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Runway Cleaning

    Airfield pavement maintenance is an important investment in an airport's major asset: runways, taxiways and aprons. Water blasting has been part of the maintenance process for over thirty years. The evolution of water blasting operations and equipment has provided an effective and efficient tool to remove rubber deposits from runways, remove paint from airfield surfaces, and prepare surfaces for new coatings.

    RUBBER REMOVAL: A new accounts manager was posting a sales report to the ledger and saw the description of the work done as "rubber removal". "You've got to be kidding!" she exclaimed. Although she wasn't thinking about the removal of rubber deposits that build up on airport runways, her reaction was typical of most who have never even considered the problem.

    The need is real at busy airports that support the "heavies", large turbo jet aircraft that lay down up to 2.5 pounds of rubber per tire per landing. If an airport's operations include 100 landings per day, the FAA recommends monthly friction surveys and the surface would most likely require bi-annual cleanings. Table 3-1, right, depicts the recommendations by the FAA found in AC 150-5320-12C.

    Runways need to be cleaned when the rubber fills in the surface texture of the pavement. The heat caused by the friction of a tire hitting the pavement and gaining "spin up speed" changes the rubber composition, melting it into a hard, carbonized layer. Pavement texture (micro texture and macro texture) are important aspects of runway safety. When the roughness of the surface (friction) drops below an established mu reading, the surface will be slippery when wet; hydroplaning can occur and an aircraft can lose control either in landing or taking off.

    During the 1960's larger aircraft became the norm for carrying an ever increasing population of travelers, and loss of friction on runway surfaces became a common cause of accidents. A team of engineers from NASA, FAA, FHWA, USAF and other agencies studied the problem and established the standards that are used today to determine safe friction levels to be maintained on paved surfaces. Even highways are monitored for proper friction, although the cause is generally mechanical wear and compression. When pavement is wet, poor surface friction can lead to hydroplaning and loss of vehicular control, either on a highway or a runway.

    As tests were being developed to monitor frictional properties on pavements, American industry was hard at work on ways to remedy the slippery problem. The first methods were low pressure water blasters, which generated a modest 8000 psi. When grouped together with several pumps, enough pressure and water volume was achieved to begin to blast away the rubber deposits and expose the rough texture of the pavement once again, restoring frictional properties sufficient to prevent slippage under wet conditions.

    As the years passed, industry worked harder to develop competitive technologies that would be better and faster. Not only were high pressure water pumps taken to new heights with better engineering, but the spray apparatus evolved as well. Initially, a "straight bar" with a series of high pressure tips arranged in a row, delivered the high pressure water stream to the pavement. An increased angle of the bar decreased the gap between the tips, yielding a clean swath of pavement approximately 12-inches wide. Although a slow process by today's standards, it was effective in cleaning runway surfaces until the advent of the rotor bar.

    Today there is a range of high pressure water blasting systems available to the airport industry. Some are better than others in removing rubber deposits, and others are better in cleaning and/or removing various materials from pavements. Matching the right equipment to the task is one of today's challenges, making sure that the water stream does little to no damage to the underlying pavement, joints, fixtures, etc.

    There have been other competing technologies developed over the years to rival the effectiveness of high pressure water in removing rubber deposits. One of them was the advent of environmentally-friendly chemicals that are designed to break down the hardened rubber from the pavement surface, causing it to dissolve into a fine powder when dry. The soapy chemical is applied to the rubber covered pavement, agitated by brooms to dissolve the rubber, and flooded with large amounts of water to wash off the detergent and rubber debris. Unfortunately, most airport operators, hearing "EPA approved chemical", chose to use the chemical to mitigate some real or perceived damage being done to the pavement by the high pressure water blasting method. What they may not have considered was the vast amount of waste water that poured into the surrounding ground area adjacent to the runway. The chemical may be environmentally friendly, the debris sometimes is not. Combined with oils and hydrocarbons, the waste water settles into the ground and accumulates with each repeated cleaning.

    Shot blasting is another method occasionally employed, but usually for re-texturing pavement when friction loss is due to other factors, such as mechanical wear and polishing caused by heavy traffic compressing the pavement. If rubber deposits are on pavement being re-textured, it will remove the rubber incidentally to re-texturing. Generally not recommended for grooved surfaces often found on commercial runways, there are some specially designed systems that are quite effective.

    High pressure water has survived the scrutiny of airport engineers over the last thirty-plus years, evolving into an efficient, clean, safe and cost effective method of cleaning runway surfaces. There are different types of water blasting:

• Discounting pressure washers as ineffective, we look first at low pressure systems, delivering up to approximately 10,000 psi. These pressures are generally adequate for removing most rubber deposits, although heavy deposits sometimes require greater pressure.
  
  
• These systems, as well as the high pressure systems, yielding up to 20,000 psi, use a high volume of water, ranging from 20-30 gpm. The result is a hydraulic effect of the water blasting out of the nozzles and pounding the pavement with the streams of water, lifting the rubber from the surface and out of the grooves (macro texture) and micro texture of the pavement. The rubber is removed from the surface re-exposing the aggregate and vastly improving the frictional properties of the pavement.
  
  
• In the 80's, another level of water blasting evolved providing pressures up to 40,000 psi, but with much less water volume, 4-8 gpm. This method, rather than creating a hydraulic effect, is better defined as a milling of the surface. Ultra high pressure (UHP) has become very useful in many cleaning operations, including runway cleaning. Many contractors use UHP for rubber removal. The one caution is that with repeated cleaning, as happens with rubber deposits that always build up in the same areas of the runway, the surface can become polished over time, sometimes creating a loss in frictional properties even after being cleaned.

    PAINT REMOVAL: UHP's better application on runway surfaces is paint removal. Often allowed to build up layer after layer of paint, the marking will begin to crack and spall, causing chunks of paint to loosen and potentially become FOD (foreign object debris) that can cause damage if ingested into aircraft engines.

    When paint is allowed to build up to this extent, continued painting only leads to failed markings (see below). It is better to remove 90% of the paint and start over. There are many different reasons why paint should be removed from airfield surfaces, but it is generally to eradicate obsolete markings. The degree of removal to be specified should be dictated by the reason for removal as well as the condition of the underlying pavement.

    Although every specification for repainting a runway includes language about surface preparation, most consider sweeping or blowing the existing markings to remove loose dirt and paint as sufficient. But if you were painting your house and the paint were flaking, would you sweep or blow the paint to prepare it for a new coat? Probably not. Would you paint over mold or mildew? Not likely. So why are runway pavements treated differently? Lots of reasons, but none of them justify the kind of paint build up pictured. On runways, the problem is particularly troublesome because of potential liability. If chunks of this paint get ingested into a jet engine, it might cause damage, sometimes enough to cause premature wear of engine parts.

    SURFACE PREPARATION: The marking shown to the right was a six-year-old, single coat of paint. It could've been painted over, but it would have been the beginning of the layering that causes markings to fail. With only 7,000 psi of high pressure water blasting, the majority of the paint that was poorly bonded came up. The result was a surface much better prepared to receive a new coat of paint. Doing proper surface preparation with water blasting, followed by vacuuming to remove the resulting debris, will prolong the life of a paint project, give the airport owner much more value for the dollars spent, and preserve the integrity of the pavement for a much longer time.

    Due to the width of runway markings, often 3+ feet wide, there is a cohesive tension created across the surface of the marking. The paint bonds to itself, as well as to the asphalt. In fact, the paint often bonds to the asphalt better than the asphalt does to itself, sometimes causing premature cracking and pavement deterioration, shown, right. Applying more paint to this kind of surface only worsens the problem. The existing paint is fractured and has many openings for water to penetrate. A new layer of paint will soon crack along the same fractures and water will continue to undermine both paint and pavement.

    Adequate surface preparation that removes the oxidized paint will help prevent the build up, the premature cracking of the pavement and will prolong the life of marking projects. So what constitutes "adequate"? Well, that's kind of "Catch-22". You'll be able to see evidence of "inadequate" surface preparation after the painting is done and the paint starts to peel worse than before, or cracks and starts to curl. It happens over time and is becoming a chronic problem on many runway surfaces around the country. A simple test of scratching a pen knife across the surface with downward pressure will give a clue to the strength of the paint bond. If it flakes or if you can pull up pieces, performing some sort of "surface" blasting (water blasting, shot blasting, sand blasting or grinding) prior to painting markings to remove 95% of loose and poorly bonded materials may begin to reverse the trend of paint chunks being FOD potential. FOD can be harmful not only to aircraft components but also to the airport itself from a liability standpoint.

    Falling under the definition of Surface Preparation is the removal of curing compound from new concrete pavement. Curing compound is sprayed onto the newly poured concrete and produces a moisture-resistant membrane film on concrete surfaces for thorough concrete hydration and strength development. It generally wears off the concrete over the course of one year, depending upon traffic. If paint is applied on top of curing compound, it will flake off as the membrane film sloughs off. Water blasting is a clean and effective way of removing the film.

    Another problem that is unique to airport pavements in warm, humid environments is green algae growth on markings. Algae is able to grow on pavement surfaces, and can seriously reduce the visibility of the markings. Airfields are particularly susceptible because the majority of the pavement receives very little or no traffic that would otherwise keep the algae at bay. Surface Preparation would include water blasting markings that have been covered with green algae.

    As a word of caution, airport operators have a heightened concern about damage to pavement and fixtures on the airfield for good reason. Part of their job is to maintain a valuable asset: their pavements, as well as keep FOD to a minimum. Water blasters in the hands of the inexperienced can cause serious pavement damage in a hurry. Making sure that operators are well trained, the equipment is well maintained, and the right equipment is used for the job is extremely important.

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