Introduction
‘Slurry Surfacing’ is the generic name for a range of cold applied
emulsion asphalt processes. This range has evolved to include ‘Slurry
Seal’, ‘Slurry Surfacing’ and ‘Microsurfacing’, providing thin,
intermediate and thick layers respectively. The thickness of a single
layer ranges from a minimum 1.5mm with slurry seal to a maximum 6mm with
slurry surfacing, and 15mm with microsurfacing. Microsurfacings are
frequently laid in two or more coats.
Pavements suitable for treatment include roads (carriageways, hard
shoulders and central reservations), car parks, airports (runways and
taxiways), footways and playgrounds. Most slurry surfacing material is
rapid setting and is mixed continuously and laid by closely controlled
machines. Appreciable amounts are also made by batch methods or are
factory pre-prepared, and then laid by hand. Slurry surfacings have a
maximum aggregate size (see Table 1) of 4mm, while microsurfacings use
larger aggregate sizes (6, 8 or 10mm). These are more durable, have
greater texture and are normally reinforced with polymer.
The main purpose of slurry surfacings is to stop fretting of the
pavement surfaces caused by loss or ageing of the binder, seal
imperfections and repairs and provide an even, consistent running
surface. Standard slurry seals/surfacings provide an excellent skid
resistance for low speed traffic but because of their relatively low
texture depth, microsurfacings are usually preferred for higher speed
traffic. Slurry surfacing is therefore preferentially used in town or
airport environments where its freedom from dust and flying chippings is
an additional advantage. Microsurfacing extends the use of slurry
surfacing by providing higher performance in terms of texture, skid
resistance and durability. It also has the ability to reshape and
reprofile existing surfaces.
Composition
Slurry
surfacings are now designed by the contractor to achieve an end
performance rather than the original recipe formulations; in accordance
with the move towards European Standards – EN12273, EN12274 – however,
the following general principals still apply to the composition of
conventional slurry surfacing.
A typical ‘mix-on-site’ slurry surfacing consists of
an aggregate to a specified grading, a rapid setting bitumen emulsion
70/100 or 100/150 pen based, to BS EN 13808 (BS434
Pt1) class C60 B7 (K3-60), water and additive. The aggregate
grading depends on the use of the slurry surfacing and the residual
binder content of the mix may vary from about 6% to 13% depending on the
substrate, aggregate grading and traffic density. Generally the
aggregate grading is coarsened and bitumen content decreased as the
expected traffic density increases. Also the thickness of the surfacing
is increased as the aggregate is coarsened. In all cases, aggregate
must be assessed to ensure that it is not too reactive to allow
insufficient time to mix and lay the slurry surfacing.
Typical aggregate gradings for
slurry surfacings are shown in the Table below:
|
Sieve Size
mm |
0/4 (CEN)
Passing % |
0/2 (CEN)Passing % |
|
5.6 or 6.3 |
100 |
|
|
4 |
80-100 |
100 |
|
2 |
50-80 |
70-100 |
|
1 |
35-60 |
50-80 |
|
0.063 |
4-16 |
5-18 |
The aggregate
incorporated into pre-packed slurry surfacing may be finer than those
given in the above table.
For ‘mix-on-site’ slurry surfacing, it is generally necessary to add
water and it is usual to incorporate additives as modifiers to adjust
the consistency of the mix, for prevention of segregation during
spreading, or to slow the rate of setting sufficiently to allow the mix
to be laid without premature breaking of the emulsion. Occasionally
additives may be used to adjust the fines content of the aggregate
grading or for pigmentation.
The composition of microsurfacings is more complex, and it is not
practicable to give information here, each formulation being unique to
an individual contractor, and involving different combinations of
polymer modified emulsions, aggregate gradings and sources, and possibly
the use of cellulose fibres or glass fibres.
Uses
The majority of slurry surfacing takes place on roads with an 85
percentile speed below 50 miles per hour and which carry less than 250
commercial vehicles per lane per day. However, microsurfacing, of
suitable design may be used in more stringent conditions where texture
depth and/or PSV of aggregate become important factors. The use of a
polymer modified binder is normally used under these conditions.
The excellent
adhesion between slurry surfacing and old concrete offers a ready
solution to an often difficult problem.
Multiple applications are practicable where circumstances demand a
heavier coating or where additional protection may be considered useful
over an existing slurry surface treatment, or
where a greater degree of regulating is wanted. Again microsurfacing is
an alternative treatment providing single pass regulating. Typical uses
include the following:
1.
Housing Estate Roads
Slurry surfacings offer the clean, dense surface desirable in these
areas where loose chippings may create a nuisance.
2.
Fretted Surfaces
Slurry surfacings may be used to restore mechanical stability to the
carpets which are deteriorating due to fretting.
3.
Sealing Basecourses
Slurry surfacings are ideal for filling the voids of, and thus sealing,
a basecourse construction. A fast-setting slurry surfacing can provide
a big advantage over other sealing methods for what is often an ‘out of
season’, i.e. winter, problem.
4.
Trenched or Patched Surfaces
Slurry surfacings may be used to provide a uniform sealing coat over the
whole surface provided the reinstatements are sound and reasonably
level.
5.
Polished Surfaces
Slurry surfacings are a means of restoring skid-resistance to general
areas.
6.
Surface Dressing Failures
Slurry surfacings offer a solution to the essentially difficult problem
of treating surface dressings which have partially stripped.
7.
Crazed Surfaces
Slurry Surfacings will go some way to restore the shape of roads where
minor foundation faults have resulted in unevenness, but existing
surface crazing may reappear. Fibre reinforced treatments may help with
this but a cure cannot be guaranteed.
8. Motorways
Slurry surfacings may be used for sealing the hard-shoulders and central
reservations of motorways. Properly designed systems can provide
texture depth where required. However, if the shoulder is to carry
large numbers of commercial vehicles in contra flow arrangements,
microsurfacing should be considered and further advice should be
sought. Coloured slurry surfacings are widely used for central
reservations.
9.
Airfields
Authorities responsible for maintenance of civil airfields have found
slurry surfacings to be a safe useful form of surfacing for runways,
taxiways, holding areas and perimeter tracks.
10. Car Parks & Playgrounds
Slurry surfacings are an effective seal on car parking areas and
playgrounds. To take full advantage of rapid set slurry surfacings,
configuration of these areas must not inhibit the manoeuvrability of
mixing/spreading equipment. The surface should be finally lightly
dusted to allow tyres to turn.
11.
Pad-coat
A slurry surfacing treatment may be used as a ‘pad-coat’ for surface
dressing in circumstances where the chippings would not otherwise embed
sufficiently to ensure long term retention, e.g. on cement concrete.
12.
Footways, Cycle Tracks and Pedestrian Areas
Coloured slurry surfacings are used widely.
Application
T
he majority of areas are treated with
fast-setting slurry surfacing systems which necessitate the use of a
mobile continuous mixer/spreader. Such a machine must be capable of
metering continuously aggregate, emulsion, water and additives into a
continuous mixer and discharging into a spreader box towed behind the
machine.
Slow setting slurry surfacings incorporating emulsion class C60 B7
(K3-60) may be prepared on site using a simple mixer such as a concrete
mixer provided proper attention is given to mix composition, i.e. there
must be adequate means of metering or weighing the separate
ingredients. Alternatively a mobile continuous mixer/spreader may be
used. Again, aggregate must be assessed for its activity before use in
a slurry surfacing.
Pre-packed slurry surfacings may require on-site agitation, sometimes
with a small addition of water, before spreading by means means of soft
brooms, squeegees or mechanical applicators.
Preparation of Existing Surfaces
The surface to be treated must be freed
from all dust and loose material by cleaning thoroughly with a
mechanical broom and/or suction sweeper, supplemented if necessary by
hand brooming. Open cracks must be cleared of vegetation and other
loose material, for which purpose a jet of compressed-air may be
useful. All debris and loose material arising must be removed.
Although not usually necessary, circumstances may make the use of a
bond-coat desirable. In such cases the emulsion should be class C40B3
(K1-40) applied in accordance with REAL Technical Data Sheet No 5, ‘Bond
Coating’.
Compaction
This is not normally required. However, with slurry surfacings which
are virtually untrafficked, one or two passes of a pneumatic tyred
roller may be given as soon as the material has set sufficiently to
ensure that rutting will not occur. In these cases the pneumatic tyred
roller should be multi-wheeled with smooth treads and with off-set front
and rear wheels giving a small overlap when rolling. Individual
wheel-loading should be between 0.7 and 1.3 tonnes and pressures such
that the tyre profile is neither convex nor concave.
After Care
It is usual for slurry surfacings to shed a small amount of their larger
aggregate particles during a short period after the treatment. This
period may vary from a few days to a few weeks depending on the extent
of the trafficking. On the public road system a routine highway
sweeping arrangement will usually suffice for the removal of these
particles since their size is such that windscreen damage does not occur
and nuisance to pedestrians is minimal.
On airfields, depending on the nature of aircraft and their movements,
sometimes it may be desirable to increase the frequency of sweeping.
Restrictions Due to Weather
(a) When using emulsion classes C60 B4 (K3) and A4 (Rapid Set):
Spreading
should not take place when air or ground temperatures are 2C or less
and falling, or during heavy rain. When treating trafficked surfaces
caution must be exercised in laying during cold, damp spells unless
drying conditions can be expected reasonably soon afterwards, since full
strength is not achieved until the film has dried once, even with
rapid-setting slurry surfacings.
(b) When using emulsion class A4 (Slow Set) or
pre-packed slurry surfacing:-
Spreading should not take place unless an
immediate 6 hour period of good weather is forecast; nor should it take
place if the air temperature is below 8C in the shade. Observe
manufacturer’s advice.
In all
cases the surface to be treated must be free from frost, ice, snow or
standing water. After long periods of dry, sunny weather it may be
beneficial to dampen the surface before spreading the slurry.
Materials Testing
(Not normally applicable to pre-packed slurry surfacings)
All slurry
surfacings are now designed to achieve an end performance, consequently
the requirement for material testing is at the design stage, and to
ensure consistency of the materials being used in the process.
Constituent materials will normally be supplied under ISO 9001 quality
assurance schemes, and further testing should not be necessary.
Aggregate:
It is essential that aggregates are laboratory tested to determine their
suitability for the process both as regards compatibility with the
emulsion to be employed and compliance with the Wet Track Abrasion Test
(BS EN 12274-5) during the design process (see BS 434 Part 2 – 2006?).
Aggregate grading should comply with, and be tested with the frequency
required by the specification in use.
Emulsion:
Emulsion should comply with the appropriate requirements of BS EN 13808
(BS434 Pt1)
Additives:
These should comply with the relevant British Standard e.g. Portland
Cement to BS EN 197-1. Testing is not required in normal circumstances.
Mixed Slurry Surfacings:
Samples may be subjected to the Wet Track Abrasion Test (BS EN 12274-5)
and analysed for binder content and grading for design purposes, but
these are not suitable tests for specification purposes or for the
acceptance of slurry surfacing. The test method used for acceptance is
the visual assessment test (EN 12274-8) (see BS434 Pt2 - 2006)
Notes
1. If binder content is to be determined
via sieving extractor, it is not sufficient to rely on silica gel to
extract the water from the system i.e. the sample should be dried first.
2. Difficulty sometimes may be experienced in obtaining a
representative sample from continuous machinery for the determination of
binder content.
Note: Descriptions of grades in accordance with BS EN 13808 may change
when UK Guidance document PD6690 is finalised. During the transition
period in changing from BS434 Pt1 to BS EN 13808, typical BS EN 13808
grade descriptions are used in the text of this data sheet, followed by
the BS434 Pt1 description in brackets. Technical Data Sheet No1 gives a
basic interpretation of grade descriptions from BS EN 13808, but for a
full understanding of the specification and variations in grade
descriptions, reference should be made to PD 6690 when it is available.
For further information see
Summary and Reference Sheets on the Association’s website
www.rea.org.uk
Revised
by the Technical Committee July 2006
Introduction