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The Idiots' Guide to Highways Maintenance
Copyright © 2000/16, C.J.Summers
SKID RESISTANT (HIGH FRICTION) PROCESSES BY PICTURES
There are two processes employed to produce skid resistant (high friction) surfaces for sites with known skidding and braking problems.
Both processes use calcined bauxite as the high Polished Stone Value (PSV) aggregate.
Calcined bauxite is also an extremely hard aggregate and retains the "sharp" edges and facets produced at the time of crushing for the life of the treatment, this property greatly enhances the skid resistant properties of these surfaces over the use of natural aggregates.
However one process uses a thermoplastic resin, i.e. when it is hot it is liquid and can be screeded on to the road surface along with the calcined bauxite aggregate contained within the resin mixture.
The second process uses a thermosetting binder which is produced by mixing separate resin components just prior to laying, and the separate calcined bauxite aggregate is applied to the resin before it begins to set.
|HOT APPLIED SKID RESISTANT / HIGH FRICTION SURFACING|
|COLD APPLIED SKID RESISTANT / HIGH FRICTION SURFACING|
HOT APPLIED SKID RESISTANT / HIGH FRICTION SURFACING
The high friction material comes ready blended in polythene bags.
The resin used is thermoplastic, i.e. it is plastic when hot and able to be screeded, but on cooling to ambient temperature becomes solid.
The coarse aggregate will be calcined bauxite, usually of RASC quality in the better products.
This type of product can be supplied as a coloured material i.e. including a suitable type and amount of pigment.
The material is heated in a thermostatically controlled "pot".
The material should not be overheated, maximum temperatures are usually printed on the bag.
Nor should the material be kept at high temperatures for long periods of time.
The above practices will harden and embrittle the thermoplastic and reduce the durability of the high friction surfacing.
Check that the heating pots are functioning correctly, and it does not hurt to check the temperature of the material on discharge from time to time.
The material is poured into an open bottomed rectangular "shoe" and screeded across the road lane.
The thickness of the surfacing is approximately equal to the nominal size of the coarse aggregate.
It is best to avoid laying it too thick as the coarse aggregate will be "lost" in the resin / filler matrix, and skid resistance will be impaired.
Although this system is regarded as less durable than systems that apply the resin and aggregate separately it is a lot less weather dependent, and can be opened to traffic within ten to fifteen minutes of the last screed being laid, which can be important.
It is my belief that a great deal of the failure attributable to this system is due to overheating the material.
hope it goes without saying that quality of workmanship, and
supervision, is paramount with hot and cold applied systems.
Often the aggregate can be a blended aggregate consisting of calcined bauxite and a high PSV natural aggregate.
The percentage of CALCINED BAUXITE in the blended aggregate can be found quite easily by using the heavy liquid, 1,2,2, -tetrabromoethane to perform a simple "sink or float" test, as described in,
TRRL Report LR 467 : Synthetic aggregates of high resistance to polishing
This test is possible because good quality calcined bauxite will have a specific gravity above 3.0, and the specific gravity of natural aggregates will be a lot lower than this.
COLD APPLIED SKID RESISTANT / HIGH FRICTION SURFACING
There are a number of proprietary formulations of resin on the market today that arrive in various types of pre-packaged systems.
The appropriate packages are combined together according to the manufacturers instructions and mixed thoroughly.
The resins are usually epoxy or polyurethane based.
All resin formulations are not equal, ask questions, know what you are buying.
Mixing the Material
Believe it or not a dustbin is usually the most convenient container for mixing the resins, with a paddle type mixer on an industrial drill providing the mixing.
Applying the Resin
Once the the resin is mixed you will only have a certain length of time for it to be spread and the surface aggregate applied before an initial set starts to take place.
The resin is squeeged to the appropriate spread rate by hand.
Spreading the Pigmented
The high PSV aggregate, usually calcined bauxite of RASC quality, is spread by hand to give slightly in excess of what is needed to give shoulder to shoulder cover of the resin.
The excess will be swept off after the resin has set and is retaining all the aggregate which it is in contact with.
In these photographs the calcined bauxite was pigmented, which is not often a long term solution to providing a coloured surface.
Even in good weather resins of this type usually take at least three hours to set completely, in cool weather it can take a lot longer, and this can present traffic management problems.
I try to refer to as few
commercial sites as possible in compiling my site, but when a site offers
particularly useful information about a subject I make an exception.
For further information on cold applied high friction surfacing press --------> HERE
Large areas of the cold applied systems are laid with fully automated machines spreading the resin and applying the high friction aggregate.
When the process is performed in this way the resins are in fact heated and the setting time is much reduced but it is still largely dependent on ambient temperature and may still take some hours before it is able to be trafficked.
Unfortunately I do not have photographs showing large scale application of the system.
information on high friction / anti-skid surfacing can be browsed
on the following pages.
Table of Skid Resistance related items, press here -----------> HERE
Skid Resistance quickly explained, press here-------------------> HERE
There will be many references to technical documents where you can obtain a good background on this subject.
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