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The Idiots' Guide to Highways Maintenance
Copyright © 2000/10, C.J.Summers

VELOCITY PATCHING

INTRODUCTION
THE PROCESS
THE EQUIPMENT
PREPARATION
 SUPPLYING THE COARSE AGGREGATE
SUPPLYING THE BITUMEN EMULSION, AND MIXING
APPLYING THE COMBINED TREATMENT
APPEARANCE OF THE LAID MIXTURE

 




INTRODUCTION
I am prompted to write this page on velocity patching at this time, although I have been aware of the process for some time, because I believe there is now sufficient competition in the industry both for providing a contractor service, and the supply of a number of pieces of plant that employ this technology, for it to be regarded as a mainstream process for highways maintenance. 
If you use Google, or the search engine of your choice, with appropriate key words, you will find a number of websites that will supply you with further information.
I would still urge engineers to look at all alternatives/processes in relation to the type of maintenance needed in particular situations.
On this website I cover many forms of maintenance procedures and materials, all of which I believe have a part to play in the highways maintenance industry. 
It is for the engineer or technician to select the most appropriate process for the particular situation, many factors have to be considered to arrive at this decision, and that includes budget considerations. 
Because of the speed at which patching can be performed I believe velocity patching has a role to play in providing a competitive alternative to shallow patching on urban and rural highways, especially if the sites are some distance from hot mix production plants.     
However I believe it is not wise to use any material or procedure inappropriately, as it only gets a good material or procedure a poor reputation, so, in my opinion "over selling" of any product is not to be encouraged.
THE PROCESS
To the left is a picture of what may be regarded as a standard velocity patcher, but there are a number of machines now on the market that have taken the same technology and adapted the machinery to perform specialised tasks, e.g. shallow haunch repair.
One of the main plus points to this process is that it is largely self contained, holding sufficient quantities of aggregate and emulsion to perform a considerable amount of shallow patching before needing re-stocking.
I could ignore the question, what is a "shallow" patch, but I will stick my head out and suggest anything less than 75mm. will fall in to this category, and hopefully most of the patching will be of less depth. 
 
However if you are a considerable distance from a hot mix supplier, and do not have other options of providing hot bituminous material on site, laying thicker layers of material using this process may be cost effective.
I expect engineers and technicians in charge of this work to be capable of making "engineering judgments" in relation to cost as well as engineering performance.
Stocks of aggregate and bitumen emulsion can be brought in to the area for restocking to reduce "idle" time, and increase the cost effectiveness of the plant.
One of the big plus points of this process is that it produces material "on demand", and although the bitumen emulsion held on the vehicle has to be treated with respect its "shelf life" is going to be much longer than that of already mixed, hot mix bituminous mixtures. 
Reference is now made to Velocity Patching in the copy of,
BS 434-2 : 2006 : Bitumen road emulsions - Part 2 : Code of practice for the use of cationic bitumen emulsions on roads and other paved areas
This recently published standard is excellent, containing very useful information on the use of materials and processes employing cationic bitumen emulsions, a useful standard to have in your technical library.
THE EQUIPMENT
I can only provide a basic description of the process, as described to me, but I believe it will help understanding what is taking place in this type of road maintenance.
I find the basic technology of this equipment refreshingly simple yet employing innovative advanced engineering in producing the "spraying nozzle" which is capable of fully coating the aggregate particles as the are "blasted" out to fill the pothole.
The equipment comprises of an air compressor with a large "holding" cylinder, the black cylinder at the front of the vehicle.
This is followed by the insulated tank containing the bitumen emulsion, the yellow rectangular item with the black base. I am told "spare" heat from the running of the vehicle engine, whilst on the move and stationary, is used to heat the emulsion to the required temperature.
The main physical item on the vehicle is the hopping holding the chosen predominantly single size aggregate.
I believe the "standard" hopper, shown on this vehicle, has a capacity of four cubic metres, and depending on the aggregate bulk density this will be approximately nine tonnes of aggregate, and hence a similar amount of "mixed" bituminous material will be produced for patching.
 PREPARATION
All potholes and defective areas are "blasted" clean and free from standing water using compressed air.
A "tack coat" is then applied to the surface using compressed air mixed with bitumen emulsion to provide a bitumen emulsion spray.

Note : It has to be said that all "hand work" pothole and patching work would benefit from the simple procedure of brushing water and "rubbish" from the defective area prior to placing any type of reinstatement material.
This does not always occur, I have seen material placed into water.
Implementing simple procedures can bring enormous benefits.
SUPPLYING THE COARSE AGGREGATE
The coarse aggregate is brought to the central large "tube" by a conveyor belt at the base of the hopper.
If you look at the picture to the left carefully, you will see a small flow of aggregate dropping in to a restricted holding area from where it is "blown" along the tube to the mixing head.

You will also see two smaller pipes attached to the larger pipe, these are delivering compressed air and bitumen emulsion to the spraying head and can be operated independently of the aggregate supply, and of each other.
SUPPLYING THE BITUMEN EMULSION, AND MIXING
It is here at the final stage that the process "comes together".
There needs to be correct choice of bitumen emulsion and aggregate, and it has to be metred at the correct amounts to produce the required mixture.
The aggregate needs to be clean and without dust.
The bitumen emulsion needs to be of the correct spraying viscosity and have a base binder of a suitable viscosity for the work in hand, usually dependent upon the nature of trafficking.
It is the base binder of the emulsion, pre modification and emulsification that will determine the final performance of the laid material.
But in fact this is usually a 200/190 pen. bitumen, a good "compromise" bitumen viscosity for shallow and small patching work.
I believe that the bitumen emulsion has to be totally "clean", i.e. without impurities to prevent blocking of the spraying head, so I would imagine emulsion filtering will be quite "sophisticated".
APPLYING THE COMBINED TREATMENT
The aggregate is finally coated with hot bitumen emulsion as it passes through a highly engineered spraying head.
This is a "metal ring" with an intricate pattern of very small "holes", from which the bitumen emulsion is blasted in a pattern that completely coats the aggregate with a coating of bitumen emulsion.
The "break" of the bitumen emulsion is almost immediate due to the heat of the emulsion and the amount of air used in the application procedure.
This means that the patch is relatively stable for trafficking very soon after laying, any movement being due to the modification of the bitumen with a small amount of volatile oils prior to emulsification.
This is not too detrimental a feature because it means it will allow some compaction by traffic before complete stabilization of the mixture occurs.  

The term "break" means to removal of the water fraction of the emulsion and the successful deposition of the bitumen on the surface of the aggregate.
APPEARANCE OF THE LAID MIXTURE
The picture to the right shows the appearance of the newly laid mixture, you will note it appears black showing that the emulsion is fully "broken".
If it were not fully "broken" it would have a brownish colouration.

The final bituminous mixture has the appearance of a 6mm. open graded bituminous macadam.
There is not such a mixture in BS 4987:Coated macadam (asphalt concrete) for roads and other paved areas.
There is a 0/6mm. size medium graded macadam surface course (Clause 7.6) which will be similar, but not the same.  
The open nature of the mixture will allow for any remaining water to disperse and allow, over time, for the evaporation of the volatile oils in the bitumen, allowing the stiffness of the mixture to increase.

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