Is pavement Quality of any Value

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

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

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

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

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

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

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


Asphalt 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.


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.


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,,



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.,




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.


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.


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.


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.


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., blog 


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 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.


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.


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,,


Fundamentals of Non-Load Associated Cracking

There has been considerable research on the engineering basis of pavement cracking. Those interested in some of the basic studies on cracking might consult volume 41 (1972) of the Proc. Association of Asphalt Paving Technologists. Many of the concepts develop there were the basis of the PG grading system with regard to low temperature properties of asphalt. While those papers are 40 years old, they lay the basis of technical progress in understanding cracking. Later studies have been oriented toward understand how cracking can be predicted.

However, it is not the purpose of this blog to go into the engineering of pavement design but rather to speak of the basic physics involved.

Failure occurs either from tensile stress or crack propagation. The maximum tensile strength of asphalt and hot mix is about 1000 psi and that only happens if the asphalt is cold or stressed at a high rate. At higher temperatures or lower rates of strain the stress at failure would be less. When cracks appear, the stresses are concentrated at the apex of the crack accelerating the formation of a crack. Thus no matter what the crack might look like, it is caused by too much tensile stress.

Literature suggests that when the temperature drops down below about 100-110°F of the softening point of the asphalt in pavement one would expect damage to the pavement. That damage accumulates eventually resulting in transverse cracks showing up. The distance between cracks is related to hardness of the asphalt. If the temperature rapidly drops to, perhaps, 150° F below the softening point, the crack may occur that day. I actually observed that in the late ‘80s. There had been a very sharp drop in temperature in Spokane, Washington on one day. I was called in for several cases where even fairly new pavements showed block (traverse) cracking, including a new tennis court. The only answer was that the temperature drop had caused it.

If we recognize that the softening point of aged asphalt might approach 200° F it can be seen that the fast drop in temperature in deserts at night could even cause damage at surprisingly higher temperatures. Pavements can reach over 170° F in the deserts.

The effect of crack propagation can be seen in parking lots where asphalt pavement is adjacent to a portland cement area where there are 90° corners. A crack will be seen radiating out of the corner even if there is no other evidence of cracking in the pavement. If small cracks are formed inside a pavement and don’t heal themselves, they will grow and eventually show up.

When cracking occurs, the asphalt in a pavement is no longer performing as a liquid, but more as a solid. It responding to stress from cooling by pulling apart horizontally. When the pavement heats up again, the crack remains, although if they are small, traffic can knead them back together. If it can act as a liquid it flows vertically upward as the temperature increases and downward as the temperature decreases. The solution to cracking is to allow the asphalt to retain its liquid properties as long as possible.

As asphalts from different crude sources behave differently, there is no golden rule. Non-electrolytic solution chemistry can be involved but that is a discussion for another day.

One of the remedies for reducing the temperature related cracking in pavements is to seal them so that the rate of hardening of the asphalt is reduced. Also the HMA needs to be protected from water, both liquid and vapors. Even in the desert water accumulates under the pavement. If the bond between the asphalt and aggregate is susceptible to being compromised by the presence of water, the bond will be broken and failure will occur. Traffic accelerates the loss of strength as water propelled by changing pore pressure scours the asphalt off of the pavement. Even water vapor has been seen to do this. Weakening the bond between rock and asphalt will then be allowed to grow under less stress.

I also like to see primes used under the pavement to discourage water from entering the mix. Reducing the rate of hardening of the asphalt so it retains its liquid properties and protecting the pavement from water damage can reduce the rate of formation of non-load associated cracks.

Robert L. Dunning,,


Tort or Penalties Called for, or the Result of Natural Aging

Once a pavement is laid it is expected to last a long time. However over time distress will occur at which time there can be a blame game, especially if tort lawyers get involved. Unfortunately the attorneys may team up with “experts” who have only limited understanding of pavement technology and absolutely no understanding of multivariate statistics upon which are based possible penalties where data collected during construction would suggest there would be future distress.

In a construction contract there are various contractors involved. One contractor may prepare the subgrade and base while another would lay the pavement. The design engineer may not have built in sufficient strength into the pavement or paid attention to the properties of the paving material with respect to the expected traffic. Also, whether the location is in a city or on a highway will affect judgments. Following are a few types of distress:

Residential Streets.

  1. There is a sunken “bird bath” in the street. There were separate contractors for the subgrade, base and pavement. There were alligator cracking in the sunken area. The thickness of the pavement is the design thickness. Who’s at fault and what can be done? Usually the paving contractor will be blamed; however the actual problem is an area in the subgrade that was not properly compacted. The pavement has to follow the consolidation that occurs in the subgrade thus the cracking comes from stretching the pavement as it sinks and is not the fault of the paving contractor. To repair it the section may be removed and reconstructed. If the pavement is to be slurry sealed, leveling can be done by the slurry seal contractor.
  2.  There is loss of matrix, called raveling, in the pavement in areas of continuing flowing water. Wet asphalt pavements are weaker than dry ones. This usually occurs on corners where there is shear stress from the tires. The source of the water needs to be identified and stopped. There are asphalt paving mixes designed for hydraulic structures, however a pavement in a street is not one of them. Another cause is the lack of use of additives to the asphalt that address the loss of wet strength. A discussion of such additives is outside of my goal at this time however it will be addressed later.



Main Streets and Highways

  1. Longitudinal Crack in the Wheel Path. This usually starts in the right wheel path and later occurs in the left wheel path. This is caused by lack of structural strength. It can be accelerated by lack of proper compaction or lack of anti stripping additives in the asphalt. It isn’t unusual for an agency to mill off two inches and repave. That is like blowing in the wind, as the lack of structural strength continues, and the cracks will soon reappear. The lack of structural strength and stripping are design problems, not construction.
  2. Damaged from Studded Tires. In areas in which studded tires are used there will be ruts, whether the pavement in asphalt concrete or portland cement concrete. The width between ruts will be consistent to that of passenger tires. That problem still lacks a solution. Some say that such rutting is caused by trucks, but that is not true for ruts described above. Rutting caused by trucks is a mix design problem that is solvable. In that case the width between ruts is that of truck tires, and the effect of the duals is obvious. This is not caused by poor construction techniques.
  3. Traffic Cause Ruts. There is a considerable effort to solve rutting by changing the binder. Adjusting the binder can affect the rate of rutting; however the true solution to rutting is to make sure that the coarse aggregate particles can interlock. Yet the specifications generally allow over-sanded mixes in which a sand asphalt matrix is supposed to stop rutting. I would suggest that both the design engineer and the supplier of the (hot mixed asphalt) HMA are equally at fault. The design engineer specified gradation limits that allow oversanded mixes, and the HMA supplier crushed and blended to a gradation that would cause ruts when in the pavement.
  4. Block (Thermal) Cracking. Often called transverse cracking, however it really occurs in blocks if the pavement is wide enough. This cracking occurs when the asphalt in the pavement is too hard to relax thermal stress fast enough. Public agencies or other owners are at fault for not sealing the pavements, which reduces the hardening rate of the asphalt.
  5. Water Damage. If the aggregate would prefer being wetted by water rather than asphalt, the asphalt will at least get weak, and probably strip off. The result is raveling. While there are additives to asphalt that helps in this area, there are data that shows that some of the more popular antistrips may lose their ability to prevent stripping over time. Once the pavement loses it strength, fatigue cracking may also be prevalent. The fault here is the design specification that does not specify proper antistrips.

Robert L. Dunning,,