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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WARM MIX ASPHALT PATENTS

I am writing this to provide information with respect to some patents that are now being litigated at the present time by A.L.M holding company against those involved in warm mixed asphalt paving. They cover about all methods of warm mix construction except those using foam technologies, all based upon testing that was done at high shear on a modified Dynamic Shear Rheometer (DSR). The litigation can be followed by looking up “H.L.M Holding litigation” . The plaintiffs are A.L.M holdings, Ergon and Meadwestvaco. The patent numbers for six of their patents are 7,815,725;, 7,981,466; 8,138,242; 7,981,952; 7.984.166; and 7,968,627. The prime inventors are Gerald H. Reinke, La Crosse, WI (US);Gaylon L. Baumgardner, Jackson, MS; Steven L. Engber’ Onalaska’ W1 (US). The patents are assigned to A.L.M. Holding Company, Onalaska.

The patent is based upon testing asphalt with and without additives in shear at very high shear rates to the point at which the viscosity decreases and a normal stress is observed. Following is a description from patent 7,815,725:

While not intending to be bound by theory, the present invention is based, in part, on the observations that the lubricating agents and additives disclosed in this application provide a warm mix having desired visco-lubricity characteristics or properties. As used in this application the term “visco-lubricity” means a characteristic of a material that it exhibits under high rotational velocity as the gap thickness of the material being tested approaches zero. As the gap thickness is reduced and as rotational velocity is increased, the material’s viscosity begins to decrease but the normal force between the plates begins to increase. A material that has good visco-lubricity characteristics will exhibit less normal force increase than one which has poor visco-lubricity. Stated another way, the ability of the material being tested to enable the plates to easily rotate relative to each other becomes more important than the viscosity of the material being tested. An example illustrating the meaning of the term “visco-lubricity” is the observed reduced requirements for the mixing and compaction temperatures of polymer modified asphalt binders compared to conventional asphalt binders. Based on purely viscosity data, polymer modified binders should require mixing and compaction temperatures that are 20-50.degree. F. higher than those which common practice have found to be adequate. Many studies have been conducted to explain this apparent contradiction however none have proven wholly satisfactory. It is now believed that these polymer systems are creating a lubricated asphalt binder having visco-lubricity properties that provide adequate mixing to coat aggregate particles and further provide mix compaction at temperatures substantially below those predicted based on viscosity alone.

The word lubricity means slipperiness. The patent implies that the lubricity, or slipperiness, is defined by the test result obtained in their DSR. There is a problem. The normal stresses are an intrinsic property of viscoelastic materials (in the constitutive equation) and would be observable at all shear rates. (It can be observed as material climbing up the shaft of a mixer during mixing of viscoelastic material). In 1967 Puzinauskas published asphalt viscosity data (Proc. Asphalt Paving Technologist, 1967). From his data, with the equipment he was using, the highest shear stress he could reach was about 1 mPa suggesting shear failure at high shear rates. He had noticed some delamination. I mentioned to him at that time that I had observed cavitation in testing with a sliding plate viscometer with high shear stresses. The data shown in the graphs in the patent could be interpreted that the observe drop in viscosity with increased shear rate is shear failure or delamination and the creation of the normal stresses were not intrinsic to the binder but rather cause by the behavior of pieces of failed binder. If this were to be the case, the patent is not valid.

I would suggest that those interested should review the patents. One will find that there is not much in this world that the inventors don’t claim to be covered by their patents.
Robert L. Dunning 509-220-1360
chemistdunning@gmail.com

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.

BIOFUELS AND BIOASPHALT

Energy Sources vs. Construction Materials

One of the hot controversies in the United States is the effect of carbon dioxide on climate change. It ranges with such vigor that some basic chemistry is neglected. It is common for many to consider that all of the components of all types of crude oil would eventually be turned into carbon dioxide, but that is not true.  Very light crude oils will contain components which will mainly be used to produce energy, however what are called heavy crudes contain asphalt, which is a construction material not a fuel. Virtually all of the carbon in asphalt is placed on the ground as roads, on roofs, on reservoirs or for other uses that benefit mankind. (One might say that tar and feathering is not constructive, but that depends on to whom it is done). Even after asphalt grows old, it can be recycled. Indeed, there are data that suggests that pavements, when recycled properly, will not age as fast as pavement using new asphalt.

The amount of asphalt in crude oil varies from none such as with Katapa crude from Indonesia to as much as 65% for some California Costal crudes. It is interesting that certain forces demand that the Keystone XL pipeline be stopped, which would ship heavy crude (high asphalt content) but are silent when we import light crude (low to zero asphalt). The amount of carbon dioxide released into the air from the use of the products made from a barrel of imported light crude oil would considerably exceed that released from the products made from a barrel of a heavy crude brought in from Canada.

Biofuels. It is quite reasonable to replace petroleum based fuels with biofuels if a reduction in net carbon dioxide production occurs. (Unfortunately in some cases exuberances trumps chemistry. In my case, the addition of 10% ethanol in our gasoline reduces the mileage of my car by 10%.) It is quite reasonable to turn spent cooking oils into a fuel for diesel engines, and such products are on the market. Eventually other biomasses will be converted into fuels or chemical feed stocks.

Bioasphalts.  I would like to suggest that developing Bioasphalts may be more complicated. First, it important to understand what happens in a refinery. All of the products in crude oil are separated simultaneously. Gasoline, kerosene, diesel (#2 fuel oil), light lubrication oil, and a heavy “lube stock” are all distilled off at the same time. Asphalt, if present, remains on the bottom (non-distilled). A refinery tries to select a slate of crude oils that allows a balance among the product since all products come off at the same time. Each distilled product must be removed timely by pumps in which case any pump that reaches its capacity will determine the capacity of that refinery even if that is below the rated capacity.  Also, if the refinery can’t sell or store all of the products it makes, it must shut down when the storage tanks for any one product is at capacity. One use of the bottoms is as a heavy fuel, but that would be the case mainly if there is no other more profitable use at the time for asphalt. Other uses of the asphalt portion from a crude is to make coke for steel production or “cracking” the asphalt to extinction in order to turn it into lighter combustible products.

Asphalt by its nature continues as “sequestered” carbon. If Bioasphalts were arbitrarily required to replace asphalt, or specified when there is an excess of asphalt, the bottoms created in refining will have to be coked or cracked, “un-sequestering” the carbon, or the refinery will have to shut down. This can be a problem in the northern part of the country. Often Customers of the refinery may put in large asphalt storage facilities that they fill during the winter with cheaper asphalt.

On the other hand, if there is a shortage of asphalt, Bioasphalts provide the opportunity to essentially sequester bio-produced carbon rather than allowing it to be made into fuel or to decay naturally which would result in the carbon returning to the air.

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.

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.

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

 

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