ROAD SURFACE DRESSING ASSOCIATION

 

 

 

 

PREPARING ROADS FOR

SURFACE DRESSING

 

 

 

 

 

                           Consultant Director and Secretary

 

                                      John Baxter

                                      Westwood Park

London Road

Little Horkesley

                                      Colchester

                                      Essex CO6 4BS

                          

 

 

 

 

 


Ó Copyright RSDA 1993

     First Published 1993

     Revised 2001

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
No part of this document may be

reproduced without the written permission

of the Road Surface Dressing Association

 
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Preparing Roads for Surface Dressing 2001

 

 

 

 

INDEX

 

 

 

Page No

 

1

Introduction

1

 

2

Siding or Edging

1

 

3

Pre-patching

2

 

4

Masking of Ironwork and Cats Eyes

5

 

5

Pre-sweeping

6

 

6

Recording Location of Road Markings

7

 

 

 

 

 

ROAD SURFACE DRESSING ASSOCIATION

 

Preparing Roads for Surface Dressing

 

1               INTRODUCTION

 

Surface dressing consists of preparation, design, execution and aftercare, and lack of attention to any one of these four elements can lead to premature failure.  The purpose of this Guidance Note is to give advice on preparation as this is the element which, in many cases, is given the least attention.

 

The preparation of roads for surface dressing involves five principal operations:

 

1          Siding or edging.

 

2          Pre-patching.

 

3          Masking of ironwork and cats eyes.

 

4          Pre-sweeping.

 

5          Recording locations of road markings, covers, etc.

 

 

2               SIDING OR EDGING

 

2.1         One of the principal purposes of surface dressing is to seal the road surface against the ingress of water and, as the main concentration of water flows are in the channel of the carriageway, it is most important that they are adequately waterproofed.

 

2.2         The requirement for siding or edging can be assessed by visual inspection.  Usually siding is no longer carried out as a routine maintenance operation and frequently routine sweeping of unkerbed roads has also been discontinued.  Under these circumstances, debris in the road channel becomes compacted to such an extent that normal sweeping operations will not dislodge it.  On lightly trafficked roads, roadside vegetation also tends to encroach onto the road itself and if both vegetation and debris are not removed it is likely that the channels will not be surface dressed and that water will, for this reason, find its way into the road foundations, leading to structural damage.

 

2.3         The quickest and easiest method of edging roads is to use a light tractor-mounted loading shovel fitted with a purpose-made rear-mounted siding plough.  The plough quickly dislodges both compacted material and vegetation which can then be picked up by the loading shovel and disposed of.  This operation should immediately be followed by mechanical sweeping, which should be continued until the carriageway surface is thoroughly clean. An additional Heavy Duty Brush attachment is also available from one sweeper manufacturer to assist with removing compacted edge debris.  Repairs to any damage to the edge of the road should also be carried out before dressing (see section 3 of this note) to ensure the road haunches are sound.

 

2.4         Consideration should also be given at this stage to whether or not it would be necessary to carry out any hedge or tree trimming to enable surface dressing equipment to pass down the road without obstruction.  This is particularly important in the case of lorries which are tipping into chipping machines where contact with branches or other obstructions could represent a danger to both operatives and the public.

 

 

3               PRE-PATCHING

 

3.1         Purpose:  It is important to remember that surface dressing alone neither restores the riding quality nor directly strengthens a road surface.  The purpose of pre-patching is to repair, before the dressing is carried out, any areas where there has been a loss of aggregate from the wearing course, damaged joints, potholes, unsatisfactory trench reinstatements, wheeltrack rutting or edge damage.

 

3.2         Sometimes, traffic over-runs the carrriageway edge and because these areas have little or no structural foundation and lack lateral support, it will often be necessary to repair them, either by providing a kerbed edge to contain traffic to the area with an appropriate foundation, or to widen the road by providing full depth construction to the encroached area.  It may be necessary to provide a concrete haunch or to pipe a roadside ditch to provide adequate lateral support.  Patching of the surface of the encroached area alone is not sufficient and would almost certainly lead to rutting and drainage problems after surface dressing.

 

3.3         Where rutting of the wheeltracks is evident, extensive patching will be necessary.  The surface dressing of roads which have ruts in the wheeltracks will seal them against the ingress of water but result in standing water during periods of wet weather.  In some cases, the application of a sandwich surface dressing to fatted, rutted wheeltracks may be an acceptable solution but this type of dressing should be undertaken with care (see Road Note 39 for advice)  Where there is a footway adjacent to a rutted carriageway, pedestrians will be splashed by passing vehicles, which is quite unacceptable.

 

3.4         Extent of Works:  Patching prior to surface dressing is usually limited to localised small areas, but there are occasions where extensive machine-laid patching, re-shaping and strengthening, followed by surface dressing can prove to be a very cost-effective alternative to overall resurfacing.  The extent of patching works will depend very much on the character of the road and the volume and type of traffic it carries.

 

3.5         The objective should, of course, be to surface dress roads before major patching becomes necessary.  Millions of pounds are wasted every year as a result of surface dressing not being applied soon enough.

 

3.6         Minor rural roads often become misshapen due to the increased axle loadings of the traffic which they carry.  In these cases, the use of 20 mm dense binder course for patching is most effective, since the required thickness can be built up in layers and the finished surface is sufficient to provide a good temporary wearing course. In shallower patches 14 mm dense wearing course can be used and this has the advantage of making a less open textured surface with less risk of binder absorption. One texture surfaces risk “ absorbing “ more of the surface dressing binder which can lead to localised failure of the dressing. Further guidance is given below under the section headed Choice of Materials.

 

3.7         With roads constructed on soft clay sub grades, seasonal wetting and drying out of the underlying clay can cause the surface to craze.  In these situations, surface dressing may have been carried out at frequent intervals in order to seal cracks which would otherwise let water into the road foundations.  Such frequent surface dressings can result in road surfaces with a very high binder content which may become soft in periods of hot weather.  In these circumstances, it may be necessary to consider cold planing to remove this soft layer before attempting a further dressing.  Where the road structure is of limited thickness, this option may not be practical and in these circumstances a possible solution is to reduce the rate of spread of binder and increase the chipping size.  This will in any case be indicated by the result of the probe tests undertaken before designing the most suitable treatment. Alternatively the use of a Sandwich Dressing could be considered.

 

3.8         Choice of Materials:  Wherever possible, pre-patching material should be selected to match the existing road surface in terms of texture, hardness and porosity, in order to provide a uniform surface to accept the subsequent surface dressing.  This is because it is impractical to attempt to vary the rate of application of binder over small patches which would be necessary to ensure that the correct amount of binder is applied to hold the chipping size selected for the overall general design of the road surface, based on road hardness and number of commercial vehicles per day. 

 

For example, if the patch is of a finer texture than the existing surface, chippings applied to it will stand more proud on the patch than on the surrounding surface, but if the patch is of a coarser texture, the chippings will be more recessed on the patch than on the surrounding surface.  If the patch is harder than the adjoining road surface, the chippings will not become embedded to the same extent as the adjoining areas, but conversely, if the patch is softer than the surrounding road surface, the chippings will become embedded causing texture to be lost and binder to rise to the road surface.  Therefore it is for this reason that cold asphalt is, in most cases, a most unsuitable material for patching prior to surface dressing.  Moreover, if the patch is more porous than the adjoining surface, more binder will be lost in the patch, leaving less to hold the chippings, probably resulting in chipping loss.  This is a problem which can arise if a heavily trafficked road is patched with a 0/10 mm open grade surface course which is subsequently surface dressed with a racked-in 14 mm/6 mm dressing, resulting from the patches being porous and the surface dressing binder passing into the patching material, leaving the 14 mm chippings with insufficient binder to hold them in place against the traffic, and both these and the 6 mm racking material are quickly lost from the patch.  For these reasons the importance of ensuring a uniform surface after patching and prior to dressing is paramount.

 

3.9         Normally it is desirable to use patching materials that match the existing road surface so that the final texture and porosity are as near as possible to the original surface.  The choice of materials for patching roads which have previously been surface dressed is more difficult, but experience suggests that the use of close graded (6,10, 14 mm) surface course to BS 4987 - 1 of similar nominal size to the existing surface dressing will give a reasonably good match in terms of texture and porosity.  However, the hardness may not be exactly the same as the existing surface, but nevertheless a good compromise.

 

3.10     Most pre-surface dressing patching materials will be hand-laid and, to enable them to be handled without segregation, binders are often “fluxed” with petroleum oils at the mixing plant, so that they remain workable for longer than machine-laid materials.  However, this also increases the time required for the volatile oils to evaporate, and it is for this reason that pre-patching work must be carried out well before the surface dressing is laid.

 

3.11     Most highway depots maintain a small stock of patching materials retained for emergency works where, for example, a pothole or trench reinstatement requires urgent attention in the interests of safety .  These materials are known as “deferred set” or “depot stock” materials and may be even softer than material manufactured for hand laying.  The majority of these materials are totally unsuitable for patching prior to surface dressing.

 

3.12     Because it is envisaged that a surface dressing should last at least seven years, or even longer when polymer modified binders are used, it is important to ensure that the materials chosen for pre-patching will perform satisfactorily for at least that period of time, and considerable care should be exercised in their choice having regard to the circumstances.  Local Highways Materials Engineers are usually pleased to advise on the choice of materials for a particular site based on the principles outlined above.

 

3.13     Organisation and Timing of Works:  Pre-patching is an important operation, and requires planning and organisation to ensure that it is properly executed and to a high standard.  This is unlikely to be the case if work is being rushed just days ahead of the surface dressing programme.  Ideally patching prior to surface dressing should be carried out in the calendar year prior to the season in which the road is to be dressed.  This has the added advantage that sufficient time is given for the volatiles in the binder to evaporate and for the patch to harden before dressing is carried out.  The worst possible situation is one where fluxed, hand-patching material is being used to fill potholes only hundreds of yards ahead of the surface dressing tanker.

 

3.14       Method of Patching:  Patching is an operation which, to describe adequately, would require a guidance note of its own.  The subject is, however, adequately covered in the document prepared by the Standing Committee on Highway Maintenance and the Department of Transport in 1988 entitled “Preferred Method of Patching”.  This is obtainable from CSS City Hall Lincoln LN11DN and carries the ISBN reference 09 02 319965.

 

3.15       Patching of minor roads can be carried out using a combination of bitumen binder (either cut back or emulsion) and aggregates.  This method can be used for small potholes, edge erosion and crazed/cracked areas.  Care must be taken to compact layers of binder and chippings when repairing potholes and edges requiring more than one application.  Sizes of aggregate will depend on depth of patch and where single layer repair is required, the type of road concerned.  Good quality sweepings retained from previous surface dressing sites are perfectly adequate for this type of repair.

 

3.16       Where, for whatever reason, patching has not been carried out well in advance of the surface dressing operation, efforts should be made to ensure that the patched areas are sealed as far as is practicable to prevent subsequently applied surface dressing binder sinking into the patch and leaving insufficient binder at the surface to hold the surface dressing chippings pending embedment under traffic.  Several techniques have been used successfully, and these include the following.

 

3.16.1   A light mist spray of bitumen emulsion and the application of 3 mm to dust crushed rock.

 

3.16.2   The slurry sealing of large patches.

 

3.16.3   The application of a mixture by equal parts of sand and cement brushed into the interstices of the patch.  Where this technique is adopted, care should be taken to ensure that surplus  sand and cement is removed from the surface during periods of wet weather as this could otherwise cause the surface to be slippery.

 

3.16.4   The application of 3 mm pre-coated crushed rock.

 

3.17    In recent years, band sealing, with a heavy cold emulsion, of joints between a patch and the adjacent road surface has become commonplace.  This is not a practice which should be employed when patching prior to surface dressing as these bands of bitumen tend to leech through the dressing and form fatty strips on the surface which, under wet conditions, can be a danger to motorcyclists.

 

 

4                   MASKING OF IRONWORK AND CATS EYES

 

4.1             The purpose of masking is to ensure that ironwork and cats eyes are returned to peak operational condition immediately the dressing has been completed.  If manhole covers are not masked, they become fixed to their frames and the keyways are filled with binder and chippings.  Similarly, if gully gratings are not properly masked, binder and chippings pass through the grating to block the gully chamber and the covers are sealed to their frames, making it very difficult to carry out the first gully emptying after dressing.  If cats eyes are not satisfactorily masked, the pads are damaged by the chippings and the action of traffic and the lenses become partly or totally obscured by binder.  It is, therefore, necessary both in the interests of safety and economy that they should be adequately masked.

 

4.2             Many methods of covering ironwork and cats eyes have been used in the past, including liquid material painted onto covers and frames preventing the binder from sticking to them.  While that particular objective is achieved, it has to be remembered that the binder and the chippings, which have fallen on the cover but are not adhering to it because of the applied liquid, must be removed before the road is opened to traffic, or these materials will be carried onto and spoil the adjacent dressing.  The most satisfactory material for masking is undoubtedly the use of masking tape, which is available in varying widths, is strong enough to stay in place under the action of traffic and relatively easily removed on completion of the dressing.  Masking materials which have been used to cover manholes, gullies and cats eyes should in all cases be removed immediately after completion of the dressing, collected and disposed of in an approved manner.  Under no circumstances should they be left on the adjoining roadside verge.

 

4.3             The efficiency of cats eyes following surface dressing must be maintained. Where 14 mm chippings have been used on a hard road surface, it is likely that the cats eyes will need raising if they are to perform satisfactorily.  This operation is best done after the surface dressing has been laid, taking into account the relevant regulations.  In such circumstances, the cats eyes should be removed before the dressing is done.

 

 

5                   PRE-SWEEPING

 

5.1             If there is to be good adhesion between a surface dressing binder and the road surface, it is essential that the road surface is clean and free from dust and debris.  For this reason, it is important that the whole width of the carriageway is swept.  One pass of the sweeper will normally be sufficient but in locations such as site entrances, bends and junctions, where vehicles brake and change direction, higher concentrations of detritus can accumulate and at such sites an additional pass of the sweeper is often necessary.  The most suitable piece of plant for this is a full width lorry-mounted brush or suction sweeper.  Pre-sweeping will normally require two distinct operations: the first as a follow-up to a siding plough or other operations undertaken to ensure the removal of debris from the road channels described in section 2.3 of this note, and carried out well before the dressing is undertaken, and the second is to ensure that the whole carriageway width is swept an hour or so before the dressing is commenced.  Remember that there is little point in paying for surface dressing chippings with less that 1% dust content if the dressing is to be applied to a road which is sufficiently dusty to prevent a good bond between the surface dressing binder and the road surface.

 

 

6               RECORDING LOCATION OF ROAD MARKINGS

 

6.1         Before surface dressing commences, a detailed and careful record should be made of the location and change points of all road markings and ironwork, that is to say lane lines, hazard lines, arrows, double line systems, “slow” road markings, “stop” and “give way” markings, manholes, hydrant covers, valves and the metal studs used to mark the node points of CHART and RMMS surveys etc.

 

The establishment of double white line systems to the required national standards is a time consuming operation and it is essential that the position of these markings is not lost.  On roads which are kerbed, discreet markings on the kerb face indicating changes in the line pattern or arrows can be neatly recorded.  On unkerbed roads, records should relate to permanent features which will not be affected by the dressing.  Where side roads meet main roads, it is essential that “stop” and “give way” markings and lines should be replaced as quickly as possible after the dressing and additional signs provided during the time that these markings are covered.  Some authorities adopt the practice of masking “give way” and “stop” markings in order that they can be immediately reinstated after the dressing, but this is very much a matter of local policy which should be clearly established before surface dressing works are undertaken.