Asphalt Film Thickness Debunked


Asphalt film thickness is often mentioned in the literature on asphalt paving technology. A few highway agencies have used minimum asphalt film thickness as a mix design and quality assurance criteria. Although the concept of asphalt film thickness sort of makes sense, the reality is a different story.

The concept of asphalt film thickness was originally proposed by Francis Hveem to estimate a starting point of asphalt content for mix designs. It is defined as a ratio of the effective asphalt volume to the surface area of aggregate, as shown in the following equation, where TF is the average film thickness (unit: μm), Vasp is the volume of effective asphalt binder (unit: L), SA is the aggregate surface area (unit: m2/kg), and W is the aggregate weight (unit: kg).

There are a number of questionable aspects about the concept. One debated aspect is the assumption that each aggregate particle is covered with a uniform thickness of asphalt. This notion has been questioned for decades. In a compacted mixture, the coating may be very thin at the points of contact between aggregate particles, but where the aggregates are close together the asphalt film is shared. Fine aggregate particles may have a much thicker coating than coarse aggregate particles. In fact, extremely fine parts of the mineral filler might simply be embedded in the asphalt coating.

Calculation of Asphalt Film Thickness

There are two critical steps involved in calculating asphalt film thickness: determination of effective volume of asphalt binder and calculation of the total surface area of aggregate. Hveem used a set of surface area factors to relate an aggregate gradation to a total surface area of aggregate. Table 1 lists these surface area factors that are commonly multiplied by the percent passing of the gradation.

Table 1: Commonly Used Asphalt Mixture Performance Tests
Commonly Used Asphalt Mixture Performance Tests

These surface area factors were based on the assumption that all of the particles are spherical, the representative particle size for aggregates passing sieve No. 200 is 0.03 mm, and the bulk specific gravity of the aggregate is 2.650. The first assumption is an obvious flaw. Secondly, it is not appropriate to specify a single representative particle size (i.e., 0.03 mm) for all kinds of aggregate passing the No. 200 sieve. Figure 1 shows the particle size distributions of the baghouse fines from various plants. As illustrated, particles smaller than 0.075 mm have a wide size distribution. Using one representative size for fines passing the No. 200 sieve grossly misrepresents the surface area contribution for most materials.

Particle Size Distributions of Baghouse Fines from Various Plants as documented by Anderson and Tarris
Figure 1: Particle Size Distributions of Baghouse Fines from Various Plants as documented by Anderson and Tarris

Thirdly, many aggregates used in asphalt mixes have bulk specific gravity values different from 2.65. Even within a given aggregate blend, coarse and fine aggregates typically have different Gsb values, so the surface area factors require different Gsb adjustments for the coarse and fine portions.

Measured Surface Areas of Aggregate Fractions

Several decades ago, a technique was developed to actually measure the surface areas of particulate materials. The Brunauer-Emmett-Teller (BET) gas adsorption technique has been widely used to measure the specific surface area (SSA) of a range of materials ranging from soils to nano materials. Recently, this technique has been applied to measure the SSA of aggregates in order to determine their surface free energy. Table 2 presents the results of the measured SSA of aggregates of individual sizes from several studies. It can be seen for each aggregate size range that measured SSA results have a very wide range. For example, the surface area of the basalt aggregates between 4.75 mm and 2.36 mm (i.e., 7.06 m2/g) is nearly 90 times larger than that of the granite aggregates (i.e., 0.08 m2/g).

Table 2: Measured Specific Surface Area of Aggregates
Measured Specific Surface Area of Aggregates

Even for the same aggregate type, the SSA results vary significantly (e.g., the SSA of gravel aggregates between 4.75 mm and 2.36 mm ranges from 0.45 m2/g to 4.76 m2/g). In addition to aggregate particle sizes, their shapes and textures have remarkable effects on SSA. Figure 2 shows the morphology difference of aggregate particles. As illustrated, the aggregate particles have significantly different shapes and textures.

Angularity of Granite Aggregates
Figure 2a: Angularity of Granite Aggregates

Textures of Two Granite Aggregates
Figure 2b: Textures of Two Granite Aggregates


Asphalt film thickness is a flawed concept primarily because the surface area factors used to estimate the specific surface area of an aggregate solely from its gradation are not valid. Measurements of specific surface areas using the gas adsorption technique have shown that the specific surface area of the particles from the same sieve size vary by nearly two orders of magnitude.


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