PAVEMENT DISTRESS. WHO’S TO BLAME

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.

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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

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LOW COST ROADS

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