Esteemed Researchers,
Please help to understand the fundamentals of stress-controlled and strain-controlled testing of asphalt binders and asphalt mixtures. I have some queries as follows:
During rheological evaluation of asphalt binder, the parameter G*/sin d (G*: complex modulus; d: phase angle) is derived assuming a stress-controlled behavior. Then why we keep the strain level constant during G*/sin d evaluation (12% strain for unaged binder and 10% strain for short-term aged binder)?
Some dynamic shear rheometers (DSR) are strain-controlled and some are stress-controlled. What does it signify? Which rheometer should be preferred?
What is the practical relevance of performing a stress-controlled or a strain-controlled test on asphalt binders/mixtures?
Thanks!
Good question,
There is always confusion between stress-controlled and stress controlled tests.
The conventional DSR PG test is strain -controlled. It means that the equipment is adjusted to apply strain (maximum or Target value=12%) and measure the stress as the output. Also, this test can be done under stress controlled mode. For more information check AASHTO T315 standard.
These two modes of loading make a lot of difference in interpreting the results. For example, in thin overlays (50 mm), strain-controlled behavior is prevailing regarding the fatigue life of mixtures while stress-controlled behavior is predominant for the thick pavements.
Hi Abhinav,
That is a relevant question. I guess there is a confusion when you say that a strain of 10-12% or 1-2% is used and you consider it as a controlled strain test.
Usually Superpave binder testings are done in constant stress mode.
The DSR can measure only two things: Torque and Displacement: rest all the other parameters are calculated/derived.
Constant Stress: Torque is constant, displacement varies
Constant Strain: Displacement of the spindle is constant, torque varies.
We need to be sure in Superpave specification testing that the binder is within the linear viscoelastic (LVE) regime. So in a constant stress mode the DSR sets the torque such that the resulting strain is approximately 10-12% for soft materials (unaged, RTFO aged) and 1-2% for hard materials (PAV aged). The DSR sets this stress value automatically. So using the value of 10-12% or 1-2% is just to set the torque (stress) so the the material property is captured within LVE range.
I hope this helps!!
Thanks :)
Nikhil Saboo, Ramya Sri Mullapudi and Hamed Majidifard : Thanks so much for the enlightening answers to the question!
Thanks to Esraa T. Al-Azawee , Mehrdad Honarmand
, Ihsan Obaid , and Yasir Mohammed Jebur for the interest in the question.It is indeed a valuable learning experience on stress- and strain-controlled evaluation of asphalt.
Respected researchers,
Can you suggest some good reference materials for this topic of stress- and strain- controlled evaluation of asphalt binders using DSR.
Hamed Majidifard
Hi Hamed,
We have always heard about controlled-strain or stress that were they are recommended. Here in the EU we have some issues when using high-modulus asphalt mixtures. As you have mentioned stress-controlled is more convenient for thick layers and stiff materials. But my problem is I can not find a reference for it. I mean a specification or standard quotation. Within EU norms, I could not find it. Do you have any references? Here ITFTcontrolled- stress mode is very common.
Ciao,
Shahin.
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