The bitumen float test is a standardized empirical method for determining the consistency of bituminous binders that are too soft or too fluid to be reliably characterized by the penetration test. The float value expresses, in seconds, the time required for water at 5 °C to break through a molded bitumen disc seated in the orifice of a floating brass dish, and it serves as a key acceptance parameter for road oils, slow-curing cutback asphalts, and soft oxidized bitumens. This article explains the technical principle, apparatus, sample requirements, step-by-step procedure, precision criteria, and practical applications of bitumen float value determination in accordance with ASTM D139, AASHTO T50, and JTG E20-2011 (T 0617).
Technical Overview
The float test is one of the classical empirical methods used to express the consistency of bituminous materials. It is applied primarily to binders whose consistency lies outside the convenient working range of the needle penetration test, such as slow-curing liquid asphalts, road oils, and soft or tacky oxidized bitumens. The method is quick, requires only a few grams of material, and provides a direct comparative index that can be written into product specifications.
In the test, a disc of bitumen is molded in a cylindrical brass collar, conditioned at a controlled temperature, and seated so that it completely seals the central orifice in the recessed bottom of a specially shaped brass float dish. The loaded dish is floated on the surface of a water bath maintained at 5 °C, and the elapsed time from the moment the dish touches the water until the water breaks through the bitumen disc is measured with a stopwatch. This elapsed time, expressed in seconds, is the float value of the binder. Because bitumen rheology is strongly temperature dependent, the float value is meaningful only when quoted together with the test temperature.
- Reported quantity: float value in seconds, determined under a standardized bath condition of 5 °C (41 °F)
- Governing methods: ASTM D139, AASHTO T50, and the Chinese highway test method JTG E20-2011 (T 0617)
- Interpretation: a higher float value indicates a harder, more viscous binder; a lower value indicates a softer, more fluid binder
- Complementary consistency tests: needle penetration (ASTM D5), softening point (ASTM D36), ductility (ASTM D113), and viscosity (ASTM D2170 or ASTM D4402)
Testing Samples
The float test is suitable for a well-defined family of bituminous products. Typical sample categories submitted for float value determination include the following.
Road Oils and Slow-Curing Cutback Asphalts
Liquid bituminous binders fluxed with slow-evaporating petroleum oils are widely used for prime coats, dust palliatives, and maintenance patching. These materials are frequently too fluid at the reference temperature for the penetration test to discriminate between grades, so the float test is used as the consistency criterion in the applicable specifications.
Oxidized (Air-Blown) Bitumens
Soft grades of air-refined or catalytically oxidized bitumens used in roofing felts, waterproofing membranes, and damp-proofing compounds are commonly controlled by float value. The test is sensitive to the degree of oxidation and therefore provides an effective check that the blowing process has produced the specified consistency.
Bituminous Coating and Joint-Filling Compounds
Industrial bituminous compounds for pipe wrapping, corrosion-protective coatings, and joint sealants may be specified by float value, particularly when the binder behaves as a very soft, slowly flowing semi-solid at ambient temperature.
Sample Intake and Conditioning Requirements
Only a few grams of material are required for each determination. The sample must be representative of the bulk lot and free from water, grit, and contamination. Before molding, the material is heated gently with constant stirring only until it is sufficiently fluid to pour, avoiding local overheating and prolonged heating, both of which alter the consistency being measured. Entrapped air bubbles must be removed before the specimen is cast.
Testing Items
The principal measured item is the float value of the bituminous material under the standard condition. Accredited third-party laboratories typically report the following package of results for a float test assignment.
- Float value at 5 °C, including individual determinations and the arithmetic mean, in seconds
- Float value at an alternative bath temperature where explicitly required by the product specification, always reported with the temperature of test
- Agreement check between duplicate determinations against the repeatability limit of the governing standard
- Optional complementary consistency profile: penetration, softening point, ductility, and viscosity determined in parallel to give a complete characterization of the binder
Testing Methods
The determination is performed in accordance with the procedures of ASTM D139, AASHTO T50, or JTG E20-2011 (T 0617), which share the same principle and differ only in minor details of apparatus and tolerances. The method is summarized below.
Apparatus and Materials
- Brass float dish with a recessed bottom and a central orifice that forms the seat for the bitumen disc
- Cylindrical brass collar serving as the specimen mold, dimensioned to match the orifice of the float dish
- Thermostatic water bath capable of maintaining 5 °C, typically within ±0.5 °C, with gentle circulation and a level, vibration-free surface
- Calibrated thermometer readable to 0.1 °C and a calibrated stopwatch
- Glass plate thinly coated with an isolating agent, such as a glycerin and talc mixture
- Hot plate, heated knife or spatula, and stirring implement for melting and trimming the specimen
Specimen Preparation
The collar is warmed and placed on the prepared glass plate. The heated, homogeneous, bubble-free bitumen is poured into the collar in slight excess, and the assembly is allowed to cool at room temperature for the period prescribed by the standard, typically fifteen to thirty minutes. The specimen is then trimmed flush with the top of the collar using a heated straightedge, producing a flat, defect-free disc of defined thickness. The specimen is subsequently conditioned so that its temperature is uniform and equal to the bath temperature before the timed run begins.
Test Procedure
- Verify that the water bath is stable at 5 °C and that the bath surface is still, clean, and free of drafts and vibration.
- Release the conditioned bitumen disc from the collar using a warmed spatula, taking care not to distort, stretch, or crack the disc.
- Seat the disc in the recess of the float dish so that it completely seals the orifice; remove any protruding excess and confirm that the seal is continuous around the full circumference.
- Gently place the loaded float dish on the surface of the bath and start the stopwatch at the instant of contact.
- Observe the disc continuously and stop the watch at the moment water breaks through the bitumen film, signaled by water appearing on the upper surface of the disc or by rupture of the film.
- Record the elapsed time in seconds, and repeat the complete determination on a freshly prepared specimen to obtain the duplicate result.
Calculation and Expression of Results
The float value of the material is the elapsed time, in seconds, recorded for each determination. The reported result is normally the arithmetic mean of at least two valid determinations that agree within the repeatability limit stated in the governing standard. The test report identifies the sample, the standard followed, the bath temperature, the individual and mean float values, and any deviations from the standard procedure, since the float value has meaning only in conjunction with the test temperature.
Precision, Quality Assurance, and Common Error Sources
- Bath temperature control is the dominant influence; a deviation of even a fraction of a degree changes the viscosity of the film and biases the time to rupture.
- Overheating or prolonged heating of the sample during preparation hardens the binder and inflates the float value.
- Entrapped air bubbles or water droplets in the disc create weak points that cause premature breakthrough.
- Distortion, stretching, or incomplete seating of the disc alters the film geometry and shortens the measured time.
- Vibration, ripples, or drafts on the bath surface disturb the float and must be excluded.
- Calibrated timing and temperature measuring devices, verified apparatus dimensions, and disciplined duplicate testing are the core quality assurance measures.
Applications
Float value determination remains a practical specification tool wherever very soft bituminous binders are produced, traded, or applied. Its main areas of use are as follows.
- Highway construction: acceptance testing of road oils and slow-curing cutback asphalts for prime coats, tack coats, and dust-laying applications, where consistency cannot be graded by penetration.
- Roofing and waterproofing: production control of soft oxidized bitumens for saturated felts, membranes, and damp-proof courses, where the float value tracks the degree of oxidation.
- Protective coatings: consistency verification of bituminous compounds for pipe wrapping, foundation damp-proofing, and corrosion protection.
- Procurement and compliance: receiving inspection by contractors and testing by accredited third-party laboratories against national and industry specifications.
- Research and development: comparative evaluation of fluxing oil content, aging resistance, and formulation changes on binder consistency, alongside penetration, softening point, and viscosity data.
FAQ
What exactly is the bitumen float value?
The float value is the time, in seconds, required for water at the specified bath temperature of 5 °C to break through a conditioned disc of bitumen that seals the orifice in the bottom of a brass dish floating on the water. It is an empirical consistency index: the longer the time, the harder and more viscous the binder.
Why is the test performed at 5 °C?
Bitumen rheology changes sharply with temperature, so a single fixed reference temperature is essential for comparability. The 5 °C condition, established in ASTM D139, AASHTO T50, and JTG E20-2011 (T 0617), produces measurable rupture times for very soft binders; if a product specification requires another temperature, the determination may be run there, but the temperature must always be reported with the result.
When should the float test be used instead of the penetration test?
The float test is preferred when the binder is too soft, tacky, or slowly curing for the penetration needle to give a meaningful and repeatable result. Typical cases include slow-curing liquid asphalts, road oils, and soft oxidized bitumens, where the float value provides better discrimination between grades.
How much sample is needed and how are duplicate results handled?
Each determination needs only a few grams of material. The reported float value is the mean of at least two independent determinations performed on separately molded discs; the duplicates must agree within the repeatability limit of the governing standard, otherwise the determination is repeated and the causes of scatter investigated.
What are the most common causes of unreliable results?
The most frequent problems are poor bath temperature control, overheating of the sample during melting, air bubbles trapped in the disc, a disc that is distorted or incompletely seated in the float dish, and vibration or drafts on the water surface. Careful specimen conditioning and strict adherence to the timing and temperature requirements of the standard eliminate most of these errors.
Which standards govern the determination?
The method is standardized internationally in ASTM D139 and AASHTO T50, and in China by the highway test method JTG E20-2011 (T 0617). Related consistency determinations are covered by ASTM D5 for penetration, ASTM D36 for softening point, ASTM D113 for ductility, and ASTM D2170 or ASTM D4402 for viscosity.