Formation Sealing
Freshly cut formations for new road
construction contain an optimum moisture content which yields a stable
condition. However, two situations can arise which lead to
de-stabilisation of the material.
(a)
Water Ingress
As new construction is often started in the winter months, rainfall is a
problem and site traffic is needlessly held up by boggy conditions. Use
of a surface dressing as a seal can enable work such as embankment and
bridge construction to continue in spite of the adverse weather.
(b)
Water Loss
Sometimes the formation is cut in hot conditions which lead to
evaporation of the moisture from the surface and it may be necessary to
dampen the surface to optimise the water content. Surface dressing is
intended to minimise this loss and the use of lightly coloured
aggregates reduces the absorption of heat from the sun’s rays.
Recommendations for Formation Sealing
The freshly cut formation should be dense and free from any loose
material and sealed as soon as possible. The bitumen emulsions used
should fast breaking emulsions such as C60B3 (K1-60), C69B3 (K1-70). ; .
Non
Trafficked Areas
For areas that will carry no construction traffic between sealing and
laying of the next pavement course, a single application of bitumen
emulsion at a rate of spread between 1.3 l/m˛ and 1.8 l/m˛ should be
used, depending on the nature of the surface and the bitumen content of
the emulsion (see note (i)). The emulsion should be allowed to break,
turning from brown to black. The surface may then be blinded with dust
at 2-4 kg/m˛.
Medium
Trafficked Areas
Where light construction vehicles will operate over the sealed
formation, a single surface dressing is recommended. In this case the
higher bitumen content emulsions are recommended at a rate of spread
between 1.4 l/m˛ and 1.8 l/m˛, depending on the class of emulsion and
texture of the surface (see note (i)). Clean, 2/6mm crushed rock or slag
aggregate conforming to EN13043, should be used at a rate of spread of
6-8 kg/m˛ and applied before the emulsion has broken followed by light
rolling (for gravel aggregate see note (ii)).
Heavily
Trafficked Areas
Where construction traffic is likely to be heavy, a double surface
dressing is recommended. The use of cement to increase the strength of
the sub-grade may be desirable (see Section 4). High bitumen content
emulsions are preferred for the surface dressings and the rate of spread
for the first dressing should be not less than 0.9 l/m˛ (suitable for a
dense close textured surface). 2/6mm crushed rock or slag chippings
conforming to EN13043 should be applied as soon as possible at 6-8 kg/m˛
and lightly rolled. Excess chippings should be removed and the process
repeated using 0.9-1.2 l/m˛ (depending on the grade of emulsion) and
2/6mm chippings. Alternatively, 4/10mm chippings may be used for the
second dressing at 9-11 kg/m˛ and the rate of spread of emulsion should
then be increased by 0.2 l/m˛ (for gravel aggregate see note
(ii)).
Note (i)
Rate of Spread
When the surface is open textured or porous or under
trees, the rate of spread chosen should tend towards the higher limit.
With these conditions the high bitumen content emulsion is preferred.
When the surface is dense and closed textured, the rate of spread chosen
should tend towards the lower limit. The rates should also reflect the
bitumen content of the emulsion used. For example, when sealing a
formation of normal texture which is to take medium traffic, C60B3
(K1-60) bitumen emulsion could be used at a rate of spread of 1.6 l/m˛
followed by the application of 6mm chippings. If it is required to
change the emulsion to hot applied C69B3 (K1-70) then the rate of spread
should be reduced to 1.4 l/m˛.
Note (ii) Gravel
Chippings
Where gravel chippings are used the rates of spread of emulsion
recommended above must be increased by 0.2 l/m˛.
2. Sealing of Wet Mix and
Other Water Bound Materials
Water-bound
sub-base such as wet mix material depends largely upon an optimum
moisture content for strength and it is therefore important to seal the
surface as soon as possible after laying and compacting. The same rates
of application and types of emulsion as recommended for formation
sealing are applicable. The use of a surface dressing is strongly
recommended as it reduces the risk of damage to the surface and
temperature effects.
3. Sealing of Cement Bound Bases, Sub-Bases and
Lean Mix Concrete
The curing of
cement stabilised materials is extremely important in order to obtain
the maximum designed strengths. The surface should be sealed as soon as
possible after compaction, preferably within one hour of laying.
Emulsion class C60B3 (K1-60) normally should be used although lower
bitumen content emulsions such as C40B4 (K1-40) may sometimes be
permitted on uniform, close textured surfaces. A rate of spread between
1.0 l/m˛ and 1.5 l/m˛ should be used depending on the bitumen content of
the emulsion and the film should be allowed to break. This is then
blinded with dust at 2-4 kg/m˛. The use of a surface dressing is
recommended as this decreases the possibility of damage to the seal.
Where heavy site traffic is to be allowed on the cured material, a
double dressing is recommended. The same specifications as detailed in
the section on surface dressing of formations may be used.
4. Curing In-situ Cement
Stabilised Soil
The in-situ
stabilisation of soils involves spreading a measured amount of cement on
the surface of the material to be stabilised, mixing it with water,
rolling, and then sealing the surface with bitumen emulsion (see Section
3). It is possible to up-grade a wide range of materials quite
dramatically and very economically by this method. The bitumen emulsion
seals the surface thereby maintaining the moisture equilibrium of the
stabilised layer and enabling it to reach its maximum strength.
5. Kerb Sealing
The concrete
backing to kerb stones may be cured with bitumen emulsion. Emulsion can
be used to seal around the base of the kerbs as shown in the diagram to
reduce the ingress of water at the interface of the kerb and the
pavement course. Bitumen emulsion may be applied liberally as it will
act as a joint sealant. Special proprietary emulsions intended for use
as joint sealants are available.
6. Bond Coating
Bond coats should
always be applied between asphalt, macadam courses and bases. Further
information is available in Technical Data Sheet No. 5, “Bond Coating”.
7. Surface Dressing
Surface dressing as
a method of providing a non-skid surface to both the footway and the
carriageway is well documented in Road Note 39 and Technical Data Sheet
No. 4. However, there remains a large potential for surface dressing as
part of the design in new construction, for example, directly to the
Base Course or on High Stone Content Asphalt. This maximises resistance
to deformation and enhances the skid resistance and macro-texture.
C69B3 (K1-70) bitumen emulsion is a particularly suitable binder for
this purpose. If the site is a high stress or high speed area then a
polymer modified emulsion would be more appropriate.
Footways
When surface dressing footways, C60B3 (K1-60) and C69B3 (K1-70) are
recommended. Full information on surface dressing footways is given in
Technical Data Sheet No. 7.
8. Curing of Pavement
Quality Concrete
Bitumen emulsions
are suitable for curing pavement quality concrete. The emulsions should
be class C40B4 (K1-40) or C60B3 (K1-60), the 40% grade normally being
preferred. Within one hour of laying the concrete, it should be given a
coating of emulsion at a rate of spread 0.5 l/m˛ to 0.9 l/m˛ depending
on the grade of emulsion and surface texture, followed soon after by the
application of sand or small grit. The road may be opened to traffic as
soon as the concrete has attained the required strength as specified by
design.
9. Sealing of Hot Rolled
Asphalt Or Macadam Base Course
In
new road construction, especially on large housing or industrial
estates, the base course may be left open for a period before the
overlaying with a wearing course. A surface dressing or slurry
surfacing is an effective method of sealing the course in the interim.
Practical Techniques
for Sealing and Curing
It is important to
agree the most suitable method of treatment at the tendering stage, so
that the correct equipment is used and provision made for the best
practicable job.
Emulsion Binder
Owing to the nature of the work it is probable that only small areas
will be sealed at any one time. This may mean that the use of bulk
distributors will be impracticable and that a spraying unit using a
small motorised sprayer or hand operated equipment is the most
appropriate method of application. This is necessary when the strength
of the base or formation is insufficient to take the weight of a bulk
distributor.
The seal is
effective provided that a continuous film of binder is formed over the
surface. To achieve an even application when working with a hand lance,
the emulsion should be sprayed with a circular action rather than a
side-to-side swing.
hen drums are used they should be set out at estimated intervals along
the carriageway corresponding to the desired rate of spread.
Chippings
It is probable that chipping spreaders, although preferred, may not be
used on most surfaces because of their weight, so small lorries or
barrows will prove more convenient to place small chipping stockpiles
along the site. The chippings used for surface dressing should comply
with EN13043. For the final dressing it is important to ensure that the
binder is well covered with chippings so that it is not picked up on
vehicle tyres.
Rolling
When surface dressing, rolling should be carried out immediately after
the chippings have been applied preferably using a pneumatic tyred
roller.
Drums of Binder
Drums must be rolled vigorously at prescribed intervals (see Technical
Data Sheet No. 2) during storage and before use. Drums must be
protected from frost.
Bulk Emulsion Binder
When the bitumen emulsion is stored in a mobile bulk storage tank, or
used through a bulk distributor, it should be circulated thoroughly
before starting work. When using emulsion binders, reference should be
made to the REAL “Code of Good Practice for the Use and Safety of Mobile
Storage Tanks.”
Health
and Safety
The manufacturer’s Health and Safety Information should be followed.
Reference should be made to Technical Data Sheet No. 2, “Recommendations
for the Safe Handling and Storage of Bitumen Road Emulsions”.
Equipment
After use, clean equipment with kerosene or gas oil. On completion of
the daily work schedule, do not leave emulsion in the sprayer pipework.
Refer to Technical Data Sheet No. 3, “Recommendations for Cleaning and
Maintenance”.
Note: Descriptions of emulsion 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 PD6690 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
