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Surface Dressing Guide, design of surface dressing using aggregate chippings and bitumen emulsion or cutback bitumen
PRACTICAL GUIDE TO SURFACE DRESSING with
INFORMATION SOURCES ON SURFACE DRESSING, FOR FURTHER STUDY, (e.g. Road Note 39)
[ click for smartphone version ] CONTENTS
INFLUENCING SURFACE DRESSING
Note for the 2004and later Surface Dressing Seasons, in relation to chipping
SURFACE DRESSING AND CONSTRUCTION PRODUCT REGULATIONS
This guide can now be downloaded in .pdf format for easy browsing off line
This is only a GUIDE - Somebody is responsible for making the decisions and hence for the quality of the work, or lack of it.
( The work does not go right on its own, and it does not go wrong on its own ! )
IS SURFACE DRESSING
(tar has not been used for 30 years) usually in the
form of an emulsion, is sprayed onto the road surface at an
appropriate rate from the spray bar at the rear of a large tanker
containing the bitumen emulsion, or the cutback bitumen.
Chippings of an appropriate size, dependent upon the hardness of the road surface and traffic conditions, are immediately applied to the
bitumen by a large spreader which usually tows behind it a lorry
containing the chippings. The lorry being backed onto, and coupled to, the spreader hopper. A 5% to 10% surplus of chippings will initially be spread to ensure there are enough chippings to completely cover all the sprayed bitumen.
The crushed premium aggregate chippings, rarely crushed gravel, will be initially compacted into the bitumen with rubber wheeled rollers. However in truth the real embedment of the chippings into the old road surface will take place under the action of the wheels of slow moving traffic.
The excess of chippings is usually programmed to be swept on the second day after the dressing was laid, with a second sweeping usually taking place after about a month
The bitumen part of the surface dressing as well as securing the chippings to the existing surface will seal the old road surface, thus preventing the ingress of water which causes deterioration of the road surface, and the road matrix.
Water if allowed to enter the road fabric, will cause severe damage to the strength/load carrying ability of the road and cause early road failure, i.e. potholes.The chippings will restore
texture to road surfaces that have become smooth with traffic
wear. A significant texture on a road surface is one of the most important aspects in preventing skidding accidents, especially on none salted/gritted roads in the winter.
Only premium quality aggregate, with an appropriate polished stone value (PSV) for the site conditions,
should be used in surface
dressing, i.e. aggregate that is known will not become smooth
under trafficking. Surface dressing will not add any strength to the road pavement, but it does maintain an already strong road in a strong condition for longer by sealing out water, and halting the fretting process caused by oxidation of the surface bitumen.
gives a great deal of information on the surface dressing
process, however it is intended to give a little more background
to the various aspects involved rather than just give a purely
technical approach. I like to think I have included some of the interesting points
other publications seem to miss. I have also tried to pull
together what I consider the more relevant aspects of the many
technical documents that are available on surface dressing. The style may be a little unusual but I hope you find it
interesting. I hope it is apparent that
I am a supporter of surface dressing, it is an excellent, and extremely cost
effective, process.
But surface dressing is ONLY good at what surface dressing is
good at. I would bring your attention to section (3) in the list of
"Factors influencing Surface Dressing'', i.e. Location of
surfaces to be dressed. For specific information
on surface dressing see the Appendix , which contains a
comprehensive list of appropriate publications. If you are serious about
understanding surface dressing, it is important to be aware of
the British Standards / technical documents included in the
appendix, and to have a reasonable idea of what they contain.
It therefore follows that you should hold copies of these
documents in your technical library for reference. It is my personal opinion
production of a Quality Assurance (Q.A.) accreditation
certificate is not sufficient in itself to replace correct
specifying to the appropriate document, and good supervision of
All contractors' Q.A. systems are NOT equal, at least, not yet. This topic is covered in a little more detail in section (11). It has to be stated that
surface dressing is not a popular process with the general public
, who see the process as rather "slap-dash'' and low tech.. However, correctly designed surface dressing carried out in an
efficient and professional manner should not cause an
unacceptable degree of inconvenience to the public. Advising the public about the process associated with good public
relations has got to be a good
idea. This will backfire if the surface dressing is not done correctly,
and authorities cannot afford for surface dressing to become an
unacceptable process if they wish to maintain their road networks in a good and
safe condition at reasonable cost, or more often within budget constraints.
major proportion of roads (I would suggest a figure of
75%) maintained by Local Authorities (so this will be the
major length of the national highway network) have a
surface dressed running surface. But, because the surface dressing has become well established and
secure it is no longer associated with the process that was
performed to achieve it. FACTORS
1) Hardness /
Softness of surface to be dressed.
2) Traffic, Numbers
and type of vehicles.
3) Location of
surfaces to be dressed.
4) Choice of
binder, Cut-back Bitumen or Bitumen Emulsion
5) Choice of
chipping, Size / Shape / Polished Stone Value,( P.S.V.) / Dust
6) Workmanship.
7) Carriageway
8) Application of
9) Application of
binder, Spray-bar tests / rate of discharge of spray-bar /
determination of forward speed of sprayer / accurate spray-rate
charts / tray-tests / tank dipping tests.
10) Rolling /
Action of traffic.
12) Weather, Current weather
conditions / Future weather conditions.
(Plus an extract from the Summer 2010 Newsletter on the importance of the time of the year of the dressing and chipping size.)
Treatment, (Don't Panic).
OF SURFACE DRESSING CHIPPINGS
FURTHER INFORMATION SOURCES RELATING TO SURFACE DRESSING )
Surface Dressing Information to be found on other pages on this website,
NEW: A RURAL 20MM. DENSE BITUMEN MACADAM (NOW CALLED ASPHALT CONCRETE) SURFACING, FOR SURFACE DRESSING, A CASE STUDY
(A study of using 20mm. DBM binder course to provide strengthening and a running surface, to be maintained by surface dressing, an extract from the Spring 2011 Newsletter.)
A FOLLOW UP TO THE ABOVE ITEM, I.E. THE RURAL 20MM. DENSE BITUMEN MACADAM AFTER SURFACE DRESSING WITH 6MM. AGGREGATE CHIPPINGS
( A reference/link to the article and images in the Summer 2011 Newsletter.)
RECENT SURFACE DRESSING VIDEOS FROM THE RSDA
(These are very informative videos relating to the modern surface dressing process, one produced in 2010, and two produced in Spring 2011, watch and learn.)
Note for the 2004 and later Surface Dressing Seasons, in relation to chipping
you have not yet acquainted yourselves with the change in the way surface
dressing chippings are specified I feel you should do so as quickly as
possible. With the introduction of, BS EN 13043 : 2002 : Aggregates for bituminous mixtures and surface treatments for roads, airfields and other trafficked areas
and PD 6682-2 : 2003 : Aggregates - Part 2 : Aggregates for bituminous mixtures and surface treatments for roads, airfields and other trafficked areas -
Guidance on the use of BS EN 13043, replacing BS 63:Parts 1 & 2, which
were withdrawn in June 2004, it has
brought a significant potential change in the nature of the chippings you could
be receiving, especially with regard to increased amounts of
"oversize" chippings.
This matter has been addressed briefly in the "Surveyor" engineering
journal ("Fears over new surface dressing standard",
22/1/2004), with
an industry spokesman reported as saying, "We do not envisage a
problem". But with a spokesman for consultants replying, "I can well understand the
concern. It is for highway authorities and their Materials Engineers to be aware
and check the quality".
At this time (29-06-08) I have not studied any guidance on the new
specifications that may be included in the recently published Road Note 39 (6th. edition), so I believe a potential problem
may still exist for purchasers of
surface dressing chippings, i.e. contractors and clients of the contractors.
Dressing Association
(RSDA) has included some guidance on this matter
on their website, the relevant information is to be found through the link at
the bottom of "Codes of Practice" page.
Maybe I am being unduly cautious, but better that, than to suddenly find you
have a significant amount of oversized chippings spread on your roads that you
have no recourse to rectify.
I urge you to check this matter out if you do not understand what I am talking
about, and to satisfy yourselves about the nature of the chippings you will be
purchasing/using.
Some of the changes introduced to chipping gradings could well affect the design
of your dressing, as the changes in some chipping sizes that the new specification
has introduced have only just been noted in the recently introduced 6th. edition
of Road Note 39 , which I believe is unlikely to be in significant use as a
reference document until the surface dressing season of 2009.
I will not detail my concerns on this page, it is for
you to decide if the new specifications cause you equal concern.
However if you wish to learn more regarding this issue I have prepared two
Guidance Notes, in .pdf format, for you to download and study, but I warn you now, they can be
confusing unless you have a background in laboratory materials, and testing.
Note 1, a more in depth summary of the effects of the new specifications for
aggregate sizes,
Note 2, a table to compare BS 63 chipping sizes and "recommended" new
chipping sizes/categories.
Be aware that the information provided on this
website is only a guide, I stress that it is important for all Engineers and
Engineering Technicians to study "official" specifications and source
documents to arrive at their own views on any item on this website page, or
indeed any other web page on this website.
If your authority is spending hundreds of thousands, possibly millions, of
pounds on your surface dressing programme I really do believe you should have
the support of a qualified and experienced
Materials Engineer, but I would,
would I not.
Road Note 39 - Design Guide for Road Surface
Dressing (6th. Edition), was published in
early 2008, see,
SOURCES ON SURFACE DRESSING
NEW - SURFACE DRESSING AND CONSTRUCTION PRODUCT REGULATIONS (CPR)
With the introduction of Construction Product Regulations (CPR), it follows that you need to know something about these regulations in relation to surface dressing.
To first get a general grasp of CPR may I suggest that you read the
Spring 2014 Newsletter, the item on CPR, there are a number of links to various official bodies where you can get information on the subject.
However with more specific reference to surface dressing may I refer you to the
Summer 2009 Newsletter which refers to the British Standard,
BS EN 12271 : 2006 : Surface dressing : Requirements
and Published Document, PD 6689 : 2009: Surface treatments - Guidance on the use of BS EN 12271 AND BS EN 12273.
Acknowledging the copyright of the BSI I reproduce the abstract/summary of BS EN 12271, "BS EN 12271 specifies a range of categories for the properties of surface dressing to enable users to select appropriate limiting values for a wide range of uses within Europe.
The specifier needs to define BS EN 12271 categories for the properties relevant to the particular end-use of the surface dressing. Some example specifications for different end-uses are given in Annex A.
BS EN 12271 covers surface dressing specified by performance of the finished product. It does not cover surface dressing works carried out to the purchaser's design, commonly called a recipe specification.
Performance categories set out in BS EN 12271 should not be included as part of a recipe specification as this might result in the recipe being correctly placed, but the performance categories not being met."
It is important that each authority, purchaser, consults their legal team to provide them with a definitive meaning of these documents before entering into a contract with any contractor.
It is my interpretation that if you are buying a total end performance product then the contractor must have CPR accreditation/approval.
It is also my interpretation, as it states in the abstract/summary of BS EN 12271, that if an authority is performing their own design and carrying out the work by their in house surface dressing team then the in house contractor does not require CPR accreditation/approval.
It follows that if a surface dressing contractor carries out work in accordance with the design provided by an authority and is not bound by an end performance element in the contract then that contactor also does not require CPR accreditation/approval, but he will be required to fulfil what is contained in the contract, i.e. if the contract states 1.5 litres per square metre, that is what is required, too little or too much is a non compliance with the contact, nothing to do with CPR.
However a client/purchaser cannot have it both ways there is no end performance element to the this form of contract, the contractor is required to provide what is in the contract/design, if the client has got the design wrong and the dressing fails that is their fault it is not the fault of the contractor.
This is one of the reasons that CPR contracts are popular with some authorities, all the responsibility to provide a successful surface dressing is with the contractor.
This is likely to cost more, i.e. an "insurance" element, and of course it is not without possible problems as to who decides what is, and is not, an acceptable dressing. 1: HARDNESS /
SOFTNESS OF SURFACE TO BE DRESSED Remember, under modern
surface dressing contracts let by local authorities it is
normally the Contractors' responsibility to design the surface
dressing. BUT you also should know
how to design the correct surface dressing for a particular site.
That way you will be able to check the contractors designs to see
if you agree, you may not. It is necessary to carry out hardness tests on the road surface
correctly and in sufficient numbers to accurately determine the
road hardness. (See the appendix) You should be aware of the
Contractors' need to select the correct size chipping for the hardness
of the surface, and he should not be tempted to choose a larger
chipping than he should, thinking he will hold it with a higher
rate of spread of binder.
The aggregate from which the chipping is sourced should be of the necessary
Polished Stone Vale (PSV) for the site conditions.
If the surface is hard,
giving little embedment to chippings, choose the smallest
chipping conversant with a reasonable surface texture.
Sizes of chippings to be used are indicated in tables in Road Note 39.
Examples, A well graded
20mm. Dense Basecourse used as a running surface is a HARD
surface, treat it as such, as is a hot rolled asphalt wearing course, it is
likely that a 6mm. size chipping will be appropriate for a first dressing.
2: TRAFFIC -
NUMBERS AND TYPES OF VEHICLES Note, a successful surface
dressing will last for ever if it does not have any traffic on
it. That is to say 20 cars (commuters) and a milk lorry each day on a
rural road. Note, a successful surface dressing will have only a limited life
when it is on a trunk road with 4000 H.G.V.'S grinding up and
down , however well you complete the dressing. The shear weight of the wheel loading on the chippings in the
wheel tracks means the chippings are starting to disappear from
the moment the dressing is complete. This is not to say surface dressing is wrong in heavy duty
situations, it may still be cost effective for a particular site,
it is to say realise it's limitations. It will be the texture that may be lost, the process will still seal and
"hold" the fretting surface of an ageing surface course.
To demonstrate, just think
of a 20 tonne quarry lorry with 10 tonne tare weight, i.e. 30
tonnes, it will probably have 3 axles with a total of 10 wheels,
on average each tyre may have a 500 sq. cms./80 sq. inch
"footprint'', so each wheel footprint of 500 sq. cms./800 sq. inches is carrying
3 tonnes. EACH SQUARE INCH OF
SURFACE DRESSING IS SUBJECT TO A FORCE OF APPROX. 80LBS.
EACH SQUARE CENTIMETRE OF SURFACE DRESSING IS SUBJECT TO A FORCE OF APPROX.
5.5KGS.
And I have not done the "maths" but the force exerted by the footprint
of a "supersingle" tyre is likely to be much greater than this.
long drawn out explanation if it makes people understand the
forces involved and therefore the thought and care needed to
produce a surface dressing that will at least be the best we can
produce in the circumstances we are presented with. Therefore, it is very necessary to know the numbers and types of
vehicle for a particular site in order that you are able to
correctly design the surface dressing.
When you have the traffic
information, plus the road hardness results, the necessary design
procedures are contained in Road Note 39.
3: LOCATION OF
SURFACES TO BE DRESSED Do not dress surfaces /
areas that are not suitable for dressing. Do not dress surfaces that
do not need dressing. Do not dress existing good
surface dressings just because they are 5 years old and adjoin
the length of road you are currently dressing. A good surface is a good
surface is a good surface, and should be left alone until it
shows signs of deterioration. We are talking surface
condition of highways here, not the strength of road pavement
where gradual decrease in strength can be predicted and added to
prior to visual failure. Areas best
to avoid when surface dressing
a) Approaches to busy
junctions, and the junction itself
b) Tight bends that take
heavy traffic, especially the traffic of HGV "trailers" that have
three back axles with "supersingle" tyres on the axles, as on a tight
bend or roundabout tyres either side of the centre axle will "scrub"
the road surface removing chippings from the surface.
c) Entrances to industrial
premises / H.G.V. depots, etc. where you have commercial vehicles
that will "screw'' across the surface dressing , tearing the
dressing and exposing fresh binder for other traffic on the road
to pick up on their wheels and "pull'' chippings from
otherwise successful dressings. This deterioration is swift and on going, and an otherwise
successful dressing can quickly become a "mess'' and cause
further damage to good surface dressing adjacent to the high risk
take the way of minimum risk
Areas excluded from
surface dressing lengths can receive alternative maintenance
either before or after the surface dressing programme.
Possible alternatives being H.R.A. on high stressed sites, and
Close Graded Wearing Courses or even Slurry Macadam on lesser
sites. This may all seem a lot of hassle but if you want to achieve good
dressings you must be aware of possible problems, plan for them,
and include the costs incurred in the programme. 4: CHOICE OF
BINDER, BITUMEN EMULSION or CUTBACK BITUMEN
This is commonly a choice
between cut-back bitumen and bitumen emulsion, the latter being
the more favoured nowadays on safety grounds i.e. its' spraying
temperature is much lower than that of cut-back bitumen.
NOTE - See the item on Road
Note 39 - 6th. Edition, it appears that cutback bitumen is no longer an option
for surface dressing, but I leave the following item for reference.)
BITUMEN (BS EN 12591 / BS 3690 : Part 1):- This binder is a base
bitumen suitably "cut-back'' by the addition of approx. 10%
of kerosene, (or other suitable volatile oil), to produce the
required spraying viscosity. It is the spraying
viscosity that determines the ability of the binder to
"wet'' the road surface and applied chippings and achieve
the initial bond of chipping to road. The spraying viscosity
will also alter the rate at which the spraying tanker will
discharge binder on to the road. The spraying viscosity
will need to be varied according to ambient temperature, i.e. in
normal summer temperatures a 100secs. will be used to provide
adequate wetting of stone and road, in HOT summer weather the
spraying viscosity will be increased to 200 secs. because it is
less lively than 100 secs. and will cause less "pick-up''
problems with chippings. The penetration /
viscosity of the base bitumen portion of the binder can be
anything between 100 pen. to 350 pen.. The best compromise for the base bitumen is a 200pen. bitumen.
The compromise being between the binder softening and
"fatting up'' in hot summer weather or becoming embrittled
and shedding chippings in the cold winter weather. It is interesting that if you ever ask a binder supplier's representative what the base bitumen is in a particular binder he
will always reply 200pen. Binder formulation is a
very complex process, if you have received good product, service
and technical backup from a particular supplier stick with him if
price differences are not great. The base binder viscosity
is the basic property of the bitumen on the road surface after
evaporation of the volatiles, the reverting to the base binder
viscosity can take some time to achieve, up to many months
according to weather conditions, ambient temperature, etc.. The purchaser has no
control of the base binder within the 100 pen. / 350 pen. range,
this is permitted within the British Standard. Note the difference when
quoting viscosities, between viscosities specified in seconds
(secs.) and those stated in penetration (pen.), take care not to
become confused. BITUMEN
EMULSION (BS 434:Part 1, Specification for bitumen road
emulsions):- This surface dressing
binder is a base binder, possibly modified with a volatile oil,
which is then emulsified; i.e. the bitumen element is suspended
as minute droplets of bitumen in water. Dispersant chemicals are
included in the water to keep the bitumen particles separate,
also all the bitumen particles carry a similar electrical charge
which also serves to keep bitumen particles separate by the
"like charge'' magnetic repulsion principle. Bit of physics there, remember your little magnets at school.) Emulsions can be ANIONIC
or CATIONIC. I shall not go into great detail on this other than
to say anionic emulsions have particles that migrate to the anode
when an electrical current is passed through the emulsion, and
cationic emulsions have particles that migrate to the cathode
when an electrical current is passed through the emulsion. Nearly all emulsions used
in surface dressing are Class K1-70. This means it is a
cationic emulsion containing 70% bitumen (minimum 67%), the base
bitumen being between 70pen. to 300pen. to BS 3690:Part 1, as already stated this may be
modified with some volatile oil prior to emulsification.
Note, BS EN 12591 : 2000 : Bitumen and bituminous binders
: Specification for paving grade bitumens , has superseded BS 3690, but reference to BS 3690 may still be present in some
note that under BS 434 the purchaser can request a
"typical sample of the penetration bitumen used as the basic
ingredient of the emulsion'' if you should so desire. This option is not
available when using cut-back bitumens. Although all "rates
of spread'' charts for emulsion take the considerable water
percentage into account, it is worth bringing it to the attention
of those who may be unaware. When, with K1-70 emulsion, you spray at a rate of 1.5
litres/sq.metre, you are in fact only putting 70% of that amount
as bitumen i.e. 1.05 LITRES OF BITUMEN,on the road surface. And although the water percentage encourages speedy
"wetting'' of road surface and chippings, it plays no active
part in retaining the chipping on the road. Emulsion is sprayed at
lower temperatures than cut-back. Emulsion can be used in
damp but NOT wet conditions. Emulsion will "wet''
road surface and chippings more readily than cut-back bitumen. Emulsion does not need
"coated'' chippings. Emulsion tends to be more
expensive than cut-back after allowing for the extra spread rate.
Emulsion NEEDS ambient
weather of a type to give good evaporation of the water present
in the emulsion. RAIN
FALLING ON AN UNBROKEN EMULSION IS A CATASTROPHE, DO NOT LET IT HAPPEN !
2:Code of practice for use of bitumen road emulsions, gives good information on the use
of emulsion binders in surface dressing.
Binders Although there are some
excellent polymer modified binders on the market, I would suggest
everybody thinks carefully before incurring the extra cost of
using proprietary binders of any type. These binders are not
backed by British Standards, and technical information offered by
some binder suppliers is limited. I recommend that anybody contemplating using a modified
(proprietary) binder read the,
Advice Note HA 33/86, Modified Binders for Surface Dressing.
CSS ENG/3-94, County Surveyors' Society report on polymer
There is a great deal of useful information in the Advice Note and the CSS
Report, and they are both well worth reading.
The success of dressings with modified binders may be associated
with the increase in care demanded by the supplier of the binder.
This increase in care is to be welcomed, BUT should be happening
with ALL surface dressing.
suggested polymer modified binders are more prone to
"INVERTED BREAKING'', see section (13).
5: CHOICE OF
CHIPPING, (BS 63 : Part 2)
shape, polished stone value (PSV) In areas with ready access
to good quality aggregate purchasers are fortunate that all
chippings offered for surface dressing are, relatively speaking,
of good quality. This may not be the situation in all areas of the U.K.. BUT it has to be said some chippings are "more equal'' than
others, this is due to the inherent qualities of the stone from
which the chippings are produced. Some chippings will have better PSV qualities, some will have a
lower "flaky'' value, and some will just be a more suitable
shape for surface dressing i.e. angular rather than cubical or
rounded. These problems can be
overcome by choosing particular sites for particular chippings. Examples of this are using angular chippings on hard surfaces
where a flat surface of the chipping will orientate itself to be
in contact with the road and provide good chipping retention with
little embedment, whereas this cannot happen with cubical and
rounded chippings and you get the phenomena of "rolling'' of
chippings on hard surfaces especially on bends and high stress
areas. BUT cubical and rounded chippings will achieve good
embedment into normal and soft surfaces and provide excellent
dressings. They also provide good 10mm. and 14mm. second dressings on
initial 6mm. dressings on hard surfaces. It is interesting to point
out what a particular chipping size is, or what it can be and
still be within a particular size specification.
E.G. 10mm. chippings according to BS 63 are required to have a
minimum of 65% of chippings passing a 10mm. sieve and be retained
on a 6.3mm. sieve. BUT this can mean all chippings are 9.9mm. nominal size or 6.4mm.
nominal size. This is of course an
exaggeration but possible, and it does explain why you hear such
expressions as a "bold'' 10mm. chipping or a "small''
10mm. chipping. This principle of course
applies to other chipping sizes, and can be a particular problem
when you have a "bold'' 14mm. chipping. "Bold'' 14mm. chippings are really quite large and cause a
serious problem if adhesion to the surface is not quickly
achieved. These differences in the
size of specified chippings should be taken into account when
considering their use. It may need a slight adjustment of binder
rate of spread, or even not using certain chippings on certain
surfaces. The ability to be able to make this kind of decision will only
come with experience, backed up by laboratory gradings and
observation. RATES OF SPREAD OF
FOR SINGLE, AND RACKED IN DRESSINGS
The following figures will be a good >>>GUIDE<<< to
estimating the quantities of SURFACE DRESSING CHIPPINGS you
require for SINGLE DRESSINGS and RACKED IN DRESSINGS :- CHIPPING SIZE .............Kg/Sq.metre
..............Sq.metres/Tonne 6mm. ........................8 ...........................125 10mm. ......................11 ............................90 14mm. ......................15 ............................65 20mm. ......................19 ............................50 Racked in 10/6mm. ...........8 and 3 .....................125
and 335 Racked in 14/6mm. ..........15 and 4 ......................65
OF SPREAD OF SURFACE DRESSING CHIPPINGS,
FOR SANDWICH DRESSINGS
The following figures will be a good >>>GUIDE<<< to estimating the
quantities of SURFACE DRESSING CHIPPINGS you require for SANDWICH
DRESSINGS :- ................CHIPPING SIZE .......Kg/Sq.metre
.......Sq.metres/Tonne 1st. LAYER .........20mm. ..............16
...................62.5 1st. LAYER .........14mm. ..............13 ...................85 1st. LAYER .........10mm. ..............10 ..................100 2nd. LAYER .........10mm. ..............11 ...................90 2nd. LAYER ..........6mm. ...............8 ..................125 2nd. LAYER ..........3mm. ...............6 ..................150 NOTE
: If you dispute the
figures above do not email me, go out in your depot yard, mark out a square
metre, weigh out the guide amount of chippings from your "source", and
spread them by hand over your square metre.
Have you got too many or too few chippings, adjust your spread rate accordingly,
allowing for some initial over-chipping to cover all binder.
It really is that simple if you are prepared to get you hands dirty, rather than
sit in your office reading bits of paper, or websites, that tell you what your
rate of spread should be. The above process was how these figures were determined, or verified, with the
sources of aggregate that we had available. If you have access to a materials laboratory it is
possible to determine quite accurately the rates of spread you need to apply for
the chippings that you are actually using. Dust
content of chippings It is important to note
different size chippings have different specified dust contents.
This is not an item in BS 63, which states the same dust
content for all surface dressing chipping sizes, but is included
in the contract documents prepared by local authorities. These different dust
contents are explained by the fact that for a given mass (weight)
of chippings the larger chippings will have less surface area for
the dust to be deposited on, (i.e. less "specific
surface''). If all chippings had a 1%
dust content you would find on observing the surface of the
chipping that with a 6mm. chipping the condition of the surface
was acceptable, but the same amount of dust ( 1% by mass ) spread
over the surfaces of a 14mm. chipping would make it extremely
dirty, and unacceptable, by putting a thicker barrier of dust
between the binder layer on the road surface and the actual
surface of the chipping surface.
The image above shows three samples of chippings submitted as 6mm. surface
dressing chippings, none of the samples actually complied with the
specification, although one was close but did not meet the percentage
requirement for the amount
of "single size".
principle of course applies to high dust contents on all sizes of
And, it is not just dust, there is aggregate dust, i.e. very fine aggregate
particles, and there is clay dust from clay overburden or inclusions in the rock
formations from when the rock is blasted. Clay dust can cause more problems than
Problems like this is why chippings are often "washed" so that they
are able to meet the specification requirements.
Dusty chippings are the root cause of many different types of surface dressing failure including "wet weather" failure and "winter shedding", there are several other reasons, but dusty chippings is definitely at the top of the list.
This is because when using dusty chippings an actual barrier of dust will be present between the layer of bitumen emulsion and the true aggregate surface of the applied chipping, the greater the dust layer the bigger the threat of some sort of failure.
The fundamental principle of surface dressing is quickly and permanently establishing a firm bond between the sprayed bitumen layer and the chipping, that is it, it should not be too difficult if you know what you are doing, and stick to the "rules".
With wet weather failures the tenuous "dusty" bond between chipping and bitumen layer is removed, when during and after rain the water will follow the dust layer around the chipping and subsequent trafficking will dislodge the chippings.
It does not help that the aggregate surface has a greater affinity for water than bitumen, i.e. actual binder stripping but that is another topic for another place.
However the answer to failures caused by dusty chipping could not be more simple, do not accept dusty chippings. You should have immediate access to a simple materials laboratory, you only need a drying oven, a clean water supply for washing the chippings, a set of sieves and an accurate balance, to constantly check the quality of chippings being purchased and used.
If you also had a simple viscometer you could quickly have a fairly accurate assessment of the viscosity of the binder that you are spraying, all pretty common practice in my day, but what do I know.
You may find it interesting to enlarge the picture above and read the side of the green van, somebody still has a Materials Laboratory (I hope) and is taking an interest in the quality aspects of surface dressing.
6: WORKMANSHIP One of the most important
factors, if not the most important factor, in producing good
quality surface dressing is good quality workmanship. The surface dressing gang,
(or operatives if you like "new speak''), at all levels ,
and in all positions should have the ability and experience to
perform their required tasks. A Quality Assurance,
(Q.A.), system will support a good gang, it will not make a bad
gang good. That is even assuming it is a good Q.A. system. This can be a problem
because surface dressing being a seasonal operation it is not
always possible to "hold'' an experienced gang together for
the following season. The problem is not helped
by the the present tendering system where it may not be until
February or March that a company will know how much work they
have been successful in tendering for. Once a company has won
contracts they are quite loathe to turn them down , after all
they would not have tendered for the work if they did not think
they were going to make money out of it. Let us be honest about this, it is the first duty of any company
that undertakes surface dressing
to make a profit or they would cease to exist. This then places a very
heavy responsibility on an experienced gang foreman or area
supervisor to instruct and control new members of the gang until
such time as they have built up the required knowledge /
experience to work without constant supervision. Also, whatever people may
think of the current state of the labour market it is not easy to
recruit workmen with the right qualities of strength, stamina,
timekeeping, and ability, to be part of a surface dressing gang
which is often going to work very long days in not always very
pleasant conditions. If you are able to recruit
men with good experience it is a bonus. 7: CARRIAGEWAY PREPARATION
It is most important in
ensuring successful dressings that the road surface is prepared
adequately prior to receiving the surface dressing. This
preparation work will include the following items
a) Patching of failed
areas of carriageway, including mini-paver work on long lengths
of road haunches to repair deteriorating haunches and
re-establish the camber of the road. This work, when possible,
will be programmed to take place 12 months prior to the surface
dressing to allow for the surface to become fully compacted. Where the patching takes place in the same year as the surface
dressing, the patches will be pre-dressed with 6mm. chippings to
satisfy the absorbency of the patch prior to the main dressing. b) Removal of heavy
contamination (e.g. large areas of compacted mud) on the road
surface will take place well ahead of the surface dressing, with
a later final sweeping of normal road dust and deleterious matter
immediately prior to the surface dressing. This is more fully covered in the contract document. c) Masking of the
ironwork/street furniture will be done accurately with self
adhesive fabric. It shall be done in such a manner to cause a
complete seal of sprayed binder from the ironwork. Masking of the ironwork
prevents water on the carriageway draining away in the event of
rain, bear this in mind because it can be the cause of localised
flooding while it is in place. The masking fabric shall
be positioned to leave a tidy edge. 8: APPLICATION OF CHIPPINGS
Chippings are deposited on
the road surface, that has already received the binder
application, by a self propelled chipping spreader. This machine spreads the chippings on the road surface in advance
of its' own wheels, so that it always travels on newly laid
chippings. The machine has the
ability of adjusting the rate of spread to what is appropriate
for a particular size of chipping. Broad rates of spread for
each size of chipping are set down in Road Note 39, and I will not repeat them here.
But correct rates of
spread are something that can be easily achieved with a
"good eye'', in as much as you need complete shoulder to
shoulder cover plus a small percentage extra as
"insurance''. A rate of spread
"chipping tray'' can be used quickly and easily to determine
rates of spread of chippings, (obvious really). Remember to put a shovel of chippings down to put the tray on. The chipping tray scale
will read off directly in kg./sq.metre, and you will find rates
of spread will alter slightly according to supplier for the same
specified size of chipping, this is due to the crushing
characteristics of each suppliers' stone. Pre-dressings
necessary to pre-dress road strip-widenings and mini-paver haunch
work with 6mm. dressings very early in the season, so that 10mm.
dressings can be laid over the whole carriageway at a later date.
Without this pre-dressing it has been found there is heavy loss
of 10mm. chippings on these areas. "Racking-in"
of chippings
Racking-in is the process
of applying a large chipping (e.g.14mm.) at a reduced rate of
spread, leaving slight gaps between chippings, these areas are
subsequently filled by a light rate of spread of a smaller
chipping (e.g.6mm.). These smaller chippings complete the cover of the sprayed binder
and perform a holding action on the larger chippings, preventing
them being disturbed and picked up by traffic. The rate of spread of
binder for this process is increased by approximately a third
over the normal rate of binder spread for the larger chipping. This process because of
its increased cost is usually confined to more highly trafficked
roads, and often a polymer modified binder is used. It has to be said the
process has been used in some interesting situations, with large
chippings, to save individual soft and fatted surfaces with
considerable success. BUT do not attempt
remedies such as these without men with experience around you,
otherwise you could come to grief. 9: APPLICATION OF BINDER
SPRAY BAR TESTS, RATE OF
DISCHARGE OF SPRAY-BAR / FORWARD SPEED OF SPRAYER / ACCURATE
SPRAY RATE CHARTS / TRAY TESTS / CONE TESTS
correct application of binder to the road surface cannot be
stressed to highly. It is of the utmost importance in
establishing successful dressings. Too little binder and you will
loose chippings, too much binder and you will have "fatted
up'' dressings. Also with an excessive
amount of binder you will have the danger of the pick-up of free
binder on traffic wheels with all the associated problems of
DRAMATIC FAILURE when binder and chippings wrap around traffic
wheels. It is not possible to
determine rate of spread of binder by "eye'' however much
experience you have, except for the two extremes i.e. if you can
see the road surface through the sprayed binder you have laid too
little, or if the binder is running off the road and down the
gullies you have laid too much. If things
are this bad give up and go home !
IS DEPENDENT UPON RATE OF DISCHARGE FROM THE SPRAY BAR AND THE
FORWARD SPEED OF THE SPRAYER.
It seems so simple so why
It is because we do not take the trouble to "know'' all that
it is possible to "know''. a) Check the speedometer
of the sprayer, is it correct? This is simply done by
walking with a measuring wheel and a stop-watch alongside the
sprayer while somebody is observing the speedometer, done over
100 meters the speed is quickly calculated. N.B. It used to be stated
in surface dressing contracts that as well as the requirement of
a road speed indicator in the drivers' cab, there should be a
second , synchronised, road speed indicator mounted in such a
position as to be readily visible to the spray bar operator and
hence any other external observer. These requirements are no longer included in many contracts.
b) Check the rate of
discharge of the spray-bar frequently, (Depot Tray Test). REMEMBER if the viscosity
of binder you are using changes the rate of discharge of the
spray-bar also changes, so check viscosity of binder frequently. REMEMBER if the pressure
in the tank changes from the pressure at which the spray bar test
was done, the rate of discharge of the spray-bar also changes. c) Check rate of spread of
binder actually deposited on the road. This can be done using
"tray tests /carpet tile tests'' and "tank dipping
tests'' Tray Test
This is carried out by putting small trays of known area and
weight on the road in front of the sprayer, the spray-bar skirts
are lifted slightly and the sprayer passes over the trays at the
prescribed speed, the trays are then lifted carefully and put in
small paper carrier bags and weighed using a hanging spring
balance. The tray and carrier bag are of known weight, so by taking this
weight from the weight plus binder the binder weight can be
Tile Test :-
The principles of this test are the same as that for the tray
test. The trays being replaced by absorbent paper squares, not in fact
carpet tiles. Then from tables the rate of binder spread in litres per sq.
metre can be found. This is a very simple little test and quite accurate.
Dipping Tests :-
This test is carried out over a large area of dressing. The sprayer is stood on level ground and the wheel positions
marked, the tank is then dipped and the binder quantity
determined and recorded. An amount of surface dressing is then
done, spraying at a full width, careful note is made of the
length of dressing performed, the sprayer is re-positioned on the
wheel marks and the tank is dipped again and the binder quantity
By establishing the area
covered by the surface dressing and the amount of binder used it
is possible to calculate the rate of spread of binder in litres
per sq. metre. BY USING A COMBINATION OF
ALL THE ABOVE IT SHOULD BE POSSIBLE TO KEEP A FAIRLY ACCURATE
ASSESSMENT OF BINDER RATES OF SPREAD. Depot Tray
To establish the rate of discharge of the spray-bar and at the
same time determine the transverse distribution, (i.e. check the
evenness of spray of the bar across its width), it is necessary
to conduct a "Depot Tray Test''. The procedure for this test, along with blank forms and
calculations, is to be found in, B.S.1707
: Hot binder distributors for road surface dressing
Basically the spray-bar is located over a large tray the full
width of the spray-bar, this tray contains many 50mm. wide
compartments, running in line with direction of travel. The sprayer is discharged at a standard temperature and pressure
for the particular binder and sprayer respectively, for a period
of 1 minute. From the quantities of binder in the compartments of the large
tray the transverse distribution of the spray bar can be
determined, also the total amount of binder discharged in 1
minute. For
further information and pictures on the Depot Tray Test press
Once you have determined the rate of delivery of binder and you
know the required rate of spread of binder needed for a
particular road surface, you can from charts determine forward
speed of the sprayer. BUT, from first principles, to calculate forward speed :-
Speed (metres per min.) =
rate of binder delivery in litres per min. / [spray width
(metres) x binder depth (mms.)] Example :-
Speed (metres
per min.) = 215 litres per minute / [2.134 metres x 1.2mms.]
= 84 metres per minute N.B. 1 litre per sq. metre
= 1.0mm. depth of binder | 1.5 litres per sq. metre = 1.5mm.
depth of binder | ETC. Rates
of Spread of Binder
Once you have established that you can
accurately distribute the required rate of spread of binder on
the road surface, it then becomes most important that the
required rate of spread is correctly assessed.
These rates are presented in table form in Road Note 39. The required rates of spread are achieved after considering the
hardness of the road and the nature of the traffic, the size of
the chipping having been previously assessed from other tables.
BUT IT IS CLEARLY STATED
NOTE 39 THAT
THESE ARE TARGET RATES OF SPREAD
Slight corrections in rate
of spread of binder can and should be considered to these stated
figures to allow for such items as :- 1) Difference in traffic
volume within the category band. 2) Porosity of the
surface, as well as hardness. 3) Time of year of
dressing. 4) Texture of surface 5) Viscosity of binder 6) Fluctuation in the size
of chipping within the chipping size parameter, i.e. "bold''
or "small'' chipping. 7) Is the length of road to dressed
The above factors, and
more, are now included in a table in Road Note 39.
10: ROLLING, ACTION OF TRAFFIC
There is not too much that
can be said about this, so I will be brief.
Steel-wheeled rollers are ruled out, (except in special
applications), by most local authority surface dressing
contracts, as they can crush the chippings, and they do not follow the transverse profile of the road, so some chippings receive no initial embedment at all.
steel-wheeled rollers are permitted, and are good where the the surface of the road is flat or on a constant cross fall. However even these rubber coated rollers do not follow the contour of the road well where there is significant undulation or camber/crown on the road, but they are unlikely to crush chippings from good quality aggregate sources.
The image to the left, which can be enlarged is typical of a rubber coated steel drum roller.
Pneumatic tyred,
multi-wheeled rollers, as shown in the image to the right, are to be recommended for the
rolling/compaction of surface dressings, as these rollers will
best follow the contours of the road and duplicate the action of
traffic. Pneumatic tyred, multi-wheeled rollers are a must for rural roads that are undulating and often heavily "crowned" as these roads will receive little traffic to embed the chippings into the binder layer.
BUT It has to be said the
best way of rolling/compacting/orientating the applied chippings
into the sprayed binder/road surface is to use the action of
traffic. This is done by keeping the traffic speed low and by using cones
to move the traffic across the complete width of the road.
Sometimes it is necessary to use a slow moving "lead''
vehicle to control traffic speed. This process is of course
only viable on roads that have a significant amount of traffic .
On rural roads and other lightly trafficked roads the rolling the
surface dressing receives is dependent upon that applied by the
roller and as such initial embedment/adhesion will not be great. (And for the "average" motorist reading this, it really is not going to damage your car if you are travelling at the correct speed. I should know, I have driven thousands of, careful, miles on fleshly laid surface dressing in my new, and getting older, cars, to take samples and perform various forms of test, without causing them any damage.
If you do get any slight damage it will be from the "idiot" either overtaking you, or driving to fast in the opposite direction, the signs are plain enough.)
Quality assurance has
become a very complicated business indeed, almost an industry in
itself rather than a supporting role to good management and
quality. It is increasingly necessary to know more than a little about
this subject, but it is too complicated to cover here, so it is
up to the reader to find his own way into this subject. For the beginner I would suggest you start with copies of the
various parts of B.S.5750, now known as various parts of the ISO
9000 series, and work your way from there, these specifications
will refer you to others you may need. Unless you are involved
almost full time in Q.A. it is virtually impossible to keep up
with the constant stream of new documentation and the revision of
existing documentation. What I will point out is
that you must NOT assume two contractors/suppliers
"certification'' will give the same standard of process, it
will not. Very simply explained
"certification'' means that the contractor/supplier has
complied AT THE TIME OF ASSESSMENT to the procedures he has
included in his quality manual. The standard of the procedures included in different quality
manuals can be markedly different. The only real way to compare
contractors/suppliers is to be able to compare their respective
quality manuals, and getting sight of quality manuals is like
trying to find rocking horse droppings. There is one good point
from this, if a contractor/supplier is prepared to show you his
quality manual he must have a reasonable amount of confidence in
it, and also confidence that he will be able to comply with it. Without knowledge of what
is in a quality manual, "Certified to B.S.5750'' ( or
whichever standard is current ) does not mean a lot. However it will mean more than not being able to make that
statement. If I have to make a statement on quality assurance, it is,
"Be careful". 12: WEATHER CURRENT WEATHER CONDITIONS
/ FUTURE WEATHER CONDITIONS. (The universal excuse !) When dressings "fat
up'' in the summer it's because it's too hot, when dressings
"strip'' in the winter it's because it's too cold, or so
some people would have you believe. BUT, not all dressings
"fat up'' in the summer, neither do all dressings
"strip'' in the winter, so weather only influences the
situation, it is not the cause of the problem. The successful dressings prove it is possible to achieve good
work even when weather conditions are a little more severe than
"normal''. Rates of spread of binder, size of chipping, need to stay as
close as possible to the original design irrespective of possible
adverse weather. Extremes of weather will be of relatively short duration, basic
hardness of the road and traffic flows will stay the same for the
bulk of the year. What can be altered in hot weather is the spraying viscosity of
the binder, by increasing binder spraying viscosity in hot
weather it is possible to decrease the problems of "picking
up'' and dressings "not going off''. The alteration in spraying viscosity will be achieved by altering
the proportion or nature of the "cutback'' volatile oil
present, the viscosity of the base binder should stay the same. After-care during hot
weather should become a part of the dressing programme and a cost
allowed for. Lively (at risk) dressings
should be treated with applications of fine grit to retain a
physical barrier between binder and traffic wheels. If this barrier can be retained during hot weather the dressing
will remain undamaged until normal temperatures prevail, and
sweeping of excess grit can take place, leaving a satisfactory
dressing. Loss of chippings in
abnormally cold weather has to be said is very often linked with
late dressings which have not had sufficient time to embed. But other simple factors like the use of too large a chipping,
insufficient binder rate of spread and dirty chippings will all
predispose the dressing to chipping loss in a bad winter. With late dressings be cautious about increasing the binder rate
of spread. I have known the binder rate of spread be increased to overcome
this late dressing problem only for the dressings to fail in hot
early summer weather by "fatting up'' the following year, so
keep to prescribed rates of spread of binder. Late August early
September dressings are best avoided if possible. SURFACE DRESSING 2010 - TIMING IS IMPORTANT (Extract from the Summer 2010 Newsletter)
Anybody who has browsed this website knows I am a big supporter of surface dressing performed correctly by Engineers, Engineering Technicians and Contractors who know what they are doing, at the right time of the year.
It is likely that in excess of 65% of most County highway networks will be have an established surface dressing that is the surface that vehicles are driving on. If we just take the rural part of a County highway network it could well be 90%. And it is only 90% because of the new bituminous mixture surfacing of 10mm. CGM's, (close grade macadams), 14mm. CGM's and 20mm. Dense Bitumen Macadam Binder courses, as running surfaces, that will be waiting to be surface dressed at the appropriate time.
So, here are a few photographs for those of you who never get out of your offices for fear of revealing what you do not know. Some would say you do not need to know about these processes, however when your department is spending around two million pounds on this treatment (and this is not an unusual amount for any large County) I think that you do.
I do not know exactly how long the dressing has been laid as I came upon it on one of my frequent country walks around the area that I live, but it had not yet received its first sweep, so it is less that 36 hours old. The date is 24th. of April 2010, so it will have been a fairly early dressing for 2010, you can see that the leaves are only just coming out on the trees.
I hope that you have not failed to notice that it is recommended that drivers only travel at 20mph. This happens to be a relatively busy rural road going from Market Bosworth to Stoke Golding and on to the south side of Hinckley. I would estimate that few vehicles, if any, were going below 30mph. and most were going a good deal faster.
One young man, with a car full of young men, in a fairly new car, probably his mum's, delighted in flooring the throttle as he went round the bend throwing chippings everywhere.
I would not be surprised if his mum put a claim into the council for bitumen on the sides of her car, her son never telling her the whole story of how it got there.
This did happen, I did not make it up, I relate it to inform the reader of some of the poor cooperation that councils have to contend with from the motoring public when conducting the surface dressing programme.
They really do not know how safe this process keeps them in wet and wintry weather on all roads, but especially on the rural highway networks.
But in fairness, the public may respond to the 20mph. boards better if they were removed after the second sweep at 28 days, not left there for many months according to the need for boards for new dressings.
Below are photographs of this surface dressing, which I take to be a 14mm. racked in with 6mm., with the 14mm. chippings being on the small size.
(Note : A 14mm. chipping passes a 14mm. sieve and is retained on a 10mm. sieve, so theoretically all "14mm. chippings could be 10.1mm. or 13.9mm. in their largest dimension, hence the terms a "bold 14mm." or a "small 14mm.", of course the actual situation is somewhere in the middle. But this is something an experienced surface dressing supervisor will bear in mind on sites where the chippings are used, in relation to both rates of spread of chippings and the rate of spread of bitumen emulsion binder.)
24th. April 2010
Above is a picture of the dressing shortly after being laid, it has not had the first sweep so it is unlikely to be more than 36 hours since laying.
You can observe there is a slight excess of chippings, mostly 6mm. chippings, to ensure complete cover of the bitumen emulsion binder layer.
2nd. July 2010
This is the same site in, to as close as I am able to estimate, the same location after almost three months. The chippings have achieved good interlocking embedment, and the dressing is excellent.
A larger picture of the view above
A larger picture of the view above.
A more detailed view of the established dressing.
A more general view of the established dressing, i.e. an excellent dressing that has sealed the road surface and is providing an excellent texture to the road surface.
Assuming that the polished stone value (PSV) and the aggregate abrasion value (AAV) is appropriate to the site (which I happen to believe is likely in this example, as I think I recognise the aggregate so I know its source and characteristics) this will be about as safe as a "normal", i.e. not high friction surfacing, can be.
This process also gives the opportunity to use a high PSV aggregate on "difficult" sites, to improve skid resistance, that do not fill the accident prevention criteria for a high friction surfacing, I have had experience of such a policy.
These factors are why good surface dressing is so important to rural road networks where not all roads will be part of the salting routes in winter. During Winter weather that consists of freezing temperatures and frost, but not completely covered with snow, providing the motorist drives accordingly, the texture provided by a good surface dressed road surface is likely to give more grip to the vehicle tyre than any other surface.
If a surface dressed road has become "fatted up", as they occasionally do over time, then the surface needs appropriate remedial treatment, but this is the exception rather than the rule.
I have added this item to the newsletter to stress how important it is to complete as much of the surface dressing programme that involves large chippings, i.e. 14mm., as early in the season as possible to allow the dressing the maximum amount of time to embed, during the warmest part of the year, with the longest day lengths at the warmer temperatures. If it is logistically possible I would suggest that all 14mm. dressing should be completed by the end of July, all 10mm. dressing by the end of August and only 6mm. dressing performed in September, if at all.
New comers to surface dressing fail to realise the importance of day length and hence the sun in the sky warming everything up, including road surfaces. In May, June and July the sun is "up" from five'ish to nine'ish, but by September it is almost half of this time, and the nights are much colder and road surfaces take longer to warm up in the morning.
Any dressing in April, although the weather may not be brilliant has four good months to embed, anything laid in late August early September has little if any opportunity to embed before the winter.
Do not be tempted to increase the binder rate of spread to "hold" the chipping through the Winter, it will only bring problems, in the hot weather of the following year.
I mention this not only to establish good practice, but because many people have never taken information like this on board when considering efficiency measures, I would urge caution if you are looking to make efficiencies by doing more work over a longer period with one surface dressing "train", consider your dressing programme very carefully, or it may not turn out as efficient as you thought.
I again make no secret of liking the surface dressing process. It is very cost effective, and leaves behind excellent surfaces when performed correctly, both sealing the road surface against the ingress of damaging water and providing a hard wearing surface with good texture and hence anti skidding properties.
I worry when the accountants are in charge and make "cost savings" with little if any knowledge of what they are doing, and unfortunately there are getting less and less experienced Engineers in senior positions with the clout to uphold engineering judgement.
"Managers" now hold sway over most day to day running of highway maintenance, but do they know what they are "managing", I think not.
13: AFTER TREATMENT I am not going to comment
on general after treatment, this is covered well enough in other
documents e.g. Road Note 39. BUT, if you have a problem soon after dressing this usually means
chippings and binder picking up on traffic wheels, causing the
surface dressing to deteriorate rapidly.
SO, DO NOT PANIC, REMOVE
THE TRAFFIC AND THINK ! Usually the only immediate
course of action open to you is to coat the surface with clean grit, or
3mm. chippings if available. This will physically cover the binder, which is causing the
problem, so that traffic can again use the road. KEEP TRAFFIC SPEEDS LOW,
With slow moving "lead'' vehicles if necessary, this allows
chippings and grit to orientate and settle, and prevents physical
disturbance of chippings by high speed wheel action.
( We have all seen chippings
streaming from the wheels of Mr.''I've got a company car, and
nobody's telling me to do 20m.p.h.'', and doing all sorts of
damage to dressings. ) SLOW TRAFFIC DOWN,
(nicely of course, remember "Customer Care'').
The reasons why the binder
is still lively are various and must be investigated at a later
date after the dressing has been made safe. BUT one, not uncommon,
problem with emulsions is "INVERTED BREAKING''. This is when the air to
emulsion surface breaks first, in effect sealing in the water
content of the emulsion with a thin layer of bitumen. This means the emulsion
stays "lively'' much longer than it should, sometimes many
hours. This problem, if recognised early, does not usually need
gritting, but does need very careful traffic management.
Traffic speed must be kept very low and moved across the whole
dressing by channelling the traffic with cones. This slow traffic movement
will cause the chippings to disturb the sealing film of binder
and allow the water in the emulsion to evaporate and hence attain
the more stable viscosity of the bitumen content.
In the most extreme cases it may be necessary to remove traffic
for a time to allow the emulsion breaking process to happen prior
to increased traffic management. Once full evaporation of
the water element of the emulsion has happened the dressing can
be treated as normal. Hooray ! CONCLUSION In some respects it is
unfortunate that surface dressing is a low cost process, (I do
not like the word "cheap'' it lets people believe that the
process is not a quality product). I am sure that if surface dressing cost �10 per square metre
instead of �1:50 pence / �3:00 per
square metre the degree of control and care exercised over the
process would change drastically. Surface dressing will be
even more cost effective if we have no remedial work, or in a
real "surface dressing world'' as little as possible, I hope
these notes contribute to making that situation possible. Doing remedials costs you
money, "skimping" in some way on the original dressing
never offsets the cost of returning to site to do work again. Also, it is a fact that if
you complete 10 miles of superb surface dressing but have just
100 metres of failed dressing it is amazing how people can only
see the failed area and not the good dressing! This is a guide to surface
dressing, highlighting some of the problems you must be aware of,
and perhaps indicating one or two interesting complications which
may not get a mention elsewhere. But, I have no intention
of "re-inventing the wheel'', there is plenty of excellent
information already published on surface dressing so it has not
been repeated in this guide.
I recommend TRRL REPORT
SR 627 : A guide to road surface dressing practice,
an older report but well worth reading to obtain a good overall description of
the process of surface dressing and what is entailed.
information on the surface dressing process see the following section for
the documents you need to study. Then get two or three
years practical experience under your belt, or better still four
or five, and only then will you really start to understand
surface dressing, and what it involves. Get it right and you will
improve the quality of your road surfaces considerably, and in a
very cost effective manner. Get it wrong and you will
receive a load of complaints from the public and their elected
representatives, and you will have wasted money. Good Luck ! [
Top of Page to Repeat this Section ]
NEW: RECENT SURFACE DRESSING VIDEOS FROM THE RSDA
I am not a big browser of "Youtube" videos, but if you use this facility appropriately there are now some excellent videos relating to various highways maintenance processes, there is also a lot of rubbish. You also need to be aware that many of the videos that are placed on "Youtube" are put there by clever marketing and public relation companies to influence you to buy their products.
However, having given the warnings I would like to bring to your attention three excellent videos on "Youtube" placed there by the Road Surface Dressing Association (RSDA). I strongly recommend that you view these videos and pay attention to the information that they contain.
It is also good to note that the two videos produced earlier this year, 2011, are sponsored by some of the leading companies in the highways maintenance industry. I do not usually name commercial companies on my pages but I will mention Nynas, Total Bitumen, and Aggregate Industries (Bardon Aggregates) who all supported the production of the surface dressing, Part 1 and Part 2, videos.
It is good to know that companies of this size and standing are openly supporting the continued use of the surface dressing process, because in my opinion over recent years some of the large companies have been trying to suggest that surface dressing is not an acceptable process in "modern" highways maintenance. Watch the videos and learn.
I will make one observation on what I viewed out of concern for my friends in local authorities, and their surface dressing procedures. In one of the later videos it states that a fresh surface dressing, after continued rolling, will be brushed prior to opening to traffic to remove surplus chippings.
This will only occur on motorways and trunk roads, if/when they are surface dressed. It is normal practice on local highway networks to wait 36 hours, or thereabouts, according to what time in the day the dressing was laid, before the first sweep will take place. (There will be a second sweep after about 4 weeks.)
This is to allow a good bond to establish between chipping and binder before the initial sweeping. This practice has proved successful over many years, the presence of the surplus chippings does encourage the traffic to obey the advisory speed limits. But it is sad to say that you can easily distinguish between company car, and commercial vehicle drivers, and drivers who own their own vehicles once the traffic control at the time of laying is removed.
RSDA Surface Dressing - Introduction (2010)
RSDA Surface Dressing Video - Part 1 (2011)
RSDA Surface Dressing Video - Part 2 (2011)
I will add a few words of warning.
It is my opinion that are now some bituminous mixtures that are being used as surface course that do not lend themselves to long term maintenance using the surface dressing process.
This is mentioned because if the surface course that is surface dressed still fails it is likely that the surface dressing will be claimed as "having failed" when it is the layer below it that has in fact failed.
Be fully aware of the nature of the surface course that you are surface dressing. It is my opinion that the thicker and denser surface course bituminous mixtures are the better option if you are going to use surface dressing as part of the programmed maintenance of your highway network.
Surface dressing is a surface treatment, it will help to preserve/maintain the existing strength of the road pavement it will not improve it.
It will seal the road surface preventing the ingress of water, and it will improve the surface texture and skid resistance, especially if an aggregate with a high polished stone value is employed.
I repeat, watch the videos, and of course read this page, there is much basic/practical information here that is not included in the videos.
NOTE 39 : Design guide for road surface dressing - (6th. Edition now current)
Published by the TRL, this excellent report provides comprehensive information
and guidance on surface dressing, however it is NOT to be regarded as a
The report provides information on such items as, hardness of surfaces, size of chippings,
polished stone value (PSV), skid resistance, rates of spread of
binder, temperatures, traffic categories, types of dressing, methods of work,
The sixth edition, was issued in early 2008, it is likely that this
document will not be relevant for the 2007 surface dressing season, but I would
suggest that you obtain copies as soon as you are able as it contains many
updates that are relevant to recent changes in specifications and the increased
use of polymer modified binders.
I acknowledge copyright by TRL
of the following summary of Road Note 39 - 6th. Edition, obtained from their
"Road Note 39 is a guide for the design of surface
dressing for roads throughout the United Kingdom (it is NOT prepared as, nor
should it be used as, a specification). This Sixth Edition of the Road Note is produced by a Panel representing all
sides of the industry, with the Chairman and Secretariat provided by the
Infrastructure Division of TRL Limited. The following extract emphasises the importance of the Road Note: "Surface
dressing can be used successfully on all types of roads, from the country lane
that carries only an occasional vehicle to trunk roads and motorways carrying
thousands of vehicles a day. It provides a simple but cost-effective form of maintenance (Carswell, 1994;
Nicholls & Frankland, 1997; Milton et al., 2001). Unfortunately, the
attention paid to design, control, supervision and aftercare is frequently less
with surface dressing than with more expensive forms of construction. Lack of
attention to detail shortens the useful life of a surface dressing; the benefits
of adequate control, particularly in the period when traffic is first allowed on
the new surface dressing, cannot be emphasised too strongly." The principal differences from the Fifth edition are: CEN standards (European
standards have been introduced for aggregates and bitumen and are being
introduced for surface dressings and so there are a lot of changes to the
standards that need to be referenced); Nomenclature for aggregates (European
standards for aggregate have introduced a nomenclature for aggregates of d/D,
where d and D are the minimum and maximum sizes, so that the use of a/b for
multiple system surface dressings would now become more complex and has been
discontinued); Nomenclature for emulsions (the European standard for bitumen
emulsions has replaced the K1 70 classification by several, more specific
classifications); Cut-back binders (cut-back binders are no longer generally
available from binder suppliers in the UK and so have been removed, leaving all
binders to be one of the varieties of bitumen emulsion); Spread rates for
proprietary polymer-modified binders (polymer-modified bitumen emulsions now
represent a major part of the market so the rates quoted in the Road Note with
an assumed binder (solids) content of 67 % can be adjusted by multiplying by 67
over the proportion declared for bitumen emulsions with a different proportion);
Traffic categories (the weight of passenger vehicles has increased, particularly
with the use of people carriers and off-road vehicles becoming more widespread
so that the proportion of vehicles over 1.5 t is increasing and the number of
roads with very low flows of medium and heavy vehicles is reducing); Simplicity
(as far as practicable, the method has been simplified without changing the
basic methodology and, in particular, the use of categories for binder spread
rates has been discontinued); and Electronic documentation (where documents are
available on the web, the web address is given that can be used as a link,
although no distinction is made between those that can be downloaded freely and
those that require payment, either for each document or as a
subscription)."
IN MY OPINION, ALL PERSONS INVOLVED WITH SURFACE DRESSING MUST HAVE ACCESS TO A COPY OF ROAD NOTE 39
SR 627 : A guide to road surface dressing practice, very sound, overall, coverage of the subject, even if it was published a while
ago, a report well worth reading if you are new to surface dressing, or wish to
TRRL REPORT SR 573 : Surface dressing, assessment of road
BS 63:Part 2 : Specification for single-sized aggregate for
Sets limits for "chippings'' e.g. grading, flakiness index,
aggregate strength,
This standard is now superseded by BS EN 13043, and was withdrawn in June
2004, but I would suggest that you do not throw it away it contains useful well
proven information and guidance, you might wish to follow this guidance by
"transposing" it into the new terminology of the new superseding
specifications, it can be done, well, as near as will not make much difference
This standard specifies the properties of aggregates and filler aggregates obtained by processing natural or manufactured or recycled materials for use in bituminous mixtures and surface trreatments for roads, airfields and other trafficked areas.
PD 6682-2 : 2003 : Aggregates - Part 2 : Aggregates for bituminous mixtures and surface treatments for roads, airfields and other trafficked areas -
Guidance on the use of BS EN 13043
BS EN 13043 : Aggregates for bituminous mixtures and surface treatments for roads, airfields and other trafficked areas", in the
EN 12271-3 : 2002 : Surface dressing : Specifications :
chippings (superseded)
12271 : 2006 : Surface dressing : Requirements (supersedes
BS EN 12271-3 : 2002)
Rate of spread and accuracy of spread of binder and chippings BS EN 12272-2 : 2003
: Surface dressing : Test methods : Part 2 : Visual assessment of defects
EN 12272-3 : 2003 : Surface dressing : Test methods : Part 3 : Determination of binder aggregate adhesivity by the Vialit plate shock method
PD 6689 : 2009: Surface treatments - Guidance on the use
of BS EN 12271 and BS EN 12273
BS 434:Part 1 : Specification for bitumen road emulsions (superseded)
Composition and properties of emulsions, including those used for
BS EN 13808 : 2005 : Bitumen and bituminous binders -
Framework for specifying cationic bitumen emulsions (supersedes
This document specifies the requirements for performance characteristics
of cationic bitumen emulsion classes which are suitable for use in the
construction and maintenance of roads, airfields and other paved areas.
3690:Part 1 : Specification for bitumens for roads and other
Composition and properties of bitumens, including cut-back
bitumen for surface dressing.
: Specification for paving grade bitumens , has superseded BS 3690,
but reference to BS 3690
may still be present in some contract documents.
1707 : Hot binder distributors for road surface dressing.
Gives definition of what is required on "sprayer'' to fulfil specification, details of Depot Tray Test, sampling recording
sheets, etc..
D.Tp.
Standard HD 28/94 : Specification requirements for aggregate
properties and texture depth for bituminous surfacings to new
(supersedes HD 21/92), although the information is the same and
is now included in Road Note 39, fourth edition.
Gives requirement for Polished Stone Values (P.S.V.) of aggregate
for specific site / traffic levels.
Standard HD 31/94,
(supersedes D.Tp. Advice Note HA 33/86) This standard includes a section on surface dressing, and gives
guidance on the use of modified surface dressing binders. Modified binders are usually proprietary formulations containing
a polymer additive of some type and are not covered by British
D.Tp, Standard HD 37/97, Part 2, Chapter 8, Surface Dressing
This standard provides excellent information on the whole surface
dressing process, it is necessary reading.
Specification for Highway Works, Clause 919 Surface
Dressing:Recipe Specification
D.Tp. Specification for Highway Works, Clause 922 Surface
Dressing:Design Specification
Comprehensive clauses containing a lot of contractual information
that client and contractor MUST be aware of for Highways Agency
work, or for those authorities who defer to these contract
documents when letting tenders. The
Authority's Surface Dressing Contract Document
This will stipulate which British Standards, D.Tp. Specification
Clauses, Road Notes, etc. are part of the Contract.
include extra requirements specified by the County, e.g.
different dust contents for chippings of different nominal size,
texture depth requirements, etc..
Technical Information/Leaflets
It is always useful and informative to obtain what knowledge you
can of the materials you are buying and using. It is sometimes more interesting to realise what you cannot find
out about materials you are buying and using. This especially applies to proprietary "modified"
of Practice for Surface Dressing, ( and other very useful information
booklets, and a video, relating to surface dressing ), from the
Road Surface Dressing Association, (RSDA).
ENG/3-2000 : RSDA / CSS Code of Practice for Signing at Surface
This code is published jointly by the Road Surface Dressing
Association and the CSS ( formerly the County Surveyors Society
It has been designed in accordance with the principles set out in
Chapter 8 of the Traffic Signs Manual, but is not meant to
supersede any part of Chapter 8.
CSS, Derbyshire County Council, Environmental Services
Department, County Hall, MATLOCK,
Derbyshire, DE4 3AG ------------ Tel. and Fax. Matlock, 01629
585730 (CSS office)
If you are using, or contemplating using, modified binders, this
Scheme Document No.13 - The Supply and Application of Surface
Dressings to Road Surfaces
I understand all contractors wishing to carry out work for the
Highways Agency need to comply with the requirements of this
Quality Assurance is a set of procedures/instructions for the
contractor to set down in his quality manual, and to employ these
procedures in his working practice to ensure work is done in a
uniform manner to a set quality. Also the contractor should be able to trace back poor working
practice or materials, when failure occurs, from his records.