Where are you on the MSCR implementation curve?

By Pete Grass, P.E., CAE

Where is your agency on the MSCR implementation curve?

MSCR – the Multiple Stress Creep Recovery test – also codified in AASHTO specification M332, has been around for many years and all but a handful of states have been testing and reviewing the impacts of implementing the specification or have made major strides to full implementation. At least 14 states report full implementation as of this writing.

Many studies have shown that the MSCR parameter, Jnr, is better related to rutting for all binders – modified and unmodified – than the current G*/sin delta parameter in the PG (Performance Grade) specification. The fact that G*/sin delta did not adequately characterize the performance of modified asphalt binders led user agencies to add elastic response tests, like Elastic Recovery, to their binder specification.

While the Elastic Recovery (ER) test is an excellent tool to establish the presence of polymer modification, it is a poor tool to evaluate how well the polymer modifier works in the asphalt binder. It also isn’t a particularly fast tool, as a typical ER test takes about four hours to prepare then test samples to generate this information.

For those agencies wanting to have an elastic response in their modified asphalt binders, the MSCR percent recovery parameter can be obtained from the same test as Jnr and can be determined much more quickly than other PG Plus tests like Elastic Recovery.

Additionally, the PG specification using MSCR relies on testing at the actual environmental temperature and adjusting the criterion instead of relying on testing at artificially high temperatures caused by grade bumping.  

All this means that the MSCR test and specification parameter, Jnr, come closer to the original intent of the PG binder specification – that the spec be performance-related and usable for unmodified and modified binders equally – than does the use of G*/sin delta as the high-temperature parameter. That reduces the likelihood of specification proliferation which unnecessarily drives up testing costs.

The Asphalt Institute supports the change to MSCR specification and our Technical Advisory Committee and our Regional Engineers are fully aligned in the effort to educate agencies, binder producers, and testing laboratories on the specifics. 

One successful model to implementation is that of Washington State Department of Transportation. On a regional basis, they’ve participated in the Pacific MSCR implementation task force which has been looking at MSCR parameters for about five years. Further, by establishing several inter-lab studies to demonstrate the repeatability and reproducibility with MSCR, this helped the department make informed decisions on MSCR implementation. The binder supplier industry was also very involved in the inter-lab studies.

Through the inter-lab studies and the work involved in fully examining the M332 specifications, it became clear that the change alleviated concerns regarding modified binders and correctly characterizing them to match actual performance. Washington State’s implementation process has been very transparent with industry, included lots of dialogue with industry and has been based on plenty of testing and shared information. 

Today we count 25 states as having implemented at least some aspect of the Multiple Stress Creep Recovery, or plan to do so this year. This count includes those states specifying MSCR requirements for all binder grades, just their modified grades or by substitution.

Where is your agency on the MSCR implementation curve? 

• Grass is the President of the Asphalt Institute