Highway pavement skid resistance testing is a critical quality control and safety assessment procedure that measures the frictional properties of road surfaces to prevent skidding accidents and ensure compliance with national and international standards. This technical article provides a comprehensive overview of skid resistance testing samples, parameters, methodologies, and applications for highway engineering professionals, contractors, and road authorities. Understanding these testing procedures helps ensure road network safety, optimize maintenance schedules, and extend pavement service life.
Technical Overview
Skid resistance refers to the frictional force developed between a vehicle tire and the pavement surface under wet or dry conditions, representing one of the most important safety characteristics of any highway pavement. Adequate skid resistance ensures that vehicles can brake, accelerate, and corner safely, particularly during wet weather when the risk of hydroplaning and loss of control increases dramatically. Statistical analyses of traffic accidents consistently demonstrate that inadequate pavement friction is a contributing factor in a significant percentage of wet-weather crashes, making skid resistance testing an essential component of highway construction acceptance, routine maintenance assessment, and network-level pavement management programs.
Pavement skid resistance is fundamentally governed by two surface characteristics: microtexture and macrotexture. Microtexture refers to surface roughness at the microscopic scale (wavelengths less than 0.5 mm), which is determined by the mineralogical properties of aggregate particles and provides the fundamental adhesion component of friction. Macrotexture refers to larger-scale irregularities (wavelengths between 0.5 mm and 50 mm) created by the arrangement of aggregate particles, which facilitates water drainage from the tire-pavement interface and provides hysteresis friction. Both characteristics must be evaluated to fully characterize the skid resistance potential of a pavement surface.
Skid resistance is not a fixed material property but varies with environmental conditions, traffic loading, seasonal changes, and surface contamination. Freshly constructed pavements typically exhibit lower initial skid resistance due to binder film coverage, which then increases during early trafficking as the film wears away, and gradually decreases over the long term as aggregates become polished. Consequently, comprehensive skid resistance testing programs established under standards such as ASTM E274, ASTM E303, EN 13036-4, and the Chinese highway field testing specification JTG E60 incorporate both construction acceptance testing and periodic network-level surveys.
Testing Samples
Asphalt Pavement Surfaces
Hot mix asphalt (HMA) and warm mix asphalt (WMA) surfaces constitute the majority of highway pavements subject to skid resistance testing. These include dense-graded asphalt concrete, stone matrix asphalt (SMA), open-graded friction courses (OGFC), and ultra-thin wearing courses. The aggregate type, gradation, and binder content significantly influence the achievable friction levels, with polishing-resistant aggregates such as basalt, granite, and artificial ceramics being preferred for high-friction surface design.
Cement Concrete Pavements
Portland cement concrete pavements are tested for skid resistance in both freshly finished and textured conditions. Surface texturing methods including burlap drag, wire comb, transverse tining, and longitudinal grooving create the macrotexture necessary for adequate wet-weather friction. Testing verifies that the specified texture depth and friction values have been achieved before the pavement is opened to traffic.
Surface Treatment and Functional Layers
Chip seals, slurry seals, micro-surfacing, cape seals, and high-friction surface treatments (HFST) applied as maintenance or corrective measures require skid resistance verification to confirm restoration of frictional function. These thin surface layers are particularly critical at locations with elevated friction demands such as horizontal curves, intersections, and downhill approaches.
Special Pavement Facilities
Airport runways and taxiways, port container terminal pavements, mountainous highway sections with steep grades, tunnel pavements, and bridge decks represent special facilities where skid resistance requirements are typically more stringent than standard highway sections. Testing programs for these facilities follow project-specific acceptance criteria in addition to general highway standards.
Testing Items
The principal measurable parameters in highway pavement skid resistance testing include the following:
- British Pendulum Number (BPN): A laboratory or field spot-value measurement of low-speed skid resistance obtained with a pendulum tester, typically reported as BPN or Pendulum Test Value (PTV) after temperature correction to 20°C.
- Sideway Force Coefficient (SFC): The ratio of sideway force to vertical load measured by a skid resistance tester vehicle traveling at a standard speed of 50 km/h, commonly expressed in units of 0.01 and used for network-level surveys.
- Friction Coefficient (Mu or f): The ratio of frictional force to normal load measured with a locked-wheel trailer using a smooth or ribbed test tire, reported as Friction Number (FN) or Skid Number (SN) at the test speed.
- Macrotexture Depth (MTD): The average texture depth determined by the sand patch method, expressed in millimeters, which indicates the water drainage capacity of the surface.
- Mean Profile Depth (MPD): A laser-based or profilometer-based texture parameter calculated from two-dimensional height profiles, convertible to estimated texture depth (ETD) for comparison with sand patch results.
- Weft-length and polishing resistance of aggregates: Accelerated laboratory polishing tests such as the Polished Stone Value (PSV) performed on aggregate samples to predict long-term friction retention.
Testing Methods
British Pendulum Tester Method
The pendulum test, standardized as ASTM E303 and EN 13036-4, employs a spring-loaded rubber slider mounted on a pendulum arm that swings across a wetted pavement surface. The energy loss due to friction is indicated by the pointer reading on a calibrated scale. Testing requires the surface to be thoroughly wetted, the instrument to be leveled and zeroed, and a minimum of five swings to be conducted at each location with water applied before each swing. Temperature correction is applied to normalize results to the reference temperature, as rubber stiffness varies with temperature. This method is suitable for spot measurements, construction acceptance, and laboratory evaluation of cores and slab specimens.
Locked-Wheel Trailer Method
The locked-wheel method specified in ASTM E274 utilizes a towed trailer equipped with a standard test tire that is braked to full lock while water is applied ahead of the tire. The horizontal drag force and vertical load are measured continuously, and their ratio defines the friction number at the test speed, typically 64 km/h (40 mph). Both smooth tires (FN40S), which are sensitive to macrotexture, and ribbed tires (FN40R), which are more sensitive to microtexture, may be used depending on the investigation objectives. This method provides continuous friction profiles over extended highway sections.
Sideway Force Coefficient Method
The sideway force method employs a dedicated skid resistance survey vehicle with test wheels mounted at an angle (typically 20 degrees) to the direction of travel and partially loaded. As the vehicle travels at a constant speed of 50 km/h with water sprayed in front of the test wheels, the lateral force generated is measured and expressed as the sideway force coefficient. This method, widely used for routine network-level surveys, measures friction without interrupting traffic and allows continuous data collection over thousands of lane-kilometers per day.
Sand Patch Method for Texture Depth
The sand patch (volumetric) method described in ASTM E965 and EN 13036-1 involves spreading a known volume of standard glass beads or fine sand in a circular patch on the cleaned pavement surface. The average diameter of the resulting circular patch is measured at multiple orientations, and the texture depth is calculated by dividing the volume by the area. This simple, low-cost method provides a reliable measure of macrotexture suitable for construction acceptance and spot verification.
Laser Texture and High-Speed Profilometry
Non-contact laser profilometers operating at traffic speeds measure surface height profiles at millimeter resolution, from which mean profile depth (MPD) is computed in accordance with ISO 13473 standards. High-speed data acquisition enables network-level texture surveys integrated with other surface condition measurements such as roughness and rutting in multifunction survey vehicles.
Accelerated Polishing Tests
Laboratory accelerated polishing using the wheel-tracking-based polishing machine subjects aggregate or pavement specimens to simulated traffic polishing, after which friction is measured with the pendulum tester. The Polished Stone Value (PSV) determined per EN 1097-8 provides a measure of the aggregate's resistance to polishing and supports material selection during mix design.
Applications
Highway pavement skid resistance testing serves a wide range of engineering and managerial purposes throughout the pavement life cycle:
- Construction quality acceptance: Verification that newly constructed or overlaid pavement surfaces meet the specified friction and texture requirements before opening to traffic.
- Network-level condition assessment: Routine skid resistance surveys supporting pavement management systems, prioritization of maintenance funding, and identification of sections with deficient friction.
- Accident investigation and site remediation: Post-accident friction measurements at crash locations to determine whether inadequate skid resistance contributed to the incident and to justify remedial treatments.
- Wet-weather accident reduction programs: Identification and treatment of high-friction-demand sites such as curves, intersections, ramps, and pedestrian crossings using high-friction surface treatments.
- Material qualification: Evaluation of aggregates, mix designs, and surface treatment products during design and procurement to ensure long-term friction performance.
- Airport pavement safety compliance: Periodic friction classification of runways to satisfy operational safety requirements for aircraft braking performance.
- Performance-based maintenance contracting: Objective measurement of friction retention as a contractual performance indicator for warranty and long-term pavement performance contracts.
- Research and standard development: Data collection supporting pavement friction prediction models, traffic polishing studies, and the calibration of investigatory and intervention friction levels.
FAQ
What is the minimum acceptable skid resistance value for highway pavements?
Acceptance thresholds vary by road class, traffic volume, and governing jurisdiction. Under typical highway maintenance specifications aligned with JTG 5210 and comparable international practice, sideway force coefficients below approximately 0.40 at 50 km/h on mainline sections are commonly treated as deficient and trigger investigation, while higher thresholds apply at high-friction-demand sites. Project specifications must always be consulted for the applicable limits.
How does wet weather affect skid resistance measurements?
Skid resistance is conventionally measured under wetted conditions because wet friction is substantially lower than dry friction and governs safety-critical performance. All standard methods apply controlled water films ahead of the test tire to ensure reproducible results. Seasonal variations also occur, with friction typically lower in late summer after extended traffic polishing and higher in winter; annual surveys should therefore be scheduled at consistent times of year for valid trend comparison.
How often should skid resistance testing be performed?
Network-level skid resistance surveys are generally conducted annually on high-traffic routes and every two to three years on lower-volume roads, subject to the policies of the responsible road authority. Construction acceptance testing is performed on completion of new surfaces, with follow-up verification after the initial wearing-in period, commonly three to six months of traffic.
Can skid resistance be restored on a polished pavement?
Yes. Restorative treatments include thin overlays with polishing-resistant aggregates, micro-surfacing, chip seals, shot blasting or bush hammering of concrete surfaces, grooving or retexturing of rigid pavements, and application of high-friction surface treatments using calcined bauxite aggregates. The selection depends on the degree of friction deficiency, structural condition, and available closure windows.
Why do pendulum test results require temperature correction?
The hardness and viscoelastic properties of the rubber slider vary with temperature, producing higher readings in warm conditions and lower readings in cold conditions. Temperature correction to a reference of 20°C, using correction factors defined in the applicable standard, is therefore required to ensure that results obtained at different ambient temperatures are comparable and reproducible.
