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PAVEMENT PRESERVATION TREATMENT CONSTRUCTION GUIDE
CHAPTER 10: ULTRA-THIN HOT MIXED ASPHALT, BONDED WEARING COURSE PROJECTS
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Chapter 10: Ultra-Thin, Hot Mixed Asphalt, Bonded Wearing Course Projects

Highway agencies throughout the world face increasing demands and decreasing resources to maintain and preserve their highway networks. The demand to “do more with less” has been an operating slogan for many of these agencies. Historically, the emphasis has been on new facility construction, and rehabilitation or reconstruction of existing facilities. However, most agencies are currently in a maintenance and/or preservation mode, a trend that can be expected to continue in the foreseeable future.

Pavement preservation is a method by which roads are treated before significant failure has occurred. This has the advantage of allowing action before user complaints, and also saving the agency money over the life of the pavement.

1.0 Introduction

A bonded wearing course (BWC) is a gap graded, ultra thin hot-mix asphalt (HMA) mixture applied over a thick polymer-modified asphalt emulsion membrane. The emulsion membrane seals the existing surface and produces high binder content at the interface of the existing roadway surface and the gap-graded mix all in one pass. The gap-graded HMA provides an open surface texture to allow water to flow through the surface. A BWC can be applied and opened to traffic quickly, usually within 15 minutes, without sanding or tracking. Bonded wearing courses are primarily used in high traffic areas as a surface treatment over HMA and PCC surfaces. It can be placed over structurally sound pavements as a maintenance treatment, and may also be used in new construction and rehabilitation projects as the final wearing course.

The BWC polymer modified asphalt emulsion membrane seals existing pavement and bonds the gap- graded mix to the surface. The thick nature of the membrane allows it to migrate upwards into the mix, filling voids in the aggregate and creating an interlayer of high cohesion. Due to the nature of the gap-graded mix and the polymer in the membrane, bleeding is not normally a concern.

The BWC gap-graded mix provides a stone-on-stone contact which provides resistance to rutting within the mix. The finished mat has very high macro-texture properties, provides good skid resistance and has a void structure that improves driving visibility by reducing back-spray and tire-splash. The void structure also reduces tire noise. Although the mix is generally laid one and a half times as thick as the largest stone in the gradation, it may be placed thicker to correct minor surface irregularities.

This document provides an overview of:

  • The materials used in construction of bonded wearing courses
  • Guidelines for project selection
  • The construction process associated with bonded wearing courses
  • A troubleshooting guide to assist the field personnel
  • Suggested construction field considerations