Determination of Skid Resistance Attenuation in Asphalt Pavements: Testing Standards, Methods, and A

Skid resistance attenuation in asphalt pavements describes the progressive loss of tire–pavement friction caused by traffic polishing, aggregate wear, binder bleeding, and environmental exposure — a degradation process that directly affects road safety, particularly under wet conditions. This article explains how asphalt pavement skid resistance decay is measured in the laboratory and in the field, which international standards govern the testing, and how attenuation data supports maintenance planning and material acceptance. Understanding these measurement techniques enables pavement engineers to predict when friction levels will fall below safety thresholds and to schedule surface restoration accordingly.

Technical Overview

Skid resistance is the frictional force developed between a vehicle tire and the pavement surface, and it is one of the most safety-critical functional properties of an asphalt roadway. A newly constructed asphalt surface typically exhibits high initial friction because the angular aggregate particles protrude through the binder film and provide strong microtexture and macrotexture. However, under repeated traffic loading, the thin bitumen film wears away, surface aggregates become progressively polished, texture channels fill with contaminants, and bleeding or rutting may develop. The result is a measurable, time-dependent decline in friction known as skid resistance attenuation.

Attenuation does not proceed linearly. In most field observations, friction drops rapidly during the early service period as the binder film is removed, continues to decline at a moderate rate as aggregate particles polish, and eventually stabilizes at a relatively low equilibrium value determined by the polish resistance of the dominant coarse aggregates. Determining this decay curve — rather than measuring friction at a single point in time — is essential for predicting the safety service life of a wearing course.

Quantitative measurement of skid resistance attenuation relies on established friction and texture indicators, including the British Pendulum Number (BPN, also reported as Pendulum Test Value, PTV) per ASTM E303 and EN 13036-4, the locked-wheel friction coefficient per ASTM E274, the sideways force coefficient measured with routine survey vehicles, and Mean Texture Depth (MTD) per ASTM E965 and EN 13036-1. Surface texture characterization following the ISO 13473 series complements friction testing, while accelerated polishing of aggregates per ASTM D3319 and EN 1097-8 provides the Polished Stone Value (PSV) that governs long-term decay behavior.

Testing Samples

In-Service Pavement Sections and Field Cores

The primary sample for attenuation studies is the pavement surface itself, tested in place at defined stations and traffic exposure intervals. Cores (typically 100 mm or 150 mm in diameter) extracted from lanes of different service age allow laboratory friction and texture measurements to be correlated with accumulated traffic passes. Cores taken at multiple ages — for example, at opening to traffic, after one year, and at three to five years — provide the data points needed to construct an attenuation curve for a specific mixture and traffic level.

Laboratory-Fabricated Slab Specimens

For pre-construction evaluation, slab specimens are compacted in the laboratory from the designed asphalt mixture using a roller compactor or wheel-tracking compactor to achieve field-equivalent air void content and surface texture. These slabs are subjected to accelerated polishing, and their friction is measured at fixed polishing intervals. This sample type allows different mix designs, aggregate sources, and gradations to be compared under identical, controlled polishing conditions.

Coarse Aggregate Specimens

Aggregate-level samples are prepared by embedding aggregate chippings in a curved mold and polishing them on an accelerated polishing wheel, after which friction is measured with a pendulum tester to obtain the Polished Stone Value. Aggregate specimens are used to rank the intrinsic polish susceptibility of mineral sources — for example, basalt versus limestone versus quartzite — before they are approved for use in surface courses.

Testing Items

Skid resistance attenuation determination typically includes the following measured and derived quantities:

• Initial skid resistance (BPN₀ or initial friction coefficient) measured at the start of service or before polishing.
• Skid resistance after defined exposure intervals — accumulated vehicle passes, service years, or laboratory polishing cycles.
• Pendulum test value (BPN/PTV) per ASTM E303 / EN 13036-4 on wet surfaces at a controlled temperature of approximately 20 °C.
• Locked-wheel friction coefficient or skid number per ASTM E274 at standard test speeds.
• Mean Texture Depth (MTD) by sand patch method per ASTM E965 / EN 13036-1, and texture profile metrics per the ISO 13473 series.
• Polished Stone Value (PSV) of coarse aggregates per ASTM D3319 / EN 1097-8.
• Attenuation rate index, calculated as the percentage loss of friction relative to the initial value: A = (BPN₀ − BPNₜ) / BPN₀ × 100%.
• Decay model parameters, typically fitted to an exponential form such as BPN(t) = BPN∞ + (BPN₀ − BPN∞)·e^(−kt), where BPN∞ is the equilibrium friction and k is the decay constant.
• Predicted time or traffic volume required for friction to fall below a specified intervention threshold.

Testing Methods

Field Friction Monitoring

In-service attenuation is tracked by periodically measuring friction at fixed, marked test locations. Portable pendulum testers provide point measurements suitable for project-level assessment, while instrumented survey vehicles measuring the sideways force coefficient or locked-wheel friction deliver continuous network-level data at traffic speed. Because friction varies seasonally — wet seasons generally yield lower readings — monitoring should be repeated in the same season each year to make valid comparisons. Texture depth is measured concurrently using the sand patch method or laser profilometry to distinguish texture loss from aggregate polishing.

Accelerated Laboratory Polishing

Laboratory attenuation testing subjects compacted slab specimens to controlled polishing that simulates years of traffic within days. Commonly used equipment includes three-wheel or linear wheel polishing devices loaded with rubber tires and an abrasive slurry, as well as rotary accelerated polishing systems. Friction is measured with a pendulum tester at intervals — for example, every 5,000, 10,000, and 20,000 polishing cycles — until the measured BPN stabilizes, indicating that the equilibrium polish level has been reached.

Attenuation Curve Analysis

The collected friction-versus-exposure data are regressed against a decay model to obtain the decay constant and equilibrium friction level. Engineers then compare the predicted curve with project-specific intervention thresholds, which are commonly set at approximately 42–45 BPN for wet pavement surfaces in many national specifications. The resulting attenuation model supports life-cycle safety assessment, optimization of aggregate selection, and scheduling of preventive surface treatments such as micro-surfacing, chip seals, or thin friction courses.

Applications

Determination of skid resistance attenuation serves a wide range of engineering and management purposes:

• Network-level pavement management: forecasting when friction will drop below safety thresholds enables proactive, budget-optimized maintenance programming.
• Mix design and aggregate approval: comparing attenuation curves of candidate mixtures identifies surface courses that will sustain adequate friction throughout the design life.
• High-risk site evaluation: intersections, horizontal curves, steep downgrades, and wet-accident black spots are prioritized for attenuation monitoring and early intervention.
• Acceptance and warranty testing: performance-based contracts may require the contractor to demonstrate a specified residual friction level after an initial service period.
• Accident investigation and litigation support: documented friction decay histories provide objective evidence of surface condition at a given date.
• Research and specification development: attenuation data from accredited third-party laboratories support the calibration of polishing tests, refinement of friction standards, and long-term pavement performance modeling.

FAQ

What is skid resistance attenuation?

Skid resistance attenuation is the gradual decline of tire–pavement friction over time, caused by the removal of the surface binder film, progressive polishing of aggregate particles, loss of macrotexture, contamination of surface voids, and distresses such as bleeding and rutting. It is quantified by tracking friction indicators at successive exposure states and fitting a decay curve to the results.

How quickly does skid resistance decline on a new asphalt surface?

Friction loss is typically fastest during the first one to two years of service, when traffic removes the binder film and exposes fresh aggregate. Thereafter the decline moderates, and within roughly three to five years most surfaces approach an equilibrium value governed by aggregate polish resistance. Surfaces built with high-PSV aggregates retain significantly more long-term friction.

What friction level triggers maintenance action?

Intervention thresholds vary by road class, climate, and national specification, but wet-surface pendulum values in the range of approximately 42–45 BPN are widely used as investigation or action levels for asphalt pavements. Once monitoring shows the projected decay curve crossing the applicable threshold, a surface restoration treatment is scheduled.

Can laboratory polishing tests replace field monitoring?

No. Accelerated polishing tests provide fast, repeatable comparative ranking of mixtures and aggregates under controlled conditions, and they are excellent for design-stage screening. However, actual decay rates depend on site-specific traffic volume, heavy-vehicle proportion, climate, and contamination, so field verification at periodic intervals remains necessary for reliable attenuation assessment.

How often should attenuation monitoring be performed?

For most network sections, annual or biennial measurement during a consistent season is sufficient. High-risk locations — intersections, curves with small radius, steep grades, and sections with documented wet-weather incidents — should be monitored more frequently, typically annually or semi-annually, with measurements taken by accredited third-party laboratories following ASTM or EN procedures to ensure traceable, comparable results.