Ann Vanelstraete

The influence of thermal history on binder rutting indicators

ABSTRACT Binder properties, believed to relate to asphalt rutting, were investigated and compared with each other. Included are: parameters derived from oscillatory tests (e.g. the “zero” shear viscosity and the SHRP rutting parameter), parameters derived from static and repeated creep tests and the traditional R&B softening point. Various binders were tested, including low and high polymer modified bitumen, and bitumen modified with commercial waxes. This study shows that for unmodified bitumen all the investigated binder parameters correlate well with each other and would give similar classifications regarding the rutting susceptibility. For modified binders, the parameters related to low frequency or long loading times, give a different ranking than those related to higher frequencies, as for example the SHRP parameter. The storing and preparation conditions prior to the rheological measurement can have a large influence, especially in the range of long loading times or low frequencies. For elastomer modified binders, the homogenization temperature and the corresponding change in micro-structure can have a large impact. For binders with semi-crystalline modifiers, the storage time and storage temperature before testing can introduce large changes in the polymer network building up. The thermal history effects in rheological tests can be related to variations in crystallinity, as shown by calorimetry, or to variations in morphology, as revealed by fluorescence microscopy. Some of the binders were also investigated in the presence of filler, to see if the thermal effects play a similar role in mastic (binder+filler) as in the neat binder.

Asphalt Production at Reduced Temperatures using Zeolites and the Impact on Asphalt Performance

ABSTRACT The potential benefits of producing and paving asphalt at lower temperatures are beyond discussion. There is already a lot of know-how on various techniques for producing warm mix asphalt, but there is insufficient data available about the impact of these techniques on the final performance characteristics of the asphalt. Nynas and BRRC are working together on a research project, to study the most important and promising warm mix techniques on laboratory and field scale. These techniques are: the use of foamed bitumen, the addition of waxes to the bitumen and the addition of zeolites to the mixture. The present paper focuses on the last technique. The performance of warm asphalt mixtures produced with different types and quantities of zeolites is compared to the performance of a reference hot mix asphalt. Several aspects have been studied: compactability, permanent deformation, water sensitivity and low temperature cracking. Small scale field trials were made to validate the findings of the laboratory research.

Field Study to Investigate the Impact of Conditions of Application of Tack Coats on the Interlayer Bond Strength

The intrinsic characteristics of tack coats play an important role in the adhesion between layers, but the conditions of application of these coats are equally crucial. In this context, the Belgian Road Research Centre actively participates in a working group on tack coats with the objective to carry out a field study about interlayer bonding while evaluating the influence of different parameters– such as type and rate of spread of emulsion, nature and preparation of the underlayer, etc. With a view to this objective, a test site was constructed and the bond strengths were investigated by direct shear and direct tensile tests according to pre-standard prEN 12,697-48. This article describes the conditions of application, the measurements made on site and the results of the interlayer adhesion tests.

Impact of hydrated lime on the durability of SMA mixtures: laboratory and field evaluation

The work presented in this paper describes the research activities of the task group ‘Properties of filler aggregates’ with respect to the durability of Stone Mastic Asphalt (SMA) mixtures, which was conducted recently at the Belgian Road Research Centre. The main objective of this study comprises the effect of hydrated lime on the durability of SMA mixtures. The methodology developed in this study includes in an initial phase the design in the laboratory of a ‘critical’ SMA10 mixture. Such a ‘critical’ SMA10 mixture was considered to be characterised by a high water sensitivity and, therefore, a low durability. During this process, attention was paid to both the SMA composition as well as to the compaction energy applied in order to provide suitable test samples. Subsequently, by making use of such a ‘critical’ SMA10 mixture, the effect of the filler nature on the durability of the SMA mixtures was established; in particular both the modification by as well as the optimal content of hydrated lime was studied. Additionally, the possible effect of hydrated lime on the workability of latter SMA mixtures was investigated. Finally, the results obtained in the laboratory were complemented by a field study and a monitoring campaign. This survey comprises the evaluation of the impact of hydrated lime on the compactibility and the water sensitivity on test sections as well as a yearly follow up of the performance of a series of SMA10 mixtures varying in the type of filler material used.

Resistance to Low Temperature Cracking of High Modulus Bituminous Binders and Mixtures

ABSTRACT The increasing growth of traffic volume and loading makes it necessary to introduce new types of asphalt with improved performances. High modulus asphalt mixtures have been developed for this purpose. These are mixtures with a high stiffness bitumen and a high binder content. A possible drawback of these types of mixtures is the low temperature behaviour which might be less good as for traditional asphalt mixtures. In this paper we will focus on the low temperature behaviour of high modulus bituminous binders and asphalt mixtures. The low temperature performance of the bitumen is investigated with the bending beam rheometer (BBR). The low temperature behaviour of the high modulus asphalt mixture is investigated with the restrained cooling test (RCT). Both results are compared in order to investigate the correlation between binder and asphalt properties.

A New Performance Test for Resistance to Ravelling by Traffic: Laboratory and Field Experience in Belgium

The use of specially designed asphalt mixtures for thin to very thin surface courses is seen as a cost-effective solution for reducing traffic noise in Belgium. However, ravelling is a major concern for this type of mixtures and there is an urgent need for a standardized laboratory test to predict resistance to ravelling caused by shear forces (also known as “scuffing”). The Belgian Road Research Centre (BRRC) uses the Darmstadt Scuffing Device, an apparatus developed in Germany for testing porous asphalt. This paper describes the experience of BRRC with this new test device. As a first step, it was verified whether the test is capable of discriminating between mixtures other than porous asphalt, the mixture type for which the apparatus was originally designed. The second step was to verify if the ranking of mixtures is in agreement with the observations on the road. Therefore, the test was applied to mixtures from different test sections that are being monitored at regular times, a work that will be continued in the coming years. While this work is going on, CEN TC 227 WG1/TG2 is in the process of drafting a European standard for the scuffing test (prCEN/TS 12697-50). The results of this research will be used to contribute to the development of a standard European test method.

Field study to evaluate different pre-normative interlayer adhesion tests

An effective and durable bond between the various constituent layers is an absolute prerequisite for the durability of a road pavement. To ensure the adhesion between successive road construction layers, cationic bitumen emulsions are the most frequently used type of tack coats. Several tests are currently available to evaluate the bonding performance between layers based on different loading modes (shear, torque and tensile) They vary in regard of test devices, test temperature, loading mode as well as sample geometry and preparation and hence they lead to different results. In this context, the European standardisation committee CEN TC227/WG1/TG2 “Test methods for bituminous mixtures” has developed a prestandard prEN12697-48 “Bituminous mixtures – Test methods for hot mix asphalt – Part 48: Interlayer Bonding” for the determination of interlayer bond strength where three main normative test methods are considered: tensile adhesion test (TAT), shear bond test (SBT), torque bond test (TBT). A site test method to evaluate the tensile bond strength making use of a practical clamping device and used for many years in Quebec, called layer adhesion measuring instrument (LAMI), was recently included in the prestandard. This article gives an overview of the different methods and discusses the results of a study in the laboratory (SBT, TAT) and on site (LAMI) in which these pre-normative interlayer adhesion tests were used. The study leads to recommendations for the methods described in prEN 12697-48 and shows the differences between the methods. The ultimate aim of this study is to come to recommendations for improving the adhesion performance of multilayer pavements and to propose values for minimum interlayer bond strength to be achieved on site.

Performance of anti-cracking interface systems on overlaid cement concrete slabs: development of laboratory test to simulate slab rocking

In the past extensive testing was performed at BRRC to determine the laboratory performance of anti-cracking interfaces in case of stresses induced by thermal variations. Several experimental sites were followed during that period to set up correct laying procedures for the different types of products and to give recommendations when applying these products. Recently this study was completed by a four year research project. Important objectives of this project were: the setup of a new test to simulate the vertical movements induced by traffic on overlaid cement concrete slabs, the evaluation of the performance of different anti-cracking interface systems with this new test and the validation of these results by experimental field trials. In this article this new laboratory test to simulate the vertical movement is described in detail and the performances of a reference and four types of anti-cracking interface systems, being a stress absorbing membrane interlayer (SAMI), a geogrid, a geocomposite and a steel reinforcing netting, are compared.

Effect of Extraction and Recovery Procedure on the Determination of PmB Content and on the Properties of the Recovered Binder

ABSTRACT In Belgium, the commonly used test method for the determination of the binder content of a bituminous mixture is based on the extraction of the soluble bitumen with a centrifuge. This step is followed by a recovery of the solvent with a rotary evaporator when the binder is yet to be characterized. The procedures for pure binders are clearly described in the European standards, but this is not the case for polymer modified bitumens (PmBs). Consequently, a research project has been launched to evaluate, in the case of bituminous mixtures with PmB both the effect of extraction on the binder content as well as the effect of extraction and recovery on the properties of the recovered PmB. In this study, the use of several solvents and dissolution methods has been compared.

Development of a Pull-off Test to Measure the Bond Strength of Bituminous Emulsions

It is now generally recognized that good adhesion between different layers of an asphalt pavement is of great importance for its lifetime. Poor interfacial bond strength leads to a higher probability of cracking, early fatigue failure, delamination, etc. Many factors have an impact on the bond strength between layers. Despite all these factors, it is important that the performance of tack coats can be evaluated independently of the asphalt complex for the purpose of comparison, optimization and selection of these products. Consequently, the development of a reproducible quantitative test method to characterize the bond strength of a bituminous tack coat is crucial. This paper describes the development and validation of a pull-off test on a bituminous tack coat film. In a first step, the test device, the breaking and curing conditions and the pull-off test conditions were optimized to ensure that failure occurs within the film and not at the interface between the film and the contacting plates of the test device. In a second step, a validation of the pull-off device and procedure was conducted by comparing the results of the pull-off tests for different emulsions with direct tensile tests carried out on multi-layer asphalt samples in which the same tack coats were applied. This paper is an overview of BRRC’s work in this field.