ROAD
SURFACE DRESSING ASSOCIATION
GUIDANCE NOTE ON
WHEN TO SURFACE DRESS
Consultant Director and Secretary
John Baxter
Little Horkesley
Essex CO6 4BS
Ó
Copyright RSDA 1993
First Published 1993
Revised 2001
reproduced without the written
permission of the Road Surface Dressing
Association
No part of this
document may be
When To Surface Dress 2001
INDEX
|
|
Page
No |
|
Introduction |
1 |
|
CHART and MARCH Conditions |
1 |
|
The Surface Dressing Season |
4 |
|
References |
6 |
|
Other RSDA Publications |
6 |
ROAD SURFACE DRESSING ASSOCIATION
Guidance
Note on When to Surface Dress
1
INTRODUCTION
This guidance note consists of two sections, the first
dealing with the selection of the optimum year to apply a dressing to a road
surface and the second which considers the times of the year in which dressings
on different categories of road should be undertaken.
2
CHART
AND MARCH CONDITIONS
CHART
and MARCH condition surveys are now widely used as a method of assessing
priorities for highway maintenance expenditure on the main road network, and
have been responsible for a more formal approach to setting maintenance
priorities.
Pavement Management Systems are also available as a tool for
decision making on road maintenance strategies.
These surveys are often supplemented by deflectograph
and SCRIM surveys, which respectively help to identify sections of road which
have insufficient strength to carry the traffic of the day and sections where
the resistance to skidding under wet conditions is below recommended limits1. These measurements are of great value to
highway maintenance engineers responsible for assessing priorities for
expenditure but do not replace the need for the application of judgement by
experienced highway maintenance engineers.
This need is particularly appropriate when deciding the optimum time to
surface dress roads and footways. Where
a blacktop surface is not deforming under the weight of traffic and where there
is no evidence of surface deterioration, the CHART and MARCH systems will not
indicate a priority, or indeed a need, for surface treatment. In some cases, a SCRIM survey will indicate
the need for surface treatment in order to restore a satisfactory resistance to
skidding, but there are a substantial number of sites where blacktop surfaces show,
under particular conditions, the first signs of deterioration unlikely to be
recorded or even noticed by a survey technician.
2.1
These signs are often only apparent after periods
of rain when the road surface is drying out.
Any hairline cracks in the surface will retain the moisture and will be
the last to dry out. By inspecting at
these times, hairline cracks can be recorded.
Similar evidence is often apparent at the time when road surfaces have
been treated with salt during the winter period when hairline cracking can, for
a few days, become even more evident than under the circumstances outlined
above. Although these first signs of
distress will not be recorded by highway assessment systems, they should alert
the highway engineer to the need to apply a surface treatment to seal the
cracks and to prevent deterioration which, if delayed, would require patching
or even more extensive repairs before a surface dressing could be carried out
satisfactorily.
3 The Local Authority Association’s Code
of Good Practice for Highway Maintenance2 published in 1989 suggests
that when fine crazing, permeable surfaces, stripping, loss of chippings, or
fatting up of existing dressings affects 20% of a carriageway area on roads in
Categories 2, 3 and 4, surface treatment should be carried out, and that where
5% of the area of a length of carriageway is suffering from major
deterioration, defined as cracking, crazing, loss of aggregate or serious
permeable problems – patching and surface treatment should be carried out on
categories 2 to 4. The categories
defined in the Local Authority Guide are as follows:
Category 1 Motorways
Category 2 Strategic routes, including local
authority motorways, primary routes and the most important urban traffic links
with more than a local significance
Category 3 Distributor roads – both main and
secondary serving a local purpose and connecting to strategic routes
Category 4 Local roads, local inter-connecting
roads, the remainder of the network
4 Enquiries made by this Association suggest
that many Highway Authorities spend more money on “patching prior to surface
dressing” than on the surface dressing operation itself. It follows, therefore, that if surface
dressing is carried out some 12 months before any patching is necessary,
substantial sums of money could be saved and far greater lengths of road could
be treated. In addition to this
financial saving, interruptions to the smooth flow of traffic resulting from
lane closures in order to carry out patching operations will be avoided, which
will represent an important safety factor, as well as avoiding increased
business costs resulting from delays.
5 A switch from a maintenance regime
determined by immediate need to one based on the principle of preventative
maintenance cannot be easily achieved in a single season, but with careful
planning should be achievable over a period of three or four years. This approach is particularly important when
local authorities are being asked by the government to restrict their
expenditure, or where highway maintenance funds are restricted in other ways.
6 Surface dressing is not a panacea for
every highway maintenance need. It
cannot, for example, repair a foundation failure or deformation such as rutting
in the wheeltracks.
These are deficiencies which may require, in extreme cases,
reconstruction, and in less serious cases, the application of a blacktop
surface to both strengthen the road and to remove rutting. Although surface dressing in itself does
little to increase the load carrying
capacity of a road in the way that a blacktop surface can do, the fact that a
dressing will make a road surface much less permeable than almost any other
treatment can result in some improvement to the capability of the surface to
carry traffic loads. This results from
keeping the moisture content at road construction and formation levels to a
minimum, where even a ½% increase in moisture content can prove very damaging.
6.1 It must, however, be noted that waterproofing
a road surface and preventing in ingress of water from the surface to the
structure and formation will be of little value if the water table extends into
those levels. In these circumstances,
the provision of permeable drainage systems on one or both sides of the road is
called for. Under English law, roadside
ditches, other than those provided by the highway authority when the road was
constructed, are normally the responsibility of the owner of the adjoining
land. Under the provisions of Section
100 of the Highways Act 1980, local authorities have the power, but not a duty,
to maintain these ditches where, in their view, this is necessary for highway
maintenance purposes. Under these
circumstances, there is a marked reluctance of either landowners or highway
authorities to maintain roadside ditches and it is perhaps regrettable that a
clear statutory duty has not been laid on either the highway authority or the
landowner.
6.2 Many highway engineers would agree that the
neglect of highway drainage systems, particularly on the minor road network,
has resulted in considerable structural damage and maintenance expenditure,
which may not have been required if ground water levels had been maintained at
an appropriate level.
7 The
cost of a surface dressing can vary from 75p per square metre to over £15 per
square metre in extreme cases involving the use of special binders and
aggregates applied at night in city areas.
This prime cost is one factor only in priority decision making, the key
factor being the efficiency of the treatment measured in terms of cost per
square metre per annum of satisfactory life.
The
most efficient maintenance system is that which has the lowest cost life index
and this is usually achieved by selecting the appropriate surface dressing
system taking account of the factors considered in Road Note 39 and
ensuring that the dressing is properly designed, executed and given appropriate
aftercare for the first few hours after the completion of the work.
It is
also important that the selected dressing is applied at the optimum time and
that time is before any pre-patching
becomes necessary. The cost of patching
10% of an existing surface, spread over a number of small patches, can easily
exceed the cost of dressing the whole area and the time taken to carry out this
patching operation can take four or five times the time taken to surface dress
the whole area.
If the
cost of pre-patching is added to the cost of surface dressing, the cost per
square metre will have more than doubled, as will the cost life index. The savings accruing by surface dressing
before patching becomes necessary would thus more than pay for a second
dressing, perhaps after a gap of seven years or so. During this whole period, the surface of the
road will have had a good texture providing resistance to skidding.
The
most cost effective maintenance treatment is to apply a surface dressing before
the point where deterioration is picked up by a maintenance survey system and
before the need for other than minimal patching is required. In this way disruption to traffic during
patching operations is avoided and the skid resistance of the road is
maintained at an optimum.
8 THE SURFACE DRESSING SEASON
The first part of this guidance note has dealt with the
advantages of applying surface dressing as a preventative maintenance
operation. We now turn to consider the
question of the best time of the year to carry out surface dressing work.
Road
Note 393 identifies the surface dressing season in the
The best periods of weather for surface dressing
lie within the period May to mid July and this is when the most important roads
should receive their dressing. Road
surfaces during this period should have become warm and somewhat softer than
during the winter period. Hot binders
sprayed onto these surfaces will not cool down as quickly as would be the case
at the beginning and end of the season, allowing a little more time to elapse
between the application of binder to the road surface and the application of
chippings on the binder film. It must
always be remembered that the ability of the binder to “wet” the applied
chippings and to stick to them quickly, falls rapidly with loss of
temperature. Main road surface dressings
carried out between May and the end of July will receive some initial embedment
under the action of traffic. It is
particularly important that such dressings using larger aggregates are executed
well within this period. Roads which
probe tests have shown to be “soft”, and are therefore to be dressed with a
larger size chipping and lower rate of spread of binder than would be the case
for roads of “normal” hardness, should be dressed early in the season in order
to achieve maximum embedment before the winter.
The rate of embedment will fall rapidly as roads cool down after
September, and it is for this reason that working beyond the end of September
with traditional binders is not recommended, irrespective of the weather
conditions prevailing at the time.
One of the problems with the British climate,
however, is that it is very unpredictable and in some years the number of days
when dressing can be carried out on main roads is substantially reduced due to
adverse weather conditions and, of course, by the need to avoid working on
these roads during the Bank Holiday periods.
In recent years, weather forecasting has become much more accurate and
by taking advice from local meteorological forecasting centres, the risk of new
dressings being spoilt by early rainfall can be substantially reduced.
In years
when the spring and summer have been badly affected by wet weather, the autumn
is often quite good and some days in October can be as good as in a normal
August or September. However, surface
dressing in October is not recommended with traditional surface dressing binders
on any category of road, simply because there will be insufficient time for
chipping embedment to occur before the onset of winter when binders become more
brittle, and chipping loss on dressings which have not become embedded are far
more likely. Although there will be no
embedment of chippings when concrete roads are dressed, dressing of concrete
roads should be undertaken during ideal weather conditions using a polymer
modified binder. An inverted double
dressing (pad-coat) should be used under these circumstances.
Year by
year, surface dressing binders are improving and the greater use of polymer
modified binders has made some extension in the surface dressing season
possible, but even using these improved binders, surface dressing on main roads
is not advisable after the second half of August.
If for
some particular reason, for example, an urgent need to improve the skid
resistance of a section of road, surface dressing is contemplated beyond the
normal surface dressing season several consequences need to be accepted. These are, firstly, that a smaller size
chipping than that identified in the design process on the basis of Road Note
39 will be required. Secondly, slightly
more binder than is normally used for this size of chippings will be necessary,
and thirdly that because of reasons 1 and 2, the total life of the dressing is
likely to be reduced because of premature loss of texture.
These
factors should be carefully discussed with your surface dressing contractor who
may well be reluctant to give the normal guarantees for work done outside the
normal season..
References
1
TRL
Research Report 296 1991
“The
Relation Between the Surface Texture of Roads and
Accidents”
by P G Roe, D C Webster and G West
Transport
Research Laboratory
2
TRL Research Report 322 (1998)
“The
Polished Stone Value of aggregates and in-service skidding resistance”
by P G Roe and S A Hartstone
3 Local Authorities Code of Practice
for Highway Maintenance 1989
Association of County Councils
ISBN 0 901 783 48 X
4 Road Note 39
Design Guide for Road Surface
Dressing (Fourth Edition) 1996
Transport Research Laboratory
ISBN 0
85235 021 X
Other R S D A publications available are:
Code of Practice for
Surface Dressing
Advice Note on Surface
Dressing Binders
Guidance Note on
Racked-in Surface Dressing
Operators’ Guide to the
Safe Use of Surface Dressing Sprayers
Preparing Roads for
Surface Dressing
Guidance Note on Surface
Dressing Aggregates