Road Skid Resistance and Texture Depth: The Pendulum Test and the Sand Patch Test

A road can be structurally sound and still be dangerous if its surface is too smooth. In heavy tropical rain, the grip between tyre and road depends on the texture of the surface at two very different scales, and both can be measured on site with simple, portable tests: the pendulum test for skid resistance and the sand patch test for texture depth.

Two Scales of Texture

• Microtexture is the fine, gritty harshness of the individual aggregate particles at the surface. It provides grip at lower speeds by allowing the tyre rubber to make direct contact through the thin film of water. Aggregates that polish under traffic lose their microtexture over time.
• Macrotexture is the larger-scale roughness created by the gaps between aggregate particles. It provides channels for water to escape from under the tyre, which becomes increasingly important at higher speeds and in heavy rain.

A surface needs both. The pendulum test is most sensitive to microtexture, and the sand patch test measures macrotexture.

1. The British Pendulum Test (Skid Resistance)

The British pendulum tester is standardised in BS EN 13036-4, Road and airfield surface characteristics, Test methods, Method for measurement of slip/skid resistance of a surface: The pendulum test, and in ASTM E303, Standard Test Method for Measuring Surface Frictional Properties Using the British Pendulum Tester.

How it works:

1. The tester is levelled on the road surface and the pendulum arm is adjusted so that its rubber slider contacts the surface over a set length.
2. The surface is wetted with clean water to represent wet-road conditions.
3. The pendulum is released and swings down, the slider sliding across the wet surface.
4. The friction between slider and surface slows the swing. The height the arm reaches on the other side is read from a scale as the British Pendulum Number (BPN). A higher BPN means more friction.

Several swings are made at each location and averaged, and the temperature is recorded because the rubber slider's behaviour changes with temperature.

2. The Sand Patch Test (Texture Depth)

The volumetric patch method, widely known as the sand patch test, is standardised in BS EN 13036-1, Measurement of pavement surface macrotexture depth using a volumetric patch technique, and in ASTM E965, Standard Test Method for Measuring Pavement Macrotexture Depth Using a Volumetric Technique.

How it works:

1. A known volume of fine, uniform material (traditionally sand, now often glass beads) is poured onto a clean, dry area of road.
2. It is spread in a circle with a flat disc, working the material into the surface voids until the circle cannot be enlarged further.
3. The diameter of the circle is measured in several directions.
4. The known volume divided by the area covered gives the mean texture depth.

A rough, open surface swallows the material into a small patch and gives a large texture depth. A smooth or bleeding surface spreads it wide and gives a small texture depth.

Where These Tests Are Used

• New surfacing: checking that a newly laid wearing course meets the texture and skid resistance requirements in the specification.
• Accident investigation: assessing the surface condition at locations with a history of wet-weather skidding crashes.
• Maintenance planning: identifying polished or bleeding sections for resurfacing or surface treatment.
• Comparing surfacing materials and aggregate sources.

Limitations

• Spot measurements: both tests measure a small area. Enough locations are needed to represent a length of road.
• Traffic management: both need a safe, closed working area on a live road.
• Condition-dependent: the sand patch test needs a dry surface, and pendulum results must be read with the test temperature and surface condition in mind.

Note: Minimum skid resistance and texture depth values depend on the road type, site category, and the project specification. Results should be assessed against the requirements in the contract.