Asphalt & Bitumen West Co .

Laboratory testing of asphalts

The properties of asphalts need to be known for a variety of reasons, including performance evaluation, mixture or pavement designs, and production and/or construction specification compliance. In situ testing of material properties in full scale trial sections or in-service pavements is impractical or uneconomical in most cases, so engineers generally have to rely on laboratory testing to characterize or predict material properties. In addition, testing may also be necessary to ensure that specified requirements are met.
Laboratory tests should reproduce the anticipated in situ conditions as closely as possible (i.e. in terms of temperature, loading time, stress conditions, degree of compaction etc.). However, in situ conditions may be the subject of change, and selection of appropriate testing conditions may be difficult. Figure shows a simplified model depicting a  representation of a pavement element, and shows the stresses to which it is subjected when a wheel load approaches. In practice, the stresses are applied three dimensionally: horizontal and shear stresses occur in planes that are perpendicular to those shown in Figure 16.1. As the wheel passes over the element, these stresses change with time, and this is shown in below Given the difficulty of reproducing such complex stress regimes accurately in the laboratory, simplified tests have been introduced that can reproduce certain aspects of the in situ behavior. Such tests are also used to correlate the laboratory mixture design with in situ performance in relation. 

to aspects such as moisture susceptibility, resistance to deformation, stiffness, fatigue and tensile strength.
Many laboratory tests have been proposed over the years. Efforts to standardize particular tests for routine usage are ongoing, and over time these have been improved by advances in equipment technology that measures material behavior more accurately. In general, laboratory tests can be divided into the following three groups
static creep test
repeated load test
dynamic stiffness and fatigue tests
indirect tensile tests.
wheel tracking tests
gyratory compaction
indirect tensile strength.

Fundamental tests

Static creep test

In the 1970s, Shell developed a simple creep test whereby a static uniaxial compressive load was applied to unconfined cylindrical samples of asphalt to assess the resistance to deformation of the material. This test gained wide acceptance due to the ease of specimen preparation, the simplicity of the test procedure and the low cost of test equipment. The only requirements for the test specimen are that it should be prismatic, with flat and parallel ends normal to the axis of the specimen, while the test procedure involves the application of a constant axial stress of up to 500 kPa to a 100 mm dia test specimen for up to 10 000 s at a constant temperature, and measurement of the resultant deformation, as described in a now with- drawn  British  Standard  (BSI, 1995).
The obvious limitation of the simple creep test is that the static loading mechanism does not simulate the dynamic traffic loading conditions to which asphalts are subjected in service, and it was found that rut prediction based on this test underestimated rut depths measured in trial pavements. Furthermore, static loading could not capture the improved performance of binder modifiers that enhanced the elastic recovery properties of an
asphalt, whereas, in contrast, this could be demonstrated under repeated loading.

Dynamic tests

Dynamic tests are more complex than repeated load tests in terms of the loading cycles and frequencies – dynamic tests apply repeated cyclic loads (usually a haversine wave) over a range of frequencies. This necessitates more accurate load application and deformation measuring   systems.
Several dynamic test methodologies have been developed to determine stiffness and fatigue resistance. Bending tests using beams or cantilevers subjected to repeated applications of load have been used since  the 1970s . In such tests, the maximum stress occurs at a point on the surface of the specimen, and its calculation, using the standard beam bending formula, depends on the assumption of linear elasticity. The European Standards (BSI, 2012a, 2012b) have brought together various dynamic fatigue resistance and stiffness test methodologies, including the flexural  bending tests  as  shown  in Table below .
In the two-point cantilever bending test, trapezoidal and square prismatic sample types are both included in the European standards (BSI, 2012a, 2012b), and different maximum aggregate sizes of the asphalts are also considered in the test sample dimensions. The base end of the sample is glued, and thus held rigidly while a sinusoidal load is applied at the head. For stiffness modulus measurements, the applied force or deflection at defined temperatures and frequencies should only cause up to 50 microstrain in the most heavily stressed part of the test sample so as to remain within the linear range. The load is applied for a minimum duration of 30 s and a maximum duration of 2 min, while the force, deflection and phase angles are measured and recorded over the last 10 s of the test. The test can be repeated for  at  least  four   temperatures  at  108C  intervals  and  three

frequencies at each temperature, in order to determine the master curve. For fatigue measurements, the European standards specify loading under constant displacement or strain for the trapezoidal samples, with at least one-third of the test set of 18 specimens able to reach one million test cycles, while the prismatic samples are tested under constant stress to a displacement of 280 mm. The French mixture design method has five levels that are chosen according to the type of asphalt, loadings and intended use, with additional tests required as the level increases. The two highest levels require fundamental tests for stiffness and fatigue using the two-point cantilever bending test. Below Figure  illustrates the two-point cantilever bending test set up with a trapezoidal   sample.
For the three-point and four-point bending tests, a beam is subjected to periodic bending through vertical movements in the central load point(s), while the vertical positions of the two end points are kept fixed. Free rotation and horizontal movement are allowed at all load and reaction  points in order to prevent the development of horizontal and torque stresses that can affect the behavior of the material during the test. The sample width and  heightshould be at least three times the maximum aggregate size of the asphalt, while the effective sample length should be at least six times the width    or    height.    The    European    standard,  EN  12697-24:2012 (BSI,
2012a), has a more specific set of sample dimensions (300 mm length and 50 mm width and height for the three-point bending fatigue test), while the American  Society  for  Testing  and  Materials  (ASTM,   2010)   specifies a 380 mm long by 50 mm thick by 63 mm wide sample for the four-point bending fatigue test. The initial  stiffness  modulus is  typically  determined at  the  50th  or  100th   cycle   (by   ASTM   D7460-10   (ASTM,   2010)  and EN 12697-24:2012 (BSI, 2012a), respectively) of the applied sinusoidal force. For fatigue, EN 12697-24:2012 specifies loading under  the  con stant strain mode for the three-point bending test, while the four-point bending test can be in either the constant strain or constant stress mode. The ASTM method  is  carried out  under  the  constant strain  mode.  Figures  illustrate the three-point and four-point bending test set ups, respectively.
The various fatigue tests are carried out until ‘failure’ occurs in the test specimen. ‘Failure’ can be an arbitrary end point, not where the test specimen literally fails. In a constant-strain test, the sample is usually deemed to have ‘failed’ when the load required to maintain that level of strain has fallen to 50% of its initial value. Because of the scatter of test results associated with fatigue testing, it is normal to test several specimens at each stress or strain level, and to plot the results plotted as stress or strain against cycles