In the field of highway engineering material testing, the bitumen solubility test is a key method used to evaluate the purity and quality of asphalt binders. According to the newly implemented JTG 3410-2025 Specifications and Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering, the test method T0607-2025 clearly specifies the requirements for glass fiber filter media used in the procedure. Similar testing principles can also be found in international standards such as ASTM D2042, EN 12592, and AASHTO T44, which are widely used for determining the solubility of asphalt materials.

However, in practical laboratory applications, misunderstandings regarding filter paper selection have led to inconsistent or systematically biased test results.

I. Official Requirement in T0607-2025

The standard clearly states that the glass fiber filter paper used in the test must have the ability to:“retain particles larger than 1.5 μm”

This requirement refers to filtration efficiency, not simply a nominal pore size.

In other words, the standard focuses on the actual retention performance of the filter medium, rather than a numerical pore-size label.

II. Common Misinterpretation in Practice

In real testing environments, some technicians incorrectly interpret this requirement as:“Select filter paper with a nominal pore size of 1.5 μm”

This misunderstanding leads to a deviation between theoretical compliance and actual filtration behavior.

Such simplification ignores the structural complexity of glass fiber filter media and can result in systematic overestimation of solubility values.

III. Structural Characteristics of Glass Fiber Filter Paper

Glass fiber filter paper is composed of randomly arranged fine glass fibers, forming a highly heterogeneous pore structure.

Unlike membrane filters with uniform pore sizes, glass fiber filters exhibit:

· Non-uniform pore distribution

· Irregular flow channels

· Statistical pore size variation

Therefore, the “1.5 μm nominal pore size” provided by manufacturers is typically an average or rated value, not a strict physical limit for every pore.

In reality, some pores may be larger or smaller than 1.5 μm, meaning:

· Fine particles may pass through even “qualified” filters

· Insoluble residues may be partially lost

· Measured solubility may appear artificially higher

This ultimately leads to distorted test results and reduced data reliability.

IV. Impact on Bitumen Solubility Results

When finer insoluble particles pass through the filter medium, the following issues may occur:

· Underestimation of insoluble content

· Overestimation of bitumen solubility

· Reduced comparability between laboratories

· Loss of test repeatability and reproducibility

This phenomenon is particularly critical in arbitration testing or quality inspection scenarios.

V. Conclusion

The key issue in the bitumen solubility test is not simply the “1.5 μm filter paper selection”, but the misunderstanding of filtration performance versus nominal pore size.

To ensure accurate and reliable results, laboratories should:

· Focus on actual particle retention capability

· Avoid over-reliance on nominal pore specifications

· Follow the intent of JTG 3410-2025 T0607-2025 rather than simplified interpretations

Correct understanding of filter media selection is essential for ensuring the scientific validity and international comparability of asphalt testing results.

Personal opinion for reference only.