Asphalt plant Design

Producing new asphalt mixtures with high content of Reclaimed Asphalt Pavement (RAP) is a major challenge in road construction for economic and environmental reasons. Although many laboratory studies addressed this issue, concerns related to the number of variables involved in the plant production process still limit hot recycling, especially when Styrene–Butadiene–Styrene (SBS) modified bitumens are used. In this sense, plant produced mixtures should be directly investigated to obtain reliable performance evaluations.

Given this background, the paper proposes the mechanical characterization of dense-graded mixtures with 40% RAP produced at the asphalt plant as part of rehabilitation activities of an in-service motorway. The Bailey Method was applied to optimize the mix design. Mixtures were prepared by using two polymer modified bitumens (with high and low SBS polymer content) and selected RAP fractions composed only of asphalt layers including SBS modified bitumen itself. An additional mixture prepared according to the current practice for binder layers of motorway pavements was produced for comparison purposes.

Advanced laboratory tests allowed the determination of the main material properties (i.e. compactability, cracking and rutting aptitude, indirect tensile strength, fatigue and self-healing). Results showed that mixtures with 40% RAP had performance comparable or even enhanced than the reference mixture especially when prepared with low modified bitumens. Consequently, this study shows that an accurate mix design and the selection of adequate bitumens overcome the potential drawbacks related to the use of high RAP percentage given the possibility to produce suitable recycled mixtures.

Keywords

• Hot recycling;
• Compactability;
• Polymer modified bitumen;
• SBS;
• RAP;
• Mechanical properties

1. Introduction

Over the last decade, in many different manufacturing fields the increasing costs of energy and natural resources as well as the need to preserve raw materials led toward the implementation of green technologies and environmentally-friendly productions. In this context, recycling represents an efficient solution to increase cost and energy savings and has gained growing acceptance also in the case of pavement industry and . In fact, since pavements are generally composed by aggregate and bitumen (no-renewable and expensive resources) the re-use of a certain amount of Reclaimed Asphalt Pavement (RAP) coming from the demolition of old flexible pavements allows the concurrent exploitation of its bituminous and aggregate component, with significant benefits in terms of environment and costs.

Among various recycling procedures currently available, the hot recycling process is widely recognized as the one which ensures better performance, but at the expense of the limited amount of RAP incorporated, usually up to 20–30% . Although over the last years many laboratory studies have focused on the optimization of hot recycling procedures, , and to increase the RAP content allowed for the production of new bituminous mixtures, many doubts and concerns still exist when mixtures are produced in plants. In fact, mixtures prepared in large scale productions usually do not perform as well as mixtures optimized in laboratory since several variables related to the production process, not adequately reproducible in laboratory, significantly compromise mixture performance. Potential detrimental effects are mainly related to RAP stockpile conditions and RAP temperature when mixed with other virgin aggregate fractions. In fact, the presence of an external thin film of aged bitumen makes it impossible to treat RAP as a virgin aggregate, forcing modifications to the production process. RAP aggregates are added into the mixer directly at ambient temperature or processed in a leased drum at low temperature in order to avoid excessive aging of reclaimed bitumen and consequent increased pollutant emissions (unavoidable if RAP comes into contact with the burner flame of the virgin aggregates drum) . However, this working temperature level does not allow the complete drying of RAP aggregates, which often retain a high percentage of water that negatively alters the interactions with other material components and, consequently, overall mixture performance. Clearly, such an issue becomes even more significant when large amounts of RAP are added to virgin aggregates making it difficult to achieve satisfactory performance despite previous laboratory mix design optimization. On the other hand, the presence of higher amounts of reclaimed aggregates pre-coated by a thin film of bitumen (RAP) could result as helpful to compensate the abovementioned drawbacks as well as other potential issues related to the stiffening contribution given by the aged reclaimed bitumen and .

As a consequence, the performance assessment of in plant produced recycled mixtures is essential to verify and, eventually, properly adapt the mix design previously optimized in the laboratory. Such a validation can demonstrate the feasibility of successfully producing hot recycled mixtures with high RAP contents without compromising material performance. It constitutes a fundamental step for a further increase in the maximum amount of RAP currently allowed for new bituminous mixtures.