Interest in the use of reclaimed asphalt pavement (RAP) has increased dramatically since the recent price increases in crude oil and energy in general.
This interest will probably continue to grow, since many studies have shown that asphalt mixtures containing RAP can have equivalent performance to virgin mixtures.
Agencies and contractors have made extensive use of RAP in constructing highway pavements, while the use of RAP on airfield pavements has been somewhat limited. Therefore, a review of the state of the current practice and appropriate application of RAP materials to airport pavements is needed.
For this purpose, a research study has recently been conducted as part of the Airfield Asphalt Pavement Technology Program (AAPTP), Project 05-06, with the objective of establishing updated technical guidance on the use and benefits of RAP in airfield hot-mix asphalt (HMA) materials and to document existing use on airport pavements.
The overall goal of the mix-design process of HMA is to recommend a mix that can withstand the combined actions of traffic and environment. Therefore, it is critical to assess the impact of the various mix components on the performance of the constructed pavement. The existence of RAP in the mix presents a challenge to the design engineer because of the complex interaction among the new and recycled components of the mix. The key to successfully including RAP in HMA is to be able to assess its impact on the pavement’s performance while recognizing the uniqueness of each project with respect to both materials and loading conditions.
The properties of RAP are largely dependent on the properties of the constituent materials, such as aggregate type, quality and size and extracted binder properties. The RAP composition is affected by the previous maintenance and preservation activities that were applied to the existing pavement. For example, in many airfield pavements, a coal-tar sealer is often applied on parking, maintenance and refueling areas to protect the asphalt pavement from possible damage by spilled fuel. In addition, sometimes RAP from several projects is mixed in a single stockpile, combining low- and high-quality materials. Using low-quality or highly variable RAP materials can lead to premature failure of the HMA pavement. Badly deteriorated pavement will lead to both foreign-object damage and rough surface creating a safety hazard for aircraft traffic on taxiways and runways. All these issues may be prevented through the implementation of an effective quality-control program.
The specifications and procedures for the use of RAP on Federal Aviation Administration (FAA) pavement projects are contained in sections 401 and 403 of Advisory Circular 150/5370-10C. A similar specification, UFGS-32 12 15, is used for military airfields.
The current FAA P-401-3.3 and P-403-3.3 sections specify that RAP can be used in lower layers only and not in surface mixes, except on shoulders. The maximum percentage of RAP allowed in the mix is 30% given that the resulting mix meets all requirements that are specified for virgin mixes.
Established in the field
Two civilian airports and one military airport were identified as currently using mixes containing RAP in the surface course. In 2001, at Boston’s Logan International Airport, a Marshall-designed HMA mix with 17% RAP and a latex-modified PG 64-22 binder was used in the surface course of a section of Taxiway November. Six years after construction, the pavement is still performing well with no signs of rutting or potential of foreign-object damage. In 2006, the center 75 ft of the northern portion of Runway 4R-22L at Logan was reconstructed with an HMA mix containing 18.5% RAP and a latex-modified PG 64-28 binder. No specific problems were encountered during construction resulting from the use of RAP in the mix. The majority of the pavement sections met the in-place density specification. After one year of service, the runway is in excellent condition with no visible rutting observed.
In 1999, the Griffin-Spalding County Airport (6A2) in Georgia used a Superpave-designed HMA mix containing 17% RAP in the surface layer of the runway and taxiway pavements. During construction, the typical Georgia Department of Transportation (DOT) requirements for conventional HMA were followed, and no problems were encountered using RAP in the HMA mix. Some pavement sections failed to meet the in-place density specifications imposed by the FAA. After eight years of service, the HMA mix with 17% RAP is still in good condition with moderate transverse cracking and cracks at the longitudinal construction joints. The pavement has moderate raveling, specifically along the longitudinal joints, but no visible rutting. The pavement does not show any signs or potential of foreign-object damage.
The HMA pavement at Taxiway Alpha of the Oceana Naval Air Station (NTU) in Virginia has been resurfaced approximately every eight to 10 years. The last resurfacing job was in 2000 where the middle 32 ft of the taxiway were milled and replaced with a 2.5-in. Navy airfield mix (almost identical to a P-401 HMA surface course) containing 20% RAP. The RAP-containing mix consisted of a 1-in. maximum aggregate size with a PG 70-22 asphalt binder. The pavement’s daily traffic is equivalent to approximately 200 repetitions of tactical aircraft (F-14 and F-18) and one repetition of cargo aircraft (C-141 or C-17).
The tactical aircraft have single-tricycle-gear geometry with a tire pressure of 240 psi. The C-141 has a dual-tandem-tricycle gear with a tire pressure of 120 psi.
Before reconstruction in 2000, the pavement consisted of a 2.5-in. HMA overlay on top of a portland cement concrete (PCC) pavement with fabric between the PCC and the HMA layer. The pavement exhibited rutting in the wheel paths at approximate distances of 8 to 14 ft left and right of the centerline.
The rutting was generally described as being up to 1 in. over the 6-ft-wide travel path of the wheel gear. Other major distresses in the pavement were reflective cracking from the underlying PCC pavement. The majority of the cracks exceeding ¼-in. width had been sealed as a part of routine maintenance.
After seven years of service, the HMA mix with 20% RAP at the taxiway is again exhibiting rutting in the wheel paths, mainly associated with the constant aircraft traffic with high tire pressures and not specifically due to the use of RAP in the mix. No difficulties or issues were reported during design or construction using RAP in the HMA mix.
This research effort reviewed the current FAA specifications (P-401-3.3/P-403-3.3) in terms of the following key elements: RAP source, RAP variability, RAP properties, RAP content, mix design and quality control.
RAP obtained from airfield aprons may be contaminated with fuel spillage and may contain coal-tar sealer, rejuvenator or material that contains coal tar. The contaminants may affect the properties of the final mix. Therefore, and since no actual study was conducted to evaluate the impact of contaminant type and amount on the final mixture’s properties, it is recommended that RAP materials shall be free of contaminants that are potentially detrimental to the mixture performance. RAP should be from pavements that were built to highway or airport standards and specifications and shall be free of contaminants such as, but not limited to, coal-tar sealer, rejuvenator, material that contains coal tar and paving fabrics.
When RAP is used in HMA, the first step in the mix-design process is to determine the average and standard deviation of the RAP binder content and gradation using samples taken from eight to 10 random locations distributed throughout the RAP pile. This information is used to estimate feasible RAP contents that will satisfy gradation and variability requirements.
Processing RAP by crushing or screening, or both, also can help to reduce the variability in RAP as reported in NAPA’s “Recycling Hot-Mix Asphalt Pavements” (IS-123). In addition, fractionating the RAP into different sizes may be necessary to maximize the percentage of RAP used in a mix and still meet the gradation and volumetric requirements. The NAPA publication provides a new and updated document on how to recycle and presents the equipment and methods that others are successfully using to reclaim, size, store and process RAP in various types of HMA facilities throughout the country.
In the case of the binder in the RAP, the two critical properties are binder content and binder properties. The recommended procedures are AASHTO T164 or ASTM D2172 for the centrifuge (method A) or reflux (method B) extraction. The recommended procedures for the recovery of the extracted RAP binder are AASHTO T170 or ASTM D1856 for the Abson method or the ASTM D5404 for the rotary evaporator method. The physical properties and critical temperatures of the recovered RAP binder should be determined in accordance with section X1.2 of AASHTO M323. The determined RAP binder PG grade will help in determining the maximum allowable RAP content in the mix.
The gradation of the aggregates in the RAP materials shall be evaluated through the solvent extraction and determined in accordance with AASHTO T30 or ASTM D5444. The bulk specific gravity (Gsb) of the extracted RAP aggregate should be determined and included in the calculation of the Gsb of the combined aggregates. The RAP aggregate Gsb may be estimated from the RAP mixture theoretical maximum specific gravity and assumed asphalt absorption for the RAP aggregate, if the absorption can be estimated with confidence. The RAP aggregate effective specific gravity (Gse) may be used in lieu of Gsb at the discretion of the engineering consultant or agency. The use of Gse may introduce an error into the combined aggregate Gsb and subsequent VMA calculations. Based on experience with local aggregates, adjustments to the VMA requirements may be needed to account for this error.
Based on the review of the various efforts on the use of RAP and applicability along with its long-term field performance on highway and airfield pavements, the recommendations on the use of RAP in airfield HMA surface and base mixes are shown in Table 1.
RAP should not be used at a percentage higher than the maximum specified in Table 1 unless the mix containing RAP is proven to have acceptable moisture, fatigue and thermal-cracking resistance.
The mix containing RAP is designed using the procedures contained in the Asphalt Institute’s Manual Series No. 2 (MS-2).
Because of moisture sensitivity concerns, the mixes containing RAP should satisfy the recommendations of Table 2 in addition to the current specification of a minimum dry compressive strength of 200 psi (ASTM D1074). Any change in the characteristics of RAP materials, such as a change in the RAP source, RAP aggregate gradation or RAP binder content, will necessitate an entirely new mix design.
For the surface mixes with RAP material, the same acceptance criteria specified in FAA P-401-5.2 for regular surface mixes will be followed. For the base mixes with RAP material, the same acceptance criteria specified in P-403-5.2 for regular HMA base mixes will be followed with additional requirements for the stability, flow and air voids of the HMA mix according to P-401-5.2.
The testing frequency should be adjusted based on the variability of materials in the RAP and the type of mix being produced.
To read the complete report on the “Use of RAP in Airfield HMA Pavements,” go to www.aaptp.us and select project 05-06.
For more information on RAP, refer to the NAPA publications “Recycling Hot-Mix Asphalt Pavements” (IS-123) and “Designing HMA Mixtures with High Rap Content: A Practical Guide” (QIP-124). Copies can be ordered at NAPA’s online store, http://store.hotmix.org, or by calling toll-free 888/600-4474.