Determination of High-Temperature Stability of Asphalt Pavement: Testing Methods, Standards, and App

High-temperature stability testing evaluates the ability of asphalt pavement to resist rutting and permanent deformation under heavy traffic loading and elevated summer temperatures. Accredited third-party laboratories perform wheel tracking, Marshall stability, Hamburg rutting, and dynamic modulus tests on asphalt mixtures in accordance with JTG/GB, ASTM, AASHTO, and EN standards. This article explains the sample requirements, test items, measurement procedures, acceptance criteria, and engineering applications of high-temperature stability determination for asphalt pavements.

Technical Overview

High-temperature stability refers to the capacity of an asphalt mixture to resist permanent deformation—rutting, shoving, and pushing—when the pavement is exposed to high temperatures and sustained wheel loads. During hot summer conditions, the temperature in the surface layer of an asphalt pavement can reach 60 °C or higher. At these temperatures, the viscosity of the asphalt binder drops sharply, the stiffness of the mixture decreases, and irreversible plastic deformation accumulates progressively in the wheel paths, forming ruts that compromise driving safety and ride quality.

The high-temperature performance of asphalt pavement is governed by multiple factors, including binder grade and modifier type, aggregate quality and gradation, air void content, film thickness, and the magnitude and repetition of traffic loading. Consequently, a complete evaluation combines binder-level tests (softening point, high performance grade via dynamic shear rheometer) with mixture-level performance tests (wheel tracking dynamic stability, Marshall stability, Hamburg wheel tracking, dynamic modulus, and flow number). The principal test methods are specified in JTG E20-2011 (T 0719, T 0709), JTG F40-2004, ASTM D6927, ASTM D36, ASTM D7175, AASHTO T 324, AASHTO T 342, AASHTO TP 79, and EN 12697-22, and are routinely performed by ISO/IEC 17025-accredited third-party laboratories.

Testing Samples

Laboratory-Compacted Slab Specimens

Rectangular slabs of 300 mm × 300 mm × 50 mm (or 300 mm × 300 mm × 100 mm for thick structural layers) are fabricated with a laboratory roller compactor from design or plant-produced loose mixture. The compaction is controlled to achieve the target density and air void content specified in the mix design, so that the specimen represents the field compacted condition as closely as possible. These slabs are the standard specimens for the wheel tracking test.

Marshall Compacted Cylindrical Specimens

Cylindrical specimens of 101.6 mm in diameter and 63.5 mm in height are compacted with a standard Marshall hammer, typically applying 75 blows per face for heavy traffic levels. The specimens are cored or sawed, and their bulk density, air voids, voids in mineral aggregate, and voids filled with asphalt are determined before stability testing.

Field Cores and Sawed Slabs

Cores drilled from newly constructed or in-service pavements can be sawed into beam or slab geometry for rutting evaluation, allowing direct assessment of the material actually placed in the field. Core geometry must satisfy the dimensional requirements of the selected test method.

Loose Plant-Mix Samples

Hot loose mixture sampled at the asphalt mixing plant or behind the paver is transported in insulated containers to maintain temperature, reheated or kept warm as permitted, and compacted into test specimens within the allowable time window to simulate the placement condition.

Testing Items

The high-temperature stability of an asphalt pavement is quantified through the following measured parameters:

• Dynamic stability (DS, passes/mm) from the wheel tracking test at 60 °C
• Rut depth and deformation development curve at specified time or loading passes
• Deformation rate between specified loading intervals
• Marshall stability (MS, kN) and flow value (FL, 0.1 mm) at 60 °C
• Marshall quotient (MS/FL) as an index of mixture stiffness
• Hamburg wheel-track rut depth (mm) at a fixed number of passes, creep slope, stripping slope, and stripping inflection point
• Dynamic modulus |E*| and phase angle across a matrix of temperatures and frequencies
• Flow number (FN) and flow number slope from repeated-load permanent deformation testing
• Supporting binder parameters: ring-and-ball softening point, G*/sinδ at the high critical temperature, and the non-recoverable creep compliance (Jnr) from multiple stress creep recovery testing

Testing Methods

Wheel Tracking Test (Dynamic Stability)

Conducted in accordance with JTG E20-2011 T 0719 or EN 12697-22, the slab specimen is conditioned in an environmental chamber at 60 ± 0.5 °C for not less than 5 hours (and generally not more than 24 hours). A solid rubber wheel 200 mm in diameter and 50 mm wide travels back and forth over the specimen at a rate of 42 ± 1 passes per minute under a contact pressure of 0.7 ± 0.05 MPa. The rut depth is recorded continuously, and the dynamic stability is calculated as DS = (t2 − t1) × N / (d2 − d1), where d1 and d2 are the deformations at 45 minutes (t1) and 60 minutes (t2) and N is the number of passes per minute. Three parallel specimens are normally tested and the average reported. Under JTG F40-2004, dynamic stability is generally required to be no less than 800–1,000 passes/mm for unmodified dense-graded mixtures, 2,400–2,800 passes/mm for polymer-modified mixtures, and up to 3,000 passes/mm for SMA and heavy-duty or steep-grade sections, depending on the climatic zone and traffic level. Higher test temperatures such as 65 °C or 70 °C may be applied for severe loading condition assessments.

Hamburg Wheel-Track Test

Performed per AASHTO T 324 or the immersion procedures of EN 12697-22, this test runs a steel wheel (203 mm diameter, approximately 47 mm wide) with a load of about 705 N over the specimen immersed in a water bath, typically at 50 °C, at 52 ± 2 passes per minute for up to 20,000 passes or until a specified rut depth is reached. In addition to high-temperature rut depth, the test identifies the stripping inflection point, making it sensitive to moisture damage combined with permanent deformation.

Marshall Stability and Flow Test

Following ASTM D6927, AASHTO T 245, or JTG E20-2011 T 0709, the Marshall specimen is immersed in a water bath at 60 °C for 30–40 minutes and then loaded diametrically at a constant rate of 50 mm/min until failure. The peak load is recorded as the Marshall stability and the corresponding deformation as the flow value, with height correction applied to non-standard specimens. Chinese specifications typically require a stability of not less than 8 kN for expressways and Class I highways, with flow values generally specified in the range of 2–4 mm.

Dynamic Modulus and Flow Number Testing

Using an asphalt mixture performance tester, the dynamic modulus |E*| is measured at several temperatures (e.g., 4–54 °C) and frequencies (0.1–25 Hz) in accordance with AASHTO T 342, and master curves are constructed for structural analysis. The rutting resistance indicator E*/sinδ at high temperature is often used to rank mixtures. Flow number testing per AASHTO TP 79 applies a repeated haversine deviatoric stress (commonly 600 kPa) with 0.1 second loading and 0.9 second rest at the design high temperature, and the number of load cycles at the onset of tertiary flow is reported as the flow number.

Binder High-Temperature Characterization

Since mixture rutting is fundamentally driven by binder softening, the asphalt binder itself is characterized by the ring-and-ball softening point (ASTM D36 / JTG E20 T 0606), the high critical temperature where G*/sinδ equals 1.0 kPa for unaged binder and 2.2 kPa for RTFO-aged binder (ASTM D7175 / AASHTO T 315), and the non-recoverable creep compliance Jnr from multiple stress creep recovery testing (ASTM D7405 / AASHTO T 350), which is particularly effective for evaluating polymer-modified binders.

Applications

High-temperature stability determination serves a wide range of engineering purposes across road, airport, and industrial pavement projects:

• Verification and optimization of asphalt mix designs before full-scale production
• Factory and field quality control and acceptance testing for expressways, Class I highways, urban arterial roads, and intersections with frequent stopping
• Effectiveness evaluation of polymer-modified, rubber-modified, and high-modulus binders
• Performance assessment of heavy-load corridors, long steep grades, bus lanes, port yards, and aircraft aprons where rutting risk is elevated
• Forensic investigation of existing rutting distress and appraisal of rehabilitation schemes
• Research and qualification of new materials, including warm-mix asphalt, reclaimed asphalt pavement (RAP) blends, and anti-rutting additives
• Provision of documented test data for compliance with design specifications and construction contracts

FAQ

Why is 60 °C the standard test temperature for wheel tracking?

Sixty degrees Celsius approximates the maximum temperature reached near the surface of an asphalt pavement on hot summer days in most climatic regions, corresponding to the standard high grading temperature for surface layers. It represents a reasonably severe but realistic service condition, and some specifications allow or require elevated temperatures such as 65 °C or 70 °C for extremely hot climates or heavy loading scenarios.

What dynamic stability value is considered acceptable?

Acceptance limits depend on the governing specification, climatic zone, and traffic classification. As a general guide under Chinese specifications, unmodified dense-graded mixtures should achieve at least 800–1,000 passes/mm, polymer-modified mixtures at least 2,400–2,800 passes/mm, and SMA or heavy-duty sections 3,000 passes/mm or more. Project-specific requirements always take precedence.

Can specimens be taken directly from the pavement?

Yes. Cores or sawed slabs recovered from the pavement can be tested to evaluate the as-constructed material, provided the specimen geometry and conditioning requirements of the selected method are satisfied. Field sampling is particularly valuable for acceptance inspection and distress investigation.

How does the Hamburg test differ from the standard wheel tracking test?

The standard wheel tracking test uses a rubber tire in air at 0.7 MPa contact pressure and reports dynamic stability in passes/mm, whereas the Hamburg test uses a steel wheel fully immersed in heated water. The Hamburg test therefore evaluates rutting and moisture susceptibility simultaneously and reports rut depth and the stripping inflection point rather than dynamic stability.

How many specimens are required and how long does testing take?

Wheel tracking normally requires three parallel slab specimens. Including the mandatory conditioning period of at least 5 hours at 60 °C and the 60-minute loading phase, a complete dynamic stability test is typically completed within one working day per batch. Marshall testing of parallel specimens is faster, with a 30–40 minute water bath conditioning before loading.

Does a higher binder grade guarantee better high-temperature stability?

A binder with a higher softening point or high performance grade significantly improves rutting resistance, but mixture stability also depends on aggregate skeleton quality, gradation, air voids, and compaction. Binder, mixture design, and construction quality must all be controlled together to achieve durable high-temperature performance.