Road Engineering Journal
Road Engineering Journal
December 1, 1997
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National Cooperative Highway Research Program (NCHRP) Report 386: Design and Evaluation of Large-Stone Asphalt Mixes

(The following information is from "NCHRP Report 386: Design and Evaluation of Large-Stone Asphalt Mixes." Reproduced here is "Chapter 1: Introduction." This chapter summarizes the findings described in the report. A copy of the complete publication can be purchased from TranSafety, Inc. for $28 plus $4 shipping and handling. The report is 141 pages in length and was published in 1997.)

Chapter 1: Introduction

This report presents a practice for the design and analysis of dense- and open-graded, large-stone mixes (LSM) and guidelines for the construction of hot-mix asphalt (HMA) pavements incorporating LSM. Work was accomplished under NCHRP Project 4-18, "Design and Evaluation of Large Stone Mixtures." For the purposes of this study, LSM are defined as HMA paving mixes containing maximum aggregate sizes between 25 and 63 mm (1 and 2.5 in.). Though typically used as base layers, LSM can serve as surface layers where rutting (permanent deformation) is a problem or concern.

In keeping with current directions in asphalt paving mix design technology (typified by the Superpave system for dense-graded mixes), the design practice provides the option of two levels of volumetric design complexity, including the use of performance-related mixture analysis tests.

The Level 1 LSM design method is appropriate for low-volume roads and requires only minimal materials testing. With this method, personnel use a spreadsheet-based computer program to estimate an optimum design on the basis of measured or reported grading of available aggregate stockpiles; the designer can select the desired air voids content (VTM) of the compacted LSM.

The Level 2 dense-graded LSM design is intended for higher volume roads. It is an iterative process that begins with a Level 1 design and ensures that the mixture provides a required resistance to permanent deformation (rutting). The objective is to design a mix with a coarse aggregate skeleton with good stone-on-stone contact capable of carrying the intended traffic load. Use of this method requires specific knowledge of the measured gradations of all aggregate stockpiles; the computer program is used to build a coarse stone skeleton and fill its voids with the smaller aggregates from succeeding stockpiles. Optimum asphalt binder content is then estimated on the basis of desired levels of voids in mineral aggregate (VMA), VTM, and other factors. Finally, a performance-related mixture test (the Superpave simple shear test [SST] procedures [AASHTO TP 7] or a uniaxial [static] compressive creep test) is used to ensure that the LSM meets minimum rutting resistance criteria.

The Level 2 LSM design method employs the Superpave gyratory compactor for specimen preparation. Depending on the maximum aggregate size in the LSM and other specific aggregate and volumetric properties, the angle of the gyration may increase substantially from the 1.25 deg specified in AASHTO TP 4 for conventional, dense-graded mixes. Other gyration compactors may be used if their operating characteristics can be adjusted to meet the requirements of AASHTO TP 4.

A Level 2 open-graded LSM design incorporates the same volumetric design principals used for dense-graded LSM to ensure that the mixture has an adequate permeability, but does not require mix testing to evaluate rutting resistance. A draindown test is used to verify or make final adjustments to the optimum asphalt content selected by the computer program.

In general, the construction of LSM pavements demands application of the same sound principles of production, placement, and compaction used for conventional (maximum aggregate size of 25mm [1 in.] or less) paving mixes. Therefore, the LSM pavement construction guidelines emphasize procedures to prevent or remedy the most frequent problems characteristic of LSM, which have slowed its adoption and use in some states--segregation, aggregate fracture, and equipment wear.

The report has two parts. The first part, Chapters 2 through 5, is intended principally for the practitioner responsible for LSM design and pavement construction and includes the following:

The appendixes form the second part of the report; they are intended principally for specialists interested in the experimental results upon which the practical products in Chapters 2 through 4 are based. The appendixes include the following:

The Level 1 computer program is written to run as a spreadsheet in Microsoft Excel, Version 5.0 or higher. It is available for downloading from the Transportation Research Board's Internet World Wide Web site at www2.nas.edu/trbcrp/229a_35a.html (go to the icon at the bottom of the status block). Instructions for using the computer program are in Annex X1 of the mix design practice in Chapter 2.



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