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



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




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. www.petroleumsciences.com, blog asphaltwaterproofing.wordpress.com 



Raveling is the loss of the mastic matrix in the surface of a pavement. This would be expected to occur with time but is aggravated by the presence of water. If the aggregate surface is not protected from water, traffic will cause raveling. This can be seen near curbs where often water is flowing. The asphalt is not pulled off but is floated off.

Water, a blessing and a bane! To get compaction in a subgrade or base, the water content must be at an option. A blessing. Even with cold recycling systems, the total liquid content (including water) must be optimum to get proper compaction. A blessing again. Obviously we love water, especially on a hot day. If we had a choice of a cool glass of water or a glass of warm lard, we would obviously choose water. Most aggregates are no different. If they have a choice, they would choose water to imbibe into their pores, not asphalt.

Our production system forcibly removes water from aggregate and equally forcibly makes the aggregate accept asphalt. That doesn’t make the aggregate happy and if it has the chance it will invite water back in through any defect in the coating and gleefully kick the asphalt off. Many aggregates have hydroxyl groups sticking out on the surface, which attracts water. In addition there may be water loving sodium and potassium exchangeable ions on the surfaces. These ions are the result of defects in the silicate and aluminate structure in the aggregate. In the silicate structure there may be an aluminum atom instead of silicon resulting in a structural negative charge. Likewise a magnesium atom may replace aluminum in the aluminate structure.

To combat this, amines or lime can be added to change the nature of the aggregate surface. Unfortunately, the protection may not last, especially if salt or magnesium chloride is used for deicing. The chemical principle of mass action can reverse the action of these antistrips. One solution has been to graph onto the aggregate surface an organosilicon material that actually becomes an integral part of the aggregate and thus cannot be dislodged. The aggregate then changes allegiance so strongly that it actually forcibly rejects water and opens its pores to the asphalt.

So to control raveling, the adverse affect of water must be controlled. This is especially important with raveling as it occurs on the surface where the pavement will be often in contact with water. The best solution is to persuade the aggregates to distain the advances of their first love and turn to a new one that is not so transparent.

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


Variables Involved

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

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

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


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

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


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

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

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

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

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

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

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

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

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


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, www.petroleumsciences.com,chemistdunning@gmail.com