LAYING A THIN BITUMINOUS WEARING COURSE

CONTENTS

6mm. Medium Graded Macadam Wearing Course
Laying with a conventional paving machine
Manhole covers and other street "ironwork"
Centre longitudinal joint, layer thickness
Points to be considered before deciding upon the thin surface course option
Surface texture

This site, four years later 
This site, six years later, and the adjacent site eight years later, summary

6mm. Medium Graded Macadam Wearing Course BS 4987 : Part 1 : Cl.7.6 : 6mm. Medium graded wearing course ( This is a "PREFERRED MIXTURE" ) At this particular site this material was chosen as most suitable. It was to be laid 20mm. thick with the only planing at the beginning and end of the work to blend in with the existing road profile. This particular material was to a BS 4987 specification but with the addition of 5% of rubber by dry weight to the bitumen component of the mixture, this means approximately 7.5% addition of rubber emulsion as the emulsion is only 67% latex.

The rubber was added as latex emulsion, being added by a pump into a batch mixer. When choosing to add a latex modifier at the time of mixing to the bituminous material I cannot stress too highly that the plant used to mix it must be a BATCH MIXING / PUG MILL type plant, and the latex emulsion must be added by a suitable pump over a 30 second period, preferably through a spray head. The material must be mixed for a further 30 seconds after the addition of the latex emulsion has been completed. This method of adding the latex is described in The Specification for Highway Works, Clause 938, Porous Asphalt Surface Course. Failure to conform with this procedure could well lead to unmixed lumps of latex being present in the bituminous mixture, and consequent problems with the laid material. The cost of a material modified in this manner will be more expensive but the increased durability of the material gained from the modifier will justify the extra cost.

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, not all, thin surface courses are a quieter surface than a, high texture depth, hot rolled asphalt and chipping wearing 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 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 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.
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) 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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