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


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.