ROAD SURFACE RETEXTURING PROCESSES

CONTENTS

INTRODUCTION TO RETEXTURING
SOURCES OF INFORMATION ON THE RETEXTURING PROCESSES
HIGH PRESSURE WATER RETEXTURING
PHYSICAL ABRASION (Rotating Discs)
PHYSICAL ABRASION (Bush Hammering)
PHYSICAL ABRASION (Shot Blasting)
WARNING
PERSONAL NOTE

CONCRETE CARRIAGEWAY RETEXTURING

INTRODUCTION TO RETEXTURING

These are physical processes that will restore/enhance the texture of sound road surfaces that have become smooth through wear, or that have lost texture through "fatting up" of binder.

One type of retexturing machine retextures by impacting the aggregate in the road surface with numerous small hardened steel hammers (often referred to as "bush hammering") to roughen the aggregate surface
Another machine impacts the road surface with free moving rotating roughened discs mounted on a revolving drum.
And there is a third method that "shot blasts" the road surface with steel balls.
I have also heard there is a "flail" technique of which I know nothing

The restoration of texture to a road surface that is smooth with excess binder can be treated by the use of a high pressure water treatment.

Although the texture can be improved with these processes it must be remembered that the Polished Stone Value (PSV) of the aggregate will remain the same.

An improvement in SKID RESISTANCE VALUE of the road surface is not to be confused with an improvement in the PSV value of the aggregate, the PSV of the in situ aggregate cannot be improved, the PSV is what it was at the time of laying and will remain that value, and it is the value of the aggregate when tested in a polished state, hence POLISHED STONE VALUE.

However the fresh fragmentation of the smooth aggregate surface creates new "sharp" angular edges which has been shown to increase SKID RESISTANCE VALUE of the road surface. 

There are various contractors in the country able to provide machinery capable of these, and other, types of RETEXTURING processes, both large lorry mounted and small hand operated units.

The effectiveness, and duration of an improvement of the skid resistance, of any physical impacting treatment to the road surface will depend upon the degree of fragmentation achieved on the surface aggregate particles and the ability of the aggregate to retain the "newly crushed" nature of the aggregate surface.
It is possible to apply exactly the same treatment to two similar road surfaces that have a different aggregate source and obtain different levels of improvement of skid resistance.
With suitable aggregate I regard this process as a very effective, and cost effective, option for maintaining skid resistance on road surfaces.
  

Some RETEXTURING processes will be more suited to site conditions than others, take advice from your Materials / Road Pavement Engineer as to which method to employ.

The cost of any of these treatments is largely dependent on the programme of work to be undertaken, but the processes are largely unaffected by weather conditions so can be used for "emergency" treatments, when the application of high friction surfacing would not be possible.

SOURCES OF INFORMATION ON THE RETEXTURING PROCESSES

"Volume 7" of the Design Manual for Roads and Bridges (DMRB), in particular,
HD 37/99 : Section 5 : Part 2 : Chapter 11.

TRL Report 298 : Mechanical retexturing of roads : A study of processes and early-life performance.

TRL Report 299 : Mechanical retexturing of roads : An experiment to assess durability.

The DMRB is now able to be accessed on the "web", a link to it is to be found on the Links Page of this site, this is an exceptional source of information on many subjects related to highways maintenance, including retexturing, and should be visited.

HIGH PRESSURE WATER RETEXTURING

"Fatted-up" Surface Dressings The most common sites where retexturing with "High Pressure Water" is undertaken are fatted up surface dressings as shown to the left. You will see on this site large areas of the road surface have completely fatted up leaving a smooth surface with no texture at all.

Before and after high pressure water treatment The photographs below are before and after sections of the fatted up surface dressing, you will see the pressure of the water has successfully removed the excess bitumen and left an excellent texture. This process is so effective that the biggest danger is "blasting" out sections of the road surface, so great care is needed by the machine operators to ensure the integrity of the road surface is not weakened by the process.

High pressure water retexturing / declogging machine
Below are pictures of a typical machine for doing this type of retexturing, it has to be able to carry large amoiunts of clean water so its basic shape is that of a "tanker".
It has a row of spinning heads at the rear of the machine which actually spray the water, there is usually just one jet on each spinning unit and complete coverage of the road is due to the spinning action.
If the head stops spinning while the jet is still spraying you will quickly have a small hole in the road surface.

Relative Hydraulic Conductivity This particular machine carried its own apparatus for testing the relative hydraulic conductivity ( porosity ) of "Porous Asphalt" when the machine was being used for "de-clogging" the open texture of that surfacing material. This test is a measure of how quickly the water will drain through the porous asphalt matrix.. Initially the test will be performed shortly after laying to check that the material/surface complies with the specification. The method of the test is to be found in BS DD 229 : 1996. It will also be performed from time to time to test that the interconnecting voids which allow drainage have not become clogged, and to test the effectiveness of any "declogging" operations. The test is also described in, Clause 938:Porous Asphalt Surface Course, of Volume 1, of the Specification for Highway Works. Texture Depth Texture depth can be tested for ( once the surface is dry ) by performing a sand patch test.

PHYSICAL ABRASION (Rotating Discs)

Description of the Process The rotating assembly of bars carrying the individual discs is positioned very accurately above the road surface so that only slight contact is made with the aggregate at the road surface. In this position the aggregate is successfully roughened without causing damage to the road surface. The accuracy of this positioning with this process cannot be over emphasised, because if it is set to low not only will the surface course layer receive damage, you may well create a road surface that has longitudinal groves that could be cause for concern for motorcycle traffic. However, performed correctly by an experienced gang this is an excellent process.

The Rotating Assembly This picture illustrates the way the individual discs are mounted on the rotating bars. The discs are not fixed but loosely mounted with the bar passing through the central hole of the disc with a considerable clearance, this is so that the once the disc has impacted with the road aggregate it will bounce up as there is no downward pressure from the machine other than centrifugal force. This is providing the rotating assembly is correctly positioned.
Close up of the discs showing  the hardened steel protrusions, which are the "active" part of the retexturing process.

PHYSICAL ABRASION (Bush Hammering)

This type of retexturing process/machine performs its task by impacting surface aggregate of the road surface with numerous small hardened steel hammers (often referred to as "bush hammering") to roughen the aggregate surface, i.e. to create new "sharp" angular edges on the surface of the aggregate in the road surface. On this particular site the 20mm. preacoats in the hot rolled asphalt surfacing. This manner of process is not subject to the problem of "grooving" of the road surface, although I am informed that on rare occasions random patterns of texture have been observed on treated road surfaces. It is likely this will not be of the same degree of a problem as "grooving", but it may result in treated areas having differing levels of skid resistance.

Hot rolled asphalt and 20mm. precoats surface course before bush hammering. In good condition but the aggregate has lost its "roughness".	Hot rolled asphalt and 20mm. precoats surface course after bush hammering, showing newly roughened aggregate surfaces

Diagram illustrating a "bank" of "bush hammers, with both the speed and pressure able to be controlled to suit the nature of the surface and aggregate type.	Single "hammer" indicating before and after treatment. Diagram shows fresh, "sharp", fragmentation to an aggregate surface that has been worn smooth by traffic. Over time this retextured "sharpness" will once again be worn smooth.  How long the retexturing will remain of benefit will depend upon amount and type of traffic, and the aggregate that has been retextured.

( Diagrams of the "bush hammering" process courtesy of Klaruw - this is not an indication of endorsement but better illustrates the process)

PHYSICAL ABRASION (Shot Blasting)

In this process small steel balls are "fired" at the road surface causing the surface of the aggregate that is impacted to break and shatter creating a "new" sharp angular surface. The steel balls are reclaimed, cleaned and used again.
This process has a slight draw back in that it is unable to work in the wet.
Some reports/journals will state that these processes create a new microtexture, others regard the actual roughness of an aggregate after "polishing" as the true aggregate microtexture. I will stay on the fence in this debate, but will say all these processes will increase the Skid Resistance Value (SRV) of the road surface to a greater or lesser degree depending upon the process, the quality of workmanship employed in carrying out the process, and probably most importantly the aggregate that is forming the road surface.

(Images of the shot blasting process courtesy of TREVALBETON Ltd.)

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 a process similar to this press HERE

PERSONAL NOTE
As with almost all highway maintenance processes it is down to the maintenance of the machinery and the knowledge and training of the operators working it, supported by good company management that results in the quality of the products, processes and treatments that the various companies offer.

And, as you are no doubt bored of hearing me say I am a great believer in having supervisory staff present at all areas of work involving highway maintenance, at least until a satisfactory level of any process/product is established and is agreed between contractor and engineers representative so that the remainder of the work will be completed successfully.

It is also not that arduous to conduct some on site before and after skid resistance testing.  The testing need not be complete coverage but spot checks with "pendulum tester" or a griptester in hand operated mode can give quick feedback that the process is being carried out satisfactorily. Traffic management is often the biggest cost with regards to any form of "performance" testing of road surfacing, so if it is already present as part of the main work, take advantage of it, and then it costs you nothing. Road closures for various forms of work, even if it is "utility" related, often present the opportunity for "free" traffic management when you want to perform road surface, including road marking, testing.

WARNING

All retexturing processes are very aggressive treatments and should only be performed on road surfaces that you know to be sound, any weakness in the road surface will quickly become apparent using these processes even when performed correctly.
At present we are fortunate in having large areas of wearing course material (my favourite HRA and precoats) on the highway network that is quite capable of receiving a beneficial retexturing process a number of times.
The repeating of the retexturing process may be necessary because the road surface is not capable of retaining the roughened state of the aggregate for long periods of time due to the site being subject to very high traffic densities, especially heavy goods vehicles.

Is a retexturing process the most appropriate course of action for the site situation ?

It is my belief that the use of a retexturing treatment may not be an option for many of the new proprietary "Thin Surfacing" materials if/when they become smooth with wear.
This is because, it is my belief that, the majority of thin surfacings will not be of a sufficiently robust nature to withstand the treatments described on this page.
But if they are sufficiently robust to withstand this process you must also consider the characteristics of porosity and "negative" texture of some thin surfacings,  these properties could be impaired by using this treatment, so consult your Materials / Road Pavement Engineer.
These processes should be able to be used satisfactorily on "original", low void content stone mastic asphalts (SMA's), laid at a conventional (40/50mm.) thickness.

For more information on road texture and skid resistance, press, ----------------------------------------------------------------------> HERE

CONCRETE CARRIAGEWAY RETEXTURING

To the right is a an image of a large concrete grinding/retexturing machine that was trialled on the A12 near Kelvedon in September and October of 2009. The purpose of the trial, as I understand it, was to determine whether this machine and the process that it is capable of carrying out, meets the claims that are made for it. The claims being that it increases skid resistance, reduces road noise generated by the tyre/road surface interaction, and increases "driving comfort". The treatment allegedly costing considerably less than overlaying with bituminous materials to bring about these same benefits. (Overlaying concrete carriageways with insufficient thickness of appropriate bituminous mixtures often giving rise to further costly repair or replacement.) The feature of this process that immediately strikes me is that the retexturing is performed in a longitudinal direction and so removes the transverse texturing pattern on the concrete surface, that if not performed correctly and with care at the time of construction is usually the source of the generated tyre noise and driver fatigue. The longitudinal direction of grinding is also claimed to to remove unevenness at transverse joints in the concrete. The bringing to your attention of a process such as this is not something that I would normally do until I had seen reference to it in main stream engineering publications. But as I have not been able to find any reference to this process I would suggest that you read the information provided on the website of Concrete Cutters (Sarum) Ltd., for further details, and a small video of the process.

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