Chiara Riccardi

A novel back-calculation approach for determining the rheological properties of RAP binder

The determination of the rheological properties of the aged binder in Reclaimed Asphalt Pavement (RAP) materials is a challenging problem. Conventionally, extraction and recovery are used to obtain the RAP binder for further experimental characterisation; however, this procedure is not entirely reliable and accurate. Alternative and more precise approaches are based on asphalt mixture tests in combination with complex and sophisticated back-calculation methods, which are costly and time-consuming. In this paper a new and simple approach to estimate the rheological properties of RAP binder at intermediate temperature is proposed. This is based on Dynamic Shear Rheometer (DSR) tests performed on mortars, composed of a selected fine fraction of RAP and virgin binder, together with a new back-calculation solution. The properties of the bituminous blend of virgin and RAP binders are obtained through the manipulation of the Nielsen model equation to take into account the effects of frequency and temperature on mortar stiffness. The Voigt model is then used to estimate the complex modulus and the phase angle of the RAP binder from the complex modulus and the phase angle of the back-calculated binder blend.

Effect of cooling medium on low-temperature properties of asphalt binder

The characterisation of asphalt binder at low temperature is of fundamental importance for selecting and designing asphalt materials with good and durable performance in regions experiencing severely cold climates. The current specification addresses this issue based on the Performance Grading (PG) system, developed during the Strategic Highway Research Program, and on low-temperature creep tests performed on asphalt binder with the Bending Beam Rheometer (BBR). Recently, an alternative experimental method was proposed to relate the complex modulus, obtained with the Dynamic Shear Rheometer (DSR) at low temperature, to the BBR creep stiffness. However, while DSR tests are performed in air, BBR relies on an ethanol bath for conditioning the binder specimens, making the relation between complex modulus and creep stiffness dependent on the specific cooling medium. In this paper, the effect of cooling medium on the low PG and on the rheological properties obtained from DSR and BBR tests is experimentally investigated and modelled. First, DSR and BBR tests, in ethanol and air, are performed on a set of different asphalt binders. Then, a relationship between the complex modulus in the time domain and the creep stiffness obtained both in ethanol and air is derived and the low PG for both cooling media is estimated. Finally, 2 Springs 2 Parabolic Elements 1 Dashpot and the Huet models are used to compare the effect of ethanol and air on the rheological properties of the asphalt binders. It is found that air results into higher creep stiffness and smaller m-values compared to ethanol. The two rheological models indicate that, only in the case of air, complex modulus and creep stiffness present the same kernel model parameters. This suggests that the low performance grade, obtained from BBR tests in ethanol, is strongly affected by the cooling medium, as well as the recently proposed procedure based on DSR tests. Based on the finding of the present research, the use of air for BBR creep tests is recommended.

Wearing Course Mixtures Prepared with High Reclaimed Asphalt Pavement Content Modified by Rejuvenators

This paper presents an experimental investigation on the combined use of high content of reclaimed asphalt pavement (RAP) and rejuvenators, for producing asphalt mixtures for wearing courses. First, the new Binder-Fast-Characterization-Test (Bitumen Typisierungs Schnell Verfahren in German) BTSV method, recently proposed in Germany, is used to determine the amount of rejuvenator required to design recycled mixtures for a specific RAP source. Then, a set of seven asphalt mixtures for wearing course is prepared with different amounts of RAP and with three types of rejuvenators. The BTSV procedure is further applied to evaluate the rheological properties of the binder extracted from the mixtures. Resistance to permanent deformation, stiffness, fatigue, and low temperature behavior tests are then performed to determine the material response. It is observed that the use of recycled material and rejuvenators results in similar or better deformation resistance, higher stiffness and enhanced low temperature properties, with an improved fatigue behavior, also for high RAP content, when compared to mixtures prepared with virgin material. These findings suggest the possibility of using rejuvenators for field tests on wearing courses.

A novel method to characterise asphalt binder at high temperature

The current experimental method used in Europe to characterise asphalt binder at high temperature is based on the Ring & Ball softening point. However, for modified binders it was demonstrated that such a method is unable to correctly characterise the material. In the present work, an alternative solution to determine the high-temperature properties of asphalt binders using the dynamic shear rheometer (DSR) is proposed: the Binder-Fast-Characterisation-Test (BTSV) (Bitumen-Typisierungs-Schnell-Verfahren, in German). Two parameters are identified for the rheological characterisation of asphalt binders: the temperature TBTSV, which is related to the Ring & Ball softening point and is an indicator of the binder stiffness, and the phase angle δBTSV, which provides information on the degree of binder modification. These two key parameters are used to discriminate among the different asphalt binder domains in the high-temperature range.

Mechanical Performance of Asphalt Mortar Containing Hydrated Lime and EAFSS at Low and High Temperatures

In this paper, the possibility of improving the global response of asphalt materials for pavement applications through the use of hydrated lime and Electric Arc-Furnace Steel Slag (EAFSS) was investigated. For this purpose, a set of asphalt mortars was prepared by mixing two different asphalt binders with fine granite aggregate together with hydrated lime or EAFSS at three different percentages. Bending Beam Rheometer (BBR) creep tests and Dynamic Shear Rheometer (DSR) complex modulus tests were performed to evaluate the material response both at low and high temperature. Then, the rheological Huet model was fitted to the BBR creep results for estimating the impact of filler content on the model parameters. It was found that an addition of hydrated lime and EAFSS up to 10% and 5%, respectively, results in satisfactory low-temperature performance with a substantial improvement of the high-temperature behavior.

Investigation on the Effect of Physical Hardening and Aging Condition on Low-Temperature Properties of Asphalt Binder Based on BBR

Low-temperature properties of asphalt binders are fundamental for designing asphalt mixture in cold regions. This is especially true for alternative technologies such as Warm Mix Asphalt (WMA), for which a temperature reduction during production may potentially lead to substantial benefits in terms of long-term aging conditions. At low temperature, asphalt binder is conventionally characterized based on creep tests conducted with the Bending Beam Rheometer (BBR) in ethanol at a single conditioning time of 1 h. However, asphalt binders undergo significant time-dependent stiffening, often referred to physical hardening, when stored at such low temperatures. In this paper, the effect of aging temperatures and physical hardening on the low temperature rheological properties is experimentally investigated and modeled. First, BBR tests are performed on four long-term aged asphalt binders, which were previously short-term aged at three different temperatures (123 °C, 143 °C, and 163 °C), after three different conditioning times: 1 h, 24 h and 72 h. Next, the creep stiffness, S(t), relaxation parameter, m-value, and difference in critical temperature, ΔTc are calculated and compared. Finally, the Huet model is fitted to the experimental data with the goal of comparing the effect of aging temperatures and physical hardening on the rheological parameters. Results indicate that physical hardening causes a significant increase in creep stiffness in the first 24 h while only moderate contribution is obtained when condition time is extended. In addition, the reduced production temperature of 40 °C can significantly improve the aging properties of asphalt binders at low temperatures while mitigating the effect of physical hardening.

Comparison of low-temperature fracture and strength properties of asphalt mixture obtained from IDT and SCB under different testing configurations

This paper presents an investigation on the low-temperature properties of asphalt mixture determined with the indirect tensile (IDT) and the semi-circular bending (SCB) experimental methods. First, IDT and notched and unnotched SCB tests are performed on a set of ten different asphalt mixtures. Then, the maximum stress (strength) along the crack surface of SCB specimens is estimated using a commercial finite-element software and cohesive zone modelling. Finally, a set of simple linear relations between the numerically determined SCB strength on notched specimens and the nominal strength value experimentally measured on IDT specimens is derived. It is found that the newly proposed relationship can be potentially used to predict the asphalt mixtures’ nominal strength of IDT specimens based on SCB notched fracture tests. No good correlation could be found between SCB strength on unnotched specimens and the corresponding IDT measurements at this stage. This is most likely due to the simple FEM approach selected and the different failure mechanisms of the SCB unnotched configuration.

Experimental Investigation of Rutting in the Different Phases of Asphalt Mixtures

Rutting is one of the most severe failure mechanisms for asphalt pavements. This phenomenon is due to the accumulation of permanent deformation in consequence of traffic loading. The behavior of asphalt mixture is highly affected by the properties of the asphalt binder used in the mix design. For this reason, the Multiple Stress Creep and Recovery (MSCR) test procedure was recently introduced with the objective of better evaluating the rutting resistance while replacing the conventional Superpave parameter, G*/sinδ. Good understanding of the rutting mechanism within the asphalt binder component is essential for correctly studying the mutual interactions of the asphalt mixture components: binder, fine aggregate and large particles. This paper presents the results of an experimental campaign consisting of MSCR tests performed on asphalt binder, mastic and fine aggregate mixture which compose a typical mixture for asphalt binder layer. All the tests were conducted using a Dynamic Shear Rheometer (DSR). The classical plate-plate configuration having 25 mm diameter and 1 mm gap was selected for asphalt binder and mastic tests. The cylindrical geometry was used for torsional tests on fine aggregate mixture presenting aggregate as large as 1.16 mm. A single testing temperature of 60 °C and three different stress levels, 100, 1600, 3200 Pa, were imposed. The results indicate that creep and recovery are functions of filler concentration and stress level.

Effects of Rejuvenator on Reclaimed Asphalt Binder: An Exploratory Study of the RILEM TC 264-RAP Task Group 3

This paper presents the preliminary experimental activity conducted to develop the round robin test (RRT) plan on the use of rejuvenators for the Task Group (TG) 3 on Asphalt Binder for Recycled Asphalt Mixtures as part of the RILEM TC 264-RAP. For this purpose, a reference Reclaimed Asphalt (RA) binder was extracted from a single RA source. This material was then fully characterised with conventional experimental methods and with Fourier Transform Infrared Spectroscopy (FTIR) for evaluating the chemical structure. The RA binder was found to be very hard compared to target binder of PEN50/70 grade and the use of virgin binder cannot restore its properties. In this condition, a rejuvenator dosage of 9% per weight of RA binder is foreseen as optimum value to restore the binder properties close to target PEN50/70 grade for the final binder-rejuvenator blend over a RA content ranging from 60% and 100%.

Reclaimed asphalt binders and mortars fatigue behaviour

Fatigue cracking is one of the most important failure mechanisms occurring in asphalt pavements, especially when mixtures incorporate considerable amount of rReclaimed asphalt pavement (RAP). In fact, aged binders contained in RAP generally make asphalt more brittle and specifically reduce fatigue resistance of the resulting asphalt mixtures. Binders and mortars play a key role in this phenomenon, considering fatigue cracking usually starts within these asphalt components. However, performance-related tests and specifications commonly regard binders and there are no sound methodologies allowing the use of mortars to predicting fatigue performance of asphalts containing RAP. For this reason, in this paper, fatigue resistance of extracted binders from high-RAP content mixtures and of RAP mortars (passing sieve with an opening size of 0.15 mm) were assessed and compared. Binders were extracted from asphalt mixtures manufactured with 30%, 60% RAP and rejuvenators. Mixtures recipes were then reproduced to manufacture mortars accordingly. Time sweep tests in stress-controlled mode were carried out on both materials (binders and mortars) and the resulting fatigue laws were compared. As a result, a strict correlation was obtained, leading to affirm fatigue-related properties of RAP mixture could be assessed by directly testing RAP mortars. This makes the recovery of RAP binders unnecessary. Moreover, a relationship between the two fatigue laws versus the percentage of fine particles in the mortar was found. This latter relationship allows determining the fatigue law of mortars corresponding to any percentage of fine particles and therefore corresponding to any percentage of RAP.