Determination of the Permeability Coefficient of Asphalt Mixtures: Test Methods, Standards, and Appl

The permeability coefficient (seepage coefficient) of an asphalt mixture quantifies the rate at which water infiltrates through a compacted asphalt surface, making it one of the most sensitive indicators of compaction quality, void structure, and long-term moisture durability. Determined with a calibrated seepage meter in accordance with procedures such as JTG E20 T 0730, JTG 3450 T 0971, and EN 12697-19, this test is essential for mix design validation, construction acceptance, and water-damage risk assessment. This article explains the testing principle, instrumentation, sample requirements, step-by-step procedures, calculation methods, and practical applications of asphalt mixture permeability coefficient determination.

Technical Overview

The permeability coefficient, commonly referred to as the seepage coefficient (CW), expresses the volume of water that passes through the surface of a compacted asphalt mixture per unit of time, reported in milliliters per minute (mL/min). It is measured with a falling-head seepage instrument consisting of a transparent graduated cylinder and a base ring that is sealed against the test surface. The test records the time required for the water level to fall between two fixed graduations, which is then converted into a per-minute infiltration rate.

For dense-graded asphalt mixtures, the permeability coefficient is an indirect but highly sensitive measure of compaction quality and the degree of interconnected air voids. A well-compacted surface course should be nearly impermeable; excessive permeability allows rainwater to enter the pavement structure, accelerating binder stripping, raveling, pothole formation, freeze-thaw damage, and weakening of underlying unbound layers. Conversely, functional drainage layers such as open-graded friction courses (OGFC), porous asphalt (PA), and drainage asphalt are intentionally designed to be highly permeable, and the test is used to verify that their drainage capacity is achieved.

The principal test procedures are the laboratory specimen method described in JTG E20 (T 0730) and the in-situ pavement method described in JTG 3450 (T 0971). In European practice, laboratory permeability of asphalt specimens is determined under EN 12697-19, while ring-type infiltration apparatuses similar in concept to the ASTM C1701 infiltrometer are commonly adapted for porous asphalt surfaces. Accredited third-party laboratories routinely perform these tests for contractors, consultants, and highway authorities.

Testing Samples

Laboratory-Compacted Specimens

Cylindrical specimens produced by Marshall compaction (typically 101.6 mm in diameter and 63.5 mm in height) or by Superpave gyratory compaction (typically 150 mm in diameter) are conditioned to room temperature before testing. Specimen dimensions and masses are recorded, and the lateral surface is sealed with a pliable sealing compound so that water is forced to flow through the specimen rather than around it. A minimum of three replicate specimens is recommended for statistical reliability.

Field Cores

Drilled cores with diameters of 100 mm to 150 mm are extracted from the compacted pavement layer, taking care to avoid drilling-induced cracking at the perimeter. Cores are cleaned, brought to a surface-dry condition, and labeled with the layer, chainage, and lane of origin. Cores containing visible cracks or material disturbance within the test zone should be rejected.

In-Situ Pavement Test Points

Field testing is performed directly on the finished, compacted asphalt surface. The test area must be dry, free of dust and loose particles, and free of visible cracks, open joints, or patching within the footprint of the instrument base. Test points are selected at spacing defined by the project specification, and the location, layer, weather conditions, and surface temperature are recorded for each measurement.

Testing Items

The primary output of the test is the permeability coefficient of the asphalt mixture or pavement surface, supported by the raw timing data and conditioning information. Typical reported items include:

• Seepage time (t1), the time in seconds for the water level to fall between the fixed graduations of the instrument, corresponding to an infiltration volume of 400 mL.
• Permeability coefficient (CW) in mL/min, calculated from the seepage time.
• Water temperature and ambient conditions at the time of testing.
• Sample identification, dimensions, layer information, and test location.
• Comparison with project acceptance criteria; in Chinese highway quality evaluation practice, typical per-point limits are 300 mL/min for dense-graded asphalt surface courses and 200 mL/min for SMA surface courses, with defined pass-rate requirements.
• Supplementary correlation with air void content when core density data are available.

Testing Methods

The standard apparatus is a pavement seepage meter comprising a transparent, funnel-shaped base ring with a sealing rim, a graduated transparent cylinder of approximately 600 mL capacity with 100 mL and 500 mL graduations, together with sealing putty, a stopwatch readable to 0.1 s, a thermometer, and a windshield screen to protect the water column from disturbance.

Laboratory Procedure (Falling-Head Method, T 0730)

• Seal the lateral surface of the conditioned specimen with the sealing compound and mount it securely with the test face uppermost.
• Position the seepage meter over the specimen and verify that no leakage paths exist around the perimeter.
• Rapidly fill the graduated cylinder with water to above the upper graduation.
• Start the stopwatch the instant the water meniscus passes the 100 mL mark and stop it when the meniscus reaches the 500 mL mark.
• Record the elapsed time t1 in seconds and repeat the measurement on replicate specimens.

Field Procedure (T 0971)

• Clean the pavement surface and apply a continuous bead of sealing putty beneath the instrument base ring.
• Press the instrument firmly onto the surface to create a watertight seal, and install the windshield.
• Fill the cylinder quickly and start timing as the meniscus passes the 100 mL graduation, stopping when it reaches the 500 mL graduation.
• For surfaces of very low permeability, record the water-level drop over a fixed three-minute interval and convert the volume loss to a per-minute rate.
• Perform replicate readings at the same point, re-seating the instrument if leakage is observed, and average the valid results for each test point.

Calculation and Expression of Results

The permeability coefficient is calculated as CW = (V × 60) / t1, where V is the infiltration volume between graduations (normally 400 mL) and t1 is the measured seepage time in seconds. For example, if the water level falls from the 100 mL mark to the 500 mL mark in 120 seconds, CW = (400 × 60) / 120 = 200 mL/min. Results are reported to the nearest 1 mL/min together with the mean and range of replicate measurements.

International and Alternative Methods

EN 12697-19 determines specimen permeability using a falling-head permeameter with lateral sealing, and results are expressed as hydraulic conductivity rather than a volumetric surface rate; empirical correlations between the two expressions are available for drainage mixtures. For porous asphalt surfaces in regions without a dedicated standard, constant-head ring infiltrometers conceptually similar to the ASTM C1701 apparatus are frequently adopted by accredited third-party laboratories.

Applications

Permeability coefficient determination supports quality management across the entire life cycle of asphalt pavements, from mixture design to maintenance planning. Key applications include:

• Construction quality control and acceptance testing of compacted asphalt surface courses.
• Validation of mix designs for SMA, OGFC, and porous asphalt, where sufficient drainage capacity is a functional requirement.
• Assessment of moisture-damage susceptibility and long-term durability of dense-graded mixtures.
• Forensic investigation of premature distress such as raveling, stripping, and potholing.
• Monitoring of clogging in porous and drainage asphalt layers during service.
• Research into the relationship between air void content, lift thickness, gradation, and pavement permeability.
• Quality assurance for airport pavements, bridge deck overlays, and other moisture-sensitive structures.

FAQ

What does a high permeability coefficient indicate?

A high value indicates an excessively open void structure, usually caused by under-compaction, segregation, insufficient lift thickness, or unsuitable gradation. Water can then penetrate the pavement, increasing the risk of stripping, raveling, freeze-thaw damage, and premature failure. For dense-graded surface courses, sharp increases in permeability typically occur once in-place air void content exceeds approximately 8%, with coarse-graded mixtures being especially sensitive.

What are typical acceptance limits?

Acceptance limits are set by the project specification. In Chinese highway quality evaluation practice, per-point limits of 300 mL/min for dense-graded asphalt surface courses and 200 mL/min for SMA surface courses are commonly applied, with defined passing-rate criteria for each inspection lot. Porous and drainage mixtures are instead evaluated against minimum permeability requirements.

How do laboratory and field tests differ?

Laboratory tests on compacted specimens or cores evaluate the permeability of the mixture itself under controlled conditions with lateral sealing, which is useful for mix design comparison. Field tests measure the combined effect of the mixture, in-place compaction, surface texture, joints, and any defects, and therefore provide the most realistic indicator of in-service water ingress.

What are the main sources of test error?

The most common errors are leakage at the sealing interface, wind disturbance of the water column, an unclean or damp test surface, cracks within the test footprint, inaccurate timing at the graduations, and large deviations in water temperature. Careful sealing, shielding, and replicate measurements control these effects effectively.

Can the test be used on porous asphalt?

Yes. For porous asphalt and OGFC, the test verifies that the intended drainage function is achieved. Because infiltration is very rapid, timing may be taken over the initial portion of the water column, and complementary laboratory methods such as EN 12697-19 or constant-head infiltrometer procedures are often used to characterize the mixture more precisely.

How often should field permeability testing be performed?

Testing frequency is defined by the project quality plan, typically at regular intervals along each lane of every compacted surface layer, with additional testing wherever density results are questionable or surface defects are observed. Combining permeability results with core density data gives the most complete picture of compaction quality.