PAVEMENT RECYCLING GUIDELINES FOR • IN-PLACE RECYCLING WITH CEMENT• IN-PLACE RECYCLING WITH EMULSION OR FOAMED BITUMEN• HOT MIX RECYCLING IN PLANT2003Comité Technique AIPCR C7/8 – "Chaussées Routières” PIARC Committee C7/8 – "Road Pavements"PAVEMENT RECYCLING GUIDELINES FOR • IN-PLACE RECYCLING WITH CEMENT• IN-PLACE RECYCLING WITH EMULSION OR FOAMED BITUMEN• HOT MIX RECYCLING IN PLANT78.02.E copyright : AIPCR – ASSOCIATION MONDIALE DE LA ROUTE La Grande Arche – Paroi Nord 92055 LA DEFENSE Cedex – France Fax : +33 1 49 00 02 02 E-mail : piarc@wanadoo.fr http://www.piarc.org PIARC – WORLD ROAD ASSOCIATION ISBN : 2-84060-154-0TABLE OF CONTENTS FOREWORD61INTRODUCTION71.1GENERAL REMARKS71.2 1.2.1 1.2.2 1.2.3 1.2.4DIFFERENT WAYS OF RECYCLING According to the place where mixing is carried out According to the temperature of the process According to the characteristics of the material to be recycled According to the type of binder8 8 8 8 82GUIDELINE FOR COLD IN-PLACE RECYCLING OF PAVEMENTS WITH CEMENT102.1ACKNOWLEDGEMENTS112.2EXECUTIVE SUMMARY122.3 2.3.1 2.3.2 2.3.3 2.3.4 2.3.5 2.3.6 2.3.7 2.3.8INTRODUCTION Definition Historical development Objectives of recycling Advantages and limitations of in place recycling Advantages Disadvantages Particular features of in place recycling with cement Comparison between recycling and overlay of the pavement14 14 15 16 16 16 16 17 172.4 2.4.1 2.4.2 2.4.3 2.4.4 2.4.5 2.4.6 2.4.7PRELIMINARY STUDIES Introduction Feasibility of the recycling Examination of the existing road Characterisation of the materials of the pavement Drainage and climate Design traffic Widening schemes and hard shoulders18 18 18 18 20 21 21 212.5 2.5.1 2.5.2 2.5.3 2.5.4 2.5.5MIX DESIGN Grading Water content Mix density Cement and other admixtures Workability time22 22 22 22 22 242.6MECHANICAL PROPERTIES OF CEMENT BOUND RECYCLED MATERIALS252.7DESIGN OF CEMENT BOUND RECYCLED PAVEMENTS272.8 2.8.1 2.8.2MACHINERY FOR RECYCLING Introduction Description of machines30 30 32 PIARC. 2 . 78.02.E - 20032.9 2.9.1 2.9.2EXECUTION OF THE WORK The recycling process Execution of works40 40 412.10 QUALITY CONTROL 2.10.1 Introduction 2.10.2 Quality control during construction 2.10.3 Control after construction49 49 49 512.11 COSTS 2.11.1 Introduction 2.11.2 Construction cost of a recycled layer 2.11.3 Cement 2.11.4 Recycling equipment 2.11.5 Auxiliary equipment and labour 2.11.6 Other costs 2.11.7 Total cost53 53 53 53 54 55 55 552.12REFERENCES573GUIDELINE FOR COLD IN-PLACE RECYCLING OF PAVEMENTS WITH BITUMEN EMULSION OR FOAMED BITUMEN603.1ACKNOWLEDGEMENTS613.2EXECUTIVE SUMMARY623.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5INTRODUCTION Presentation of the process of in-place recycling with emulsion or foamed bitumen The different forms of introduction of the new binder Objectives and fields of application of in-place recycling Advantages and limits of in-place recycling Historical development of the technique63 63 63 64 64 653.4 3.4.1 3.4.2 3.4.3PRELIMINARY STUDIES Field investigations Characterisation of the materials in-place Feasibility of in-place recycling67 67 67 683.5 3.5.1 3.5.2 3.5.3 3.5.4 3.5.5MIX DESIGN Content and steps of the mix design study Grading considerations Choice of the new binder Investigation of the affinity between the binder and the aggregate Laboratory study for mix design for recycling70 70 71 72 75 763.6 3.6.1 3.6.2MECHANICAL CHARACTERISTICS OF THE RECYCLED MATERIAL Materials recycled with bitumen emulsion Materials recycled with foamed bitumen82 82 823.7 3.7.1 3.7.2 3.7.3DESIGN OF PAVEMENTS USING RECYCLED MATERIALS Geometrical aspects Mechanical aspects Input parameters for pavement design84 84 84 85PIARC. 3 . 78.02.E - 20033.7.4Wearing course863.8 3.8.1 3.8.2IN-SITU RECYCLING WORKS Elementary tasks Recycling in one or two operations88 88 893.9 3.9.1 3.9.2EQUIPMENT FOR IN-PLACE RECYCLING The different recycling plants Performance criteria for the equipment91 91 963.10 EXECUTION OF THE WORK 3.10.1 Organization of the job-site 3.10.2 Practical guidance98 98 983.11 QUALITY CONTROL 3.11.1 Specifications and QC/QA 3.11.2 Before the works 3.11.3 During the works 3.11.4 After the works101 101 101 101 1023.12 COST CONSIDERATIONS 3.12.1 General considerations 3.12.2 Direct costs to be considered for a comprehensive evaluation103 103 1033.13 ILLUSTRATION OF THE TECHNIQUE 3.13.1 Example of application in Canada-Quebec 3.13.2 Example of application in KwaZulu Natal (South Africa) 3.13.3 Example of application in Cape Town (South Africa) 3.13.4 Example of application in Casablanca (Morocco) 3.13.5 Example of application in Louisiana (U.S.A) 3.13.6 Example of application in Maryland (U.S.A) 3.13.7 Example of application in Germany105 105 105 106 106 107 107 1073.14CONCLUSIONS1093.15REFERENCES1103.16APPENDIX - COLD RECYCLING IN PLANT WITH EMULSION OR FOAMED BITUMEN1124GUIDELINE FOR HOT MIX ASPHALT RECYCLING IN PLANT1134.1ACKNOWLEDGEMENTS1144.2EXECUTIVE SUMMARY1154.3 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7 4.3.8 4.3.9HIGHLIGHTS FOR DECISION MAKERS Pyramid of demands Specification of requirements on level 5 Composition and production of ARA Data given in the tender documents Mix composition by the asphalt producer Milling and storage of RA Production and quality-control of RA Transport and laying of ARA Costs116 116 117 117 119 119 120 120 120 1204.4 4.4.1INTRODUCTION The place and relevance of Hot Mix Recycling of Asphalt in Plant122 122PIARC. 4 . 78.02.E - 20034.4.2 4.4.3 4.4.4 4.4.5Policy concerning hot mix asphalt recycling in plant Historical development of hot mix recycling The growth of hot mix recycling during the last decade Quantities of RA and ARA123 123 124 1254.5 4.5.1 4.5.2 4.5.3 4.5.4 4.5.5 4.5.6 4.5.7 4.5.8 4.5.9PRELIMINARY STUDIES, TESTING AND CLASSIFICATION OF RA Calculation of quantities and properties of the old and new binder in ARA How to deal with PAHs in RA How to deal with modifications in the binder of RA How to deal with gravel in RA Grading of minerals Polishing Stone Value (PSV) and strength of stones in RA Quantity of strange particles and contaminants Homogeneity of RA Testing and classification of RA126 126 128 129 129 129 129 129 130 1304.6MIX DESIGN1314.7 MECHANICAL CHARACTERISTICS OF RECYCLED MATERIALS AND PAVEMENT DESIGN 133 4.8 EQUIPMENT FOR RECYCLING 4.8.1 Types and number of mixing plants 4.8.2 Plants for destruction of tar in RA and production of ARA with cleaned RA (TCU process) 4.8.3 Approvals procedures for ARA production 4.8.1. Capacity of plants during ARA production134 134 137 137 1384.9 4.9.1 4.9.2PRODUCTION OF ARA The preparation and storage of RA RA with PMB139 139 1414.10QUALITY CONTROL1424.11COSTS1434.12REFERENCES1454.13 ANNEX 4.13.1 New Developments Case Study: TCU 4.13.2 Philosophy of designing and producing asphaltPIARC. 5 . 78.02.E - 2003147 147 147FOREWORD This document is not intended as a specification document or to be a state of the art report. Its purpose is to provide: - Up-to-date information about the uses of the corresponding techniques around the world; - Some guidelines drawn from the experience gained in the different countries for a proper application of the discussed recycling techniques. For more and detailed information, the reader is invited to refer to the various documents listed in the reference sections. The guidelines have been prepared within the PIARC technical Committee C7/8 “Road Pavements” under responsibility of the subgroup SC5, Pavement recycling and re-treatment chaired by Jan.Th. van der Zwan (Netherlands). The three guidelines have each been written by a separate working group under the leadership of Carlos Jofré (Spain), Jean Francois Corté (France) and Jan van der Zwan (Netherlands) (see the different guidelines for the participants in those working groups. In the subgroup the following C7/8 members participated. Abdelhkim Jakani (Marocco), Alf Vollprachts (Germany), Allan Bell (Australia), André Jasienski (Belgium), Andrus Aavik (Estonia) , Asghar Naderi (Iran), Bronislaw Jefimow (Poland), Carlos Jofré (Spain), Carlos Kraemer (Spain), Claude de Backer (Belgium), Jean-François Corté (France), Elias Ndlovu (Zimbabwe), Figueiredo Mourao (Brasil), Guilermo Templeton (Mexico), Jan Kudrna (Tcheque Republic), Jean-Gabriel Hammerschlag (Switzerland), Jean-Pierre Marchand (France), John Williams (United Kingdom), Jorge Freire (Portugal), Jorge Nobre Santos (Portugal), Maria Da Conceicao Azevedo (Portugal), Reinhard Gruening (Germany), Rudi Bull-Wasser (Germany), Safwat Said (Sweden), Sally Ellis (United Kingdom), Yasumasa Torii (Japan), Yves Guidoux (France). Grateful acknowledgement is due to Allan Bell (Australia), English-speaking secretary of PIARC Technical Committee C7/8 on Road Pavements, for his kind editorial revision of the English version and to André Jasienski, Claude de Backer, Bertrand Guelton, (all Belgium) Pieter Pols (the Netherlands) and Jean-Francois Corté (France) for their translation into French of the English version of the guidelines. The editing of the guidelines was done by Jean-Francois Corté (France).PIARC. 6 . 78.02.E - 20031 INTRODUCTION This document contains three different guidelines, cold in place recycling of pavements with cement, cold in place recycling of pavements with bitumen emulsion or foamed bitumen and hot mix asphalt recycling in plant. After a general introduction the three techniques will be discussed.1.1GENERAL REMARKSRoad construction is a major consumer of materials. On the other hand, due to maintenance and reconstruction works, a lot of materials are released annually. Recycling, that is to re-use existing road materials and the use of waste materials in road construction is a very old custom. In fact the Romans already used in their roads all kinds of waste products. Reasons for recycling can differ all over the world but the awareness that a sustainable development is needed, has a strong influence on recycling of materials. Nevertheless, of course, economical conditions dominate the actual recycling. In general it can be stated that a lack of suitable aggregate sources, a lack of landfill area’s, economical benefits and combinations of these factors have stimulated recycling in the last decades. Recycling of road materials is defined as the reuse of existing road materials in road construction, with or without changing the characteristics of the material. There are many options for recycling: to recycle in place or in plant, to use the reclaimed materials with or without adding new materials, to change or not the function and the characteristics of the materials. The choice to be made is depending on technical, environmental and economical issues. Approaches are different in the various countries based on national needs, requirements, technologies, resources, etc. In this guideline the choice for the recycling technique is not answered. In each country one has to select a specific technique based on technical needs for rehabilitation or maintenance, the new function of the road, national policies aiming at a sustainable development and economical considerations. This report is based on the most recent state-of-the-art of in place recycling of pavements with cement, in place recycling of pavements with emulsion/foamed bitumen and hot mix asphalt recycling in plant. Only subjects with practical importance for all countries are described more in detail; a lot of technical details (of the plants for instance) are described in many manuals and therefore are not reproduced here.PIARC. 7 . 78.02.E - 20031.2DIFFERENT WAYS OF RECYCLINGSeveral classifications of the main types of recycling can be made according to: - The place where mixing is carried out, - The temperature of the process, - The characteristics of the material to be recycled, - The type of binder. 1.2.1According to the place where mixing is carried outIn-situ The milled materials and the binder are blended in the highway itself. Here, the materials of the existing pavement are the main constituents, sometimes with the addition of some new aggregate. The binder is either spread on the surface of the pavement (in case of cement or lime) by appropriate spreaders (manual distribution is acceptable only in works of low importance) or pumped into the recycling equipment from a tanker (cement slurry, bituminous emulsion) and then mixed thoroughly with the milled material. Water, if necessary, is usually added during the mixing process. In-plant The milled or pulverized material is stockpiled, then processed to obtain an appropriate grading, and finally mixed in a plant to produce a new cement – treated or bituminous mixture. Mixers can be either continuous or batch wise. Next, the recycled material is transported to the site, where it is mechanically laid and compacted. 1.2.2According to the temperature of the processCold recycling Cold recycling (without heating the existing pavement materials) is generally carried out in place but it can also be performed in plant. Hot recycling When recycled in-plant, the milled material is hot-mixed with bitumen, and new aggregates added for gradation correction. Mixtures usually contain less than 40% of recycled material, but may consist of up to 100% recycled material. When performed in place, special heating machines elevate the temperature of the pavement to facilitate its milling and mixing. 1.2.3According to the characteristics of the material to be recycledRecycling can be limited to a relatively homogeneous layer (for example, a granular layer covered by a surface dressing or a reduced thickness of bituminous mixture) or it may use two or more layers of different materials (for example, a granular layer covered by an important thickness of bituminous mixtures, in consequence of successive overlays). 1.2.4According to the type of binderCement Cement content is adjusted to obtain strength similar at least to that of soil cement, although, depending on both the characteristics of the material to be recycled and the percentage of cement, much higher values can be obtained. For instance, in the case of relatively clean granular materials, the characteristics and strength of the mix after recycling will be similar to those of a cement – treated base.PIARC. 8 . 78.02.E - 2003Lime and cement With very plastic materials, such as certain granular sub-bases polluted with clays, a combined treatment with lime and cement can be convenient. Each binder has its role: - Lime flocculates fine particles, with a quick reaction of ionic exchange. Moisture content is reduced at the same time; - Cement quickly increases mechanical strength. Bituminous emulsion The pulverized material is mixed with emulsion and the necessary amount of water. Once spread and compacted, a mix is obtained with characteristics similar to those of an emulsion – treated granular layer or a dense cold bituminous mixture. Foamed bitumen Foaming is produced by injection of a controlled quantity of cold water (usually, about 2 to 3% in weight) and air in the hot bitumen. The bitumen viscosity is so substantially decreased, allowing it to be mixed with the material from the milled pavement. Cement and emulsion or foamed bitumen With the combination of both binders, the purpose is to obtain a mixture with an increased strength but, to the effect of the emulsion or foamed bitumen, the mixture remains flexible and has a smaller shrinkage than cement-treated aggregates. Bitumen Hot mix recycling of reclaimed asphalt pavements in plant uses bitumen as a binder.PIARC. 9 . 78.02.E - 20032 GUIDELINE FOR COLD IN-PLACE RECYCLING OF PAVEMENTS WITH CEMENTPIARC. 10 . 78.02.E - 20032.1ACKNOWLEDGEMENTS The present report has been prepared within the PIARC Technical Committee 7/8 “Road pavements” by a working group chaired by Carlos Jofré (Spain) with the participation of the following experts: Jesús Díaz Minguela (Spain), Sally Ellis (United Kingdom), Tornstein Fröbel (Germany), André Jasienski (Belgium), David Jones (United Kingdom), Stelios Kolias (Greece), Carlos Kraemer (Spain), Bernard Marx (Germany), Hermann Sommer (Austria), George Vorobieff (Australia), Contributions were also received from: J.-P. Drevet (Belgium), Jules Egli (Switzerland), Maurice Lefort (France), Tony Lewis (South Africa), Zdenĕk Nevosád (Czech Republic), Etsuro Noda (Japan), Jan R. Prusinski (USA), Luc Rens (Belgium), Alain Sainton (France), Pierre Sion (Belgium), Harry Sturm (Canada), Yasumasa Torii (Japan), Benoit Verhaege (South Africa), Pierre Vincent (Belgium), Graeme Warren (United Kingdom), Tom Wilmot (Australia). The translation into French of the English version of these guidelines was done with the support of FEBELCEM (Belgium).PIARC. 11 . 78.02.E - 20032.2EXECUTIVE SUMMARYIn place recycling with cement presents many advantages for the rehabilitation of fatigued pavements needing a significant increase in the bearing capacity. Technical advances mean that it has become a solution, which should always be considered, together with the classical treatments of overlaying or reconstruction. In many cases in place recycling with cement is the most economical alternative. Many countries already have extensive experience in recycling with cement. Results available indicate that the process is satisfactory for low traffic roads, main highways with a high volume of commercial vehicles and airports. In principle, there are no problems in using cement-recycled layers on high traffic roads provided they are covered by an adequate thickness of bituminous mix. It should be remembered that the material obtained after recycling with cement is similar to soil cement or a cement-treated base, which are allowed for all traffic categories. This is confirmed by the results obtained from roads with a high volume of commercial vehicles. Recycling equipments is currently available which can treat satisfactorily up to 35 cm of the existing pavement in a single pass. In most cases, the capacity of the compaction equipment limits the thickness treated. On the other hand, the target thickness of cement recycled layers should not be less than about 20 cm, to avoid the presence of zones with a reduced thickness that could result in a premature fatigue failure. The criteria for ascertaining the feasibility of recycling a pavement is to identify whether its fatigue comes mainly from the poor quality of the pavement itself (insufficient thickness, granular layers contaminated with clay, debonded bituminous layers, etc.) or from problems related to the subgrade. In the former case, recycling with cement is generally a better option. The lack of technical specifications or recommendations has meant that several Road Administrations have not used this technique. However, several countries (Spain, Belgium, France, United Kingdom) have recently published reports on these topics, supplementing earlier Japanese publications. Good results have been obtained with recycled materials with a design compressive strength not less than 2.5 MPa at 7 days. However, some circumstances may require higher strengths, for example to provide resistance to freeze/thaw. In principle, all types of cements can be used for recycling. However, those, which will perform best, are those with a content of active additions, which can increase the setting and the workability time. Both from laboratory results and from the experience with cement recycled pavements, it can be concluded that, in many cases, it is possible to recycle with cement without removing entirely or partially the existing bituminous layers. As with other cemented layers, cement recycled materials show transverse cracks as a result of the combined effect of shrinkage, traffic loads and thermal gradients. Precracking of cement recycled layers by means of notches created in the fresh material about 3 m apart is the most effective system to minimize reflection cracking in the upper bituminous layers. Several precracking methods have been successfully employed. Precracking is always advisable, although the minimum traffic level at which it is necessary is not yet established. The performance of other recycled sections in the same network, subjected to different types of traffic, may help to answer this question. PIARC. 12 . 78.02.E - 2003A number of countries (e.g. Spain, UK) have published information on the thickness design of cement-recycled pavements, and Australia has published details of thickness design based on multilayer analyses. However, when adopting other countries’ experience care must be exercised, taking into account their traffic composition (axle load), properties of the materials more commonly recycled, the climatic conditions and, most important, local experience.PIARC. 13 . 78.02.E - 20032.3 2.3.1INTRODUCTION DefinitionRecycling of pavements is a technique whereby an existing degraded pavement (Figure 2.1) is modified and transformed into a homogeneous structure that can support the traffic requirements. More specifically, it involves reusing the materials from the existing pavement for the construction of a new layer, including: - Pulverisation of the existing pavement up to a certain depth, and - Addition of a binder (cement and/or bituminous emulsion), water (for hydration, mixing and compaction), - Aggregates if necessary (for grading correction or other purposes) and admixtures.Figure 2.1. Rehabilitation of degraded pavement by in-situ recycling Mix design is based on the results of performance tests on trial mixes. The homogeneous mixture is spread (Figure 2.2), compacted (Figure 2.3) and cured appropriately, resulting in a base or a layer with a greater structural contribution to a new pavement.Figure 2.2. Recycled material A sealing coat is then applied on the recycled layer to protect it against traffic during the works, with one or two layers of asphalt concrete later applied to ensure the functionality and bearing capacity of the pavement. PIARC. 14 . 78.02.E - 2003Figure 2.3. Compaction of recycled material Recycling of pavements can be carried out either in plant or in place, the latter being the most used nowadays, and is a rehabilitation procedure of great interest from a technical, environmental and economical point of view. 2.3.2Historical developmentThe structural design of pavements provides highways with an appropriate bearing capacity to resist the expected traffic loads. Both the composition and the strength of the base layer are as important as the quality of the wearing course. In this respect, the development of treated bases was a great advance, as this avoided many of the problems caused by granular untreated layers. In search of new procedures to improve bearing capacity whilst seeking to reduce both costs and the use of new materials, the existing pavement began to be reused and improved by means of milling and the addition of a binder. A forerunner of recycling techniques can be found in the United Kingdom where, after the Second World War, the “Retread Process” [3] was developed to repair the secondary roads. This comprised: - Pulverisation of the pavement, - Addition of a small quantity of aggregate if needed, - In place mix of the milled material with the new one by means of a grader or a disc harrow. A bituminous emulsion with low binder content was then sprayed on the resulting material, and both were immediately mixed using agricultural machinery. During the first day the mixed material was only slightly compacted, since it contained a lot of water, and the following day compaction was finished. It was a very simple procedure which, properly executed, provided acceptable results. Since the middle of the 1980s, the option of in place recycling to rehabilitate existing highways has reappeared. On this occasion it has been a remarkable success, mainly due to the following factors: - A better knowledge of the mechanical characteristics of cement, treated materials and the performance of semi-rigid pavements [1]; - The use of new, more powerful equipment, with both a greater output and work depth, and capable of providing a better quality final product [6, 23]; - Growing ecological awareness due to its environmental benefits, particularly in the light of exhaustion of the existing aggregate sources and the difficulty of opening new onesPIARC. 15 . 78.02.E - 2003At the present time, in place cement-bound recycling is used in several countries like United States [6], Australia [29], Germany [27], South Africa [18], Spain [20] and France [9]. In the latter, about 2 million m2 of pavement have been recycled with cement some years. In Germany, increasing environmental concern has made recycling compulsory to fulfil the Government's guidelines to avoid, reduce and reuse debris and asphalt waste materials. 2.3.3Objectives of recyclingThe fundamental objective when in place recycling a pavement is to improve its characteristics and performance under traffic to: - Transforming a degraded and heterogeneous pavement into a consistent and more homogeneous structure, - Increasing the bearing capacity, adapting it to the requirements of traffic, - Improving the durability: reduced water susceptibility and increased strength to reduce the erosion, - Protecting the subgrade and the lower layers of the pavement, whose characteristics are sometimes substandard. Recycling of pavements competes with the classic rehabilitation technique of overlaying the existing pavement with bituminous mixtures, cement bound granular materials or concrete. Both methods increase the pavement’s structural capacity, but recycling benefits from the reuse of the existing materials and less modification of the surface levels. It could be considered as a stabilisation with additives, since a binder is added to the existing material, thereby changing the physical and chemical characteristics of the mixture. A specific advantage of in place cold recycling is that the materials do not need to be transported to a mixing plant. 2.3.4Advantages and limitations of in place recyclingThe advantages and drawbacks of in place recycling in comparison with the conventional technique of overlaying are listed below. 2.3.5AdvantagesRecycling of a pavement in place leads to: - Reuse of the aged, polluted or inadequate materials of the existing pavement, - Homogenisation of the pavement, both in strength and in geometry, - Reduction in waste and extraction of aggregates from quarries or pits, with the associated environmental advantages, - The possibility of rehabilitating individual lanes of roads with two or more lanes, where deterioration is frequently restricted to those supporting the heaviest traffic, - Decrease in nuisance caused by the conventional repair work traffic, - Fewer possibilities of deteriorating the adjacent network of secondary roads as a result of the reduced volume of new materials to be transported, - Smaller costs of rehabilitation of worn pavements [31], - Maintenance of the surface to a level which will not need hard shoulders and curbs to be raised, and will not reduce the clearance under bridges, - Opportunities for simultaneously widening the existing road, a frequent situation when upgrading a road. The construction of narrow wedges often associated with the widening can be avoided. 2.3.6DisadvantagesRecycling of a pavement in place can lead to: - Less homogeneity than in a new mixture, - Possible appearance of longitudinal cracks if adjacent strips are not correctly bonded, PIARC. 16 . 78.02.E - 2003- Longer rehabilitation time than that required by a simple overlay with bituminous mixtures where no milling is necessary. 2.3.7Particular features of in place recycling with cementIn place recycling of pavements with cement allows the rehabilitation of worn or damaged pavements and their adaptation to support the traffic requirements. If pavements show high deflections, then special studies must be carried out to calculate the required overlay thickness. Cement bound recycling allows a homogeneous and stable layer of an improved thickness to be obtained, with mechanical characteristics similar to those of a soil/cement or a cement-bound base. Therefore, with the cement bound recycling the bearing capacity is substantially increased or conversely pavement deflections, subgrade stresses and strains are greatly decreased. Moreover, existing ruts can be appropriately corrected if the thickness of the bound layer is increased. Shrinkage of the cement bound material may lead to reflective cracking in the surface. Reflective cracking can be minimised or eliminated by precracking at short distances in the cement bound recycled layers [22]. Thick bituminous layers on top of the recycled ones will also help. However, for low volume roads, fine cracks less than 0.5 mm in width, will not adversely affect the performance of the pavement if they are appropriately treated. 2.3.8Comparison between recycling and overlay of the pavementThe selection between one of these two possible solutions must be based on a technical and economical study, which should consider the following: - The total costs, including that of widening of the pavement and hard shoulders, of construction for functionally equivalent solutions, - The expected results of recycling, based on both the study of the existing materials in the pavement and the composition of the new mixture, - The final quality of the new pavement, with regard to its adequacy to the traffic requirements and the design life, its performance in relation to climatic conditions and foreseen maintenance costs, - The availability of local materials and the costs of new aggregates if needed, - Problems related to bridge clearance, side accesses, etc. The choice to be made is depending on technical, environmental and economical issues. There are in different countries also different approaches based on national needs, requirements, technologies, resources, etc.PIARC. 17 . 78.02.E - 20032.4 2.4.1PRELIMINARY STUDIES IntroductionPrior to recycling of pavements it is necessary to carry out a series of studies to: - Verify the feasibility of the recycling, - Select the type of recycling, - Determine, by means of laboratory tests, the characteristics of the materials to be recycled material and to perform the mix design. 2.4.2Feasibility of the recyclingIn order to ascertain the feasibility of recycling a pavement, it is necessary to identify whether distresses come mainly from a bad quality of the pavement itself (insufficient thickness, granular layers contaminated with clay, debonded bituminous layers, etc.) or from problems related to the subgrade. In the first case, cement recycling is generally a viable option. In place recycling uses the existing pavement as a source of aggregates. To verify the possibility of recycling them it is necessary to know, in advance, the characteristics of the existing materials in the pavement and the thickness of the layers. Most materials in existing pavements can be recycled except those of discontinuous grading, e.g. macadam. Materials of discontinuous grading will require a grading corrector (fine aggregate) or the adjustment of both the speed of the rotor and the position of the breaking plates. On the other hand, pavements with materials with a particle size greater than 80 or 100 mm may require special machines to disintegrate the pavement. The presence of certain substances such as organic matter, sulphides (pyrites) or sulphates (gypsum) can disturb or stop cement setting. Recycling feasibility should be established from the knowledge of the structure of the pavement and of the characteristics of the materials present in it. For this purpose it is necessary to examine the road, to determine the characteristics of the materials of the pavement and to collect data on the traffic and the climate. 2.4.3Examination of the existing roadCollection of data of road construction and maintenance operations An obvious starting point is to collect the reports of all the operations carried out on the road to be inspected. It can save time and may reduce the number of samples and test pits necessary for the required information. This includes the composition of the layers of the pavement, their thickness and the characteristics of the materials. It can also be helpful to preliminarily subdivide the road into relatively homogeneous sections (of the greatest possible length) according to the characteristics of the layers of the existing pavement and the traffic movement on it. However, it is necessary to bear in mind that all the data may not be available, and if it is so, sometimes it could be erroneous. Inspection of the existing pavement A pavement inspection will evaluate the structural condition of the pavement, as well as the nature and the thickness of its layers. It will include a visual evaluation of the distresses of the pavement. All distresses should be recorded, especially: - Their severity and extent, - Details of sections showing the same types and levels of deterioration, - Sections needing corrections to their grade or alignment, - Potential problems with kerbs, gullies, manholes, accesses and safety barriers, - Isolated spots severely deteriorated that must be treated separately, - Problems related to the subgrade and embankments, and especially those connected with drainage, PIARC. 18 . 78.02.E - 2003- Clearance limitations. It is also necessary to collect information on the composition and condition of the hard shoulders, and accurately locate and record the depth of all underground services. If a pipe lies within 150 mm of the bottom of the pavement, it should be deemed at risk. The extent of the structural assessment of the pavement (deflections, rutting, thickness, composition, etc.) will depend on the importance of the work. Occasionally it may not be easy to ascertain the composition of the pavement to be recycled. In such a situation, ground-penetrating radar can be used to estimate the consistency of the pavement layers throughout the site. It is possible to operate this equipment at traffic speeds of 6080 km/h. The detection of the changes in the layers by means of the ground penetrating radar is based on the emission of an electromagnetic wave and analysis of the reflected wave. However, this device cannot determine by itself either the composition or the thickness of the layers, and its results need to be correlated with core and pit data. From the information collected, the road can be divided into homogeneous sections with similar traffic volumes, type and condition of pavement, and strengthening requirements. Core sampling and test pits Sections with similar properties will serve as a basis to formulate a program of core extractions and excavation of test pits (Figure 2.4) to determine and/or confirm the thickness and the characteristics of the materials of the pavement and the subgrade, in both the carriageway and in the hard shoulders.Figure 2.4. Test pit Sample collection in each section should be enough to obtain representative data, the number of samples to be extracted depending on the type and importance of the work. In general two cores and a test pit per kilometre are sufficient. Transverse test pits are generally excavated in the outer wheel path of the traffic lane. They must include the subgrade, since it is necessary to define its characteristics as well as those of the layers of the pavement. Particular attention should be paid to old pavements with areas where repairs, widenings or camber corrections have been made, since they are likely to contain variable materials, some of them of poor quality.PIARC. 19 . 78.02.E - 2003Experience from a great number of recycling works indicates that there are frequently significant variations in the thickness of the layers, in deflections levels or in the condition of the subgrade. This may necessitate further division of the sections into more homogenous stretches. However, it is recommended that the length of each section should not be less than 500 m. 2.4.4Characterisation of the materials of the pavementPavement samples must be analysed in a laboratory to identify: - The composition and moisture content of the subgrade, - The composition and moisture content of the materials in the different pavement layers, - The presence of any products that can alter or even prevent the setting of cement. Consequently it is necessary to undertake the following operations. Identification of the materials Grading: although this will depend on the depth of recycling (the deeper it is, the higher the amount of unbound granular materials), the grading obtained will help to decide whether a grading corrector is needed (Figure 2.5). The samples should be prepared to simulate as closely as possible the gradation of material achieved during the actual milling process.Figure 2.5. Material requiring a grading corrector to be recycled Study of fines: The amount and the activity of the fine clayey particles present in the materials must be identified either by their plasticity index (PI) or by their methylene blue value (MBV). The PI or MBV values will give a first indication of whether or not it is necessary to perform a combined recycling with lime and cement, in order to counteract the effect of the clay. Usually this will be necessary only for PI values greater than 15 and for materials with a high plastic fine content. Even in these cases the need for a lime treatment must be confirmed by means of laboratory tests. Determination of the moisture content of the materials A property of great importance in a recycling project is the moisture content of the materials. It should be determined in the following way: - The natural moisture content, wnat, of the materials to be recycled, bearing in mind that it depends on the climatic conditions, - The optimum moisture content, wopt, which is that at which a precise amount of compaction produces the maximum density, dmax. Both parameters, wopt and dmax, are determined by meansPIARC. 20 . 78.02.E - 2003of the Modified Proctor AASHTO test [15] or other standardized moisture - density tests. They are used as reference values to control the compacting process. By comparing the natural and optimum moisture contents, the volume of water to be added in place can be deduced. If wnat is higher than wopt, it will be necessary to aerate the milled material before adding the binder. Identification of occasional setting inhibitors In addition to the above, the laboratory study should carry out tests to investigate the presence of substances that may impair the binding action of cement. The materials most likely to affect the cement are sulphates (gypsum), sulphides (pyrites) and organic matter. Once the work described above is completed, there should be sufficient information to judge whether recycling the cement can be carried out and, if affirmative, to accurately design the mix design. 2.4.5Drainage and climateAs in all rehabilitation works, it is necessary to investigate and to repair highway drainage failures including: ditches, surface drainage, underground drains, etc. It may also be convenient at this stage to correct isolated areas showing a low bearing capacity. With regard to the weather conditions, in place recycling with cement is less sensitive to the weather than other types of recycling, because it can be carried out at temperatures over 2ºC. If freezing is expected during the night, the recycled layer should be protected with a polyethylene sheet. If rain is likely to significantly affect the moisture content during compaction, works must be stopped. 2.4.6Design trafficSince recycling is performed on roads, which are already in service, estimating traffic is generally easier than for new roads. Depending on the design method, the traffic in one direction may be defined in several ways as: - The average daily traffic of commercial vehicles during the first year after the rehabilitation, - The cumulative number of commercial vehicles over the design life, - The cumulative number of equivalent standard axles. If data is not available, traffic counts can be carried out before the rehabilitation. Special attention must be paid to low traffic roads, such as agricultural, forest roads, etc. where a seasonal intensive use may take place. 2.4.7Widening schemes and hard shouldersThe road inspection should include the existing hard shoulders. Therefore, the test pits in the carriageway must be continued through the hard shoulders to measure the thickness of different layers and to take samples for laboratory testing. Frequently, the pavement rehabilitation by in place recycling is also used to widen the carriageway and to increase the bearing capacity of the hard shoulders. For widening, a new granular material should be used, with characteristics comparable to those of the material to be recycled. If this is not economically feasible, widening can be carried out with soil cement mixed in plant. The ultimate objective should be to obtain a pavement as homogeneous as possible in the transverse direction after widening and recycling.PIARC. 21 . 78.02.E - 20032.5MIX DESIGNRecycling the pavement should not begin until the mix design testing has been completed and approved. The design mix specification must include the following: 2.5.1GradingThe aim of the sieve analysis is therefore to decide whether or not the grading is acceptable. If it is not acceptable then decisions have to be made as to what corrective steps should be taken, as for example, the addition of imported material. It should be remembered that the material to be recycled is obtained by milling and this operation will produce different material gradings. If the obtained grading requires a correction, importing a suitable material is the simplest solution, but it is not the only one. Bigger particles can be removed or crushed. Some recycling machines are provided with either an adjustable impact plate or a crushing bar that will allow reducing the maximum particle size. It must be emphasised that lumps of bituminous mix may cause the grading to vary greatly. The depth to be recycled can also influence the grading considerably (the greater the recycling depth, the higher the proportion of granular layers with no asphalt). In comparison, the top layers will normally be bituminous mixes (hot or cold) of different gradings. In cases where the exact depth to be treated is not known, it is recommended that the grading for different recycling depths be determined. There are several references for a desirable grading, and these include those specified for cementtreated bases and the Talbot curve [21]: y = 100 x (d/D)0.4 where: y is the passing through sieve d in % d is the sieve size in mm D is the maximum aggregate size in mm. 2.5.2Water contentThe water content of the material at the time of mixing must be defined. 2.5.3Mix densityIn principle, the average density should not be less than 97 % of the reference value (modified Proctor AASHTO test). 2.5.4Cement and other admixturesOptimum cement content The cement content should be the optimum amount, which will economically achieve the required strength and will keep the shrinkage cracks as fine as possible. To determine the optimum cement content at least three series of specimens will have to be made. These will be made with the material supposed to be obtained after scarifying the existing pavement, with the addition of imported material if deemed necessary, and with a range of cement contents. The specimens should be compacted to the minimum density required on the site [34] using the optimum moisture content (as determined by the relevant moisture-density test) and tested in compression at 7 days. Once a preliminary cement content has been chosen, sensitivity analysis should be carried out on specimens compacted to densities varying by ± 2 % from the one previously used. This method will allow the minimum cement content that will sufficient guarantee that the strength requirements will be met on the site. For large works it is advisable to carry out also compressive strength tests at 28 PIARC. 22 . 78.02.E - 2003and 90 days, as well as indirect tensile strength tests. The elastic modulus in compression will also be useful information. Types of cement In recycling, the type of cement is less important than the content and density obtained. Most of the cements available on the market can be used and in many cases the choice will depend on their availability and price. However, points to be noticed are listed here. Generally, if cements of different strength classes are available, those of medium strength (e.g. 32.5 class of European Standard EN 197: Part 1) {13] should be preferred. Binders with a not very high strength will increase the workability time, reduce the heat of hydration and help to reduce the number of shrinkage cracks. A cement of higher strength (e.g. 42.5 class of EN 197: Part 1) should be used only under special situations, for example when recycling at low temperatures. However, it should be acknowledged that the use of high-strength cements might result in extremely low cement contents (2 to 2.5 % weight of dry material). Although such small cement content will allow the strength requirement to be met, it will not be possible to guarantee a suitable uniformity of mixing within the recycled material. Furthermore, such low cement contents will require the cement to be fed into the mixer in the form of a slurry. A design mix with cement of higher strength will have a shorter workability time for compaction purposes. If cements of strength class 32.5 or less are employed, the binder content usually ranges from 3 to 6 %, which usually does not pose problems with regard to the homogeneity of mixing. Therefore, the cements most suitable for recycling (and, in general, for all cement-bound materials compacted by rolling) are those with a high content of additions such as natural or artificial pozzolans or granulated blast furnace slag, e.g.: - Pozzolanic cements (CEM IV), - Portland-composites cements (CEM II), - Blastfurnace cements (CEM III), - Composite cements (CEM V) according to European Standard EN 197, - Blended hydraulic cements covered by ASTM C 595 Standard {35]. In addition to the above-mentioned advantages associated with these lower strength cements, the presence of pozzolans and/or slag will improve the performance of the recycled material in an aggressive environment. Slag cements will also be more resistant to sulphate attacks. Composite cements hydrate slowly and as a consequence their rate of water evaporation is higher. If not cured properly, the ultimate strength will be lower than required. This will not pose a problem on the recycling site if an appropriate measure, such as a curing seal of bituminous emulsion is applied as soon as compaction is completed. The recycled material has to be kept moist until the curing seal is applied. In addition, the curing seal may require protection from damage by work traffic, and this can be achieved by covering it with sand or chippings. European cements specified in the EN 197 standard should not be mistaken for ASTM Portland cements with the same denomination included in ASTM C 150 Standard [33]. The latter have no additions, whereas the only European type with this feature is the CEM I type. Other binders especially suitable for pavement recycling and soil stabilisation are called the ‘special road binders’, covered by the CEN standard ENV 13282 [14]. These are factory-produced cements, which may only contain a small amount of clinker, and they are available ready for use. They are coarser than the common cements and exhibit a very slow setting. As a result workability periods over 10 h are easily achievable. They are manufactured in several countries including France [19], Germany [27] and Austria [26], [28]. There have been some recent developments in hydraulic road binders: PIARC. 23 . 78.02.E - 2003- Dust-proofing cements, intended to reduce environmental pollution caused by works in urban areas or other sensitive locations. Several cements are now available including nodulized, teflon-added, and alcohol-added cements [24]; - Special binders with the overall effect of a combined treatment with lime and cement are also available for recycling pavements contaminated with a high proportion of clay [17]. 2.5.5Workability timeThe workability time of a cement-bound material is defined as the time available before the material sets. The workability time to compact the recycled material must be deduced on the basis of the expected air temperature during works. This will allow compaction of the material without impairing its mechanical properties when hardened. When cement starts to hydrate, it binds the aggregates. The forces applied during compaction by the rollers could destroy these links between the particles. Furthermore, the more rigid the material becomes, the more difficult it is to compact. Therefore, the compaction should be completed before the bonds between aggregates have been formed. The workability time should not be mistaken for the setting time of the cement. It is only one of the many parameters affecting the workability time. The characteristics of the materials, the water content or the climatic conditions are also influential. For instance, other factors being equal, the workability time at a temperature of 30°C is more or less half its value at 20°C. There are a number of different methods to determine the workability time. The simplest one is based on the decrease in the density of the samples compacted at increasing times after mixing [8]. The workability time is considered to be over when the density is 98 % of that of the specimens made immediately after mixing. Another method is based on the reduction in the time of propagation of the ultrasonic pulses during the setting of a continuously monitored specimen [7]. The workability time is considered to be over when the time of propagation is reduced to 60% of its initial value. These tests should be carried out at the average temperature expected on the site. It is also important to complete the compaction of a strip before the workability time of the previously laid adjacent strip has elapsed. By doing this, the compacting rollers will not produce any damage to the material previously laid, and a cold joint between both strips will be avoided. If any trimming is required it should be carried out before compaction is completed. It is also important that the final passes of the rollers are made before the workability time is over.PIARC. 24 . 78.02.E - 20032.6MECHANICAL PROPERTIES OF CEMENT BOUND RECYCLED MATERIALSThe knowledge of the mechanical characteristics of cement bound recycled materials is essential both for mix design and thickness design, i.e. the thickness of both the recycled layer and of the bituminous surfacing. Therefore, the mechanical properties of the recycled material must be known before construction starts. This is the major difficulty with recycling, since the materials are relatively heterogeneous and it is difficult to estimate the grading of the recycled materials that will be obtained after pulverisation. Due to this heterogeneity, ranges of variation of the different mechanical properties of cement bound recycled materials are wide. Tensile strength values between 0.4 and 2 MPa at 1 – 2 years have been obtained on cores extracted from cement-recycled pavements. The corresponding moduli of elasticity span between 3,500 and 37,500 MPa [9]. Therefore, some cement bound recycled materials are similar to cement treated granular bases, whilst some others are closer to a soil cement, in spite of the fact that, in many cases, cement contents (4.5 – 6%) exceed those usual in cement treated bases. Several factors can contribute to this different performance [5]: - Low quality of aggregates present in the pavement, - Pavement granular layers polluted with clay, - A portion of the milled bituminous materials consists of asphalt mortar particles, resulting in low strength “aggregates”, - If cement bound layers are milled, the resulting particles are composed of aggregates totally or partially coated in cement mortar, and consequently showing a lower strength. In addition to the great differences in the materials to be recycled, some other factors contribute to the large scatter in the values of the different mechanical properties: - Cement content, - Effectiveness of the milling and mixing process, - Density obtained after compaction, - Moisture content, - Time elapsed since the recycling operation. Anyhow, cement recycled layers have a high modulus of elasticity and consequently a high bearing capacity, which result in an important decrease in both deflections and the stresses and strain in the subgrade compared with those before recycling. In this regard, mention should be made that, in France, values of modulus of elasticity at one year between 11 000 and 20 000 MPa has been proposed for the design of cement recycled layers [9], as shown in Table 2.1. Table 2.1. Values of modulus of elasticity (365 days) proposed for the design of cement-recycled layers Characteristics of recycling Existing materials Execution Aggregates of good quality High performance binder and with homogeneous spreaders and recyclers grading High performance binder Other cases spreaders and recyclers Aggregates of good quality Other cases and with homogeneous grading Other cases Other cases PIARC. 25 . 78.02.E - 2003Modulus of elasticity E (MPa) 20 000 16 000 16 000 11 000An important remark about the values in Table 2.1 is the influence not only of the properties of the existing materials, but also of the equipment used in the work and, in short, of the accuracy of the recycling process. E values in Table 2.1 are only indicative. Several laboratory studies have made clear the influence of the content of milled bituminous mix, both on mechanical strengths and moduli of elasticity [25]. It appears that when the content of bituminous materials is below 50%, the flexural strength, which is most relevant to the performance of the cement recycled material, is not greatly affected, although a reduction in the modulus of elasticity have been noticed. This should result in mixtures less prone to cracking. However, these findings need to be confirmed by further research. From these results, and from the experience with recycled pavements, it can be concluded that, in most cases, it is possible to recycle a distressed pavement with cement without removing entirely or partially the existing bituminous layers [16]. A thickness of bituminous layers not exceeding 1/3 of the total recycled depth seems to be a good compromise [5]. More information about the relationship between the different types of mechanical strength (compressive, flexural, splitting) of cement-recycled materials should be collected. With regard to fatigue performance of cement recycled materials, it can be deemed similar to that of vibrated concrete or cement bound granular bases, i.e. the fatigue curve representing the relationship between stress ratio and load repetitions has a relatively low slope [1], [5]. In consequence, a slight increase in the stresses in the recycled layer, caused for instance by a reduction in the actual thickness, results in a considerable decrease of the traffic volume the pavement can withstand. In consequence, the target thickness of cement recycled layers should not be less than about 20 cm, to avoid the creation of thinner zones which would be prone to premature fatigue. Fatigue relationships developed for these materials are usually presented in the form σ / σ0 = 1 – (1/a) · log N where σ : σ0 : a : N :flexural stress causing failure after N cycles flexural strength (kN) a coefficient varying from 12 to 16 number of cycles of loading.PIARC. 26 . 78.02.E - 20032.7DESIGN OF CEMENT BOUND RECYCLED PAVEMENTSThe design of a cement bound recycled pavement concerns the determination of the thickness of both the recycled layer and the total thickness of the asphalt surface layers. In general hot bituminous mix is used to protect the cement-recycled layer, except with low traffic roads (less than 50 commercial vehicles per day). In such cases, cold bituminous mixes or double surface dressings may also be used. Cold in-situ recycling with cement is particularly useful when the pavement to be rehabilitated exhibits severe deterioration, with high deflections that need special attention. It is important to emphasise that recycling with cement is a total rehabilitation technique. This means that the design of the recycled pavement does not depend on the deflections of the existing pavement. In place evaluation is useful in deciding whether it would be appropriate to recycle the existing pavement, to divide it into sections and to detect stretches possibly needing some preliminary repair (e.g. drainage). However, once the decision to recycle has been made, the thickness of the recycled layer and the thickness of bituminous layers mix on top of it are not dependent either on the observed deflections or on the condition of the existing pavement. Cement bound recycled layers have a high bearing capacity, as a result of both the high modulus of elasticity and the thickness. It should also be taken into consideration that in many cases the subgrade under the pavement has experienced a significant additional compaction as a result of the traffic. For this reason the bearing capacity of the support of the recycled layer, i.e. the subgrade plus the layers remaining untreated if any, is often considered as having a CBR value over 20%. If however there are any doubts, the bearing capacity of the support can be determined carrying out penetrometer tests on test pits [10], or by means of surface deflection measurements (in most cases, the deflection of the most distant geophone from the centre of the load plate of a Falling Weight Deflectometer depends exclusively on the characteristics of the subgrade) [2]. Consequently, the design of the recycled pavement will depend mainly on the commercial vehicle traffic and on the characteristics of the material obtained after recycling. The latter are a function of both the characteristics of the unbound granular base material and the thickness and the bitumen content of the existing bituminous layer. It should be mentioned that it is not difficult with cementrecycled materials to obtain strengths similar to those of soil-cement and, in favourable cases, near to those of a cement treated base. Therefore, thickness design of a cement-recycled pavement must be undertaken as for a new pavement. There is a range of different approaches, which can be adopted. Mechanistic method Here, the pavement and the subgrade are modelled as a multi-layer linear elastic system. It is necessary to estimate the modulus of elasticity and Poisson’s ratio both of the recycled layer and the subgrade, as well as the parameters of the existing layers that are to remain untreated, if any. As indicated previously, the range of values of the moduli of elasticity E of cement-recycled materials is very broad. If the content of milled bituminous elements is small (e.g. less than 10%) and the granular materials are of good quality, the recycled layer can be similar to a cementtreated base and E can exceed 20,000 MPa [9]. In case of unbound layers contaminated with clay, the recycled material is more similar to soil cement and values between 6,000 and 10,000 MPa after recycling are to be expected. With regard to Poisson’s ratio, it is usually assumed to be equal to 0.25 [11].PIARC. 27 . 78.02.E - 2003Structural coefficients A different approach is to assign a structural coefficient to the recycled layer, and then apply some design method relying on this concept, e.g. the AASHTO one [2]. For instance, a coefficient of 0.5 is recommended in Japan [30] for materials having a minimum compressive strength at 7 days of 2.5 MPa. This value is slightly lower than that of 0.55 adopted for a cement-treated base, for which a higher compressive strength (3.0 MPa) at 7 days is specified. Catalogues Finally, design methods presented either as catalogues of pavement sections or design curves have been issued in several countries (e.g. Spain [11], UK [12] and Australia [4]). Table 2.2 presents the pavement sections as proposed in Spain for cement-recycled pavements, assuming a compressive strength of the cement-recycled layer equal to 2.5 MPa at 7 days. These sections have been designed for a maximum single axle load of 130 kN. Table 2.2. Pavement sections with cement-recycled layers recommended in Spain (minimum compressive strength at 7 days: 2.5 MPa) Average daily truck traffic*Cement-recycled layer (cm)800 - 2000 35 400 - 800 35 200 - 400 30 100 - 200 25 50 - 100 25 25 - 50 22 12 - 25 20 < 12 20 * In the design lane in the first year in serviceBituminous mix (cm) 15 12 12 12 10 8 5 Double surface dressingIn all cases, the application of these methods must be carried out carefully, taking into account the particular traffic composition (axle loads), the characteristics of the materials found in the pavements to be recycled and local climatic conditions. It should be mentioned that thickness of cement recycled layers should not be less than about 20 cm, to avoid the presence of zones with a smaller thickness that would be prone to premature fatigue. Conversely, it is not advisable to design cement-recycled layers thicker than 35 cm. Both the recyclers currently available and the compaction equipments impose this limit. Keeping mind on these limits, it can be stated that, in principle, there are no problems using cement-recycled layers on high traffic roads when they are covered by an adequate thickness of bituminous mix. It should not be forgotten that the material obtained after recycling with cement is similar to soil cement or a cement-treated base, which are used for all types of traffic categories. This is confirmed by the results obtained from roads with a high volume of commercial vehicles. If the characteristics of the existing pavement are such that there is not enough material to obtain the required recycled thickness, several options are available: - Spreading of additional granular material on top of the existing pavement until the required thickness can be achieved (Figures 2.6 and 2.7), - Increasing the thickness of asphalt surfacing on top of the recycled layer, - Combining both of the above.PIARC. 28 . 78.02.E - 2003Figure 2.6. Granular material spread on top of the existing pavement to improve gradingFigure 2.7. Granular material spread on top of the existing pavement to achieve the required treated thicknessPIARC. 29 . 78.02.E - 20032.8 2.8.1MACHINERY FOR RECYCLING IntroductionThe process of in place recycling with cement of an existing pavement can be divided into two main stages: - Milling the pavement layer(s) to the required depth, and mixing the pulverized material with additives such as cement and water; - Compacting, including trimming, the cement bound recycled material and applying a bituminous emulsion curing seal if required. The method applied can differ considerably between recycling low volume roads where the thickness to be recycled is usually reduced and other roads carrying medium and high traffic volumes. In the latter, the pavement is generally pulverised and the material mixed with cement and water in a single high-performance machine (a recycler). On low traffic roads, different machines (including agricultural equipment) may be used to carry out the individual steps. Recyclers are also feasible on low traffic roads, particularly if the size of the work is considerable, resulting in a fast and high-performance operation. After mixing the pulverised materials with the cement and water, the individual steps are the same as those for cement-bound bases or soil stabilisation. There is little difference between the machines used on large or small construction sites, although the compaction plant can vary with depth of treatment. Tables 2.3. and 2.4. respectively show the individual steps and the machinery required when recycling with cement on low or high volume roads.PIARC. 30 . 78.02.E - 2003Table 2.3. Machinery & operational steps for recycling of low volume roads Operational steps Pavement scarification(1)Eliminating large elementsLevelling Adding imported aggregate (if needed)Moistening (2) Binder distributionAimTypical machineryLoosening the existing pavement Eliminating large elements over 80mm: - by crushing - by removal Distribution of milled material - Improvement of aggregate grading - Cross slope correction