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Notes for Guidance on the Use of Stone Mastic Asphalt (SMA) in Hand Lay Situations

07th March 2003


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 UK 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 UK specification of 8% maximum for what is envisaged to be hand lay work.

HAPAS approved thin surfacing may also comply with the HAUC UK 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 UK 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.

There has been a suggestion that for hand lay work the aggregate nominal size should be one size lower than for machine laid work. This may be a good idea and would undoubtedly aid compaction but specified texture still needs to be achieved. 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, so far, anecdotal. Staffs CC staff are hoping to carry out work to better establish the range of surcharge that may be required.

 

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