippings, In very warm weather, cut-back binders canbecome too fluid; this problem may be ameuorated by thechoice of viscosity ofthe binder. Weatherferecasts shouldalways be obtained before carrying out. surface dressingoperations.It isdifficult to obtain good adhesion between cut-back:binders and uncoated chippings at low temperatures, par-ticularly when high viscosity binders are used. The use oflightly-coated chippings with cut-back bitumen enablesadhesion to be established (Sub-Sections 6.1.2 and 6.1.3)This isparticularly the case if the road surface temperatureat the time of dressing is likely to fall below 15C Lightly-coated chippings are advisable when a cut-back binder isbeing used on roads in traffic categories A to D inclusive.4.4 PARAMETERS FORDETERMINING THE STAGE 2

BINDER-SPREAD CATEGORY4.4.1 Chipping shapeThe shape of the aggregate is important in terms of theamount of binder needed to fill the interstices between thechippings, For the purposes of the design. the aggregateparameters may not be known until shortly before construe-

15


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TABLE 4Minimum PSV of Cbippings

Traffic (e,vll/d) at design lifeSite Site 0 101 251 501 751 1 1 0 0 1 1251 1501 1751 2001 2251 2501 2751Category Definition IO to to to [0 to [0 to to to to to to Over

100 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3250 3250I A Motorway

(main. line)B Dual carriageway

(all purpose) S5 57 60 65 68non-event sections

0 Dual carriageway(all purpose)minor junctions

1 1 C Single carriageway Inon-event sections I45 50 53 55 57 60 63 65 68

E Single carriagewayminor junctionsill F Approaches to and

across majorjunctions (all limbs)G1 Gradient 5% - 10%,

longer than SOm(Dual downhill; 50 55 57 60 63 65 68 over 70single uphill anddownhill)

HI Bend (oot; subject to40mph or lowerspeed limit)

Iradius 100 - 250mL Roundabout

IV G2 Gradient> 10%,longer than 50m(Dual downhill;single uphill anddownhill) 55 60 63 65 68 over 70

H2 Bend (not. subject to40mph or lowerspeed limit)radius


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*~,

------ - ~ - -

TABLESIdeal Pavement Temperature Range for Surface Dressing

Binder Grade Minimum Temperature *Uncoated CoatedChippings Chippings

Maximum TemperatureTraffic Categories Traffic ~gories I

AtoE FtoGBitumen EmulsionCut-Back Bitumen

Tar-bitumenblends

Kl-70 lOOCro -c s-c15C ro-c18e l3Crs-c s-c15C 13C

18C

35C 40C35C 40C35PC 40PC35PC 40PC3SOC 40PC35C 40PC35C 40C

50 sec100 sec200 sec100 sec200 sec300 sec

Seek advice from the supplierSeekadvic;e from the supplier

TABLE 6Surfa~ Dressing Season

applicable when the temperature is steady or-rising.a suitably increased value should be used ifthe temperature isfalling to ensure that the tempeFatUre does not fa U below the minimum.

Modified BindersCured Resin

Latitude TrafficCategory Category

(Figure 4,ISub-SectionI 4.2.1)

(Table 3,Sub-Section4.2.4)

I North, AB&CD&EF-H

Central AB&CD&EF-2H

South AB&eD&EF-H

Unmodified binders Modified biadecs(indicative only, refer to supplier)

Cut-backBitumen & tar-bitumen blends

BitumenBmulsien

Cut-backBitumen & tar-bitumen blends

May to mid-Aug. May to Aug.May to mid-Aug. mid-Apr. to m.i~-Sep.

mid-May- to mid-JulyMayto}uly

May tomid-Ang,May temia-Aug.

May to mid-Aug.May to Aug ...

May toAug.mid-Apr ..to niid-Sep.

May toJD11May to Jnly

May tp mid-Aug.May1~oAug.

Apr. to Aug.Apr. to mid-Sep,

Apr. tomid-Sep ..Apr.. to Sep.

May to mid-Aug.May to Aug ..Apr ..to Aug.

Apr. to mid-Sep,

BitumenEmulsion

mid-May to JulyMay to mid-Aug ..May to Aug ..

mid-Apr. to mid-Sep,May tomid-Aug.Mayto mid-Aug.May to Aug.

mid-Apr. to mid-Sep,May to mid-Aug.May to Aug.

Apr. to mid-Sep ..Apr. to Sep,


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tion and, therefore. adjustments to the rate of spread ofbinder for th e sbape are included in the determination of theStage 2 binder-spread category (Section 7.2.2). Forchippings of lOmm nominal size and above, the flakinessindex is used as an indication of shape whilst, for 6m mchippings, the proportion passing the 5mm sieve is usedand, for 3mm chippings, the middle category is assumedalways to-apply. The following three categories are used:

Category u: Flakiness index less than 10 percent (l Omm, 14mm and 20mmchippings) orMore than 60 per cent retained onSmm sieve (6mm chippings);Flakiness index IOto 25 per cent(lOmm, 14mm and 20tnmchippings) or40 to 60 per cent retained on 5mmsieve (6 mmchippings) oral13mm cbippings; andFlakiness index more than 25 percent (10mm, 14mmand 20tinnchippings) orLess than 40 per cent retalnedon 5mm sieve (6 mm chippings).

Category b:

Category c:

For specification purposes, BS63: PartZ (BSI, 1987b) setsout a maximum flakiness index for chippings, These cat-egories are included in order to determine more preciselythe binder demand for the shape of aggregate that is actuallybeing used.4.4.2 Type of chippingThe majority of chippings are of crushed rock or slag, andthe values for the Stagel binder-spread categories (Section7.2) are based on their use. However, gravel can be used onroads in traffic categories F. G and H and a correction isapplied in the determination of the Stage 2 binder-spread.category (Sub-Section 7.2.2).4.5 PARAMETERS FOR

DETERMUilNGTHESTAGE3ADJUSTMENT FACTORS4.5.1 Surface conditionLocal variations in the condition of the existing surfacingneed to be allowed for in the rate of spread of binder. Thevariations should be such that. for all conditions. thereshould be:

sufficient binder ispresent for the initial retention ofthe chippings prior to longer term embedment; butexcess binder is avoided which could fat up.

For the purposes of local corrections in Table 15 (Sub-Section 7.2.3), the five categories of surface condition are:

very binder rich; binder rich; normal; binder lean; and very binder lean.

As for the overall surface condition (Sub-Section 4.2.6),allocatian to a particular category is a subjective assess-ment which should be carried out by an experienced person.Allowance should be made for macadam substrates lessthan 1year old, which tend to have a relatively high binderdemand. The texture depth of the existing surface, deter-mined in accordance with. BSS98: Part 105 (BSI. 1990a)can be used to assess the extent to which an asphalt surfaceis open-textured, thus requiring more binder (Sub-Section6.2.1.3).4.5.2 GradientThe gradient of the road affects stresses imposed on itssurfacing.ln the Design Guide. three categories of gradientare used for the purposes of local corrections in Table 15(Sub-Section 7.2.3):

10 per cent (1 in 10) and over uphill gradient; less than 10 percent gradient; ~and l 10per cent and over downhill gradient.

10 addition, further categorisation into less than 5 per centand 5 to 10 per cent is required for selecting the polishedstone value of the chippings from Table 4 (Sub-Section4.3.1).The gradient can be obtained from surveys by the High-speed ROaQ Monitor (HRM) or on site using a 1 metrestliaitghtedge and measuring the distance between thebottom of the straightedge and the road when the straightedge is.tfeltl in. a level position. The distance expressed in .centimetres is equal to the gradient in percent.4.5.3 ShadeAreas of asphalt" road surface shaded by trees, or in theshadow ofbuildings, bridges or tunnels, tend to be coolerand thus more resistant to cropping embedment than areas

4 In this Report and its Appendices. the term "asphalt" is taken to be any mixture of aggregate andeither bitumen. L1 r or a.combinati.onthereof (each either modified or unmodified) and therefore includes mixture types such as rolled asphalt . macadam, mastic asphalt,stone mastic asphalt and thin surfacing materials.18


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open to the sun. To account for this effect. the rate of spreadof binder should be increased in shady areas by the amountshown in Table 15 (Sub-Section 7.2.3). This correction isnot appropriate for concrete surfaces.4.5.4 Local trafficSome areas of a road may be subject to significantly lesstraffic than the rest, but may not be sufficiently large towarrant a separate design. Such areas include hard shouldersand edge strips on dual carriageways (unless a contraflow isplanned) and sizeable areas with hatched lines to excludetraffic. All these areas are effectively untrafficked and the rateof application of binder should be increased in order tocompensate for the lack of embedment of chippings,

5. SELECTION OFTYPE OFDRESSING

5.1 SECTIONS WITHIN A SITEA site requiring surface dressing can include sections withdifferent values for one ormore of the parameters (Section4.2). The site can be divided into sections longitudinallyandlor transversely; the latter often occurs when dressingdual catriageways because different lanes will carry differ-ent numbers of commercial vehicles. Ifa she can be dividedinto sections. a separate design should be carried out foreach section.5.2 TYPE OFDRESSING FOR ASECTIONThe types of surface dressing available are described inSection 2.2. The choice of an appropriate type of surfacedressing depends on a number of factors; Figure 6 gives asimplified flow diagram or decision tree to aid selection,For those options with modified binders, tbe choice ofintermediate or premium grade modified binders will de-pend on the severity of the traffic stresses that leads to theneed to use a binder modifier.Whilst Figure 6 can be used to identify one type that issuitable for consideration, the type selected is not necessar-ily the only one that can be used in the circumstances.Further, there may be reasons, other than those included inthe decision tree. for using a different type. The typeindicated may be regarded as over- or under-design, inwhich case consideration should be given to a more or lessexpensive option, respectively. Possible reasons could in-clude when the road has a limited structural life, when thetraffic intensity is expected to change in the foreseeablefuture or when the road ha s a strategic importance forreasons other than traffic Bow. AU these considerationsshould be taken into account when choosing the mostappropriate type.

5.3 RATIONALISATION OFDRESSING TYP.ESFOR A SITEThe detailed design of the surface dressing should becarried out for each identified design section but it is notpractical to change designs too often on a site. Therefore.having selected the most appropriate type of surface dress-ing for each identified section, the variation in the typeshould be rationalised within the following constraints:(a) the changes should be more towards the.robust types

rather than in the opposite way;(b) the changes may be allowed fo r in the detailed

design for that section (e.g. the selection of aggre-gate sizes can be adjusted by one nominal size fromthat indicated in Section 6.2); and

(c) no change should be too extreme (e.g. acured-resin-binder dressing should not be changed to a singledressing with unmodified binder, or vice versal),

6. CHIPPINGS6.1 TYPEOFClllPPINGS6.1.1 GeneralAn chippings of 6mm nominal size and above shouldcomply with the general requirements for size, shape andstrength included in 8S63: Part 2 (BSI. 1987b)~ 3mmnominal size chippings for multi-layer types should com-ply with the requirements of BS63: Part 1 (BSL 1987a)together with a maximum limit onthe proportion by weightpassing the 75micronBS test sieve "of1per cent in order tolimit the dust and silt contents. Samples of chippings to beused should be tested for compliance as deliveries arereceived6.1.2 Uncoated chippingsThe cleanliness of uncoated chippings is important inachieving good adhesion to the binder film. In 8S63,cleanliness is specified by limiting the proportion passinga 75 micron sieve. A thin layer of dust on dry uncoatedchippings will interfere with adhesion, This problem ismore acute at low temperatures and with the larger sizes ofchippings. When using cut-back bitumens. adhesion maydevelop more slowly in damp conditions because thechippings need to dry out before adhesion can occur.Uncoated chippings will usually work well with emulsionbinders.6.13 Lightly -coated cbippingsLightly-coated chippings have a thin film of binder, prefer-ably 50 pen grade bitumen, applied at a coating plant; the

19


24/58

Tbe order of surface dressing systems is selected to ease the logic of the flow chart and is not intended to indicateany ''league table" of quality.

Where there are boxes witb several altemati ve criteria, the UNo" branches should be used ifnone of them are met whilethe "Yes" branches should be followed ifone or more of those criteria is met The sets of criteria are ammged. so that

the harshest co~ditions dictate the type to be used, thereby minimising the risk of failure.I STt ITRAFFIC CATEGORY A or BorTOTAL TRAFFIC MORE THAN 10,000 vehllld

or(TRAFFIC CATEGORY C and

APPROACH TO JUNCTION OR CROSSING) SANDWICH SURFACE+ No to all Yes to any DRESSING WITHquestions question _! Yes MODIFIED BINDE'RSURFACE CONDITION v. binder ri~ I I SURFACE CONDITION v. binder rich, tYes ,. No SANDWICH SURFACENo DRESSING WITHUNMODIFIED BINDER

RADIUS OF CURVATURE UNDER 100 m Yes to elther_ ..or HIGH-FRICTIONAPPROACH TO JUNCTION OR CROSSING question SURFACING ,J Noto.both Eitheruestions, RACKED-IN SURFACEYesRADIUS OF CURVATURE UNDER 100m DRESSING WITHMODIFIED BINDERi- No r No rRADIUS OF CURVATURE UNDER 250 m

or SURFACE CONDITION _ _ . . DOUBLE SURFACETOTAL TRAFFIC MORE THAN 5,000 vehilld Very Yes DRESSING WITHor binder lean MODIFIED BINDERTRAFFIC SPEED MORE THAN 100 kmhlorGRADIENT GREATER THAN 10 PER CENT DOUBLE SURFACE

Nota all Yes to any _ .. DRESSING WITH, , UNMODIFIED BINDERquestions question Ves - .:-~II 'I ROAD HARDNESS CATEGORY hard or very hard ~ No _ _ . . _ _ . . RACKED-IN SURFACE

Either DRESSING WITHYes UNMODIFIED BINDER

ROAD HARDNESS CATEGORY very-hard L . .No Yes INVERTED DOUBLEI VARIABLE ROAD HARDNESS I EXTENSIVE PATCHES I DRESSING WITHMODIFIED BINDER~ No' 1 1 'TRAFFIC CATEGORY C or D Yes to either SINGLE SURFACEor _ .. _ _ .

ROAD HARDNESS CATEGORY hard ~ DRESSING WITHquestion MODIFIED BINDER,. No to bothquestions INVE.RTED OOUBLE

VARIABLE ROAD HARDNESS IEXTENSIVE PATCHES I Yes_. . DRESSENG WITHUNMODIFIED BINDERNo,

SINGLE SURFACEDRESSING WITH

INMODIFlEDBlNDER. , rlane . rda ,-

~ I

vehNd - total vehicles pe pe .,20 Fig.6 Selection of Typeof Surface Dressing


25/58

-- - - -, - - -_ ~ -

chippings should comply with Sub-Section 6.1.1 prior tocoating. The binder film eliminates surface dust and helpsto ensure rapid adhesion to the surface dressing binder.Lightly-coated chippings are used to improve adhesionwith cut-back bitumens, particularly in the cooler condi-tions at the extremes of the season. However, they are oflittle advantage with.emulsions becausethe shielding effectof the-coating can delay the break of the emulsion. Further,the use of lightly-coated chippings present problems whenused in sandwich surface dressings.The binder film should be as thin as possible a nd need not becontinuoos,becauseafew'!pinholes"willnotaft'ect the perform-ance. Coatings which are too thick can result in chippingssticking together in wann weather and not flowing evenlythrough the chipping spreader or gritter. The amountofbinderrequired tocoat the chippings is largely a matter of experienceand will vary from one source of aggregate to another, higheradditions will be required for porous aggregates such as slag.The values given in Table 7 serve as a guide.

TABLE 7Guide to Binder Contents for Lightly-Coated ChippingsNominali sizeof chippings

(mm)Bitumen

(approximate percent by mass)

6101420

1.00.80.60.5

Care should be taken during mixing to ensure that thebinder film is not hardened sothat itcarbonises, This wouldbe apparent on inspection as a black dust.The coating on a lightly-coated chipping overcomes mostof the adhesion problems associated with uncoatedchippings. Filler dust incorporated in the binder film willnot interfere with adhesion. However, if an extensive layerof dust is present on the aggregate beneath the bindercoating, adhesion of the coating could be affected and thiscondition should be avoided ..Contamination of the coatedchippings with. dust afterproduction should also be avoided.as this can lead to poor adhesion to the surface dressingbinder. Care should be exercised in the siting of stockpilesto avoid contamination by wind-blown dust.6.1.4 Heated chippingsThe use of bot chippings may be specified under certaincircumstances. Uncoated or lightly -coated chippings shouldthen be applied at a. temperature within the range 60C to120C. Particular care should be taken to ensure that

additional dust is not generated during the beating process.Heated chippings can cause problems with emulsions,giving localised break of part of the emulsion around thechippings and delaying the break of the remainder.6.1.5 Aggregate selection for primary layerChippings for single dressings and for the primary layer inmultiple dressings should have the required minimumpolisbed stone valiue (PSV) (Sub-Section 4.3.1). However.the attainment of the relevant PSV limit does not imply thatthe aggregate bas suitable properties to withstand crushingand abrasion, the latter being a surrogate for aggregate wear(durability). Therefore, limits on the minimum aggregateabrasion value (AA V) andlorthe maximum ten per centfines value of the aggregate may be required for certainsites and traffic intensities.6.1.6 Aggregate selection for secondary layerThe secondary layer of smaller chippings used.in racked-inand double dressings pack round the chippings in theprimary layer, "locking" them in position. Strength is ofminor importance and there are advantages in baving arelatively weake-r aggregate. such as some types of slag.Because the smalleraggregate in a racked-in dressing hasless contact with the vehicle tyres, PSV is of less impor-tance; this is not the case with double dressings and thechippings need to have the appropriate PSV.6.1.7 Chippings for very bard surfaciogsOn very hard surfacings,the resistance to crushing is animportant property. There may be special circumstanceswhere ten per cent fines values higher than. those requiredby BS63: Part2 CBSI, 1987b) should be specified. Aggre-gates with an AAV not exceeding 10 should be specified foruse in single surface dressings on roads intraffic categoriesA and B, but it must be recognised that this degree ofabrasion resistance may not be attainable with aggregatesin the highest PSV classes and multiple dressings will berequired. Further advice is given in Guidance Note onSurface Dressing Aggregates (RSDA, 1995b).6.1.8 Artificial aggregateThere are several types of artificial aggregates includingblastfumaceand steel. slag and calcined baux.ite. Slags aregenerally used as alternatives to natural aggregates inconventional surface dressings, particularly for the second-ary layer in multiple dressings. Some artificial aggregateshave very high polished stone values required for high-friction dressings, of which calcined bauxite is the aggre-gate predominantly used. There ace several sources ofcalcined bauxite, principally Guyana (from where it isgenerally blue or grey in colour) and China (from where itis usually yellow or buff coloured), which do not all havethe same PSV properties.

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- ----- -------===~~-~~--~~-- -~

6 . 2 SIZE OF CHIPPINGS6.2.1 Singledressing6.2.1.1 GeneralThe recommended size of drippings to suit the amount oftraffic and the hardness of the substrate of the existing roadsurface is given in Figure 7; alternative sizes of chip pingsmaybe used provided they are given for those conditionsinTable 10. The sizes recommended are related to the mid-pointofeach traffic category: lighter traffic conditions maymake the next smaller size more appropriate. Dressingswith larger-size chippings should be carried out early in theseason in order to ensure adequate embedment before theonset of cold weather ..The size of the chippings may alsobe changed to assist in rationalising the different designsrequired at a site.20mm chippings are not included in Figure 7. but can beused as an alternative for certain conditions. However, theyshould be used with the utmost care and, in particular, noloose chippings should.remain on the surface when the roadis opened to unrestricted traffic because of the high risk ofwindscreen breakage.

~ Conditions not suitable

~ ::] sizechipping~ recommended 6mm

Increasing trafficvolume increasesembediment

Lane trafficcategory

6.2.1.2 Hard shoulders and out,side lanes ofmotorways

Hard shoulders of all motorw ays and outside lanes of three-and four-lane motorways nominally take no commercialtraffic unless contraflow or other temporary traffic condi-tions are in place. Therefore, they will be in traffic categoryHwith a 6mm recommended size of chipping irrespectiveof the surface hardness. However, when temporary traf-ficking is introduced, a 6mm chipping will be inadequate.Therefore. for hard shoulders and other similar situations,the recommended chipping size is increased to lOmm.6.2.1.3 Adjustment for use over open-textured

asphalt surfacesSome of the binder will penetrate into an open-texturedsurface and, unless allowance is made for this loss. insuf-ficient binder will be left on the surface to hold larger sizesof chippings. Accordingly, unless aninverted double dress-ing is used. the rate of spread of binder for any particularcombination of lane traffic category and road surfacehardness should be chosen as for an. impervious surface(Section 7.2) and the size of chipping should then. bereduced to one size smaller except. in the case of 6mmchippings.

Softer substrateincreases embecliment

J

Fig.7 Recommended Sizeof Chippings for Single Surface Dressings

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6.2.2 Racked-in dressing 6.2.2.2 Hard shoulders and outside laDes ofmotorw.ays6.2.2.1 General No recommended sizes are given for traffic category H. butfor hard shoulders of all motorways and outside lanes ofthree- and four-lane motorways which nominally carry nocommercial. traffic (Sub-Section 6.2.1.2), dressings withIOl3mm sizes are recommended.6.2.3 Doable dressingThe recommended chipping sizes are as for racked-indressings inFigure 8.6.2.4 .Inverted double dressing

The primary chippings are uncoated when emulsion bind-ers are used and generally coated when cut-back binders areused, as forsingle dressings. while the secondary chippingsare almost always uncoated.The recommended size of chippings to suit the traffic andthe hardness of the substrate is given inFigure 8; altemati vesizes of chippings may be used because of particular noiseor texture requirements. provided they are given for thoseconditions in Table 11. The sizes of chipping recom-mended are related to tbe mid-point of each traffic cat-egory; lighter traffic conditions may make the next smallersizes more appropriate. Dressings with 14/6mm sizechippings should be carried out. early in the season in orderto ensure adequate embedment before the onset of coldweather. The sizes of the cbippings may be changed toassist in rationalising the number of different designsrequired at a site.

~ Conditions not suitable

14mm&]mm .. size chippingsrecommended.1Omm&3mm

Aacked-in systemnot cost-effective

Increasing trafficvolume increasesembediment

Lane trafficcategory

The recommended dripping size for the first layer ofchippingsin an inverted double dressing is traditionalty6mm.The recommended chipping size for the second layeris as for single dressings (Sub-Sections 6.2.1) for thesurface hardness category after the first layer of 6mmchippings has been applied.

Softer substrat.eincreases embe(jrment

Fig.8 Recommended Sizes of Chippings for Racked-In and Double Surface DresSings

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6.2.5 Sandwich dressingThe recommended chipping sizes are as for racked-in anddouble dressings in Figure 8 (Sub-Section 6.2.2), chosen toreflect the extent of the "excess" binder on the surface.6.2.6 High-friction dressing6.2.6.1 Cured-resin.":binder dressingThe calcined bauxite, or other aggregate with suitably highP8V, in a cured-resin-binder dressing is usually in a sizerange with not more than 5 ' per cent being retained on a3.35mm test sieve and not more than 5 per cent passing a1.18mm test sieve.6.2.6.2 Bituminous-binder dressingCalcined bauxite can be obtained in 5 to 8mmand 8, tol .Omm sizes for use in high-friction premium grade modi-fied. bituminous-binder dressings; this is designed in ,ac-cordance with the relevant type of conventional surfacedressing ..6.3 RATEOF SPREADOFCHIPPINGS6.3.1 Single dressingThe quantitY of ebippings applied must be sufficient tocover thefilm of binder, The chipping-s .sbould be spread ata rate to achieve 100 to 10S~tcent shoulder-tb-shouldercoverage as determined by BSS:~8:Part 108 CBS!, 1990b")or pr ENI2272-1 CBS1,1996). The ql,lantityrequired willdepend upon the size, shape and relative density of thechippings selected, but guidance for estimating quantitiescan be obtained from Table 8.

TABLESTypical Range of Rate of Spread of Chippiags

for Single DressingsNominal Sizeof Chipping

Range of Rates of Spread(kg/m") (m2/tonne)

6mm10mm14mn120mm

7-89-1312_ 1615 - 20

143 - 100111-7783- 6266- 50

6.3.2 Racked-IndressingThe primary chippings should be spread to provide about90 per cent shoulder-to-shoulder coverageas measured by

24

B8598:. Part 108 (BSI, 1990b) orprEN12272-1 (B~I.1996). Ifother combinations of cbippings are usedin whicbthe secondary cbippings are large compared with the pri-mary (e.g, 10mm and 6mm),. the rate of spread pI primazychippings may need to be reduced in order to l~ve inter-stices for the secondarycbippings,The secondary chippings should be spread so that there isan excess. The amount necessary will depend on the ratioof the two sizes used, the location and the method ofspreading. The extent of theexcess is best left to theexperience of the contractor to obtain optimum results ..Atminor jnnetionsand other locations where traffic will tumacross the new dressing, it is advantageous to u se a greaterexcess, of smaller ehippings in order to reduce the possibil-ity of d;amage from turning traffic in the early life of (hedressing ..6.3.3 Double dressingThe rate of spread of the first layer should be such that itachieves atwiJ.t95 per cent shoulder-te-shoulder coverageas measured by BS598: Part 108 (BSI, 1990b) orprENI2272-1 CBSI, 1996), whilst that for the second layershould be 100 to 105 per cent. The quantity required willdepend upon the size . shape and relative density of thechippings selected, as for racked-in dressings (Sub-Section63..2).6.3.4 Inverted double dresslngBoth layers of chippings should be spread to achieve 95 toIQO:_P-er'Centhoulder -to-shoulder ,"overage a s measured byBS598: .fan 108 {BSI, 199Gb) orpiENl2272-1 CBSl.1996J. as for single dressings: (Sub-Section 6.3.I) ..Ifthereare SUl'pluschippingsfiom th e first layer, lthese should beremoved beFore t h e second layer of larger drippings areapplied.6.36 Sand.wieb dressingThe rate of spread of chippings IS as for d.ouble dressings(Sub-Section 6.3.3).

'Cured-r~in-bind~r dressingWhen applied separately to the binder, the chippings areapplied to excess so as to complet.ely cover the surface.once the 'binder hascured, the excess cbippings should beswept off as for other surfacedressing systems.6.3.7 CheckingThe rate of spread can be checked on site using the spreadbox me.tliodinJ'l'ENI2272-1 (B8I, 1996).


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. ..

7. BINDER7.1 BINDERTYPES7.1.1 Unmodified binderThe unmodified binders available for surface dressing are:

bitumen emulsion to BS434: Part 1, Table 2 (BS!,1984); cut-back bitumen to BS3690: Part 1.Table 2 (BS!,

1989a); andtar-bitumen blend to BS369O: Part 3, Table 2 (BSI,199Oc).

However, bitumen emulsion andcut-back bitumen are thepreferred binder types.Surface dressing binders are classified in terms of theirviscosity. Different measures of viscosity are used for thedifferent types of binder. as follows: Emulsion (hot) Seconds Redwood IIt 8$OC;

The recommended viscosities are based on seasonal normsinthe UK, but consideration should be given to exceptionalweather conditions that may occur and to differences inclimate between regions of the UK. Higher viscosity bind-ers are generally used on the more heavily trafficked sites,although traffic category A isconsidered to be a special casefor which unmodified binders are unacceptable; unmodifiedbinders are also generally unsuitable for roads in trafficcategories B and C. Penetration grade bitumens and cut-backbitumen binders with. viscosities higher than 200 sec may beused, but it is difficult to obtain good adhesion at lowtemperatures, Therefore, 200 sec cut-back. bitumen shouldnot be used with uncoated chippings if the road surfacetemperature during application is likely to fall below 15C(Table 5 in Sub-Section 4.2.3). The use of lightly-coatedchippings enables adhesion to take place more rapidly andtheir use is recommended with cut-back bitumen.On roads of traffic category DtoHwhere a high proportionof the traffic travels in excess of lOOkmlb (6Omph), emul-sions with base binders of suitably high viscosity or 200 secgrade cut-back bitumens should be used in order to resistdisplacement of chippings by high-speed traffic. The use of.lightly-coated chippings is recommended if high-viscositycut-back binders are used.

Cut -back bitumen Standard tarviscQmeter(STV)seconds at 40C; and 7.1.2 Modified binder

Residual bitumen Penetration at 25C Modified binders are generally proprietary products madeby the addition of polymers or other materials to biturnen.These products should be permitted for use in surfacedressing under the conditions pertaining to the HighwayAuthorities Products Approval Scheme (HAPAS)S runthrough the British Board of Agrement. The scheme recog-nises two levels of modification, intennediate grade andpremium grade, with premium grade modifiers being usedat themore highly stressed locations. In addition, variousdiscriminatory tests are under development and/or beingassessed. J

More discus sion ofbinder viscosity isgiven inAdvice Noteon Surface Dressing Binders (RSDA, 1995c) and TheShellBitumen Handbook (Whiteoak. 1990).

The bitumen emulsion class and the cut-back bitumenviscosity grades generally considered suitable for surfacedressing carried out in the United Kingdom are: Bitumen emulsion KI-70; Cut-back bitumen 100 second or 200 second The special applications of certain modified binders are

summarised. in Table '9 .

Type of binder Type of modifierModified Binders for Use in Surface Dressing

Where likely to be used

TABLE 9

Bitumen emulsion Various polymersCut-back bitumen Natural and synthetic rubbers

Mineral fillersVarious polymers

More heavily stressed sites (including urban)Surface dressing on concreteAUareas (often using heated chippings)More heavily stressed sites (including urban)

5 At the time of publication, the HAPAS for modified binders is being set up..Prior to its implementation, modified binders will haveto be, assessed from any available data on past experience in similar situations,

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The addition of polymers to bituminous binders modifiesthe performance in a number of ways depending on thepolymer used. Typically, improved performance in one ormore of the following areas is possible: reducedtemperature susceptibility in service; improved low temperature adhesion and elasticity; improved elasticity to bridge hairline cracks;

improved early "grip" on the aggregate; improved long term cohesion of the system; and

improved durability as thicker films are possible.Premium grade modifiers are designed to provide a greaterimprovement in performance than intermediate grade modi-fiers. Technical details of the improvement claimed bymanufacturers should be sought, if not included with theHAPAS approval documentation.7.1.3 Adhesion agentProprietary chemical compounds may be mixed with cut-back bitumens toimprove their "wetting" properties and sopromote adhesion between the chippings and binder inunsettled weather; adhesion is particularly impOrttmt ifdiedressing is to be laid on a road carrying heavy traffic. Someadhesion agents are thermally stable inbitumen, whilst othersundergo thermal degradation when stored in hot bitumen fo rprolonged periods. Excessive additions can change the vis-cosity of the binder. However, surface dressing of roads inpoor or unsettled weather is not recommended.A simple test for determining the optimum quantity ofagent to be added to the binder for any particular combina-tion of binder and aggregate is given in Appendix C.Changes in binder viscosity, arising either from a change ofgrade or a cbange of road temperature, hayean effect on theconcentration of the adhesion agent required to ensure goodadhesion. At road temperatures below 10C, adhesionagents are unlikely to be economic.7.1.4 Resin-based binderResin-based binders should be pan of a high-friction dress-ingaccredited with the Higbway Authorities ProductsApproval Scheme (HAP AS)6 run through the British Boardof Agrement. The HAPAS for high-friction surfacings isbased on laboratory tests with monitoring of the site appli-cation and performance.Specification requirements have been developed for anepoxy-resin modified binder. The binder is a two compo-nent. chemical-set system comprising a resin componentand a bituminous component containing the hardener.

These two components are kept separate until the time ofspraying. and are proportioned according to the manufac-turer's recommendations. The cured binder can be testedfor its mechanical properties using BS2782: Part 3,method320A (BSI. 1976).Proprietary thermoplastic polymerisedrosin-esterand ther-mosetting acrylic-resin and polyurethane-resin binders havebecome available as alternatives to epoxy-resin, The ad-vantages oftbe thermoplastic binders over epoxy-resin a rethat they are not multi-part systems and do not require aminimum temperature to effect a cure; however. they aremore difficult to apply to larger areas and they may not beas durable. The polyurethane-resin binder is a two or threecomponent system applied in. similar way to epoxy-resinsystems. The durability of these systems is still beingassessed, although experience of different resin-based bind-ers is increasing (Nicholls & Carswell, 1997).7.2 RATE OFSPREAD OF

UNMODIFIED BITUMINOUSBINDER7.2.1 STAGE 1Binder-Spread CategoryThe Stage I binder-spread. category should bederived from; Table 10 for a single dressing and {or the second

layer in an inverted double dressing using the hard-ness category attained with the first layer in place;

Table 11 for a racked-in dressing;

Table 12 for adouble dressing and, usingtheeatego-ries for the second layer only. for sandwich dress-ings; and

Table J;3 for the first layer in an inverted. doubledressing.

For double dI:ssings. the Stagel binder-spread categoriesfor the individual layers in Table 12 may be varied forspecific binders and site conditions. bu t if the Stage Ibinder-spread category is, say, increased for one layer, theStagel binder-spread category for the other layer should bereduced by the same number of steps.The Stagel binder-spread categories for a particular chip-ping size rend to decrease with increasing traffic flowsbecause of the extra embedment that will occur. Theexception is for the harder surfaces, where (here will belittle or no embedment. irrespective of traffic flow, andadditional binder may be required to deal with the extrastresses imposed by higher levels of traffic.Where traffic isintennediate between two traffic catego-ries, the values can be interpolated from the relevant Table.

6 At the time of publication, the HAPAS for high-friction surfacing systems is being set up. Prior to its implementation, resin-basedbinders will have tobe assessed from any available data on past experience in similar situations.

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TABLE 10STAGE 1 Binder Spread Category for SINGLE SURFACE DRESSING and the second layer

in an INVERTED DOUBLE DRESSINGStagel binder-spread categories given inBold type are recommended for specific chipping sizes,

' Binder ChiPPing Traffic Hardness categorysize category Very Hard Normal Soft VeryHard SoftKl-70 20mm* A- Allbitumen Bt Allemulsion c: AlOD A9

14mm At AU A10Bt Al2 AIO AIOCt Al2 AlO A~O A9D AlO All AlO A8E AIO All AIO A9F All AIO A9G AU A9

IOmm At AllBt All A9Ct All A :9 A9D AIO A9 A9 A9E AIO A9 .A9 A9 A8F AlO A9 A9 A9 ASG AlO A9 AlO A9 A8H AJO A9 AJO AIO A9

6mm Bt ASCt A8D A8 AS A7E A8 AS A7F AS A8 A7G AS AS AS A7 A6H AS A8 AS A7 A7

Cut-back 20mm* At ASbitumen Bt A7Ct A6D AS

14mm A t A8 A7Bt AS A7 A6Ct A7 A7 A7 ASD A7 AS A6 A4E A? AS A6 ASF A8 A6 ASG A7 AS

lOmm At A7Bt A7 A6ct A6 A6 A6D A6 A6 A6 A4E A6 A6 A6 AS A4F A6 A6 A6 AS ASG A6 A6 A7 A6 ASH A5 A.6 A S A 7 AS

6mm Bt ASct ASD AS AS A4.E AS AS A4F AS AS ASG AS AS AS AS A4H A4 AS AS A S A5

* 20mm chippings can be used for these conditions, but only in exceptional circumstances. Particular care should be takenwith 20mm chippings to ensure that no loose chippings remain on the surface when the road is opened to unrestricted 27traffic due to severe risk of wind-screen breakage.t Polymer-modified binders are preferred for these traffic categories,


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TABLE 11STAGE 1 Binder Spread Category for RACKED-IN SURFACE DRESSING

Stagel binder-spread categories given in Bold type are recommended for specific chipping sizes.Binder Traffic "Chipping Hardness category

sizes category Very Hard Nonnal Soft 'Very t.Hard Soft

J < . 1 - 1 0 14mm At AU AU AUbitumen with Bt AU A13 AU AIOemulsion 6mm C A l3 AU Al2 AUD Al3 A13 AU All A llE A l3 A13 _AU AU AllF A l3 A l4 AU AllG A l4 Al3 A l3H A l3 A l3

lOmm A t A l2 A l2 A l2with Bt A l2 A l2

Documents

Design Guide for Road Surface Dressing by j c Nicholls (4th Ed)