|Direct Tension (DTT)
Used to measure failure properties of an asphalt binder at low (i.e., -12ºC) temperatures. Provides an indication of the failure stress and strain of an asphalt binder. The Direct Tension Test was originally used as a referee test when the m-value from BBR results passed but the estimated stiffness failed and was between 300 and 600 MPa. If the average strain value from the DTT was 1% or more, it could be assumed the binder was sufficiently elastic at the tested low temperature range.More recently, the Direct Tension is being implemented as one test in the determination of low temperature critical cracking. Used in conjunction with the Bending Beam Rheometer and software that incorporates the data from both tests we are able to better characterize low temperature binder properties. The critical cracking temperature is considered to be the point at which the maximum stress at a given temperature exceeds the strength of the material.
Bending Beam Rheometer (BBR)
Used to measure the flow properties of liquid asphalt binders at low (i.e., -12ºC) temperatures. Is analogous to the DSR – which is not used at low temperatures because of the torque capability of the equipment. Operates on engineering beam principles. A fixed static load is applied to an asphalt binder beam of known dimensions. The resulting deflection is measured and the flexural stiffness reported as a function of time. Provides an indication of the low temperature stiffness and cracking potential of an asphalt binder.
Dynamic Shear Rheometer (DSR )
Also known as an oscillatory shear rheometer. Is used to measure the flow properties of liquid asphalt binders at intermediate (i.e., 20ºC) to high (i.e., 64ºC) temperatures. Works by applying a sinusoidal shear stress to produce a resulting shear strain. The complex shear modulus (G*) is a ratio of the applied shear stress ( t ) to the resulting shear strain ( g ). The phase angle, d , (related to the time lag between input and output signals) provides a relative indication of the viscous and elastic behavior of the asphalt binder.Materials with a phase angle of 90 degrees are completely viscous; while materials with a phase angle of 0 degrees are completely elastic. At intermediate temperatures, such as 20ºC, asphalt binders are said to be viscoelastic (phase angle near 45 degrees).
Rolling Thin Film Oven (RTFO)
The RTFO simulates the ‘thin films’ of binder as they are exposed to heat and forced air during the mixing process and placement. A sample is poured into a cylindrical bottle and rotated horizontally at approximate mixing temperatures. As the bottle rotates and air is blown over the sample, new thin films are exposed simulating the binder coating on the aggregate during mixing.
Lighter oils are driven off and some oxidation occurs. A mass change determination is made and because RTFO-aged asphalt binder is approximately 2-3 times the viscosity of the unaged asphalt binder, it is tested to determine whether the desirable viscoelastic properties have been maintained.
Pressure Aging Vessel (PAV)
One of the limitations of some of the older grading systems is the inability to simulate actual aging conditions of asphalt binder as it goes through production, mixing placement and, particularly, long term aging. After mixing and placement, further oxidation will occur over the service life of the pavement. The PAV was refined during the Strategic Highway Research Program (SHRP) to subject asphalt binders to the long-term aging expected after 5-15+ years of service in an asphalt pavement.
Asphalt Binders are exposed to high temperature (100ºC) and air pressure (2.07 MPa) for 20 hours to simulate long-term aging. Resulting viscosity of the PAV-aged asphalt binder is approximately 8+ times the viscosity of the unaged asphalt binder.
Traditional method of determining the viscosity of asphalt binders at 60ºC. Uses viscosity tubes and vacuum. Viscosity is calculated from the time required for the asphalt binder to flow between two successive marks. Also used to check continuity in source and process. An element of the Viscosity Grading system.
Traditional method of determining the viscosity of asphalt binders at 135ºC. Similar to absolute viscosity except that an oil bath is used (instead of a water bath) and no vacuum is required. Another method to check continuity.
Apparent Viscosity of Non-Newtonian Bitumens (ASTM D4957)
Measures viscosity at different shear rates. Performing absolute viscosity (AASHTO T202) assumes that the material is newtonian or that its viscosity does not depend on the applied shear rate. Apparent Viscosity is performed in a similar fashion as Absolute but uses several points or vacuum levels.
Used to measure viscosity (kinematic) at high temperatures (i.e., 135ºC). Used in mix designs to provide representative temperatures for mixing and compaction of asphalt mixtures. Operates as a rotating cylinder within a fixed sample cylinder (within a thermal-controlled chamber). Resistance to flow is measured as the torque applied to a spring and converted to viscosity.
An empirical measure of asphalt consistency. Based on this test, asphalt cements have been classified into standard grades. In testing, a container of asphalt cement is heated to the standard test temperature, 25 o C, in a temperature controlled water bath. A needle is allowed to bear on the surface of the asphalt cement for 5 seconds. The distance that needle penetrates is the penetration measurement.
Used primarily as a consistency check for modified asphalts. Samples of asphalt loaded with steel balls are confined in brass rings suspended in a beaker of water, glycerine or ethylene glycol. The liquid is heated at a prescribed rate. As the asphalt cement softens, the balls and asphalt sink a plate. When the asphalt touches the plate, the temperature of the water is considered the softening point.
Used to determine the temperature at which asphalt binder fumes first may flash, or spark. For common paving asphalt binders, this temperature is usually 230ºC or higher. May also be used to determine the fire point (temperature at which the asphalt binder ignites and burns).
Solubility (ASTM D2042)
A measure of purity of the asphalt cement. Active cementtious constituents will be soluble in trichloroethylene while non-cementing matter is not. A prerequisite of eligibility to be graded in the Superpave system, insolubles cannot exceed 1%.
Used to determine the ductile (ability to stretch) behavior of asphalt binders at intermediate temperatures (4, 25ºC). “Dog-bone” shaped specimens are pulled at a constant rate until the sample breaks. An indication of temperature susceptibility.
When a tire passes over a section of pavement, it is desirable for that pavement to have the ability to “give”, but it is equally important for it to recover to its original shape. The inability to do this results in permanent deformation. The elastic recovery test determines these characteristics by using the ductility apparatus to stretch a sample. After the sample has reached a specified elongation, it is cut and allowed to recover.
Utilizing the ductility apparatus, force ductility is a method to determine the stress, strain characteristics of a given binder. Primarily used on polymerized binders, a computer measures stress at time intervals as a sample is stretched.
Used by selected D.O.T.s, a method of determining the homogeneity of a sample. The specimen is poured through a sieve and retained particles are noted.
Thin Film Oven
This test produces mass change data and a residue that approximates changes that occur during mixing and the condition of the asphalt as placed in the pavement. TFO is an element of both the Penetration and Viscosity grading systems.
After lengthy storage at elevated temperatures, some binders can separate and cause properties to vary from the top to the bottom of the storage tank. To determine if a binder is susceptible to separation, this method uses tubes that have been filled with binder and stored for a period of time in an oven. The tube is frozen and cut so that measurements can be made on the top and bottom thirds of the material.
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