Search

Technical specifications for the asphalt concrete properties are developed to be able to specify mixtures that will perform well in pavement applications. Being able to identify and determine properties related to pavement performance in practice is crucial for both road authorities and contractors, since it allows for design and risk management by determining design life times and reliability. However, the properties of Asphalt Concrete (AC) change over its lifetime and since most pavement layers last for a decade or more these changes are crucial in determining the performance in practice. For many of the standard materials the effect of aging is implicitly dealt with in the safety factors that also account for other effects such as the variation rest periods/healing and variations in traffic and weather in the design methods and specifications. Rapid changes in the materials used (increasing percentages reclaimed asphalt, bio-bitumen, rejuvenators, waste materials) and in the production of both bitumen (new refining methods resulting in different composition of bitumen) and asphalt concrete itself (warm mix asphalt, porous asphalt concrete, rubber asphalt mixtures) lead to increased uncertainty in the effects of aging. As a result, the uncertainties in pavement performance increase, which means the prediction of maintenance and the necessary budgets is getting more inaccurate. In order to maintain the ability to reliably design and maintain pavements and determine the most cost-effective solutions for a given situation, a better understanding of the aging processes and objective methods to take into account aging effects on material properties is needed. This need is widely recognized, in the USA the Mechanical Empirical Design Guide takes aging into account through aging tests on the bitumen used and in Europe CEN TC227 works on establishing a method to assess the aging sensitivity of asphalt mixtures. This symposium aimed at combining the existing information and insights from ongoing research into recommendations that will allow the development of methods to determine aging sensitivity and the impact on pavement performance, facilitate the exchange of obtained data and stimulate further developments The resulting recommendations are: •Do make long term aging sensitivity of binders part of the bitumen standards and take the results from the aging sensitivity of binders into consideration when assessing AC properties. Be aware that RTFOT testing only gives an indication of the sensitivity of a penetration grade binder to aging during hot mix production and construction, it doesn’t work for hard grades, PMB’s or warm mixes. •Because of the many variables involved, developing one test method to characterise aging sensitivity seems improbable. However, PAV aging is both practical and, if tests at various conditions are carried out, able to give kinematic properties. A PAV protocol for testing at two temperatures and time intervals could provide practical characterisation information for the short term and enable model development and validation on the long term. RCAT and other aging procedures could also be used in this sense, but considering the availability of equipment and the wide spread experience, PAV is the best candidate to allow the rapid development of international experience with the approach. Based on the current standards and the work presented during the symposium, PAV tests at 90 and 100 degrees Celsius and 20 and 40 hours, respectively, are suggested. The low values for temperature and duration are based on the current standards and fit both the USA and CEN procedure, while research shows that after 40 hours at 100 degrees the chemical (FTIR) and rheological (DSR) properties of laboratory aged and field samples were similar (Section 6.5 and 8.5). At 100oC the temperature is low enough so that the effect of secondary reactions is negligible. As such, these conditions are appropriate for kinetic expressions for in service pavement performance. For high temperature processes and possibly also for repeated recycling (very long term) more sophisticated methods are needed. •Set-up and maintain field monitoring of temperature and UV radiation in various climate zones, as well as regular sampling over time and height to keep checking the predicted changes (from both tests and models), versus the actual changes in properties order to ensure reliability of the data as well as the applicability for pavement performance prediction. In setting up field tests, it is important to get both the composition of the virgin bitumen and the composition after mixing, transport and placement in the pavement. These compositions provide the starting points from both the material and pavement structure point of view and can be used to assess the development of aging products over time. There is a lot of discussion about the impact of binder recovery methods on the observed composition, so until it is proven that this does not have an influence, for comparisons the same recovery method should be used. •Set up a coordination and support action on AC-Aging to continue to exchange information and experiences, both in research and in construction projects. •develop an IR testing protocol, to facilitate the exchange of results and information. •Compare the bitumen composition that is found through various recovery methods to establish if there is an effect and if so, develop a procedure to address this. •To further understanding of aging, a Round Robin test on the differences in test conditions between US and EU, allowing better access to each other’s data and knowledge would be useful. •When developing aging tests for AC, it would be useful to look at the US experience. As long as there is no fundamentally correct method for assessing the aging, it would be preferable to standardize it as much as possible in order to allow cooperation and exchange of data.

In the past years a wide discussion has been held among asphalt researchers regarding the existence and interpretation of observed microstructures on bitumen surfaces. To investigate this, the RILEM technical committee on nano bituminous materials 231-NBM has conducted a round robin study combining differential scanning calorimetry (DSC) and Atomic Force Microscopy (AFM). From this, methods for performing DSC and AFM tests on bitumen samples and determination of the influence of wax on the observed phases, taking into account thermal history, sample preparation and annealing procedure, are presented and critically discussed. DSC is used to measure various properties and phenomena that indicate physical changes such as glass transition temperature (T g) and phase transition such as melting and crystallization. In the case of existence of wax, either natural or synthetic, it can further indicate the melting point of wax, that could be used to determine wax content. The results from seven laboratories show that T g temperatures obtained from the heating scans are more repeatable and easier to obtain in comparison to the cooling scans. No significant difference was noted for T g's obtained from the first and second heating scans. AFM is an imaging tool used to characterize the microstructures on a bituminous surface. Using AFM three phases in the materials with wax could be distinguished. The changes in the phases observed with AFM for increases in temperature were correlated with the DSC curve, and it could be established that the so called "Bee" structure disappeared around the melting peak in the DSC curve. Thus, this research has confirmed the relation between the microstructures on a bitumen surface and the wax content. cop. 2013 RILEM.

In a previous paper presented at the 2008 E&E Congress, the French mirror group (BNPé/P04/GE1) of the CEN/TC336 WG1 committee (bituminous binder) has reported the results of a round robin test on DSR complex modulus measurements involving two paving grade bitumen (10/20 and 50/70 classes) and three modified binders (plastomer, elastomer and cross-linked elastomer). This paper presents the outcome of the follow-up studies which had been decided at the time. In a first step, a number of dedicated investigations have been made to identify the potential contribution of different factors to the variation in measurement results. In particular, the impact of waiting time after test sample preparation, gap setting procedure, bonding temperature, conditioning times and temperature sweep procedure have been investigated. How to establish the limits of the linear domain of visco-elasticity is another issue which has been dealt with. This has allowed to define more stringent operating conditions which have then been followed by the participating laboratories for a final round robin limited to a pure bitumen (50/70) and the most difficult PmB (high level of plastomer modified product). In comparison to the first round robin, reproducibility was indeed improved, even for the modified product. The BNPé/P04/GE1 work does thus further open the way for the definition of binder performance indicators based on DSR measurements. This will however only be possible (especially for modified binders) if operating conditions are strictly defined and followed.

This paper presents the results of a round robin test (RRT) on binder fatigue, with data received from 11 participating laboratories. Most data were obtained with a Dynamic Shear Rheometer (DSR) equipment, one laboratory used a tension-compressiontest. In a first stage, the RRT consisted of three binders, two unmodified binders and onepolymer modified binder. These three binders had almost the same stiffness level at the testing conditions, 10°C and 10 Hz. Fatigue tests were conducted at three strain levels, and each strain level was measured in three repeats. For each test it was asked to follow the small strain stiffness during an equilibration time of 30 minutes. A second stage of this RRT including two more binders is foreseen. The same three binders were also tested in two and in four point bending mix fatigue tests. The analysis was separated into two parts, in a first part the repeatability and reproducibility of the small strain equilibrium stiffness was evaluated and in a second part the fatigue data were analyzed. Small strain stiffness data showed that the repeatability and reproducibility of G* is still not very high, and possible reasons are under evaluation. The analysis of the fatigue data showed that the overall reproducibility was not good. But when all participants that did in some way not follow the preparation procedure or that had problems with recording, were excluded, it was found that for 6 of the participating laboratories the fatigue properties gave good repeatability and reproducibility levels, for the two unmodified binders in this study. Each laboratory could distinguish the better fatigue life of the polymer modified binder compared to the unmodified binders as was also confirmed by mixture tests. This shows that the binder behavior plays an important role in the fatigue behavior of the mix, and that a binder test should be able to evaluate this role. The binder fatigue test can be used as a surrogate test for mixture fatigue, implying considerable time savings in sample preparation, however the testing of the binders is still time consuming.