Transport and Road Research Laboratory Overseas Unit

Department of TransportOverseas Development Administration

Overseas Road Note 2

Maintenance techniquesfor District Engineers(2nd Edition)

Overseas UnitTransport and Road Research LaboratoryCrowthorne Berkshire United Kingdom1985

ACKNOWLEDGEMENTS

This Note was drafted by Mr L S Hitch, Mr T E Jonesand Dr R Robinson of the TRRL Overseas Unit.It is based on an original text produced by Scott WilsonKirkpatrick and Partners, consulting engineers.

First published 1981Second edition 1985Reprinted with minor revisions 1987

OVERSEAS ROAD NOTES

Overseas Road Notes are prepared principally for roadand road transport authorities in countries receivingtechnical assistance from the British Government. Alimited number of copies is available to other organisations and to individuals with an interest in roads overseas, and may be obtained from:

Transport and Road Research LaboratoryCrowthorne, Berkshire RG11 6AUUnited Kingdom

© Crown Copyright 1985Limited extracts from the text may be producedprovided the source is acknowledged. For moreextensive reproduction, please write toHead of Overseas Unit,Transport and Road Research Laboratory

CONTENTS

Page

1. Introduction 11.1 Scope of this Note 11.2 The Engineer’s responsibilities 1

2. Methods of undertaking maintenance 22.1 Labour and equipment 22.2 Direct-labour and contract 3

3. Safety measures and traffic control 53.1 Principles 53.2 Edge working 53.3 Lane closure 53.4 Centre line working 73.5 Diversions 73.6 Development of safety measures 9

4. Maintenance of road furniture 94.1 Traffic signs 94.2 Rails and posts 94.3 White lines and reflective studs 104.4 Maintenance gangs 10

5. Maintenance of drainage, shoulders and slopes 115.1 The drainage system 115.2 Side drains 115.3 Fords and drifts 145.4 Culverts 145.5 Bridges 155.6 Shoulders 155.7 Slopes 165.8 Maintenance gang 16

6. Maintenance of unpaved roads 176.1 Types of maintenance 176.2 Grading 17

6.2.1 Types of grading 176.2.2 Crossfall 186.2.3 Steep hills 186.2.4 Grader operation 186.2.5 Grading gang 20

6.3 Dragging and brushing 216.3.1 Design of drag 216.3.2 Brushing 216.3.3 Method of operation 216.3.4 Dragging gang 24

6.4 Regravelling 246.4.1 The task 246.4.2 Quality of gravel 246.4.3 Organisation and equipment 246.4.4 Spreading gravel 256.4.5 Compaction 256.4.6 Continuous working 26

6.5 Filling and patching 266.6 Dust prevention 26

6.6.1 The dust problem 266.6.2 Remedial treatment 26

Page7. Maintenance of paved roads 27

7.1 Types of maintenance 277.2 Local sealing 277.3 Crack sealing 277.4 Patching 27

7.4.1 Procedure 277.4.2 Materials 287.4.3 Patching gang 28

7.5 Mechanised surface dressing 287.5.1 General principles 287.5.2 Design of surface dressing 29

7.5.2.1 Type of existing road surface 297.5.2.2 Traffic categories 297.5.2.3 Chippings 297.5.2.4 Binder 29

7.5.3 Equipment 307.5.4 Application 33

7.5.4.1 Preparation of the surface 337.5.4.2 Application of binder 337.5.4.3 Spraying temperatures 337.5.4.4 Jointing strips 347.5.4.5 Application of chippings 347.5.4.6 Rolling 34

7.5.4.7 Aftercare and opening to traffic 347.5.5 Control 34

7.6 Manual surface dressing 357.6.1 Applicability 357.6.2 Chippings 357.6.3 Binder 357.6.4 Application of binder 357.6.5 Application of chippings 367.6.6 Rolling 367.6.7 Gang size 36

7.7 Slurry sealing 36

8. References 37

Appendix A – Additional notes on surface dressing 37

A.1 Chippings 37A.1.1 Size 37A. 1.2 Average least dimension 37A.1.3 Flakiness 39A. 1.4 Adhesion and pre-treatment 40A.1 .5 Stockpiling 41

A.2 Binder 41A.2.1 Grade 41A.2.2 Viscosity 41A.2.3 Measuring the rate of spread of the binder 42

A.2.3.1 Average rate 42A.2.3.2 Longitudinal variation 42

A.3 References 42

1. INTRODUCTION

1.1 SCOPE OF THIS NOTE

This Note describes the principal techniques that will beused to maintain roads within a District in a developingcountry. Particular attention is paid to the planning anddesign aspects of the work, as it is in this area that theEngineer will make the largest contribution. Practicalexecution is not described in detail.

The Note first discusses the merits of using labour orequipment-based operations and examines the scope forusing local contractors for maintenance work. Theimportance of safety during maintenance work is thendiscussed and each of the various maintenancetechniques is described in turn.

1.2 THE ENGINEER'SRESPONSIBILITIES

One of the key persons in any maintenance organisationis the professional engineer who is responsible forrunning the organisation at District or operational levelAlthough the District or Maintenance Engineer willusually be constrained to work within whatever manage-ment system is operated by his Department, there isinvariably scope for improving the cost-effectiveness ofthe use of the resources available for road maintenance.

By adopting the same professional approach towardsroad maintenance as to any other engineering activity, hecan inspire Ids staff to improve their own performance.The Maintenance Engineer should therefore show apersonal commitment to maintenance work by regularlyinspecting the roads under his control and making Insstaff aware of his interest. By getting out of the officeand into the field as much as possible, the Engineer willbe able to get to know his road network thoroughly andwill readily identify trouble spots and other areas ofdifficulty. He will be able to assess priorities and willgain first-hand knowledge of what maintenance hasactually been carried out, instead of having to rely onreports from others. He will also see the quality of themaintenance work carried out and will be able to use hisprofessional skill and expertise to solve problems on-the-spot as they arise. Seeing him actually on site will give aboost to staff morale and this will result in animprovement in both the quality and quantity of workdone. If there is one single factor which influences thestandard of road maintenance more than all others, it isthe attitude of the Engineer responsible.

The Engineer will find it difficult to follow this advice ifhe is overburdened with routine administrative work andit is essential that he delegates such activities to clericaland administrative staff as much as possible.

The Engineer will require adequate trained staff to carryout Ids instructions and he will need to arrange for super-visers, foremen and specialist artisans to attend suitabletraining courses. Accordingly the Maintenance Engineermust arrange for on-the-job training for labourers andother junior staff.

The supply and maintenance of equipment and vehiclesis usually organised as a separate Departmental functionand will be beyond the control of the MaintenanceEngineer. lack of suitable vehicles is always a majorfactor in maintenance organisations that are inefficient.The Maintenance Engineer must be aware of the impor-tance of proper maintenance of equipment and vehiclesand should actively concern himself with theorganisation of this, as far as Departmental proceduresallow.

The work of the District or Maintenance Engineer fallsinto three categories:-

(i) Assessment of requirements: establishingwhat needs to be done in physical terms;

(ii) Allocation of resources: estimating the men,materials and equipment required fordifferent tasks, determining priorities andallocating resources to ensure the most cost-effective results;

(iii) Monitoring: checking that the work doneproduces the desired results.

The management aspects of this work are described inmore detail in a separate Note (TRRL Overseas Unit,1981).

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2. METHODS OF UNDER-TAKING MAINTENANCE

2.1 LABOUR AND EQUIPMENT

Most maintenance operations offer considerable scopefor the application of labour-based methods and some areonly practicable by such methods. No recommendation ismade here for either equipment-intensive or labour-intensive methods, but Table 1 indicates the potential forequipment and labour-based methods in differentmaintenance operations.

In choosing between equipment-based and labour-basedmethods, consideration should be given to the standard ofwork achieved by each method as well as to costs and tothe way in which the work is organised. It is not alwaysnecessary for labour-based operations to have the samestandards of finish that can be

obtained by equipment. For example, on low-volumeroads maintained by labour-based methods, there isunlikely to be any economic justification for insisting onthe close tolerances of level and smoothness that can beachieved by the use of equipment.

Equipment and labour cannot be substituted directly foreach other as, in addition to the question of standardsalready mentioned, it will usually be necessary to makechanges in work organisation. The following pointsshould be borne in mind when considering the use oflabour-intensive maintenance methods: -

(i) It is necessary to check carefully that labourwill be available in the actual place where it iswanted, and at the time it is wanted.

(ii) Adequate organisation and management arecritical considerations for large scale labourintensive works.

TABLE 1POTENTIAL FOR EQUIPMENT AND LABOUR

POTENTIAL FORACTIVITY

EQUIPMENT LABOUR

Ditch cleaning and cuttingCleaning and minor repair to culverts and bridges

Good (*)Poor

Good (*)Good

Building scour controlsRepair of structures

Grading unpaved surfaces

Dragging and brushing of unpaved surfacesPatching sanding or local sealing of bituminous surfaces

PoorPoor

Good(Skilled) (ΨΨ)GoodPoor

GoodGood

Impracticable

PoorGood

Filling on unpaved surfaces and slopesGrass cuttingManufacturing signs

Repairing and replacing traffic signsRoad line markings

PoorGood (**)Fair (Ψ)

PoorGood

GoodGoodGood (Ψ)(Skilled) (ΨΨ)GoodFair

Stockpiling gravelRegravelling gravel surfacesStockpiling chippingsSurface dressing

GoodGoodGoodGood(Skilled) (ΨΨ)

FairFairPoorFair(Skilled) (ΨΨ)

NOTES:(*) The potential in these activities is dependent upon suitable design of the ditch cross-section. 'V'-shaped

ditches are suitable for maintenance by grader whereas flat bottomed ditches are suitable for maintenance by handor by mechanical shovel.

(**) The potential in this activity is dependent on the width of the shoulder and presence of obstructions such as roadfurniture and culvert headwalls.

(Ψ) Some methods of manufacture may require the use of specialised plant (eg. vacuum application of reflectivesheeting to sign plates).

(ΨΨ) The expression 'skilled' implies that specific training of operatives is essential.

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(iii) The health and nutrition of the labour force arevery important and improvements can increaseproductivity.

(iv) Standardised tools of good quality are vital inorder to achieve high levels of output.

(v) It is normally inefficient to have labour andmachinery working alongside each other on thesame task.

(vi) Arrangements may need to be made totransport labour to and from the work sites. Forlarge labour forces, this can be a substantialtask.

2.2 DIRECT-LABOUR AND CONTRACT

Road maintenance organisations usually carry out muchof their work by direct-labour. However, some mainten-ance activities can be done by small local contractorsand, where Departmental policy permits, the Mainten-ance Engineer should consider whether the use of suchcontractors is advantageous.

The use of contractors can reduce the burden on scarcegovernment staff and also bring lower costs as a result ofcompetitive pressures on efficiency which are unlikely tobe obtained within a government department. However,contract work does require the preparation of detailedcontract documents and a high degree of supervision, andthe staff requirements and extra work involved for thisshould not be underestimated. There is also a danger thatcontractors bidding for maintenance work on a regularbasis might introduce ‘price fixing' to increase theirprofitability, and this will result in an increased cost tothe maintenance department.

The following activities should present no seriousproblems in the control of quality or quantity if carriedout by contract on the basis of competitive tender:-

Supply of MaterialsNatural gravelScreened gravelRock aggregate (for subsequent crushing by Depart-mental crusher)Crushed rock aggregate (for surface dressing andconcrete)Washed sand (for concrete)Precast concrete blocksPrecast concrete culvert rings

Maintenance OperationsExcavation of side drains and turnoutsConstruction of culvertsRegravellingSurface dressingGrass-cutting and bush-clearing

In addition, the supply or maintenance of equipment andvehicles can be undertaken by specialist contractors.

The Maintenance Engineer should satisfy himself thatany contractor asked to tender is capable of completingthe work satisfactorily, that he has the necessaryequipment and staff, and that he has sufficient knowledgeof estimating (or of current market rates) to be able tosubmit realistic prices.

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Fig.1 Standard signs

3. SAFETY MEASURES ANDTRAFFIC CONTROL

3.1 PRINCIPLES

When work is being carried out on or close to thecarriageway, it is the Maintenance Engineer'sresponsibility to see that adequate measures are taken towarn and protect road users and maintenance workers.He should instruct all supervisors and foremen in safetymeasures, including traffic control, and the use oftemporary road signs.

In selecting and positioning temporary road signs, thefollowing principles should be applied:-

(i) only standard signs should be used,

(ii) the signs must be clean and in good condition,

(iii) the standard signs should be displayed in astandard layout,

(iv) the layout used must give drivers time tounderstand and respond to the informationwhich the signs convey.

Recommended standard signs are shown in Fig. 1.

It may not be possible for the Engineer to purchase orobtain the necessary signs from Departmental stores, butthe manufacture of adequate signs should be within thecapabilities of a central signs workshop. Each roadmaintenance gang should be provided with signsappropriate to the work it is carrying out and all foremenand supervisors should be trained in their use and layout.All temporary signs must be removed as soon as thework they relate to is complete. If they are not the valueof the signing will be reduced.

From the point of view of safety and traffic control, roadmaintenance work may be divided into four categories:

(i) work which does not affect the carriageway,such as cleaning out side drains and cuttinggrass on verges (Section 3.2);

(ii) work requiring partial closure of thecarriageway, such as repair work to surface orbase which is restricted to one lane whiletraffic continues to use the other lane (Section3.3);

(iii) work on the centre line, such as white linepainting (Section 3.4);

(iv) work requiring total closure of the road, withconstruction of a temporary diversion, such asthe reconstruction of a damaged culvert(Section 3.5).

Whenever possible during maintenance work, yellow

or orange safety vests should be worn by the supervisorand all of the workforce. All vehicles and equipmentshould be painted yellow or orange and should carry redand white striped marker boards front and rear. Allvehicles and equipment should work with headlightsswitched on and, where possible, should carry yellowflashing warning lights. If warning lights are notavailable, vehicles and equipment should carry a yellowor orange flag.

3.2 EDGEWORKING

Where road works are being carried out which do notaffect the carriageway, or where works such as gradingor dragging are being carried out, the sign layout shownin Fig. 2 is recommended.

Warning signs should be placed before work starts andmust be placed in the following order:

1. 'Men working' signs should be placed at theapproaches to the work area.

2. 'Road clear' signs should be placed at the ends ofthe work area.

When the work has been completed, signs should beremoved in reverse order. Signs must not be left on theroad or at the roadside overnight. They should beremoved and returned to the depot.

3.3 LANE CLOSURE

For repairs to the carriageway such as patching whichrequires closure of one lane, the sign layout shown inFig. 3 is recommended.

Before work starts, warning signs, barriers and conesmust be placed around the work area. Work will becarried out on one side of the road at a time allowingtraffic to pass on the other. Signs must be placed in thefollowing order:

1. 'Men working' signs should be placed 200 metresin front of the work area.

2. 'Road narrows' signs should be placed 100 metresin front of the work area.

3. 'Keep left/right' arrows should be placed at thestart of the work area.

4. Barriers should be placed at each end of the workarea.

5. 'Keep left/right' arrows should be placed next tothe barriers.

6. Cones should be placed in a taper at theapproaches to the work area and at a spacing of10 metres along the middle of the road next to thework area.

7. 'Road clear' signs should be placed 200 metresbeyond the work area.

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Fig.2 Sign layout for edge working (drawnfor driving on the left)

Fig.3 Sign layout for lane closure (drawn fordriving on left)

Traffic controllers should stand opposite the barriers onthe other side of the road holding reversible 'stop/go'signs. One controller should be appointed by thesupervisor as the leader. He should decide when tochange the direction of the signs and the other controllershould follow his lead. In this way, the controllers workclosely together to ensure that only traffic from onedirection is allowed to pass at a time. When the workarea is short, only one traffic controller may be needed.The Maintenance Engineer should train these men in theuse of the signs and only the trained men should beallowed to operate them. Police assistance in trainingmay be helpful.

On low traffic roads, the Maintenance Engineer mayapprove the use of a simpler system of traffic control.

Patching work should not be left unfinished over-nightbut, if this sign layout is being used for repairs to aculvert or a washout, then at night the approaches mustbe adequately lit. Kerosene lanterns may be adequate onlightly-trafficked roads or where speeds are low, buthigh-intensity lamps should, if possible, be used onheavily trafficked roads. A watchman should always bein attendance to ensure that the lamps are working andare not interfered with or stolen. Lamps should show ayellow light. Flashing lamps are best as they consumeless power and are more easily seen.

When the work has been completed, signs should beremoved in reverse order.

3.4 CENTRE LINE WORKING

When painting centre line markings on the road,considerable care must be taken and it is most importantto pay a great deal of attention to safety measures. Thesign layout shown in Fig. 4 is recommended.

Before work starts, warning signs, barriers and conesmust be placed around the work area. They must beplaced in the following order:

1. 'Men working' signs should be placed 200 metres infront of the work area at the side of the road.

2. 'Road narrows' signs should be placed 100 metres infront of the work area at the side of the road.

3. 'Keep left' arrows* should be placed in the centre ofthe road at the start of the work area.

4. Barriers should be placed behind the 'keep left'signs.

5. Cones should be placed at a spacing of 10 metres oneither side of the work area.

* Assumes driving on the left.

6. 'Road clear' signs should be placed 200 metresbeyond the ends of the work area at the side of theroad.

When the work has been completed, signs should beremoved in reverse order. Signs must not be left on theroad or at the roadside overnight. They should beremoved and returned to the depot.

3.5 DIVERSIONS

A diversion will enable maintenance work to be carriedout more efficiently and more safely. In particular,diversions are needed for regravelling work and majorculvert repairs. If traffic is to be diverted for more than aday or two, or the work is being carried out during thewet season, the diversion should be constructed with anadequate base and surfaced with gravel. A small gangshould be allocated to keep the diversion in goodcondition. Diversions should be wide enough to allowtwo lorries to pass.

After the diversion has been completed and before workstarts, warning signs, barriers and cones must be placedaround the work area. The layout shown in Fig. 5 isrecommended. Signs must be placed in the followingorder:

1. 'Men working' signs should be placed 200 metres infront of the work area.

2. 'Turn left/right ahead' arrows should be placed100 metres in front of the work area.

3. Cones should be placed diagonally across the roadto lead into the diversion.

4. 'Keep left/right' arrows should be placed at bothends of the lines of cones.

5. Barriers should be placed behind the lines of cones.

6. 'Road clear' signs should be placed 200 metresbeyond the ends of the diversion.

Diversions will usually be in operation at night as well asby day and the approaches must be adequately lit.Kerosene lanterns may be adequate on lightly-traffickedroads or where speeds are low but high-intensity lampsshould if possible be used on heavily trafficked roads. Awatchman should always be in attendance to ensure thatthe lamps are working and are not interfered with orstolen. Lamps should show a yellow light. Flashinglamps are best as they consume less power and are moreeasily seen.

When the work has been completed, signs should beremoved in reverse order.

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Fig.4 Sign layout for centre line working(drawn for driving on the left)

Fig.5 Sign layout for diversion (drawn fordriving on the left)

3.6 DEVELOPMENT OF SAFETYMEASURES

In many developing countries, no safety measures areprovided at all during maintenance works. TheMaintenance Engineer must understand that the safety ofhis workers and of the road user during maintenanceoperations is his responsibility. He must train his staff inmethods of signing and traffic control and make themunderstand the need for safe working.

Initially it will probably not be possible to provide all thesafety equipment recommended here. However, simplesigns can be easy and cheap to make and it is possible toimprovise cones and barriers. Turfs of grass cut from theroadside can be used instead of cones and effective roadmarkers can be made from old oil drums painted withblack and white bands and filled with sand, stones orwater. If signs cannot be provided, a maintenance vehicleshould be parked on the road between the on-comingtraffic and the maintenance workers with its lightsswitched on.

When traffic volumes are very low, the Engineers mayallow his workers to relax these standards. In particular itmay not be necessary to use 'stop/go' signs. However, byrelaxing standards, road users and maintenance workersmust not be placed in a hazardous position.

4. MAINTENANCE OF ROADFURNITURE

4.1 TRAFFIC SIGNS

Traffic signs are the principal means of conveyinginformation about the road to drivers and, as the roadnetwork becomes more extensive, the number of trafficsigns increases. As traffic flows increase, an increasingeffort on their maintenance is needed.

The sizes, colours and layouts of signs have, in mostdeveloping countries, been standardised in accordancewith international protocol and incorporated in Depart-mental standards. It is important that all signs shouldcomply with these standards.

Signs which are clean and in good repair can be easilyseen and understood and inspire confidence, that theirmessage is accurate and reliable. Damaged or missingsigns should, for the same reasons, be replaced promptly.Signs which are no longer needed (e.g. because of roadimprovements) should be removed, as should temporarysigns upon completion of the maintenance works towhich they are related. The provision and care of signs isvery cheap and cost effective.

Signs should be inspected and cleaned at least twice ayear. If there are enough signs, it may be worth setting upa small team which tours the District, covering all roadsin rotation, which carries out this work as well asrepainting posts and replacing damaged signs. Thecleaning and inspection team will need to be providedwith equipment such as that listed in Section 4.4.

Major repairs, particularly to sign faces, are better carriedout in a central workshop which is equipped to do thiswork under good conditions and supervision. (This isparticularly important in relation to signs which havelegal force.)

It is worthwhile keeping records of traffic signs; theyshould be included in an inventory and transferred to asigns register (which may be a card-index) in whichinspections, repairs and replacements are recorded. It isuseful if their location is recorded on a strip-map.

4.2 RAILS AND POSTS

Guard rails and parapet rails are provided to protect roadusers guard rails to prevent vehicles from running overhigh embankments and parapet rails on bridges tosafeguard pedestrians. They should be repaired promptlyif damaged, and kept clean and repainted regularly so asto maintain their visibility and prevent corrosion.

Kilometre posts provide both drivers and the mainten-ance organisation with the basic reference for theposition of any point on the road. Kilometre posts

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should be kept clean and repainted regularly, andvegetation should be cleared so that they can easily beread from a moving vehicle.

Similar requirements apply to bridge and culvert markerposts. It is recommended that each bridge and culvert isprovided with a marker post (or posts), serially numberedwithin the kilometre in which it is located, thus:79/3 is the third culvert or bridge in km 79. Thesenumbers should be used as references in the culvert andbridge registers.

Delineator posts are usually provided only on bends.They should be kept clean and repainted regularly,preferably with reflective paint if reflectors are not fitted.Vegetation should be cleared so that they can be easilyseen.

4.3 WHITE LINES AND REFLECTIVESTUDS

White lines are being used increasingly on principal two-lane roads in developing countries for centre line, laneand edge markings, and for 'stop' or 'give way' lines.They are particularly useful as an aid to night driving. Anote on their maintenance is therefore appropriate.

White lines may be of paint or hot-applied plastic. Bothmaterials may be reflectorised with small glass beads(ballotini). Emulsion paint gives a good performance onsurface dressed roads with a good surface texture but, onpremix roads or roads with less texture depth paint has ashort life (perhaps only a few months under heavytraffic) and will require renewing regularly. Hot-appliedplastic has a much longer life and should require muchless maintenance effort. Its application requires the use ofspecialised equipment and properly trained operators.Paint, on the other hand, can be applied manually,although this will be a very slow procedure and the useof a portable paint sprayer is preferable. Whichevermethod is used, road markings should always be appliedusing templates or a straight edge. Such equipment willusually be provided through the central equipmentorganisation.

Reflective road studs may be used in conjunction withwhite lines. They should be replaced promptly if they arelost or damaged and protected by masking when surfacedressing is being carried out. They should be installed bymen who have been suitably trained. Particular attentionshould be paid to safety measures when men are workingin the middle of busy roads (see Section 3.4).

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4.4 MAINTENANCE GANGS

The composition of a typical road furniture gang is givenbelow. Such a gang would be reponsible for field repairsto traffic signs, guard rails, parapet rails, marker anddelineator posts. They should also be responsible forcleaning signs. The Maintenance Engineer may find thatlocal circumstances mean that a different compositionwith different tools and equipment are needed.

Personnel1 Overseer/Gang leader1 Driver2 Labourers

Vehicles and equipment1 Truck or van, equipped for carrying signs without

damageAssorted signsAssorted tools, including spanners, etc

1 Pick-axe1 Spade or shovel1 Saw1 Hand rammer Dry mixed concrete1 Small ladder

WaterDetergentPaintsBrushesSolvent for brush cleaningSafety equipment as recommended in Section 3.

A typical gang for road marking might consist of thefollowing. These would be responsible for painting whitelines and replacing reflective studs.

Personnel1 Overseer/Gang leader1 Driver2 Painters or labourers

Vehicles and equipment1 Small truck

Stencils30 metre tapeStringPaintsBrushesSolvents for brush cleaningSafety equipment as recommended in Section 3.2.

If thermoplastic paint is being used, it must be appliedusing a specially equipped truck and skilled operators inplace of the painters or labourers. The truck should bepainted yellow or orange and be equipped with flashingyellow warning lights.

5 MAINTENANCE OFDRAINAGE, SHOULDERSAND SLOPES

5.1 THE DRAINAGE SYSTEM

One of the most important aspects of the design of a roadis the provision made for protecting the road fromsurface water or ground water. If water is allowed toenter the structure of the road, the pavement will beweakened and it will be much more susceptible todamage by traffic. Water can enter the road as a result ofrain penetrating the surface or as a result of theinfiltration of ground water. The road surface must beconstructed with a camber so that it sheds rain-waterquickly and the formation of the road must be raisedabove the level of the local water table to prevent it beingsoaked by ground water.

Water can also have a harmful effect on shoulders,slopes, ditches and other features. High water velocitiescan cause erosion which, when severe, can lead to theroad being cut. Alternatively, low velocities in drainagefacilities can lead to silt being deposited which, in turn,can lead to a blockage. Blockages often result in furthererosion.

A good road drainage system, which is properlymaintained, is vital to the successful operation of a road.It has four main functions:

(i) to convey rainwater from the surface of thecarriageway to outfalls (streams and turn-outs);

(ii) to control the level of the water table in thesubgrade beneath the carriageway;

(iii) to intercept surface water flowing towardsthe road;

(iv) to convey water across the line of the road ina controlled fashion.

The first three functions are performed by side drains andthe fourth by culverts, drifts and bridges.

Common drainage problems include:

(i) blocking of drains by debris or vegetation;

(ii) silting: the deposition of silt in the bottom ofdrains and culverts, often reducing thegradient;

(iii) erosion of the bottom of side drains inerodible soils or on steep gradients, particu-larly where insufficient turn-outs result inlarge flows in drains;

(iv) erosion at culvert outfalls, resulting fromhigh discharge velocities;

(v) erosion of shoulders and side slopes.

Even if the drainage system of a new road has beencarefully designed, it is likely that for several years afterconstruction it will be necessary to observe itsperformance closely and to make additions and amend-ments to it. Reference is therefore made here to designmatters where these can be seen as forming part of themaintenance function.

Both in the design and in maintenance of drainage, it isimportant to interfere as little as possible with the naturalflow of water. Culverts on natural water-courses shouldfollow the existing alignment as closely as practicableand re-alignment (often resulting in sharp changes indirection) should be avoided. The surface flows in drainsand culverts should also be kept to a minimum by the useof frequent turn-outs where side drains cannot bedischarged to existing water courses. In side-longground, where discharge from the side drain on the highside passes to the low side, it is best to use frequent smallculverts rather than occasional large ones. In such cases,the spacing will be governed by the maximum flowacceptable in the side drains and the capacity of theculverts will not usually be a constraint as the minimumrequirements for access for maintenance (often taken as600mm diameter or 600mm x 600mm box) will ensureadequate capacity.

5.2 SIDE DRAINS

The level of the water table beneath the carriageway is amajor influence on the strength of the subgrade. Thebottom of side drains should normally be maintained at alevel at least one metre below formation level (theunderside of the sub-base). If side drains have beenconstructed too shallow and they are not performingproperly, they should be deepened as part of themaintenance operation. The performance of side drainsshould be monitored over time by the MaintenanceEngineer to determine designs and dimensionsappropriate to local topographical, climatic and soilconditions.

Side drains are usually built to the same gradient as theroad. This may result in high velocities and erosion onsteep gradients and silting on flat or reductions ingradient. Volumes in the side drains can usually bereduced by constructing frequent turn-outs.

In highly erodible soils, additional measures may beneeded to prevent or control erosion. Grass should beencouraged to grow in drainage ditches as this helps bindtogether the topsoil and inhibits erosion. Where theerosion is only just starting, the most effective control islikely to be to dam the side drain at frequent intervals andto construct additional turn-outs. More severe erosionmay need check-dams as shown in Fig. 6.

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Fig.6 Check-dams for side drains

Fig.7 Layout of turn-outs

Dumping rock into an erosion gully is not usuallyeffective unless a filter of finer material is also used.Rocks in steel wire mesh baskets or gabions may bemore effective. These are shown in Fig. 6. Gabionsshould be extended well into the sides of ditches toprevent scour around their edges. Protection should alsobe provided at the foot of check-dams either by rock rip-rap or gabion mattresses as found appropriate. In verysteep sections on highly erodible soil, it may benecessary to line the ditch with concrete, masonry or rip-rap. In time, erosion prevention measures maythemselves require repair.

Removal of silt should form part of the routinemaintenance programme. Silt should be thrown at leasttwo metres clear of the side drains away from the road. Itmust never be used to repair road surfaces. If silting ofside drains persists, it may be advisable to realign theditches to increase water velocities, if this is possible.

The discharge from side drains should, whereverpossible, be taken to existing natural watercourses.Where this is not possible, the side drains should beinterrupted at intervals by dams, and the flow dischargedto turn-outs (see Fig. 7). Here the water will be dispersedby seepage, evaporation, or by surface flow. Turn-outsshould be built so that they have a longitudinal gradientof about 1 in 50 to 1 in 100 (ie. nearly parallel to thecontours) and of such length that they run out to zerodepth. The spoil from turn-outs should be thrown to thelow side. The junction of the side drain and the turn-outshould have a radius of at least five metres. This avoidsthe tendency for scour to occur at this point. Spacing ofturn-outs depends on several factors including theerodibility of the soil, the width and gradient of road andcross-slope of the ground. On highly erodible soils, thespacing may initially be based on those indicated inTable 2. Spacings may be increased on soils with greaterresistance to erosion. Observation of the performance ofthe side drains and turn-outs in practice will indicate ifany changes are necessary.

TABLE 2

TURN-OUT SPACING

Gradient Spacing(metres)

1 in 100 or less1 in 100 to 1 in 501 in 50 to 1 in 201 in 20 to 1 in 10more than 1 in 10

5040251510

14

In agricultural areas, the construction and maintenance ofturn-outs can cause difficulty with farmers. The mattershould be discussed at the outset with the farmer as it isoften possible to integrate road and farm drainage. In thelast resort, it may be necessary to ask for the assistanceof the Civil Authorities. Compensation may need to bepaid.

Turn-outs should receive the same attention as sidedrains during maintenance. Both can be maintained byhand or with a grader. The cross-section of the ditch orturn-out determines the most appropriate method ofmaintenance.

The use of cut-off drains should be avoided as theysometimes cause small land-slips. Because they are oftendifficult for maintenance crews to locate, they are oftenneglected and become ineffective.

5.3 FORDS AND DRIFTS

Fords and drifts are often used on lightly trafficked roadsat watercourses with seasonal flows. They may becovered by deep water for short periods when traffic maynot be able to pass, or by shallow water for longerperiods. For much of the year they may be dry.

Maintenance of fords and drifts should be carried outafter flooding and much of it consists of maintenance tocarriageway, earthworks and culverts as for a normallength of road. In addition, warning signs, marker postsand depth indicators should be checked, and repaired orreplaced as necessary. The crossing should be checked toensure that the carriageway has not scoured or washedout, and any damage should be repaired. Erosion down-stream of the ford is a common problem and, ifnecessary, a masonry or concrete apron should beconstructed. An alternative may be to use stone gabionsor mattresses. Any silt or debris must be cleared from thecarriageway.

Some drifts have a culvert passing under them. These cancause the deposition of silt up-stream of the drift whichtends to block the entrance to the culvert, making itinoperative. If culverts are used in drifts, they should belarge enough and steep enough to be self-cleaning.

5.4 CULVERTS

Culverts are provided to convey water from the upstreamside of the road to the downstream side. They may bebuilt on the line of existing watercourses or to carry thebuild-up of water which results from the presence of theroad. In either case, silting, choking by debris orstructural collapse will usually result in over-topping anddamage to the road. Maintenance comprises keeping thewaterway clear, controlling scour and repairing structuraldamage.

Erosion of outlet channels from culverts is a commonproblem and if not dealt with promptly is likely to resultin damage to the culvert and the road. This is caused byhigh discharge velocities from the outlet, and solutionswhich do not take account of this are not likely to bepermanently effective. If the discharge velocity cannot bereduced by increasing the area of cross-section of theculvert and reducing its gradient, perhaps with a drop-inlet, then some type of energy-dissipating outlet shouldbe used. As a first step, a fan discharge constructed withmasonry or concrete should be tried. Stone gabions ormattresses downstream of the outlet may be a cheapalternative. Drop outlets are good, but on many sitesthere is insufficient height to construct these.

The checking and removal of debris from culverts can bedifficult, particularly if the culverts are small. Long-handled shovels or shovels attached to rods are useful forclearing out culverts which are too small for a man to getinside. Trees or branches blocking culvert entrancesshould be sawn into convenient sized pieces to help theirremoval and carting away. Culverts which get regularlyblocked by debris should have a grill constructed at theirupstream entrance.

Culverts made with corrugated metal pipes can beabraded by water carrying silt and sand which can wearaway the protective zinc coating. This will result in theculvert pipe rusting away. If the culvert is large enoughfor a man to enter, the rust can be removed with a wirebrush and the pipe coated with a thick layer of hotbitumen or tar. Alternatively, a flat concrete invert can beconstructed.

Culverts made from concrete rings can be subject todifferential settlement. This is a construction fault andmajor settlement problems can only be corrected byreconstruction. Minor mis-alignments should be repairedby grouting the joints in the pipes with concrete toprovide a waterproof seal.

5.5 BRIDGES

Bridge inspection and maintenance requires specialistknowledge and skills, some of which will only be held bya qualified bridge engineer. The maintenance of largebridges is beyond the scope of this Note. However, somerepairs to small bridges can be accomplished by a normalroad maintenance unit and notes on these are includedhere.

The maintenance of the waterways under bridgesinvolves the same principles as that for culverts. Themain operations involve keeping the waterway clear,controlling scour and repairing structural damage.Maintenance methods are similar to those describedunder the maintenance of side drains, fords, drifts andculverts (Section 5.2 to 5.4). Debris should be removed

as soon as possible after floods, and eroded and scouredareas should be repaired. These should be filled withcompacted gravel and then protected with rip-rap,concrete or gabions. Deposits of silt and sand should beremoved where necessary to restore the original channel.

Simple repairs can be carried out to the decks of timberbridges. Loose plans can be re-fixed using screws ornails whose length is two or three times the thickness ofthe plank. Defective planks should be replaced with newplanks of the correct thickness, length and width, whichshould be treated with wood preservative. All nail headsshould be driven flush with the surface. Whenever planksare replaced, the condition of the timber underneathshould be checked.

Small painting jobs can be undertaken such as therepainting of railings or, occasionally, steel beams. Alldust, dirt, rust and old paint scale should be removed,where possible with an oxy-acetylene burner, and thenwith a wire brush. The steelwork should be given coatsof primer, undercoat and topcoat. Only good qualitypaint should be used and it should be brushed thoroughlyinto the steel. Ample time should be allowed betweencoats for the paint to dry. Brushes should be washedthoroughly after use with thinners, petrol or kerosene.The colour of the topcoat should be as bright as possiblefor better visibility and safety.

5.6 SHOULDERS

Shoulders provide lateral support for the carriageway.They also provide accommodation for stopped vehiclesand, on narrow roads, provide room for passing vehicles.Besides being used by vehicles, they are also used bypedestrians, cyclists and for animal driving. Shouldersmay be paved, gravel, earth or may be grassed.

Paved shoulders are subject to the same type of defectsas paved carriageways and are repaired using the samemethods as described in Section 7 (Maintenance of pavedroads). It will usually be convenient to carry out repairsto this type of shoulder at the same time as the repairs tothe carriageway with the same maintenance gang.Periodically, it is necessary to reseal shoulders and thisshould be carried out in the same way as for manualsurface dressing (Section 7.6).

Similarly, maintenance of earth and gravel shoulders isessentially the same as for unpaved roads. Most defectsare corrected by grading, although ruts and pot-holes canbe repaired manually (see Section 6.5). One of theobjects of grading is to retrieve gravel lost to theshoulder and place it back on to the carriageway. On noaccount, must sediment from the ditch be graded on tothe shoulder. The grader should cut to the bottom of alldefects in the shoulder and spread the material so that,when compacted, it is level with the carriageway edgeand slopes away to the ditch at a

15

steeper crossfall than the carriageway. For unpavedroads, this grading should be carried out at the same timeas grading of the carriageway and in the same way (seeSection 6.2).

If shoulders are worn away by the action of traffic andthe level falls below that of the carriageway at the edge,water can collect at the edge of the road and infiltrate thebase. This considerably weakens the structure of the roadand usually results in severe deformation of thecarriageway. In the case of paved roads, the edge starts todisintegrate and break away. This type of edge damage isdifficult to repair effectively, but must be patched usingmethods similar to those described in Section 7.4. Edgerepairs must be accompanied by the placing andcompacting of new material on the shoulder or therepairs will be ineffective. Periodic regravelling of theshoulders should be carried out in a similar way to thatdescribed in Section 6.4.

Grassed shoulders can cause problems if the grass trapsmaterial washed off the road, leading to the shoulderlevel building up and trapping water at the edge of thecarriageway. If this occurs, the shoulders should begraded-off as described above. Where there are bushesand high grass growing on the shoulder which interferewith lines of sight, these should be cut back as short aspossible. This can be done using a tractor-mower or byhand using scythes and cutlasses. Where there is muchroad side furniture, hand cutting will be easier than usinga machine.

When there is extensive damage to shoulders as a resultof erosion or washouts, repairs are needed urgently andshould be carried out as described in Section 5.7.

5.7 SLOPES

For all slips and settlements of cut and fill slopes whichaffect the road, the maintenance gang should placewarning signs, cones and barriers around the siteaccording to the layouts described in Section 3. Severedamage, where the road is cut by a landslide or awashout will usually have to be repaired by special gangsoften needing heavy construction equipment. Less severedamage can often be repaired by the maintenance gang.Although slips and settlements can occur as a result ofslopes being too steep, they are much more usuallyassociated with the presence of water in the soil.

In the case of landslide material blocking the road, all thematerial should be removed and carted away on a tippertruck or on wheelbarrows and dumped where it cannotaffect the road. Roadside ditches should be cleared of alldebris and ditch shapes should be

16

reinstated. When shoulders have been cleared, theirlevels should be checked to ensure that water can flowfrom the carriageway uninterrupted into the drain. Whenthe failed slope is not to be reinstated, the sides of theslip area should be rounded off.

Where there has been severe erosion or slips which haveremoved part of the carriageway or shoulder, or iferosion of slopes threatens the road structure, repairs areneeded urgently. Loose material must be removed andthe damaged area should be cut back to sound material.New material must be placed and compacted in 50-7 mmlayers using small vibrating rollers or hand tampers. Itmay be necessary to add water to the fill material toassist compaction. When reinstating slopes in thesecases, gab ions can be used for all or part of the repairwork. In all cases, protection measures should be taken tostop the slip occurring again. Thin will probably involveimprovements to the drainage system and may requirepaving of the slope itself using rip-rap, masonry orconcrete. The use of gabions for filling material removesthe need for slope protection. Establishing grass or othervegetation on the slope may be a cheaper form ofprotection. It may also be necessary to surface theshoulder with gravel or with a bituminous surfacedressing.

5.8 MAINTENANCE GANG

For general maintenance work to drainage features,shoulders and slopes, the detailed composition of themaintenance gang will depend on whether the work is tobe carried out by labour4ntensive or equipment. intensivemethods. A suggested basic gang size is given below, butthis will need to change for different operations andsituations, and the Maintenance Engineer should develophis own gang compositions to meet his own localcircumstances.

Personnel1 Overseer/Gang leader1 Driver

Several labourers

Vehicles and equipment1 Small truck or tractor-and-trailer1 Hand-held vibrating roller (0.25 Mg) plus a plank to

help load onto truck or trailer, or one hand rammerfor each labourer used on compaction work

1 Pick-axe for every two labourers1 Broom for every two labourers1 Shovel or hoe for every two labourers plus

extension rods for culvert clearing1 Rake for every two labourers1 200 litre drum for water1 Bucket or watering can1 Axe

Safety equipment as recommended in Section 3.

Materials

Patching materials as indicated in Tables 3 and 4 forunpaved shoulders and slopes and as indicated inTables 5-7 for paved shoulders.

Cement, sand and ballast may be needed for makingconcrete for repairs to culverts and paved ditches. Water,if not obtainable on site, must be carried.

Grass cutting work can be done by this same gangequipped with scythes and cutlasses or by a specially-equipped tractor mower. Shoulder grading should becarried out with a grader of at least 135 horse powerworking in conjunction with compaction equipmentwherever possible.

6. MAINTENANCE OFUNPAVED ROADS

6.1 TYPES OF MAINTENANCE

The principal operation in maintaining earth and gravelroads is grading. Dragging or brushing may also becarried out with the objective of controlling the develop-ment of corrugations, and light or routine grading is alsocarried out for this reason. Heavy grading is used toreshape the road surface and to restore it to its correctcamber or crossfall and to provide a smooth runningsurface. Heavy grading can be combined with regravell-ing to restore the thickness of the gravel surface. Fillingor patching are labour-intensive operations to deal withthe worst defects on low-volume roads for which theexpense of grading or other machine activities cannot bejustified.

6.2 GRADING

6.2.1 Types of grading

Light grading is a light trimming of the surface of theroad which should be carried out on a routine basisparticularly in the dry season to control roughness andcorrugations. When undertaking light grading in the dryseason, loose material should be bladed towards the edgeof the road. If several graders are available, it is moreefficient to use them together on the same job. In thiscase, they should work one behind the other covering thewhole width of the road. In the wet season, materialshould be graded towards the centre of the road. Lightgrading maybe carried out by motor graders, but a morecost-effective technique is to use tractor-towed graderswhich are capable of similar outputs and standard ofwork on properly constructed roads.

Heavy grading consists of scarifying and cutting to thebottom of deformations and then reshaping the surface. Itusually requires the use of a 135 horse power motorgrader but, in some cases, the tractor-towed grader couldalso be utilised. Heavy grading operations should alwaysbe carried out at the beginning of the wet season toensure that the road has the correct profile for effectivelyshedding water during the rains. When possible, it shouldalso be carried out at the end of the wet season when themoisture content of the surfacing material is still highenough to help recompaction and prevent loss of fines.This is particularly important when heavy grading isneeded to remove ruts and potholes. Scarifying to thedepth required to remove these will result in theproduction of a considerable depth of loose materialsand, in the dry season, this cannot be recompacted unlesslarge amounts of water are added. The surface will thenbe

17

quickly deformed and fines will be scattered by traffic.Heavy grading of gravel is inadvisable without theprovision of additional surfacing material if theremaining thickness of gravel is less than 75mm.

The frequency at which grading should be carried outwill depend upon the traffic, the climate and the nature ofthe surface material. Gravels of average quality willprobably need grading after 12,000-15,000 vehiclepasses and good quality gravels may sometimes be leftfor 25,000 vehicle passes. For roads that are liable tocorrugate, grading may be needed after the passage ofonly 1,500-2,000 vehicles. Some gravels, particularlythose which are self-cementing or which contain largesize material, are not suitable for grading as this results inthe surface being torn up. Patching or regravelling isneeded to repair these. The frequency of grading willalso depend upon the daily traffic level as, at high trafficlevels, a higher level of service may be expected,requiring more frequent grading. More guidance on thechoice of grading frequency is given in Overseas RoadNote 1.

6.2.2 Crossfall

Earth and gravel roads require steeper crossfalls thanbituminous surfaces if rainwater is to be shed satis-factorily. If the grader operator has been trained onconstruction work for paved roads, it will probably benecessary to ensure that he understands the differentrequirements for unpaved surfaces. Crossfall on graveland earth roads should be between 1 in 25 and 1 in 15(4-6 per cent). It is very important to ensure correctcamber on steep alignments. 'Flat' cambers are frequentlythe cause of the longitudinal gullying commonly foundon such alignments.

Crossfall should be checked on site using a simplecamber board, such as that illustrated in Fig. 8 which canbe carried on the grader. Use of the camber board isillustrated in Fig. 9. It should be placed on its edge acrossthe road with its narrower end pointing towards thecentre line. If the level bubble is central, then the camberis correct. Checks

should be made at approximately 100 metre intervalsalong the road and if the camber is too steep or too flat,then the road must be graded again.

6.2.3 Steep hills

Steep hills on unpaved roads, where the longitudinalgradient is steeper than the crossfall, are prone to severeerosion in the wheel paths, particularly when thesecoincide in the centre of the road. Considerable attentionmust be paid to maintaining adequate cross-fall in thesesituations as this will minimise the erosion. If severedamage persists, consideration should be given to pavingthe gradient either by surface dressing or preferably witha concrete pavement.

6.2.3 Grader operation

The quality of workmanship in maintaining earth andgravel roads depends to a great extent on the skill andjudgement of the individual grader operator. Carelessoperation can cause extensive damage to a road, forexample by flattening the crown so that rainwater is notdischarged, by cutting too deeply in dry weather or byblading plastic material from side drains on to thecarriageway.

The Maintenance Engineer must get to know the abilityof each grader operator and should find out the capabilityof each type of grader in use in his District. Themanufacturers of graders publish manuals which explainthe correct use of their machines in various applicationsand these should be obtained and studied by theEngineer. A particularly useful document is GradingIllustrated (Aveling-Barford, undated). With thisinformation he will be better able to explain hisrequirements to grader operators and to ensure that theyare carried out.

Arrangements must be made for graders to be checkedmechanically, greased and fuelled before each day'swork. The Maintenance Engineer must make operatorsunderstand the need for preventative maintenance of theirmachines and must try and ensure that equipment is notmishandled, misused or abused by operators in

Fig.8 Five per cent camber board

Fig.9 Use of camber board

Fig.10 Transition of camber on approach to bends

the field. This can only be achieved by proper trainingand regular site visits by the Engineer. Arrangementsmay also need to be made to refuel graders on the job ifnecessary. This may be done either with a refuellingtruck visiting the working site, or with an arrangement torefuel at a maintenance camp.

For heavy grading, the grader works on one side of theroad at a time, if at all possible, and works in passes ofabout 200 metres in length. Graders must not stop onjunctions or on bends where they will be a danger totraffic. On straight stretches of road the operator shouldaim to develop a crown on the road. The surface shouldbe cambered to fall away from the crown with a slope of4-6 per cent. The shape of the road must be maintainedacross culverts but, on sharp bends, the surface must besuperelevated and must be flat from shoulder to shoulderwith the outer shoulder higher. Any crown on a bend canbe very dangerous to traffic. On the transition from bendsin the road to straight sections, the camber on the outsidelane should be gradually reduced until the normal cross-section shape is obtained again (see Fig. 10).

For heavy grading, it is important to cut to the bottom ofsurface defects and, if the road surface is hard, thegrader's tines should be used to loosen the material. Thegrader should start from the edge of the road and worktowards the centre. Gravelled shoulders should be treatedas part of the running surface. The first and secondpasses cut to the bottom of the surface irregularity anddeposit a windrow just beyond the centre line. If water isto be added, then the water tankers should spray the roadat this point. The windrow is then spread back across theroad depositing all the material on the carriageway togive the correct camber. The material may need to besprayed again with water during this operation. After thecamber has been checked, the other side of thecarriageway is graded in a similar way to complete thework and leave a smooth even surface. This is illustratedin Fig. 11.

It is essential that the grader does not make a final passdown the centre of the road with the blade horizontal.This flattens the centre of the road and causes water topond. This leads to rapid deterioration of the surface.Windrows must not be left in the middle of the roadovernight as this is a danger to traffic.

If compaction equipment is available for use, it mustfollow up closely behind the grader but must only workon sections where grading has been completed to avoidinterference with the grading operation. Rolling shouldstart at the edge of the road and work towards the middle.Providing that the work is carried out in the wet season,watering of the road will not be necessary before rolling.Otherwise water should

20

Fig.11 Grading

be added, as necessary, during the rolling operation togive the correct moisture content for compaction. Therollers should aim to progress from section to section atthe same rate as the graders.

In the dry season, grading is essentially a draggingoperation to remove loose dry material from the surfaceof the road and to fill in potholes and ruts (see Section6.3).

6.2.4 Grading gang

For heavy grading, the grader needs to be powerfulenough to cut to the bottom of the deformation in theroad and a minimum size of 135 horse power is usuallyrecommended. However, if the wearing course does notcontain an excessive amount of oversize material, atractor-towed grader could be used instead. The operatorwill normally be assisted by a machine attendant whohelps direct traffic and grader turning, and removes largestones and other unwanted material from the path of thegrader. He should work well ahead of the grader to stopit being delayed. The grader should carry a camber boardand traffic signs as indicated in Section 3.2. Rollersshould be used if available and water tankers should beused in conjunction with rollers if necessary.

For light grading, up to three graders can be used as thisis more efficient for supervision, maintenance andrefueling. The graders can be less powerful than thoseneeded for heavy grading. Traffic signs should be used asdescribed in Section 3.2.

All graders should be fitted with yellow flashing warninglights, and these plus headlights should be switched onwhen the graders are working.

6.3 DRAGGING AND BRUSHING

Regular and frequent dragging can be used, in the dryseason, to delay the formation of corrugations on earthand gravel roads by removing loose material from thesurface. Dragging will not remove severe corrugationsonce they have formed, nor will it restore camber or lostmaterial. These defects must be corrected by heavygrading.

6.3.1 Design of drag

Typical drag units are shown in Fig. 12. The first consistsof a metal 'A' frame constructed from 100 x 65 x 3mmchannel on to which are bolted used grader blades. Theleading one of these is angled to the direction of travel.The second unit is constructed from 100 x 75 x 3mmchannel, but has additional blades for re-distributing thewindrow. The approximate weights of these units are 250and 375 kg respectively and they are relatively cheap tomanufacture. Many other forms of drag have been usedin various countries, such as railway rail, roIled steeljoists, tolards, timber baulks, etc. However, the two typesillustrated in Fig. 12 are recommended because of theirproven performance in quantified field trials and theirease of manufacture. They should be towed at speedsfrom 5-8 kilometres per hour and are capable ofmaintaining roads carrying up to about 100 vehicles perday. They are also effective for dealing withcorrugations.

6.3.2 Brushing

Brushing is generally effective only on very lightlytrafficked roads with surfaces containing loose material.Typical tractor drawn brushes constructed from locallyavailable material are shown in Fig. 13. The actualbrushes used on the first type illustrated are made frombrushwood tied tightly together. These must not be madeout of old steel cable which is dangerous if broken offand left lying on the road. The tyre sledge illustratedconsists of old tractor or heavy lorry tyres cut in halfaround their circumference and bolted or chainedtogether. It is important that all the tyres are in contactwith each other, as in the illustration, to ensure properdistribution of the loose material. Brushes made out oftrees dragged behind a vehicle are not very effective forredistributing loose material on the road.

6.3.3 Method of operation

The basic method of operation for both dragging andbrushing is the same.

The frequency with which dragging should be carried outdepends on the traffic loading, the rate of development ofcorrugations and the soil type. A road carrying 100vehicles per day may need to be dragged every twoweeks using the metal drags illustrated in Fig. 12. Onedragging pass will probably be necessary every 3 to 4weeks for roads carrying 50 vehicles per day and everyfour to six weeks for traffic levels of 25 vehicles per day.Simple experiments should be carried out by theMaintenance Engineer to determine the optimumfrequencies for different conditions.

The drag should be designed where possible so that itswidth adequately covers half the road. This enables themaintenance to be carried out with a single pass in eachdirection.

For the best results, four main adjustments can be madeaccording to the conditions; namely depth of cut, angleof cutting blades relative to direction of traffic, towingangle of drag and weight of drag. The drags illustrated inFig. 13 have height adjustments at each end of the cuttingblades and the position of the blades on the drag can alsobe changed to obtain the required volume of windrowedmaterial. Varying the towing angle between tractor anddrag can also achieve different volumes of windrows, butthis is more difficult to control. The depth of cut can alsobe varied by weighting the drag. The level of surfaceroughness and size of the gravel wearing course willlargely dictate the optimum weight of drag. However, thedesigns in Fig. 13 can still be utilised by changing thethickness of the channel sections.

The tractor or grader towing the drag should always workin the same direction as traffic and should not stop onjunctions or on bends. Drags should be towed at speedsof 5-8 km/h depending on the type of drag and on thecondition of the road surface. Care must be taken not todrive too fast or tile drag will skip over the surfaceirregularities and will also generate a lot of dust. Withbrushes made of thorn scrub or brushwood, the operatormust ensure that pieces which may break off the drag arenot left lying on the road surface where they will be adanger to following traffic. Pass lengths should be aslong as possible, preferably of the order of severalkilometres.

6.3.4 Dragging gang

Dragging can be carried out by a small crew of a tractordriver and/or grader operator and a machine attendant,depending on the availability of equipment. It is most

21

Fig.12 Metal cutting drags

Fig.13 Types of brush drag

efficient to use several graders working in a team, onebehind the other, spreading across the whole width of theroad.

Machinery should, where possible, be fitted with flashingyellow warning lights, and these plus headlights shouldbe switched on when working. If warning lights are notavailable, machinery should carry flags. Traffic signsshould be used as described in Section 3.2.

6.4 REGRAVELLING

6.4.1 The task

The surfacing material of gravel roads is worn away bytraffic, eroded by rain and blown away as dust. Wherethis occurs the subgrade will be exposed particularly inruts and depressions. Before all the material has been lostand the subgrade loses shape and gets damaged, the roadrequires regravelling. Regravelling is also used to correctloss of shape, ruts, potholes and erosion gullies, whenthese have become severe.

Before regravelling work is carried out, it is important tomake any necessary repairs or improvements to thedrainage system of the road. If this is not done, the newgravel surface will deteriorate very quickly.

6.4.2 Quality of gravel

Most Roads Departments have standard specificationsfor gravels for surfacings (and for bases, where these areused). In practice, what is used will depend largely onwhat is available, and it may be necessary to use lower-grade material than is commonly specified. In dryclimates, a fairly high proportion of clay binder isdesirable to prevent the surface from ravelling andbecoming corrugated. In wet climates, the presence ofclay in the material is a disadvantage since it makes thesurface slippery and prone to soften and rut under traffic.Suggested specifications in terms of grading andplasticity characteristics are given in Tables 3 and 4.

6.4.3 Organisation and equipment

Regravelling will be the major item of expenditure on themaintenance of gravel roads and its organisation shouldbe carefully planned to ensure maximum efficiency.

The following will provide a basis for an estimate of theplant required for regravelling:

Gravel production: 1 bulldozer1 loading shovel1 grader8 tipping lorries 1 6/8 tonne steel-wheeled roller and6 labourers 1 pneumatic-tyred roller

24

TABLE 3

PARTICLE SIZE DISTRIBUTION FORGRAVEL SURFACINGS

Percentage passing (*)Nominal maximum sizeB S sieve

size 37.5mm 19mm 9.5mm 37.5 19 9.5 4.75 2.36 425µm 75µm

10080-10055-8040-6030-5015-305-15

-100

80-10060-8545-7025-4510-25

--

10080-10050-8025-4510-25

(*) Not less than 10% should be retained between eachpair of such successive sieves specified for use,excepting the largest pair.

TABLE 4

PREFERRED PLASTICITY CHARACTERISTICSFOR GRAVEL SURFACINGS

Climate

LiquidLimitnot to

Exceed(%) (*)

PlasticityIndexrange

(%) (*)

LinearShrinkage

(%)Moist tropicaland wet tropical

Seasonal wetTropical

Arid and semi-arid

35

45

55

4 – 9

6 – 20

15 – 30

2 – 5

3 – 10

8 - 15

(*) Higher limits may be acceptable for some lateritesor concretionary gravels that have a structure that isnot easily broken down by traffic. Lower limits maybe appropriate for some other gravels that are easilybroken down by traffic. Any variation from theselimits should be based on carefully collated localexperience.

Production can be estimated as 450-500m3/day on a 5kilometre average haul. If the haul exceeds 5 kilometres,additional lorries should be provided so that the otherplant is fully utilised.

Regravelling: 1 grader1 6/8 tonne steel-wheeled roller and1 pneumatic – tyred roller

2 water tankers, if water is available1 water pump1 lorry

20 labourers

Production can be estimated as 300-350m3/day.

To provide 100mm of gravel on one kilometre of road7.5 metres wide will require 750 m3 of gravel (compactedvolume) so that the above team could regravel about halfa kilometre of road per day.

Well in advance of the work, a start should be made tostockpile gravel at the borrow pit or quarry. In addition,arrangements must be made to obtain water close to theregravelling site. Whenever possible, a bulldozer orgrader should open up a diversion track adjacent to theroad. If a traffic diversion can be opened adjacent to thework site, it will enable the job to be carried out moreefficiently and more safely. If traffic is heavy, thediversion may need to be gravelled and a grader assignedto keep the surface in good condition. After the diversionhas been completed and before the work starts, warningsigns, barriers and cones must be erected around thework area as described in Section 3.5. If it is not possibleto open a diversion, 'lane closed' signing must be used asdescribed in Section 3.3.

6.4.4 Spreading gravel

It is advisable to reshape the existing surface beforeplacing additional material. If this is not done it is likelythat existing deformation will be reflected in the newsurface. A hard surface should be scarified with a graderto a depth of about 50 mm to ensure a good bondbetween the new and existing material. The edges of theroad should be 'boxed-out' to provide support for the newmaterial. The camber of the graded surface should bechecked to ensure that it is between 4 and 6 per cent.

At the quarry, the tippers should be loaded for transportto site. The supervisor at the quarry should ensure thatgravel is taken from the right place and that the trucks areloaded correctly. Tippers circulate continuously betweenthe quarry and the site. Usually the gravel is supplied inadvance and tipped in heaps on one side of the road atthe correct spacing to give the required thickness ofmaterial when spread across the road. If the diversion hasnot been opened, material must be tipped onto theshoulder and warning signs placed at either end. If thegravel is fairly moist, this will not cause any seriousproblems, but dry gravel is likely to segregate during theloading, tipping and subsequent spreading operations. Abetter method of spreading gravel is to use a spreaderbox towed by a lorry. This is much faster than spreadingusing a

grader, but does require a continuous feed of lorries orthe method becomes very ineffcient.

Ideally, the moisture content of new gravel should beadjusted to optimum for compaction. In practice, thismay be impracticable in view of the cost of providing,transporting and applying large amounts of water.However, it is usually possible to take advantage of theclimate and to carry out regravelling work at thebeginning of the dry season when the natural moisturecontent of gravel from borrow pits is close to optimum. Itis important that the work is planned to obtain maximumbenefit in this way. If additional water is required andavailable, the tankers should spray the road initially,before the new gravel is spread. The new gravel is thenspread right across the road using the grader. The newmaterial is alternately spread by the grader and wateredby the tanker until its moisture content is correct forcompaction. The tankers circulate continuously betweenthe site and the source of water.

Once the material has been spread evenly across the roadand it is at the correct moisture content, it should begraded to shape as described in Section 6.2.3. Finally, thecamber should be checked with the camber board and, ifthe required standard has not been reached, the gradingshould be repeated.

6.4.5 Compaction

Compaction should not be left to the action of traffic asthis quickly becomes concentrated in the wheeltracks andleads to deformation of the road. Rollers should be usedif available as, even though it may not be possible toachieve full compaction, the limited compaction obtainedwill improve the quality of the surface. Four passes of aroller will give a worthwhile degree of compaction atoptimum moisture content, while eight or more will beneeded to bring relative compaction up to that required.

Compaction should start as soon as the grader hasfinished a section. The rollers should start at the edge ofthe road and work towards the centre and continuerolling until full compaction has been achieved. Thisshould be organised to finish at the same time as thegrader finishes the next section. It is essential to checkthe thickness of the compacted layer.

6.4.6 Continuous working

The work should continue along the road in sections. Aseach section is completed, the traffic signs, cones andbarriers are moved along the road to the next section.This opens the road at the completed

25

end for traffic, and closes it at the other end to allow newgravel to be dumped. As the work proceeds, it will benecessary to open new diversions.

6.5 FILLING AND PATCHING

These are manual operations which can be used forrepairs to the surface where defects develop on a smallscale and heavy grading or regravelling is not justified.They can also be used when equipment is not available.They can be used to repair pot-holes, ruts, soft-spots anderosion gullies. The operations are sometimes known asspot regravelling. Patching may also be needed on self-cementing gravels or gravels containing large lumps ofmaterial as, in these cases, grading will only cause moredamage to the surface. Filling and patching are notsatisfactory methods of repairing corrugations anddragging or grading is needed to remove these. Wherethere are large numbers of pot-holes, the road will needheavy grading and possibly regravelling.

The quality of the material used should be the same asthat used for regravelling (Tables 3 and 4). It should bestockpiled at the nearest maintenance camp or dumpedby the side of the road near where it will be used.

Before work starts, signs must be set up as described inSection 3.3. Loose material and standing water shouldthen be brushed from the area to be repaired. large ordeep pot-holes should have their sides cut back to bevertical and should be deepened to reach sound material.If the material is dry, the area to be repaired should besprinkled with water and it is then also useful to mix thepatching material with water as well. The patch shouldthen be filled in layers of about 50-70mm at a time. Eachlayer should be compacted with hand rammers or withsmall vibrating compactors. It is not advisable to rollwith the wheels of the truck or tractor as insufficientcompaction can be obtained in this way. The layers of thepatch should be built up in this way and, finally, thepatch is filled with gravel to approximately 30mm abovethe level of the road surface and is spread and raked tothe correct shape. The patch is then compacted to give asurface which is slightly above the level of thesurrounding road. Both large and small areas are repairedin the same way.

Patching work started must not be left unfinished over-night. At the end of each day, tools and traffic signsshould be taken back to the maintenance camp and thesite must be left clean and tidy with no stockpiles ofmaterial left on the road.

Details of the maintenance gang and equipment neededfor filling and patching work are given in Section 5.8.The District Engineer will need to modify this basic unitto meet his own local conditions and for differentsituations.

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6.6 DUST PREVENTION

6.6.1 The dust problem

In the dry areas of many developing countries, unpavedroads are often affected by dust. This may have beengenerated by the interaction between vehicular tyres andthe unbound surface or by the effect of wind. Dust is amaintenance problem because it results in the loss ofmaterial from the road surface which has to be replaced.It is a contributory factor to road accidents because of thereduction in visibility and it also pollutes the atmosphereclose to the road.

It has been shown that well constructed and adequatelymaintained unpaved roads carrying 100 vehicles per dayin dry areas can lose over 25 tonnes of dust per kilometreeach year. In addition, it has been found that, when thewearing course contains an excessive amount of materialfiner than l0mm, the annual dust loss can be greater than33 tonnes per kilometre. Losses on older roads withlower levels of maintenance will be much higher. Thisloss of material increases the permeability of the surfacelayer and results in the early development of pot-holes,all of which accelerate the need for regravelling.

6.6.2 Remedial treatment

The major types of remedial treatment for dust proofingunpaved roads are as follows:-

(i) application of a bituminous seal

(ii) chemical stabilisation and coating withdeliquescent salts

(iii) application of waste local materials such asoiled gravels, sulphur liquors, molasses,palm oil, vegetable oil, bamboo oil, lime,charcoal, etc

(iv) addition of water.

With the exception of the bituminous seal, all of thesetreatments provide only temporary improvements. Manyof the additives are soluble in water and will requirerenewal at the end of the rainy season. Apart from theuse of local materials, most treatments are alsoexpensive.

If dust treatment is being considered, careful costingsshould be carried out to ensure that the costs of repeatedapplications of the additive over several years arecheaper than the more permanent treatment by surfacedressing.

7. MAINTENANCE OFPAVED ROADS

7.1 TYPES OF MAINTENANCE

Maintenance operations on paved roads can be dividedinto those used to repair local failures and those used toimprove the whole carriageway.

Local surfacing failures such as cracking (withoutrutting), stripping and fretting may be repaired by localsealing. Fatting-up of bitumen over small areas is not aproblem unless the bitumen is 'bleeding' when it shouldbe repaired by sanding. Localised structural failure,where there is rutting and cracking, or pot-holing shouldbe repaired by patching.

Whilst surface dressing is essentially a waterproofingoperation some minor surfacing defects such as loss oftexture, fine cracking and fretting can often be correctedby this process. An alternative to surface dressing isslurry sealing. This is almost always carried out byspecialist contractors and only brief notes are given on ithere. It is usually more costly than surface dressing. Sandsealing is a form of surface dressing, but it should onlybe used where there is a shortage of aggregate and anabundance of sand. Its use is not recommended in mostcases and no notes are given on it here.

Reflection cracking sometimes occurs on roadsconstructed with cement or lime stabilised bases. It ischaracterised by a regular rectangular crack pattern andis caused by shrinkage of the base. It is not a sign offailure, but the surface should be sealed to keep outwater. If the cracks are widely spaced (greater than 1metre), they can be sealed with a bituminous binder. Ifthey are closely spaced, it is probably better to surfacedress.

Where there is extensive structural failure, the road willneed to be overlaid or reconstructed. For overlays to beeconomical, they must be placed when the road is in a'critical' condition and before complete failure hasoccurred. Such major repairs are very expensive and usetechniques more similar to construction thanmaintenance. Their use is beyond the scope of this Noteand they are not described.

7.2 LOCAL SEALING

This may be used to seal relatively small areas ofsurfacing failure where the size of the works does notjustify using a bulk bitumen distributor (eg. lengths ofroad less than 100 metres long). The method used willneed to take account of the equipment, materials and menavailable to do the work.

Frequently the process relies on the manual applicationof binder using watering cans, hand-pumps and lances,

etc. Such considerations automatically limit the range ofbinders which can be used to bitumen emulsions and lowviscosity cut-backs. Control of the bitumen applicationrate will often be poor and requires careful super-visionby an experienced foreman. Uneven distribution ofbitumen is a common fault when it is applied using hand-lances. Typical productivities for hand-sprayingemulsion are 600-1000 litres per day for a unit of threemen with one sprayer.

Cover aggregate should be of a small nominal size suchas 6 mm chippings or coarse sand and should be lightlyrolled.

This work should be carried out by the patching gang(see Sections 7.4.3 and 5.8).

7.3 CRACK SEALING

This is used where reflection cracking has occurred andthe aim is to fill the cracks as completely as possible withbituminous binder to keep out water.

The method and materials used will depend on theequipment available for the work. Viscous binders willneed a hand-lance with a relatively fine jet capable ofgetting the binder into the cracks. The use of a wateringcan for viscous binders is not effective, besides beingwasteful and untidy. If sophisticated spraying equipmentis available, it should be capable of filling cracks withpenetration grade binders such as 80/100 or 60/70. Lesseffective equipment can only use bitumen emulsion forsealing the cracks. Sealed cracks may be blinded withquarry fines.

This work should be carried out by the patching gang(see Sections 7.4.3 and 5.8).

7.4 PATCHING

7.4.1 Procedure

The need for patching is usually the first sign of majorpavement deterioration. It may be carried out on arecurrent basis or prior to the application of a surfacedressing or overlay. The cause of the local failure thatrequires patching will often be related to problems of on-road or off-road drainage, and it is most important thatthese defects are identified and remedied before thepatching operation begins.

With all patching work, it is important to remove thefailed area entirely and to cut the road back to soundmaterial. The sides and bottom of the patch should besquared-off to provide a firm coherent surface. Wherebituminous patching material is to be used, the excavatedand trimmed area should be carefully brushed, moistenedslightly with water and painted with bitumen emulsion orrapid-curing cut-back so as to provide a good bond withthe in-filling material.

27

Recommended materials for patching are described inSection 7.4.2. The material should be compacted into thehole in 50-70mm layers using hand-rammers or a smallvibrating roller. The surface of the completed patchshould be slightly higher than the road surface so as topermit final compaction by traffic. The patch should besealed using bitumen emulsion or cut-back and blindedwith sand, crushed rock fines or small chippings.

7.4.2 MaterialsThe materials used for patching will depend on thosepresent in the existing road. Recommendations are givenin Table 5.

TABLE 5

RECOMMENDATIONS FOR PATCHINGMATERIALS

Layer Existing material Repair materialGravel Gravel of at least as

good quality as theoriginal

Base

Cement stabilised,bitumen stabilised,crushed rock orbitumen macadam

Graded crushedrock plus fines, soilcement or premixmacadam

Surface dressing(new, thin)

Seal with binder andsmall chippings

Surface

Surface dressingswhich have built up toseveral centimetresthickness and premixsurfaces

Premix macadamplus seal of bindercovered with sand,crushed rock finesor small chippings

Specifications for suitable gravels, which may be usedfor patching, have been given earlier in Tables 3 and 4.These are also suitable for stabilisation with 5-10 percent of cement. If crushed rock is used for the patchingmaterial, gradings can be again taken from Table 3 oralternatively from Table 6.

TABLE 6

ALTERNATIVE GRADING FOR CRUSHED ROCKPATCHING MATERIAL

BS sieve % passing

37.5 mm201052.36600µm75µm

10080-10055-8040-6030-5015-30

5-15

28

Premix macadam should comply with the requirementsgiven in Table 7.

TABLE 7

REQUIREMENTS FOR PREMIXED MACADAMUSED FOR PATCHING MATERIAL

14mm nominal sizeBS sieve(mm) % passing

201410 6.3 3.3575µm

10090-10055-7525-4515-252-6

Binder content 4 – 5%

The binder to be used for premix patching material willusually be a soft penetration grade bitumen or a fairlyviscous medium-curing cut-back, such as MC800 orMC3000, depending upon local conditions and thepossible need to stockpile patching material. Premixmacadam patching material must be mixed using apugmill-type mixer. This may be at a central mixingplant where quality can be most easily controlled,although purpose-built mobile mixers are now availableto permit cold or hot-mixed material to be manufacturedon site.

The mixes described above are adequate for small-scalepatching work but, for larger-scale work. it is desirable touse higher-grade materials under close quality control.

7.4.3 Patching gang

Details of the maintenance gang and equipment neededfor patching work are given in Section 5.8. The DistrictEngineer will need to modify this basic unit to meet hisown local conditions and for different situations.

7.5 MECHANISED SURFACE DRESSING

7.5.1 General principles

Surface dressing consists of spraying the road surfacewith a film of binder followed by the application of alayer of stone chippings which is then rolled. Surfacedressing has three main purposes:

(i) to seal the road surface against water,(ii) to prevent disintegration of the surface,(iii) to provide a non-skid wearing surface.

Surface dressing will not restore the riding quality ofmis-shapen roads nor will it significantly strengthen theroad structure.

The use of mechanised surface dressing methods offersimportant advantages over manual methods. Mechanicalbitumen sprayers allow the close adjustment of rates ofapplication so that these can be accurately controlled andthe adverse affects of excessive, insufficient or variableamounts of binder can be avoided. The rate of progressof a mechanised unit is much higher than can beachieved by manual methods.

A separate document (TRRL Overseas Unit, 1982) givesdetailed guidance for specialist surface dressingoperations.

7.5.2 Design of surface dressing

The design should take into account the type of existingroad surface, the traffic, the available chippings and theclimate.

7.5.2.1 Type of existing road surface Beforedeciding on the specification for the surface dressing, asite visit is essential to assess the condition and hardnessof the old road surface. For all practical purposes,surfaces may be divided into the following categories:-

TABLE 8

ROAD SURFACE TYPES

Very hard Surfaces such as concrete or exceptionallylean bituminous mixtures with dry stonysurfaces into which there will be negligiblepenetration of chippings under very heavytraffic.

Hard Surfaces containing some hard bituminousmortar into which chippings will penetrateonly slightly under heavy traffic.

Normal Surfaces into which chippings will pene-trate moderately under heavy and mediumtraffic.

Soft Surfaces into which chippings willpenetrate considerably under heavy andmedium traffic.

Very soft Surfaces into which even the largest chippings will be submerged under heavytraffic. Such surfaces are usually rich inbinder.

7.5.2.2 Traffic categories The volume of trafficthat the road is required to carry is an important factor inthe design of the surface dressing. Heavy vehicles embedchippings into the road surface and the amount of thisembedment depends on the number and weights of thesevehicles. Traffic should be considered in terms of thenumber of commercial vehicles a day in the lane underconsideration. Lane traffic categories are shown in Table9.

TABLE 9

LANE TRAFFIC CATEGORIES

Category

Approximate number of vehicles withunladen weight greater than 1.5 tonnescarried per day in the lane underconsideration

12345

Over 20001000-2000200-100020- 200

Less than 20

7.5.2.3 Chippings The chippings should comply inall respects with the requirements of BS 63:Specification for single sized roadstone and chippings(BSI, 1971), the main points of which are summarised inAppendix A. Samples of the chippings to be used shouldbe tested before the start of the work and subsequently asnew deliveries are received. The sampling and testingshould be in accordance with the methods described inBS 812 (BSI, 1975).

The size of chippings should be chosen to suit theamount of traffic and hardness of the existing roadsurface, as given in Table 10.

The quantity of chippings needed must be sufficient tocover the entire surface of the binder film after rolling.The most reliable way of ensuring a complete cover is tolay a slight excess of chippings. The rate at whichchippings should be spread depends on their size, shapeand specific gravity, but rates can be estimated using Fig.14.

The least dimension of at least 200 chip pings should bemeasured, using the method described in Appendix A,and the 'Average Least Dimension' (ALD) determined.The Average Least Dimension is then entered in Fig. 14to intersect line AB and the approximate rate ofapplication of chippings can be read off. This rate shouldonly be used as a guide for supply purposes; the actualrate of spread should be adjusted as required on sitewhen the spreading characteristics of the chippings canbe observed.

7.5.2.4 Binder Surface dressing binders should havea viscosity of between 104 and 5 x 105 centistokes at theprevailing road temperature. At higher viscosities, stonewill not be wetted by the binder and will be lost by whip-off. At lower viscosities, the binder will be too fluid tohold the stone. Figure 15 shows the viscosity/ roadtemperature relationship for a wide range of binders. Daytime road temperatures in the tropics range generallyfrom 300C to 700C, from which it will

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TABLE 10

RECOMMENDED CHIPPING SIZE (mm)

Lane traffic category

Approximate number of commercial vehicles currently carried per dayin the lane under considerationType of surface

(1)Over 2000

(2)1000-2000

(3)200-1000

(4)20-200

(5)Less than 20

Very hardHardNormalSoftVery soft

10 mm1420****

10 mm141420***

6 mm10141420**

6 mm6

101414

6 mm66

1010

* Unsuitable for surface dressing.** 20 mm chippings may be used for remedial treatment where traffic speeds are low. Very particular care should be

taken when using 20 mm chippings to ensure that no loose chippings remain on the surface when the road is openedto unrestricted traffic as there is a high risk of windscreen breakage.

Note:The size of chippings specified is related to the mid-points of lane traffic category ranges 2-5: lighter traffic conditions maymake the next smaller size of stone more appropriate.

TABLE 11

CONDITION CONSTANTS

Traffic

Very LightLightMediumMedium HeavyHeavyVery Heavy

Vehicles/day

0-5050-250

250-500500-1500

1500-30003000+

Constant

+3+10-1-3-5

Types of Chippings

Round/dustyCubicalFlakyPre-coated

Constant

+20-2-2

Existing surface

Untreated/primed baseVery lean bituminousLean bituminousAverage bituminousVery rich bituminous

+6+40-1-3

Climatic conditions

Wet and coldTropical (wet and hot)TemperateSemi-arid (dry and hot)Arid (very dry and very hot)

+2+10-1-2

be seen that the most appropriate binders are MC 3000 orpenetration grades up to 80/100. The choice may berestricted in practice either as a result of Departmentalpurchasing policy or by what is commercially available.

In order to design the rate of application of binder, anappropriate constant should be selected from Table 11for each of the four sets of conditions listed. The fourconstants are then added together to give the appropriatefactor.

30

The Average Least Dimension of the chippings and thefactor obtained from the condition constants in Table 11are then entered in Fig. 14 to give the rate of applicationof binder.

7.5.3 Equipment

Few districts will have a length of bitumen-surfaced roadlarge enough to justify a permanent, full-time surface

Fig. 14 Surface dressing design chart

*1 For slow traffic or climbing grades steeper than 3 per cent, reduce the rate of spreadof binder by 10 per cent

2. For fast traffic or down grades steeper than 3 per cent increase the rate of spreadof binder by 10 to 20 per cent

Fig.15 Surface temperature/choice of binder for surface dressing

dressing unit. It is more likely that a mobile unit,comprising the specialised machines and operators, willmove from district to district in accordance with acentrally-planned schedule, with non-specialised plantand labour provided by the district in which the unit isworking. However, a large part of the work involved inorganising and carrying out the surface dressingoperation will fall to the District Engineer. The operationof a mechanised surface dressing unit will probablyrepresent the largest single item of road maintenanceexpenditure and should receive corresponding attention.

A typical surface dressing unit might comprise:

1 bitumen distributor (capacity 5000-8000litres) with pressurised spray bar and oil-fired heaters

1 or 2 bitumen preheaters, oil-fired

8 or more tipper lorries (supplied by District)

1 or 2 chipping spreaders, if lorries are not fittedwith spreaders

2 rubber-tyred rollers

1 loading shovel (unless hand loading is used).

In determining the make-up of the surface dressing unit itis important to keep a balance between the various itemsof equipment so that each is utilised to the maximumpossible extent. A common fault is that there are too fewlorries hauling chippings to keep up with the output ofthe bitumen distributor. Spraying must stop when nochippings are available for spreading.

Such a unit is potentially capable of single-coat applica-tion at rates of up to two kilometres of 2-lane road perday, using some 20,000 litres of bitumen and 200 tonnesof chippings. In practice progress would fall short of thisas a consequence of imbalances in equipment,interference by traffic and non-productive time, but therate of use of materials will still be very high and it isimportant to ensure that stockpiles are adequate both inextent and location.

7.5.4 Application

7.5.4.1 Preparation of the surface A surfacedressing does not contribute directly to the strength of aroad pavement, neither does it improve the riding qualityof the existing surface. Therefore it is essential that theunderlying material has sufficient strength to withstandthe expected traffic loading and is also within thetolerance required for the level of the final runningsurface.

Any area of the road showing signs of structural weak-ness should be strengthened and any depressions in the

surface should be filled. Patching should be done well inadvance of the surface dressing. For best results, thisshould be completed during the previous season toensure that as uniform a texture as possible is avail-ablefor the subsequent dressing.

It is important that the surface to be treated is clean. Dustand loose aggregate must be swept from the surface sothat the binder film can adhere easily.

7.5.4.2 Application of binder The final quality ofthe surface dressing will depend largely on the efficiencyof the method of application of the binder, which must beapplied uniformly over the road surface. The mechanisedbitumen distributor must therefore be fitted with meansof controlling the rate of spread of binder longitudinallyon the road. In addition, the transverse rate of applicationacross the spray bar should be checked at least once ayear in the depot.

Because the distributor is mounted on springs, the spraybar will tend to rise as the load of bitumen in the tankdecreases. The correct spray pattern is dependent on thespray bar being kept at the correct height above the roadsurface and the spray bar must therefore be adjustable inheight, under the control of the operator.

The amount of binder actually sprayed for any single runof the distributor must be within plus or minus 10 percent of the amount calculated by multiplying the areasprayed by the specified rate. In addition to checking theaverage rate of spread over a substantial area of road bymeans of readings from the dipstick fitted to thedistributor, the actual rate of spread on the road should bedetermined by means of a tray test as described inAppendix A.

7.5.4.3 Spraying temperatures Thermometersmust be fitted to the bitumen distributor to measure thetemperature of the binder delivered to the spray bar. Thespraying temperature for any given grade of bitumen willbe related to the design of the spray bar nozzles. It isgood practice for a thermometer also to be fitted to thespray bar as inaccurate readings are likely to occur whenthermometers within the tank become exposed as thelevel of binder falls.

The application temperatures for different grades ofbitumen are given in Table 12 for each type of jet.Whirling spray jets atomise the hot binder and expel it asa finely divided spray in the shape of a hollow cone. Ahood is required to eliminate wind effects. No specialsetting is required for these jets and the transversedistribution is fairly insensitive to the height of the barabove the road. An unexpected characteristic of these jetsis that the rate of spread is increased as the temperatureof the binder falls. Slotted jets expel

33

the hot binder in a thin fan-shaped film and consequentlya hood is not required over the bar to protect it fromwind. Slotted jets need to be set with the slots at an

TABLE 12

SPRAYING TEMPERATURES

Spraying temperature

Whirlingspray jets

°CSlot jets

°CCutback gradesMC30RC/MC70RC/MC250RC/MC800RC/MC3000

Penetration grades400/500280/320180/20080/100

50-6065-80

95-115115-135135-155

160-170165-175170-190180-200

40-5055-7080-90

105-115120-130

140-150150-160155-165165-175

angle to the horizontal of 15 to 30° so as to permitoverlap of adjacent fan patterns. The slots in these jetsare not easily blocked and have a fairly high rate ofdelivery thus the road speed of the machine whenspraying is higher than that of a machine fitted withatomising, whirling spray jets.

Because of the inflammable nature of the solvent used inRC-type cutbacks, application temperatures for RCgrades should be restricted to the lower parts of theranges given in Table 12. No smoking or naked flamesshould be allowed when heating, pumping or sprayingany cutbacks. Fire extinguishers should always be readilyat hand.

7.5.4.4 Jointing strips It is most important toensure that the thickness of the binder film is not reducedat longitudinal joints. There must be a clearunderstanding of the jointing technique appropriate to thetype of distributor used.

For example, on some machines the intensity ofapplication falls away towards the edges of the stripbeing sprayed. Adjacent strips should then be overlappedsufficiently to give the required uniformity of spread.This means that the first strip laid must be left uncoveredby chippings for a width of several centimetres along theedge to be overlapped.

Other types of machine are designed to apply the fullthickness of the binder film to the extreme edge of thestrip. With these, great care must be taken to make

34

a good butt joint as an inaccuracy will lead to chippingsnot adhering.

A good clean transverse joint may be obtained by the useof a strip of building paper spread across the road onwhich to start and stop spraying.

7.5.4.5 Application of chippings Chippingsshould be applied uniformly over the freshly sprayedbinder film by a mechanical spreader which shouldfollow closely behind the sprayer. Greater uniformity ofspread will be achieved with spreaders fitted withmetering devices. Any thinly chipped areas found afterthe passage of a mechanical spreader should be chippedover by hand to obtain shoulder-to-shoulder cover. Anysurplus chippings accidentally spilled on to the roadsurface should be removed.

7.5.4.6 Rolling Rubber tyred rollers are stronglyrecommended for rolling surface dressings. Rollingshould start immediately after the chippings have beenspread. The faces of the chippings in contact with thebinders should be pressed into it, so that maximumadhesion occurs and slow-moving rubber tyres are idealfor this purpose. Steel-tyred rollers tend to crushchippings and to bridge local depressions. If steel rollershave to be used, they should be the lightest available andin no case should their weight exceed 8 tonnes.

7.5.4.7 Aftercare and opening to traffic Slowmoving traffic can help to roll and align the chippingsimmediately after surface dressing, but vehicle speedsmust be restricted to 30 km/h or less until there issufficient adhesion to ensure that the chippings will notbe plucked from the surface. This period of time willvary from a few hours to a day or more, depending on theclimatic conditions and the type of binder employed.Excess chippings should be swept up and taken awaybefore opening the road to unrestricted traffic. Signs maybe used to warn of possible damage to windscreens.

7.5.4 Control

The success of any surface dressing depends to a largeextent on the control of all aspects of the design,application and quality of materials. Table 13 is a check-list of the items that require checking in chronologicalorder. This should be read in conjunction with AppendixA.

7.6 MANUAL SURFACE DRESSING

7.6.1 Applicability

Labour-based techniques for surface dressing may be

TABLE 13

CONTROL OF SURFACE DRESSING

Stage when actionshould be taken Action Required

1. Before start ofjob

(i) Check that any pot-holesor edge-failures have beenrepaired.

(ii) Check grading, shape and(if necessary) the resistanceto polishing of aggregate.

(iii) Ensure that chippings arenot dusty or dirty.

(iv) Check that binder is ofcorrect type and viscosity.

(v) Check availability andcondition of plant.

(vi) Ensure that traffic controlmeasures are adequate.

2. Before startingdaily work

(i) Ensure that the surfaceto be dressed has beenadequately swept andcleaned.

(ii) Check binder temperature.(iii) Test spray bar jets.

3. During work (i) Check rates of spread ofbinder.

(ii) Check that chippingspreading follows closelybehind binder distributorand that coverage is correct.

(iii) Check that rollers followimmediately after thespreading of chippings.

(iv) Check binder temperature atintervals.

3. After surfacedressing

(i) Limit traffic speed whenroad is first opened totraffic.

(ii) Remove surplus chippings.

used on small or remote projects, where the use of plantis too expensive or impracticable. The design principlesand standards described in the preceding section relatingto mechanical methods apply generally to labour-basedwork. The following paragraphs describe differenceswhere these are justified, as well as outlining someaspects of manual techniques.

7.6.2 Chippings

Chippings for surface dressing will normally be smallerthan 20 mm nominal size. Production of these relativelysmall sizes by hand-breaking is uneconomic even at low

wage rates and is not recommended. Hand-screening ofriver gravel, however, may be worth consideringprovided that the product complies with the requirementsnoted in Appendix A. Notes on hand-screening are givenin a separate document (Robinson, 1979).

7.6.3 Binder

For small-scale work it may be more convenient to usebitumen-emulsion, applied at the ambient temperature,rather than cut-back or penetration grade which requirepre-heating. Cationic emulsions are now probably morecommonly used than anionics as they are claimed to bemore effective in depositing a binder film on to acidicaggregates. However, as with anionic emulsions, theprincipal break mechanism is the evaporation of water.Disadvantages common to both types are:

(i) Poor adhesion to dry or dusty surfaces.This may be overcome by slightlydampening the surface before spraying.

(ii) Low effective bitumen content (usually notmore than 60%), which must be allowedfor when calculating rates of application.Emulsions containing up to 70 per centbitumen have been developed, but have tobe heated (to 70-800C) before spraying,thus losing the advantage of use withunheated equipment.

(iii) Low viscosity emulsions tend to drain fromthe crown of the road before the breakoccurs; cover aggregate is therefore poorlyheld at the crown, while excess binderaccumulates at the edges.

(iv) Some emulsions tend to coagulate if storedor transported in unsuitable conditions. Ifemulsion has to be stored for long periods,the drums should be turned overoccasionally.

7.6.4 Application of binder

Bitumen (cut-back, penetration grade or emulsion) maybe applied manually using hand-operated sprayers or bymeans of watering-cans. The choice of binder type willneed to be related at least partly to the applicationequipment available. In either case, the road surfaceshould be marked out in a rectangular grid such that thearea to be covered by each full container (drum or can) isdefined. This may be done by placing stones (about fist-size) at the corners of the area to be covered.

Provided that the work is set out in this way and thebitumen applied systematically, it should be possible toget acceptably uniform rates of application. It will benecessary to supervise the work carefully and to instructoperators in detail in the methods to be used. Unevendistribution of bitumen is a common fault when bitumenis applied by hand-lances.

35

Bitumen for labour-based surface dressing work shouldbe brought to the site in drums and distributed along thework in accordance with the requirements for rate ofspread.

Typical productivities for hand-spraying emulsion are600-1000 litres per day for a unit of three men with onesprayer.

7.6.5 Application of chippings

Chippings should be delivered to the site in dumps atintervals of about 400 metres, from where they are takento the work in wheelbarrows, with an average haul of 100metres. The chippings are usually spread by a swingingmovement with the shovel and this operation alsorequires close supervision if full coverage is to beachieved. It is necessary to apply a greater excess whenspreading chippings manually than when mechanisedspreaders are used, the excess being recovered bybrooming after rolling and trafficking. Productivity forspreading chippings, including haulage from dumps bywheelbarrows, is about one m3 /man-day.

7.6.6 Rolling

There is no practicable alternative to mechanical rolling.The low outputs from labour-based work will result inunder-utilisation of self-propelled rollers, and it may bebetter to consider the use of tractor-drawn rollers. Anyroller should preferably be rubber-tyred, as explainedpreviously.

7.6.7 Gang size

The following gang sizes have been found to bepracticable:

Spraying bitumen 500 m2 /day - 3 men plus 1 sprayerSpreading chippings 500 m2/day - 6 men plus 4 wheel-barrows

Loading/unloading drums, cleaning surface and otherodd jobs - 7 men

Total - 16 men plus 1 foreman.Also required:

(i) periodic use of truck or tractor/trailer fordelivery of chippings

(ii) tractor-drawn rubber-tyred roller.

7.7 SLURRY SEALING

Slurry seals are mixtures of fine aggregates, bitumenemulsions and additional water. When freshly mixed,they have a thick creamy consistency and can be spreadto a thickness of 5 to 10 mm.

The principal application of slurry seals is as a mainten-ance treatment for old bituminous surfaces. Because oftheir low viscosity, they can readily penetrate surface

36

voids and cracks and hold together surfaces that arestarting to fret or ravel. Costs are higher than for surfacedressing, but the resulting layer can improve the ridingquality of the road which a surface dressing would not.The higher costs may therefore sometimes be justified,particularly for badly cracked or lean bituminoussurfacings. However, a slurry seal will not have suchgood surface texture as a surface dressing and maytherefore be slippery in wet weather.

Both anionic and cationic emulsions may be used, butcationic emulsion is normally used in slurries containingacidic aggregates. Anionic slurries may be premixed in astatic mixing plant but, because emulsions in cationicslurries break relatively quickly, these have to beprepared in a purpose-built mixing and laying machine.A contractor usually supplies this equipment, itsoperators and the bitumen binder; the maintenanceorganisation will usually supply the aggregate, water andthe labour force. The laying technique can range fromsimple 'squeegees' to modern mixer-spreader units.

The following specification is reproduced by permissionof Colas (East Africa) Ltd.

Aggregate Gradations (all percent cumulative, passing)

Sieve Fine Seal General Seal CoarseSurface

10mm 100 100 1006.3 mm 100 100 -5.0 mm 100 85-100 70-90

2.36 mm 100 65-90 45-701.18 mm 65-90 45-70 28-50600 µm 40-60 30-50 19-34300 µm 25-42 18-30 15-25150 µm 15-30 10-21 7-1875 µm 10-20 5-15 5-15

Slurry CompositionPer m3 Percent by mass

Aggregate 1m3 81-83Cement (catalyst) 15kg 1

Terolas A3 bitumenemulsion * 280-330 16-18

litres

Water As required

* Type A3 is a slow setting anionic emulsion, 55 percent bitumen (minimum)

Coveragem2 /m3

Old surfaces 130-250

New surfaces:On 20mm aggregate 130 – 170On 14mm aggregate 170 – 240On 10mm aggregate 180 – 250On primed base 150 – 180 (in 2 layers)

8. REFERENCES

AVELING-BARFORD, undated. Grading illustrated.Technical Publication TP 549. Grantham: Aveling-Barford Ltd.

BSI, 1971. Specification for single-sized road-stone andchippings. BS63: 1971 (Part 2). London: BritishStandards Institution.

BSI, 1975. Methods for sampling and testing of mineralaggregates, sands and fillers. BS812: 1975 (Parts 1, 2and 3). London: British Standards Institution.

ROBINSON, R, 1979. Some manual methods of screen-ing aggregates for labour-intensive road construction.TRRL Supplementary Report 503. Crowthorne: Transportand Road Research Laboratory.

TRRL OVERSEAS UNIT, 1981. Maintenance manage-ment for District Engineers. Overseas Road Note 1.Crowthorne: Transport and Road Research Laboratory.

TRRL OVERSEAS UNIT, 1982. A guide to surfacedressing in tropical and sub4ropical countries. OverseasRoad Note 3. Crowthorne: Transport and Road ResearchLaboratory.

APPENDIX A

ADDITIONAL NOTES ON SURFACEDRESSING

A.1 CHIPPINGS

A.1.1 Size

The chippings for a surface dressing should be of single-sized crushed rock, roughly cubical in shape and cleanand free from dust. The nominal sizes used may varybetween 6 mm to 20 mm according to the local require-ments. The chippings should comply with BS63.-Specification for single sized roadstone and chippings(BSI, 1971). It is most important that the chippingsshould be single sized. Graded chippings make designand construction much more difficult and their use,without the much closer control which is required, willlead to loss of the larger sizes of chipping and to fatting-up of the surface dressing. Single sized aggregate may bemore expensive to produce than graded aggregates, buttheir higher cost can be offset by better performance. Theremaining sizes of stone can be absorbed into othersectors of the road construction programme.

Recommendations for the grading of surface dressingaggregate are given in Table A1.

The use of rounded-gravel aggregates should, if possible,be avoided because it is difficult for the binder film tohold them in place and because of their poor surfacefriction properties. If their use cannot be avoided,adjustments must be made to the rate of spread ofbitumen. The rounded aggregates do not interlock andmore bitumen is required to hold the particles firmly tothe road surface than is required for cubical aggregates.

A.1.2 Average least dimension

Once the surface dressing is laid, the traffic will orientatethe chipping to lie as flat as possible with the leastdimension vertical. Therefore the Average LeastDimension (ALD) is the measurement which bestclassifies the chippings and gives a guide to the requiredrate of spread of bitumen and the rate of spread ofchippings. Two methods for determining the AverageLeast Dimension of the chippings are given below:-

Method 1The ALD of chippings may be determined by taking arepresentative sample, say 200 chippings, and measuringthe least dimension of each chipping with a pair ofcalipers. The average of a series of such measurements isthen the ALD of the chippings.

Method 2A sieve analysis is first carried out on a surface-drysample of the chippings and the results plotted on a

37

38

TAB

LE

A1

GR

AD

ING

AN

D S

HA

PE O

F C

HIP

PIN

GS

Fig. A1 Average least dimension chart

large scale grading curve. The median size of chipping isthen determined as that theoretical sieve size throughwhich 50 per cent of the material will pass. The flakinessindex is determined as in Section A.1.3. Then usingFigure Al the median size is marked on scale A andflakiness index on scale C. A straight line is drawnbetween the two marks and the Average LeastDimension read off on scale B.

A.l.3 Flakiness

The flakiness index of chippings should not exceed 35.The method for determining flakiness index is givenbelow. The chippings should also satisfy at least one ofthe following strength criteria:-

Aggregate crushing value - less than 25 (Values up to 35may be permitted for lightly trafficked roads)

Aggregate abrasion value - less than 12

Ten per cent fines value - not less than 8

Aggregate impact value - less than 30

Descriptions of these tests are given in BS812. Methodsfor sampling and testing mineral aggregates, sands andfillers Part 3: Mechanical properties (BSI, 1975).

The flakiness index of an aggregate is the percentage byweight of particles in it whose least dimension (thick-ness) is less than three fifths of the nominal size. The testis not applicable to material passing a 6.30 mm sieve.

Apparatus consists of:-

(i) A metal thickness gauge of the pattern shown inFigure A2. The width of the slot used in thegauge shall be the dimension specified in the'thickness gauge' column of Table A2 for theappropriate fractions.

(ii) BS test sieves as shown in Table A2 (BSI, 1969).

(iii) A balance accurate to 0.5 per cent of the weightof the test sample.

1. Sample quantities are given in Table A2.Allowance should be made for the later rejectionof particles retained on 63 mm BS

39

Fig.A2 Thickness gauge

TABLE A2

DIMENSIONS OF THICKNESS GAUGES

Aggregate size-fraction

BS test sieve nominal aperture size

100% passing 100% retained

Thickness gaugeWidth of slot

Minimum massfor subdivision

mm

63.050.037.528.020.014.010.0

mm

50.037.528.020.014.010.0

6.30

mm

33.9 + 0.326.3 + 0.319.7 + 0.3

14.4 + 0.15 10.2 + 0.15

7.2 + 0.1 4.9 + 0.1

kg

503515 5 2 1

0.5

test sieve and passing 6.30 mm BS test sieve. Thetest sample shall be obtained from the laboratorysample by quartering, and dried.

2. The sample shall be weighed and separated into theappropriate size fractions from Table A2, columns1 and 2, by sieving, rejecting material retained on a63 mm sieve or passing a 6.3 mm sieve. Record thesum of all fractions M1, discard those which are 5per cent or less of M1 and record the remainder asM2.

3. Each appropriate fraction shall be gauged in turnfor thickness on the thickness gauge.

4. The total amount M3 passing the thickness gaugeshall be weighed to an accuracy of at least 0.5 percent of the weight of the test sample.

5. The flakiness index is the total weight M3 of thematerial passing the various thickness gauges,expressed as a percentage of the total weight M2 ofthe sample gauged to the nearest whole number,and shall be reported together with the sieveanalysis.

M3 x 100Flakiness Index =

M2

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A.1.4 Adhesion and pre-treatment

The most critical period for a surface dressing occursimmediately after the chippings have been applied. Atthis stage the chippings have not formed an interlockingmosaic and they are held mainly by adhesion forces inthe binder film. This adhesion takes time to develop andis completely stopped if the chippings remain wet.Adhesion is very slow to develop if the chippings aredusty. However in hot climates the chippings may bewashed with fresh or salt water just prior to spreadingsince the chippings dry out in a few minutes after theyare spread on the road and the development of adhesionis not significantly delayed.

If adhesion problems are experienced it may be useful totreat the chippings with a light spray of diesel oil orkerosene prior to spreading. Enough should be added torender the surface of the chippings slightly damp.

A.1.5 Stockpiling

Chippings should be stored in stockpiles which are largeenough to supply four to five days work without haulsexceeding 15 kilometres.

Stockpiles will typically be about 2000 tonnes and willrequire an area of about 1000 m2 if formed by tippingfrom lorries. If suitably located, these areas will be re-used at intervals of three to five years, and somepermanence in their construction will be justified by areduction in wastage of chippings.

They may be surfaced, either with a two-coat surfacedressing, bricks or concrete, with a compacted gravelbase in each case. A concrete or brick wall about onemetre high on three sides will define the stockpile, avoidcontaimination and simplify measurement.

A.2 BINDER

A.2.1 Grade

The performance and qualities required of a surfacedressing binder makes the choice of the binder critical.

The binder must:-

(i) be sprayable at a reasonable temperature,

(ii) 'wet' the surface of the road and remain in acontinuous film waterproofing the roadstructure,

(iii) not run off a steep gradient or cambered roador form pools of binder,

(iv) 'wet' and adhere to the stone chippings at roadtemperature,

(v) be strong enough to resist the traffic force and

retain the chippings even at the highest ambienttemperatures,

(vi) be flexible at the lowest ambient temperature,neither cracking and allowing water to enternor brittle thus allowing the chippings to breakfree,

(vii) resist excessive weathering and hardening oncethe initial hardening has taken place.

Surface dressing work will normally be carried out withcut-back or penetration-grade bitumens. Cut-backbitumens are likely to be of the high-viscosity, medium-curing type. The advantage of these binders is that thecutting agent (normally kerosene) lowers the viscositysufficiently to obtain good wetting of the chippings bythe binder. Also the small quantity of cutting agentpresent results in a reasonably rapid set to hold thechippings after initial adhesion has been achieved.Penetration-grade bitumens may be used in the drier.hotter regions, depending upon actual road temperaturesat the time of working.

With mechanised methods, the rate of consumption ofbitumen is high and it is desirable that it is supplied inbulk rather than in drums, with their associated problemsof handling and leakage. It is more economic to transportbitumen in bulk than in drums and the pre-heaters can, iflarge enough, act as holding tanks and ensure thatoccasional interruptions to the supply do riot disrupt thewhole operation.

It is not usually necessary to test the grade of the bitumenon site, reliance being placed on the supplier's certificate.

A.2.2 Viscosity

Surface dressing binders should have a viscosity ofbetween l04 and 5 x l05 centistokes at the prevailing roadtemperature. At higher viscosities, stone will not bewetted by the binder and will be lost by whip-off; atlower viscosities wetting will occur but the binder will betoo fluid to hold the stone. Figure 15 shows therelationship between binder viscosity and road tempera-ture for a wide range of binders and provides a means ofselecting a suitable binder for use with a particular roadsurface temperature. Daytime road surface temperaturesin dry weather in the tropics are likely to range from300C to 700C according to region and season. Thissuggests that the most suitable binders for surfacedressing will be between MC 3000 and 80/100 penbitumen. The use of a more viscous bitumen than 80/100pen is not recommended. Occasionally chippings have tobe spread manually instead of by mechanical gritters. insuch cases it may be advisable to use a slightly lessviscous binder than would otherwise have been required.(See also Section 7.6 on manual surface dressing).

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It is unlikely that the complete range of possible binderswill be available to the engineer and therefore forpractical reasons it will probably be necessary to selectperhaps two binders for general surface dressing use. Inthis case MC 3000 and 180/200 pen are likely to be themost suitable: MC 3000 for roads with prevailing surfacetemperatures less than 450C and 180/ 200 pen whenprevailing surface temperatures are 450C and above.

A.2.3 Measuring the rate of spread of thebinder

A.2.3.1 Average rate There are two methods ofmeasuring the average rate of spread of the binder. In thefirst the volume of binder sprayed is calculated fromweighbridge records and this is divided by the measuredarea covered in a day. Although this is useful in checkingquantities for payment, it does not reveal differences inrates of spread along the road and is therefore of limitedvalue.

A better method, which should be used on every site, isto measure the average rate over an area of 500 - 800square metres. If the distributor is fitted with an accuratedipstick, this method can be of considerable value eventhough it does not give a measure of maximum variation.To make the test, the distributor is placed on a levelstretch of road and the position of the wheels are marked.Dipstick readings are taken, repeating the dips untilconsistent readings are obtained. Binder is sprayed by thenormal technique until about 10 per cent of the totalvolume in the tank has been used. Dipstick readings aretaken with the distributor in the same position as before.

The area covered is then measured accurately and theaverage rate of spread calculated after allowance hasbeen made for any variation in the intensity of appli-cation at the longitudinal joint (see jointing of strips,Section 7.5.4.4).

A.2.3.2 Longitudinal variation Longitudinalvariation in the rate of spread of the binder can bechecked with a tray test. In this test, light metal traysabout 200 mm square by 5 mm deep, and numbered, areplaced at intervals along the road in the path of the binderdistributor. After the distributor has passed, the trays areremoved, wrapped in weighed sheets of paper and takenaway to be weighed; the rate of spread can then becalculated for each point on the road. A balance accurateto 0.1 gramme installed in a car, van or mobile laboratorypermits the results from five trays to be obtained within10 minutes. The spacing and number of trays used can bevaried to suit the circumstances of the particularinvestigation, but at least five

42

trays should be used. When using this test, care must betaken to raise the protective hood on the distributor, iffitted, so that it does not touch the trays, but not so muchas to let any wind disturb the spray.

The tray test only gives an approximation to the averagerate of spread. If it is to be used, it must be rememberedthat the maximum variation shown by individual trayswill be greater than that of groups of trays. The meanresult from three or more trays, spaced at intervals of notless than 10 metres, should fall within 10 per cent ofspecification.

A.3 REFERENCES

BSI, 1969. Specification for test sieves. BS4IO: 1969.London: British Standards Institution.

BSI, 1971. Specification for single sized road-stone andchippings. BS63: 1971 (Part 2). London: BritishStandards Institution.

BSI, 1975. Methods for sampling and testing mineralaggregates, sands and fillers. BS812: 1975 (Parts 1, 2and 3). London: British Standards Institution.

NOTES

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