Rutting of asphalt pavements subjected to high traffic volume, heavy wheel loads, and high tire contact pressures is a major concern. Several approaches to mitigating this problem are being used; one that shows potential is the use of large stone mixtures (LSM). The introduction of larger sizes and higher proportions of the coarse aggregate fraction is intended to change the basic structure of the mixture such that the loads are supported by direct stone-on-stone contact. LSM performance may be less dependent on binder type and content than conventional mixtures. Higher void content, however, could lead to stripping problems. Field experience has indicated that there are concerns associated with constructibility, such as incomplete coating of aggregates, segregation, aggregate breakage, and inadequate compaction.
Potential benefits have led several states to construct a number of LSM pavements. However, a variety of mixture design methods, aggregate gradings (e.g., well-graded, stone-filled, or open-graded), and construction procedures were used with mixed results. Because of the growing interest in this concept, there is an urgent need for laboratory and field evaluation of performance characteristics to determine the effectiveness of LSM in minimizing plastic deformation. Additionally, a definitive design procedure needs to be developed for LSM.
The objectives of this study were (1) to evaluate the effectiveness of LSM in resisting plastic deformation in asphalt pavements, (2) to recommend a design procedure for the proportioning of aggregate and binder, and (3) to provide guidance on constructibility. The research effort shall consider the effects of aggregate shape, grading, and binder characteristics on LSM performance. The scope of this research included LSM having a maximum aggregate size between 1 in. and 2 1/2 in., inclusive.
The final report was published as
NCHRP Report 386, "Design and Evaluation of Large-Stone Asphalt Mixes." A copy of the Level 1 computer program written as a spreadsheet for Microsoft Excel, Version 5.0 or higher is available by double-clicking this icon.
NCHRP 418 (xls)
