General
SMA is a high stone content,
gap graded material which has the voids between aggregate
particles essentially filled with a mortar of bitumen, sand,
fine crushed rock and filler.
There is no British Standard
for SMA's. They are all proprietary mixes and are very rich
in bitumen to the extent that measures have to be taken to
prevent bitumen from 'draining' from the mix during
transport. Most commonly bitumen drainage is prevented by
addition of either polymer modifiers or cellulose fibre.
True SMA is intended to be virtually impermeable and has
very good resistance to deformation by virtue of its 'stone
to stone' skeleton. The level of texture depth achieved is
largely a function of the material design, texture depth
achieved is therefore likely to be more consistent than with
HRA, where the application rate of coated chippings is
critical.
Specification
For works being carried out
against the HAUC reinstatement specification the material
should be manufactured in accordance with the requirements
of SHW cl 937, but with fixed binder contents of 6.0+/- 0.5%
for 14mm and 6.2+/- 0.5% for 10mm. Although this clause is
refers to binder course and regulating course its use has
been considered appropriate. Note that the clause requires
compaction to achieve a mean maximum in-situ void content of
4% whilst the HAUC specification of 8% maximum for what is
envisaged to be hand lay work.
HAPAS approved thin surfacing
may also comply with the HAUC requirements but
there is no guarantee. Some HAPAS materials give very high
void contents and will consequently give contractors
problems achieving the specified 8% maximum. Purchasers
should make it clear to suppliers that the work has to
comply with an air void specification.
Transport
To allow effective placement
and compaction it is most important that temperature loss is
minimised during transport and storage. The high bitumen
content of mixtures means that provided material
temperatures remain elevated then compaction is relatively
easy!
Always
transport in well insulated, double sheeted lorries or hot
boxes.
Try to use the material
direct from lorry or hot box. If practicalities of the
operation do not allow this then at least discharge into a
wheel barrow and deliver to the excavation without delay.
Never tip loads
on to adjacent carriageway for use sometime later!
Preparation
Normal good practice (S6.5)
applies but the importance of some elements of preparation
work may need to be reinforced:
All vertical faces of
adjacent carriageway, ironwork etc should be effectively
coated with hot bitumen or treated with an appropriate edge
sealing system. Treating the surface of the joint with
bitumen is no more acceptable on SMA than on any other
surface.
The permitted void content
may mean a higher degree of permeability than achieved with
HRA, therefore the importance of a well applied tack coat or
bond coat cannot be over stressed. The HAUC specification
only requires the same application of K1-40 as any other
surface course material but a heavier application of tack
coat or polymer modified bond coat would be prudent when
using SMA.
Laying
One of the biggest
differences between placing SMA and placing more familiar
materials is the degree of surcharge required. Typically,
HRA SC is placed with a surcharge of approximately 30 to 40%
to allow full compaction to be reached as the surface
profile of the reinstatement matches that of the adjacent
carriageway. When using SMA the degree of surcharge is much
less, as mentioned previously the mixes are proprietary and
will vary from one supplier to another, but a surcharge in
the region of 15 to 20%(1) is usually adequate.
SMA mixes are sticky! Wipe hand tools regularly with a
diesel soaked rag or brush but do not use diesel to excess.
Do not rake the material, level with a shovel, metal screed
or the back of a rake. Hand raking will cause segregation
and give problems. Any segregated aggregate should either be
dragged on to the adjacent carriageway and discarded or
pulled back over the main body of the reinstatement to allow
a better chance of incorporation into the mat. Segregated
material should not be allowed to remain at the
reinstatement edge where it will hinder the effectiveness of
the edge seal and allow water ingress to the lower layers.
Remember that void testing is not permitted within 75mm of
the reinstatement edge.
Compaction
There is nothing peculiar to
SMA with respect to compaction technique. All usual good
practice applies:
"Pinch" the
reinstatement edges first. Then overlap successive passes by
approx. 50%.
Ensure that vibrating
equipment is properly maintained and is operating in
accordance with the manufacturer's guidance.
Always roll at recommended
speed, usually 4 to 6km/hr. For any given frequency of
vibration a slower speed will increase the compactive effort
applied.
SMA is binder rich, if the
use of vibrating roller results in bitumen "flushing
up" then operate without vibration. No vibration will
mean using a larger dead-weight roller to achieve acceptable
compaction.
Note.
As the specification stands
at the moment only 14mm and 10mm are specified and each has
to be used as appropriate to match the nominal size of the
existing. Any variation to this has to be by agreement.
Note (1)
The degree of surcharge
required for various SMA mixes is to be established.
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