WEAKNESS OF AASHTO ASPHALT MIX DESIGN

 

Prior to SHRP the mix designs in use were the Marshall and Hveem procedures. They were developed my user agencies and performed well for many decades. The Marshall design is still being used. The SHRP mix design was developed by academics who would not have had the field experience that state agencies would have had. The universities have provided many great advances in paving; however they do not have the experience of personnel with years of experience in road building. However they often have the power to place academic theories into practice. Following are certain problems with the specifications.


Incorrect Use of Maximum Density Line.
 The maximum density line shown in the specifications is based on the maximum aggregate size rather than the nominal size (screen size that first retains aggregate.). The aggregate retained between the maximum size and the nominal size would act in conjunction with that of the material between the nominal size and the next screen size smaller as there is not enough material to interlock. The actual maximum density line that pertains to the mix design is from the nominal screen size to zero. (Using the 0.45 power of the sieve size on the x axis. Note, Rudy Jiménez at The University of Arizona, believed that it should be the 0.50 power; that is, the square root, and he was probably correct.) To properly make judgments about the gradation of the mix, one needs to have the maximum density line that corresponds to the actual aggregate to be used. I was taught this by Vaughn Marker when he was Asphalt Institute Engineer in California. Properly used, it can stop mix problems, such as tender mixes and rutting, from happening.

Forbidden Zone of the Gradation.  This was placed in the specification by academics using the maximum density line from the maximum size gradation not the nominal size gradation. Also it had no value with respect to quality .

Specifications Allow Over-Sanded Mixes. All mix designs allow gradations that will cause tenderness and accelerate rutting. If the proper maximum density line is used, such mixes are readily detected, however that is not so with the worthless maximum density line in the present design procedure. Rutting is highly dependent upon where the VMA in a mix comes from also, which I will discuss in a future blog.

Asphalt Grading Specifications

 The grading specification should be on the RTFO residue as that is what is in the road. Also, the RTFO test should realistically be such that it approximates the properties of the asphalt in the mix in place. The TRFO was designed to mimic the increase in viscosity of the asphalt that is mixed in a batch plant at 320°F with the oxygen partial pressure the same as air. Things are different in a drum mixer. If the air in the drum mixer is 4 times that needed to burn the fuel, the oxygen partial pressure will be decreased by 25% from the combustion reducing the rate of oxidation. Also if moisture is present, the partial pressure of the oxygen will b further decreased. Also if the mixer runs at a temperature less that 320° F, the rate   of oxidation will be further reduced.

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