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OCR for page 26
26 A Manual for Design of Hot Mix Asphalt with Commentary and other major construction projects, reliability levels of at least 95% should be used when selecting performance-graded binders. Practical Selection of PG Binder Grades for HMA Mix Design Although the LTPPBind computer program is very useful, in practice most highway agencies have, through experience, developed their own systems for selecting binder performance grades depending on traffic level and location. This has been done in part because refineries are able to produce only a limited number of binder grades, so engineers must determine two or three per- formance grades that can be used to meet most or all of the paving needs in a given region. This is sometimes referred to as a "binder slate" for a given state or region. For example, a common binder slate in the Mid-Atlantic states involves only three performance grades: 58-28, 64-22, or 76-22. Other binders might be occasionally used in this region, but typically only for small demonstration projects. Engineers selecting performance-graded binders for paving applications should refer to the appropriate specifications for their state or, if there are none, to those in neighboring states with similar climates and conditions. Binder producers may also be useful in providing information concerning what binder performance grades are available locally and which might be most appropriate for a given application. Engineers and technicians using the LTPPBind program without referring to the binders used by the local highway agency may find that the binder they have specified for a given application is not locally available. In selecting performance-graded binders from an available slate, it must be remembered that a given performance grade will meet the requirements of many less extreme situations. For example, in many areas of the Mid-Atlantic, LTPPBind (version 3.1) indicates that a PG 58-22 binder should be used for light traffic. However, this binder may not be available in some Mid-Atlantic states. If the PG 58-22 binder cannot be found (or found at a reasonable price), a PG 64-22 binder would be selected and would perform perfectly well, since its extreme high and low temperature ratings meet or exceed those for these applications. Care should however be used in selecting binders that are much stiffer than required for a given application. Recently, many highway agencies have noticed an increase in surface cracking in HMA pavements. Although such top-down cracking is not yet fully understood, using unnecessarily stiff binders may contribute to the problem. Additional details concerning the selection of asphalt binders for HMA mixtures are given in Chapter 8 of this manual. Bibliography AASHTO Standards M 320, Performance-Graded Asphalt Binder M 323, Superpave Volumetric Mix Design PP 42, Determination of Low-Temperature Performance Grade (PG) of Asphalt Binders R 28, Accelerated Aging of Asphalt Binder Using a Pressurized Aging Vessel (PAV) R 29, Grading or Verifying the Performance Grade of an Asphalt Binder R 35, Superpave Volumetric Design for Hot-Mix Asphalt (HMA) T 48, Flash and Fire Points by Cleveland Open Cup T 49, Penetration of Bituminous Materials T 51, Ductility of Bituminous Materials T 53, Softening Point of Bitumen (Ring-and-Ball Apparatus) T 202, Viscosity of Asphalts by Vacuum Capillary Viscometer T 240, Effect of Heat and Air on a Moving Film of Asphalt (Rolling Thin-Film Oven Test) T 313, Determining the Flexural Creep Stiffness of Asphalt Binder Using the Bending Beam Rheometer (BBR)

OCR for page 26
Asphalt Binders 27 T 314, Determining the Fracture Properties of Asphalt Binder in Direct Tension (DT) T 315, Determining the Rheological Properties of Asphalt Binder Using a Dynamic Shear Rheometer (DSR) T 316, Viscosity Determination of Asphalt Binder Using Rotational Viscometer Other Publications The Asphalt Institute, Asphalt Binder Test Manual (MS-25). The Asphalt Institute (2007) The Asphalt Handbook (MS-4A), 7th Ed., 832 pp. The Asphalt Institute (2003) Superpave Performance Graded Asphalt Binder Specifications and Testing, 3rd Ed., 72 pp.