The SCRIM® machine was introduced in the 1970s to provide a method for routinely measuring the skid resistance of the road network. This machine uses the sideway force principle to measure skid resistance. A freely rotating wheel fitted with a smooth rubber tyre, mounted mid-machine in line with the nearside wheel track and angled at 20° to the direction of travel of the vehicle, is applied to the road surface under a known vertical load. A controlled flow of water wets the road surface immediately in front of the test wheel so that, when the vehicle moves forward, the test wheel slides in the forward direction along the surface.
The force generated by the resistance to sliding is related to the wet road skid resistance of the road surface. Skid resistance is not a constant, but is influenced by various factors, including temperature, test speed and weather conditions, plus longer-term effects such as seasonal variation and changes in traffic flow. For the purpose of this Standard, SCRIM® machine measurement will be made under standardised conditions to control these effects as far as possible, including:
• limiting the testing season to a specific time of year;
• specifying a standard test speed;
• specifying the test line to be followed;
• specifying the ambient conditions under which acceptable measurements may be made.
All measurements are taken on a wet road surface and water is fed at a virtually constant rate on to the road in front of the angled wheel.
SFC varies with the speed of the machine, so the SCRIM® vehicle is normally operated at a standard speed of 50 km/h or 80 km/h on dual carriageways.
The skid resistance of a pavement surface varies with the time of the year assuming a constant volume of traffic so standard practice is to measure during the summer season (May to September). Details of the testing procedure and seasonal correction process are in the Design Manual for Roads and Bridges HD 28/04.
SCRIM is a registered trademark of W.D.M Limited.