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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.

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 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.

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).

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.

- 1. Senior/Graduate HMA Course General Rheology Asphalt Binder General Rheology 1
- 2. Rheology • Rheology: – the study of flow and deformation • Constitutive relations – fundamental relationships between force and deformation Asphalt Binders General Rheology 2
- 3. Rheometers • Equipment used to measure rheology • Shear rheometers – Drag flow – Pressure driven flows • Rheometers for measuring stiffness and strength – Bending beam – Direct tension Asphalt Binders General Rheology 3
- 4. Shear Rheometers Asphalt Binder General Rheology 4
- 5. Drag Flow Rheometers • Sliding plate Asphalt Binders General Rheology 5
- 6. Schematic of Sliding Plate Viscometer Chart Recorder (not to scale) Asphalt Binders General Rheology 6
- 7. Assumptions • Steady, laminar flow, isothermal flow • No radial or vertical flow • Negligible gravity and end effects Asphalt Binders General Rheology 7
- 8. Drag Flow Rheometers Concentric Cylinder τ Mi Rθ = 2 π Ri2 L γ = θ R Ro - Ri Narrow gaps: Ri / Ro > 0.99 Asphalt Binders General Rheology 8
- 9. Drag Flow Rheometers • Cone and Plate Similar triangles for shear flow Homogeneous flow Asphalt Binders General Rheology 9
- 10. Drag Flow Rheometers • Parallel Plate Shear flow varies with gap height and radius Non-homogeneous flow Asphalt Binders General Rheology 10
- 11. Assumptions • Steady, laminar, isothermal flow • Negligible body forces • Cylindrical edge Asphalt Binders General Rheology 11
- 12. Oscillating Plate B C A Fixed Plate B Time A Test operates at 10 rad/sec or 1.59 Hz A 360o = 2 p radians per circle 1 rad = 57.3o C 1 cycle Asphalt Binders General Rheology 12
- 13. Elastic Viscous B Strain A Time A C Strain out-of-phase δ = 90o Strain in-phase δ = 0o Asphalt Binders General Rheology 13
- 14. Complex Modulus, G* Viscous Modulus, G” δ Storage Modulus, G’ Complex Modulus is the vector sum of the storage and viscous modulus Asphalt Binders General Rheology 14
- 15. Master Curves • DSR testing can be used to develop master curves – Need range of temps and frequencies at each temperature • Information – Affect of loading time on asphalt binder stiffness – Estimate of thermal coefficient of expansion Asphalt Binders General Rheology 15
- 16. Equipment Example Asphalt Binders General Rheology 16
- 17. Equipment Examples Asphalt Binders General Rheology 17
- 18. Equipment Example 25 mm Plate with Sample Asphalt Binders General Rheology 18
- 19. Pressure Driven Rheometers • Vacuum Viscometer Velocity Profile Asphalt Binders Shear Stress Profile General Rheology 19
- 20. Viscometer Tube • Asphalt Institute Tube Asphalt Binders General Rheology 20
- 21. Stiffness and Strength Rheometers Asphalt Binder General Rheology 21
- 22. Bending Beam Rheometer Deflection Transducer Computer Air Bearing Load Cell Asphalt Binders General Rheology Fluid Bath 22
- 23. Bending Beam Rheometer • S(t) = P L3 4 b h3 δ (t) Where: 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 Asphalt Binders General Rheology 23
- 24. Bending Beam Rheometer • Evaluates low temperature stiffness properties – Creep stiffness – Slope of response (called m-value) Log Creep Stiffness, S(t) 8 15 30 60 120 240 Log Loading Time, t (sec) Asphalt Binders General Rheology 24
- 25. Direct Tension Test Load Stress = σ = P / A σf ∆L ∆ Le Strain Asphalt Binders General Rheology εf 25
- 26. Equipment Example Fluid Bath Loading Ram Cooling System Asphalt Binders General Rheology 26
- 27. Direct Tension Test Asphalt Binders General Rheology 27
- 28. QUESTIONS? Asphalt Binder General Rheology 28

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