Chemical: There are at least 3 primary factors to be considered when comparing rubber removal with chemicals to rubber removal by captive water blasting. They are the Application Process, the Environmental and Infrastructural Impact, and the Cost.
Application Process: The cleaning of a runway with chemicals requires at least 3 steps and sometimes as many as 5. In any case, the amount of equipment and personnel required for the process will always be 3 or 4 times as much as required by proper water blasting.
1) Initial Flushing: The process sometimes begins with an overall flushing of the runway to bring the surface temperature to a level recommended by the chemical manufacturer. This is, typically, only necessary in hot climates.
2) Chemical Application: The application of the chemical itself requires a specially designed truck or trailer. Typically, this equipment incorporates long, boom like outriggers for the spraying of the chemical across the width of the runway. Even the largest equipment used for the applying chemicals requires 2 or more passes to cover the area being cleaned.
3) Brushing: After the chemical has been given time to soften and break down the rubber deposits, the third step typically involves multiple passes with a rotating, steel bristled broom. The rotary action of the broom is meant to separate the softened rubber from the runway surface.
4) Rinsing: The rinsing of the runway is meant to keep the aircraft rubber, now mixed with the chemical, from settling back into the grooves and pores of the asphalt until the vacuum trucks make their pass.
5) Vacuum Recovery: The vacuum recovery of the residue that remains on the runway is usually the last step. However, vacuum trucks engineered for working on city streets, are not capable of lifting the remaining rubber and chemical mix from within the grooves of the runway. There is a growing concern that the long-term use of chemicals breaks down the bitumen in runway asphalt. This creates a softening of the runway surface, ultimately reducing the useful life of the grooves.
The application process typically requires 3 or more trucks with as many or more operators. Furthermore, the runway being cleaned is not available for emergency landings during the cleaning process.
Environmental Impact: A critical result of the rinsing process is that a large amount of the rubber and chemical mix is washed off of the runway and into the adjoining grass and ground. Even if a bio-degradable detergent is used, when mixed with the petroleum-based composition of the rubber, the runoff is still hazardous to any groundwater beneath the surface. A major international airport in the northeast part of North America is surrounded on 3 sides by water. Because the large numbers of fish in the surrounding bodies of water, this airport is forbidden from using chemicals or detergents for their runway cleaning by the federal environmental agency. They are only able to use water blasting and representatives from the environmental agency must be on the site when rubber removal is performed. Several other major airports have on their own accord, determined that runway cleaning by chemical or detergent is hazardous to the airport environment.
Structural Impact: In the brushing process, the rotary action of the broom is meant to separate the rubber from the runway surface. However, it is physically impossible for the steel bristles of a rotary broom to reach into the grooves of a grooved surface runway. Consequently, even though the chemical may break down the composition of the rubber deposits in the grooves, the rotary broom is not capable of lifting the rubber out of the grooves. Therefore, a measure of the rubber, mixed with the chemical will always settle and remain in the grooves.
A second, long-term impact of the brooms is a polishing effect on the runway aggregate. This ultimately reduces the base level friction coefficients for the surface of the runway.
Costs: Studies show that the combined costs of the chemicals, the equipment required to support the process, the number of employees required to operate the equipment and the many hours of time required to complete the process, all add up to costs that far exceed the cost of water blasting. Studies also show that runways cleaned with chemicals or detergents must be cleaned more frequently then runways cleaned by properly engineered water blasting equipment.