ROAD SURFACE DRESSING ASSOCIATION

 

 

 

 


GUIDANCE NOTE ON

SURFACE DRESSING

 

 

 

 

 

 

BINDERS

 

 

 

 

 

 

                                              

 

 

                                               Consultant Director and Secretary

 

                                      John Baxter

                                      Westwood Park

London Road

Little Horkesley

                                      Colchester

                                      Essex CO6 4BS

                                     

 

 

 

 

 

 

 


Ó Copyright RSDA 1993

     First Published

     Revised 2001

     Second Revision 2004

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
No part of this document may be

reproduced without the written permission

of the Road Surface Dressing Association

 
 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


R S D A Binders Guidance Note 2004

 

 

 

 

 

INDEX

 

 

 

Page No

 

1

Introduction

1

 

2

The Functions and Properties of Binders

2

 

3

Binders Available

3

 

4

Properties Associated with Binders

3

 

5

Selection of Binders To Meet Site Requirements

5

 

6

Handling of Surface Dressing Binders

6

 

7

Changing the Type of Binders used in a Spraying Machine

6

 

 

Appendix A: Test methods for bitumen and surface dressing binders

 

8

 

 

Appendix B: Glossary of terms relating to surface dressing binders

17

 

 

 

ROAD SURFACE DRESSING ASSOCIATION

 

Surface Dressing Binders

 

 

1.     INTRODUCTION

 

Surface dressing comprises spraying the road surface with a film of binder followed by the application of a layer of stone chippings.  After rolling and light trafficking, any excess of chippings is swept up.  After the dressing has stabilised it is opened to traffic.  The main functions of surface dressing are:

 

1.                  To seal the road surface against ingress of water

2.                  To arrest any surface disintegration

3.                  To improve skid resistance and texture

 

It is the combination of chippings of suitable quality and size, held in place by an effective binder, which results in satisfactory surface dressing.  Road Note 39 provides sound guidance on surface dressing design and practice. 

 

In Road Note 39, which should be considered as essential reading, the selection of the correct size of chippings and the required rate of binder application is based on:

 

1.            The number of heavy commercial vehicles of more than 1.5 tonnes unladen weight per lane, per day.

2.            The hardness of the road

3.            The degree of difficulty – i.e. to what extent it is subjected to braking, turning and accelerating stresses

4.            The existing condition of the road to be re-surfaced.

5.            Precise requirements or specification of the finished dressing.

 

It should be noted that Road Note 39 contains essentially general and “target” recommendations providing sound overall basic guidance.  These recommendations must be considered, together with any specialised information available on binders or local conditions, in order that the best possible work specification is devised.

 

 

2.           THE FUNCTIONS AND PROPERTIES OF BINDERS

 

2.1        Functions:

 

The functions of the binder in surface dressing are

 

-                      to provide adequate adhesion between the chippings and the road surface

-                      to seal the surface of the road against ingress of water

-                      To arrest disintegration of the existing road surface

 

Any other function of the binder is dependent on the nature of the road and the traffic stresses applied to the surface dressing.  It is unlikely that all binders will prove to have properties which will give optimum performance under all conditions.  Nevertheless, to achieve the benefits of these functions, certain properties will be common to all binders.

 

2.2.     Properties

The following properties are necessary if binders are to be used successfully for surface dressing:

 

-                      they must be sufficiently fluid to spray

-                      they must be sufficiently fluid to “wet” the road surface and the chippings

-                      they must be sufficiently stiff (i.e. have sufficient cohesive strength) to hold the chippings against traffic forces in both the early stages and longer term

-                      they must not become so brittle in cold weather that chipping loss occurs

-                      they must not become so soft in hot weather that loss of chippings and bleeding occurs

 

2.2              Viscosity

 

The word “fluid” as used in the paragraph above indicates the ease with which a binder can flow.  Fluidity can be considered in terms of viscosity. 

 

The method of measurement of viscosity varies depending on the type of material being applied:

 

Cut-back bitumens                S.T.V. seconds @ 40°C

Hot bitumen emulsions           Redwood II seconds @ 85°C

                               Or       Brookfield viscosity

                               Or       Rheological measure

 

Currently, only the British Standards grade (non-proprietary) materials are specified, i.e. BS3690 part 1 and BS434 part 1. (Note: at the time of writing, these standards are undergoing modification or may soon be superseded by European standards.)  The binder supplier will specify proprietary materials.

 

The viscosity of surface dressing binders varies with temperature.  As the temperature rises, the viscosity decreases.  Use is made of this property in order to select the appropriate temperature for spraying each binder. The spraying temperature is regulated to provide the correct viscosity, to achieve good transverse distribution across the spraybar and to secure “wetting” of the road surface and the chippings (i.e. to achieve a good “bond”). 

 

If the viscosity is too high, it is unlikely that an even transverse distribution will be achieved.  Furthermore, good wetting will not occur and chippings will be lost.  However, if the viscosity is too low the material could flow off the high points of the road and pool in the low spots, or run off the road altogether.  Once again, the transverse distribution could be compromised.

 

3.     BINDERS AVAILABLE

 

The first binders used at the beginning of the century were tars, which were a by-product of the production of coal gas.  Today, these materials are usually obtained by distillation from coal used either in the production of coke for the steel industry, or in the production of smokeless fuel. 

 

With the advent of the oil industry, bitumens became available as a heavy residue from the distillation of crude oil.  Blends of both bitumen and tar were introduced in the late 1950s. 

 

In order to achieve a workable viscosity for application, bituminous binders were treated in two main ways:

 

1)           By the addition of volatile oils to reduce the viscosity during transport and application (allowing good transverse distribution and wetting of road and chipping).  The volatile oils then evaporate, leaving a stiff residual material.  Such materials are called “Cut-back” materials

(section 4.1).

2)           By emulsifying the stiff bitumens, creating very small droplets of bitumen dispersed in water.  The low viscosity of the water aids application, yet once applied, the water is lost leaving only the stiff bitumens.  Such materials are called emulsions (section 4.2).

 

4.           PROPERTIES ASSOCIATED WITH BINDERS

 

The following properties are associated with the different binders:

 

4.1        Cut-Back

 

Cut-back binders can contain base bitumens which are straight run binders or binders which are modified (section 4.3).  Their use tends to be limited to localised areas.

 

For the dressing that used unmodified cut-backs, BS3690 part 1 defines the specifications.  These specifications are empirical, looking at basic characteristics such as viscosity, distillation characteristics, etc.  with little or no attempt to predict site performance.

 

Cut-back binders are defined in terms of their flow characteristics (viscosity STV at 40°C), namely 50 seconds, 100 seconds or 200 seconds.

 

Very often the cut-back bitumen contains adhesion agents to assist in the development of the chipping/binder bond.

 

4.2        Emulsions

 

Emulsions are traditionally defined by chemical nature, stability and bitumen content.  Such that a K1-70 emulsion is cationic or acidic in nature (K), rapid break (1) and contains nominally 70% bitumen (70).  BS434 part 1 (or BSEN 13808 to be published) more fully defines the specification.  Once again, the specification is fairly basic, covering bitumen content and viscosity (Redwood II @ 85°C).

 

Bitumen emulsions have the benefit of:

 

-                      enabling a binder of higher viscosity (once the emulsion has broken) to be applied to the road at a lower temperature than cut-back binders

-                      can be used on damp (but not wet) roads using damp chippings.  The bond is achieved once the emulsion is “broken”.

 

The process by which the bitumen emulsion reverts to its original water and bitumen components is known as “breaking”.  The break of an emulsion is often indicated by a progressive change in colour from brown to black.  This break is a complex process in which, initially, bitumen separates from the water in the emulsion onto the road and onto the applied chippings.  The final stage of break involves the loss of water by evaporation so that a continuous adhesive film of binder is produced.  The loss of water from an emulsion will be delayed if the road surface is very wet or if there is a high level of moisture in the air (i.e. if it is very humid).  These conditions will unduly prolong the breaking period.  Because emulsions contain water, they must be protected from frost whilst in storage.

 

4.3        Modified Materials

 

The majority of materials sprayed today contain modification in many forms, chiefly of synthetic elastomers, plastomers or natural elastomers.

 

The benefits of such modification can be seen in the fact that they exhibit:

 

-                      higher resistance to shear

-                      increased tenacity of bond

-                      reduced susceptibility to extremes of temperature

 

In simple terms, this increases the performance of the binder in service particularly in the following areas:

 

a)           increasing the temperature at which the binder begins to soften, reducing the occurrence of bleeding and fatting at stress areas

b)           improves the low temperature adhesion and elasticity during the serviceable life of the binder (reduced embrittlement)

c)           improved elasticity to bridge hairline cracks

d)           improved early stability in the dressing

e)           improved long term adhesion of the dressing

f)             improved longer term durability of the dressings as thicker binder films can be applied

g)           extends the life of the dressing, particularly when high quality aggregate is used

 

4.4        Modified Emulsion Binders

 

In addition to the benefits listed in 4.3, modified emulsion binders also:

 

a)           initially perform better in damp conditions

b)           perform better in high humidity

c)           perform better in areas of higher annual rainfall

 

4.5        Thermosetting binders

 

The hardening of thermosetting binders relies on a chemical reaction which is irreversible.  Once this process has taken place it is said to have “cured”.  The main properties of these binders are that they:

 

a)           have a very high resistance to stress

b)           do not rely on embedment of chippings

c)           have a long life if applied to a sound surface

d)           are very durable at low temperatures

 

5.     SELECTION OF BINDERS TO MEET SITE REQUIREMENTS

 

Good results can be achieved with all the binders referred to above, given proper preparation, design, execution and after-care.  However, the matching of binders to sites representing different degrees of difficulty requires care. 

 

5.1              SPECIFICATION

 

Non-modified cut-back are currently specified in BS3690 part 1.

 

Emulsions are currently specified in BS434 part 1.

 

At the time of writing, prEN 13308 covering both emulsions and cut-backs (modified and non-modified) is being written.

 

The British Board of Agrement run a HAPAS scheme (Highway Authority Product Approval Scheme) under SG4 for modified binders for surface dressing, bond coats and microsurfacings.  This scheme is a five stage assessment of the binder covering lab performance to specific tests, Quality Assurance of the supplier and monitored test site trials.

 

The laboratory testing involves more performance type testing than is demanded in the non-modified specification.  Key tests are:

 

a)           Vialit Pendulum Cohesion test – measures of the cohesivity (bitumen to bitumen strength) of the recovered bituminous material (recovered from the emulsion) over a range of temperatures.  Typically report peak cohesion strength, temperature at which peak cohesion is achieved, and the range of temperature over which a cohesion threshold is reached.

 

b)           Determination of the complex shear modulus and phase angle by the DSR method.  This method determines the stiffness characteristics of the recovered bitumen binder over a range of temperatures and oscillation frequencies.  It also provides indications of the visco elastic behaviour of the bitumen at different temperatures and oscillation frequencies.

 

Typically quoted are equi-stiffness temperatures at 2MPa and 2kPa (to provide a feel for temperature susceptibility) and complex moduli at 0.4Hz frequency and various temperatures.

 

c)