The segregation test is the primary method for evaluating the storage stability of polymer-modified asphalt, revealing whether the polymer phase remains uniformly dispersed in the base binder during hot storage. This article provides a comprehensive analysis of modified asphalt segregation testing, covering the underlying separation mechanisms, sample types, test procedures based on standards such as ASTM D7173 and JTG E20 T 0661, result interpretation, and practical strategies for improving binder homogeneity. Engineers, laboratory technicians, and pavement material specifiers will find actionable guidance for quality control and product development.
Technical Overview
Polymer-modified asphalt (PMA), particularly binders modified with styrene-butadiene-styrene (SBS) block copolymers, is widely used in high-grade pavements, bridge decks, and heavy-traffic roadways because of its superior high-temperature rutting resistance, low-temperature cracking resistance, and fatigue performance. However, the polymer modifier and the base asphalt are not always thermodynamically compatible. During prolonged hot storage in transport tanks or storage silos, the lighter polymer-rich phase can migrate upward while the heavier asphalt-rich phase settles downward, a phenomenon known as segregation or phase separation.
Segregation is problematic because the performance of the modified binder becomes non-uniform: material drawn from the top of a tank may behave differently from material drawn from the bottom, leading to inconsistent pavement quality. The segregation test, also called the hot storage stability test, quantifies this tendency by conditioning a vertical column of binder at elevated temperature for an extended period, then comparing the properties of the top and bottom portions of the column. In Chinese practice, the test is specified in JTG E20 (Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering) as test method T 0661, while internationally similar procedures are described in ASTM D7173 and EN 13399.
The most common acceptance criterion is the difference in softening point between the top and bottom sections of the conditioned specimen. For SBS-modified asphalt, Chinese specification JTG F40 typically requires the top-bottom softening point difference to be no greater than 2.5 °C. A larger difference indicates poor storage stability and a binder that requires continuous agitation or reformulation.
Testing Samples
SBS-Modified Asphalt Binders
Styrene-butadiene-styrene modified asphalt is the most frequently tested material, typically containing 3.5 % to 6.0 % SBS by mass. These samples are evaluated for storage stability before acceptance on construction projects and during production quality control at mixing plants and binder manufacturing facilities.
Crumb Rubber and SBR-Modified Binders
Styrene-butadiene rubber (SBR) latex-modified binders and terminal-blend or wet-process crumb rubber modified asphalts are also assessed. Crumb rubber systems can show particle settlement, and the segregation test helps verify whether the rubber has been sufficiently devulcanized or reacted to remain in suspension.
Plastomer-Modified Binders
Binders modified with polyethylene (PE), ethylene-vinyl acetate (EVA), or similar plastomers generally exhibit higher densities and a stronger tendency toward phase separation, making segregation testing especially critical for these products.
Research and Formulation Samples
Laboratory-prepared formulations used in modifier development, compatibility studies, and stabilizer optimization are tested to screen candidate designs before field production. Samples taken from different depths of storage tanks may also be submitted to verify homogeneity in ongoing operations.
Testing Items
The segregation test program typically includes the following measurements and evaluations:
- Softening point of the top section of the conditioned binder column, determined by the ring-and-ball method (ASTM D36, GB/T 4509, or JTG E20 T 0606).
- Softening point of the bottom section of the conditioned column, determined by the same method for direct comparison.
- Calculated segregation value, expressed as the absolute difference between top and bottom softening points, with a typical acceptance threshold of 2.5 °C or less.
- Optional supplementary tests on the top and bottom portions, including penetration, ductility, and dynamic shear rheometer (DSR) characterization, to provide a fuller picture of performance drift.
- Optional polymer content distribution analysis of top and bottom sections using infrared spectroscopy or solvent extraction techniques during failure investigations.
Testing Methods
The standard segregation test procedure follows a consistent sequence across the major standards. A brief description based on JTG E20 T 0661 and ASTM D7173 is provided below.
Specimen Preparation
An aluminum or glass tube, commonly about 25 mm in diameter and 150 mm to 160 mm in length and closed at one end, is filled with approximately 50 g of the homogenized modified asphalt sample. The tube is sealed and placed vertically in an oven maintained at 163 °C ± 5 °C, or at 135 °C where a lower conditioning temperature is specified, for 48 hours ± 1 hour. The binder must not be disturbed during conditioning, and the oven must provide stable, uniform temperature without significant convection currents.
Sectioning and Sample Recovery
After conditioning, the tube is removed and immediately cooled, typically in a freezer, until the binder column is fully solidified. The frozen tube is then cut into three approximately equal sections. The top section and the bottom section are retained for testing, while the middle section is discarded. Each retained section is melted separately in its own container and stirred gently but thoroughly to ensure homogeneity within that section without entraining air bubbles.
Determination of Segregation Value
Ring-and-ball softening point tests are performed on the top and bottom samples. The segregation result is the absolute difference between the two softening point values:
- Segregation value = | Softening point (top) − Softening point (bottom) |.
- A value of 2.5 °C or less generally indicates acceptable storage stability for SBS-modified asphalt.
- A value greater than 2.5 °C indicates that the polymer has migrated significantly and the binder is prone to separation during hot storage.
Key Quality Control Points
- The sample must be homogeneous before filling the tube; pre-stirring of the submitted container is essential.
- The tube must remain perfectly vertical during conditioning, as tilting distorts the separation gradient.
- Oven temperature calibration and uniformity verification directly affect result reliability.
- Sections must be cut cleanly and equally, since unequal sectioning biases the calculated difference.
- Parallel determinations, often two tubes per sample, are recommended, with the average reported and both values checked for agreement.
Applications
Segregation test results support a wide range of engineering and commercial decisions:
- Product acceptance for SBS, SBR, PE, EVA, and crumb rubber modified binders supplied to highway, expressway, and airport pavement projects.
- Factory production control at binder manufacturing plants, including verification of shearing parameters, development time, and stabilizer dosage.
- Storage and logistics management, determining whether tanks require continuous agitation, recirculation, or limited storage duration before use.
- Research and development of compatibility improvements, including evaluation of stabilizers, sulfur crosslinking agents, and base asphalt selection based on aromatic content.
- Failure investigation when pavement sections exhibit inconsistent performance, linking binder non-uniformity to construction quality issues.
- Compliance demonstration in tenders and specifications that reference JTG F40, ASTM D7173 combined with ASTM D36, or EN 13399 storage stability requirements.
FAQ
What does a large softening point difference between top and bottom sections indicate?
It indicates that the polymer modifier has migrated within the binder column during hot storage. A top section enriched in polymer usually shows an elevated softening point, while the depleted bottom section shows a lower value. A difference exceeding the specification limit, commonly 2.5 °C for SBS-modified asphalt, means the binder is not storage-stable and may deliver inconsistent performance in the field.
Is segregation reversible?
Physical segregation caused by density differences can often be reversed by re-heating and mechanical agitation, which re-disperses the polymer. However, repeated thermal cycling can accelerate binder aging, and relying on stirring alone does not correct an inherently incompatible formulation. Re-testing after agitation is advisable to confirm uniformity.
What are the main causes of segregation in modified asphalt?
Primary causes include poor compatibility between the polymer and the base asphalt (often related to insufficient aromatics in the base), inadequate shearing intensity or development time during production, excessive storage temperature or prolonged storage duration, low or absent stabilizer, and high polymer dosage beyond what the base binder can sustain.
How can storage stability be improved?
Effective measures include selecting base asphalts with compatible chemical composition, optimizing polymer particle size through proper high-shear milling, adding dedicated stabilizers or crosslinking agents that form a stable polymer network, controlling storage temperature within the recommended range, and implementing gentle recirculation or agitation in storage tanks.
How does the segregation test differ from ordinary softening point testing?
An ordinary softening point test characterizes a single homogeneous sample, whereas the segregation test conditions a vertical column of binder under heat, then compares softening points of the top and bottom portions. The purpose is not to measure the absolute softening point but to quantify the degree of phase separation that develops during simulated hot storage.
Which standards govern the segregation test?
In China, the test is performed according to JTG E20 T 0661 with acceptance limits set in JTG F40. International equivalents include ASTM D7173 for determining the separation tendency of polymer-modified asphalt and EN 13399 for bitumen storage stability, with softening points measured per ASTM D36, GB/T 4509, or equivalent ring-and-ball methods. Accredited third-party laboratories routinely conduct these tests in accordance with the applicable standard.