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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USING ASPHALT PAVEMENTS AS A GARBAGE DUMP

Is pavement Quality of any Value

Over the past 50 or more years I have read about adding all sorts of waste materials to pavements or to the material beneath the pavements. Some of the materials have benefits, some are just garbage.

Research has shown that including reclaimed asphalt in new pavements has a benefit with respect to quality as well as cost. If the resulting blend of new and recycled asphalt meets the specification requirements, the pavement should be at least as good as pavements made with new asphalt. One value of the recycled material (RAP) is that the rate of oxidation of the old asphalt on the aggregate will be less than that of new asphalt on new aggregate. That is because the rate of oxidation of new asphalt on aggregate deceases with time, (except perhaps for an asphalt from one particular crude source). The RAP asphalt has already experience the rapid oxidation phase.

Recycled asphalt shingles (RAS) are now being used. At the present time I am not comfortable with that although future testing may find it works well. Shingles consist of an air blown saturate in a felt on which a filled coating asphalt is placed. The softening point of the coating is above 200° F. Past experience has shown that the presence of air blown asphalt can accelerate non-load associated cracking. I don’t know if the addition of elastomers helps with this problem or not. Non-load associated cracking occurs when the binder cannot relax thermal stresses before they reach the failure stress. Time will tell.

Pavements have been used to get rid of glass. This is a novelty as there isn’t enough glass around to have an impact. It can work, however it must be realized that glass likes water better than asphalt resulting in possible areas of water damage.

I have heard that some agencies are adding reclaimed oil to asphalt. That is a very bad idea as paraffins and asphaltenes are incompatible. Asphalt naturally contains some paraffins which are kept in solution by the aromatics and polar materials in asphalt. Loading up the asphalt with more paraffins can cause phase separation, which would be expected to cause non-load associated cracking. Before adding such oils to asphalt it might be well to read up on the research done by Rostler et al. half a century or more ago. Refineries have had corrosion problems with the addition of such oils to their crude feed.

Reclaimed tire buffings have been added to asphalt for many years with success. Truck tire buffings (natural rubber) and passenger tire buffings (SBR) will react differently. There can be a problem in QA testing. A contractor may specify that they have added a certain amount of the tire buffings but testing on a sample taken from construction may indicate that there was less than what the contractor said there was. The tire buffings would be expected to contain some processing oils which would be extracted out, and if there was natural rubber in the buffings, it might have broken down some as cis-polyisoprene (natural rubber) is not as heat stable as SBR. It would be well for the contractor to tell the owner how much of what was added would not be found.

The original specifications for asphalt and hot mix were based upon unmodified asphalt and aggregate. Experience has shown that those specifications can still be valid with the addition of certain polymers and with the addition of lime to the aggregate. However adding other materials to the pavement simply to get rid of them doesn’t mean there won’t be unforeseen consequences ever if such mixes meet specification requirements. Early non-load associated cracking is especially difficult to predict.

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.

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

WHAT IS THE IMPORTANCE OF ASPHALTENES IN ASPHALT?

What are asphaltenes? While often discussions about the composition of asphalt will define asphaltenes in chemical terms, the basic definition is that asphaltenes are material in asphalt that is insoluble in certain solvents. For some, asphaltenes appear to be something that is in the way that must be tolerated. That is far from the truth. First, as mentioned above, the definition of asphaltenes is simply material insoluble in either pentane, hexane, heptan or octane, depending on the method used. Often the compositions are described as being saturates, aromatics, polar materials and asphaltenes. In fact asphaltenes might be considered to be simply the part of polar materials that are insoluble in some arbitrary solvent. If an asphaltenes-free asphalt is exposed to light, new asphaltenes will be formed. In fact pictures have been taken using asphaltenes-free asphalt as the “film”. Upon exposure to light a picture is formed.

What do they do? Asphaltenes have three important functions: 1) a bodying agent; 2) forming a complex structure that aids in performance in conjunction with the other polar materials; 3) and helps to reduce hardening with time.

Bodying agent. There was an asphalt a number of years ago that was essentially asphaltenes free. It acted almost like lubricating oil and was such a problem that the agencies insisted that it be blended with different more suitable asphalt. Normally once a hot mix is made, and cools, a gel like structure is formed that aids in the setting of a hot mix. With some asphalts there is a setting problem because the formation of that structure is slow or weak and mixes made with them are tender. Other asphalts result in hot mixes that set very well and are not considered to be tender. At one time California DOT had a test using what they called the cohesiograph that measured tenderness.

Performance.  Tender mixes tend to rut easier and to be easily marred from power steering. If the aggregate gradation in the hot mix has too much sand, the mix will be very hard to compact. If the asphaltenes bodies up the mix well, compaction will go well even with oversanded mixes.

Aging. When the relationship between the polar-asphaltenes is optimum, oxidation will be controlled. In the asphalt “micelles” (a term coined by Dr. Claine Petersen, one of the foremost asphalt scientist in the world in my opinion) are formed such that oxidation is restricted for the material in the micelle so that the rate of oxidation decreases with time. With asphalts in which the asphaltenes are too well dissolved and micelles do not form, the rate of oxidation continues resulting in very hard asphalts in the pavement in a relatively short time. We have run across some cracked pavements that were not very old but the asphalt was very badly oxidized. The asphalt used was from a crude oil in which the micelles do not occur.

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