ASPHALT / BITUMINOUS MIXTURE REINFORCING

INTRODUCTION TO "ASPHALT REINFORCING" USING A MESH AS THE REINFORCING
I have created this page to give an idea of the process most commonly known as "asphalt reinforcing", although their are other terms, some of them proprietary.
There are a number of types of "asphalt reinforcing", employing several types of mesh/grid.
Most of these meshes are of a plastic or fibre-glass nature although some are of metal/wire.
I am not going to attempt to make any judgment on any of these processes, as I freely admit I know to little about these processes/procedures.
What is concerning is that I am aware of a number of people and organistions who are using these options who know equally as little as I do, perhaps even less. 
This page may at least show them what the procedure entails, from a practical point of view, with one type of product, if you are thinking of using other types of asphalt reinforcing you must look elsewhere.
I have been involved in the use of the type of mesh shown in the photographs on several sites. 
The use of the the reinforcing was part of a substantial resurfacing that included the plane off and replacement of surface course, binder course and in some cases an amount of the road base.
At the particular site shown on this page the reinforcing was being used between areas of old and newer construction to prevent the problem of induced cracking in the surface, which had previously occurred.
At other sites cracks were coming through the surfacing for different reasons, and the mesh was used to prevent this occurring.
On all three sites the cracks did reappear after a number of years, with one site not showing signs of cracking for approximately ten years.
In some cases the individual cracks were sealed and a further period of life of the surface was achieved, in other cases resurfacing in the form of patching was necessary.
The question is whether the inclusion of the mesh extended the life of the surface before the cracking appeared and did it reduce the amount and severity of the cracking, the answer is I do not know, comparison trials were not conducted.
I am unaware of any British Standards appropriate to mesh used in asphalt reinforcing, although there is a great deal of specifying information for geogrids in relation to soil and formation strengthening.
I am also unaware of any independent testing or trials on any of these materials by an independent laboratory, if anybody can supply appropriates sources of independent comparison trial information I would be glad to include them.
My own opinion, for what is worth, is that I may consider the use of an appropriate mesh on sites where only a relatively thin (100mm.) inlay after planing is possible, especially if it is a busy site where working is very difficult and disruptive, so that periods between resurfacing need to be extended to the maximum possible, e.g. city centres.
I would still want to use premium surfacing materials, I always prefer BS 594 Hot Rolled Asphalts, (BS EN 13108-1:2006 FROM 01-01-2008) as you are aware, but even these can be improved by the addition of appropriate polymer modifiers.
In "open" or rural areas where levels are not critical and the work will be an overlay I would prefer to spend the budget on increased thickness of overlay and/or "improved" bituminous mixtures.
If you decide to use a mesh for asphalt reinforcing, where it is placed in the road pavement is extremely important.
I have heard, but I hope it is not true, of the mesh actually being laid directly on to the planed surface, which I would imagine is unfair to the mesh and likely to create voiding between the planed surface and the layer immediately on top of the mesh.
What ever you choose to do, think about it, if you have any knowledge at all on road construction and maintenance, couple this with imagination and you should at least avoid the obvious problems.
Also, and importantly, do not forget to think about any implication the type of mesh/grid you may be using will have on subsequent planing and recycling operations, what may appear to be a saving now could be costly in the future.
This page is not a recommendation for or against the use of the various grids and meshes now on the market, this page is to provide information on one particular type of product that may be appropriate to particular situations, and to increase your knowledge on the process commonly referred to as asphalt reinforcing.
Budgetary considerations also have to be taken into account. These products and the procedures for using them have considerable cost, you must determine if they are cost effective in relation to maintaining a total highway network.
It is likely that TRL would be more than happy to do trials on these products at their road pavement testing facility if somebody were to fund it. 

APPLYING THE MESH/GRID The mesh/grid being applied to the basecourse surface. This mesh is of a fibre glass type and is coated with an adhesive so that it will attach itself to any flat surface that it is applied to with a firm bond. Rolls of the material can be seen in the foreground. In this case the rolls are not particularly wide, so a significant overlay needs to be achieved to prevent any crack propagation at the mesh joints.

An empty roll of mesh showing the nature of its structure.

UNROLLING THE MESH This operation is not quite as easy as it appears because of the strong adhesive nature of the material, and if not kept under control this material can seem to have a "will" of its own. The Engineer in charge of this site and in consultation with the contractor and the supplier of the grid felt maximum benefit from the inclusion of the mesh would be achieved by placing it at the interface of the binder course and surface course.

ENSURING ADHESION OF MESH TO SURFACE It is important to ensure full contact and adhesion of the mesh to the applied surface prior to the paver passing over it. The considerable "pushing force" of a paver laying the surface material over the laid mesh can dislodge it if application has not been done correctly. You will also have a heavy lorry reversing over it to access the paver hopper for discharge of the bituminous material. The process in the photograph may seem simple but it was performed conscientiously  and it worked, and that is what is important. There are automated procedures for laying the mesh using suitable plant, that are employed on larger sites.

ADHESION OF SURFACING COURSE Here a joint is being cut in the surface course prior to the next rip of adjoining surface course. I observed that it took considerable effort to remove the laid surface course, but it did detach from the mesh and underlying surface without damage to the grid, except where the jack hammer had cut into it. This tells me that the mesh was well attached to the underlying surface, and that the surfacing material was well attached to the mesh and underlying surface, but was able to be removed "cleanly" from the interface. I therefore draw the conclusion that this particular type of mesh is able to be used in a practical way when involved in the "normal" procedures of surfacing providing good working practices have taken place. Note the tack coat on the binder course and mesh, this was applied prior to laying the surface course.

APPLICATION AROUND IRONWORK Applying this type of mesh around ironwork does not present a problem. Basic work procedures for installing and backfilling to the ironwork must be followed, prior to applying the mesh.

AN AREA OF LAID MESH ON BINDER COURSE It is my opinion that a well chosen binder course (or other receiving layer), which must be well laid and compacted is essential if the full benefit of any asphalt reinforcing is to be achieved. In this case the binder course was a 50/20 HRA Binder Course to BS 594:Part 1, with a 50pen binder. If you do not know what this means look in the British Standard. Prior to laying the surface course the binder course and mesh was sprayed with a K1-40 bitumen emulsion tack coat, in accordance with BS 594:Part 2. Earlier editions of the standard used to quote 0.35 to 0.55 litres per square metre, current editions now quote application rates as the weight of residual bitumen, a much quoted figure being 0.15kg. per square metre.

If you are using proprietary bond coats these figures may be increased, according to the manufacturers instructions. Note - The British Standards BS 4987, Parts 1 & 2, and BS 594, Parts 1 & 2 were superseded by the "family" of specifications, BS EN 13108 : 2006:Bituminous mixtures - Material specifications, on the 1st. of January 2008. More details on the changes will be found HERE. However BS 594 and BS 4987 contain a wealth of useful information in relation to bituminous mixtures, such as mixture composition, bitumen content and viscosity, temperatures, storing, transporting, compaction, testing and other related items. In my opinion these documents are worth retaining for advice and guidance even though you will not be able to use them for specifying bituminous mixtures or work practices. This information will be particularly useful in specifying comparable mixtures from the "range" available in the new, replacing, BS EN 13108 "family" of specifications.

LAYING THE SURFACE COURSE The surface course is hot rolled asphalt laid 45mm. thick and applied high Polished Stone Value (PSV) 20mm. precoats, all to BS 594. Here you can see the prepared surface with the applied mesh and a rip of HRA surface course being laid. The three point roller is on its first pass of this area of laid asphalt ensuring the precoats are "pushed" firmly into the mat as soon as the chippings have been distributed by the chipping spreader. An aside to the main subject, but chippings need to be "pushed" into the asphalt while it is hot enough to melt the bitumen coating on the precoat to ensure a positive "glued" bond between asphalt and the actual aggregate chipping.

I try to refer to as few commercial websites as possible in compiling my site, but when a site offers useful information about a subject I make an exception.
For further information relating to the use of asphalt reinforcing, some able to be downloaded,  press, ----> HERE

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