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Stone mastic asphalt (SMA) mixtures (NGA2.2)


  1. SMA is a high stone content, gap graded material where the voids between aggregate particles are essentially filled with a bitumen-rich mortar.
  2. SMAs are very rich in bitumen to the extent that measures have to be taken to prevent bitumen from draining out of the mix during transport. Most commonly bitumen drainage is prevented by adding polymer modifiers or cellulose fibre.
  3. True SMA is intended to be virtually impermeable and has very good resistance to deformation because of its stone-to-stone structure. The level of texture depth achieved is largely a function of the material design, and is therefore likely to be more consistent than with HRA, where the application rate and the embedment of the surface applied coated chippings is critical.


  1. There is a European Standard for SMAs, BS EN 13108-5 and, in the UK, a Published Document PD6691 “Guidance on the use of BS EN 13108 Bituminous mixtures – Material specifications”.
  2. There are also proprietary mixes that fit the generic description of an SMA.
  3. Product Acceptance Scheme approved thin surface course systems may also comply with the HAUC requirements but there is no guarantee. Some Product Acceptance Scheme approved materials can have void contents in excess of that permitted in this Code. Purchasers should make it clear to suppliers that the work has to comply with the air void specification.


  1. As with all asphalt materials it is important that temperature loss is minimised during transport, handling and storage to allow effective placement and compaction. The high bitumen content of mixtures means that provided material temperatures remain elevated, then compaction is relatively easy.
  2. Transporting should comply with the requirements of this Code and BS 594987.
  3. To minimise the risk of segregation and temperature loss it is preferable to use the material direct from the delivery vehicle or hot box. Material should never be tipped on to adjacent surfaces for use sometime later.


Preparation should comply with this Code and BS 594987. A polymer-modified bond coat is preferable when using SMA.


  1. Placement of the SMA should be done in such a way as to avoid segregation of the mix. Where possible this is to be accomplished by careful use of a shovel. The use of a rake is to be avoided. Care must be taken with the use of some “hot boxes” as these can also cause segregation of mixes through the discharge augers.
  2. Areas showing segregation should be removed as these are not acceptable at any time.
  3. Where initial skid resistance of >40 SRV is required (all carriageway applications) or in areas where equestrian usage is likely, the surface should be gritted using a clean, dry, crushed quartzite, or a comparable very hard angular aggregate to a grading similar to that shown in Table NGA2.1. Alternatively, a 3 mm steel slag should be applied evenly to the surface during initial rolling, i.e., whilst the material is still warm. The rate of application should be set to provide about 80% surface coverage (approximately 1000 g/m2). Where the authority uses a lightly coated grit for the treatment of SMA, this may also be suitable.
  4. After final rolling all surplus aggregate should be removed before the reinstatement is opened to traffic.
  5. It is important that the application of grit should not reduce compliance with any texture depth requirement.
    Table NGA2.1 Recommended aggregate grading for gritting SMA surfaces
    BS test sieve: % passing
    6.3 mm 100
    5.0 mm 95-100
    3.35 mm 66-90
    1.18 mm 0-20
    0.60 mm 0-8
    0.063 mm 0-1.5


Compaction of the material is best carried out using a smooth wheeled roller. Vibration may be applied provided that this does not bring about excessive movement of the bitumen to the surface of the layer, i.e. “bleeding”. On narrow reinstatements compaction equipment in accordance with Table A8.1 may be used but care must be taken to ensure that there is no excessive loss of surface texture or bleeding.