ROAD SURFACE DRESSING
ASSOCIATION
GUIDANCE
NOTE ON
SURFACE DRESSING
AGGREGATES
Consultant Director and Secretary
John
Baxter
Little Horkesley
Essex CO6 4BS
Ó
Copyright RSDA 1993
First
Published
Revised 2001
Second
Revision 2004
reproduced without the written permission of the Road Surface Dressing Association
No part of this
document may be
R S D A
Aggregates Guidance Note 2004
INDEX
|
|
|
Page No |
|
1 |
Introduction |
1 |
|
2 |
|
2 |
|
3 |
Polished
Stone Value PSV |
2 |
|
4 |
Availability
of High PSV Aggregates |
3 |
|
5 |
Flakiness
Index FI |
4 |
|
6 |
Aggregate
Abrasion Value AAV |
4 |
|
7 |
Immersion
Tray Test |
5 |
|
8 |
Texture
and Resistance to Skidding |
6 |
|
9 |
Department
of Transport and Local Standards of MSSC |
7 |
|
10 |
Relationship
between MSSC, PSV and Commercial Traffic Levels |
8 |
|
11 |
Storage
of Surface Dressing Aggregates |
9 |
|
12 |
Loading |
9 |
|
13 |
Colour |
9 |
|
14 |
Selection
of Surface Dressing Aggregates |
10 |
|
|
References |
10 |
ROAD SURFACE DRESSING ASSOCIATION
Guidance Note on Surface Dressing Aggregates
INTRODUCTION
There are three types of aggregates used for
surface dressing:
Crushed rocks
Crushed gravels
Artificial aggregates
Pending the introduction of
European standards, the requirements for single size aggregates to be used in
surface dressing in the
BS EN 13043:2002 has a
corresponding Published Document PD 6682-2:2002, this document gives guidance
on the use of the standard and in Table 4 gives the preferred grading
requirements for the different sizes. Annex A of the PD 6682-2:2002 gives an
example specification for surface dressing aggregate with suggested values for
aggregate physical properties.
The correct size of chipping for
use in a surface dressing on any particular section of road should be
determined in accordance with the recommendations contained in Road
Note 39, 5th edition published in November 2002.
The principal factors are the road hardness
measured by a Hardness Probe(1), the number of commercial vehicles
carried each day, total traffic volume, traffic speed and altitude above sea
level.
Size is not the only
consideration, and other qualities required for surface dressing chippings are
listed below, the values required being dependent upon the circumstances at
each site:
(a)
Resistance to fragmentation
(b)
Resistance to polishing
(c)
Satisfactory shape
(d)
Resistance to abrasion
(e)
Affinity to surface dressing
binder
These Qualities
are measured and expressed in the following terms, as detailed in British
Standards or elsewhere and described in the following pages:
(a)
Los
Angeles Test, a maximum value of 30 would normally be specified
(b)
Polished
stone value – PSV
Values
of 45 to 70, depending on the risk rating of the site.
(c)
Flakiness
index – FI
A maximum
flakiness index of 20 would normally be specified.
(d)
Aggregate
abrasion value – AAV.
Values of less
than 12 are usually specified.
(e)
The
immersion tray test – degree of
binder adhesion expressed as a
percentage
(not applicable to emulsified binders)
1
LOS ANGELES TEST
With the implementation of
harmonised European Standards, the Los Angeles Test has replaced the Ten
Percent Fines Value test as a measure of rock “strength”. The test procedure is
given in BS EN 1097 Part 2 and measures an aggregate’s resistance to
fragmentation rather than its compressive strength.
The test procedure is to place an
aggregate sample in a revolving steel drum along with a charge of large steel
balls. By carrying out a grading of the sample before and after the prescribed
test period, a determination of the degree of fragmentation created during the
test can be established and is reported as the Los Angeles Value.
2
POLISHED STONE VALUE PSV
Freshly laid surfaces using almost
any type of aggregate will initially provide similar resistance to
skidding. Some aggregates however
quickly polish under the action of traffic which reduces the resistance to
skidding. Aggregates which quickly
polish under the action of traffic have low PSV’s
whilst those that resist polishing have a high PSV.
The PSV test attempts to reproduce
in the laboratory the polishing effect of vehicle tyres on aggregates. In order to carry out this test which is
described in BS EN 1097 Part 8, 6.3/10 mm sized samples of the
aggregate to be tested are prepared in a curved mould and set in epoxy resin. The prepared samples are then mounted on the
circumference of a wheel as shown in Figure 1, the wheel is then rotated,
and a loaded rubber wheel is applied to the samples. Water and coarse emery is then fed between
the samples and the rubber wheel for a given time. The samples are then washed and the process
repeated using emery flour and water for a further period of time. The samples are then subjected to a pendulum test, see Figure 2, where a pendulum with a standard
rubber slider is swung over the sample under controlled conditions. The polished stone value is measured by the
degree of retardation of the pendulum caused by the friction between the rubber
pendulum and the sample.
Although this test is reasonably
reproducible for a given sample of aggregate, aggregates from a quarry or
gravel pit vary and for this reason the polished stone value of a particular
aggregate should refer to the mean of the three most recent consecutive results
carried out in the previous six months, or by the average range of values
excluding the maximum and minimum values.
In this way an aggregate may be referred to as having a PSV of 55 – 60
or 60 – 65. At sites carrying high
volumes of heavy commercial vehicles, where there are tight bends, steep
gradients, and where there are traffic lights at junctions or pedestrian
crossings, PSV’s 65 -70 are often specified. At sites carrying up to 750 commercial
vehicles per lane per day on main line sections of motorway and other dual
carriageways, where heavy braking and turning is unlikely to occur, a PSV of
about 55 should normally be satisfactory and on single carriageway minor roads
of similar character, a PSV of about 50 is normally acceptable.
In general terms, as the volume of
heavy commercial vehicles increases and events such as junctions, bends and
gradients are present, higher polished stone values are necessary to maintain a
satisfactory resistance to skidding under dry conditions. The maintenance of skidding resistance under
wet and dry conditions is also dependent upon surface texture
(see Figure 3) and vehicle tyre treads(2).
Figure 3[1]
4 AVAILABILITY
OF HIGH PSV AGGREGATES
At the present time, only a
limited number of quarries in the United Kingdom consistently produce chippings
for surface dressing with an accepted PSV of 68 or above, and four or five
produce surface dressing aggregates with a PSV of 65 or above. Calcined bauxite
imported from Guyana and China have been shown to have PSV’s
of about 70, but they are significantly more expensive than the best surface
dressing chippings available within the United Kingdom. In spite of this, their use at the limited
number of sites where they are needed to maintain satisfactory levels of
skidding resistance can be justified in accident reduction terms. On the other hand, the use of aggregates with
PSV’s in excess of those required to meet particular
site conditions cannot be justified in financial terms and also represent a
waste of a scarce national resource.
In recent years, the demand for
high PSV chippings exceeded the supply and it seems likely that this will
continue to be the case in the foreseeable future as commercial traffic volumes
grow. Unless the PSV selected is kept to
the minimum level necessary for particular site conditions, there will be a
shortage for some sites requiring higher levels of PSV, these then having an
unacceptable accident risk under wet conditions.
5 FLAKINESS INDEX FI
This test is described in BS EN
933 Part 3. In the test a flake sieve
consisting of steel rods of specified dimensions is used in order to determine
the percentage by weight of flaky chippings. This characteristic is important
in surface dressing because after initial rolling and compaction by traffic,
individual chippings will tend to lie on their longest dimension and a flaky
chipping will not stand above the binder film to the same extent as a more
cubically shaped chipping.
Figure 4[2]
Flakiness Sieve
6
AGGREGATES ABRASION VALUE AAV
This test is described in BE EN 1097
Part 8 and is used to simulate the abrasion
caused by traffic on chippings in a road surface. In the test, nominal 10/14 mm chippings are
mounted in a shallow tray and subjected to wear by means of a lapping machine
fed with sand as the abrasive, at a constant speed, for a given time. The percentage loss in weight on the sample
at the end of this period is used to calculate the abrasion value.
7. IMMERSION TRAY TEST
The immersion tray test is not
applicable to emulsion binders. This
test has two objectives. The first is to
determine the compatibility of the aggregate and binder selected for the
dressing, and the second is to indicate the quantity of adhesion agent required
to improve the bond between chipping and binder to an acceptable level. The test, which is described in Road Note 39,
consists of applying a film of the binder at a thickness of about 1½ mm
onto a small tray and allowing it to cool to about 20° C.
It is then immersed in water to a depth of 25 mm, the water having
previously been heated to 20°. Six or more
pieces of aggregate, usually of 14 mm nominal size, are then applied to
the binder film under water and lightly pressed into the binder film. After 10 minutes, the tray of binder is
removed from the water tank and the chippings carefully removed and the
percentage of binder retained on the chippings is visually assessed and
recorded.
Where good adhesion between
chippings and binder is not indicated by this test and cut back binder is to be
used, adhesion can be improved by one or more of the following methods:
(a)
lightly
coating the chippings with a thin film of binder,
(b)
chemically
coating the chippings,
(c)
spraying
adhesion agent on top of the binder film before the chippings are applied to
the road surface,
(d)
adding
an adhesion agent to the binder in the spray tanker before spraying commences,
or
(e)
heating
the chippings before application to the binder film.
Lightly coating
chippings with bitumen ensures a quick bond between the chippings and the
binder film to which they are applied and is a technique to be recommended
where cut back binders are being used on roads carrying fast moving
traffic. It is most important that after
coating, chippings are free flowing and this is far more important than a
requirement that the chippings should be not less than 90% coated. Many different grades of binder have been
used but it is recommended that a binder of 40/60 Pen is used. As a rough guide 0.5%-1% of bitumen by weight
should be enough to provide a satisfactory coating but it is necessary to
ensure that the binder applied to the chippings has not been burnt or
carbonised. They should not be used with
emulsions as this will seriously delay the break of the emulsion resulting in
an unstable dressing during its early life.
Coating chippings
with a chemical designed to enhance adhesion has been used successfully. The presence of the coating cannot, however,
be visually detected. As with coated
chippings, their use is not recommended with emulsion binders as this will
seriously delay the break of the emulsion.
Applying a fine
spray of adhesion agent directly onto the binder film onto the road immediately
before the chippings are applied is probably the most effective method other
than precoating of chippings and applies the adhesion
agent only at the point where it is required.
It is also possible to start and stop the operation quickly as
circumstances demand.
Adding adhesion
agents to bitumen tankers before spraying commences is expensive – some £15 per
thousand litres treated at 2000 prices.
The agent has to be added to the tanker and circulated for a minimum of
20 minutes, thus losing productive time.
The effectiveness of some adhesion agents added to binders in this way
diminishes quite rapidly with time and treated binders lose some of the effect
when used on the following day. The
adding of adhesion agent will also lower the viscosity of the treated binder.
8 TEXTURE AND RESISTANCE TO SKIDDING
Methods for
measuring texture depth are given in BS EN 13036-1. While the PSV of the aggregate in a surface
dressing plays a significant part in the resultant levels of skidding
resistance, the texture of the surface as a whole is also important. As traffic speeds increase, surface texture
becomes more important, because, under wet conditions, the texture provides a
space into which water can be displaced as vehicle tyres pass over the
surface. In this way contact is
maintained between the chippings in the road surface and vehicle tyres. Tyres with
poor treads on roads with very smooth textures can, under wet conditions,
result in aquaplaning, ie the vehicle ceases to
respond to steering or braking. Good
textures are required on roads where vehicle speeds are likely to exceed
50 km per hour.
Prior to 1990,
the traditional way of measuring texture was by spreading a known volume of
sand, of a standard grading, in a circle on a dry road surface until it could
be spread no further, as the sand filled the texture on the road surface. The texture depth was then expressed as a
relationship between the volume of sand and the area of the patch. Under this method, known as the sand patch test, large diameter circles
indicate poor texture, while on a road with a good texture, sand patches are
relatively small, although to measure the texture depth accurately it is
sometimes necessary to alter the volume of sand. The test method for this
particular test has now been amended to use glass beads instead of sand and
hence is simply known as the Patch Test.
The fact that
this method of measuring texture was slow led the Department of Transport’s
Research Laboratory to develop the high
speed texture meter (HSTM). This
apparatus consists of a trailer which can be drawn behind a vehicle travelling
at normal traffic speeds. Lasers and microprocessors continually record details
of the road texture and other characteristics including gradient, crossfall and curvature.
Texture measurements resulting from the high speed texture meter are not
identical to those measured using the patch method, but they do provide a
reliable comparison between different textures, and the speed at which the
apparatus operates allows regular monitoring of the texture of the most heavily
trafficked roads.
A less
versatile, and consequently cheaper, mini laser texture meter has also been
developed by the TRL. This apparatus
measures the average Sensor Metered
Texture Depth (SMTD) over 50 metre test lengths.
At the present
time the Patch Test is the only test contractually acceptable in the case of
dispute.
The skidding
resistance can be assessed by measuring the sideways force co-efficient (SFC).
The apparatus used is the sideways
co-efficient routine investigation machine (SCRIM). This vehicle is a lorry with a test wheel
mounted between the axles of the vehicle, at an angle of 20° to the forward direction of the
vehicle. The forces acting on the wheel
when the vehicle is travelling at 50 km per hour on a wet
surface are used to calculate the SFC50. The wetting of the road is achieved by
applying water in front of the test wheel from a storage tank mounted on the
vehicle. Because the SFC of a road
surface varies throughout the year, SFC measurements are taken during the
summer when values are at their lowest.
A series of tests are taken on each section of road to be assessed
between the months of May to October.
These measurements give the mean
summer sideways co-efficient at 50 km per hour – MSSC50. Department of Transport Advice Note HA/36/87
has established that there is a rough relationship between SFC50 and
the portable skid resistance pendulum tester which is used for determining the
polished stone value. The pendulum
produces a skid resistance value (SRV)
and SRV is approximately equal to 105 x SFC50.
A more mobile
and versatile method of measuring skid resistance is by use of the Griptester. This is
a trailer mounted piece of equipment having a test wheel geared down from the
trailer axle, the resultant drag created on the test wheel is converted into a Gripnumber. There is
a correlation between Griptester and SCRIM although,
currently, SCRIM is the only contractually acceptable method of measurement.
9 DEPARTMENT OF TRANSPORT AND LOCAL
STANDARDS OF MSSC50
In 1987,
desirable minimum MSSC50 levels were set down for the first time by
the Department of Transport for trunk roads and motorways under their control,
and this was followed a little later by advice from the Local Authorities’
Association(3) on the levels appropriate for the less important
roads. The values which have been
indicated in these documents are described as investigatory limits, below which consideration should be given to
the action necessary to improve the skidding resistance of the road in
question. It is likely that
investigatory levels are to be the subject of continual review in the light of
experience and increases in traffic volumes.
10 RELATIONSHIP BETWEEN MSSC50,
PSV AND COMMERCIAL
TRAFFIC LEVELS
Table 6.3.1 of
Road Note 39 (Fifth edition) which is reproduced below with the permission of
the TRL, shows minimum PSV requirements for different sites and commercial
vehicle trafficking. Research undertaken
by many highway authorities has indicated that immediately after the completion
of a surface dressing, the SFC is at its maximum but that it falls over about
the first 12 to 18 months but thereafter maintains a more or less constant mean
summer average as long as the road remains in a satisfactory condition.
Figure 5[3]
11 STORAGE OF SURFACE DRESSING AGGREGATES
Because peak
demand for surface dressing aggregates exists during the surface dressing
season, it is necessary for highway authorities and contractors to take
delivery of chippings over a period of months before the surface dressing
season starts. For this reason, it is
vital that chippings are stored in carefully selected locations, on
well-drained hard-standings, free from dust of other fall-out, including
leaves, and in bays where different sizes and different types of aggregates can
be separated. Such storage areas should
be located with easy access to main roads, and ideally at 10 or 12 mile centres
for maximum efficiency. It is important
that adequate space for lorries to turn and to load is available within the
storage area, which should be secured against trespassers.
Even when all
the above precautions have been taken, the long term storage of chippings can
cause problems. Rain can wash small
quantities of dust from chippings near the top of the pile and concentrate the
dust content at the bottom of the pile, contaminating these chippings and
rendering them out of specification. The
degree to which this happens depends very much on the nature of the chippings,
but needs to be taken into account. Contamination of chippings and stockpiles can
also occur when loading shovel drivers attempt to “tidy up” the site and by
collecting chippings which have become deposited on the manoeuvring area of a
chipping dump and replacing them on the stockpile, thus contaminate the
stockpile. Loading shovel drivers should
be made aware of this problem and the danger of “over handling” chippings at
storage areas.
12 LOADING
The more
surface dressing chippings are handled, the greater the risk of abrasion
resulting in an increased dust content.
For this reason, mechanical loading or handling should be kept to a
minimum, particularly when using high PSV aggregates, which, by their nature,
are less resistant to abrasion.
13 COLOUR
Surface
dressing chippings are available in a wide range of colours including white,
buff, grey, black, green, pink and red.
These are
listed in Road Note 59 Sources of White and Coloured Aggregates in Great
Britain. During the intervening time
some of these sources have changed hands, whilst others have ceased
production. Reference to the Directory
of Quarries and Pits published by the Quarry Managers Journal Limited, 7 Regent
Street, Nottingham, NG1 5BY, can be helpful in identifying whether or not the
sources referred to in the Road Note are still available.
Light coloured
aggregates can be an aid to safety, particularly at night in areas with a poor
standard of street lighting. Colour can
be used to differentiate minor roads from main roads, and through roads from
housing estates or parking areas, and hard shoulders from carriageways. A careful combination of coloured aggregates
can be both environmentally attractive and technically sound but, inevitably,
the cost of coloured aggregates for any site will be heavily influenced by the
distance between the site and the quarry.
14
SELECTION OF SURFACE DRESSING
AGGREGATES
There is a need
to exercise engineering logic in the selection of surface dressing
aggregates. There has been a temptation
in recent years for some engineers to demand that surface dressing aggregates
for use in their area should conform to more stringent requirements than those
set out in BS EN 13043, particularly with regard to dust content. Unless that requirement is accompanied by
similar extraordinary standards for the storage and handling of surface
dressing chippings in perfect conditions and the pre-sweeping, or even washing,
of roads before the application of binder, these requirements are illogical.
Improvement in
the quality of surface dressing is much more likely to occur by greater
attention to design, supervision and aftercare than to higher standards for
either the aggregate or the binder used.
Natural aggregates are a non-renewable resource and the best available should
be regarded as a valuable national resource.
REFERENCES
1
Hardness
Probe
S M E, 173
Handley Road, New Whittington,
Chesterfield,
Derbyshire S43 2Eep.. Tel: 01246 450 518
2
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
3
T
R L Research Report 322 (1998)
“The Polished
Stone Value of aggregates and in-service skidding resistance”
by P G Roe and
S A Hartstone
4
Local
Authorities Code of Practice for Highway Maintenance (1989)
Association of
County Councils
ISBN 0 901 783
48 X