Emulsified Asphalt Viscosity Test: Standards, Procedures, and Applications

The emulsified asphalt viscosity test is a critical quality control procedure used to evaluate the flow resistance and pumpability of bitumen emulsions before field application. This article provides a comprehensive technical overview of the test, covering relevant standards such as ASTM D7496, AASHTO T 59, EN 12846, and JTG E20 T 0622, along with sampling requirements, testing procedures, and practical applications in road construction. Understanding emulsion viscosity helps engineers ensure proper spraying, coating performance, and long-term pavement durability.

Technical Overview

Emulsified asphalt, also known as bitumen emulsion, is a two-phase system in which bitumen droplets are dispersed in water with the aid of an emulsifier. Viscosity is one of the most important physical properties of the emulsion because it directly governs pumping behavior, spray patterns, storage stability, and the final film thickness achieved on the pavement surface. The emulsified asphalt viscosity test measures the resistance of the emulsion to flow under controlled temperature conditions, typically expressed as Saybolt Furol viscosity in seconds or as efflux time according to European practice.

Unlike neat bitumen, whose viscosity is measured at high temperatures, bitumen emulsions are tested at ambient or moderately elevated temperatures because the water phase dominates the flow behavior. If the viscosity is too low, the emulsion may run off the road surface or fail to achieve adequate binder film thickness. If the viscosity is too high, the emulsion may be difficult to pump, clog spray nozzles, or fail to properly coat aggregates. Therefore, viscosity specifications are embedded in virtually every national emulsion standard.

Testing Samples

The viscosity test applies to a wide range of emulsion grades. Common sample types submitted to accredited third-party laboratories and internal quality control facilities include:

Cationic Emulsions

Cationic rapid-setting emulsions (CRS-1, CRS-2), cationic slow-setting emulsions (CSS-1, CSS-1h), and cationic medium-setting emulsions (CMS-2) are widely used in chip seals, tack coats, and slurry systems. These grades typically reference ASTM D2397 or AASHTO M 208 specifications.

Anionic Emulsions

Anionic rapid-setting (RS-1, RS-2) and anionic slow-setting (SS-1, SS-1h) emulsions are used in prime coats, fog seals, and dust control applications. These grades generally follow ASTM D977 requirements.

Polymer-Modified Emulsions

Emulsions modified with polymers such as SBR latex or SBS (for example CRS-2P and CSS-1P under AASHTO M 316) exhibit elevated viscosity and improved elasticity, making viscosity verification especially important before use in high-performance surface treatments.

Specialty Emulsions

Micro-surfacing emulsions, cold-recycling emulsions, and prime-coat grades designed for cold patching mixtures may have project-specific viscosity requirements defined by the purchaser or the relevant highway authority.

Testing Items

Depending on the governing specification and the intended application, the following viscosity-related determinations are commonly performed:

• Saybolt Furol viscosity at 25°C, applicable to slow-setting and medium-setting emulsions
• Saybolt Furol viscosity at 50°C, applicable to rapid-setting and high-viscosity emulsions
• Efflux time of bituminous emulsions at 40°C using the standard efflux viscometer per EN 12846-2
• Viscosity of the residual binder recovered from the emulsion, typically measured by rotational or capillary methods such as ASTM D4402 or ASTM D2170
• Verification of emulsion temperature conditioning and stability of the measurement conditions during the test

Typical specification limits include 20 to 100 seconds for CSS-1 and SS-1 grades at 25°C, and 20 to 100 seconds for CRS-1 or 100 to 400 seconds for CRS-2 grades at 50°C, although exact limits must always be confirmed against the applicable project specification.

Testing Methods

Saybolt Furol Viscometer Method (ASTM D7496 / AASHTO T 59 / JTG E20 T 0622)

The Saybolt Furol method is the dominant procedure for emulsified asphalt viscosity in North America and China. A calibrated Furol viscometer is brought to the specified test temperature in a constant-temperature bath. The emulsion sample is stirred gently to ensure homogeneity without entraining air, filtered if required by the standard, and poured into the viscometer tube. After allowing the sample to reach thermal equilibrium, the operator releases the cork stopper and records the time required for exactly 60 mL of emulsion to discharge through the calibrated orifice into a receiving flask. The recorded efflux time in seconds is reported as the Saybolt Furol viscosity, corrected to the standard temperature if necessary.

Efflux Viscometer Method (EN 12846-2)

In European practice, the viscosity of bituminous emulsions is determined using a standard efflux viscometer with a 10 mm jet. The conditioned sample is allowed to flow from the filled cup, and the time for the material to reach a calibration mark is recorded as the efflux time. The test is normally performed at 40°C, and results are reported in seconds together with the test temperature and the specific viscometer used.

Key Procedural Considerations

• Maintain the bath temperature within plus or minus 0.1°C of the target value, since viscosity is highly temperature sensitive
• Avoid vigorous agitation or shearing of the sample, which can break the emulsion or alter its flow properties
• Verify the calibration of the viscometer orifice and receiving vessel before testing
• Conduct the test promptly after sample conditioning to prevent water evaporation and creaming of the bitumen droplets
• Report the test temperature explicitly, because values measured at 25°C and 50°C are not directly comparable
• Perform duplicate determinations when required by the specification and report the mean result

Applications

Emulsified asphalt viscosity testing serves multiple purposes across the pavement construction and maintenance industry:

• Quality control during emulsion production, ensuring each batch conforms to specification limits before shipment
• Acceptance testing by contractors, highway agencies, and accredited third-party laboratories for incoming emulsion deliveries
• Optimization of spray application rates for tack coats, prime coats, and fog seals
• Design and field control of chip seals, slurry seals, and micro-surfacing systems, where viscosity influences aggregate retention and mix workability
• Assessment of cold in-place recycling emulsions and cold mix asphalt binders
• Evaluation of storage and handling characteristics, since excessive viscosity changes can indicate instability, evaporation, or premature breaking of the emulsion
• Research and development of new emulsion formulations, polymer-modified systems, and rejuvenating emulsions

FAQ

What is the Saybolt Furol viscosity of emulsified asphalt?

Saybolt Furol viscosity is the time, in seconds, required for 60 mL of emulsified asphalt to flow through the calibrated orifice of a Saybolt Furol viscometer at a specified temperature. It is an empirical measure of flow resistance specifically suited to low-viscosity bituminous materials.

Why are some emulsions tested at 25°C and others at 50°C?

Slow-setting and medium-setting emulsions are fluid enough to be tested at 25°C. Rapid-setting emulsions and heavier grades may be too viscous at ambient temperature to give a measurable efflux time, so they are tested at 50°C to obtain a result within a practical range. The specification for each emulsion grade designates the required test temperature.

What happens if the emulsion viscosity is out of specification?

An emulsion with viscosity below the specification minimum may drain off the surface, fail to build an adequate binder film, and produce bleeding or poor aggregate retention. An emulsion above the maximum may be difficult to pump and spray, clog distributor nozzles, and produce non-uniform application. Corrective actions typically include adjusting the emulsifier dosage, water content, or milling conditions during production.

Which standards govern emulsified asphalt viscosity testing?

The most widely referenced standards include ASTM D7496 and AASHTO T 59 for the Saybolt Furol method, EN 12846-2 for the efflux viscometer method used in Europe, and JTG E20 T 0622 within the Chinese highway testing framework. Product requirements for specific emulsion grades are found in standards such as ASTM D977, ASTM D2397, AASHTO M 208, and AASHTO M 316.

How should emulsion samples be handled before testing?

Samples should be stored in sealed containers at moderate ambient temperatures, protected from freezing and direct sunlight. Before testing, the sample is gently stirred to restore homogeneity, taking care not to entrain air or shear the emulsion excessively, as either condition can distort the measured viscosity.