Arianna Stimilli

Performance evaluation of a cold-recycled mixture containing high percentage of reclaimed asphalt

Cold recycling of asphalt pavements proved to be an effective maintenance and rehabilitation technology for both environmental and economic reasons. Nevertheless, the use of cold-recycled (CR) asphalt mixtures requires a careful assessment of their mechanical properties, especially when they are designed to replace traditional hot-mix asphalt concrete (AC) mixtures. In this study, the potential use of a CR asphalt mixture as base course of an Italian motorway was evaluated. The studied mixture was produced in a central plant employing high-reclaimed asphalt (RA) content and used to construct two experimental sections along an in-service Italian motorway. In particular, a special mixing procedure, involving the use of water vapour and bituminous emulsion, was tested. A third experimental section was constructed with the same layer thickness using the AC mixture currently used in rehabilitation projects, incorporating 30% of RA. Volumetric properties, stiffness, resistance to permanent deformation and fatigue behaviour of mixtures were investigated by performing tests on samples cored from the three test sections and on laboratory-compacted samples. Results of the mechanical tests showed that CR mixtures provide lower stiffness modulus and lower resistance to repeated loading, but better resistance to permanent deformation when compared with AC. This behaviour can be explained due to the presence of cementitious bonds that reduce thermal sensitivity and viscous response.

Effect of Healing on Fatigue Law Parameters of Asphalt Binders

Asphalt binder has the ability to self-heal during rest periods when repetitive loading is applied. Studying the effect of rest on fatigue law parameters provides useful insight into the healing capabilities of asphalt binders. Currently, standard testing and analysis procedures to quantify asphalt binder healing capability are limited and difficult to implement in practice. Fatigue is known to depend on both traffic loading and pavement structure. Power law relations (e.g., Nf = Aγ−B) are commonly used for fatigue analysis of pavement materials. Power laws are used to estimate fatigue life (i.e., number of cycles to failure, Nf) as a function of load amplitude (e.g., strain, γ), which is a reflection of the pavement structure. In this study, testing consisted of strain-controlled time sweeps in the dynamic shear rheometer with a single rest period inserted at a specified damage level. With the selected test, the effect of healing on the relationship between fatigue life and strain was investigated. Nine neat and modified binders were tested. Healing testing was conducted at multiple age levels and strains. Healing that resulted from a single rest period had an insignificant effect on fatigue performance compared with modification and oxidative aging. Although this paper highlights the challenges of using few rest periods to predict healing potential, preliminary results of testing with multiple rest periods show the importance of healing. Further investigation is needed to verify the effect of multiple rest periods on binder fatigue.

New method to estimate the “re-activated” binder amount in recycled hot-mix asphalt

The magnitude of the binder re-activation within the reclaimed asphalt pavement (RAP) fraction is a major concern when recycled hot-mix asphalt (HMA) is produced, since it directly affects the final performance of the mix. Currently, no reliable methods to predict the RAP binder re-activation are available and the lack of such information prevents the quantification of the proper amount of virgin binder to be added to the recycled HMA.This study proposes a new method to estimate the re-activated binder amount based on the surface area (SA) of RAP aggregates. Laboratory results demonstrated the effectiveness of the SA approach in the case of RAP amounts typically used for hot-recycled HMA. Moreover, the experimental investigation suggested that revised coefficients are needed for the evaluation of the SA to account for the presence of clumps when high amounts of fine RAP aggregates are used.

In Plant Production of Hot Recycled Mixtures with High Reclaimed Asphalt Pavement Content: A Performance Evaluation

Nevertheless hot recycling process is nowadays a widespread technique, many doubts related to the in plant recycling process effects on the performance of recycled mixtures still exist and limit the maximum allowable amount of Reclaimed Asphalt Pavement (RAP). Therefore, the feasibility of an efficient production of plant hot recycled mixtures characterized by high RAP content and suitable performance should be properly addressed. To this aim, the overall performance of hot recycled asphalt mixtures produced in asphalt plant and containing high RAP content were assessed in this study. The mixtures were prepared with two different bitumens (high and low content of SBS polymer modifier) and 40 % of RAP only deriving from asphalt layers containing polymer modified bitumens. The aggregate grading curve was previously optimized through a specific laboratory study by applying the Bailey Method and using selected RAP. A third mixture, currently used for binder layers in motorway pavements, was also studied for comparative purposes. Compactability, stiffness, cracking and rutting resistance and fatigue behavior were investigated. Results of the mechanical tests suggest that mixtures containing 40 % RAP are suitable for the production of new asphalt pavements, especially when low modified bitumens are used. In fact, the performance of such mixtures were comparable or even higher than those of the reference mixture. In particular, the specific and accurate mix design allowed the potential drawbacks due to higher RAP content to be balanced.

Innovative Testing Protocol for Evaluation of Binder-Reclaimed Aggregate Bond Strength:

The durability of asphalt mixtures is strongly related to the adhesion properties developed at the interface between binder and aggregates. The loss of adhesion implies a rapid deterioration (e.g., stripping or raveling) of pavement layers under traffic loads, especially when the pavement is affected by the presence of moisture. Adhesion is a complex phenomenon related to the mineralogical and morphological nature of aggregates as well as to the chemical binder composition and the environmental conditions. The evaluation of adhesion has become even more complicated as an increasing percentage of reclaimed asphalt pavement (RAP) is used in the production of new asphalt mixes. Therefore, adhesion properties are also related to the mechanisms developed at the interface between virgin binder and aged binder that coats the RAP aggregate surface. An innovative procedure to evaluate the compatibility of the system virgin binder–RAP aggregate was proposed in this study. This procedure allowed the substrate of a RAP aggregate to be simulated in the laboratory and could integrate the binder bond strength test currently used to investigate bonding properties and water sensitivity of the system binder and virgin aggregates. Tests were performed with various aggregate sources, several modified binders, and two conditioning types (dry and wet). It was found that the procedure was able to discriminate different test configurations and variables. In particular, the artificial reclaimed aggregate substrate ensured higher adhesion performance compared with the virgin aggregate, especially in the wet condition, regardless of the modification level of the virgin bitumen.

Performance Assessment of Plant-Produced Warm Recycled Mixtures for Open-Graded Wearing Courses

Sustainable solutions, such as the combination of reclaimed asphalt pavement (RAP) as a partial substitution of virgin materials and warm-mix asphalt (WMA) additives to decrease production temperatures, represent the new research frontier in the asphalt industry. Specific investigations must evaluate the consequences of recycled WMA performance, especially in the case of open-graded (OG) mixtures, given that lower production temperatures can affect the adhesion properties and durability of those materials. This paper describes an experimental effort that involved OG mixtures produced in plant at warm temperatures (130°C) with two WMA chemical additives characterized by different compositions. In addition, an equivalent mixture (used as a control for comparison purposes) was produced at standard temperatures. Each material was prepared with a polymer-modified binder and 15% RAP. The main objective of the study was to characterize volumetric and mechanical properties of the investigated mixtures with particular attention paid to compactability aptitude and durability. To this end, gyratory-compacted specimens were subjected to several laboratory tests after dry and wet conditioning (i.e., indirect tensile strength, Cantabro test, semicircular bending, and repeated indirect tensile loading). Moreover, plant production of the investigated mixtures made it possible to evaluate the feasibility of large-scale processes. The WMA mixtures showed significant water susceptibility, although they guaranteed good compactability and satisfied mechanical acceptance requirements and international recommendations for raveling resistance in dry conditions. The chemical composition of the WMA additives was found essential to reduce the water damage. Surfactants and adhesion enhancers included within one of the investigated WMA additives ensured better water resistance than the other additive classified as a viscous regulator.