M.F.C. van de Ven

First Observation of Blending-Zone Morphology at Interface of Reclaimed Asphalt Binder and Virgin Bitumen

One of the challenges in designing recycled asphalt mixtures with a high amount of reclaimed asphalt pavement (RAP) is estimating the blending degree between RAP binder and added virgin bitumen. The extent of blending is crucial because asphalt concrete response is influenced by the final binder properties. This paper focuses on the evaluation of interaction and extent of blending between RAP binder and virgin bitumen by studying the microstructures of the blending zone with atomic force microscopy (AFM). AFM is used to probe the change of microstructural properties from a RAP binder and virgin bitumen to the blending zone of these two. Averaged microstructural properties have been observed in thin-film blends of RAP binder and pure bitumen. The morphology of the blending zone (spatial extent of about 50 μm) exhibits domains of a wide range of microstructure sizes (160 nm to 2.07 μm) and can be considered to be a completely blended new material that has been observed directly for the first time. The fully blended binder properties are found to be between those of the two individual binders, as could be inferred from the averaged microstructural properties derived from AFM images of the blending zone. This finding is also consistent with the results of mechanical tests by dynamic shear rheometer on the same materials. Finally, a design formula is proposed that relates the spatial dimensions of the blending zone to temperature and mixing time, which will eventually allow the results of this study to be extended from small-length scales up to the engineering level.

Development of (half-) warm foamed bitumen mixes: state of the art

Foamed bitumen mixes are produced by mixing foamed bitumen with aggregate at ambient conditions. However, the variations in the temperature of mineral aggregate at the time of mixing have a profound influence on the properties of the foamed bitumen mixes produced. A feasibility study previously undertaken to investigate the possible benefits of heating the aggregate moderately (above ambient temperature but below 100°C) before foam treatment, highlighted the improvement of mix properties that is achievable. The properties that can be enhanced include particle coating; mix cohesion and tensile strength, as well as a degree of compaction when compared with the equivalent properties of conventional cold foam mixes (CMA). This has been found to be applicable to, in particular, reclaimed asphalt pavement (RAP) and densely graded crushed aggregates. The feasibility study was undertaken exclusively at laboratory scale. A second phase, more focussed research project was subsequently launched in the Netherlands using the mixes that show the most potential to benefit most from the “Half-warm Foamed Bitumen Treatment” process viz, STAB (crushed stone ashalt concrete used in the Netherlands as a base course layer) and RAP combinations. This investigation was undertaken at laboratory level as well as full scale production trials. This paper discusses the findings of the entire investigation into half-warm foamed (HWF) bitumen treatment, carried out in South Africa and the Netherlands. As special development, the reuse of porous asphalt (PA) RAP in HWF PA, is also discussed. With up to 40% less energy consumption than that required for manufacture than hot mix asphalt (HMA), HWF asphalt mixes can provide comparative monotonic properties at higher test temperatures, similar fatigue properties and a lower phase angle at low loading frequencies. The improvement of mix properties relative to CMA is significant. This indicates that the HWF process holds the potential for successful implementation in pavement layers.

Turning Back Time: Rheological and Microstructural Assessment of Rejuvenated Bitumen

Countermeasures to the aging of bituminous asphalt binders is a highly important topic for service-life extension of asphalt in the field and for recycling old pavements into new structures with similar functional requirements as the original structure. Countermeasures are usually achieved by applying additives that restore the adhesive and mechanical properties of the original bituminous binder. The additives are commonly termed (asphalt) rejuvenators. This study examined the performance of two very distinct rejuvenating agents. The effectiveness of rejuvenators is usually measured by comparing the penetration and softening point of the rejuvenator-aged bitumen blend with reference values of the virgin binder. The study used a dynamic shear rheometer to evaluate the rejuvenating capabilities of the two additives. The microstructures of the virgin binder and the rejuvenated blends were obtained by atomic force microscopy. Subsequently, the rheological results were related to the microstructure morphologies. From the rheological experiments, both rejuvenators exhibited the desired softening and property-restoring performance. However, there was a strong difference in the amount of rejuvenator needed to achieve complete rejuvenation. By correlating rheology to the microstructural observations, the effects of the rejuvenators were found to be distinct at microscopic length scales: rejuvenation was achieved by distinct chemophysical mechanisms. One of the rejuvenators restored the virgin microstructure, whereas the other rejuvenator generated a new morphology. Thus, the study demonstrated that by combining rheological and microstructural techniques, the mechanism and performance of rejuvenation can be understood. This finding may help guide future designs and optimization of asphalt-rejuvenating agents.

Investigating the Self Healing Capability of Bituminous Binders

ABSTRACT In the Netherlands, the loss of stone at the surface of porous asphalt wearing courses (ravelling) results in a limited service life. Ravelling is caused by repeated traffic fatigue loadings and climatic influences. Bituminous materials exhibit a healing capability which is helpful to extend the service life. However, aging is expected to limit this positive effect. Therefore, upgrading the self-healing capability of the bitumen in time is of major importance. Inspired from mimicking nature, several chemical treatments can be used to improve the self healing capability of asphalt mixtures when the binder starts to harden due to aging, or when micro cracks have initiated. Possibilities of self-healing improvement are discussed in this paper. Preliminary results of measurements of the self healing capability of bituminous binders are also presented.

Experimental study into the fundamental understanding of blending between reclaimed asphalt binder and virgin bitumen using nanoindentation and nano-computed tomography

The properties of asphalt mixtures containing a high amount of reclaimed asphalt are presumed to be strongly influenced by the degree of blending between old and virgin bitumen. As part of a PhD project, fundamental testing at micro- and nano-levels was undertaken to achieve better understanding of the blending and diffusion phenomenon between rheologically and chemically different bitumens. Three different techniques were used to observe the interface zones of bitumen–bitumen and aggregate–bitumen zones. Nanoindentation, nano-computed tomography (nano-CT) scanning and optical microscopy were done on three different groups of prepared samples. Nanoindentation was conducted to evaluate the difference of surface stiffness of aggregate binder interfaces. Nano-CT scanning techniques were used to observe slight differences in density of the interface in binder zones. Optical microscopy was conducted on thin sections of an asphalt mixture to detect the interface zone between two binders. With nano-CT scanning, it is possible to see the separation zone between the hard- and soft-grade bitumen. Nano-CT scanning images on two different bitumens clearly showed the two bitumen layers due to their varying densities but it was not possible to detect two different binders inside the mixture. Initial results from the nanoindentation study illustrated the benefit of using nanoindentation in detecting two different binders. It was not possible to detect hard and soft bitumens from optical microscopy images on thin sections. Differences in enhanced colours in some points were caused by their difference in thickness. The three-dimensional analysis of bitumen nano-CT images seems to be in reasonable agreement with the assumed bitumen microstructure but further research is needed to confirm this.

New developments with half-warm foamed bitumen asphalt mixtures for sustainable and durable pavement solutions

Internationally, a growing health, safety and environmental awareness of the public can be observed. In this context, significant efforts are underway to develop advanced technologies to reduce the use of non-renewable fossil fuels, in order to reduce emissions and human exposure. Also, in the road industry, the search for new sustainable solutions is eminent. The introduction of asphalt mixtures that can be produced at lower temperatures is an important development in this direction. Lower temperature means in general that lower than 140°C. In this article, developments with the so-called half-warm foamed bitumen mixtures produced at temperatures below 100°C and even as low as 90°C are discussed. Mixtures used in binder and base layers are considered. The role of life-cycle assessment as used in the Netherlands is described for half-warm foamed bitumen mixtures. The performance properties according to the CE marking are also discussed at the same time. Finally, the workability of these mixtures is studied, showing how compaction of this type of mixtures can be managed. In all cases, a reference hot base course mixture is used for comparison.

Evaluating Laboratory Compaction of Asphalt Mixtures Using the Shear Box Compactor

Laboratory produced test specimens are usually obtained with devices like Marshall compaction, gyratory compaction, or roller compaction. However, with these methods it is difficult to control the sample-to-sample variation of the final density of the test specimens, which can strongly influence the results of performance testing. It is very important to have a repeatable and efficient production method of test specimens available in the laboratory. The shear box compactor was recently developed to simulate field compaction with a constant compressive force and a cyclic shear force with constant maximum shear angle applied to the asphalt mixture. The shear box compactor produces asphalt blocks with a size of 450 mm in length, 150 mm in width, and 145–185 mm in height. Test specimens like beams or cylinders can be obtained from the block for laboratory performance testing. In this paper, the compaction results with the shear box compactor are reported for asphalt mixtures with different gradations and binder types. Asphalt specimens with different mixture compositions, shapes, sizes, and sampling positions were investigated by volumetric properties. Finite element modeling was introduced to obtain more understanding of the compacting process of the shear box compactor. The results indicate that the decreasing of voids content of asphalt mixtures during compaction process is dependent on the gradation than the binder type. The asphalt mixture specimens obtained from the same asphalt mixture block has a variation in voids content of less than 1 %. Test specimens obtained from the upper part of the asphalt block are more compacted than specimens from the lower part. And the specimens obtained close to the side of the block are less compacted due to lack of shear stress. As a result, the shear box compactor provides a reliable means of sample preparation, making it very suitable for producing specimens with constant volumetric properties.

Simulation of HMA Compaction by using FEM

This paper introduces a simulation tool for the compaction process of hot mix asphalt (HMA) using a roller under varying external conditions. The focus is on the use of the finite element model (FEM) with code DiekA, on its necessary requirements and on the presentation of simulation results. The approach requires the availability of a suitable material model, equipment to measure material parameters and a laboratory-testing program for fitting the correct material parameters. Subsequently, in discussing the simulation results attention is paid to; the principal stresses, strains and shear stresses inside the material during rolling, and the incremental displacements of the material. Furthermore, the simulated density path as a function of depth inside the layer is presented for a series of applied roller passes. In conclusion it can be stated that FEM approaches in general are well suited to simulate compaction processes of HMA on condition that the selected FEM comprises the right features.

Comparison of Uniaxial and Four-Point Bending Fatigue Tests for Asphalt Mixtures

The fatigue properties of asphalt mixtures estimated with fatigue tests in the laboratory are difficult to compare with each other. One reason is that for so-called inhomogeneous tests that do not have a uniform stress–strain distribution, the measured stiffness is a weighted overall stiffness that does not really represent the stiffness of the damaged zone. For homogenous fatigue tests, the stress–strain field within the specimen is uniform in theory. In this case, the measured stiffness corresponds to a material property. It is supposed that the stiffness evolutions in the inhomogeneous tests might be described by the results from the homogenous test. In this paper, the results of the uniaxial tension and compression (UT–C) fatigue test and the four-point bending (FPB) fatigue test are described and compared. The evolutions of the stiffness and phase angle are simulated by means of the partial healing model. With the model parameters, the local stiffness for each volume unit of the beam is calculated. It is found that, on the basis of the measured overall stiffness, the classical fatigue life Nf,50 of the beam in the FPB fatigue test is larger than that of the cylinder in the UT–C fatigue test. However, in the midsection of the beam, the local stiffness evolution is similar to that observed in the UT–C fatigue test. On the basis of the local stiffness concept, comparable fatigue lines can be obtained from the UT–C and FPB fatigue tests.

Mechanical Impedance Measurement on Thin Layer Surface With Impedance Hammer Device

The mechanical impedance describes the ability of a structure to resist motion when subjected to a given force. It has received increasing attention as a parameter that could influence the tire/road noise in the medium frequency range (630–1600 Hz). In this study, the impedance hammer device is used for measuring the mechanical impedance on thin layer surface mixture samples in the lab. In the test, the excitation is induced by hitting the sample surface with the hammer. Values of the driving force and acceleration of the vibrating slab are recorded. The mechanical impedance can then be calculated from the ratio of the force to the velocity that is obtained by integration of the acceleration. The influence of the material properties were investigated by comparing the results on different samples. A further measurement of dynamic stiffness on the same material is also discussed and related to the hammer measurement.