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:

• A practice, intended for laboratory engineers and technicians, for design and analysis of LSM, presented in the format of an AASHTO provisional standard (Chapter 2);
• Three test methods that support the mix design practice, presented in the format of AASHTO provisional standards (Chapter 3);
• A manual, intended for field personnel including inspectors responsible for quality control and quality acceptance (QC/QA), for the construction of large-stone HMA pavements (Chapter 4); and
• A brief, introductory summary of the research results of NCHRP project 4- 18 and the conclusions drawn from them that form the basis for the LSM design practice and pavement construction guidelines (Chapter 5).

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 detailed research approach; experimental findings; and their interpretation, appraisal, and application to practice (Appendix A);
• A summary of the performance, structural, and material properties of LSM pavements included in the SHRP Long-Term Pavement Performance (LTPP) general pavement sections (GPS) inventory (Appendix B);
• Detailed results of material, volumetric, and performance tests on laboratory-compacted specimens and field pavement cores in the course of research (Appendix C); and
• The results of accelerated performance testing (APT) conducted on LSM specimens with the full-scale accelerated loading facility operated by the Indiana DOT’s Division of Research at West Lafayette, Indiana (Appendix D).