Paving airport runways High speeds, heavy loads and rigid safety requirements for aircraft traffic demand the use of special materials and tight controls by the paving contractor
JOHN D. GESSAMAN TECHNICAL DIRECTOR AMERICAN CONCRETE PAVING ASSOCIATION
t is impossible to place speed restrictions on a runway. It is impossible to detour a landing aircraft in the middle of the runway. Expensive new airplanes have less tolerance for pavement roughness than highway vehicles and they impose heavier loads on the pavements. The high speeds of a i rc raft on runways result in the need for higher skid resistance to maintain directional control and provide for safe stopping. These are some of the reasons why there must be tighter control of materials and construction procedures for airport runway paving than in road construction.
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Smoothness Runway smoothness is becoming a much more critical re q u i re m e n t for safe, efficient airline operations. Rough runways add to the stresses imposed on already heavily loaded landing gear. Additional stresses to aircraft parts due to rough runways are believed by some to result in the early retirement of many modern airplanes. Problems such as porpoising and skipping induced by
Equipment for grooving to precise specifications. Specially constructed machine passes a head containing a number of spinning diamond cutting blades across the surface to be grooved.
undulating runway surfaces add to the control difficulties encountered by pilots during critical phases of the takeoff and landing operations. Rough runways also slow a plane’s acceleration on take off. The Federal Aviation Administration (FAA) currently specifies that surface deviations shall not exceed one-quarter inch from a 16-foot straightedge placed in any direction, including placement both along and spanning longitudinal construction joints.
Subgrade The construction of smooth pavements starts with the subgrade. The FAA requires that the cutting of the finished grade of the natural subgrade or the subbase on which
concrete pavement is to be placed shall be controlled automatically by taut lines erected and maintained by the contractor. Stabilized subbase is generally required for all pavements designed to accommodate dual tandem gear aircraft in excess of 200,000 pounds gross aircraft weight. This would include pavements on most commercial airp o rt s. The stabilized material must be plant mixed. Pug mills or central plant concrete mixers are used in producing cement stabilized material and hot mix plants are required for producing bituminous stabilized bases. Often an existing pavement that needs strengthening for heavier aircraft serves as the base for a new concrete surface. If the new concrete is to be partially bonded to an
existing concrete pavement little advance preparation is re q u i re d other than cleaning old sealant from joints and cracks and sweeping the older slab. Where bond is to be prevented between two concrete slabs a separating layer of material such as building felt is placed on the old slab. Where an existing uneven asphalt or concrete pavement is to be resurfaced a leveling course is constructed before paving. Special surface preparation is required for complete bonding of a concrete overlay to an existing concrete slab. The existing surface must be thoroughly cleaned and etched with acid before a bonding grout of cement and sand is applied. The new concrete must be spread while the bonding grout is still moist.
Slipforming As in the case of highways, the trend in concrete paving on airports is the use of slipform pavers. The FAA has just issued a supplemental specification permitting this method of paving. The agency had deferred endorsement of slipform paving equipment due to concern for the longitudinal construction joint integrity. Howe ve r, recent projects have shown this method to be satisfactory when proper control is exercised. Vertical edges up to 20 inches thick have been successfully constructed by slipform paving on runways. Generally the width of paving lanes has been 25 feet. Howe ve r, a runway at Seattle-Tacoma International Airport was paved in four 37.5-foot widths last year. At the new Dallas-Fort Worth Regional Airport, optional paving lane widths will be 25 feet or 50 feet for the 200foot wide runways and 100-foot wide taxiways. Concrete for airport projects is almost always mixed in large central plant mixers. Because of the close control that can be achieved in producing concrete at a central point, these mixers turn out high volumes of concrete having the uniformity required for constructing smooth pavements.
Another requirement for smooth pavements is effective consolidation of the concrete in place. The FAA requires that “the paver shall vibrate the concrete for the full width and depth of the strip of pavement being placed. Vibration shall be accomplished by internal spud vibrators with an impulse range variable between 6,000 and 10,000 impulses per minute. The number, spacing, f re q u e n c y, and eccentric weights shall be approved by the engineer, and shall be changed as necessary to achieve an acceptable concrete density and finishing quality.”
Skid resistance Improved directional control and stopping capability of runway pavements are important goals of the aviation industry. The surface of a concrete runway must provide fine texture (grittiness) contributed by the fine aggregate, and coarse texture formed by the method of finishing. To maintain a satisfactory surface, the concrete must be resistant to abrasion (polishing). Therefore, available materials should be carefully examined from the standpoints of durability, toughness and gradation before being approved for use in runway pavement. For heavily used airport pavements only the highest material standards are employed. The FAA requires tra n s ve r s e brooming or combing with stiff bristle brooms, steel or plastic combs for concrete runways at commercial airports unless immediate grooving is intended. Machines have been developed for providing uniform textures of this type and new materials and configurations for combs are under development. Runway grooving is currently the best method of decreasing hydroplaning, poor directional control and unsafe braking conditions. Howe ve r, grooving is not a substitute for the use of high quality materials and good workmanship in pavement construction. The FAA has urged airport sponsors to g ro ove those runways where an analysis indicates a potentially haz-
ardous situation. The recommended groove is onequarter inch by one-quarter inch with a center-to-center spacing of 1 1 ⁄4 inches. The grooves are evenly spaced not less than 11⁄8 inches and not more than 2 inches. Ge n e ra l l y the grooves are cut in the hardened concrete by diamond saw blades. Howe ve r, pre-formed grooving in plastic concrete is a re c o g n i ze d method in the development stage. A contractor desiring to groove plastic concrete normally is required to construct a test section and demonstrate that his equipment will provide the required groove and surface configuration. Concrete paving contractors, materials producers and equipment manufacturers are aware of their important role in improving airport safety. They are a part of the aviation community and must work closely with government agencies, engineering consultants, airc ra f t m a n u f a c t u re r s, airline pilots, and airfield operators and users in this role.
PUBLICATION #C710513 Copyright © 1971, The Aberdeen Group All rights reserved
