1 PREAMBLE
1.1 General
To the
highway engineer, surface dressing offers a quick, efficient and cost-effective
way of maintaining skid-resistant and waterproofing road surfaces. To obtain the best results it is necessary to
give careful consideration to a wide range of detail and to plan and design the
work carefully. The speed of the surface
dressing operation and the short duration of time during which motorists are
inconvenienced is also an important issue.
The
purposes of surface dressing are to waterproof the road surface, to arrest
disintegration, to provide texture, and provide a skid-resistant surface. This latter quality can play a major part in
accident reduction and was highlighted by the initiative of the Department of
Transport in 1987 when the Minister introduced minimum mean summer SFC values
for motorways and trunk roads. The
importance of surface texture as provided by surface dressing has been highlighted
by TRL report LR 286, which stresses that texture depth is important under both
wet and dry conditions.
The
purpose of this Code is to identify the important aspects of the process, and
to refer to other documents relating to good surface dressing practice.
1.2 Health, Safety and Environment
All those involved in
preparing and executing surface dressing operations have a legal duty of care
for the health and safety both of the operatives carrying out the works, and
those who come into contact with the operation whilst in progress and during
aftercare.
The planning and organising
for health, safety and environmental issues must commence as soon as a surface
dressing programme is envisaged. The
Construction Design and Management Regulations will apply to most surface
dressing operations and therefore clients, designers and planning supervisors
must plan and prepare the information and documentation necessary to inform the
prospective contractors of the specific hazards identified on the various sites
and the level of risk which is envisaged.
This must take into account the nature of the site, the materials to be
used, the traffic management requirements and any special health, safety and
environment issues that have become evident during the pre-tender stage.
The client should employ a
competent contractor. The procedure of
selection can be carried out at tender list stage by a questionnaire. Once the contractors have been selected, the
Outline Health and Safety plan contained in the tender document should be
detailed enough for the prospective contractors to take account of the health,
safety and environment issues in their tender submission.
On the appointment of the
Principal Contractor to carry out the surface dressing operations, it will be
his duty to prepare a Detailed Health and Safety Plan of that particular
contract or works. This must itemise the
methods to be employed to overcome the specifically identified hazards and risk
reduction measures that will be in force on this contract.
Once the works commence the
Principal Contractor has the control of health, safety and environment matters
but liaison with the client, police and the general public on issues of
congestion, diversions or closures must be ongoing throughout the contract.
The Principal Contractor has duties under other pieces of
legislation to look after the health and safety not only of his own employees
but of other persons who work alongside them and also of the passing
public. Written risk assessments must be
prepared which can be used to identify control measures for both physical and
chemical hazards. The measures must form
the Contractor’s safe systems of work which enhance the safe behaviour of the
workforce as well as protect the general public during the various stages of
the works.
Account must also be taken of
environmental factors with pollution from fumes, noise and dust being the main
concern during the work phase. Disposal
of waste and protection from spillage and contamination are other
considerations when looking at the overall surface dressing activity.
1.3 Training
The
design of surface dressing and its execution is dependent on a wide range of
factors. Many engineers and technicians
have been involved with surface dressing, but only a limited number of them
have had any formal training in the design, specification and execution of the
work. This lack of training often
results in inappropriate selection of materials, unsatisfactory preparation,
incorrect rates of spread and lack of traffic control and aftercare. All these can lead to premature failure and
increased costs.
A useful way of comparing the
effectiveness of a dressing, or other maintenance work, is to express it in
terms of a ‘cost life index’. This is
the cost per square metre of the work divided by the satisfactory life in
years. It provides a measure of the
“value for money” which the highway authority is achieving.
A low ‘cost life index’ and
“high value for money” is the result of high-quality work. This requires that all personnel involved in
any operation should be appropriately trained.
It is the Association’s view that this principle should apply to all
personnel involved in surface dressing and is a requirement under ISO
9002. The RSDA runs regular training
courses details of which can be obtained from the Consultant Director and
Secretary.
1.4
Quality Assurance.
The
Road Surface Dressing Association continues its commitment to quality assurance
and has been instrumental in producing the Sector Scheme for Surface Dressing
which is number 13a in the series of Sector Scheme Documents.
The Sector Scheme Document,
which was first published in 1998, is available on the UKAS website www.ukas.com .
The Sector Scheme Technical Advisory
Committee is currently chaired by the Association's Consultant Director and
Secretary, from whom further details are available.
1.5 Planning and Coordination
Careful and detailed planning
before work commences is an essential element of successful surface
dressing. There should be close
co-ordination between contractors and their clients at every stage, commencing
with a pre-works meeting, the purpose of which is to ensure total understanding
of the way that the programme will proceed.
Contract
prices will be provided by contractors based on an indication of the likely
size of the client’s programme.
Significant reduction in the size of the indicated programme will
increase the contractor’s overhead costs per square metre of work undertaken
and contracts should make provision for compensating contractors under these
circumstances. It is in the interests of
both contractors and clients that the programme of works flows smoothly from
site to site without the need to travel many miles for the purpose of dressing
small areas of road.
2 DEFINING THE SITE AND ITS CHARACTERISTICS
Before
any design, planning or dressing is undertaken, it is important to identify the
lengths of road to be dressed and to draw up a schedule. A clearly understandable system such as a
line, arrow and job number on the road surface is recommended.
3 DETERMINING THE SPECIFICATION
3.1 The selection
of the right type of dressing, size of chippings and rate of spread of binder
is as important as the design of other engineering works. Each site must be considered in the light of
its unique characteristics, including the nature of surface, geography, volume
and speed of commercial and other traffic using the section of road. Advice on the design of surface dressing is
contained in the fifth edition of Road Note 39 "Design Guide for Road
Surface Dressing” (2002) published by TRL Limited, Old Wokingham Road,
Crowthorne, Berkshire RG45 6AU.
3.2 The
design considerations of surface dressing are particularly important in order
that the dressing applied to the road surface is right for the circumstances in
which it has to operate and the job it has to do. This is equally the case if the design is
done by the Client or Contractor.
3.3 In recent years, there
has been a movement towards "End Performance Specifications". End Performance Specifications transfer the
responsibility for the design of the dressing to the selected contractor who is
also responsible for the execution of the dressing and generally guarantees the
dressing for a specified period of time.
3.4
Advice on "End Performance" and "Recipe"
specifications can be found in clauses 922 and 919 respectively of the Highways
Agency Design Manual for Road & Bridgeworks available from HMSO.
4
SITE INFORMATION
It is important that the person in charge of surface
dressing operations on site fully understands the type and extent of the work
required. One way of achieving this is
the preparation of a schedule and map indicating the sections of road to be
dressed, the length and average width of each section, the area to be dressed,
the type of binder, rate of spread of binder, size of chipping and location of
chipping storage area – The Information Sheet.
5 PLANNING
THE EXECUTION OF THE WORK
In
addition to compiling the site information, the person supervising the dressing
may need to decide which width of spraybar and type of chipping machine to use
and where joints are to be located. Most
modern equipment can operate in standard lane widths and, where possible, the
equipment selected should be compatible so that the spraying width and chipping
width at each pass are the same (see Section 6 – Spraybars, Section 7 – Joints,
and Section 12 – Chipping Applications).
Unless
they are made with proper care, joints can become weak points on any
dressing. For that reason, the number of
joints made should be kept to a minimum.
5.1 On narrow roads, it is better to
spray the whole width in one pass, after making appropriate arrangements for
temporary diversions. Where wider roads are being dressed, there are also
advantages to be gained by introducing traffic diversions while the work is in
progress. This causes minimum
inconvenience to road users and separates traffic from the surface dressing
operations.
5.2
Temporary
diversions should not be introduced casually and will involve consultations between
contractors and the highway authority.
5.3 Poor planning can result in low daily output, increased costs and
public criticism. The information sheet
mentioned in paragraph 4 also enables supervisory staff to give proper
consideration to the order in which various sections are dressed, the number of
lorries required to haul chippings from storage sites and when and where
additional binder deliveries are required.
Wherever possible, operations that result in a break in the spraying
operation can be planned to coincide in order to minimise “down time”. Such operations include moving from one site
to another, transferring binder from one tank to another and heating the
binder.
6
SPRAYBARS
Most modern tankers are fitted with adjustable spraybars
enabling the distance across the spraybar to be varied, whilst in motion, to
cater for any changes in road width.
This enables the spraybar to be operated from 0.1m up to its maximum
width.
Adjustable spraybars come in two types:
1) Expandable: These can be expanded
to the particular width required.
Typical machines available in the UK have maximum widths of 4.0m – 4.3m.
2) Extendable: These can be extended
by bolting on extra sections.
Fixed width spraybars are available in several sizes, from
2.25m to 2.75m. These are often mounted
on smaller tankers and are ideal for narrow roads, particularly housing
estates, where manoeuvrability is essential.
The spraying width is also variable from 0.9m upwards.
In order to achieve the desired rate of spray of binder,
two systems of control are used:
1) The driver uses a pre-calibrated
chart to select the correct forward speed and drives to that speed as indicated
on a special low-speed speedometer.
2) The spraybar is equipped with
automatic ground speed control with the operator pre-setting the required rate
of spread and, the controller then electronically controls the forward speed of
the machine.
Note: for the highest levels of accuracy demanded by Clause 922 (see section
21), automatic speed control is required.
As it is essential that the
correct amount of selected binder is applied onto the road surface, it is also
necessary to check that the spraybar is working correctly. Member companies of the Road Surface Dressing
Association are required to have all their spraybars tested by a
NAMAS-accredited testing house at the beginning of each season. Contractors will also undertake additional
tests to BS 1707 to calibrate their spraybars for use with various binders.
BS 1707 requires spraybars to be
tested in depots that are suitably equipped, where binder is sprayed for a
given duration over test trays divided into sections, each of which is 50 mm
wide. A satisfactory spraybar will
deliver the same amount of binder into each section (plus or minus 15% of the
average) and a spraybar that does not achieve this standard requires either
adjustment or replacement of jets.
6.1.1
Checking Output
Although a successful test result
will show that any particular bar is capable of operating within the limits
specified in the British Standard, checks should be made at the start of each
day and during the day as appropriate, for example if there has been a long
break in the continuity of the work, to ensure that the jets are continuing to
operate satisfactorily.
Spraybars are fitted with either
swirling or slot jets. The output from
any jet is affected by both the temperature of the binder and the spraybar
pressure. The rate of spread of binder
on the road surface is additionally affected by the speed at which the spray
tanker moves.
Some machines are fitted with
equipment that automatically adjusts the output from the spraybar to compensate
for variations in the forward speed of the spraybar. Filters are fitted in the pipework feeding
binder from the tanker to the spraybar to prevent any solid material reaching
the spraybar where it could cause the total or partial blockage of a jet. It is essential that these filters are
checked at the start of each day as well as during the work, particularly if a
drop in pressure is observed during spraying.
For bars
fitted with swirling jets, a site test indicating whether or not all the jets
are operating correctly can be carried out as follows:
All the
jets, with the exception of the two that are at the extreme ends of the bar,
should be closed and the bar opened momentarily. The tanker should then be moved forward about
a metre and the process repeated, closing the end jets and opening the
penultimate jets at each end of the bar.
This process should continue to be repeated progressively along the bar
until all the jets have been operated. This should produce a pattern as
indicated in Diagram 1 and the photograph in appendix E.
Where all the jets are working
properly each of the patches of binder on the road surface will be circular and
of similar appearance. Where a jet is
partially blocked, the shape of the spray from that jet will be irregular. It should be removed, cleaned and retested. This test may not be appropriate for bars
fitted with slot jets but where it is possible, the spray pattern from a
blocked or misaligned jet is noticeably different from jets operating
correctly.
On-site testing of transverse
distribution can be carried out using carpet tiles or similar material. In this test, tiles forming a continuous
strip, the full width of the spray bar and taped together, are laid in the path
of the spraybar. For a spraybar
operating correctly across its full width, the difference in the weight of each
tile before and after spraying with binder should be similar.
Once it has been established that
a spraybar is operating correctly in a transverse direction, longitudinal
distribution is normally controlled by the vehicle’s forward speed.
The rate of spread of binder over
any section of road can also be calculated by comparing the area of the section
dressed with the amount of binder used as determined by dipp
