Testing and Characterization of Sustainable Innovative Bituminous Materials and Systems

One of the most important aspects of asphalt pavement deterioration is the ingress of water in pavement which leads to loss of the material characteristics, even material integrity with loss of aggregates. Thus the behaviour of asphalt mixture under moisture conditions is one of the key parameter for specifications. It’s a complex phenomenon which is influenced amongst other things by materials properties with wetting, cohesion and adhesion of bituminous binder and by environmental conditions with temperature, moisture, loading and layer type. It has been a research subject for a very long time and still not precisely described. A large number of test methods is available to estimate the affinity between aggregates and bituminous binders. These test methods can be subdivided in different ways; a first distinction can be based on the presence or absence of water during the test procedure. If water is present, the evaluation is in fact referred to as water sensitivity or moisture damage testing. Another distinction can be based on the type of sample that is evaluated. The test sample can be loose aggregates coated with a bituminous binder or a compacted asphalt mix sample. Lastly the individual components, bitumen and aggregate, can be tested separately through intrinsic properties. Furthermore, test results can also be based on the quantification of the test results, whether this is based on a qualitative or a quantitative evaluation. In RILEM TC 237 SIB, TG1 the main purpose was to evaluate common test methods, used to assess the affinity of bitumen to aggregate surfaces, to determine, if possible, the repeatability and reproducibility and to give recommendations for improvement. In this study both binders and aggregates have been considered. Three bituminous binders, two unmodified from different sources, one polymer modified binder, and four aggregate types, with different mineralogy, have been selected. The test methods considered in the study include the rolling bottle test, the boiling water stripping test and the bitumen bond strength test; also surface energy was investigated. This chapter presents the results of these tests and their accuracy.

This chapter focuses on the three-dimensional linear viscoelastic (3D-LVE) behaviour of bituminous mixtures and in particular on the measurement and modelling of the complex Young’s modulus and Poisson’s ratio (PR). In the first part of the chapter, the LVE definition of PR is reviewed and experimental measurements of the LVE PR carried out over the last 40 years are summarised. The second part of the chapter is devoted to the description of a RILEM round robin test (RRT) organized by Task Group 3 (TG3) “Mechanical testing of bituminous mixtures” of RILEM TC 237-SIB. Within the RRT uniaxial cyclic (sinusoidal tension/compression or haversine compression) tests at different temperatures and frequencies were carried out on cylindrical specimens cored from laboratory compacted slabs. Two types of bituminous mixtures, GB3 (continuously graded) and GB5^® (gap-graded), were analysed. Five laboratories participated in the RRT, each laboratory measured axial and transverse (or diametral) strains using different sensors and configurations. In particular, transverse strain was measured along two orthogonal directions in order to evaluate the effect of compaction-induced anisotropy on PR. Results confirmed that PR of bituminous mixtures is a complex function of temperature and frequency and that the time-temperature superposition principle can be applied (for absolute value and phase angle). For the studied mixtures the norm of the complex PR ranged between 0.22 (low temperatures/high frequencies) and 0.60 (high temperatures/low frequencies) whereas the phase angle was less than 6°. A small difference (less than 0.05) was found between measurements carried out in two orthogonal directions. This small difference is probably related to measurement accuracy and not to the anisotropic behaviour of the material. Comparison of data between the different laboratories, which could not be performed at exactly the same temperatures and frequencies, was performed using a common reference given by the 3-dimensional formulation of the 2S2P1D linear viscoelastic model. This model provides a good simulation of experimental data. Based on close results from all participating laboratories, it is possible to conclude that cyclic uniaxial test could be a good candidate to become a standard test for evaluating the 3D-LVE behaviour of bituminous mixtures.

Nowadays, road pavements are subjected to steadily increasing traffic volumes generating accelerated functional and structural distresses that require frequent and expensive maintenance. On the basis of such needs, in recent years, practical applications and theoretical studies have proved that the service life of flexible pavements can be extended by installing geosynthetic reinforcements. In particular, grids can be placed at the interface of bituminous layers for both new constructions and rehabilitation of existing pavements, in order to improve repeated loading and rutting resistance and to prevent or delay reflective cracking. However, the presence of an interlayer reinforcement may also hinder the full transmission of horizontal shear stress between asphalt layers (debonding effect), penalizing the overall efficiency of the pavement system. For the above-mentioned reasons, both laboratory and in situ investigation are needed in order to better understand the real role played by the grid reinforcement. The achievement of such objective is the main goal of the RILEM TC 237-SIB/TG4 that carried out an interlaboratory experiment focused on the “Advanced Interface Testing of Geogrids in Asphalt Pavements”. In this context, the participating laboratories were involved with a twofold objective: to compare the predictive effectiveness of different experimental approaches and to analyze the behavior of different grid types. For this purpose, two experimental reinforced pavement sections were realized with the same materials and construction techniques. The first pavement section was used to prepare samples for the interlaboratory experiment, the second one was specifically designed and instrumented to analyze the field performance of the grids under heavy traffic conditions. The objective is the characterization of the mechanical behavior of grid reinforced interfaces in asphalt concrete pavements using different test methodologies and the analysis of the relationship between laboratory test results and actual field performance. To this purpose, the laboratory research activities were based on the analysis and comparison of the results obtained following specific testing protocols proposed by the participating laboratories that combine performance-based tests (e.g. interlayer shear tests, static and dynamic bending tests, tensile-bending tests), in order to investigate the overall behavior of double-layered asphalt systems. The role of the instrumented pavement section was complementary and oriented towards an improvement in the existing design and testing approaches. Such goal was attained by analyzing the actual stress-strain response of grid-reinforced systems under vehicular loads, also monitoring the natural and induced field cracking evolution. Despite the variety of the testing equipment and protocols adopted by the participating laboratories, all test results were consistent. Moreover, such experimental results contributed, together with the data analysis collected on the instrumented pavement section, to the correct understanding of the grids performance that were characterized by specific peculiarities making them appropriate for different applications.

Recycling of old pavements is becoming a widespread technique; in order to preserve the natural resources of aggregates and binder, the materials reclaimed from the damaged layers of the pavements maintained are generally reincorporated in the material for new maintenance layers. However, recycling induces some technical challenges. In the case of the Hot mix asphalt (HMA), the most commonly used road construction material, one of the main concerns is ageing. Along with the heterogeneity of the RA properties in stockpiles, it is one of the main factors limiting the recycling rates in HMA. Therefore, the characterisation of ageing through reliable ageing indexes and of its impact on the bitumen physical properties is of paramount importance to improve the recycling techniques. The RILEM TC 237 SIB TG5 inherited a database from the previous work of the RILEM TC 206 ATB TG5 on a protocol for bituminous mixture ageing in laboratory. Taking advantage of these data, the work of the Task Group was first dedicated to a state of the art and an evaluation of the chemical ageing characterization through indexes based on Fourier Transform Infrared Spectroscopy (FTIR). In a second step, an attempt was made, in order to bring out the essential trends of the rheological data with ageing, by using a Huet modified rheological model. Then, exploring the potential of a recently developed method, an interpretation of these trends in terms of asphalt structure is suggested. Finally, possible empirical relationships between the chemical ageing indexes and the rheological parameters are investigated.

Pavement engineers have in front of them multiple challenges linked to addressing issues related to social development and society’s expanding needs. One of the most substantial of these issues is perhaps how to effectively rehabilitate and/or maintain the existing road network while preserving and sustaining limited natural resources. The re-usage of existing pavement materials to reconstruct/rehabilitate our future pavements is the solution that is now more and more selected by the different road administrations around the world. However, upon closer inspection, one can find many areas and details, not negligible issues, that are simply extensions of HMA technology (i.e. mix design process in cold recycling) or empirical arrangements; in particular RAP still does not have an internationally recognized classification. So SIB – TG6 decided to develop a classification protocol of RAP, depending on its intended application. The objective has been followed by considering the procedures generally utilized to classify the natural aggregates: tests able to identify the main components (i.e. the geometrical and mechanical properties of aggregates and the characterization of recovered bitumen for RAP) and provide information on their behaviour under specific conditions, near to real life usage (e.g. the Los Angeles test for aggregates gives an idea of the potential behaviour of aggregates under the action of a roller compactor). The following sections illustrate and explain the actions of the TG in order to achieve the goals outlined above: the review of current standards, the protocol designed to classify RAP and the round robin tests carried out to validate the protocol.