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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QUALITY CONTROL

Reliability of Data

In a previous entry I showed that the basic concepts of quality control, which depends upon the laws of probability (statistics), are surprisingly simple. All that we are trying to do is measure lengths of lines. The equations used to calculate the mean and standard deviation are those that describe only two lines so that no matter how many samples are tested, the calculations of those parameters result in just those two lines which are independent of each other. While “n” data points occupy “n” dimensions, the mean and standard deviation occupy only two. We can use the standard deviation as the ruler to measure the lengths of interest.

What makes things difficult is the fuzziness of those lines. In quality control the first thing we want to determine is the length of the distance from the measured length (sample mean) to some desired length. To do that we use a ruler in which the standard deviation is set to be one. For convenience, and because the standard deviation is defined as the second moment around the mean, the targeted mean is subtracted from the data points so that the resulting length of the data vector is reduced to the difference between the sample mean and the target. That length is then divided by the standard deviation. The resulting length is then measured not in inches or millimeters but rather in units of the standard deviation ruler. As an example, assume that 100 was the target value, the measured mean was 85 and the standard deviation was 10. We are not interested in what the actual measured mean is, but rather how close it is to the target, based upon the standard deviation ruler:

1. (100-85)/10 = a distance of 1.5 SD units. In some cases the measurement is not from the desired target, but to upper and lower limits.

However, the mean value is fuzzy and the standard deviation may or may not be fuzzy. The data generated in calculating the mean make up a random variable (X= (x1, x2, —, xn)) in vector space. How fuzzy it is depends upon the length of the SD, and the type of distribution. While there are many distributions, if the SD is not fuzzy, what is called the normal distribution is often used. Because of the uncertainty in the mean, the distribution function tells us the chances of the mean actually being somewhere else.  In example 1 with only the mean being fuzzy, and using the normal distribution, we can say that there is a 6.68% chance that the true mean of the data is the desired mean.

Unfortunately, the SD often is fuzzy too and is thus also a random variable. The square of the SD is called the variance, and has its own distribution function called the chi squared distribution. While the normal distribution is independent of the number of data points defining the random variable, the form of the chi squared distribution depends upon the degrees of freedom. The chi square distribution with one degree of freedom is the square of the normal distribution. That distribution may be used to determine whether two measured standard deviations are really the same.

How the fuzziness or uncertainty is handled will be covered later. Although the mathematics gets more complex, especially when multivariate sets of data must be considered, the goal is still to simply measure lengths with a specific ruler.

 

ASPHALT, A REALY INERESTING LIQUID

Resisting Failure if Treated with Care

 

A pavement is about 93-96% rock, by weight, however it seems that there is a strong belief that by properly modifying the asphalt all problems can be solved. Asphalt or more properly, asphalts have served us well, even before modification. The properties of asphalts are primarily determined by their crude sources, however blending crudes or asphalts can at times produce an asphalt that performs better than either of the components. Modifying asphalts can also enhance their properties. However, it is important that we keep in mind that its performance depends to a great extent to its ability to flow, and its ability to suppress hardening as time goes.

Rutting is Not an Asphalt Failure. Asphalt is a liquid whose job is to flow in response to stress. If a pavement ruts, it is either ground by studded tires, or the aggregate size or the gradation is improper. If the stress is greater than the aggregate can handle, rutting occurs with the asphalt doing what it is designed to do, flow. Modifying the asphalt can affect how fast the flow occurs, however it is the aggregate properties that affect the rutting.

Many Aggregates Prefer Water to Asphalt. Asphalt doesn’t work well if it can’t stick to aggregate. Water can interfere with adhesion. One cause can be in the asphalt itself. If it is produced from crude oil that had been treated with caustic soda, it will contain soaps that will make the asphalt itself water sensitive. That has been solved by lime treating the crude. Antistrips are used to aid adhesion; however it has been shown that with some antistrips the effect wears off which allows water to lift the asphalt off of the rocks. There is one antistrip that combines chemically to aggregate and provides long term durability.

Non-load Associated Cracking Occurs when the Asphalt Cannot Relax Stresses. The fluidity of the asphalt is essential to prevent cracking. Trying to make the asphalt stronger only makes the matter worse as its maximum tensile strength is about 1000 psi. Portland cement cannot defeat thermal stress so don’t expect asphalt to do so. The solution is to have a binder that can relax stresses faster than they build up.

Pavement Slippage. Slippage occurs when of tack coats and primes are not used properly.

Fatigue Failure. There are suggestions that asphalt could be modified to increase its stiffness so that the pavement thickness could be reduced. Again it must be remembered that it is the aggregate that carries the load, in compression, not the asphalt. However fatigue failure occurs in tension, and again the tensile strength of asphalt is much less than that of aggregate. The pavement is stretched underneath the wheel path, and between the wheel paths. However, tensile failure is often really crack propagation, thus additives that stop crack propagation such as tire buffings may be of value.

chemistdunning@gmail.com, http://www.petroleumsciences.com

REDUCING HOT MIXED ASPHALT COSTS

Controlling Voids in Mineral Aggregate (VMA)

Considerable effort is being made to reduce costs and amount of hydrocarbons that go into hot mixed asphalt (HMA) pavements. One such effort is to find ways to mix and compact at a lower temperature thus reducing the amount of fuel required. However, saving fuel can also be obtained by reducing the amount of asphalt used as asphalt can also be sold as a component of heavy fuel oil or cracked to make diesel, gasoline etc.

Mix Design.

Irrespective of the type of mix design or the amount of modification of the asphalt, the basic properties for an acceptable product remains the same. If we get down to basics, we want the gradation to be such that it inhibits rutting, want the gradation in the # 30 sieve size to be such that there isn’t a lack of material in that area and want the composition of the binder to be such that the film thickness is somewhere between 7 and 10 microns (based upon our experience. Idaho specifies 6 microns as a minimum) and, for example for a ½” nominal design, an effective asphalt content of 4-5%.

Trade off between % Asphalt and VMA. As the VMA increases, the % asphalt  required increases at a rate of about 0.25% per each percent of increased VMA, the exact amount depending on the actual specific gravities of the aggregate and asphalt. For a 400 ton an hour plant, the reduction of the VMA of 1% would reduce the asphalt by one ton per hour or a savings of $500/hour if asphalt is $500/ton.

Silliness of the “Forbidden Zone”. Some Superpave gradation specifications have a “forbidden zone” for the gradation through which the gradation must not go. It is supposed to be on the maximum density line (on the 0.45 power gradation curve) of the aggregate; however, in addition to being silly, it doesn’t even fall on the actual maximum density curve for the job mix formula.

Effect of RAP on VMA. With the introduction of SUPERPAVE the VMA, which used to be 13% if one was used, was increased to 14%. We were having problems in being able to make the 14% with granite aggregate, and found that we had to control this by blowing out -#200 material. On one project I used a factorial experimental design to aid in adjusting the gradation with considerable success. This allows evaluating the effect of numerous variables on mix properties. Of course saving money by reducing the VMA was not an option. With the introduction of RAP, however, the VMAs rose by as much as 2%, requiring as much as 0.5% more total asphalt (including that in the RAP).

Reducing VMA to Reduce Cost

A number of years ago I did a Gram-Schmidt orthogonalization on gradation data. I found that there were only three truly independent variables, one of which was the % -#200 material. By using three independent aggregate criteria and % asphalt as a fourth variable we should be able to determine what changes should be made in the mix to minimize the VMA within the specification criteria, thus minimizing cost. I would suggest the use of a 24 factorial design with triplicate centerpoint to find the most economical gradation. The following would be for a ½” nominal mix design. For variables I would use: 1) the % of the gradation between the ½” and the #4 screens; 2) the % of the gradation between the #4 and #30; 3) the  % -#200; and 4) the % asphalt. We have found that a Hveem compaction at the recommended compaction temperatures for a 75 gyration Superpave design give the same results as the gyratory compaction. We would suggest that this be done, therefore, with the Hveem compactor as it uses only 1/4th as much aggregate and asphalt as does the 6” gyratory design however gyratory compaction could be used. The advantage of the Hveem is we can also get as a bonus the stability. I would stipulate that one of the boundary limits would be that no gradation point should be above a line on the gradation curve (0.45 power graph) from the % passing through the first sieve that retains aggregate (1/2”) to the % passing of the #200 sieve. This would provide the information needed to minimize the VMA within the specification. The results could provide the starting gradation and asphalt needed for a gyratory design.

Decreasing the VMA from 16.5 to 14.5% for 100,000 tons of mix would save $ 250,000 of $500/ ton asphalt.

Petroleum Sciences, Inc. has the equipment and mathematical knowledge (as there is considerable mathematics involved) to provide a service should a contractor wish to reduce costs. We can set up the experiment to be done in the contractors own facility and then evaluate the results or do the complete project in our facilities.

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

 

 

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

GUARANTEEING UNPRODUCTIVE RESEARCH ON RUTTING

Expecting Binder Research to Solve the Problem

In a previous article “Fundamental Causes of Cracking, Potholes, Raveling, and Rutting in Asphalt Pavements” I touched on some of the causes of rutting. I wish to expand on this subject. However I wish to exclude rutting from studded tires as that problem has not been solved at this time.
Prior to the establishment of the Strategic Highway Research Program (SHRP) I attended a meeting in which it was stated that the goal was to develop an asphalt that could solve all of the problems that occur in pavements. The philosophy that problems reside primarily with the asphalt is still deeply encountered, however, in my opinion it is all “vanity and blowing into the wind”. That does not mean that there isn’t a place for asphalt research because great strides have been made in resolving pavement problems with modified asphalts. In fact, with rutting, I am sure many will state that with such and such binder, the rut tester shows an improvement in rut resistance. And I am sure that their data is correct. However I would suggest that those modifications only affect the rate at which rutting occurs not the basic cause. The misconception is that it is the properties of the asphalt that allows rutting. That is false. A properly performing asphalt is a liquid and is purposely designed to not resist rutting or any other stress that might prevent it from flowing. In fact one of the solutions to low temperature cracking is to modify the asphalt so that it can flow to relax thermal stresses before they reach the point where the asphalt fractures.

If one wishes a life time research project on rutting, concentrate only on the binder and work only with oversanded aggregate gradations. Do I mean that the aggregate gradation is part of the problem? Yes. In fact the aggregate gradation is the problem; and the present gradation specifications specifically allow oversanded mixes, thus, allowing the construction of tender and rut prone pavements to be built. I learned this from Vaughn Marker and Went Lovering of the Asphalt Institute back when I had more hair, and it was not so grey. (Went Lovering also had worked for CALTRANS and was a great source of knowledge and wisdom. He was instrumental in the development of the Hveem Design.)

How can we get oversanded mixes. First draw a straight line on the 0.45 power gradation chart from the % passing of first sieve that retains aggregate to that of the – #200. If you want an oversanded mix, make sure that the -# 4 gradation is above that line. If it is below the line, you can still meet that goal of an oversanded rut prone mix if the gradation in the -#30 range goes above that line. It is true that messing with the ability of the binder to flow will help reducing the rate of rutting, but, of course, non-load associated cracking is associated with the lack of ability for the binder to relax thermal stresses. In this manner research can be continually funded so that one can be an expert on how to not to stop rutting and tenderness.

You do want an oversanded mix for hydraulic structures, however.

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

IS THE SUPERPAVE MIX DESIGN COST EFFECTIVE?

Variables Involved

The introduction of the Superpave Mix Design for asphalt pavements was herald as one of the greatest improvements of the 20th century by some. The effort in developing the new design parameters was scattered among some of the most skilled asphalt technologists in the world.

I was involved in the SHRP program, and have experience in actually using the Marshall, Hveem and Superpave (Gyratory) mix designs for use by various agencies in doing so have reach the conclusion that perhaps we have not gained anything with the Superpave design and perhaps the benefits do not outweigh the deficiencies. In this blog I wish to outline the variables involved and expand on them in later blogs.

It should be mentioned that many people concentrate on the properties of the asphalt as being the determining factors defining performance. While the asphalt properties, especially the low temperature ones, are indeed important, the properties of the aggregate is vital, especially its gradation and its ability to resist the stripping of the asphalt off of the aggregate by liquid and gaseous water. A case can be made that the asphalt properties are over-defined for a well made pavement, but that is for other blogs.

Benefits

Mold Size. One of the benefits for some of the mix designs with large sized aggregate with mixes on the coarse side is the 6” mold. Smaller molds can result in over-estimating the asphalt requirement caused by bridging of the aggregate. This can especially be true with the 4” Marshall Design; however with the use of the 6” Marshall design, this problem has been overcome.

Record of Continuous Compaction History. In problem solving the record of the compaction by gyrations allows one to obtain much better information of how things are compacting, and to predict how other variables will affect the compaction.

Disadvantages

Too “Academicized”. It seems to me that if the procedure would have been turned over to state technicians to proof test, the procedure would have been better.

VMA Requirement too High. The VMA requirement was taken out of the Marshall design procedure. For a nominal ½” mix that is 14%. The old requirement for VMA from the FHWA Hveem specifications was 13%, and many successful pavements have been made in that range. As a result with certain mixes it is needlessly necessary to blow out fines just to meet the VMA requirements. (In my opinion neither the VMA or voids filled are the best specification parameters. I prefer effective asphalt and film thickness but would report the VMA for information purposes.)

Gradations Allow for Tender and Rut Prone Mixes. It amazes me that the effort to control rutting is to concentrate on the properties of the asphalt and neglect the true controlling factor: the gradation. The properties of the asphalt can affect the rate of rutting, however the gradation controls the extent of rutting. No one seemed to have listened to Dick Davis (Retired from Koppers) at meetings where he explained the importance of letting the aggregate carry the load. Both my son (Dr. Michael R. Dunning) and I know how to set the gradation to greatly reduce rutting (except from studded tires) and tenderness.

Lack of Understanding of the Gradation in the – #30 + #100 Range. Some mixes need a fine sand to be added as a filler, otherwise asphalt will have to be used to fill the void.

Lack of a Measurement of Strength. I frankly don’t understand leaving out a strength measurement. Adding an indirect tensile strength would have been easy.

Insufficient Emphasis on Water Damage. Many aggregates prefer to be wetted by water than by asphalt. Even though the rocks may be coated with asphalt, if there is any break in the film, water will get inside and lift the asphalt off. Even water vapor can cause damage and at one time was measured by the Moisture Vapor Susceptibility Test.  For protection, amines and lime has been used, however there are data that suggest that the protection may be transitory, the mechanism for which will be described in a later blog. There is a new product that overcomes that problem.

Another problem can be that the test criteria for measuring stripping are not severe enough.

The research done by the SHRP projects was very valuable, however some have found that the Marshall and Hveem designs have features not found with the more costly Gyratory designs. The purpose of this blog is to suggest areas of concern from one who has practical experience.

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