During the last decades, extensive research has been carried out on using reclaimed asphalt pavement (RAP) material in the production of hot recycled mix asphalt. Unfortunately, the aged, stiff, and brittle binder in the RAP typically increases the mixture stiffness and can therefore cause fatigue and low-temperature damages. In the scientific literature, there are many studies concerning the aging and rejuvenation of bitumen, but there is a lack of up-to-date reviews that bring them together, especially those facing the phenomena from a chemical point of view. In this paper, a recap of the chemical aspects of virgin, aged, and rejuvenated bitumen is proposed in order to provide a useful summary of the state of the art, with the aim of both encouraging the use of an increasing quantity of RAP in hot mix asphalt and trying to give indications for further research.

In road construction, it can happen that, for different reasons, the time between hot-mix asphalt (HMA) production and paving is extended to some hours. This can be reflected in several problems such as mix cooling and temperature segregation, but also in an extremely severe bitumen ageing due to its prolonged exposure to high temperatures. This paper deals with the investigation of these phenomena both in the laboratory and on site. In particular, the first part of the research aimed at observing the influence of the conditioning time, when the loose HMA is kept in the oven at a high temperature, on the mix properties. The second part focused on the ageing/cooling that happens on site during HMA hauling, as a function of time and type of truck. Temperatures were monitored using a thermal camera and different probes, and gyratory compactor specimens were produced by sampling some HMA from the trucks every 1 h for 3 h. The results showed that HMA stiffness rises if the time when the loose mix stays in the laboratory oven before compaction increases. However, on site, the HMA volumetric and mechanical properties do not change with hauling time up to 3 h, probably because the external material in the truck bed protects the HMA core from the access of oxygen, hindering bitumen oxidation and loss of volatiles. The temperature monitoring highlighted that temperature segregation, after 3 h hauling, can be higher than 30 °C but it can be reduced using insulated truck beds.

Heavy traffic loads require the replacement of damaged pavements, so a huge amount of reclaimed asphalt pavement (RAP) material is now available and must be recycled in order to avoid landfill and to achieve both environmental and economic benefits. The most common and profitable solution to reuse RAP is associated with the hot recycling technique, as it allows recovering both solid and binding components of RAP. Several factors influence the performance of hot recycled mix asphalt (HRMA). Among those, this paper focuses on the role played by the origin of the virgin bitumen, i.e. the oil-distillation process, and by the mixing temperature adopted during HRMA production. The objective was to evaluate the rheological properties of mixtures produced using a high amount of RAP (50%), two different rejuvenators, two mixing temperatures (140 °C or 170 °C) and two neat bitumen types derived from different distillation processes (visbreaker and straight-run). The results showed that the addition of RAP led to an increase in the dynamic modulus and a decrease in the phase angle, while the use of rejuvenators partly tended to rebalance these characteristics. The visbreaker bitumen showed a higher sensitivity to short-term aging than the straight-run, determining higher mix stiffness and lower viscous features. The higher mixing temperature also determined an increase in the complex modulus and a reduction in the phase angle as a result of the higher mobilization of the aged bitumen from the RAP.

The increasing bitumen cost, the growth of the traffic loads and the need to find more sustainable paving practices are forcing agencies to identify new ways to maximise the re-use of Reclaimed Asphalt (RA). If RA could theoretically be 100% recycled, its content in bituminous layers is still restricted in most countries mainly due to legislation limitations and technical issues. In particular, the stiffer and more brittle mixes, usually resulting from RA addition, have raised concerns about the long-term properties of the pavement. In order to further increase the amount of usable RA, rejuvenating additives can be included in the mixtures. The paper deals with the analysis of seven additives, with particular focus on their ability in mitigating the effects of the presence of RA in the mix as a function of production temperature. In particular, FTIR analysis was carried out on the additives to evaluate the differences in their chemical composition. Moreover, the mechanical properties of the mixtures including 50% RA and produced at 140°C or 170°C were investigated. The results showed the reduction of the mixing temperature from 170°C to 140°C determined a slight increase of the air voids content but also allowed a significant decrease of mix stiffness and strength with a contemporary increase of ductility. The mixtures containing 50% RA and the additive had an intermediate behaviour between the mix without RA and the mix with 50% RA and no additive. The additives investigated in this research showed, in most cases, different chemical structures, which reflected in their rejuvenating performance. The comparative analysis of the results from mechanical and chemical tests provided remarkable information about the characteristic properties of a good rejuvenator, particularly the presence of esters and the high flashpoint.

The paper aims at analysing the evolution of the rheological behaviour of the bitumen extracted from reclaimed asphalt (RA) with rejuvenation (blending of virgin and RA bitumen, with addition of a rejuvenator) and re-ageing. The complex modulus of the binders was measured through a DSR device, while RTFOT and PAV protocols were followed to reproduce the bitumen ageing. Three rheological models were used in order to study the evolution of their parameters in the various steps. In addition, the apparent molecular weight distribution (AMWD), determined through δ-method, was analysed. The results showed that the rejuvenator could reduce RA bitumen stiffness but had low effect on the phase angle. After long-term re-ageing, the rejuvenated RA bitumen showed comparable rheological behaviour with respect to the bitumen initially recovered from RA. However, the AMWD analysis showed that the rejuvenator did not adequately disrupt the asphaltene clusters in the aged bitumen.