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20MM. HOT ROLLED ASPHALT PRECOATS -
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.
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.
two most common causes for chipping loss from the the surface of the hot
rolled asphalt are :-
The two most common causes for chipping loss into the laid asphalt mat are :-
Information is to be found here
process of laying hot rolled asphalt and precoats.
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