Olard-Gaudefroy-1-LABORATORY ASSESSMENT OF MECHANICAL PERFORMANCE AND FUME EMISSIONS OF LEA ® HALF-WARM MIX ASPHALT VERSUS TRADITIONAL HOT MIX ASPHALT

With the current trend in favour of reducing pollutant emissions and energy consumption, the road industry and bitumen manufacturers have also been pursuing a path of sustainable development. A series of new medium temperature coating and mixing technologies, under the label "warm and half-warm asphalt mixes" have come to the fore. A study, carried out in the laboratory within the framework of a partnership established between EIFFAGE Travaux Publics and IFSTTAR, aimed at correlating potential fume emissions with mixing parameters for asphalt mixes, in relation to the mix mechanical properties and to bitumen ageing. This study demonstrates that a decrease in manufacturing temperature leads to a global emissions reduction as well as slower bitumen ageing. Results obtained highlight the influence of mix parameters and processes, especially regarding fume emissions for both halfwarm and hot mix asphalts (hma). As for the mechanical performance and durability of LEA® mixes, their complex modulus and fatigue resistance characteristics are similar to those of traditional hma. h al -0 08 45 89 2, v er si on 1 18 J ul 2 01 3 Olard Gaudefroy -3INTRODUCTION The topic of sustainable development is of prime importance to European road research, which has come to be based on a multidisciplinary approach to scientific problems, involving the environment, physics, chemistry, biology, the social sciences and societal issues. Road construction companies, such as EIFFAGE Travaux Publics or its subsidiary LEA-CO, have adopted an approach that seeks to accelerate convergence between environmental demands and the development of energy-saving materials, called "warm and half-warm mix asphalts" (wmas and h-wmas) (1)(2)(3)(4). Within the framework of a scientific collaboration organised between EIFFAGE Travaux Publics and the French Institute of Science and Technology for Transport, Development and Networks (IFSTTAR, formerly LCPC) on half-warm mix asphalts (< 100°C) since 2005, research efforts have been devoted to the mechanical and environmental assessment of LEA, as compared to its hot mix counterpart. This article is intended to present the main results obtained during this half-warm mix asphalt study. BACKGROUND The bibliographic references currently available regarding fume emissions during asphalt mixture manufacturing process are rather limited. Previous studies (5)(6)(7) have been conducted solely on fumes produced by bitumen with a focus on Polycyclic Aromatic Hydrocarbons. More recently, Paranhos (8) and Gaudefroy (9) have carried out at IFSTTAR a laboratory characterisation of Hot Mix Asphalt (hma) fumes during mixing in association with mechanical performance. The purpose of this research was to identify the effect of bituminous material design on bitumen emissions depending on various manufacturing factors, such as aggregate temperature and bitumen film thickness. As part of this laboratorybased study, a characterisation methodology combined with an assessment of mechanical and environmental performance was developed by IFSTTAR in order to differentiate the various mix formulae in the laboratory. Study results highlighted the strong influence of hma volumetric composition on emissions generated during manufacturing. The method derived consists of sampling the gas emissions generated during asphalt mixing. It should be pointed out that this process, which is comparable to a channelled source, makes it possible to analyse the pollutant emissions of bituminous mixes due to individual components alone, especially bitumen. Much more bibliographic data are currently available on laboratory assessments devoted to the mechanical behaviour of warm and half-warm mix asphalts (2)(7). The originality of the present study lies in the fact that it is intended to correlate potential fume emissions with mixing parameters for mix asphalts, in focusing on both the mix mechanical properties and bitumen ageing. STUDY OBJECTIVES This study, jointly performed by IFSTTAR and EIFFAGE Travaux Publics teams, has been aimed at determining the influence of manufacturing temperature and additives on the mechanical and environmental properties of LEA® asphalts, assessed in the laboratory by drawing comparisons with traditional hma, which is used herein as a reference material. EXPERIMENTAL METHODOLOGY Manufacturing processes ha l-0 08 45 89 2, v er si on 1 18 J ul 2 01 3 Olard Gaudefroy -4In this study, two processes have been employed: a traditional hma, used as the reference; and three variants of h-wma LEA. The originality of this h-wma process stems from the ability of hot bitumen to foam or emulsify when placed in contact with residual aggregate moisture at right around the water vaporization point of 100°C, which therefore allows for coating at lower temperatures (2)(3) (see Fig. 1). As a result of water dispersed inside the bitumen, the spontaneous volume expansion of bitumen leads to a thicker binder film around aggregates (from the mixing and coating stage inside the plant to the paving and compaction stage at the jobsite), which in turn enhances mix workability. Some specific additives have been introduced in this study to improve the foaming and coating capability of the binder. Materials The formula selected is a dense, bituminous 0/14 mixture for base courses (according to the French designation "Grave Bitume", denoted "GB") with diorite aggregates, originating from France's La Noubleau Quarry (see Table 1). The filler is limestone from the Airvault Quarry (France). TABLE 1: Design of the mix used in this study Mineral composition (%) Formula Filler Sand (0/2) Aggregate (2/6) Aggregate (6/10) Aggregate (10/14) Bitumen (%) GB 0/14 0.8 33.6 14.4 13.4 33.6 4.2 In this study, hma and h-wma were prepared in the laboratory using bitumen heated to 160°C. This bitumen (class 35/50, according to the NF EN 12591 Standard) is paraffinic and displays the following characteristics: a pen grade of 42 (1/10 mm) (NF EN 1426), a ring and ball softening point of 52.6°C (NF EN 1427). Two proprietary additives of vegetal origin (O and G) have also been incorporated into the bitumen just before conducting the experiment (with an additive content of 0.5% by weight of bitumen). For this study, an initial water content of 1.5%/dry aggregate weight was adopted for the LEA® fabrication. Presentation of this study's mixing principle The hma was prepared with bitumen and aggregates heated to 160°C. In this specific study, the three LEA® variants (Fig. 1) have been adapted to the laboratory constraints for manufacturing asphalt mixtures (i.e. a mixing time of approx. 120 s) at a final temperature in the range of 95°-100°C with bitumen heated to 160°C and aggregates heated at different drying temperatures: the drying stage at 160°C only affects an initial fraction of the aggregates (75% of the entire aggregate skeleton), then coated by all of the hot bitumen. The remaining cold and wet portion (25% of the entire aggregate skeleton), which has retained its initial moisture, is then added (the second fraction typically corresponds to either the cold and moist sandy fraction or the cold and moist RAP aggregates). All mix components are then mixed (LEA1); the drying stage at 160°C affects just an initial fraction of the aggregates (75% of all aggregates), which are then mixed, prior to the coating stage, with the remaining cold and wet portion (25% of all aggregates) that has retained its initial moisture level (LEA2), the drying stage at 150°C applies to all aggregates and is carried out in a way that allows retaining a fraction of the initial moisture (whereby a controlled amount of water is simply added). This stage is followed by coating (LEA3). In the laboratory, hma and h-hmas were manufactured at low temperature in a classical mixer with a stirring speed of 20 rpm. During coating, the fumes generated inside the mixer were channelled towards a stainless steel stack developed by IFSTTAR (9). Experiments were ha l-0 08 45 89 2, v er si on 1 18 J ul 2 01 3 Olard Gaudefroy -5performed in accordance with laboratory manufacturing conditions, as specified in the inplant manufacturing protocol: hmas and h-wmas (approx. 80 kg) are mixed for just 2 minutes at 20 rpm, and then the materials are held in the batch mixer (at a temperature of 160°C for hma and 105°C for h-wma) for another 28 min, a period that corresponds to the time of transport (storage or resting position) from plant to jobsite. STAGE 4: Aggregates, bitumen & residual water (liquid/steam) in thermal equilibrium 3 MAIN SEQUENTIAL DRYING & COATING METHODS STAGE 3 Coating with the hot bitumen + LEA® N°1 STAGE 1 Chippings + sand, together at 100°C