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CHAPTER 10 Design of Gap-Graded HMA Mixtures Gap-graded HMA (GGHMA) consists of two parts: a coarse aggregate skeleton and a mortar. The coarse aggregate skeleton consists of crushed coarse aggregate particles; these make up about 70 to 80% of the total aggregate blend. The mortar consists of asphalt binder, fine aggregate, and mineral filler and fills the voids in the coarse aggregate skeleton. Stone matrix asphalt (SMA), one particular type of GGHMA, is widely known for its durability and rut resistance. SMA has been used in Europe for over 30 years. The first U.S. project that used this high-performance HMA was constructed in 1991. Since then, the use of SMA has steadily increased within the United States. The GGHMA discussed in this chapter is similar to SMA in many ways, but there are some differences, so to avoid confusion or arguments over whether or not the mix design presented in this chapter is truly an "SMA," the term GGHMA is used instead. In Europe, SMA mixtures have primarily been designed by recipes. It was not until 1994 that a formalized mix design procedure was available in the United States This mix design procedure was developed by a Technical Working Group established by the FHWA. This procedure was based on the Marshall mix design method, since this was the method used to design SMA in Europe. In 1994, the National Center for Asphalt Technology (NCAT) began a 4-year study to develop a mix design system to design SMA using the concepts and methods of the Superpave mix design system. Results from this research project were published in 1998, and, along with more recent experience and research, they are the basis for the GGHMA mix design system described in this chapter. The philosophy of GGHMA mix design is not complicated. The first principle is that a gap-graded blend of aggregate is needed so that the coarse particles will have stone-on-stone contact. The second principle is that the voids within the coarse aggregate skeleton must be filled with fine aggregate, mineral filler, and asphalt binder. In order to provide increased durability, GGHMA has a relatively high asphalt binder content. This leads to the third principle of GGHMA mix design, which is that the aggregate must have a high VMA value--typically 18 to 20%. This relatively high asphalt binder content can result in an increased potential for draindown if not properly taken into account. Draindown can be a common problem in GGHMA; it occurs when the asphalt binder and fine aggregate separate from the coarse aggregate during storage, transport, or placement. The fourth and final principle of GGHMA mix design is that small amounts of stabilizing additives, such as mineral fibers or cellulose fibers, are usually needed to prevent draindown. The sections below describe in detail how to design GGHMA to achieve the unique properties and excellent performance for which this mix type is known. Overview of GGHMA Mix Design Procedure The mix design procedure for GGHMA contains five primary steps (Figure 10-1). First, suitable materials must be selected to compose the GGHMA. Materials needed include coarse aggregates, fine aggregates, mineral fillers, asphalt binder and stabilizing additives. The second step is to blend 174
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Design of Gap-Graded HMA Mixtures 175 Design of GGHMA Mixtures Identify Materials Step 1 Select Aggregates Stabilizer Materials Asphalt Binder Determine VCA of Select Trial coarse aggregate in dry - Gradations rodded condition Step 2 Low % passing Break Medium % passing Break High % passing Break Point Sieve Point Sieve Point Sieve Add 6.0% - 6.5% Within asphalt binder Specifications and compact Analyze data Step 3 and select optimum gradation Fix gradation and vary asphalt binder content Step 4 Adjust asphalt binder content as needed Conduct draindown, moisture susceptibility, and performance testing. Step 5 No Meet all specifications ? Yes End Figure 10-1. Flow diagram illustrating GGHMA mix design methodology. three trial gradations. For each trial gradation, asphalt binder is added and the mixture compacted. After each trial gradation has been compacted, mixture volumetric data for each trial mixture is evaluated as the third step in order to select the best gradation. The fourth step is to fix the selected gradation and compact mixtures with varying asphalt binder contents. The asphalt binder content that produces 4% air voids is selected as optimum asphalt binder content. The final step in the mix design procedure is to evaluate the moisture susceptibility, draindown sensitivity, and rut resistance of the designed mixture.