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.

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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

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