The mixing of SMA is performed in conventional mixing plants, with a preference to batch mixing plants as opposed to drum mixing plants.
However because of the many variations in basic plant design some drum mixers are more able to mix bitumen rich mixtures than others.
It is reasonable to note that in Germany, where SMA was developed, the material is primarily produced in batch mixers.
Inadequate mixing effort may produce material of variable consistency which may not comply with the specification although feed to the plant is accurate.
SMA is laid with a conventional paver without any problems.
The ride quality of SMA is similar to a bituminous macadam (BS 4987), or a high stone content asphalt (BS 594) depending on the nominal aggregate size of any of these materials.
Hand laying of SMA can leave an "untidy" surface appearance from raking what is a predominantly single sized material.
This hand laying problem will be influenced by the grade (viscosity) of bitumen used in the SMA design, the stiffer the bitumen the greater the difficulty of hand laying.
The design of an SMA is critical in providing an aggregate grading that will accept the high bitumen content that provides durability without binder drainage.
If this design is not correct it will result in binder drainage in the lorry bodies and fatted areas of the surfacing.
Conversely an aggregate design that requires a lower binder content to prevent binder drainage will result in a bituminous mixture that will be less durable and have a reduced life.
SMA was developed to be laid at a conventional thickness, here it is being laid as a nominal 45mm. layer.
SMA laid at this thickness will add strength to the road pavement.
A conventional SMA surfacing course, with its high binder content, will also be impervious to the ingress of water into the road pavement.
It is necessary to be cautious when Thin Surfacing materials are offered to you as an SMA as these materials may not possess the basic design concept of an SMA, which is that it shall be a binder rich mixture with a load carrying aggregate matrix and bitumen filled voids.
Note the good coating of tack /bond coat on the existing surface.
This photograph shows the surface characteristics of this material as it emerges from behind the paver and before rolling, so the texture shown here will reduce an amount after rolling.
This material however will have significant "negative" texture, probably above the 1.5mm. (by sand patch) required by the Highways Agency on motorway and trunk roads.
Below is what I believe a "genuine" stone mastic asphalt should look like, a load bearing aggregate matrix, with not a high amount of fine aggregate filling the voids, but the voids filled with binder to create a binder rich, impervious, durable bituminous mixture.
But there are some aggregates fines (grit) in the "mastic" which provides initial micro texture to the road surface.
The scale is 1cm. squares, so you will see this was laid almost 50mm. thick, ( although the nominal thickness was specified at 45mm ), however with a 50pen. binder there have been no problems with deformation after three years.
With a correctly designed SMA, sufficient surface texture is achieved without the need for a porous matrix.
