SKID RESISTANCE AND HIGH FRICTION SURFACING

CONTENTS

INTRODUCTION
SOME WIDELY USED TERMINOLOGY AND ITS MEANING
MEASURING SKID RESISTANCE
Portable Skid Resistance Tester (Also known as the "Pendulum Tester")
SCRIM, Sideways Force Coefficient Routine Investigation Machine
Griptester
Road, Pavement Friction Tester (PFT)
VERY USEFUL REFERENCE SOURCES
TEXTURE
SITES REQUIRING A PARTICULARLY HIGH RESISTANCE TO SKIDDING
METHODS OF ACHIEVING / RESTORING A GOOD SKID RESISTANCE SURFACE
HIGHWAY AUTHORITY PRODUCT APPROVAL SCHEME ( H.A.P.A.S.)
WARNING !

INTRODUCTION

The types of skid resistance, and factors affecting skid resistance that this guide will give information on are those applicable to road surfaces normally encountered in highway maintenance and road construction.

It is important to understand that all road surface skid resistance monitoring relates to wet/damp road surfaces, and the testing takes place after the road surface has been pre-wetted.

Although testing of dry road surfaces does take place from time to time by individual testing laboratories, it does not form any part of the routine skid resistance monitoring that takes place on the UK road network.

It can be assumed that, in dry conditions all clean surfaced roads have a high skidding resistance 
This is documented in, 
DfT. Design Manual for Roads and Bridges, Vol.7 : Design Manual HD 36/99 : Surfacing Materials for New and Maintenance Construction : Section 5-Surfacing and Surfacing Materials : Chapter 3-Texture and Aggregate Properties

SOME WIDELY USED TERMINOLOGY AND ITS MEANING

Polished Stone Value, PSV

This is a value of an individual aggregate, found by subjecting the aggregate to a standard polishing process and then testing the aggregate with the Portable Skid Resistance Tester.
The testing procedure and description of the process is set out in,
B.S. 812:Part 114.

PSV IS A VALUE APPLICABLE TO A PARTICULAR AGGREGATE, NOT THE ROAD SURFACE.

Aggregate that has a PSV value over 60 is regarded as a High Skid Resistant Aggregate.
The higher the PSV figure the greater resistance the aggregate has to polishing, and the greater the ability the aggregate has to retain its own natural very fine texture, (roughness).

THE PSV PROPERTY OF AN AGGREGATE IS OFTEN REFERRED TO AS MICRO TEXTURE.

Skid Resistance Value, SRV

This is the value obtained from the actual road surface, measured using the Portable Skid Resistance Tester.
The resistance to skidding of a road surface, i.e. its' SRV, is dependent on the PSV of the aggregate in the wearing course material AND the large texture, (roughness), of the surface of the wearing course material.

THE TEXURE / ROUGHNESS OF THE ROAD SURFACE IS OFTEN REFERRED TO AS MACRO TEXTURE

MEASURING SKID RESISTANCE

Portable Skid Resistance Tester (Also known as the "Pendulum Tester")

This is the piece of test equipment for measuring PSV's and SRV's.
The nickname originates from the pendulum action of the rubber slider that contacts the surface to be tested.
This equipment is hand operated for individual results and is not suitable for testing long lengths of road.
Instructions for use and information on the portable skid resistance tester can be found in,
ROAD NOTE 27 from the Transport and Research Laboratory.

I personally still very much like this piece of equipment, even if you can only use it for relatively small lengths of road that need testing. 

The apparatus is quite basic, quite robust, and does not easily go out of calibration if treated with the respect a precision instrument should be given.
The process is quite simple, the pendulum is released from the horizontal position by a quick release button, it swings down with uniform force each time, and the rubber slider at the bottom of the pendulum contacts the road surface for a fixed length that you have previously set by highering or lowering the height of the pivot of the pendulum.
The degree to which the pendulum will rise up the the calibration on the left-hand side of the image will be dependant on the friction / resistance the rubber slider meets on the road surface. 
The more friction / resistance the less the pendulum will rise and the higher the Skid Resistance Value (SRV) of the road surface.
Swinging with the pendulum is a pointer that cannot be seen on the photograph, and as the pendulum falls back the pointer will be left in place indicating the SRV.
The other good, practical, thing about using this apparatus is that when you are on your knees operating the pendulum tester you get a very good look at the road surface and the aggregate that constitutes the surface, and this can be very revealing.
It has to be said operating the pendulum tester is beneath the dignity of many highway engineers, even in their training days, that is why some of us actually know more about road surfaces than others. 

See :-
BS EN 13036-4 : 2003 : Road and airfield surface characteristics - Test methods - 
Part 4 : Method for measurement of slip/skid resistance of a surface - The pendulum test

This standard describes a method for determining the slip/skid resistance of a surface using a device which remains stationary at the test location. The slip/skid resistance is measured by means of a pendulum incorporating a rubber "slider" which is dragged across the road surface as the pendulum swings.
The "swing" of the pendulum will be retarded to a greater or lesser extent depending upon the friction characteristics of the road surface.
The pendulum swings with a pointer that remains against a scale of SRV's when the pendulum swings back, and a value is able to be recorded.

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 the PORTABLE SKID RESISTANCE TESTER, press ------> HERE

SCRIM, Sideways Force Coefficient Routine Investigation Machine

This machine is a lorry chassis with a large water holding tank similar in size to a "gully emptyer", but it has mounted in its mid-section left side wheel track, a test wheel that is set at 20° to forward travel.
This creates a pressure on the wheel related to the skid resistance of the road surface.
This pressure can be measured and is processed to give figures, SFC's, that represent the road surface skid resistance.
The SCRIM normally travels at 50 km/h and so is capable of surveying many kilometres of road in a day.
This speed has led to the use of SCRIM to conduct large surveys of road network to ensure adequate skid resistance of the road surface.

SFC, Sideways Force Coefficient

The SFC is the value of the skidding resistance of a road surface obtained using the SCRIM, and be careful with the terminology SRV and SFC, because SFC's are not the same as SRV's, they are almost the same but not quite, but for practical purposes they may be considered so.

See :-
BS 7941-1 : 1999 : Methods for measuring the skid resistance of pavement surfaces
Part 1 : Side-way force coefficient routine investigation machine

This standard describes a method for determining the wet-road skid resistance of a surface using the sideway-force coefficient routine investigation machine (SCRIM). 
The method provides a measure of the wet-road skid resistance properties of a bound surface by measurement of the sideway-force coefficient at a controlled speed. 
The method has been developed for use on roads but is also applicable to other paved areas such as airport runways.

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 SCRIM, press ------------------------------------------------------------> HERE

Griptester

This is an excellent small three wheeled device with the "normal" axle being connected to the recording wheel by a pair of gears which causes a braking effect on the axle of the third wheel which can be measured as a "Grip Number". If you are able to obtain a copy of, TRL Project Report RR/H/58/93 this will explain and compare the apparatus to a SCRIM. This equipment can be used pushed by hand on a pre-wetted road surface for small area surveys. As the Griptester has developed in to a more precise piece of equipment it has been taken up by many organisations as their standard means of assessing roads and runways for skid resistance, and a number of user groups are well established to further develop and support its use.

Griptester attached to towing vehicle	Large plastic water tank with  metering pump connected to laptop	Griptester in use

See :-
BS 7941-2 : 2000 : Surface friction of pavements - 
Part 2 : Test method for measurement of surface skid resistance using the GripTester braked wheel fixed slip device

This standard describes a method for determining the skid resistance of a surface using the GripTester continuous reading braked wheel fixed slip device. 
The method is for measurement of skid resistance along a continuous surface on external paved surfaces, or indoors. 
Test speeds can vary from 5km/h to 130km/h depending upon application. The measured values can be affected by the test speed.

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 GRIPTESTER, press --------------------------------------------> HERE

Road, Pavement Friction Tester (PFT)

This device differs from the already described methods by actually employing the "locked" wheel principle and measuring the friction between the fully braked wheel and the road surface.
The TRL now owns a PFT, and pictures and description of the apparatus and how it is used can be found in :- 

TRL Report 367 : High and low speed skidding resistance : the influence of texture depth

The PFT is the standard apparatus for testing the friction of road surfaces in the USA, it is a towed trailer with both wheels having hydraulically applied disk brakes which are activated by compressed air. 
It uses the locked-wheel principle in accordance with ASTM Standard E274 (1990)

I believe the machine owned by the  TRL  has only the left side wheel fitted with the braking system.

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 the PAVEMENT FRICTION TESTER, press ---------------> HERE

VERY USEFUL REFERENCE SOURCES

DfT. Design Manual for Roads and Bridges, Vol.7 : Design Manual HD 28/04 : Skidding Resistance (recently updated)
DfT. Design Manual for Roads and Bridges, Vol.7 : Design Manual HD 36/99 : Surfacing Materials for New and Maintenance Construction

County Surveyors Society Reports ENG/7-94, and ENG/3-95 :
The Achievement of Skidding Characteristics for Road Pavements,
Volume 1:An Overview,
and
Volume 2:PSV values for Aggregate in UK Roads

TRRL Report 510 : A guide to levels of skidding resistance for roads
TRRL Report 466 : Aggregates for resin bound skid resistant surfacings
TRRL Report 296 : The relationship between surface texture of roads and accidents
TRL Report 367 : High and low speed skidding resistance : the influence of texture depth
The book "ROAD AGGREGATES AND SKIDDING" by Roger Hosking, available from H.M.S.O.
and
BS EN 13036-4 : 2003 : Road and airfield surface characteristics - Test methods - Part 4 : Method for measurement of slip/skid resistance of a surface - The pendulum test
BS 7941-1 : 1999 : Methods for measuring the skid resistance of pavement surfaces - Part 1 : Side-way force coefficient routine investigation machine
BS 7941-2 : 2000 : Surface friction of pavements - Part 2 : Test method for measurement of surface skid resistance using the GripTester braked wheel fixed slip device

For an excellent report from the Highways Agency's website, covering most aspects of road surface skid resistance, written by someone who obviously knows their subject for those who need to know, look under the sub-heading, "Composition and condition of the road surfacing", after pressing ---------------------------------------------------------------> HERE 


TEXTURE

 Texture is measured by the Sand Patch Method on small areas, i.e. areas of new bituminous surfacing surfacing prior to allowing it to be trafficked.
The method is fully documented in B.S. 598:Part 105, however this method has already, or is soon to be superseded by a very similar method but using a standard sized glass bead in place of sand.
However for practical purposes either method will give you a fairly accurate assessment of road surface texture. 
The principle is fairly obvious the greater the texture the more the sand will be taken up by it and the smaller the circle that can be achieved from the standard quantity of sand. 
There is a simple formula in the British Standard that allows you to determine an average texture depth from the diameter of the sand circle.

But there is an area of controversy regarding the measurement of texture in positively textured road surfaces, e.g. hot rolled asphalt and precoats, and surface dressing,
and
negatively textured road surfaces such as porous asphalt, and Thin Surfacings that are porous in nature.

It is suggested that sand or glass beads will fall into the voided structure of a porous material and so produce a result for surface texture that is not related to the texture that is able to be exploited by the contact area of the vehicle tyre.
I will leave you to read the literature that is available on this subject and to ponder on whether this is a reasonable hypothesis.

Large areas/lengths of road that need assessing for texture depth are tested by the,
High Speed Road Monitor or the High Speed Texture Meter.

These devices employ lasers pointing at the road surface to measure texture, and the measurement recorded in this way is known as Sensor Measured Texture Depth, (SMTD).

Be careful SMTD results cannot be compared directly with the texture depth figures obtained by Sand Circle texture depth testing.

E.g. 1.5mm texture depth by sand circle = 1.1mm texture depth by SMTD, and is THE SAME ACTUAL TEXTURE DEPTH, with regard to skid resistance requirements

SITES REQUIRING A PARTICULARLY HIGH RESISTANCE TO SKIDDING

Although all roads require good skid resistance there are some situations that will require a higher level of skid resistance than others:-

1) Approaches to Pelican Crossings, and some junctions, islands and bends

These sites need the highest possible skid resistance available.
This will usually mean an anti-skid process using a resin as the binder and Calcined Bauxite chippings of approx. 2mm. to 3mm. size as the high PSV aggregate.
This can be achieved in two ways :-

a)
by applying the resin, (an epoxy resin for trunk roads), to the road surface and applying Calcined Bauxite chippings onto the binder to give complete shoulder to shoulder cover for maximum texture.
For large areas this process is normally machine applied but resins capable of being mixed and applied by hand are now available.

b)
by applying the resin and calcined bauxite together operation in a hot applied operation. The thermoplastic resin and calcined bauxite aggregate is premixed in poly bags and then heated in a large "boiler" to the required temperature.
The molten material is screeded on to the road surface using a 300mm. wide bottomless "shoe" at right angles to the direction of traffic, in lane widths.

These processes gives good MACRO TEXTURE, and the PSV of Calcined Bauxite is about as high as you can get, approximately.
75 PSV for Guyanan (grey),
70 PSV for Chinese (buff coloured).
These processes should give SRV's or SFC's in the range 70 to 80 depending on the process, and the quality of the workmanship.

2) Trunk Roads

The D.O.T. have a requirement in their specification that road surfaces shall have a minimum texture depth, (MACRO TEXTURE), of 1.5mm., (by "Sand Patch" not by SMTD).
This was normally achieved by having a wearing course of 30%/14mm HRA and applying a layer of 20mm. precoated chippings which are rolled into the asphalt while it is still hot.
The PSV of the precoat will be a minimum of 55 for straight sections of road with no junctions, bends or other hazards.
It is more usual to specify a minimum PSV of 60 for all average sites, and where there are tight bends or hazardous junctions a minimum PSV of 65 is required.
(See D.O.T. Standard HD 28/94 for further information.)
The above surface treatments will give SRV's and SFC's of 55 to 65.

The use of Hot Rolled Asphalt Wearing Course with Precoats is not now permitted as a wearing course option on motorways and trunk roads in England.
All wearing course now has to be proprietary "Thin Surfacing".

3) Principal Roads and other highly trafficked roads

This group of roads can still use the Hot Rolled Asphalt wearing course and Precoats option.

METHODS OF ACHIEVING / RESTORING A GOOD SKID RESISTANCE SURFACE

1) Resin / Calcined Bauxite processes as described, ------------------------------------------------------------------------------------------> HERE

2) Specify bituminous wearing course material that will have a high SRV, e.g. hot rolled asphalt wearing course with 20mm. precoted chippings of a high PSV applied at the correct rate to achieve a high texture, see, ---------------> HERE

3) Surface Dressing with a high PSV chipping is an excellent process for restoring skid resistance to the surfaces of all classes of road, on sections that are suitable for the treatment, shown, ------------------------------------------------------------> HERE

4) Re-texturing of a sound road surface by a mechanical abrading process, (best for surfaces with a high aggregate content), or a high pressure water method, (best for binder rich surfaces), as described, ----------------------------------> HERE

A compilation of individual items of information related to Skid Resistance can be accessed by pressing,-------> HERE

(NEW ) 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 excellent information on the skid resistance of roads and runways, that is able to be downloaded in .pdf format,  press, -----------------------------------------------------------------------------------------------------------------------------------------------------> HERE


HIGHWAY AUTHORITY PRODUCT APPROVAL SCHEME ( HAPAS)

This is a recently introduced organisation to "approve" new products for use in highway maintenance and construction, so that individual authorities do not need to undertake their own trials.
It has been set up by the Highways Agency (HA), CSS (formerly the County Surveyors Society) and the British Board of Agrement (BBA), with the involvement of bodies representing private industry.

I am concerned about products and processes that gain approval / certification at national level with little true participation of local authorities or local contractors.

The development of HAPAS is administered by the BBA, closely advised by the Highways Technical Advisory Committee (HiTAC), certificates of approval are issued by the BBA acting on recommendations from HiTAC.

Specialist groups, with a strong industry representation, are created for each product type.

Products and processes are put forward for consideration by the group who will report back its findings to HiTAC, who will in its turn make recommendations to the BBA as to whether the product / process should receive HAPAS approval.

Full details of the particular schemes and the criteria for assessment are available from the British Board of Agrement.

British Board of Agrement ------------------------------------------------------------------------------------------> www.bbacerts.co.uk

A list of High Friction Surfacing products and installers that have HAPAS approval / certification will be found on the above site.
Copies of certificates of some of the products that have received approval can be downloaded in "pdf" format from this same site.

THIN SURFACINGS have now become products/systems that are able to receive HAPAS approval, replacing Highways Agency approval, but the mechanism by which THIN SURFACINGS gain approval has been the cause for some debate, as "systems" of THIN SURFACINGS are receiving approval rather than individual products.
Copies of certificates of some of the products/systems that have received approval can be downloaded in "pdf" format from the BBA site.

For more information on Highways Authority Product Approval Scheme, press ---------> HERE

For more information on proprietary Thin Surfacings, press ---------------------------------------> HERE

WARNING !

Sites that have developed a low skid resistance, for whatever reason, must be picked up and dealt with quickly by a suitable remedial treatment.
If this cannot be done "Slippery Road" sign boards should be prominently displayed.
Claims against Highway Authorities have been upheld in court where the authority has been thought to have been negligent in respect of road surface skid resistance, failing to investigate a possible danger, or failing to inform of a possible risk after investigation.

You may wish to obtain and study :-

Interim Advice Note IAN 50/03 : Investigation of sites identified from skid resistance measurements
Volume 7 - Pavement Design and Maintenance Manual - Section 3 : Pavement Maintenance Assessment
and,
Interim Advice Note IAN 49/03 : Use of warning signs for new asphalt road surfaces
Volume 7 - Pavement Design and Maintenance Manual - Section 5 : Surfacing and surfacing materials

[ Top of Page ]