This module will provide a description of the rheological concepts that are used to characterize asphalt binders. All of the asphalt binder rheological tests as well as some information on the traditional viscosity test used for the AC grading system are covered.
Various configurations of rheometers have been or are currently being used for the characterization of asphalt binders. Rheometers are designed so that fundamental engineering parameters of stress, strain and/or strain rate can be measured.
There are two general configurations of shear rheometers that are used: drag flow and pressure driven. Various uses of these general configurations are discussed in the following slides. Rheometers are also used to determine the tensile characteristics of asphalt binders. Stiffness can be assessed by applying a load in the center of a simply supported beam then using standard mechanics equations. Strength, a property evaluated at failure, can be obtained by directly pulling a sample apart.
This section covers shear rheometers typically used to characterize asphalt binders.
One of the oldest drag flow rheometers used to characterize binder properties is called a sliding plate rheometer. This configuration was used as early as the mid 1950’s to determine the viscosity of asphalt binders at around room temperature.
The steps involved in testing are simple: A heat lamp is used to warm a small glass plate which is then coated with a couple of grams of asphalt binder. A second glass plate is then put on top of the asphalt binder and a weight applied to the glass plate so that the asphalt binder is squeezed out of the sides of plates and a prescribed film thickness is obtained. The plates and sample are cooled to room temperature and the sides of the plates cleaned. One plate is fixed in a holder in a constant temperature water bath. A weight is hung on the second plate and the speed at which the plate slides is documented on a chart recorder. The speed and weight used to cause the plate to slide is then used to calculate the viscosity of the asphalt binder.
One of the important designs in this type of rheometer is the configuration of the bottom of the inner cylinder. In the case of the Brookfield rheometer commonly used for asphalt testing, the bottom of the cylinder is tapered like the one shown on the left hand side of this slide. This configuration is needed so that the drag on the bottom of the cylinder matches the drag on the sides of the cylinder. That is, a true measurement of the engineering properties of the asphalt binders is obtained.
Another type of drag flow rheometers is the concentric cylinder. This is the configuration used in the rotational viscometer required for asphalt binder testing at high temperatures. This slide shows the equations for shear stress and shear strain based on the geometry. One of the key elements to the design of these rheometers is the narrow gap between the cylinders. This type of configuration requires that the size of any additives (e.g., crumb rubber, filler) be less than 1/10th the diameter of the gap. In the case of crumb rubber, the discrete rubber particles are large enough to be easily seen in the asphalt binder. This type of modified asphalt binder should usually not be tested with this type of rheometer.
The cone and plate configuration has also been used in the past for characterizing asphalt binder properties. One of the main advantages to this configuration is that the shear flow from the center to the outside edge is homogeneous.
This style uses two circular plates with a film of asphalt binder in between. One plate is held stationary with the other is either continuously turned or oscillated back and forth (sine wave). The second method of loading of this configuration is used for the dynamic shear rheometer (DSR). While it is easier to set up the test specimen with this configuration, the shear flow is non-homogeneous from the center to the outside edge. The values reported and used in asphalt binder testing use the shear stress and strain at the outside edge of the parallel plates.
The accuracy of the results depend on these assumptions being true. The requirement for cylindrical edges is critical and is the reason for so much care being required in trimming the edges of the samples prior to DSR testing.
The main advantage to this test is that it can be run using a sinusoidal wave form. The elastic and viscous component of the material response can then be determined by evaluating the strain response to an applied stress (see next two slides). The time for one cycle has been set at 10 rad/sec (1.59 cycles per second). This is generally representative of the time for one cycle of loading due to 55 mph traffic.
If a material is elastic, then the strain response will be in-phase with the applied stress. If a material is viscous, then the response will be 90o out of phase.
When a material has both an elastic and viscous component to its behavior, this type of testing can sort out the contribution of each to the total response. Delta is the phase angle, that is, the degrees that the strain response is out of phase with the applied stress. The complex modulus, G*, is the vector sum (Pythagorean's theorem). If delta is 0, the G* equals the storage modulus. In other words, the response is all elastic. If delta is 90o, then the response is all viscous (G* = viscous component).
This is just one example of DSR units. There are a number of manufacturers of this type of equipment.
This slide shows a close up of one style of cone and plate testing area. Also included is a typical unit. These rheometers are typically small and portable.
This photograph shows a sample of asphalt binders placed on the upper DSR plate. This plate will be mounted in the rheometer and the plate moved downward to achieve the desired gap. The asphalt binders will be squeezed out the sides and the excess trimmed off.
The most common type of pressure drive rheometer used for evaluating asphalt binders is the Asphalt Institute viscometer tube. This tube, shown on the next slide, has a vacuum applied to one side of the tube then the time for the asphalt binder to flow through the tube is evaluated. (see previous module for description of historical viscosity testing)
This figure shows the configuration of a typical Asphalt Institute tube. The asphalt binder is added to the large side of the tube until it reaches the line near the bottom of that side of the tube. A vacuum is then applied to the small side, the time it takes for the asphalt to pass each of the marks on the small side is recorded, and the tube constants for each section are used to determine the viscosity in Poise.
This section covers the rheological concepts associated with the bending beam rheometer and the direct tension rheometer. These tests are usually conducted at cool to cold temperatures so that the asphalt binder is more likely to maintain the desired geometry for testing.
This test applies a static load to a simply supported beam of asphalt binder. Temperature is held constant using a liquid bath. A computer provides both equipment control and data acquisition.
The equation used to determine the change in stiffness with time is that for a simply supported beam. The geometry parameters remain constant throughout the test. The only values that change are the deformation of the beam due to the static load and the stiffness calculated using this time-dependent deformation.
The software collects deformation measurements at 8, 15, 30, 60, 20,and 240 seconds and the corresponding stiffness is calculated. The stiffness is then plotted versus the log of time. Since deformation increases the longer the load is left on the sample (and deformation is in the denominator of the preceding equation), stiffness decreases with time. The parameters used in the specification are the creep stiffness at 60 seconds and the slope of the tangent line at this point (called m-value). There is a maximum requirement on the binder stiffness to ensure that the binder is sufficiently soft at cold temperatures. There is a minimum set on the slope requirement. This is to ensure that the material can relax (deform) quickly enough to prevent cracking.
Regardless of the type of equipment used, a sample of asphalt binder is molded into a “dog bone” shape with a uniform center cross section. The sample is pulled until the it breaks in the middle. The stress and strain at failure are recorded. This test requires a minimum strain before the sample fails.
The entire system is comprised of the computer hardware, BBR,and chiller system located under the counter (liquid circulates from the chiller through the BBR fluid bath).
This photograph shows the samples in the bath with the one in the upper part of the picture ready to test.
Block 3 SP 14
– the study of flow and deformation
• Constitutive relations
– fundamental relationships between force
• Equipment used to measure rheology
• Shear rheometers
– Drag flow
– Pressure driven flows
• Rheometers for measuring stiffness and strength
– Bending beam
– Direct tension
Schematic of Sliding Plate
(not to scale)
• Steady, laminar flow, isothermal flow
• No radial or vertical flow
• Negligible gravity and end effects
Drag Flow Rheometers
2 π Ri2 L
Ro - Ri
Narrow gaps: Ri / Ro > 0.99
Drag Flow Rheometers
• Cone and Plate
Similar triangles for
Drag Flow Rheometers
• Parallel Plate
Shear flow varies with
gap height and radius
• Steady, laminar, isothermal flow
• Negligible body forces
• Cylindrical edge
Test operates at 10 rad/sec
or 1.59 Hz
360o = 2 p radians per circle
1 rad = 57.3o
δ = 90o
δ = 0o
Complex Modulus, G*
Viscous Modulus, G”
Storage Modulus, G’
Complex Modulus is the vector sum of the
storage and viscous modulus
• DSR testing can be used to develop master curves
– Need range of temps and frequencies at each
– Affect of loading time on asphalt binder stiffness
– Estimate of thermal coefficient of expansion
25 mm Plate with Sample
• Asphalt Institute Tube
Stiffness and Strength
Bending Beam Rheometer
Bending Beam Rheometer
• S(t) =
4 b h3 δ (t)
S(t) = creep stiffness (M Pa) at time, t
P = applied constant load, N
L = distance between beam supports (102 mm)
b = beam width, 12.5 mm
h = beam thickness, 6.25 mm
d(t) = deflection (mm) at time, t
Bending Beam Rheometer
• Evaluates low temperature stiffness properties
– Creep stiffness
– Slope of response (called m-value)
Log Loading Time, t (sec)
Stress = σ = P / A
Direct Tension Test