Michael P. Wistuba

The German segmented steel roller compaction method – state-of-the-art report

Out of the European methods, the steel roller method (EN 12697-33) is preferred in Germany. A segmented roller is used, as specified in the German Standards in detail (acc. to TP Asphalt-StB, part 33; in German only). It is appropriate for laboratory compaction of any hot rolled asphalt mix types (such as asphalt concrete AC, or stone mastic asphalt SMA) either manufactured in the laboratory or in a mixing plant. And it realizes properties that are similar to those of specimen taken from the road. The segmented roller compaction method as described in the German Standards has been developed in the frame of several studies conducted at the Braunschweig Pavement Engineering Centre (see, e.g. Arand & Renken, 1990, 1999; Renken, 2001, 2002).

The European regulations for specimen fabrication prior to performance testing of asphalt mixtures

For fundamental performance testing of asphalt mixtures, specimens have to be produced from an asphalt mixture that is compacted in laboratory. For this purpose, different laboratory compaction methods are available today, which are based on the effects of impact, kneading or rolling action and/or vibration. It is well known that the method used for compaction considerably influences the properties of the specimen, and in turn, the specimen properties significantly influence the outcome of fundamental asphalt mix testing. The choice of compaction method is therefore of utmost importance. Hence, there is general consensus among experts that high priority must be given to the harmonisation of laboratory methods for the production of specimens prior to mix testing. Asphalt mix requirements must not be specified independently from the detailed compaction requirements. In Europe, a harmonised quality assurance system for asphalt mix properties was established recently. In order to guarantee its practicability, also a harmonised approach for the fabrication of specimens for fundamental testing becomes indispensable. In this paper, an overview is given on how standardisation of specimen production has progressed in Europe so far, and the need for further harmonisation is pointed out from a European point of view.

Comparison of rheological parameters of asphalt binders obtained from bending beam rheometer and dynamic shear rheometer at low temperatures

In this paper, an investigation was performed to determine if the complex modulus obtained from frequency sweeps performed with the dynamic shear rheometer (DSR) can be used to accurately predict the creep compliance obtained experimentally using the bending beam rheometer (BBR). Two sets of asphalt binders were tested at low, intermediate, and high temperatures and the results were analysed using 2S2P1D and DBN rheological models. DSR and BBR testing was performed in two different laboratories using different equipment manufacturers. It was found that significant differences are observed between the creep stiffness obtained with DSR and BBR devices, most likely due to the different preparation and conditioning of the test specimens in different cooling media.

Consideration of climate change in the mechanistic pavement design

This paper is about a parameter study on the influence of climate change on pavement design using the linear-elastic multi-layer theory. An approach is presented considering every single hour of the design period for the calculation of traffic-induced and temperature-induced stresses and strains in the pavement structure. This is realised by bringing in line the time-variation curves of pavement temperatures and the corresponding time-variation curves of traffic load repetitions. This approach is especially suitable for studying the effect of temperature-related influence factors on the pavement design. As an example for application, a parameter study on the influence of climate change on pavement design is presented in this paper. For the example of different climates, it is investigated if the pavement design approach, which is applied for layer thickness design to resist bottom-up fatigue cracking, needs to be adopted when climate change is considered.

Evaluation of hot-mix asphalt susceptibility to temperature-induced top-down fatigue cracking by means of Uniaxial Cyclic Tensile Stress Test

A dominant failure mode in hot-mix asphalt (HMA) layers is material fatigue, occurring when the asphalt layer is repeatedly loaded by tensile stresses. The maximum stress arises at the bottom of the asphalt layer and, in consequence, crack development is initiated at the bottom. Various techniques are known that simulate bottom-up fatigue cracking in the laboratory. Fatigue failure is also observed on top of the pavement. Especially in cold climates, temperature-induced top-down cracking initiated at the pavement surface is a well-known failure mode. Usually, horizontal tensile stresses in the surface layer are smaller than stresses in the bottom layer. At cold temperatures and when temperature falls within a short period of time, the traffic-induced stress is superposed by temperature-induced stress, and the total stress may come up to the tensile strength of the material. In order to test the materials susceptibility to temperature-induced top-down fatigue cracking in the laboratory, the Uniaxial Cyclic Tensile Stress Test (UCTST) was established and recently introduced in European Standards (prEN 12697-46). In this test, a prismatic shaped HMA beam is subjected to a constant tensile load (representing the temperature-induced stress) which is superposed by a sinusoidal tensile load (representing the traffic load). This loading situation results in a visco-elastic strain response on the one hand, and in an accumulation of visco-plastic strain on the other. Different failure modes are observed in UCTST. In this paper, the failure modes occurring in UCTST, i.e. cracking of the specimen, stiffness decrease, and creep, are discussed based on the results from laboratory testing of nine different HMA and on variation of loading conditions.

Development of simple relationship between asphalt binder and mastic based on rheological tests

The relationship between the different phases of asphalt materials, from asphalt binder to mastic and mixture, has received significant interest over the years. The Shift-Homothety-Shift in time-Shift (SHStS) transformation developed by the research team of the École Nationale des Travaux Publics de l’État has been found to provide a very effective tool to relate the rheological properties of binders to those of the corresponding mixtures. In this paper, the possibility of using the SHStS transformation to link the behaviour of asphalt binder to that of mastic is investigated with the objective of proposing a simple expression of the SHStS transformation parameter, α, as function of the filler content in the mastic. First, an experimental campaign consisting of dynamic shear rheometer tests is conducted on a set of binders and mastics to obtain the material response at different temperatures and frequencies. Then, the rheological 2S2P1D (2 springs, 2 parabolic elements, 1 dashpot) model is used to evaluate the experimental measurements. Finally, a simple relationship associating the characteristic times of asphalt binder and mastic as function of the filler content is developed. The proposed expression allows to easily and efficiently predict the complex modulus of mastics from the binder complex modulus and vice versa.

Adhesion in Bitumen-Aggregate-Systems. New Technique for Automated Interpretation of Rolling Bottle Tests

ABSTRACT The loss of adhesion between the binder and the aggregate in the presence of water is a major mechanism of deterioration in bituminous pavement materials. For consideration and evaluation of factors influencing the adhesion properties, a number of testing techniques have been developed. Among these the Rolling Bottle Test according to the European Standard EN 12697-11 is most common to sort out good from bad bitumen-aggregate systems. Even though the boundary conditions of the test are specified in detail, the residual degree of bitumen coverage is estimated from visual inspection. This is usually complained as the main disadvantage of the Rolling Bottle Test. Thus, for the elimination of any subjective manipulation in interpreting Rolling Bottle Test results, and for a reduction of manpower needed for test analysis, a computer aided analysis technique is suggested in this paper.

Microstructural Analysis and Rheological Modeling of Asphalt Mixtures Containing Recycled Asphalt Materials

The use of recycled materials in pavement construction has seen, over the years, a significant increase closely associated with substantial economic and environmental benefits. During the past decades, many transportation agencies have evaluated the effect of adding Reclaimed Asphalt Pavement (RAP), and, more recently, Recycled Asphalt Shingles (RAS) on the performance of asphalt pavement, while limits were proposed on the amount of recycled materials which can be used. In this paper, the effect of adding RAP and RAS on the microstructural and low temperature properties of asphalt mixtures is investigated using digital image processing (DIP) and modeling of rheological data obtained with the Bending Beam Rheometer (BBR). Detailed information on the internal microstructure of asphalt mixtures is acquired based on digital images of small beam specimens and numerical estimations of spatial correlation functions. It is found that RAP increases the autocorrelation length (ACL) of the spatial distribution of aggregates, asphalt mastic and air voids phases, while an opposite trend is observed when RAS is included. Analogical and semi empirical models are used to back-calculate binder creep stiffness from mixture experimental data. Differences between back-calculated results and experimental data suggest limited or partial blending between new and aged binder.

GPR as a crack detection tool for asphalt pavements: Possibilities and limitations

Cracking in asphalt pavement is a distresses often requiring urgent measures. GPR has been proved by numerous scientists as a fast and reliable asphalt pavement characterization tool. Whether GPR can be applied as a crack detection tool is a question to be answered. To find out the influencing parameters, e.g., crack fill, atmospheric condition, etc., GPR surveys were carried out on asphalt test pavements. Multiple techniques were used for data processing. Possibilities and limitations are documented.

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.