BEST PRACTICE GUIDELINES FOR SPECIFICATION OF MODERN NEGATIVE TEXTURE SURFACES (NTS) ON LOCAL AUTHORITY HIGHWAYS

Transcription

1 BEST PRACTICE GUIDELINES FOR SPECIFICATION OF MODERN NEGATIVE TEXTURE SURFACES (NTS) ON LOCAL AUTHORITY HIGHWAYS

2 Contents Page No 1 EXECUTIVE SUMMARY 1 2 INTRODUCTION Overview and Objectives Code of Practice for Highway Maintenance Management Scope of Application and Use Terms and Definitions References Summary 7 3 PROCESS AND MATERIALS DEVELOPMENT Development History Generic Systems Proprietary Systems References Summary 15 4 SITE CONSIDERATIONS AND INSTALLATION General Serviceability Substrate Existing Pavement Bond Coat Early Life Surface Characteristics Skid Resistance Characteristics Horses on the Highway Installation Installers and Inspection References Summary 26 5 THE RIGHT MATERIAL FOR THE RIGHT SITE Site Evaluation Chart A Chart B Chart C Chart D Chart E 36 6 EXPIRED LIFE ENGINEERING Introduction Failure Groups Types of Deterioration Deterioration Model References Summary 52 7 LOCAL AREA REINSTATEMENT Performance Characteristics 53 i

3 7.2. Reinstatement Size and Position Installation and Compaction Street Furniture and Ironwork Summary 58 8 ASSET MANAGEMENT Asset Management Plan Whole Life Costing Winter Maintenance Distress Mechanism and Forecast References Summary 62 Figures Figure 1 - Structure and relationship of Best Practice Guidelines 5 Figure 2 - Schematic diagram of positive and negative textures. 6 Figure 3 - Chart A 31 Figure 4 - Chart B 32 Figure 5 - Chart C (1 of 2) 33 Figure 6 - Chart C (2 of 2) 34 Figure 7 - Chart D 35 Figure 8 - Chart E (1 of 2) 37 Figure 9 Chart E (2 of 2) 38 Figure 10 Fattting-up 41 Figure 11 - Fretting 45 Figure 12 - Mini paver machine 53 Figure 13 -Whole Life Cost 60 Tables Table 1 - British Standard surfacing mixtures for thin layers 9 Table 2 - Classification of Bond Coats by Vialit Test Value 20 Table 3 - Classification of bond coats by torque bond strength 21 Table 4 - Selection of Bond Coat 21 Table 5 - Site evaluation checklist 29 Table 6 Elements of a Deterioration Model 44 Appendices Appendix A - Functional Carriageway Hierarchy 64 Appendix B - Glossary of Terms and Definitions 65 Appendix C - Questionnaire 67 Appendix D Summary of Responses 68 Appendix E - Regional Workshops 71 Appendix F - BBA Scheme Guidance 72 Appendix G - Investigation Protocols 73 Appendix H Specifying Proprietary Materials 77 Appendix I Installers Observations 81 Acknowledgements 84 ii

4 1 EXECUTIVE SUMMARY These Best Practice Guidelines for Negative Texture Surfaces (NTS) provide a methodology for site evaluation and material selection, to ensure that the right material is installed in the right site, together with a structured approach to the factors which may have a bearing on distress mechanisms. Enquiry responses have indicated generally that NTS is performing equally well or better than traditional materials in most circumstances on local authority highways. Substrate condition evaluation is essential in the selection of NTS on evolved local authority highways. The surface layer on thin local highways can constitute a significant proportion of the sound bituminous material in the pavement structure. The wide variety of proprietary system surfaces which are available broadens the scope of sites that are potentially suitable for their application. However, there are many situations where generic types of surface material may provide serviceability at best value. A rational approach to the engineering selection of NTS is provided as a series of flow diagrams. A checklist together with a series of decision charts enables a consistent approach for site evaluation by consideration of five distress modes for the substrate. An investigation of the existing carriageway is particularly important for thin flexible local pavements to ensure optimum maintenance designs. Investigation protocols have been developed to support this. A surface material decision chart considers key engineering performance requirements to ensure strain compatibility between surface layer material and support substrate. A process of bond coat selection is developed to ensure good inservice adhesion between the surface and the substrate. All sites for surfacing should be inspected and evaluated by experienced and knowledgeable personnel with joint client/contractor inspection as appropriate. An assessment should be undertaken of the period of time required for removal of binder film in early life through vehicle wheelovers. Attention to detail in the location and formation of joints, especially in highly stressed areas will avoid premature distress in an otherwise serviceable surface. Fretting and cracking were found to be the most significant mechanisms of deterioration in NTS. Five factors of influence for causes of distress of NTS have been determined and related to a mechanistic deterioration model. The influencing factors do not always behave in isolation and are frequently inter-dependent. For local area repair the care and preparation of the area to be reinstated is at least as important as the installation of the surface material itself. Failure of NTS, when it occurs, can be a swift process. A procedure to predict the onset of distress by forecasting has not been 1

5 identified. This is a significant area for future study, since the absence of an appropriate deterioration model could result in lack of certainty in asset management planning. 2

6 2 INTRODUCTION 2.1. Overview and Objectives Negative Texture Surfaces (NTS) comprise a family of modern asphalt materials which have been developed to provide safe, durable and quieter highway surfaces. The common feature of NTS is that surface texture, an essential component of skid resistance, is provided in a downward (negative) direction beneath the vehicle tyres. The NTS family comprises a suite of proprietary surfaces, known collectively as thin surfacings and generic stone mastic asphalt. Thin surfacings were developed to provide safety through the adequate provision and retention of negative texture over time. Generic stone mastic asphalts used in the UK have been modified from the original European materials to provide a sufficient installed surface texture The Roads Liaison Group (RLG) was established in 2001 in order to advise local authorities and Central Government on highway maintenance issues. Members of RLG include the Department for Transport (DfT), the Local Government Association (LGA), and representatives of national and local highway authorities in England, Scotland, Wales and Northern Ireland. The RLG is supported by four boards - Roads, Bridges, Lighting and Traffic Management. This project was commissioned by the DfT on behalf of the UK Roads Board and these Best Practice Guidelines have been developed for application on the local authority highway network A major element of local road maintenance relates to the surface course of the highway. Hot rolled asphalt (HRA) surface course was the standard road surface material throughout the country for very many years but this traditional, virtually impermeable, positive surface texture material has swiftly given way to NTS The terms thin surfacing (TS) and stone mastic asphalt (SMA) have been in common use for about a decade and consequently they have been retained in this document but all such materials belong to the highway surface performance family of NTS materials The impact of NTS has been so great that some authorities have virtually ceased to use traditional rolled asphalt surfaces. The rapid introduction, however, has meant that there has been little time for longer term performance lessons to be learnt on the network Local authority enquiry responses undertaken during development of these Guidelines have indicated, overall, that NTS is performing as well or better than, traditional materials in most circumstances where the right material has been installed in the right site. Development of innovative surface materials will continue in the future with advancing technology and therefore these Guidelines are not timeless. These Guidelines should be reviewed within an elapsed period of five years to accommodate any new developments, incorporate performance feedback, consider the implications of further research and address the introduction and embedment of European Standards for asphalt surface materials. 3

7 The core objective of these Best Practice Guidelines is to define distress conditions of NTS to aid both engineering understanding and asset management planning for the network. This will assist in a managed change from reactive to programmed maintenance as recommended in the Code of Practice for Highway Maintenance Management. These Guidelines have been developed to enable a rational decision making and selection process for NTS materials Code of Practice for Highway Maintenance Management The Code of Practice for Highway Maintenance Management, Well Maintained Highways, describes good practice and sets out a series of recommendations for highway authorities to implement Figure 1 illustrates the relationship of these Best Practice Guidelines for NTS with the Code of Practice, asset management and local transport planning These Guidelines should be read in the context of these documents, as they provide more detailed guidance for the application of NTS in the delivery of the core objectives of the Code of Practice: Safety; Serviceability; Sustainability; Customer service. 4

8 Government Transport Policy Local Transport Planning Guidance CSS Framework for Highway Asset Mnaagment Codes of Practice Highway Maintenance Management Management Of Highway Structures Highway Lighting Management Best Practice Guidelines Negative Texture Surfaces (NTS) Figure 1 - Structure and relationship of Best Practice Guidelines The Code defines carriageway hierarchy functionality which is reproduced in Appendix A. It is recommended that this hierarchy should be developed by an authority to take into account current and expected traffic characteristics. It is this relationship between traffic and hierarchy that is developed later in these Guidelines as the basis for selection of surfacing. This approach is consistent with the Code and local transport planning. The hierarchy and subsequent design approach adopted for surfacing should also be consistent with adjoining authorities, to meet road users reasonable expectations In the selection of surfacing, authorities should consider both the future maintenance implications of the surfacing together with any environmental implications. The Code provides designers with check lists to assist with the development of designs for each of these aspects. These checklists should be used by surfacing specifiers in the context of these Guidelines. Local Transport Plan guidance identifies that additional maintenance costs arising from all new and improved infrastructures should be explicitly identified and taken into account in evaluating the whole life cost of the scheme. This will include the surfacing aspects of the maintenance or improvement works. 5

9 2.3. Scope of Application and Use Distress which has occurred in NTS has generally been attributed to inappropriate site conditions, selection of material or installation practices. These factors are much more significant on the local network than on the strategic network. This is due to the evolved nature of the local network, its more variable construction, geometry, junctions, layout and the frequency and scale of highway maintenance works The development of these Guidelines was undertaken through dialogue with a number of stakeholders, including clients, specifiers, installers and highway maintenance engineers, together with examination of the surfacing failure mechanisms. This was conducted with the objective of producing robust guidance of value to the profession The early life surface frictional characteristics of negatively textured surfaces are not within the scope of these Guidelines and are covered in detail elsewhere. However, the overall mechanisms are reviewed insofar as they inform a distress condition The focus of the Guidelines is the right material for the right site to ensure value for money and maximum longevity in service. The guidelines are articulated through engineering performance and materials science factors. Decision charts provide a process for site evaluation and surface materials selection. No attempt is made to define which material should be used in particular circumstances but the Guidelines provide a broad mechanism for structured evaluation and choice of surfacing type in a highway maintenance context Terms and Definitions The provision of surface texture on a pavement is a key difference between traditional surfacing, such as HRA, and the family of NTS materials. Positive texture as provided by HRA with chippings and negative texture as provided by NTS is illustrated in Figure 2. Figure 2 - Schematic diagram of positive and negative textures. 6

10 There is no universally agreed categorisation for TS, even the limits of the term thin surfacing are ill-defined as the materials have developed and evolved. Terms and definitions are described in Appendix B and are based upon those published previously (Nicholls, Carswell and Langdale, 2002) but adapted and extended to address the wider scope of surfacing solutions that are the subject of these Guidelines References 1. Well Maintained Highways. Code of Practice for Highway Maintenance Management (2005), The Stationery Office, London. ISBN Walsh I.D. (2000), Out of the skid pan. Surveyor, 9th November 2000, pp Nicholls J.C, Carswell I. and Langdale P.C. (2002), Durability of thin asphalt surfacing systems. Part 1 Initial findings TRL Report 557, TRL Limited Summary These Best Practice Guidelines provide underpinning support for Well Maintained Highways. Surfacing selection should be based upon highway functionality. Future maintenance implications should be considered in surfacing selection. Surfacing distress is related to inappropriate choice of site, installation and materials selection. Local highway networks have evolved with variable construction, geometry, junctions and layout. The scope of the Guidelines includes both generic and proprietary surfacing. The Guidelines were developed through extensive consultation with stakeholders. Focus is the right material for the right site. 7

11 3 PROCESS AND MATERIALS DEVELOPMENT 3.1. Development History Before the availability of NTS the traditional surface course materials for pavements in the UK were HRA with high polish resistant precoated chippings, dense or close graded bitumen macadam (DBM) and surface dressing (SD). High stone content asphalt (HSCA), which has no surface applied pre-coated chippings, was also a frequently used surface material Selection of the surfacing material was dependent on: Traffic levels; Availability and cost of suitable crushed rock aggregate; Condition of substrate; Intended laying season The service life of HRA surfaces was generally between ten and twenty years and in some specific cases significantly longer. After this period it could be overlain by a new HRA surface course if levels permitted and this added significantly to the structural strength and flexibility of the pavement as a whole DBM (or close graded macadam) surfacing was a common choice on minor highways. It was generally expected to have a typical service life of between eight and fifteen years although during this period it may require surface dressing (SD) treatment. As with HRA it could be overlaid at the end of its service life and would continue to contribute to the structural strength of the pavement Many contractors developed variations on the British Standard BS 4987 close graded mixtures with more exacting control limits on the grading and special binder types. Such materials were used successfully in urban areas where thin overlays were required to address difficulties with access levels. Although many proprietary systems and local specifications provided for relatively durable surfacing down to 25mm nominal thickness, as did BS 4987, materials which could confidently be laid on more heavily trafficked or highly stressed sites were not available. Table 1 illustrates the BS surfacing mixtures which can be installed at thicknesses of 35mm or less. 8

12 British Standard BS BS BS Table 1 - British Standard surfacing mixtures for thin layers Description 15% Stone content 30% Stone content Fine graded surface course Aggregate size (mm) Thickness (mm) Reference 0/10 30 Table 6, Col 6/2 0/10 35 Table 6, Col 6/3 0/ Clause 7.7 BS BS BS BS Medium graded surface course Dense surface course Porous asphalt surface course Close graded surface course 0/ Clause 7.6 0/ Clause 7.5 2/ Clause 8.2 0/ Clause 7.4 Note: The above table is a means of illustration only and should not be taken as implying suitability of a particular surface A revision of Chapter 8 of the Traffic Signs Manual, in the early 1990s introduced a requirement for a safety barrier zone between operatives, plant and active traffic. For most rural and many urban highways this safety zone requirement meant that the machine application of chippings to the surface of HRA could not be used without a complete road closure At about the same time as the introduction of the increased construction site safety measures, some asphalt manufacturers were developing SMA and derivative materials. These products were based on continental practice, particularly German splittmastixasphalt. Other manufacturers chose to take out licences for the production of similar surface products which had already been developed overseas. The adoption and use of these materials reduced the contractual risk associated with chipping on HRA and also needed less resources for installation. Thus, the use of HRA, which was the favoured surface course material for many situations, became problematic and fell into reduced use Early NTS products included the Jean Lefebvre ULM thin asphaltic concrete, originally licensed to A. McAlpine Ltd, and Safepave, a paver-laid surface dressing licensed to Associated Asphalt Ltd. As these products were already fully developed they were capable of immediate larger scale application and were quickly adopted on a trial basis by many authorities The development, and client acceptance, of mixtures based on the German splittmastixasphalt, known in English speaking countries as SMA, lagged slightly behind the acceptance of the licensed systems. 9

13 In Northern Continental Europe the requirement was for durable dense surfacing mixtures for heavy traffic conditions. The delivery of optimum skid resistance, whilst desirable, did not assume the same priority as in the UK. The German practice was to use a relatively stiff bitumen binder with high bitumen content and high stone content. The lack of fine aggregate and filler in the mixture could lead to bleeding of bitumen during transport and laying. To guard against this the splittmastixasphalt mixture normally included cellulose fibres to partially immobilise the bitumen before laying The UK skid resistance requirements necessitated the provision of significant macro-texture depth. The specifications were relatively easily met by HRA with pre-coated chippings but with splittmastixasphalt formulations the stone matrix was virtually fully filled with bitumen. In developing UK equivalents, suppliers needed to be able to reduce the relative volume of binder-filler matrix. This was to allow adequate texture to remain on the surface after compaction. To ensure low permeability and therefore good durability, the bulk of the layer was fully filled with bitumen mastic. This compromise was satisfactorily achieved at bitumen contents in the order of 6%-7% by mass. The mixtures were particularly stable with aggregate interlock providing good resistance to wheel track rutting. Previous experience of bitumen rich SMA in Northern Europe had shown rutting under heavy traffic conditions Early versions of SMA in the UK suffered with development problems when the ratio of layer thickness to nominal stone size was less than two. Most of the currently available SMA and Thin Asphalt Concrete (TAC) variants now have a minimum layer depth close to, or in excess of, twice their nominal stone sizes Based upon trials and the use of SMA as a high stability surface course material (Nunn, 1994), many authorities developed generic specifications for the material and its installation Owing to the many proprietary surface systems which had entered the market, a product type approval evaluation system was developed by the Highways Agency. This process required the system proprietor to lay a trial site, usually on a trunk road. The manufacture and installation was monitored together with laboratory testing and upon completion of the trial a panel of experts inspected the site. The inspection process was repeated at regular intervals for at least two years Requirements for bituminous surface material and techniques to be used on the strategic network are encapsulated in the Design Manual for Roads and Bridges (DMRB) HD 37/99. Site trials confirmed the German experience that generic SMA mixtures are sensitive to small variations in aggregate grading and binder content. This can result in a reduction in surface texture as evidenced by a localised patchy appearance and fatting of the surface. Potential inconsistency in terms of texture retention, which is essential for high speed skid resistance means that SMA produced to a generic specification must not be used on the Highways Agency network. However, the constraints of traffic management and congestion on the local network are not so great and generic SMA has continued to be used. Additionally, there are differing 10

14 speed limits which are not so texture dependant enabling the installation of mixtures, which may be less exacting in this performance parameter than is the case for trunk roads The type approval scheme, although used by authorities as a specification requirement for NTS was not entirely appropriate for nontrunk road application. In 1995 the CSS and Highway Agency established the Highway Authorities Product Approval Scheme (HAPAS) to enable independent certification of the performance of proprietary surfacing products and systems. The British Board of Agrément (BBA) was appointed to administer the scheme. A specialist advisory group (SG3) was established in 1996 to broaden the scope of the product approval scheme. SG3 developed Guidelines for the Assessment and Certification of Thin Surfacing Systems. Various proprietary thin surfacing systems were assessed with the first certificate issued in By November 2005 twenty seven proprietary systems had been certified. Throughout these Guidelines the term product is taken as referring to the manufactured material and the term system to the product and its installation protocol The introduction of BBA certification did not make the use of generic specifications redundant. Over half of all authorities specify SMA surfacing by generic means for their network applications. These generic SMA specifications have generally been evolved in collaboration with local asphalt manufacturers to suit the local availability of mineral resources and local network needs. Some authorities use both generic specification and BBA certification selectively as the site requirements dictate Generic Systems British Standards, BS 594 for HRA and BS 4987 for Coated Macadam (asphalt concrete), provide a limited spectrum of surfaces suitable for layer thicknesses of less than 40mm Tables 3 and 4 of BS provide only a 0/2mm nominal size aggregate designed asphalt mixture for laying at 25mm thickness. The sand carpet mixture is unsuitable for carriageway surfacing application due to low surface macro-texture and potential mix instability. Table 6 of BS594-1 provides recipe specifications for 15% stone content 0/10mm nominal size mixtures for laying at a thickness of 30mm and a 30 % stone content, 0/10 nominal size mixture for 35mm thickness. The former material is generally only regarded as suitable for footway and very light vehicle use. The 30% (0/10) mixture can be used for light traffic application but the non-availability of a design mix equivalent inhibits its wider acceptance, particularly in areas where the native sand tends to produce unsuitable mixtures. However, with critical formulation and the use of modern polymer bond coats, the 30% (0/10) mixture may well be capable of meeting many of the requirements of a thin surfacing system, if designed in accordance with the principles of Specification for Highway Works (SHW) Clause 943. Since Clause 943 does not include for modification to the 30% (0/10) mixture the resultant material would need to be specified as a special material or manufactured as a proprietary product. 11

15 BS 4987 has been the traditional source for the specification of thin surfacing layers. Many of these British Standard formulations perform well and when specified and installed in appropriate situations in the network provide good value for money. BS , Table 6 specifies nominal and minimum layer thicknesses for coated macadam indicating seven mixtures capable of being laid at depths of 35mm or less. The 0/10 close graded surface course is suitable for general highway use but the 0/6 dense surface course material cannot be relied upon to provide adequate surface texture depth for higher speed applications. The open graded mixtures can provide adequate surface texture but can fret in high lateral stress areas. The 0/10 close graded mixture is unlikely to produce a texture depth suitable for areas with a speed limit above 30mph. As with the 0/10 HRA mixture, the use of a premium (high performance) bond coat could make the use of conventional material more reliable under relatively heavy trafficking The generic SMA specifications used by authorities include the following essential requirements and are based upon the style of specification in PR 65 (Nunn, 1994). However many authorities adapted their specifications locally with bespoke modification of composition and other aspects to reflect serviceability, resource constraints and installation requirements, which have been developed over time in collaboration with local asphalt manufacturers. The core features comprise: A permitted range for the target grading envelope; A range of potential tolerances to be applied to the suppliers chosen grading curve; Maximum and minimum binder content; An air void requirement in the designed mix Forthcoming European Specifications for asphalt will impact on current British Standards specifications for surface materials. The relevant parts of the emergent technical standards are: EN Bituminous mixtures Part 1 Asphalt Concrete; EN Bituminous mixtures Part 2 Very Thin Layer Asphalt Concrete; EN Bituminous mixtures Part 5 Stone Mastic Asphalt; EN Bituminous mixtures Part 7 Porous Asphalt All of the above four parts of EN are progressing through the European Standards body, CEN, and are likely to be implemented in the UK by EN makes reference to an intended installed layer thickness of between 20mm and 30mm. Asphalt concrete surfacing greater than 30mm thick will be specified to EN Layers less than 20mm in thickness will also be covered by EN Asphalt concrete products 10mm to 20mm in thickness may be capable of submission for product approval certification Porous Asphalt is specified in the Specification for Highway Works, (Nov 2004 amendment), and a similar material, but less definitively described, is included in BS Both materials have 6/20 mm 12

16 nominal size aggregate and are placed at 45mm to 50mm in thickness. BS 4987 also includes a 2/10mm nominal size mixture, which may be laid at thicknesses of 30 to 35mm. Porous asphalt has advantages over close graded mixtures in respect of noise and spray reduction. However, it is little used on local roads because of limited structural contribution, winter maintenance management and relatively poor durability Proprietary Systems Thin surfacing systems may be certified by BBA through the HAPAS mechanism. All thin surfacing systems for use on trunk roads must be BBA certified. Some authorities also make certification mandatory on their network whilst others require BBA certification only for selected parts of their network To achieve certification a system proprietor must satisfy the process requirements of the BBA Assessment and Certification of Thin Surfacing Systems for Highways. The certification is described in full in Section 3 of the BBA protocol and is summarised as follows: Stage 1: Assessment of applicant submitted data Company details; System details and history; A quality plan covering; binder type(s); aggregate source(s), characteristics etc; ancillary products detail; final products, thickness, composition etc; Details of total quality structure and system; Installation method(s) Stage 2: Assessment of factory production control: Subsequent site and plant inspections by BBA Stage 3 Laboratory testing (Mandatory): PSV and AAV test on proposed aggregates; Wheel tracking; Sensitivity to water; Torque bond test for assessing the adhesion at the layer to the substrate; Visual assessment; Texture depth Stage 3 Laboratory testing (Optional): Optional tests carried out by the BBA, based upon in-house protocols, to evaluate properties within a menu which an applicant system proprietor may elect to have included on their certificate. The properties that may be covered include: 13

17 Stiffness; Retained stiffness after fuel immersion; Ageing; Resistance to binder stripping; Noise reduction relative to HRA; Regulating ability; Hydraulic conductivity; In service skid resistance Stage 4 Installation Trial: Construction of a section of surfacing witnessed by the BBA; The installation is subject to laboratory testing as required by BBA Stage 5 Performance Trial: Each trial section is monitored and tested for a period of two years and any loss of texture recorded; Expert panel inspections are carried out at the end of the two year evaluation period Stage 6 Certification Certificates verify the system compliance with BBA requirements, define the systems assessed, the conditions of application, likely performance and the results of laboratory and site tests; Once certified a system may not be changed without the approval of BBA; The acceptability of any proposed changes to a system will be evaluated by further testing as BBA require; Audits of each system are conducted at least annually; Full review of each system is undertaken by BBA every five years BBA certificates which have been issued for proprietary thin surfacing systems are available at References 1. British Standard Institution, (2005). BS Hot rolled asphalt for roads and other paved areas. Specification for constituent materials and asphalt mixtures BSI, London. 2. British Standard Institution, (2003). BS Hot rolled asphalt for roads and other paved areas. Specification for transport, laying and compaction of rolled asphalt BSI, London. 3. British Standard Institution, (2005) BS Coated macadam (asphalt concrete) for roads and other paved areas. Specification for constituent materials and mixtures BSI, London. 4. British Standard Institution, (2005) BS Coated macadam (asphalt concrete) for roads and other paved areas. Specification for transport, laying and compaction BSI, London. 5. Nunn M.E. (1994) Evaluation of Stone Mastic Asphalt (SMA): A high stability wearing course material TRL Project Report 65, TRL Limited. ISSN

18 6. Highways Agency. Manual of Contract Documents for Highway Works, Volume 1: Specification for Highway Works, The Stationery Office, London. 7. Highways Agency. Design Manual for Roads and Bridges. Volume 7, Section 5 Bituminous surfacing materials and techniques. HD 37/99 Amd No1, The Stationery Office, London Summary Introduction of improved safety standards and reduction of risk associated with installation of NTS, led to the decline of traditional HRA as the preferred surface material. NTS materials, originally based upon continental European practice, have been modified to provide adequate surface texture for use in the UK. Proprietary thin surfacing in the UK has evolved and is now certified by the BBA through the HAPAS protocol. A summary of the certification process is provided for ease of reference and understanding. 15

19 4 SITE CONSIDERATIONS AND INSTALLATION 4.1. General A thorough understanding of the engineering performance of the existing pavement substrate upon which the surface is to be placed is a key factor for maintaining serviceability Key elements comprise: A suitably resilient substrate on which to place the surface material; Selection of an appropriate surface material or system An enquiry questionnaire presented in Appendix C was issued to all highway authorities to establish local aspects for surfacing related to: Site situation; Materials; Performance prediction; Serviceability and performance The summary findings from the questionnaires are presented in Appendix D and illustrate: Cracking and fretting to be the most common failure mechanisms; The key role of the substrate; The importance of bond with the substrate; Approximately equal use of generic and proprietary specifications; Surface serviceability, in general, was good or better than traditional materials, but performance in high stress situations was indicated not to be quite so good Analysis of the enquiry questionnaire response led to regional interactive participatory workshops in Birmingham, London and Huddersfield. The format for these workshops is summarised in Appendix E and provided the opportunity for practising maintenance engineers to address the issues of: Site evaluation; Materials selection The views of a cross section of installers with experience in NTS were sought in parallel with authority enquiries. These observations are summarised in Appendix I and contributed to the subsequent development of decision charts for substrate evaluation and material selection which followed the participatory workshops. 16

20 4.2. Serviceability The local highway has generally evolved over time. For a typical length of carriageway various structural strengthening, surfacing or repair treatments have been undertaken in the past. These measures have resulted in a network which is variable both in terms of thickness and types of construction materials. Consequently, these pavements are significantly different from designed highways such as motorways which perform in a stable and long life manner. In comparison, the local highways are much thinner and more susceptible to structural changes resulting from increased traffic loading and subgrade moisture conditions Over time the local highway has come into equilibrium with its surroundings including underlying soil conditions, traffic loading and drainage. Many of the materials used in previous construction and repair are not now in common use and as a result of just in time maintenance practices may have become distressed through cracking and embrittlement. The relative proximity of the subgrade results in the thinner pavement being intrinsically more flexible in its structural behaviour under traffic loading. At the surface there are many traffic braking, accelerating and turning manoeuvres and consequently any surfacing material has to be capable of absorbing these forces in addition to working in harmony with the underlying pavement Compatibility of strain is important if the surfacing is to perform well in service. The inclusion of a stiff brittle layer in an otherwise relatively flexible structure is unlikely to yield longevity of performance. The selection of surfacing therefore, needs to consider the ability to provide strain compatibility, as well as the polishing resistance and texture characteristics, which are necessary for safety. The nature of the evolved pavement and the condition and type of pre-existing materials requires that careful consideration should also be given to the porosity of the new surface material. The introduction of water through a more porous surface can lead to serviceability distress, especially when the pavement is in an existing state of limiting equilibrium. This can be ameliorated with the use of bond coat and an impermeable binder course, providing there is an effective drainage system for the carriageway Substrate Existing Pavement The condition of the existing pavement which will form the support platform for the surface is a key factor in the overall performance Five distress features form the basis of condition evaluation for the existing road: Oxidisation; Fretting; Cracking; Texture; Rutting. 17

21 Oxidisation of the surface results in a dull or burnt appearance due to the degradation of the exposed hydrocarbon binder. This can result in wrinkle cracks and the road surface has a tired appearance since the binder component has reached the end if its ductile life. The depth of distress is probably not great unless the oxidisation is well advanced or other deeper seated distress factors are also present Fretting of the aggregate and/or matrix from the pavement surface occurs when the micromechanical bond between binder and aggregate reaches a critical point. The mechanisms which cause this to occur are complex and frequently interactive but are likely to be triggered by environmental factors. Rapid failure can occur with water pressure and suction effects on the surface resulting from the passage of vehicle tyres. In a matrix dominated material, such as HRA, this process occurs slowly as these materials are generally impermeable and environmental intrusion is very limited, but in aggregate dominated materials, such as NTS, once the lateral support of one particle is lost, fretting can occur swiftly and progressively Cracking of an existing surface is frequently the most difficult to define. From a surface material selection perspective it is important to establish if the cracking is surface initiated and is propagating downward or has been initiated at depth and propagating upwards. If the root cause of the cracking incorrectly diagnosed then a replacement surface may simply crack again after a relatively short period of service life. Three principal types of cracking have been established: Longitudinal; Map; Transverse Dependent upon severity and crack propagation direction, a single layer, two layers or multi layer solution may be required and in some cases the inclusion of a Stress Absorbing Membrane Interface (SAMI) may provide additional safeguard against crack propagation Surface texture is a key safety feature for high speed skid resistance. Any texture loss or gain will only occur within the existing surface layer and is a defect which can be corrected relatively easily with a single layer solution Rutting can owe its origin either in the surface course (non-structural rutting) or from a deeper seated origin, and it is important to identify the root cause. Non-structural rutting, which is caused by deformation within the surface, is generally accompanied by horizontal pushing of material at the margins of the longitudinal rut. Deeper seated rutting has a more general depressed area beneath the overall surface level. Both surface material rutting and deeper seated rutting can yield the same numerical measure of rut depth but cause must be understood in order to ensure an effective surfacing solution The addition of an overlay, wherever practicable, will add to the overall structural capacity of the pavement. There is little published information on the extended structural life for overlays which are less than 40mm in thickness. However, it is considered that overlays of 18

22 30mm thickness do offer some structural contribution to thin evolved pavements The structural contribution effect of an overlay 40mm or greater in thickness can be considerable (Kennedy and Lister, 1978). In situations where the pavement can be overlain with a new surface then this should be the first choice option Where access levels or other physical features prevent the addition of an overlay, then an inlay may be the only practical solution. This is frequently the case in urban areas where the existing defective depth of pavement may have to be removed and new surfacing material installed in its place. Inlay construction generates aged material from the existing pavement which may be recycled in-situ or elsewhere. However, the inlay itself is unlikely to provide much direct improvement in structural terms, other than degraded material being replaced with new material, resulting in the exclusion of surface water by a newer and less permeable surfacing In some urban kerbed areas it may be possible to channel plane close to the kerb edge to enable the new surfacing to be installed to the previous road level at the kerb. The remaining central portion of the road is not milled and in effect there is an inlay at the edge and overlay in the centre. The key area is the transition from inlay to overlay at the milled step in the existing pavement. When a new surfacing is installed in this fashion the thickness can be at its thinnest at this point and this may be directly in alignment with the nearside wheel track Where planing operations in the existing pavement are required to enable inlay surfacing, care should be taken to ensure that the planed depth is such that each old layer which is disturbed is removed in its entirety. The thickness of the existing layers and their condition will be known from investigation or local knowledge. Remnants of old layers which are not removed can have a detrimental effect on the in-service performance of the inlay Bond Coat All pavement layers need to be bonded to the layers above and/or below to assure the maximum structural efficiency of the whole pavement structure under vertical loading An effective bond between layers enables horizontal shear stress between layers to be transmitted. Greatest efficiency is obtained where the horizontal shear strain in a bond coat is small and approaches that in the adjacent asphalt layers. Horizontal shear stresses are small by comparison with the tensile stress in the overall pavement but nonetheless need to be fully transmitted Adhesive bond becomes critical in near surface layers. At points remote from an applied wheel load normal stresses can be tensile and thus no frictional transmission of horizontal shear is possible and total reliance must be placed on the bond coat. Additionally, at shallow depths the horizontal shear forces due to braking, turning, traction on gradients, and differential thermal movements are far more concentrated than at greater depths. 19

23 The efficient transmission of horizontal shear stress is critical to the function of the pavement as a whole. The bond coat is essential to ensure the survival of the surface course. Fretting, cracking and delamination have been recorded to be the most serious and frequent forms of deterioration of NTS and since fretting and delamination are frequently initiated by cracking it follows that a good bond coat is also beneficial in slowing failures due to cracking Virtually all thin surfacing systems are more permeable than the HRA material they have replaced. The ingress of water particularly in undesigned pavements is the major cause of deterioration. Thick bond coats can act as a water barrier and assist in extending the life of a pavement. However, the existence of a horizontal water barrier puts particular importance on the performance of the surface layer when saturated BS594 and BS4987 define tack coat as K1-40 or K1-60 bitumen emulsion conforming to BS434-1 and bond coat as proprietary material having performance characteristics certified by BBA. When certifying a complete thin surfacing system, rather than a bond coat, BBA also differentiate between tack coat and bond coat. The certified torque bond values on a BBA certificate only usually relate to a single tack/bond coat even when the certificate covers a number of bond/tack alternatives The Notes for Guidance on the SHW Clause 920 Bond Coats, Tack Coats and other Bituminous Sprays suggests a classification into three categories, based upon the Vialit Pendulum Energy Loss Test as illustrated in Table 2. Table 2 - Classification of Bond Coats by Vialit Test Value Classification Vialit pendulum, peak cohesion value (J/cm 2) Conventional (U V ) >0.5* Intermediate (I V ) >1.0 Premium (P V ) >1.2 *Inferred from Table NG Alternatively, bond coats may be classified by the torque bond test. The torque bond strengths in Table 3 are derived for the BBA certificated values. Test variability combined with substrate variability make the application of this approach difficult for site specific application and therefore, the use of the values in specifications is not recommended until further research is completed. The ranges in Table 3 are based upon best available knowledge and may benefit from revision in the future but illustrate three categories of classification for bond coat. If site measured values of torque bond strength are less than forty percent of the values shown in Table 3 then further investigation should be carried out. 20

24 Table 3 - Classification of bond coats by torque bond strength Classification Torque bond strength (kpa) Conventional (U T ) >250 Intermediate (I T ) >500 Premium (P T ) >1000 Note: The tabulated values are for guidance only - no correlation with the values in Table 2 is implied nor established. Selection Selection is dependent primarily on: Substrate condition; Surface thickness; Surface type; Site stress conditions Suggested grades of bond coat which are likely to be appropriate are presented in Table 4. These are grouped into three categories; conventional (U), intermediate (I) and premium (P) For those surfaces which are covered by generic specifications and the site condition criterion indicates differing requirements for minimum bond coat, the highest overall grade should be selected For those surface systems which are proprietary in nature the BBA certification should take precedence but the authority should still evaluate the site against the condition criteria in Table 4. If this results in an anomaly with the proprietary proposals then it should be brought to the attention of the system proprietor. Table 4 - Selection of Bond Coat Condition Substrate sound and rugous Substrate smooth / polished Substrate lacking bitumen Substrate slight cracking Surface thickness 30-40mm 20-30mm <20mm Site stress level (NG942) Recommended coat U I P P U I P U I P P bond 21

25 Bond coats should be spread at the rates in the BBA certification for proprietary systems or at the rates in BS 594 or BS 4987 for generic requirements as appropriate. There should be a continuous even spread without puddles of coating and the rate of spread should be regularly checked Spraying should not be too far in advance of the paving process and if there is any sign of picking up by delivery trucks the bond coat should be lightly gritted. Self-propelled sprayers or integral paver mounted spray bars should always be used where appropriate. When integral spray bars are used the bond coat formulation adopted should ensure virtual instant breaking of the emulsion. For the best quality of work and least traffic disruption integral spray bars applying intermediate or premium grade bond coat are preferred to other systems Early Life Surface Characteristics Previously much of the surfacing on the local network was of matrix dominated HRA type. This surfacing displays positive texture as chippings are embedded in the material to provide the skid resistant surface The film of bitumen which coated the chippings in the surface of HRA was very thin (circa 2 microns) and was generally unmodified residual petroleum bitumen. The passage of vehicle tyres removed this thin film rapidly due to the scrubbing effect of the constantly flexing tyres, the relative roughness of the surface and the positive texture encountered in resistance to rolling. The aggregate mineralogy of the chipping was exposed swiftly to deliver the in-service skid resistance Negative texture surfaces as provided by NTS are significantly different in their early life characteristics. There is less resistance to rolling due to the negative texture and the tyres do not impart the same scrubbing effect as the wheel travels forward. The tyres pass over the surface with only the vertical component of load having a wear effect The binder film is much thicker (circa 10 microns) for SMA negatively textured surface material. This results from the need to formulate the thin surfacing material as a whole to produce a stable mastic structure to support the mineral skeleton of aggregate. To enhance the binder stability and durability and to prevent binder drainage during transport, additives such as cellulose fibres or polymers are often used. These binder modifications result in a greater tenacity and adhesion between the binder and the mineral aggregate. With the combination of a thick binder film and the inclusion of modifiers, a substantially greater number of wheelovers are required to wear the binder film and expose the aggregate as the skid resistant surface Under the action of vehicle braking in an emergency on NTS, the thick binder film may be mobilised, either by melting or shearing. This plays an active role in effecting the maximum level of friction that can be generated between a locked and sliding tyre and pavement surface during the braking manoeuvre. 22