Aging effects on recycled WMA porous asphalt mixtures
Introduction
Environmental friendly asphalt pavements represent a priority goal for administrators and industries gaining interest from the scientific community. The use of Reclaimed Asphalt Pavement (RAP) as partial replacement of virgin aggregates is a widely recognized sustainable solution since it allows reduction of waste materials and preservation of natural resources as well as savings in construction costs. In addition, lower production temperatures for asphalt mixtures are needed in order to limit emissions and energy consumption as well as ensuring better working environment during construction phases both at asphalt plant and worksite. In this sense, Warm Mix Asphalt (WMA) technologies are recognized as proper solution since they allow significant reductions of mixtures production temperatures without affecting their mechanical performance in the field. Nowadays, the use of RAP is often combined with WMA technologies in order to produce warm recycled mixtures with several benefits in terms of environment, cost and mechanical performance [1], [2], [3], [4]. In fact, high production temperatures generally used to produce Hot Mix Asphalt (HMA) mixtures cause additional aging of the already aged bitumen within RAP that not only compromises its rheological and mechanical behaviour but also generates harmful emissions during production.
Several studies have been carried out to investigate the feasibility of warm recycling in case of porous asphalt mixtures (PA) [5], [6], [7], [8], [9]. Wurst and Putman [6] showed that WMA porous asphalt mixtures exhibited greater durability than HMA porous asphalt mixtures after long term aging. Moreover, Goh et al. [5] found beneficial effects of coupling recycled PA with WMA technologies in terms of complex modulus and indirect tensile strength. Concerns still remain related to water susceptibility of warm PA mixtures; it was found that the use of chemical WMA additive tends to limit such a problem with respect to other WMA additives even though it does not guarantee the same performance of PA produced at conventional temperature [8]. Furthermore, short-term and long-term aging effects are two aspects that need careful investigation in case of warm mixtures [10], [11], [12]. In fact, during mixing and compaction WMA mixtures are exposed to lower temperatures compared to conventional HMAs so leading to limited short term aging and premature rutting failure. In addition, concerns still exist related to in-service and long term performance since WMA technologies have been developed over the last decade and long term performance data are not available yet. Further investigations are needed in order to fully understand long term behaviour of WMA mixtures, especially in case of PA that are more sensitive to the detrimental effects of climate and traffic that lead to brittleness as well as ravelling [13].
Given this background, the objective of this experimental study is to evaluate aging effects on recycled porous asphalt mixtures produced at reduced temperatures using different WMA additives and including 15% of RAP. In this sense, long term aging was simulated in laboratory by means of an innovative test procedure (Viennese Aging Procedure – VAPro) [14], [15]. Mechanical laboratory tests were carried out on mixtures as well as bituminous components in order to evaluate possible links between bitumen and mixtures performance.
Section snippets
Materials
Four different porous asphalt mixtures were produced in laboratory using the same mix design that was optimized in previous research studies [16], [17] including 15% of selected RAP aggregates from milled porous asphalt surface layers. Basalt aggregates, filler and coarse RAP aggregates (8/16 mm) were combined to obtain the final grading curve showed in Fig. 1.
Moreover, a blend of 70% cellulose and 30% glass fibres dosed at 0.3% by aggregate weight were added to the mixtures. The total bitumen
Aging effects on extracted bitumens
Complex modulus values G∗ were measured for bitumens extracted and recovered from specimens before (i.e. Short Term aging – ST) and after VAPro aging (i.e. Long Term aging – LT) by means of frequency sweep tests through DSR equipment. Results were used to determine master curves at a reference temperature of 10 °C, considering valid the Time-Temperature Superposition Principle. The modified CAM Model [27] (Christensen Anderson and Marasteanu) was adopted to analyse test data and to represent the
Conclusions
Based on the results obtained during the experimental program, the following conclusions can be drawn:
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WMA bitumen and WMA mixtures are characterized by lower stiffness values than the HMA suggesting that reduced production temperature could affect stiffness properties of warm mixtures due to the lower temperature-induced oxidation during the production phase;
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extensive long term aging effects are measured in case of WMA bitumens and mixtures contrarily to HMA materials that are not significantly
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