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The Idiots' Guide to Highways Maintenance
Copyright 2000/14, C.J.Summers

20MM. HOT ROLLED ASPHALT PRECOATS - SPREAD RATES

A good, well maintained "chipper" which I believe is making an excellent job of applying 10kgs. to 12kgs. of 14mm. precoats per square metre, but remember it is texture depth that is the criteria. All these chippings have been placed by hand in laboratory conditions for the specific purpose of demonstrating what differing rates of spread look like in perfect conditions.

NOTE - The spread rates, in kgs. per square metre, for 14mm. precoated chippings will be lower.
With smaller nominal size chippings you obtain the appropriate surface cover with less mass of chippings than you do with a larger size chipping.
You will see evidence of this fact if you look at the rates of spread of the various sizes of chipping for surface dressing.
It is an easy enough procedure to determine rates of spread of chippings per square metre for any source of chipping, you only need a measured square metre and an accurate balance. 
Rates of spread of chipping will vary slightly according to the density of the aggregate, and the nominal size of the chipping within the permitted size range, e.g. within the 14mm. to 20mm. size range for 20mm. precoats.
This is why you get the terms "bold" or "small" applied to a particular nominal size chipping, i.e. are the bulk of the chippings nearer a 20mm. size or a 14mm. size.

BUT, do not forget, it is the TEXTURE DEPTH of the surface that is critical, it is possible to spread the correct amount of precoats and then roll all of them in to the mat leaving little if any surface texture.
This is why the nature of specifying was changed from a required rate of spread of precoats to a texture depth requirement.

A surface texture of 1.5mm., by "sandpatch" was the usual criteria for trunk roads and motorways whether it was Thin Surfacing or HRA and precoats, however this has recently decreased to 1.2mm. for low speed roads. 

For current guidance on texture depth requirements relevant to different surfacing materials and highway locations see,
Specification for Highway Works-Series 900-Road Pavements-Bituminous Bound Pavements (Amendment August 2008)
Clause 921 - Surface Macrotexture of Bituminous Surface Course - Table 9/3

The recommendations here, for motorways and trunk road are often deferred to for local authority work, at least on the major roads within that network, but it is not compulsory.

( NOTE - The texture depth for HRA and precoats, for high speed roads, remains at 1.5mm., while on low speed roads it has been decreased to 1.2mm.
However for thin surface course systems (TSCS) the requirement for high speed roads has been dropped to 1.3mm., and for low speed roads to 1.00mm. 
I do not follow the engineering reasoning for this, other than to improve the durability of proprietary "Thin Surfacings" rather than maintain the wet skid resistance properties of these surfacing materials.
This topic was discussed in the
Summer 2008 Newsletter and the Autumn 2008 Newsletter.)

If the above table of texture depth requirements, or some similar specific requirement is not inserted into the contract document, then the default requirement will be that contained in,
BS 594987 - Asphalts for roads and other paved areas - Specification for the transport, laying and compaction and type testing protocols
The actual specification of texture depth is stated to be "site specific and outside the scope of this standard", however typical examples of texture depth requirements are included, for guidance, as a "Note", see Chapters 7 & 8 for further information.

It is worth pointing out that there is in BS 594987 a large amount of excellent guidance contained in the many "Notes" included throughout the document. These "Notes" cannot be used as part of the specifying text, but will assist in the inclusion of appropriate specifying paragraphs in you contract/works document, should you think it necessary.

20mm. precoated chippings shown at 11.5 kgs. per square metre
20mm. precoated chippings shown at 7.2kgs. per square metre
20mm. precoated chippings shown at 5.75kgs. per square metre
20mm. precoated chippings shown at 3.2 kgs. per square metre
Diagram of a hot rolled asphalt wearing course layer with embedded precoated chippings With a 35% HRA mixture a 45mm. layer of hot rolled asphalt wearing course is laid with a paver and then the appropriate rate of precoats is applied to the asphalt surface, with a chipping spreader, to achieve the required texture depth.
A higher stone content asphalt (35%) is a "stiffer" material and other things being equal will provide more resistance to the embedment of the precoated chippings providing an increased surface texture.
It is most important to realise the precoats are held in the asphalt because the bitumen coating has been melted by the heat of the laid asphalt and the chippings are then positively bonded to the surface, as well as being partially compacted into the surface, to a greater or lesser degree.

 

The two most common causes for chipping loss from the the surface of the hot rolled asphalt are :-
  • Chilling of the surface of the laid asphalt mat while waiting to receive the application of precoated chippings, so that there is insufficient heat in the surface of the asphalt to melt the layer of bitumen coating the chipping, that should provide that "glueing" bond of chipping to asphalt.
  • Overheating the chippings and bitumen at the time of precoating so that the bitumen binder is "burnt" and rendered inert, this can happen even after the coating process has taken place, i.e. when/if the hot precoated chippings are stockpiled in deep stockpiles rather than spread into relatively thin layers to assist cooling.
    "Burning" / "coking" / "carbonising" of the bitumen coating around the chipping means that there is no "active" binder to melt and bond the chipping to the asphalt.

The two most common causes for chipping loss into the laid asphalt mat are :-

  • The laid asphalt is too hot, i.e. above permitted rolling temperatures (and is likely to have been mixed too hot so that the asphalt has already been "damaged" and will have a reduced life) and the chippings will be pushed completely into the asphalt, and will not provide any texture.
  • The supplied asphalt contains moisture in the sand element of the mixture due to insufficient drying of the sand. This makes the asphalt unstable, and can even make it impossible to lay to correct level through the paver because of this instable nature. The applied chippings are quickly lost into the asphalt layer when rolling takes place. This phenomena often takes place after periods of overnight heavy rain when the sand stockpiles at the quarry / production plant are not under cover, and the speed of aggregate/sand drying has not been altered to take account of the increased moisture.
Hot rolled asphalt wearing course surface with chippings at about 12kgs. per square metre

Information is to be found here describing,  The process of laying hot rolled asphalt and precoats.

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