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.

QUALITY CONTROL II

Students “t”

The purpose of this and previous blogs is to show that statistics is basically linear algebra and intrinsically simple. Thus in previous posts I showed that data gathered can be simply expressed by lines, the lengths of which represent the mean and the standard deviation of the data respectively.  It was also established that the two lines are independent of one another. If we actually knew that they absolutely represented the true values of each, we would be through. However, that is not the case. The mean is always sort of “fuzzy” in that we can’t be sure that what we measured is the true value. Measuring uncertainty is where it gets complicated. There is usually uncertainty with the standard deviation, but not always.  Data sources from facilities that routinely manufacture a product may have sufficient data on the standard deviations to be able to assume their data represents the true value.

True Value of the Standard Deviation is Known.

 In this case the normal distribution is all that is needed to evaluate the uncertainty of the mean.

 

True Value of the Standard Deviation is Not Known

Usually standard deviations are also fuzzy thus both the mean and the standard deviations can be considered to be random variables. While the mean is normally distributed, the square of the standard deviation (variance) is distributed according to the chi squared distribution. (The chi squared distribution with one degree of freedom is the square of the normal distribution.) However, the distribution that we want is that of the mean divided by the standard deviation, both of which being random variable.

Derivation of the t” Distribution

 The complication is what we need now is the distribution of the normal distribution divided by the square root of the chi square distribution. The equation for that is:

 f(z) = Integral |x|f(x)f(zx)dx

where zx is the normal distribution, x is the square root of the chi square distribution and z is the “t”  distribution.

 The calculation of the derivation of the “t” distribution may be found in Statistical Inference, Vijay K. Rohatgi, John Wiley & Sons, 1984.

So we see that while the basic concepts of statistics is simple, the problem of uncertainty is complex.

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 QUALITY CONTROL

Means and Standard Deviations as Lengths

When we talk about quality control we hear about distributions, such as the poisson, hypergeometric, binomial, normal, “t”, chi-squared and “F”. How complicated! And we are told to worry about things being independent, are inundated with words like variance, mean, median, mode, standard deviation, whether the standard deviation is homo or hetroscedastic (whether the standard deviation is constant or not), confidence limits, and such things as Type I error, Type II error, null hypothesis etc. It cannot be denied that all of these have their place. However, to get to the basics, all we are really trying to do is measure lengths. Statistics is really simply analytical geometry or linear algebra, depending on one’s outlook. Let’s look at the mean and standard deviation.

Mean (one type of average). We are told that it is the first moment around the origin.

Mathematically it is the integral of xf(x)dx between some limits where f(x) is some distribution  function. Yet it is still length.

Consider a set of “n” data points, X= (x1, x2, —, xn). Then visualize a graph of n dimensions with a single location, X, representing those data. Also visualize a line in that n dimensional space that is equidistant from each axis, i.e. It goes through (1,1,—–,1) etc. Drop a line perpendicular from X to that equidistant line. Call that point M=(µ, µ,—-, µ).  Divide every point by the square root of n, the number of data points to introduce the number of tests into our considerations.

The line (δ ) from the X to M would be the vector (x1– µ, x2– µ, —, xn– µ) while the line (µ) from the origin to M would be the vector (µ, µ,—-, µ). Since the two lines are perpendicular, their scalar (or inner or dot) product would be zero:

((µ, µ,—-, µ))·((x1– µ, x2– µ,—, xn– µ)/ )= 0

x1, + x2, +—-,+ xn – nµ = 0

µ= (x1, + x2, +—-, + xn)/n, which is identical to the form for the mean.

That is, the length of the line µ from the origin to M is equal in value to the mean of the data points.

Standard Deviation. The length of the line, δ, from X to M is the square root of (1/n)*((x1)2+ (x2)2+—-,+ (xn)2 – nµ2). (1/n)*(x1)2+ (x2)2+—-,+ (xn)2 is the square of the length of the line from the origin to the data, X,  while (1/n)*(nµ2) is the square of the length from the origin to the point of M.

δ = ((1/n)*((x1)2+ (x2)2+—-, + (xn)2 -nµ2))0.5

Thus the equation of the length of the line δ is identically to one of the equations used for calculating standard deviations (where the standard deviation is not a random variable. If the sample standard deviation (s) is a random variable, 1/n would be replaced with 1/(n-1)).

Rulers. To measure lengths we need a ruler. We use miles in the United States, in Canada they use kilometers while in Russia, the Verst may be used. In statistics the ruler used is the length, “δ”, if the standard deviation is known or, “s” if the standard deviation is a random variable.

The many terms mentioned above and the sophistication of the mathematics are important in establishing the reliability of the data, still, basically we are only measuring lengths.

EFFECT OF WATER DAMAGE ON CRACKING

Chemistry of Stripping

 

We talk about things being as solid and eternal as a rock. But how durable are rocks? Especially with the onslaught of water and carbon dioxide? As our roads are rocks mixed with a cement, either portland cement or asphalt cement, this is an important issue. In this blog I am discussing asphalt concrete, i.e., roads, particularly those with rocks made of granite and basalt.

For clarity, let me describe an almost marriage ending disaster from working with bentonite, which has a composition not that different from granite or basalt. I needed about a pound of sodium bentonite but had to buy 50 pounds. I had just started my business and was working in my garage. What to do with the excess 49 pounds? Well, spread in my wife’s garden of course. Bentonite is a mucky clay, which turned her garden in a field of muck. Fortunately I knew that adding lime would turn the sodium bentonite into calcium bentonite which is friable, eliminating the muck. Bentonite consists of platelets of an aluminate layer sandwiched between two silicate layers. Within the crystal structures of the aluminates and the silicates are other atoms such as potassium, sodium, calcium, magnesium, iron etc. These impurities leave “holes” in the crystal structures that carry a negative charge, which must be neutralized with what are called exchangeable ions, which is what saved my marriage. Bentonite doesn’t care what is on the outside as long as it is positive! Calcium from lime is positive.

Clays are the weathering product of rocks.

The challenge with asphalt pavements is to keep the asphalt stuck to the rocks to prevent loss of strength in the pavement.  That loss of strength can come from absorption of water by the asphalt (very rare) or the asphalt becoming unglued from the rocks. As some rocks like water much better than they like asphalt, this is a challenge.

Like bentonite clay, there should be exchangeable ions on the surface of the aggregate; ions that really, really like water. There are products, however, that can stop water sensitivity of the asphalt and which can also make the rocks like asphalt.

Sticking Asphalt to Rocks

What happens at a surface of a rock when water and oil (asphalt) is very complex. I shan’t dwell on the chemistry, much of which is discussed in papers on drilling of oil. It essentially depends on the energies. There are several forces that can come to bear. The weakest are called van der Waals bonds. These bonds are from the natural cohesive forces of molecules causing them to pack closely together.

Wetting of a surface is the result of adhesive and cohesive forces involved, and the energies involved.

The next binding forces are ionic, that is, positive molecules attracted to negative molecules. Although these binding energies can be very high, in solution these ions are mobile, and can be exchanged if they are on the surface of a rock.

A third bond is called covalent, in which atoms share electrons. The bonds that hold rocks together are covalent.

The loss of the bond between asphalt and rocks is called stripping.

There are several materials available to help the asphalt stick to the aggregate with aggregates that have a stripping problem.

Amines. Some of the common antistrips are based upon amines. If the problem is the result of the asphalt, the amines would react with any organic acids, neutralizing the problem. They also would replace sodium and potassium ions on the rock, thus providing resistance to stripping. There are data, however suggesting that that resistance could be lost over time, especially in the presence of salt or magnesium chloride. That replacement might occur from what is called mass action in chemistry.

Lime. Lime also provides stripping resistance, and also can react with the aggregate. There are data suggesting that the ability of the lime to provide protection can diminish with time, however it has generally performed well.

Latex Adding a polymer latex to the aggregate prior to entering the dryer and adding the asphalt has performed well.

Organosilicate. A fourth approach is to bond an organosilicon molecule that is un-wetable directly to the rock with a covalent bond that is as strong as the rock itself. That type of antistrip has performed well even in the presence of salt.

If the HMA cannot be protected from water damage, no other mix property has meaning. With traffic, water damaged pavement comes apart.

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

 

SOME BASIC CAUSES OF PAVEMENT FAILURE

Basics

INTRODUCTION

There are certain basics with respect to pavement failure that have existed since the first pavements were laid. Pavements crack, pavements slip, water damages them, and pavements rut. Irrespective of the tests used to evaluate pavements, failures have the same basic causes.

CRACKING

No matter where the cracking occurs, it is caused by the inability of the asphalt to relax the stresses, and must rupture.

Fatigue Cracking. Stress and strain are what are called tensors, which means that a pavement can be under compression and tension at the same time, but in different directions. While a tire compresses a pavement downward, it forms a deflection basin which causes the pavement to go into tension in both horizontal directions. Many years ago we used data from deflection testing and, assuming a parabola, did a line integral to calculate strain. If the pavement is not strong enough, the asphalt is stretched too far, separates and a crack forms in the wheel track. Also a crack may form between the wheel tracks.

Longitudinal Cracking on Joints. The joint between two passes are especially week. Inside any one pass of the paver, some aggregate willbe on both sides of any plane or slice inside of the pavement. In fact, when sample undergoes an indirect tensile test such as is done in stripping tests, rocks actually fracture. A joint, however, is held together only by the asphalt layer, which has a tensile strength of about 200-1000 psi, depending on the temperature and shear rate. If the asphalt in the mix can flow vertically in response to thermal stresses, the crack won’t form. However, if the stresses exceed that at the joint, a crack forms. As a result the pavement on either side of the crack can shrink or expand independently. Often what happens then is that the pavement sections shrink away from each other in the cold, but do not expand completely back together in the heat. For that reason it is crucial to follow proper technology of forming a joint.

Thermal Cracking.  The mechanism of formation of thermal or non-load associated cracks is again the lack of the asphalt to be able to relieve thermal stresses by flowing vertically up when the pavement is hot and vertically down when the pavement is cold.

PAVEMENT SLIPPAGE

From time to time the pavement will shift. In one project I has on at the LAX airport, a 2” lift was slipping on a 4” lift from landing of air traffic. A core was made of the section so it waw possible to observe a daily slippage. Two sources of the problem. First, it was supposed to be 4” over 2”. Secondly, if there was a tack coat, it had been ruined as a result of a dust storm. To prevent slippage a prime needs to be used between the base and pavement, and a tack coat between two lifts.

RUTTING

There are two causes of rutting, improper aggregate gradation and studded tires.

Gradation.  Asphalt itself is too weak to stopthe flow of the mix by itself. If the coarse aggregate in the mix cannot interlock themix has to rely on a mastic composed of the fines and asphalt, which cannot carry the load. The solution is a coarse gradation with no humps in the fine mastic area.

Studded Tires. Research is under way on how to solve this problem. Harder aggregate has helped, but no solution is available now.

WATER DAMAGE

If the pavement is not protected from water damage, all of the above is blowing in the wind. There are data that suggest that even pavement protected by amine or lime antistrips will lose much of its strength thus cannot complete its design life. Many aggregates are wetted by water better than asphalt so that if the surface cannot be permanently altered to prefer wetting by asphalt, eventually water will replace the asphalt.

Robert L. Dunning. www.petroleumsciences.com, blog asphaltwaterproofing.wordpress.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

MEETS SPECIFICATION BUT FAILS TO WORKS AS WELL AS BEFORE

Asphalt Construction Nightmares

You have a project going well when all if the sudden things change. How can this happen? You are buying the asphalt from the same source, it meets specification, but it just doesn’t work well. What could have happened?

There are various factors that can address that problem:

Change of Crude Oil Source. First, every crude produces its own distinct asphalt. York and Halstead characterized a multitude of asphalts in the late 1950s and Rostler and White produced a set of punch cards as “finger prints” of the various asphalt sources. References available by request. I have those cards. I mention this as a refinery may switch crudes. The asphalts would still meet specifications, but may behave entirely different. In addition, many asphalts are being modified with polymers (rubber) in different ways with each system having their own distinct set of properties. Some examples.

Asphalt Susceptible Water. In the middle 1970s a contractor was having a problem with stripping which even 1% antistrip would not help. The mixes when soaked turned brown which indicated that water was being absorbed. It turned out that the crude from which the asphalt was produced had been treated with caustic soda which made the naphthenic acids into soaps. As asphalts from caustic treated crudes made lousy emulsions, they also had non-caustic treated crude in stock. Switching to the non-treated crude solved the problem. Asphalt from the treated asphalt caused other problems so the refiner switched to lime treating of the crude. That greatly improved the asphalt making it resistant to stripping.

Changing Grading System. When I first got involved in asphalt technology (1959), asphalts were graded by penetration at 77°F. (How far a needle would penetrate the asphalt in dmm.) The grade mainly used was 85/100. Later the grading system was changed to viscosity at 140° F after an aging test to mimic the effect of mixing on the asphalt consistency. The contractors were told that the grade AR 4000 would replace the 85/100 grade, implying that that no changes in practice needed to be made. However in Southern California the contractors claimed that the AR 4000 asphalt didn’t work the same. I was at a meeting with the contractors, oil companies and the asphalt institute, the latter two of whom stated flatly that the asphalts were the same. Period. In a broad sense they were telling the truth. Averaging all of the available asphalts, the AR 4000 was the same as an 85/100. However with the asphalts in California, the range of penetrations for AR 4000 from the various crudes varied from 40/50 to 120/150. In Southern California a AR 4000 was actually a 60/70 penetration grade. Once the contractors were finally told that, they were fine with it because they also would have known how to handle the 60/70 asphalt if someone would have told them that was what it was. In the roofing industry some specifications actually specify the crude source. It might be well for a contractor to get a guarantee that the crude source will not be changed.

Product Trading among Oil companies. Oil companies trade products, primarily to reduce shipping costs. One example was one company trading California Coastal crude to another for asphalt emulsion in the northwest. But they can also trade asphalts if there are supply problems at the refinery.

Replacing an Approved Product with Another during Construction. Another thing that can happen is that the supplier submits one material for approval then switches the ingredients to lower the cost. The product may still meet specifications, however will not perform that well in the field.

If retains are kept during construction and there is a costly problem, the asphalts could be analyzed. One method is the Rostler analysis ASTM D 2006, (removed) and the Clay Gell procedure ASTM 2007. (We only do the Rostler procedure because the solvents from this method are easier to handle.) There are other new unpublished procedures that can be used including Fourier transform infrared analysis.

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

THE IMPORTANCE OF GRADATION IN ASPHALT PAVEMENTS

Superpave Myths

There is a lot of attention of binder properties on the performance of asphalt pavements. Various types of polymer are added to asphalt in the drive to improve properties.
It is easy to forget about gradation in evaluating a new additive to, say, reduce tenderness and rutting. However the prime reason that polymers are added to asphalt is to change the temperature susceptibility so that the binder appears to be more viscous at the higher temperatures than neat asphalt and softer at lower temperatures than neat asphalt. These additives may make have other benefits, however gradation is extremely important. Here are some Superpave Myths:

  • Rutting is solved by asphalt modification
  • A Maximum Density Line drawn from 100% passing to Zero has value
  • There is a “forbidden area” that the gradation curve must avoid
  • Gradations that go above the “forbidden area” are good as those that go below

Rutting

Rutting is a gradation problem, not a binder problem. While modifying the binder with additives may show delayed rutting in the tests, the only sure solution is to assure that the coarse aggregate carries the load. There are forest roads that are open graded mixes bound together with a CMS-2s asphalt emulsion (an emulsified cutback) that perform very well. While modifiers may slow the rutting down, the only sure thing is the strength of interlocking rocks. If asphalt is modified so that it cannot stress relieve by flowing, non-load associated cracking will surely occur.

Maximum Density Line

Another myth, in my opinion after decades of providing mix designs, is that a “maximum density” line going from the 100% passing point to zero has value. It does not. Let’s call this the false maximum density line (FMDL) as it has no relationship to the gradation in the mix designs and is thusly of no value.

Forbidden Area

The myth of the so-called “forbidden area” of the gradation is essentially silly and does not provide any benefit. It was based upon an FMDL which has no resemblance to the reality of the gradations in mix designs. It was an academic after-thought that ended up in the specifications. Aggregates with a high rugosity may very well have gradations that could go through that area yet perform very well.

Specifying Gradations above the FMDL

Such gradations will have a hump in them. That is an invitation for rutting since the large aggregate cannot interlock to provide strength. The load has to be carried by a sand mastic.

The above myths may have a nice academic feel, but are useless in solving problems in the field.

Mixes that are Strong and Compact Easily

The tool we have found to be beneficial for strong easily compacted mixes is based upon gradations in which the coarse aggregate interlocks and carries the load. We first draw a straight line from the sieve size that first retains aggregate to the -#200 on the 0.45 power graph. For this discussion, let’s call that the true maximum density line (TMDL). That line should be the specification maximum, i.e., under no circumstance should the gradation plot go over that line. The slope of the line drawn from the sieve that first retains aggregate to the #4 should be greater (coarser) than that of the TMDL. It should not be too coarse, however, in order to avoid segregation problems during construction.  Ideally the slope from the – #4 to the – #200 should be close to a straight line, with a slope less that the TMDL.

If the gradation in about the # 30 sieve goes over the TMDL, the mix will be tender and tend to rut. We call such mixes “over sanded”. There was a pavement a number of years ago that was rutting during construction. By using the above principals, not only was the rutting stopped, when the principals were place into specifications, the rutting specification for the agency could be lowered below the national norm. With certain non-modified asphalts, compaction is nearly impossible with over sanded mixes.

Another problem area in the compaction curve is below the # 30 and above the #200.If there is an overabundance of some material and lack of other, there will be a “hole” in the gradation which needs to be filled, usually with a fine sand. Without the sand as a filler, additional asphalt has to be used resulting in compactions problems.

I did a Gram-Schmidt orthogonalization of gradation data and found that only about 3-4 sieve sizes were truly independent thus having only a few sieve sizes in the specification is wise, especially when statistical specifications are used. However it is also wise for those who do mix designs to have used as many sieve sizes in the gradations as practical so that problems in the -#30 to -#200 region of the gradation can be identified and fixed.

 Criteria for a Rut Resistant, Easily Compacted Economical Hot Mix

First, select a gradation that meets the TMDL guide lines. Within these guidelines for, say, a ½” nominal mix, adjust the gradation and -#200 material so that the effective asphalt content is between 4-5 %, the film thickness is 7-10 microns and the VMA is no higher than 0.5% above specifications. For every 1% the VMA is above the specifications about 0.4 % additional asphalt is required. (For the nominal ½” mix, the VMA specification is higher than it should be at 14%. Hveem mixes for this mix specified by the FHWA was 13% for decades and there is no technical reason for it to be higher.)

For more information or help, contact Robert L. Dunning at chemistdunning@gmail.com  or www.petroleumsciences.com.