High-Modulus Asphalt Mix - A Case for Using Recycled LDPE

Transcription

1 High-Modulus Asphalt Mix - A Case for Using Recycled LDPE Walter J Tappeiner, Pavement Consultant, wjt.pave@gmail.com

2 1. Long-Life Asphalt Pavements In the quest for designing and constructing durable asphalt pavements for heavy highway and airport traffic, engineers look for good quality aggregates and special bituminous binders, and for advanced methods of asphalt mix design and performance based evaluation. Many of the traditionally used asphalt binder and mixture specifications, and related test methods, have been developed during the first half of the 20th Century. Bitumen grades and asphalt mix properties characterized using simple empirical tests are not adequate for design and construction of heavyduty long-life pavements. To be able to tailor more efficiently asphalt mix properties to structural and functional performance requirements of individual pavement layers, special bituminous binders and mixtures are needed. Engineers who face this challenge look increasingly at polymer modified asphalt as a potential solution. Design and construction of long life asphalt pavements requires usually the following: A strong and uniform foundation. Design of high quality asphalt mixtures the performance of which is tailored to structural and functional requirements of individual pavement layers (layer function concept); high modulus rut resistant asphalt mix is a good choice for upper (binder) and lower base course. Specify appropriate construction tolerances and ensure effective quality control during asphalt mix production and pavement construction. Pavement courses must be uniform in composition and density. Specify and verify adequate bonding between and good joint construction within pavement courses. Select pavement structures based on catalogues developed using mechanistic pavement design, or design individual pavement structures using mechanistic design parameters appropriate for long pavement life. Figure 1: Principle of Long-Life Asphalt Pavement Principle of Long-Life Asphalt Pavements Wearing Course: functional and structural Properties; renew as required (for example, every 10 to 15 years) Upper (or binder) and lower base course; primary structural pavement courses; design and construct to meet long-life requirements (40 to 50 years) Strong and uniform foundation (sub-grade) Page2

3 This paper reviews and summarizes materials and performance requirements of high modulus asphalt mixtures. In France high modulus asphalt (Enrobé à Module Élevé, or EME) has been used for more than 20 years as a solution to reducing rutting and increasing the stiffness of pavement structural courses for better load distribution. These types of mixtures are recommended for design and construction of long-life pavements. In France and in other countries hard paving grade bitumen (EN 13924: PEN 10/20 and 15/25) is widely used to achieve the properties specified for ELE mixtures. Specially formulated polymer modified bitumen (PMB) are used effectively as alternative when hard paving grade bitumen is not available. Hard PMB grades that are suitable for EME mixtures can be formulated and produced wherever PEN or similar standard bitumen and additives that increase initial asphalt binder and mixture stiffness are available. 2. Bituminous Binders for High-Modulus Asphalt Mix a. Shear Strength and Deformation Resistance of Asphalt Mixtures Huschek, S. reviews in his paper The Deformation Behavior of Asphalt Concrete under Tri-Axial Compression (AAPT, 1985) deformation and shear resistance of asphalt mixtures. Referring to the dissertation by Nijboer, L.W. (Plasticity as a Factor in the Design of Dense Bituminous Carpets, 1948) he postulates that total deformation resistance is made up of the following three components: Initial resistance Internal friction Viscous resistance Real and apparent cohesion and aggregate particle interlock are key contributors to initial resistance to deformation and to asphalt mix elastic modulus. Asphalt binders that exhibit high cohesive strength at low strains confine the aggregate structure of asphalt mix. By increasing confinement of the aggregate structure, friction between aggregate particles is increased. The combination of high cohesive strength at low strains and of increased internal friction enhances shear strength and modulus value of asphalt mixtures. Little, D.N. provides in his paper Analysis of the Influence of Low Density Polyethylene Modification of Asphalt Concrete on Mixture Shear Strength and Deformation Resistance (ASTM STP 1108, 1992) an example of the contribution that polymers and PMB can make to increase shear strength and modulus of asphalt mix (see Table 3 of Little s paper, reproduced as Figure 2 below). The effect of PMB formulated to obtain high initial cohesive strength and shear resistance of asphalt mix is further demonstrated in Figure 3, which shows unpublished test results obtained at the Texas Transportation Institute (1988). Page3

4 Figure 2: Excerpt from Paper by Little, D. N ( ), 1992 Figure 3: Stress-Strain Response of Different Asphalt Mixtures (unpublished, based on Research conducted by Little, D. L. (1988) Stress-Strain Response of Asphalt Mixtures PE SBS Control Page4

5 Christensen, D.W. and Bonaquist R. (Rut Resistance and Volumetric Composition of Asphalt Concrete Mixtures, AAPT, 2005) use the term asphalt mix resistivity to describe the effect of aggregate structure and binder viscosity on shear strength and rut resistance of asphalt mix. Table 17 of their paper shows that binder grade is the most significant contributor to rut resistance of asphalt mix. Binder grade (in this case expressed as PG-grade) is related to binder stiffness. To achieve asphalt mix properties typically specified for high-modulus asphalt mixtures, hard unmodified bitumen or special PMB is required. Polymer type, properties and dosage are all very important for such special applications. The importance of polymer type is also reflected in AUSTROADS Technical Report AP-T41/06: Specification Framework for Polymer Modified Binders and Multigrade Bitumen. For high modulus and highly shear resistant asphalt mixtures, use of PMB formulated with appropriate plastomeric polymers is recommended. This recommendation is based on extensive laboratory and accelerated field tests (ALF). In the European framework of PMB specifications cohesive strength (or deformation energy) based on force-ductility or direct tensile strength testing is an important specification parameter. The deformation energy parameter is calculated as the work expended to stretch a PMB sample from 200 to 400 mm (Figure 4 below). While this parameter provides useful information about cohesive strength of a binder under these specific test conditions (applicable to elastomer modified PMB), it is not useful to assess a binder s contribution to initial strength and stiffness. For this purpose, the area under the primary peak of the force-elongation curve should be taken into consideration, which is closely linked to initial resistance and modulus. Figure 4: Force Ductility and Deformation Energy Page5

6 Hard paving-grade bitumen shows excellent initial resistance to deformation, but does not exhibit high deformation energy between 200 and 400 mm extension. The same applies to asphalt modified with polyethylene and/or natural rock asphalt (asphaltite), both of which are used in France for EME 2 mix. Elastomeric polymers can also be used, but require high dosages of highmolecular weight polymer, which tends to yield PMB that is difficult to work with. Addition of special viscosity-reducing polymers such as SASOBIT would be an option, but this increases further costs of modification. Plastomers, such as LDPE, impart on asphalt mixtures high initial stiffness without affecting workability of asphalt mix. b. Hard Paving Grade Bitumen Hard paving grade bitumen is now standardized in Europe under EN (-1) and is widely used for EME 1 and EME 2 asphalt mixtures. The UK Specifications for Road Works (Series 900: Road Pavements Bituminous Bound Materials, 2010) specify in Section 930 use of PEN 10/20 or PEN 15/25 for EME 2 asphalt mixtures. Other grades of bitumen or special grades of PMB may be used subject to approval by relevant authorities. c. Polymer Modified Bitumen for EME Mix In France PE is frequently mentioned as additive for EME type mixtures. A selection of relevant literature references is provided below: TRB Circular 503: Perpetual Bituminous Pavements (2001): includes a contribution by Jean- Francois Corte, LCPC (Development and Use of Hard-Grade Asphalt and of High-Modulus Asphalt Mixes in France). In this article polyethylene and asphaltite are mentioned as suitable additives to achieve the special properties of EME. Figure 5 below (reproduced from Corte s paper) shows the effectiveness of recycled PE for achieving rut resistant (and high modulus) asphalt mixtures. Figure 5: Effect of Bituminous Binder on Rut Resistance of Asphalt Mix Page6

7 In the paper Special Bitumen by Jean-Noël Onfield use of polyethylene and other plastomers for high-modulus asphalt mixtures is mentioned several times. Contractors and material suppliers in France have developed proprietary asphalt mixtures that meet specifications for high modulus asphalt. For several of these special asphalt mixtures polyethylene is used as additive. In Spain CICLOPLAST (a non-profit society, which is committed to promoting recycling and use of recycled plstic0 and DITECPESA (a company specialized in development and use of asphalt products) have entered into an agreement to evaluate and promote use of recycled plastic in asphalt pavements. Under this agreement more than 2000 tons of plastic (recycled film used as greenhouse cover) was used in 2010 in a highway project in Almeria. Comprehensive research carried out about ten years earlier in several European countries (sponsored in part by the European Community) on use of recycled plastic evaluated a large number of PMB formulations using virgin and recycled polyethylene. The four-year research program concluded that select recycled plastic, especially recycled film used in horticultural and agricultural applications0 is suitable as asphalt additive. More information about recycled PE characterization and selection criteria for use as asphalt additive is provided in Section 4 below. 3. Use of Recycled LDPE as Asphalt Modifier Over the last thirty years several hundred papers and reports were published around the world on use of polyethylene and other plastics as asphalt modifier. Until the late 1990s selection criteria were usually limited to a few basic plastic properties, such as density and melt flow rate; applications at project level were often based on trial and error. Because of this largely empirical approach some of the projects did not perform as expected. Comprehensive research carried out since the late 1990s on use of virgin and recycled polyethylene as asphalt additive focused on developing performance based application criteria (for example, Performance Characteristics of PMB The Contribution of Recycled Polyethylene by Legnani, G. and Tappeiner, W.J. in Rassegna del Bitume, 2003; in Italian language; Study of Recycled Polyethylene Materials as Asphalt Modifiers by Ho, S. and others, 2006; NRC, Canada; Behavior of Reclaimed Polyethylene Modified Asphalt Cement for Paving Purposes, by V.S. Punith et.al., Journal of Materials in Civil Engineering, ASCE, 2011). These studies show that when properly selected and used as asphalt modifier, recycled PE offers attractive performance and cost advantages. Use of recycled PE in high modulus asphalt mixtures offers the following advantages: Recycling and re-use of plastic offers environmental benefits; recycled PE is readily available at reasonable costs. Asphalt mix produced with recycled PE (typically less than one percent by mass of mix) can be safely recycled. Good performance when used within the framework of the layer function concept; highmodulus asphalt mix plays a key role in this concept. Page7

8 High-modulus asphalt mixtures allow reducing layer thickness by 15 to 30 % when compared to traditional base and binder courses. This offers important savings on raw materials such as aggregates and bitumen. Thickness reduction in conjunction with high-modulus asphalt mix is now widely accepted by road authorities, and is incorporated in pavement design in France (SETRA/LCPC) and in the UK (DMRB HD 26/06: Pavement Design). 4. Recycled LDPE Selection Criteria and Quality Control Recycled plastic, including film from agricultural and horticultural applications, is readily available in many countries. Hurley, S. presents in Agricultural Plastic Film Use and Recycling in California (2008 ) results of a study carried out on use and recycling of agricultural plastic film. Costs of recycling and lack of recycling facilities were listed as impediments to broader acceptance of recycling by film users. These perceived problems can be overcome if PMB production or construction companies that are interested in this special application organize collection, transport and sorting/cleaning of such film. Using recycled material from selected sources and industrial applications allows more effective utilization of such material than would, for example, be possible when household waste is recycled. During the first two decades of using PE and other plastic as asphalt modifier, simple polymer properties were employed as basis for selection. Properties such as density and Melt Flow Index (MFI) are useful but do often not provide sufficient Information about performance potential of recycled plastic as asphalt additive. One of the reasons for this limitation is that density and MFI are often masked by the presence of processing agents, color pigments, and other additives used at the time of manufacturing plastic goods. Based on research carried out from the lte 1990s onwards one or more of the following tests is recommended for assessing the suitability of REC PE as asphalt modifier: o Molecular Weight and Molecular Weight Distribution Weight-Average Molecular Weight M w Number-Average Molecular Weight M n Polydispersity Index M w /M n o Fourier Transform Infra Red (FTIR) Spectroscopy for Identification of Impurities (polymers other than PE) o Differential Scanning Calorimetry (DSC)/Differential Thermal Analysis for identification of different types of PE (LDPE; LLDPE; MDPE/HDPE) o Shear-Rate dependent Melt Viscosity (at 190 C) for evaluating interaction (and miscibility) of REC-PE with bitumen. Critical values were identified below which good quality PMB can be formulated with PE. Page8

9 5. Summary Traffic volumes and traffic loads are increasing faster in many developing countries than in industrialized countries. In most developing countries, hard paving grade bitumen (EN 13924), which is used in Europe for high modulus, highly rut resistant asphalt mixtures, is not available at a competitive price. Use of special grades of PMB, which can be formulated and produced using locally available raw materials, offers a sound and overall cost-effective alternative. Use of special asphalt mixtures within a sound layer function concept improves performance, longevity and cost effectiveness of asphalt pavements. The technology required to design and produce high modulus asphalt mixtures is readily available. Use of selected recycled LDPE in accordance with current know-how and practices offers performance, cost and environmental benefits. Long Life (Perpetual) Pavement Good Foundation (Sub-Grade) Superior Paving Materials Europen Layer Function Concept EME 2 for Binder/Base Course Functional/Structural Wearing Course PEN or Special PMB for EME 2 Limted Supply of PEN 10-20/15-25 As Alternative use special PMB, such as PMB formulted with recycled PE In IM-40 Perpetual Asphalt Pavements A Synthesis published by the Asphalt Pavement Alliance key recommendations for further investigations include development of high-modulus asphalt pavements and of mix designs for high-modulus asphalt mixes. The EME technology is already well developed in Europe. Adaptation of this proven technology to climate and traffic conditions of other regions should be feasible; because of performance, cost and environmental benefits consideration should be given to using recycled plastic to achieve the high modulus values and superior rut resistance required for these mixtures. Page9