Laying with a conventional paving machine A conventional paving machine that is in good repair and able to lay to a tight tolerance is really all you need to lay suitable bituminous mixtures in a thin layer. There are now available paving machines that have the ability to lay the "bond coat" of bitumen emulsion as part of the paving process. I am not familiar with these machines, but I personally prefer machinery that is not cluttered with too many features, then there is less to breakdown, as the construction site is pretty hard on all machinery.

Manhole covers and other street "ironwork" The ironwork was raised 20mm. to ensure good ride quality of the finished work. Kerbs were not raised. One of the main reasons the thin overlay option had been chosen was to specifically save the cost of raising kerbs. Please note the thorough tack coating of the existing road surface which is most essential for thin overlay work. All thin surface courses must adhere strongly to the surfaces they are applied to if they are to be durable. Specified rate of spread of tack coat is 0.35 / 0.55 litres per square metre.

Centre longitudinal joint, layer thickness This picture shows the thin nature of this type of surface, and it is not difficult to realise that the life of this type of surface will be less than surface courses that are laid at a greater thickness. It is also generally recognised that a surface course laid above 40mm. thick will add strength to a road pavement, and you may want to choose the option of laying a thick surface course as a cost effective option of strengthening a road

It is my opinion you choose a thin surface course for the benefits it offers, not as a direct replacement for a different material that offers different qualities.

Points to be considered before deciding upon the thin surface course option

1) It is only necessary to remove a minimum of existing surface material, perhaps none at all if the kerb upstand is sufficient, giving a substantial cost saving by not having to raise kerbs, and you leave existing strength in the road pavement.

2) If you are laying bituminous mixtures thinner, you are using less tonnage, this gives a cost saving.

3) Most, but not all, thin surface courses that have a small nominal aggregate size (e.g. 6mm. and 10mm.) cause less tyre noise to be generated than a high texture depth surface course, such as a hot rolled asphalt and 20mm. precoat surface course.

Lower noise generation can be a significant reason to use this material in urban areas, but do remember noise from road surface and tyre interaction is dependent to a great degree on vehicle speed, so you may not get the (any) benefit you expect in a 30/40 mph speed restricted area.

4) Thin surfacing courses tend to give a smoother/even ( not more slippery ) ride quality.

5) Thin surface courses should only be used as an overlay where you already have a strong road pavement as they will add little if any strength to the road.

6) Thin layer surface courses are usually porous as an open texture is needed in these materials to provide the (negative) texture of the surface course.
Therefore it is most important for it to be laid upon an impervious basecourse, or existing surface made impervious by a thorough tack coating.
Failure to lay this material on an impervious layer will allow water to gain access to the lower road pavement materials, and this will weaken the road and may subsequently lead to failure of the road pavement.

7) It is very important to stress that you must have a good application of tack coat / bond coat to ensure a complete bonding of the surface course to the underlying road surface. This is because the surface course must become an integral part of the existing road surface, rather than a thin layer of material sitting upon an existing surface. If a "no bond" situation exists failure of thin surfacings is usually quite swift.

8) In high stressed areas you must consider materials with modified bitumen binders to give increased material strength and durability, this will increase material cost substantially.

Surface texture The surface texture of a 6mm. medium graded wearing course is a lot greater than you might think. The picture shows a close view of the road surface, with a £1 coin to give scale to the texture. This material is porous, surface water can be displaced through the material, as well as to the sides by the passages in the surface texture and the tread pattern of the vehicle tyre. With an aggregate of suitable polished stone value (PSV) it is my opinion this material may be considered for low speed ( 30mph ) urban situations.

For more information on British Standard bituminous mixtures that can be laid thinly, press ----------------> HERE

For more information on proprietary "Thin Surfacings", press ----------------------------------------------------------------> HERE

For access to a table comparing and contrasting many bituminous surfacing materials, press --------------> HERE 

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