Extending the Life of Pavements

Author: 
H.D. Campbell Jr., P.E.
Published in: 
October
2012

Extending the Life of Pavements
 

Each year, airports throughout the world spend significant funds to maintain, rebuild and/or add new square footage to their runways and other critical airfield pavements. Without this most critical physical infrastructure, operations are simply not possible.
 

Current and near-term constraints, economic and environmental, are shifting the emphasis toward extending the life of existing pavements - a true effort toward sustainability. Of the two main types (bituminous and Portland cement concrete), bituminous pavements are receiving the most attention regarding lifecycle extension.
 

There is something very different about the bituminous pavements constructed in the last 20 to 30

H.D. Campbell Jr., P.E. 
 

H.D. Campbell Jr., P.E., is chief executive officer of Campbell and Paris Engineers. During his 30+ years as a consulting engineer here and abroad, he has managed more than 100 major airfield development projects, with a total construction value of more than $700 million.

years. Since I am not a petroleum engineer, I cannot explain the differences in the current liquid asphalts that act as the cementing agent in bituminous pavement mixes. However, as a professional engineer who has designed and observed bituminous airport pavements for the past 40+ years, I know that current bituminous pavement mix designs are not performing as well over the long term as their predecessors.
 

The basic consensus is that advancements in the refining processes used to produce petroleum products are stripping out higher-grade petroleum materials once left in liquid asphalt. It appears that the absence of these high-quality materials makes the pavement less resistant to atmospheric oxidation, or "greenhouse gases," in the current vernacular.
 

The increased use of recycled asphalt pavement is another main factor. While this is an excellent way to reuse existing pavement, recycled materials further reduce the virgin liquid asphalt material in bituminous mixes. If a current mix design calls for an asphalt cement content of 5% and recycled bituminous pavement is used, the percentage of the higher-quality virgin liquid asphalt may be as little as 3%.
 

Despite such changes, current liquid asphalt must still meet required ASTM performance grade ranges. The question is: Where in the ranges do the current bituminous products fall, and how do the range variations affect the long-term performance of the mix designs? Some also question the validity of current standards. The original ASTM tests were performed on materials that included higher-grade distillates. Are the metrics of the original tests valid for current products?
 

Regardless of exactly what is decreasing the longevity of bituminous pavements, the industry has come up with ways to extend their lifecycle. While FAA advisory circulars on the matter are written for pavement designs that last for 20 years, most industry experts agree that bituminous airport pavements perform eight to 12 years, at best, before needing major remediation.
 

Rejuvenators have consequently become popular. These products, mostly proprietary in nature, are akin to a solvent-based stain. Many rejuvenators contain coal tar oils suspended in liquid solvents that are sprayed onto the pavement. The solvents soak into the bituminous pavements, soften the mix and allow the coal tars to penetrate. Once the solvents oxidize or "flash off," the rejuvenated pavement is resistant to fuel spills and accelerated oxidation from atmospheric/greenhouse gases. Since rejuvenators do not appreciably affect the frictional component of the surface, they are used on runways.
 

Coal tar emulsion sealcoats are a prevalent option for taxiways and aprons. Like rejuvenators, they are sprayed onto the pavement. After the water evaporates, coal tar, latex and fine sand remain on the surface, but add no appreciable thickness. They are also fuel-resistant and may reduce oxidation to a minor extent. Because coal tar emulsions contain fine sand, which reduces friction, they are not used on runways.
 

Thermoplastic coal tar slurries are another standard longevity extender. These sealcoats are a combination of coal tar emulsions suspended in polymer additives, water and an appreciable volume of aggregate. Because of the aggregate, these products are applied with a spreader box. After the water evaporates, an abrasive surface about 1/8- to 3/16-inch thick remains. This surface treatment is also fuel-resistant, may retard surface oxidation to a minor extent and can be used on pavements where there is fueling and significant traffic, such as aprons. It is not used on runways due to the friction loss associated with the fine aggregate.
 

Although sealants can help prevent oxidation, they do not replace the properties supplied by the high-quality distillates that are currently being removed from asphalt cements. Surface treatments simply cannot achieve the same pavement life factor. This is the problem that needs to be truly identified.

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