CHAPTER 7: INTEGRATING SKID RESISTANCE AND TIRE/ROAD NOISE
7.1 Overview of the Existing Porous Mixture Design Methods
7.1.1 United States Design Method
Porous pavement is known as open-graded friction courses (OGFC) in United States, although the porosity level of a traditional OGFC is slightly lower than that of a regular porous asphalt surface. With the adoption of modified asphalt in mix design, the new generation OGFC now possesses a similar porosity level as the porous surfaces out of United States. The first formalized design procedure in United States was developed by the Federal Highway Administration (FHWA) in 1974. There are currently more than 20 different approaches used across the United States (Putman and Kline, 2012), among which, the method developed by the National Center for Asphalt Technology (NCAT) in 2000 (Mallick et al., 2000) is the most influential and was standardized by ASTM in 2004 (ASTM, 2013a). Four major steps are considered in the porous mixture design: 1) selection of appropriate materials; 2) development of aggregate gradation; 3) determination of optimum asphalt content; and 4) evaluation of mix performance.
In mixture design, the materials that need to be selected include aggregates, asphalt binders and stabilizing additives. The requirements on aggregate quality for porous mixtures are similar to those for stone matrix asphalt (SMA) mixture.
Polishing resistance and durability have been identified as the most important properties (Cooley et al., 2009). The angularity, abrasion resistance, particle shape and cleanliness are also considered. The selection of asphalt binder should be based on environment, weather and traffic at the site, as well as the expected performance of the porous surface. A wide range of asphalt binders could be used and graded in accordance with either Superpave performance grading system, viscosity grading procedure or penetration grading system. Modified asphalt cements can significantly improve porous mixture performance, so are highly recommended. The functions of
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stabilizing additives used in porous mixture include reducing draindown potential, increasing mixture strength and improving durability. Cellulose fibers, mineral fibers, polymers or rubber particles may be used as additive. Fiber stabilizers are typically added into the mixture at a content of 0.2% to 0.5% of the total mixture mass. Table 7.1 summarizes the material selection criteria specified by ASTM standard (ASTM, 2013a).
The next step in porous mixture design is to develop an aggregate gradation that guarantees the desirable air void content in the total mixture and the existence of stone-on-stone contact in coarse aggregate skeleton. The porous mixture gradations may potentially be characterized by the nominal maximum aggregate size (NMAS) as defined in Superpave. Highway agencies in the United States have developed various recommended master gradations for different NMAS (most commonly 9.5, 12.5, 19 and 25 mm). Examples are shown in Table 7.2. In the design process, engineers first determine the adopted NMAS and a specified master gradation is then selected. Three trial gradations within the recommended grading range should be selected and mixed with a trial asphalt content (typically between 6.0% and 6.5%). Additive should be included if it is going to be used in the actual mixture. Specimens are then prepared using the Superpave gyratory compactor with a compaction effort of 50 gyrations.
The condition of stone-on-stone contact is defined achieved when the percent voids among the coarse aggregates of compacted mixture (VCAmix) is less than that of coarse aggregates obtained by the dry-rodded test (VCADRC), with coarse aggregates defined as the fraction larger than the No.4 sieve. The air void content of compacted porous mixture can be determined based on its bulk specific gravity and the theoretical maximum density measured on loose sample. Of the three trial gradations, the one with the highest air void content (minimum acceptable is generally 18%) and a VCAmix equal to or less than VCADRC is considered optimum and should be adopted as the desired gradation.
261 Although there is no specific approach that can identify an absolute optimum asphalt binder content, methods resulting in a range of allowable binder contents have been widely developed. The procedures used in the United States can be divided into three categories: methods using compacted specimens; methods using oil absorption test; and methods based on visual observation. Compacted specimen approach is the most popular and scientific methodology which is adopted by most highway agencies.
In this method, the optimum asphalt content is determined based on test results of air voids, asphalt draindown and durability. Specimens compacted with 50 gyrations are produced using the selected gradation and at least three asphalt contents in increments of 0.5%. The air voids calculated from bulk specific gravity and theoretical maximum density should be at least 18%, and higher air void contents are desirable. The asphalt draindown is tested at a temperature 15 °C higher than the anticipated production temperature and the maximum permissible draindown is 0.3% by total mixture mass.
Cantabro abrasion test may be used to examine the durability of designed mixtures.
The average abrasion loss on unaged specimens should not exceed 20%. For aged porous mixture, the average loss should be below 30% and the loss for any individual specimen should not exceed 50%. Laboratory permeability test is optional in this step, with a magnitude greater than 100 m/day being recommended. The optimum asphalt content could be determined based on the above requirements. If none of the designs satisfies all the criteria, the mixture needs to be further adjusted.
The last step of porous mixture design involves performance evaluation. The predominant type of performance testing to date is moisture sensitivity test using the modified indirect tensile test with five freeze/thaw cycles. The desired retained tensile strength ratio (TSR) should be at least 80%. The moisture susceptibility may also be evaluated through the boil test or a loaded-wheel tester (Cooley et al., 2009). If the mixture fails to meet the requirement of moisture susceptibility, anti-strip additives
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such as hydrated lime may be used. The stiffness of porous mixture, rutting resistance and short-term raveling resistance may also be examined in the design.