The dry friction coefficient of asphalt pavement is a fundamental indicator of surface skid resistance and traffic safety, describing the tangential force developed between a tire and the pavement under dry conditions. This article explains the testing principles, specimen requirements, measurable parameters, and internationally recognized methods — including ASTM E303, EN 13036-4, ASTM E274, ASTM E1911, and JTG E60 (T 0964) — used for the determination of dry friction on asphalt surfaces. It provides practical guidance for highway engineers, quality-control professionals, accredited third-party laboratories, and pavement researchers.
Technical Overview
The friction coefficient (μ) of an asphalt pavement surface is defined as the ratio of the tangential resisting force (F) developed between a rubber slider or tire and the surface to the applied normal load (W), expressed as μ = F / W. For dry asphalt surfaces, peak friction coefficients typically range from approximately 0.6 to 0.9, depending on aggregate mineralogy, mixture type, texture depth, tire rubber properties, sliding speed, and temperature. Determining the dry friction coefficient is essential for evaluating skid resistance performance, accepting newly constructed surfaces, monitoring polishing over the service life, and supporting accident reconstruction analyses.
Two surface texture scales govern pavement friction. Microtexture, with wavelengths below approximately 0.5 mm, is produced by the asperities of individual aggregate particles and dominates adhesion friction under dry, low-speed conditions. Macrotexture, with wavelengths between 0.5 mm and 50 mm, is governed by mixture gradation and compaction and controls water drainage and hysteresis friction at higher speeds. Because dry friction is dominated by microtexture, dry friction coefficient testing is a sensitive indicator of the polishing state of the surfacing aggregate.
Several internationally recognized test procedures exist, including the British Pendulum Tester method (ASTM E303, EN 13036-4, and JTG E60 T 0964), the locked-wheel skid trailer method (ASTM E274), the Dynamic Friction Tester method (ASTM E1911), side-force measurement (ASTM E670), and the International Friction Index harmonization approach (ASTM E1960). Accredited third-party laboratories routinely perform these tests for construction acceptance, safety audits, and research programs.
Testing Samples
In-Situ Pavement Sections
Field testing is performed on representative sections of the asphalt pavement, typically covering the outer wheel path, the inner wheel path, and the area between wheel paths. Test points are usually spaced at intervals of 5 to 20 meters along the lane, with a minimum of five measurement locations per evaluation section. Prior to testing, the surface must be swept clean of dust, loose aggregate, and debris, and it must be visibly dry. Testing is generally avoided during rain, fog, or immediately after wet weather unless the surface has been force-dried. Surface temperature is recorded at each point because rubber friction properties vary significantly with temperature.
Laboratory Compacted Slabs
Laboratory evaluation uses asphalt mixture slabs compacted by a rolling wheel compactor, a linear kneading compactor, or a laboratory wheel-tracking device. Common slab dimensions are 300 mm × 300 mm × 50 mm or 305 mm × 305 mm × 50 mm, which provide a sufficient planar area for multiple pendulum or drag-type friction measurements. Slabs are conditioned at the specified test temperature, typically 20 °C ± 2 °C, for at least 4 hours before testing to ensure thermal equilibrium.
Drilled Core Specimens
Cores of 100 mm or 150 mm diameter extracted from the pavement can be tested in the laboratory to evaluate the friction characteristics of an existing surface. Core surfaces must be cleaned with a soft wire brush and compressed air, inspected for polishing or contamination, and mounted on a rigid, level base so that the test surface remains flat and immovable during measurement.
Testing Items
The principal parameters determined in a dry friction coefficient test program include:
- Dry friction coefficient (μ), a dimensionless ratio of tangential force to normal load.
- British Pendulum Number (BPN) measured on a dry surface, representing low-speed skid resistance.
- Skid Number (SN) at a standard test speed of 64 km/h, obtained with locked-wheel equipment.
- Dynamic friction coefficient as a function of speed, typically from 20 km/h to 80 km/h, using a Dynamic Friction Tester.
- Static friction coefficient determined by drag-type (sled) methods on laboratory specimens.
- Ancillary measurements, including surface temperature, air temperature, and macrotexture depth by the sand patch method (ASTM E965) or laser profilometry (ISO 13473-1), which are required for friction index conversion.
- Statistical results, including the mean value, standard deviation, coefficient of variation, and temperature-corrected values reported to the reference temperature.
Testing Methods
British Pendulum Tester Method (ASTM E303, EN 13036-4, JTG E60 T 0964)
A pendulum with a spring-loaded rubber slider at the end of its arm is released from a horizontal position, and the slider sweeps a fixed contact length across the pavement surface. The energy lost to friction is indicated by the pointer on a graduated scale, yielding the BPN. Before testing, the instrument is leveled, the slider contact path and contact length are verified, and calibration is performed. At each test point, at least five valid swings are conducted and the median value is reported. For dry testing, no water film is applied; the surface is tested in its clean, dry state. Results are corrected to the reference temperature where required. The pendulum method is economical, portable, and widely used for construction acceptance and laboratory studies.
Locked-Wheel Skid Trailer Method (ASTM E274)
A towed trailer fitted with a standard test tire — a ribbed tire per ASTM E501 or a smooth tire per ASTM E524 — is pulled along the pavement at a standard speed of 64 km/h. The test wheel is momentarily locked, and the frictional drag force and vertical load are measured to compute the skid number, where SN = 100 μ. Although primarily used for wet skid resistance surveys, dry measurements can be performed for research purposes; extended dry locked-wheel testing accelerates tire wear and must be controlled carefully.
Dynamic Friction Tester Method (ASTM E1911)
The Dynamic Friction Tester uses three parallel rubber sliders attached to a horizontal rotating disk. The disk is accelerated to approximately 100 km/h and then allowed to decelerate freely, during which the sliders contact the surface and the friction force is measured continuously, providing friction coefficient values at effective speeds from 20 km/h to 80 km/h. The device is compact, suitable for both dry and wet testing, and commonly applied on airport pavements, research projects, and spot safety investigations.
Side-Force and Fixed-Slip Devices (ASTM E670)
Side-force friction measuring devices assess pavement friction continuously at a constant slip angle while the apparatus is towed at traffic speed. These systems measure the lateral force developed between angled test tires and the surface and are used primarily for network-level skid resistance surveys of long road sections.
Laboratory Drag and Static Friction Methods
For small specimens and comparative material studies, a drag sled fitted with a standard rubber pad is pulled across the dry surface under incrementally increasing normal loads. The force required to initiate sliding yields the static friction coefficient, and the force required to sustain uniform motion yields the kinetic friction coefficient. These methods are simple, repeatable, and well suited to evaluating mixture formulations, surface treatments, and anti-skid additives.
Applications
Dry friction coefficient determination supports a wide range of engineering and safety applications:
- Construction acceptance and quality assurance for new asphalt surfacing, including SMA, OGFC, and surface treatment courses.
- Periodic skid resistance surveys for pavement management systems and maintenance prioritization.
- Safety evaluation of high-risk locations such as horizontal curves, intersections, long downgrades, ramps, and tunnel decks.
- Runway, taxiway, and apron friction monitoring for airport pavement maintenance programs.
- Accident reconstruction and forensic evaluation of pavement skid resistance conditions.
- Research into aggregate polishing resistance, including polished stone value testing (EN 1097-8) and accelerated polishing with subsequent friction measurement (ASTM D3319, EN 12697-49).
- Development of anti-skid surfacing materials, high-friction treatments, colored pavement systems, and specialized industrial or sports pavements.
FAQ
Why measure the dry friction coefficient when wet skid resistance is usually the governing safety concern?
Dry friction isolates the contribution of microtexture and reveals the polishing state of the surfacing aggregate without the confounding influence of a water film. It establishes a baseline for the surface, supports research into material durability, and is directly relevant to high-speed braking and heavy-vehicle maneuvers on dry roads. A comprehensive assessment normally combines dry and wet measurements.
What is a typical acceptable dry friction coefficient for asphalt pavement?
Dry asphalt surfaces generally exhibit friction coefficients between 0.6 and 0.9. Many acceptance specifications require dry values above approximately 0.5 to 0.6, while Chinese highway specifications such as JTG F40 require pendulum values of BPN 45 to 56 or above on the wet surface depending on road class and climatic zone. Because results depend on the test method, speed, and tire or slider rubber, values should always be compared against the criteria defined for the specific method.
How does temperature affect the results?
The hardness and viscoelastic behavior of the rubber slider or test tire vary with temperature, which changes the measured friction. The applicable standards provide temperature corrections, typically normalizing results to a reference temperature of 20 °C. Testing should be avoided when the surface temperature falls outside the range permitted by the standard.
How many measurements are required at each test point?
For pendulum testing, at least five valid swings are performed at each location and the median is reported, with test points distributed along the evaluation section to capture surface variability. For trailer and continuous-measurement devices, readings are taken over defined test sections and averaged. Reproducibility is checked through the coefficient of variation of replicate measurements.
Can pendulum results be converted into a friction coefficient?
Pendulum numbers represent low-speed skid resistance and do not translate directly into a speed-dependent friction coefficient. By combining the pendulum value with macrotexture measurements, the International Friction Index defined in ASTM E1960 allows harmonization between different devices and speed conditions, enabling results from different test methods to be compared on a common basis.