Asphalt Compositions Vary.

Those skilled in the art of asphalt technology have known that the composition of an asphalt depends primarily on the crude source. Secondary effects are oxidation and modification either by the addition of polymers or air blowing, which is controlled oxidation to make roofing, pond linings etc. The properties of an asphalt therefore can also vary according to the crude source. Back in the 1960s Rostler, White and others compiled a list of properties and compositions of a very large number of asphalts. It turns out that the properties of blends of asphalts from different sources are sometimes not predictable.

Blending Predictions

The plot of the loglog(viscosity) vs. log(absolute temperature) of an asphalt generally is a straight line. Special graph paper has been available for decades. It turns out that in blending petroleum products, including asphalt, using that graph paper with 0% of an oil at 100° F and 100% at 300° will generally be linear also. At times the X axis may be assumed to be linear rather than the log(absolute temperature). (In ASTM D4887, the X axis is linear.) The resulting plot is not always linear, however, depending upon the composition of the second ingredient. As an example, when blending recovered asphalt from RAP with an aromatic oil, such as Dutrex® 739 or Reclamite® base stock, the viscosity may drop faster than predicted. On the other hand, if a paraffinic oil is used, the actual viscosity may be higher than that predicted from the plot.

We had found that blending 50% 85/100 asphalt from California costal crude with 50% 85/100 asphalt from San Joachim Valley crude resulted in an asphalt with a penetration in the 130s. The same thing was found with a blend of Dubai asphalt with LA Basin asphalt. There are thermodynamic reasons for this based upon non-electrolyte solution chemistry.

Recycled Shingles (RAS)

Roofing asphalt is manufactured by air blowing fluxes containing added lube stock. This changes the composition. An asphalt shingle contains two different air blown products. One is used to saturate the felt or fiberglass while the other is a more viscous asphalt (more air blown) and used in the coating. These two asphalts might be incompatible as the coating asphalt, though harder, contains more oil. If the oil from the coating migrates to the felt or fiberglass the coating might slide off. There is a test used to measure compatibility. Also ferric chloride or phosphorus pentoxide might be used as a catalyst. As the use of air blown asphalt in paving has been correlated with non-load associated cracking, care should be taken in recycling such asphalt. Cracking occurs when the asphalt cannot relax stresses as fast as they build up. A low temperature ductility test is valuable in detecting asphalts that are prone to crack.

Recycled asphalt shingles (RAS) are now being used in paving. In recovering the asphalt from shingles the saturant asphalt and the coating asphalt are blended. It will be interesting in following the performance of pavements using RAS and RAS/RAP added asphalt. As mentioned above, historically, air blown asphalts in pavements are more prone to crack.


It is therefore important to understand that the terms “asphalt” or “bitumen” describe a broad set of materials as does the word “vehicle” in describing a set of transportation equipment. Just because two asphalts are black does not mean that they are compatible. And just because two asphalts are of the same grade, does not mean that a blend will be the same grade. Also, the oxidation process that occurs over time in the pavement is not the same as that which happens in the hot plants, and which is mimicked by the Rolling Thin Film Oven test (RTFO). The RTFO oxidation is the same process that occurs in air blowing. That implies that the chemistry of the oxidation of the asphalt in RAP is different than the chemistry of the asphalt in RAS.


Superiority of the AR Grading System


AR Grading. The Asphalt Residue (AR) grading system used in the Western part of the United States for decades grew out of the fact that the asphalts in this area differed greatly. While various grades were in use, the workhorse grade was AR 4000 which meant that the asphalt in the pavement, irrespective of crude source, would have the same consistency. AR 4000 meant that the viscosity at 60° C of the asphalt after the RTFO test would be 3000 (2500 in Washington) to 5000 poises. A viscosity of 4000 poise was selected as it was found that at 4000 poises tenderness in oversanded mixes was easier to handle.  60° C is used as in most cases that is about the highest temperature the pavement reaches although in the deserts it can reach considerably higher temperatures. On the other hand, the viscosity at 60° C from the RTFO of equivalent asphalts graded by the AC grading system (2000 ± 400 poises based on original viscosity at 60° F) or by penetration grading system (85/100 based on penetration at 25° C) can vary greatly. For the 85/100 penetration grade, the range of the 60° C viscosity after the RTFO of those asphalts evaluated during the development of the AR grading system varied from about 1600 to over 7000 poises. For an AC 2000 grade asphalt, the probable viscosity after the RTFO aging would range over about 4000-8000 poises, depending on the crude source. The equivalent PG grade is PG 64-XX.

PG Grading. There is an astounding number of PG grades, 7, and up to 6 subgrades within each grade, based upon low temperature properties. If there was consistency within the grades it might make sense, but we have regressed even back beyond the AC grading system. These grades were set up primarily to control tenderness and rutting even while leaving the gradation specification so open that gradations that would allow grievous rutting are included. The equivalent PG grade is based upon the Dynamic Shear test of G*/sinδ of 1.00 kPa at 64° C with no maximum. For a sinδ of 1.00 (close to that of unmodified asphalt) the viscosity is G*· sinδ or 1000 poises. The G*/sinδ value from the RTFO test would be 2.20 kPa min or 2200 poises with sinδ = 1.00 and again there is no maximum. Sinδ for modified asphalts is less than one thus that drops the specification minimum viscosity below that of non-modified asphalt.` In other words, for the asphalt as placed in the pavement, the AR 4000 specification is 3000-5000 poises at 60° C. For the PG 64-XX , the-in place viscosity at 64° C can vary from somewhat less than 2200 poises to as high as one wishes.



Philosophical Inconsistency of the PG Grading System. I am only addressing the grading system, not the value of the low temperature specification. I am not suggesting that there is anything wrong with the use of the DSR, as it is a handy tool. I am suggesting that the grading should have been based upon the consistency of the RTFO residue whether viscosity tubes are used or the DSR. The value of the DSR data is that we can get information about the effect of polymer modification from the phase angle, sigma (δ).

We have shown above that the range of the allowed viscosity from the RTFO test of any particular PG grade is greater than that of any previous grading system even though there is are 7 specific grades in order to control rutting. The implication is that controlling rutting requires fine tuning. Yet, at the same time there is a movement to use warm mixes, one of the benefits of which is that the asphalt will have a considerably lower viscosity than the intention of the grade.

Controlling Rutting. The prime control of tenderness and rutting should be with aggregate gradation.  As long as the gradation specification allows badly oversanded mixes, rutting will be a problem.

Robert L. Dunning,,