Chip Seals

The application of a seal coat has a number of functions however one of the most important is to waterproof the pavements, protecting them from water damage and oxidation. If pavements were sealed early in their life, e.g. within a year, the pavements would last a lot longer. Chips seals are used especial on highways.

Chip Seal Emulsion. The emulsified asphalt used for chip seals are specially designed to break very fast on contact with aggregate. Emulsions can be either anionic (basic) or cationic (acidic) although the cationic are very popular. With asphalts from some crude oils the amount of emulsifier required for anionic chip seal emulsions is very small, approaching zero as a result naphthenic acids in the asphalt which serve as emulsifiers when neutralized with caustic soda.

Special Seal Emulsion. There is a product called PASS that has the ability to re-seal cracks and regenerate pavements.

Where to Use. A chip seal does an excellent job as a seal. While it can be used in cities, in my opinion a slurry seal would be better, unless it is a Capeseal in which a slurry is placed over the chip. The disadvantage of use in cities is that the chips can spread over lawns, in driveways, etc.

Mix Design. It is very important that a mix design is done, otherwise there can be failures.

Problems. One of the causes of failure is dirty aggregate. The chip seal emulsions are designed to break immediately on a surface thus when it hits the dust it breaks on the dust and not on the surface of the aggregate. An emulsion type called High Float is more tolerant of dust. Not enough emulsion can cause loss of chips while too much emulsion can called bleeding.  Also when used in cities, loss of chips can occur at the centerline as along the centerline there can be less asphalt as a result of less overlap of the spray. For rural roads this isn’t a big problem as there is not that much turning stress on the aggregate, however in the city, there can be turning traffic out of driveways. Also, there is another important problem; it is difficult to skate on chips.

It isn’t a good career move for a director of public works to place a chip seal on streets in expensive neighborhoods, especially if chips end up on the lawns, sidewalks and driveways.

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


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


Spreadsheet for Comparing McLeod and Kearny Designs

Chip seals are widely used for maintenance of pavements. Two of the popular design methods are those developed by Dr. Norman McLeod and Mr. J. P. Kearny. I am finishing the development of a spread sheet that can be used to compare the two procedures, and that can be taken out in the field during construction so that changes can be made with regard to pavement conditions, traffic during construction, % oil in the binder or other changes that might occur.

The spreadsheet may also be used by agency engineers to establish budgets and for contractors in bidding projects.

At the present time the Kearny method on the spreadsheet does not include the modifications that Jon Epps et al. have made, but those will be added soon.

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


Using Local Materials

Roads are absolutely necessary for economies to succeed. Yet in these perilous economic times, funds are not available to build them to luxury standards. However there is technology available that allows the construction of very usable roads using materials already in-place.

Asphalt Emulsion Stabilized Bases. With soils with a plasticity index of 6 or less asphalt emulsions could be considered for base stabilization. This technology has been around for decades. I published a paper in the 1965 Proc. Asphalt Paving Technology on asphalt emulsion stabilized bases which included a mix design. We had found that one inch of stabilized base could replace about 1 ¼” of crushed aggregate base. For roads with low truck traffic a chip seal might be used as a wearing surface. A word of caution, the same care must be taken for compaction as with soils, and in calculating the maximum density; the liquid would be the sum of the emulsion and added water. There are some sophisticated emulsion formulations in which the emulsion “sets” and kicks out water, however they are not available everywhere.

Another caution. Just because an emulsion is labeled “slow set” does not mean it will necessarily mix with all in-place soils. We once were working with a particular slow set emulsion that was working quite well. On this project we first treated the soil with lime then stabilized it with a slow set emulsion. To save money, the contractor switched to another slow set emulsion, which didn’t work. In emulsion stabilization the mixed soil should be brown. In this case it came out the same color as it was before mixing, indicating that then emulsion was coagulating and balling up rather than coating.

Emulsion Based Macadams. When I was in Panama many years ago I witnessed the construction of a macadam using CRS-2 asphalt emulsion. The emulsion was manufactured by a company for whom I was doing consulting on asphalt emulsion manufacturing. A typical macadam construction technique was used. First a layer of large stone was place followed by a layer of asphalt emulsion. Following that were consecutive layers of aggregate and emulsion with each aggregate size ½ the size of the proceeding one. The last layer was sand. Since CRS-2 emulsions break as soon as it contacts the aggregate, it appeared to work in the tropics.

Lime Stabilization. For soils too plastic for using asphalt emulsions, lime stabilization might be the selection.

Cold In-Place Recycling.  Cold in-place recycling is being used in the United States especially in place of hauling new aggregate base. For low truck traffic the wearing surface could be a single or double chip seal. For heavier traffic, however, hot mix should be used.

This short piece was to suggest that there may be lower cost options for constructing roads in rural areas. For any question contact me at chemistdunninng@gmail.com in English or Spanish. (I have also had a couple of years of Russian but that was a lifetime ago, but I can read the Cyrillic alphabet. Although my knowledge of Russian has retreated to the far reaches of my brain, we do have a large Russian population here so we can accept Russian inquiries.)

Robert L. Dunning. www.petroleumsciences.com



There are a lot of risks in construction. One problem is that once something is built it is around a long time. Consider asphalt. If it is a component of heavy fuel oil, it is purchased and used in a short time. If there are problems, they are obvious, almost immediately found and taken care of. If the asphalt is used in pavements, on the other hand, it is around for years, and can be subjected to tort suites throughout its lifetime. One area where legal actions have been used extensively is with gated communities. What happens is that a team of attorneys and civil engineers contact the managers of gated communities and arrange to inspect the facilities, even after many years from construction, then sue all the contractors for one problem or another, real or imaginary. What is supposed to take place is that all of the contractors are expected to bring their insurance agent to a meeting and settle, then the attorneys and gated community share in the settlement. The attorneys can get real testy if the contractors don’t immediately capitulate.

I have had experience as a consultant on the side of a paving contractor, in which we demonstrated that one does not have to submit to such extortion.

It was very interesting with the projects with which I was involved that the engineering consultants on the attorney’s team did not even know the vocabulary of the field of asphalt technology nor were they to truly able to identify causes of stress or determine which contractor was at fault. In one case they were complaining that a product that had been applied years before had been out of specification. Unfortunately they were comparing that product to a specification of a completely different product but didn’t know enough about asphalt products to be able to recognize their error. Once I finally got that across to the attorney in a deposition, he immediately shut down the deposition.

The lesson to the contractor is to confront the attorneys with personnel who are truly experts in the field; not those with only limited academic training, but those with true experience with the field. While there can be problems that have occurred during construction that need to be addressed, problems of aging and lack of maintenance are not the contractor’s problem no matter how strong attorneys and unknowledgeable “experts” may make demands.

The lesson for the managers of gated communities is to engage someone who has real practical knowledge in the various areas of technology without the intervention of attorneys. It is very important that a maintenance program includes sealing the pavements to prevent oxidation and water damage which can greatly affect the life of a pavement.

In other areas of construction damage can be avoided. As an example, waterproofing stucco walls with certain excellent products can avoid water damage and mold problems.

And finally, a lesson for those writing the specification for asphalt paving in gated communities is to be sure that the paving specification requires that the paving mix be tested for water resistance.

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


Slurry Seals

The application of a seal coat has a number of functions however one of the most important is to waterproof the pavements, protecting them from water damage and oxidation. If pavements were sealed early in their life, e.g. within a year, the pavements would last a lot longer.

Under slurry seals I am including standard Slurry Seal, Microsurfacing, the Rubberized Emulsion-Aggregate Slurry (REAS), a special seal developed to stop emission of radon gas from uranium tailing piles, and the parking lot seals.

None of the above seals will prevent crack propagation when place upon cracked pavements. However there are other seals which will be discussed in a later blog that will repair cracked pavements.

Standard Slurry Seals. Slurry seals are mixes of asphalt emulsions and a well graded aggregate that are placed over a pavement to waterproof and to replace the fines that have raveled away. They come with three gradations, Type I which is the finest, Type II, which is the one mostly used and Type III, which is the coarsest. The exact gradations are usually those specified by the International Slurry Seal Association (ISSA), however often certain agencies will adjust them. The first slurry seal that I saw was mixed in a concrete mixing truck and was made with an SS-1h asphalt emulsion. Today the emulsions that we work with are quick set, which means that in a short time after mixing with the aggregate, the mix sets, kicking out the water and allowing traffic on the slurry. They are now mixed and placed by special trucks that can carry water, emulsion, aggregate and additives. The chemistry is intriguing, but too complex to discuss here.

Microsurfacing. Microsurfacing is a more robust type of slurry which uses primarily the Type III coarse gradation however Type II will also be used. It is designed to fill in areas in which there is more severe damage. The test criteria are more challenging than for a simple slurry seal.

REAS. The Rubberized Emulsion-Aggregate Slurry was developed to be able to be able to use tire buffing in a slurry seal like surface coating. The formulation is quite complex and covered by certain patents. It is specified in the Standard Specifications for Public Works Construction (Green Book) of Southern California.

Seal to Stop Emission of Radon Gas from Uranium Tailing Piles. This was developed on a special project but has not been put into practice as far as I know. It is based upon Slurry Seal quick set technology. Radon has a half life of only about 3 ½ days. If it takes more than 30 days for radon to diffuse through the coating, it is essentially blocked. We used helium gas in our research then turned the technology over to our client who confirmed our data with radon.

Parking Lot and Drive Way Seals. These seals are designed to primarily water proof, fill in minor loss of matrix fines and, actually, be black. The compositions are proprietary but essentially are emulsion based with fine fillers. A specification for this type of seal is also in the Standard Specifications for Public Works Construction (Green Book) mentioned above, however there are other specifications available from suppliers. They do an excellent job in preserving pavements.

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