OVERSEAS_ROAD_NOTE_3

Overseas Road Note 3 (2nd Edition) A guide to surface dressing in tropical and sub-tropical countries ORN 3 (2 nd Edition) First Published 2000 ISSN 0951-8797 Copyright TRL Limited 2000. Subsector: Transport Theme: T2 Project title: A guide to surface dressing in tropical and sub-tropical countries Project reference: D251 This document is an output from a project funded by the UK Department for International Development (DFID) for the benefit of developing countries. The views expressed are not necessarily those of the DFID. IRL is committed to optimising energy efficiency, reducing waste and promoting recycling and re-use. In support of these environmental goals, this report has been printed on recycled paper, comprising 100% post-consumer waste, manufactured using a TCF (totally chlorine free) process. ii ACKNOWLEDGEMENTS This Overseas Road Note was produced in the International Division of TRL (Director Mr S W Colwill) on behalf of the Department for International Development. The research has been carried out with the active collaboration of highway authorities in many countries and their help and cooperation has been essential to the success of the project. The TRL project team responsible for this Road Note were Mr H R Smith (Project Officer), Dr J Rolt and Mr W G Ford. The helpful comments of the World Bank are gratefully acknowledged. Where necessary, use has been made of work published by other research and road authorities. OVERSEAS ROAD NOTES Overseas Road Notes are prepared principally for road and transport authorities in countries receiving technical assistance from the British Government. A limited number of copies is available to other organisations and to individuals with an interest in roads overseas, and may be obtained from: International Development Advisory and Information Unit TRL Limited Crowthome, Berkshire, RG45 6AU United Kingdom Limited extracts from the text may be reproduced provided the source is acknowledged. For more extensive reproduction, please write to the address given above. iii iv CONTENTS Page Preface 1 1 Introduction 3 2 Types of surface dressing 3 Single surface dressing 3 Double surface dressing 3 Triple surface dressing 4 Racked-in surface dressing 4 Other types of surface dressing 5 3 Chippings for surface dressings 5 4 Bitumens 6 Prime coats 6 Bitumens for surface dressings 7 Penetration grade bitumens 7 Bitumen emulsion 7 Cutback bitumens 8 Polymer modified bitumens 9 Adhesion agents 10 5 Design 10 Existing site conditions 10 Selecting the binder 12 Choice of binder and timing of construction work 13 Designing the surface dressing 13 Basis for the design method 13 Determining the average least dimension of chippings 14 Determining the overall weighting factor 15 Determining the basic bitumen spray 15 Spray rate adjustment factors 15 Adjusting rates of spray for maximum durability 17 Surface dressing design for low volume roads 17 Spread rate of chippings 17 6 Plant and equipment Methods of distributing binder 18 Constant volume distributors 18 Constant pressure distributors 19 Principal components of binder distributors 19 Spray bars and spray jets 19 Binder pumps and air pumps 20 Tanks and burners 20 v Page Distributor speed control and calibration 20 Chip spreaders 21 Rollers and other equipment 21 7 The surface dressing process 22 Planning 22 The surface dressing operation 23 After-care 23 8 Other surface treatments 24 Slurry seals 24 Otta seal 25 Sand seals 25 Synthetic aggregate and resin treatments 25 Applications of light bitument sprays 25 Fog sprays 26 Enrichment sprays 26 9 References 26 Appendix A: Requirements for grading and particle shape (Reproduced from BS 63: Part 2: 1987) 28 Appendix B: Recommended polished stone values of chippings for roads in Britain 29 Appendix C: The immersion tray test for determining the concentration of adhesion agent required 30 Appendix D: The probe penetration for test for measuring road surface hardness 31 Appendix E: Example of a surface dressing design 32 Appendix F: Tests for uniformity of transverse distribution of binder (depot tray tests) (reproduced from British Standard 1707:1989) 33 Appendix G: Bitumen distributor: rate of spread/speed calibration 35 Appendix H: Miscellaneous equipment required for a surface dressing unit 35 vi Preface This Road Note is the 2nd edition of Overseas Road Note 3 'A guide to surface dressing in tropical and sub-tropical countries' The revisions include descriptions of a wider range of types of surface dressing, current materials specifications and more detail on the use of bitumen emulsions. An equation has been introduced to allow the design spray rate to be calculated as an alternative to reading from a nomograph. A simplified presentation of spray rate adjustment factors, related to different site conditions, has also been provided. The durability of surface dressings, particularly for low volume roads, is discussed and additional spray rate adjustment factors have been suggested for these roads. The use of Otta seals has also been suggested for low volume roads. It is important that this Road Note, is not regarded as a specification. However, the advice contained in this Road Note together with local experience of relevant materials and surface dressing performance should be of value to those drawing up specifications. 1 2 1 Introduction 1.1 Surface dressing is a simple, highly effective and inexpensive road surface treatment if adequate care is taken in the planning and execution of the work. The process is used throughout the world for surfacing both medium and lightly- trafficked roads, and also as a maintenance treatment for roads of all kinds. 1.2 Surface dressing comprises a thin film of binder, generally bitumen or tar, which is sprayed onto the road surface and then covered with a layer of stone chippings. The thin film of binder acts as a waterproofing seal preventing the entry of surface water into the road structure. The stone chippings protect this film of binder from damage by vehicle tyres, and form a durable, skid-resistant and dust-free wearing surface. In some circumstances the process may be repeated to provide double or triple layers of chippings. 1.3 Surface dressing is a very effective maintenance technique which is capable of greatly extending the life of a structurally sound road pavement if the process is undertaken at the optimum time. Under certain circumstances surface dressing may also retard the rate of failure of a structurally inadequate road pavement by preventing the ingress of water and thus preserving the inherent strength of the pavement layers and the subgrade. 1.4 In addition to its maintenance role. surface dressing can provide an effective and economical running surface for newly constructed road pavements. Existing roads with bituminous surfacings, carrying in excess of 1000 vehicles/lane/day, have been successfully surfaced with multiple surface dressings. For sealing new roadbases traffic flows of up to 500 vehicles/lane/day are more appropriate, although this can be higher if the roadbase is very stable or if a triple seal is used. A correctly designed and constructed surface dressing should last at least 5 years before resealing with another surface dressing becomes necessary. If traffic growth over a period of several years necessitates a more substantial surfacing or increased pavement thickness, a bituminous overlay can be laid over the original surface dressing when the need arises. 1.5 The success of a surface dressing depends primarily on the adhesion of the chippings to the road surface, hence both the chippings and the road surface must be clean and free from dust during the surface dressing process. Inappropriate specifications, poor materials, and bad workmanship, can also drastically reduce the service life of a surface dressing. 1.6 This Road Note is a general guide to the design and construction of surface dressings in tropical and sub-tropical environments and draws attention to some of the more common mistakes that are made. It provides a framework on which the engineer can base more specific decisions made to suit particular local conditions thereby producing cost effective results. It also contains brief descriptions of certain other types of surface treatment. 2 Types of surface dressing 2.1 Surface dressings can be constructed in a number of ways to suit site conditions. The common types of dressing are illustrated in Figure 1. Single surface dressing 2.2 When applied as a maintenance operation to an existing bituminous road surface a single surface dressing can fulfil the functions required of a maintenance re-seal, namely waterproofing the road surface, arresting deterioration, and restoring skid resistance. A single surface dressing would not normally be used on a new roadbase because of the risk that the film of bitumen will not give complete coverage. It is also particularly important to minimise the need for future maintenance and a double dressing should be considerably more durable than a single dressing. However, a 'racked-in' dressing (see paragraph 2.7) may be suitable for use on a new roadbase which has a tightly knit surface because of the heavier applications of binder which is used with this type of single dressing. Double surface dressing 2.3 Double surface dressings are robust and should be used when: · A new roadbase is surface dressed. · Extra 'cover' is required on an existing bituminous road surface because of its condition (e.g. when the surface is slightly cracked or patched). · There is a requirement to maximise durability and minimise the frequency of maintenance and resealing operations. 2.4 The quality of a double surface dressing will be greatly enhanced if traffic is allowed to run on the first dressing for a minimum period of 2-3 weeks (and preferably longer) before the second dressing is applied. This allows the chippings of the first dressing to adopt a stable interlocking mosaic which provides a firm foundation for the second dressing. However, traffic and animals may cause contamination of the surface with mud or soil during this period and this must be thoroughly swept off before the second dressing is applied. Such cleaning is sometimes difficult to achieve and the early application of the second seal to prevent such contamination may give a better result. 3 Figure 1 Type of surface dressings 2.5 Sand may sometimes be used as an alternative to chippings for the second dressing. Although it cannot contribute to the overall thickness of the surfacing, the combination of binder and sand provides a useful grouting medium for the chippings of the first seal and helps to hold them in place more firmly when they are poorly shaped. A slurry seal may also be used for the same purpose (see paragraph 8.2). Triple surface dressings 2.6 A triple surface dressing (not illustrated in Figure 1) may be used to advantage where a new road is expected to carry high traffic volumes from the outset. The application of a small chipping in the 4 third seal will reduce noise generated by traffic and the additional binder will ensure a longer maintenance-free service life. Racked-in surface dressing 2.7 This system is recommended for use where traffic is particularly heavy or fast (TRL, 1996). A heavy single application of binder is made and a layer of large chippings is spread to give approximately 90 per cent coverage. This is followed immediately by the application of smaller chippings which should ‘lock-in' the larger aggregate and form a stable mosaic. The amount of bitumen used is more than would be used with a single seal but less than for a double seal. The main advantages of the racked-in surface dressing are: · Less risk of dislodged large chippings. · Early stability through good mechanical interlock. · Good surface texture. Other types of surface dressing 2.8 'Sandwich' surface dressings are principally used on existing binder rich surfaces and sometimes on gradients to reduce the tendency for the binder to flow down the slope. 2.9 'Pad coats' are used where the hardness of the existing road surface allows very little embedment of the first layer of chippings, such as on a newly constructed cement stabilised roadbase or a dense crushed rock base. A first layer of nominal 6mm chippings will adhere well to the hard surface and will provide a 'key' for larger l0mm or l4mm chippings in the second layer of the dressing. 3 Chippings for surface dressings 3.1 The selection of chipping sizes is based on the volume of commercial vehicles having unladen weights of more than 1.5 tonnes and the hardness of the existing pavement. Ideally, chippings used for surface dressing should be single sized, cubical in shape, clean and free from dust, strong, durable, and not susceptible to polishing under the action of traffic. In practice the chippings available usually fall short of this ideal but it is recommended that chippings used for surface dressing should comply with the requirements of BS 63: Part 2 (1987) for the nominal size of chipping selected by the engineer. In this standard, some control of shape is ensured by the limits set for the flakiness index for each nominal size (except 6mm). Part of BS 63 is reproduced in Appendix A. 3.2 Samples of the chippings should be tested for grading, flakiness index, aggregate crushing value and, when appropriate, the polished stone value and aggregate abrasion value. Sampling and testing should be in accordance with the methods described in British Standard BS 812 (1985,1989a, 1989b 1990a, 1990b). 3.3 Specifications for maximum aggregate crushing value (ACV) for surface dressing chippings typically lie in the range 20 to 35. For lightly trafficked roads the higher value is likely to be adequate but on more heavily trafficked roads a maximum ACV of 20 is recommended. 3.4 The polished stone value (PSV) of the chippings is important if the primary purpose of the surface dressing is to restore or enhance the skid resistance of the road surface. The PSV required in a particular situation is related to the nature of the road site and the speed and intensity of the traffic (Salt and Szatkowski, 1973). The resistance to skidding is also dependent upon the macro texture of the surface which, in turn, is affected by the durability of the exposed aggregate. This property is measured by the aggregate abrasion value (AAV). Appendix B gives recommended values of PSV and AAV for various road and traffic conditions in Britain and provides an indication of the required aggregate properties. 3.5 The nominal sizes of chippings normally used fo r surface dressing are 6, 10, 14 and 20 mm. Flaky chippings are those with a thickness (smallest dimension) less than 0.6 of their nominal size. The proportion of flaky chippings clearly affects the average thickness of a single layer of the chippings, and it is for this reason that Jackson (1963) introduced the concept of the 'average least dimension' (ALD) of chippings. 3.6 In effect, the ALD is the average thickness of a single layer of chippings when they have bedded down into their final interlocked positions. The amount of binder required to retain a layer of chippings is thus related to the ALD of the chippings rather than to their nominal size. This is discussed further in Section 5 where guidance is given on the selection of the appropriate nominal size of chipping and the effect of flakiness on surface dressing design. 3.7 The most critical period for a surface dressing occurs immediately after the chippings have been spread on the binder film. At this stage the chippings have yet to become an interlocking mosaic and are held in place solely by the adhesion of the binder film. Dusty chippings can seriously impede adhesion and can cause immediate failure of the dressing. 3.8 The effect of dust can sometimes be mitigated by dampening them prior to spreading them on the road. The chippings dry out quickly in contact with the binder and, when a cutback bitumen or emulsion is used, good adhesion develops more rapidly than when the coating of dust is dry. 3.9 Most aggregates have a preferential attraction for water rather than for bitumen. Hence if heavy rain occurs within the first few hours when adhesion has not fully developed, loss of chippings under the action of traffic is possible. where wet weather damage is considered to be a severe risk, or the immersion tray test, described in Appendix C, shows that the chippings have poor affinity with bitumen, an adhesion agent should be used. An adhesion agent can be added to the binder or, used in a dilute solution to pre-coat the chippings. However, the additional cost of the adhesion agent will be wasted if proper care and attention is not given to all other aspects of the surface dressing process. 5 3.10 Improved adhesion of chippings to the binder film can also be obtained by pre-treating the chippings before spreading. This is likely to be most beneficial if the available chippings are very dusty or poorly shaped, or if traffic conditions are severe. There are basically two ways of pre- treating chippings: · Spraying the chippings with a light application of creosote, diesel oil, or kerosene at ambient temperature (NAASRA, 1986). This can be conveniently done as the chippings are transferred from stockpile to gritting lorries by a belt conveyor or. alternatively, they can be mixed in a simple concrete mixer. · Pre-coating the chippings with a thin coating of hard bitumen such that the chippings do not stick together and can flow freely. 3.11 Chippings which are pre-coated with bitumen enable the use of a harder grade of binder for construction which can provide early strong adhesion and thus help to obtain high quality dressings. The binder used for pre-coating need not necessarily be the same kind as that used for the surface dressing; for example, tar-coated chippings adhere well to a sprayed bitumen film. Pre-coating is usually undertaken in a hot-mix plant and the hardness of the coating, and thus the tendency for the chippings to adhere to each other, can be controlled by the mixing temperature and/or the duration of mixing; typical coating temperature are about 1400C for bitumen binders and 1200C for tar binders. Table 1 indicates the amount of binder recommended for lightly coating chippings. Table 1 Binder contents for lightly-coated chippings Target binder content (per cent by mass) Nominal size of Bitumen Tar chippings (mm) (TRL, 1996) (TRL, 1992) 6 1.0 1.2 10 0.8 1.0 14 0.6 0.8 20 0.5 0.7 Reproduced from Road Note 39 3.12 Pre-coated chippings should not be used with emulsions because the breaking of the emulsion will be adversely affected. 3.13 In some countries adhesion agents or pre-treated chippings are often used in an attempt to counteract the adverse effect of some fundamental fault in the surface dressing operation. If loss of chippings has occurred, it is advisable to check whether the viscosity of the binder was appropriate for the ambient road temperature at the time of spraying. The effectiveness of the chipping and traffic control operations should also be reviewed 6 before the use of an adhesion agent or pre-treated chippings is considered. 4 Bitumens 4.1 It is essential that good bonding is achieved between the surface dressing and the existing road surface. This means that non-bituminous materials must be primed before surface dressing is carried out. Prime coats 4.2 Where a surface dressing is to be applied to a previously untreated road surface it is essential that the surface should be dry, clean and as dust-free as possible. On granular, cement or lime-stabilised surfaces a prime coat of bitumen ensures that these conditions are met. The functions of a prime coat can be summarised as follows. · It assists in p