ASPHALT BLENDING

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.

Caution

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.

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ASPHALT AND ASPHALTENE

Neither One is a Single Material

 

Asphalt and asphaltenes are names that show up in articles and papers discussing paving and roofing materials. Especial with people not very familiar with technical field, discussions often sound like each is a single well define material such as salt or water. However that is far from the fact. Some may even feel that asphaltenes are something in the way that needs to be isolated or corralled. Yet they are vital in controlling the properties of an asphalt. Also researchers may reach conclusions on an asphalt from a particular crude source and believe that those conclusions pertain to all asphalts.

Asphalt.

Asphalt is the part of crude oil that is left when all the other hydrocarbons have been removed. There are two main ways of separating the asphalt from the gasoline, kerosene and oils; distilling, and solvent extraction.

Source. The properties of a particular unmodified asphalt are controlled by the source of the crude oil. The differences can be profound. In California there are three crude sources that produce entirely different asphalts: California Valley, Coastal and LA Basin. Within those broad designations are subgroups such as the coastal crudes; Santa Maria and San Ardo. A specification can be developed such that it can be met by asphalts from all three sources however they will perform differently. There are some asphalts that have very poor cold temperature performance and others that perform very badly in hot weather.

Distillation. In the distillation of crude oil, one pipe goes into the distillation towers, and a number of pipes come out. Each tower system is designed for a particular crude or crude blend and there are pumps removing the products. What is left over is asphalt on the bottom of the tower also. Some crude oils have no asphalts while others may contain as much as 65% asphalt. If any one of the storage tanks gets full, the refinery has to shut down.

Propane Extraction. The other method is to extract the non-asphalt portion with propane.

Asphaltene

One of the components of asphalt is the asphaltenes. Here we have two problems: the misconception that asphaltenes are significantly different than other asphalt components, and the basic definition. While some methods define asphaltenes as n-pentane insoluble material, other methods use hexane or heptane or even iso-octane as the solvent. n-Pentane will produce the largest amount. Because certain asphaltenes are precipitated by a solvent doesn’t mean that there aren’t still other materials in the asphalt that are very similar to asphaltenes. Asphaltenes give body to the asphalt. If the asphaltenes are completely solvated, the asphalt won’t perform well. On the other hand, if they are in a second phase, again the asphalt may cause problems. In some cases, the asphaltenes will be at least solvated sufficiently at ambient temperatures for a single phase to be present, however they may form two phases in cold conditions, resulting in cracking in winter.

Modification

The addition of polymer modifiers further complicates the situation. Adding a polymer to any asphalt will result in two phases no matter how well the asphaltenes are solvated. When polymer modification was young problems with phase separation was a problem that had to be resolved. It can be seen that with a wide range of properties in asphalts, polymer modification can be more of an art than a science. One question I have is how well modified asphalts will perform in low temperatures even though they pass all of the low temperature test. For pavements to resist low temperature cracking the binder must be able to stress relax faster than thermal stresses build up. If the binder becomes more like a plastic with a yield force necessary, the pavement will crack.

Robert L. Dunning, chemistdunning@gmail.com, www.petroleumsciences.com

WORLD OF PAVEMENT CRACKING

Fundamentals of Non-Load Associated Cracking

There has been considerable research on the engineering basis of pavement cracking. Those interested in some of the basic studies on cracking might consult volume 41 (1972) of the Proc. Association of Asphalt Paving Technologists. Many of the concepts develop there were the basis of the PG grading system with regard to low temperature properties of asphalt. While those papers are 40 years old, they lay the basis of technical progress in understanding cracking. Later studies have been oriented toward understand how cracking can be predicted.

However, it is not the purpose of this blog to go into the engineering of pavement design but rather to speak of the basic physics involved.

Failure occurs either from tensile stress or crack propagation. The maximum tensile strength of asphalt and hot mix is about 1000 psi and that only happens if the asphalt is cold or stressed at a high rate. At higher temperatures or lower rates of strain the stress at failure would be less. When cracks appear, the stresses are concentrated at the apex of the crack accelerating the formation of a crack. Thus no matter what the crack might look like, it is caused by too much tensile stress.

Literature suggests that when the temperature drops down below about 100-110°F of the softening point of the asphalt in pavement one would expect damage to the pavement. That damage accumulates eventually resulting in transverse cracks showing up. The distance between cracks is related to hardness of the asphalt. If the temperature rapidly drops to, perhaps, 150° F below the softening point, the crack may occur that day. I actually observed that in the late ‘80s. There had been a very sharp drop in temperature in Spokane, Washington on one day. I was called in for several cases where even fairly new pavements showed block (traverse) cracking, including a new tennis court. The only answer was that the temperature drop had caused it.

If we recognize that the softening point of aged asphalt might approach 200° F it can be seen that the fast drop in temperature in deserts at night could even cause damage at surprisingly higher temperatures. Pavements can reach over 170° F in the deserts.

The effect of crack propagation can be seen in parking lots where asphalt pavement is adjacent to a portland cement area where there are 90° corners. A crack will be seen radiating out of the corner even if there is no other evidence of cracking in the pavement. If small cracks are formed inside a pavement and don’t heal themselves, they will grow and eventually show up.

When cracking occurs, the asphalt in a pavement is no longer performing as a liquid, but more as a solid. It responding to stress from cooling by pulling apart horizontally. When the pavement heats up again, the crack remains, although if they are small, traffic can knead them back together. If it can act as a liquid it flows vertically upward as the temperature increases and downward as the temperature decreases. The solution to cracking is to allow the asphalt to retain its liquid properties as long as possible.

As asphalts from different crude sources behave differently, there is no golden rule. Non-electrolytic solution chemistry can be involved but that is a discussion for another day.

One of the remedies for reducing the temperature related cracking in pavements is to seal them so that the rate of hardening of the asphalt is reduced. Also the HMA needs to be protected from water, both liquid and vapors. Even in the desert water accumulates under the pavement. If the bond between the asphalt and aggregate is susceptible to being compromised by the presence of water, the bond will be broken and failure will occur. Traffic accelerates the loss of strength as water propelled by changing pore pressure scours the asphalt off of the pavement. Even water vapor has been seen to do this. Weakening the bond between rock and asphalt will then be allowed to grow under less stress.

I also like to see primes used under the pavement to discourage water from entering the mix. Reducing the rate of hardening of the asphalt so it retains its liquid properties and protecting the pavement from water damage can reduce the rate of formation of non-load associated cracks.

Robert L. Dunning, chemistdunning@gmail.com, www.petroleumsciences.com