SUPERPAVE: bitumen mixes fundamental properties determination
Superpave performance requisites
Under the Strategic Highway Research Program (SHRP), an initiative was undertaken to improve materials selection and mixture design by developing:
- A new mix design method that accounts for traffic loading and environmental conditions.
- A new method of asphalt binder evaluation.
- New methods of mixture analysis.
There are test methods and relative laboratory equipment which currently exist, complying with standards, and allowing to identify the performance requisites necessary to guarantee extreme and durable quality of the surfacing.
Fundamental tests - performance based
The specificity of the approach is provided by the introduction of tests which measure the physical magnitudes directly linked with the performance characteristics.
The most important are:
- Resilient modulus
- Dynamic modulus and phase angle
- Resistance to fatigue
- Resistance to permanent deformation
- Low temperature cracking
- Fracture energy
These dynamic tests are conducted by automatic machines which operate both under load control and displacement control and which are contained in a thermostatic cabinet.
It should be emphasised that the bitumen mix, as a viscoelastic material, behaves in a manner heavily dependent upon temperature and load in terms of load type, frequency and application time; these parameters must therefore be well-defined in the tests.
Indirect Tension Test for Resilient Modulus of Bituminous Mixtures - AASHTO TP31.
Cylindrical specimens with diameter in the range 97.8÷105.4 mm are tested by indirect tension repeated loading at some different ambient temperatures and different vertical loading frequencies, within a loading range compatible with visco-elastic behavior, and specimen deformation data are used for resilient modulus calculation.
Resilient modulus of Bituminous Mixtures by Indirect Tension test, ASTM D7369
The test is similar to ASTM D4123, but the substantial difference consists of the displacemen transducer configuration that is on sample type. This makes more feasible the measurement of Poisson’s ratio and hence of the instantaneous and total resilient modulus. This test measures the Poisson’s ratio rather than assure a value like in other test.
Dynamic Modulus and phase angle
Determining Dynamic modulus of Hot Mix Asphalt (HMA) -AASHTO T 342 and AASHTO TP 79
Cylindrical 4"diameter specimens of compacted bituminous mixture are subjected to sinusoidal loading at different frequencies generally ranging from 0.1Hz to 25 Hz and at different temperatures ranging from 14 to 130 °F (-10 °C to 54.4 °C). Specimens are obtained by coring and cutting 6" diameter samples compacted by gyratory compactor. Vertical deformation is measured by on-sample displacement transducers.
Resistance to fatigue
The fatigue strength test consists generally of assessment of the number of pulses, usually of the sinusoidal type, necessary to achieve a specific reduction in resistance of the specimen subjected to repeated flexural bending, according to ASTM D7460 or AASHTO T321.
The specimen consists of a prismatic beam of compacted bituminous mix, 380 mm (14.96”) long by 50 mm (1.97”) thick by 63 mm (2.48”) wide. In the case of dynamic tests conducted with preset deformation (controlled strain), the criteria consists in counting the pulses, obviously lower than the ultimate tensile stress, necessary to reach a given decrease usually 50% of the stiffness modulus compared with the initial one of the test.
Resistance to Permanent Deformation
Determining the Flow Number for Asphalt Mixtures Using the Asphalt Mixture Performance Tester - AASHTO TP 79; NCHRP 9-29 (1).
Beyond the Dynamic Modulus and phase angle determination, that has been used for the rutting criteria in MEPDG program, Mechanistic Empirical Pavement Design Guide, the second section of AASHTO TP 79 is dedicated to the permanent deformation characteristics of paving materials by the "Flow Number Test". A cylindrical sample, 4" diameter * 6" height, is subjected to pulse load cycles consisting each of 0,1s load followed by 0,9 s rest period, in confined or unconfined configuration, where the magnitude of vertical load, or deviator stress in confined configuration, is in the order of 10 to 30 psi (30 and 200 kPa).
Resistance to Permanent Deformation
Hamburg Wheel -Track Testing of Compacted Hot-Mix Asphalt (HMA) - AASHTO T324
By this test, the rutting and moisture susceptibility of a compacted hot mixed asphalt specimen is measured.
A specimen slab, or a 150 mm diameter cylinder specimens pair, submerged in thermostatic bath in the temperature range from 40°C to 50°C, is subjected to a repetitive loading by a 705 ± 4.5 N loaded steel wheel, by means of a relative reciprocating movement, at the rate of 50 passes/minute. The depth of the produced impression is measured continuously and plotted
versus the number of wheel passes, and give indication to rutting tendency of the compacted mix.
The rut formation testing on submerged samples allows also to evaluate the moisture susceptibility of hot mixed asphalt (HMA).
Low temperature cracking
Determining the Creep Compliance and Strength of Hot-Mix Asphalt (HMA) Using the Indirect Tensile Test Device. AASHTO T322.
The test configuration consists of an indirect tensile loading kit which acts on a 150 mm diameter compacted hot asphalt cylindrical specimen. Vertical and horizontal deformation are measured by on sample displacement transducers . From the test it is possible to evaluate the tensile creep compliance, defined as time-dependent strain divided by the applied stress, the Poisson's ratio and the tensile strength.
Determining Fracture Energy of Asphalt – Aggregate Mixtures Using the Disk-Shaped Compact Tension Geometry. ASTM D7313
The test configuration consists of a machine applying a tension loading on a 150 mm diameter disk shaped specimen via a clevis tool at relatively low temperature, 10 ° C or less.
The disk shaped specimen is notched and a clip-on gauge is glued astride of the notch to measure the relative crack mouth opening displacement (CMOD) during the test.
TSRST Thermal Stress Restrained Specimen Tensile Strength
Standard Test Method for Thermal Stress Restrained Specimen Tensile Strength, AASHTO TP 10 (3).
A cylindrical or prismatic sample of compacted bituminous mix is epoxy glued restrained by a pair of platens of a dedicated testing frame that is housed in climatic cabinet. As much as the temperature decreases the shrinkage effect into the sample increases and the developed tensile stress is measured until breaking of sample occurs.
Cylindrical specimens are usually prepared by following a volumetric mix design approach by means of gyratory compactor, to AASHTO M 323 "Superpave Volumetric Mix Design".
The slab specimens that are mainly used for wheel tracking test (Hamburg test) are obtained by
compaction of slab or prisms by mean of linear 14 Automated Asphalt Saw kneading compactors or equivalent compactor such as roller or wheel compactors or shear box compactor.
Prisms for flexural fatigue resistance tests to AASHTO T321 and ASTM D7460 can be prepared by sawing the compacted slabs or prisms.
Superpave Gyratory Compactor
Gyratory compactor has been standardized by ASTM D6925 and AASHTO T312 "Standard test methods for Preparation and Determination of the Relative Density of Hot Mix Asphalt (HMA) Specimens by means of the Superpave Gyratory Compactor".
It is well known that the gyratory test defines the mix by its volumetric characteristics. A given mass of asphalt mixture, heated to its optimum compaction temperature, is poured into a cylindrical steel mold and then subjected to: vertical static compression and horizontal gyratory rotation with an axial offset, creating an internal gyratory angle of 1.16°.
Bitumen - Asphalt binder performance grading
As known, bitumen is the active binder of the bituminous mix. It is present on average as 5% of the mixes and determine their rheological behavior to a large extent.
In the past, up to about twenty years ago, asphalt binders were classified by mean of traditional empirical tests.
This system is no longer commonly used in the United States. The penetration test only is still alive as valuable indicator of consistency of source and as commercial denomination.
"PG classification" has been standardized as AASHTO M320 and ASTM D6373.
The tests adopted are:
- Flash Point, Rotational Viscosity (ASTM D4402 and AASHTO T316),
- DSR (dynamic shear rheometer test,( ASTM D7175/ASTM D7405 and AASHTO T315),
- Rolling Thin Film Oven Test RTFOT,
- DSR test after RTFOT
- Pressure Aging Vessel (PAV, ASTM D6521 and AASHTO R28),
- DSR test after PAV,
- Bending Beam Rheometer test (BBR, ASTM D 6648 and AASHTO T313) after PAV and Direct Tension test,
- DTT (AASHTO T314 and ASTM D6723) after PAV