Asphalt Mixture Specification and Testing

Cliff Nicholls

• Publisher: CRC Press
• Year: 2017
• Print ISBN: 9781498764056
• eBook ISBN: 9781351788663
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

This guide reviews the way asphalt mixture can be specified, with particular emphasis on the test methods used to measure performance. The advantages and limitations of the tests are described for measuring the desired property, and engineers can specify a test according to the material's use.

The book starts with a resume of specifications and their relative advantages and disadvantages for different situations. Then different properties are discussed in terms of: their specification; the test methods that can be used (primarily the EN 12697 suite of European methods, of which the author has been responsible for drafting); the extent to which the results predict performance; the levels that can be achieved with different asphalt mixes and types; what levels, if any, should be specified in various situations and pavement layers; and which other properties are adversely affected by enhanced performance. The final section covers various aspects of sustainability, with a strong emphasis on durability.

Better understanding should enable clients and consultants who specify pavements to produce durable asphalt pavements more economically, and also help asphalt producers and students trying to understand the black art of asphalt.

Chapter 1

Introduction

1.1Objective

Bitumen is a ‘hydrocarbon product produced from the refining of crude oil’ that is ‘a thermoplastic, viscoelastic liquid that behaves as glass-like solid at low temperatures and/or during short loading times and as a viscous fluid at high temperatures and/or during short loading’ (Hunter et al., 2015), making it one of the most complex construction materials. When it is combined into asphalt, which is a ‘surfacing material consisting of bitumen, mineral aggregates and fillers and may contain other additives’ (Hunter et al., 2015) for which the aggregates can have a variety of shapes, gradings and compositions, that asphalt becomes an even more complex material that has been, and is being, used successfully on roads, airfields and other paved areas. However, the properties asked of asphalt pavements (the number of which seems to be continually increasing with time) differ both for different layers within the pavement and for different circumstances and, therefore, require different asphalt mixtures.

The objective of this book is to review the ways that asphalt can be specified with particular emphasis on the test methods used to measure the performance of the various properties. Therefore, the tests are described in terms of their advantages and limitations as far as measuring the principal desired properties. It is hoped that this approach will help engineers define what is required of the material needed for their specific situations without requesting any properties, or levels of properties, that are not necessary for that situation.

As such, a résumé of specifications and their relative advantages and disadvantages for different situations is given. Then, different properties are discussed in terms of

•Their specification

•The test methods that can be used (primarily the EN 12697 suite of European methods)

•The extent to which the results predict performance

•The categories that can be set for the test when assessing different asphalt types

•The precision that has been found for different tests

•The other properties that are adversely affected by enhanced performance

Finally, various aspects about sustainability are discussed, with a strong emphasis on durability. It is hoped that better understanding of the sustainability needs will encourage improved and economical implementation of the means to produce durable and sustainable asphalt pavements.

1.2Terminology and units

Oscar Wilde described the United Kingdom and United States as ‘two nations divided by a common language’ and nowhere is this statement more relevant than for asphalt technology. The two countries use different terms to mean the same thing for several items, and these differences have been further exacerbated by the harmonisation of terms across Europe requiring the United Kingdom to change some terms that were previously the same as those used in the United States. Therefore, it is important to clarify which terms are being adopted in any publication.

The main differences are with regard to the materials and the layers in which that material is used. The term ‘asphalt’ is used in Europe solely to mean the mixture including the aggregate whereas in America it can be used to mean either the mixture, when it is generally introduced as ‘asphalt concrete’, ‘asphaltic concrete’ or ‘hot mix asphalt (HMA)’ or the binder, when it is generally introduced as ‘asphalt cement’. In Europe, ‘asphalt concrete’ is a specific mixture type (Section 2.6.2) which can cause confusion when it is uncertain whether the term relates to the mixture type or to all/any of them, while the binder is known as ‘bitumen’.

With regard to layers, the European terminology is ‘surface course’ at the top, ‘binder course’ next and ‘base’ (which may be split into upper base and lower base), whereas the American terms, which were previously used in the United Kingdom, are ‘wearing course’, ‘basecourse’ and ‘roadbase’. There is potential for confusion if the term ‘base course’ is used as to whether it is the second layer with the space omitted or the bottom layer with ‘course’ erroneously added.

The terms are listed in Table 1.1 for quick reference. For this book, the European terms will generally be used unless stated otherwise.

Table 1.1Conflicts in terminology

Another term for the mixture, used in both America and Europe, is ‘HMA’. However, the term has become more specific with the recent rise of asphalt mixtures that are mixed and laid at temperatures lower than those that were previously standard. These new reduced temperature mixtures are generally categorised as warm mix asphalt if still mixed above 100°C, half-warm mix asphalt if mixed below 100°C and cold mix asphalt if mixed around ambient temperatures, as shown in Figure 1.1. However, the temperature at which the asphalt is mixed does not affect the properties required of them in service, so these terms will not be used in this book.

Figure 1.1Categories of reduced temperature asphalt.

With regard to units, this book has been written with all units converted to SI units (which excludes centimetres, despite the unit sometimes being mistakenly used in standards) whenever possible to keep the units consistent. Therefore, where American tests are referenced, it is the metric conversion that has been quoted rather than the imperial units.

Reference

Hunter, R N, A Self and J Read. 2015. The Shell Bitumen Handbook. 6th edition. London: ICE Publishing.

Chapter 2

Specifications for asphalt

2.1Development of specifications

Asphalt specifications are designed to define an asphalt mixture with the capability to both perform for all required properties and not to fail in any of the potential failure mechanisms. Early asphalt specifications were based on attempts to replicate mixtures that had already proven themselves in practice. This approach implies specifying the same constituent materials that are mixed in the same proportions using the same procedures for mixing, transportation, laying and compaction under the same temperature and climatic conditions. This approach is generally called a recipe or recipe-type s...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Dedication
• Contents
• List of tables
• List of figures
• Preface
• Author
• 1. Introduction
• 2. Specifications for asphalt
• 3. Composition
• 4. Surface characteristics
• 5. Surface course properties
• 6. Structural properties
• 7. Serviceability properties
• 8. Sustainability
• 9. Summary
• Index