| 1. |
- Ghafoori Roozbahany, Ehsan, 1982-, et al.
(författare)
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Modelling the flow of asphalt under simulated compaction using discrete element
- 2019
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Ingår i: Construction and Building Materials. - : Elsevier Ltd. - 0950-0618 .- 1879-0526. ; 227
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Tidskriftsartikel (refereegranskat)abstract
- The flow differences between the particles of asphalt mixtures compacted in the laboratory and in the field have been identified as one of the reasons for the discrepancies between laboratory and field results. In previous studies, the authors developed a simplified test method, the so-called compaction flow test (CFT), for roughly simulating the flow of particles in asphalt mixtures under compacting loads in laboratory. The CFT was used in different studies to examine its capability of revealing the differences between the flow behavior of different asphalt mixtures under various loading modes. The promising results encouraged further development of the CFT by investigating the possible impacts of simplifications and boundary conditions on the results of this test. For this reason, discrete element method (DEM) was utilized to investigate possible impacts of the mold size, geometry of the loading strip as well as the loading rate on the results of the CFT. The results of the simulation indicate that in case of wearing course layers with nominal maximum aggregate size of 11 mm, the length of the CFT mold can be increased from 150 mm to 200–250 mm for reducing flow disturbances from the mold walls. However, since the majority of the flow of asphalt mixture particles is expected to take place within the first 100–150 mm length of the mold, reasonable results can still be obtained even without changing the size of the CFT mold. Moreover, comparing results with different loading strip geometries and loading rates indicates that the current CFT setup still appears to provide consistent results.
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| 2. |
- Jelagin, Denis, et al.
(författare)
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Experimental and numerical modelling of shear bonding between asphalt layers
- 2023
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Ingår i: International Journal on Road Materials and Pavement Design. - : Taylor & Francis. - 1468-0629 .- 2164-7402. ; 24:S1, s. 176-191
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Tidskriftsartikel (refereegranskat)abstract
- Interlayers in asphalt pavements are potential structural damage initiators. In order to better understand the quantitative role of interlayer parameters, such as surface roughness, binder type, binder content and loading type on interlayer shear strength, this paper focuses on the effects of particle interlock and contact conditions on interlayer strength through experimental and numerical modelling. Experimentally, interlayer shear box strength tests on a model material consisting of stiff binder blended with steel balls are performed with and without normal force confinement. A Discrete Element method model of the test is developed using measurements of the model material for calibrating the contact law and for validating the model. It is shown that this model captures adequately the measured force-displacement response of the specimens. It is thus a feasible starting point for numerically and experimentally studying the role of binder and tack coat regarding interlayer shear strength of real asphalt layers.
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| 3. |
- Olsson, Erik, 1986-, et al.
(författare)
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A numerical framework for modelling settlements of railway ballast layers
- 2024
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Ingår i: Transportation Geotechnics. - : Elsevier. - 2214-3912. ; 44
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Tidskriftsartikel (refereegranskat)abstract
- Permanent deformation in ballast layers is a major contributing factor to the railway track geometry deterioration. In spite of a considerable amount of research on understanding and predicting performance of ballast layers, accurately capturing their settlements remains a challenge. In order to contribute to solving this important issue, a new numerical method for predicting ballast settlements is presented in this paper. This method is based on the finite element (FE) method combined with a constitutive model that captures permanent deformation accumulation in unbound materials under cyclic loading. This allows predicting permanent deformations of large structures and at large number of load cycles in a computationally efficient manner.The developed constitutive model is validated based on triaxial test measurements over wide range of loading conditions. Stress state in ballast layers has been examined with a 3D FE model, for several embankment structures and traffic load magnitudes. The determined stress distributions and loading frequencies were used as an input of the constitutive model to evaluate permanent strains and settlements of ballast layer. The influence of embankment structural designs and traffic loading magnitudes on the ballast layers settlements is examined and the results obtained are compared with the existing empirical performance models.
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| 4. |
- Saliko, Denis
(författare)
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Validation of heavy vehicle loading responses and temperature predictions in flexible pavements using field data
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is well established that both traffic-related loading and environmental conditions influence the structural behaviour of pavements. Pavement design methods aim to consider the effect of traffic loading and environmental variables on pavement structure, foresee their changes during the lifetime of the pavement and predict the resulting distresses and pavement life. Newer models are required to further advance the development of pavement design methods. Validations using reliable and representative data are required prior to incorporating these models in pavement design methods.The impact of environmental factors such as temperature, moisture content and freeze-thaw cycles on pavement behaviour have been examined in this doctoral thesis. Furthermore, the impact of increased loading by new long heavy vehicles on low-volume roads subjected to large variations of the environmental conditions has been investigated. The findings presented in this thesis have been based on field data collected on roads that have been and currently are in day-to-day operation. The collected data on mechanical response, temperature, moisture and frost was used to validate models on mechanical behaviour and thermal behaviour, as well as the effect of their interaction in pavement structures. The models developed and validated in this study are aimed to be integrated into a new mechanistic-empirical pavement design framework that is currently under development in Sweden. The work done for this thesis is presented hereby in the form of 5 papers and a short summary part. This thesis is a continuation of a licentiate thesis previously published at KTH Royal Institute of Technology. Part of the material published in the licentiate thesis has been included in this doctoral thesis.In paper 1, air temperature data recorded over a span of 10 years from 44 meteorological stations and temperature data from built-in sensors in 49 pavement structures located in different locations throughout Sweden were analysed. The data was used to statistically correlate the freezing index, calculated using the mean daily air temperature and the frost penetration depth in the cross-section of the pavement. Comparisons were made for the results obtained for various climatic zones in the country. The output of the paper is a country-specific empirical chart obtained through exponential interpolation and nonlinear prediction limits that indicates a range of expected frost penetration depth based on historical air temperature data. Paper 2 presents a study in which the structural response of a test section was evaluated using built-in sensors. The instrumentation consisted of asphalt strain gauges (ASG) recording the tensile strain in the bottom of the asphalt layer, strain measuring units (εMU) recording the vertical strain in the granular layers, and soil pressure cells (SPC) recording the vertical stresses in the granular layers. Falling weight deflectometer (FWD) measurements were performed on the structure to backcalculate the stiffness of the layers from the measured surface deflections. The aim of the study was to evaluate the structural response of the structure under loading by three long heavy vehicles (LHV) weighing ~64 tonnes, ~68 tonnes, and ~74 tonnes and compare the resulting estimated accumulated damage from each vehicle. The main finding from the paper was that the damage caused to pavements by long heavy vehicles was slightly larger than the damage caused by shorter vehicles with fewer axles but higher axial loading and tyre pressure.Paper 3 focuses on the effect of environmental factors and their variation on the structural behaviour of a thin pavement structure. Loading by long heavy trucks was applied to a test section at four different measurement campaigns performed at different seasons over one year. The variation of temperature and moisture in the structure was monitored continuously for the entire duration for which the study was performed. Thermocouples embedded in the asphalt layer and a frost rod placed in the granular layers were used to monitor the temperature variation in the structure. The moisture variation was monitored using time-domain reflectometer (TDR) probes. The correlation of the changes in temperature and moisture to the changes in mechanical stiffness of the layers was investigated. The instrumentation used to monitor the mechanical response, temperature variation, and moisture variation in the structure was found to be reliable for collecting data over the entire duration of the study. The main finding of the study is that it is possible to model the mechanical behaviour of thin pavement structures using multilayer elastic theory (MLET) calculations modelling, using linear-elastic material models if the stiffness of the asphalt layer is adjusted based on temperature and the stiffness of the granular layers is adjusted based on moisture levels.In Paper 4, the same response testing procedure as in Paper 3 was performed for a second pavement structure with a thicker asphalt layer. Data from response testing results for 2 pavement structures on 4 different dates, with a focus on the spring thaw period, were considered in the paper. Three different strategies for material modelling were used to investigate the mechanical response of the pavement structures. The layers were initially modelled using linear material parameters and the response results were compared both to calculations in which a viscoelastic model was used for the asphalt layer and to calculations in which a nonlinear K-Theta model was used for the granular layers. Comparisons were made between the calculated response using each modelling strategy and the measured response values. It was found that the viscoelastic and nonlinear models provided only marginal improvements in the range of 1%-4% in predicting the mechanical response of the structures. Based on the results, it was concluded that the linear elastic model was sufficiently accurate in capturing the mechanical behaviour of both pavement structures, including at the critical locations.Paper 5 presents the development and validation of a one-dimensional finite control volume (FCV) model capable of predicting temperature in pavements. The model is intended to be implemented into a new mechanistic-empirical pavement design framework currently under development in Sweden. The model uses easily obtainable meteorological data for air temperature, solar radiation, and wind speed for the three main modes of heat transfer, namely conduction, convection and radiation. To validate the model and estimate its accuracy, comparisons were made between the measured temperature and the calculated temperature values, using the FCV model. Comparisons were made for the pavement surface temperature, the temperature within the asphalt layer, and the temperature in the granular layers for 4 pavements located in different climatic zones in Sweden. In general, good agreement was found between the measured and calculated temperature values. Points for future improvements include better consideration of the surface properties, including the latent heat transfer in the calculations, and coupling the model to a moisture transfer model.
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| 5. |
- Bekele, Abiy
(författare)
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Evaluation of Low Temperature Damage in Asphalt Mixtures with Non-Contact Resonance Testing
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Thetemperature induceddamage in asphalt mixtureshas always been a major distress that requires a substantialconsiderationin the asphalt industry. One of the most important aspects of studying temperature induceddamage is developing a practical test method for evaluation of the material’s resistanceto it. Hence, there is a growing interest in developing testing methodologieswhich are more efficient, less expensive and simpler to perform than the conventional test methods. Impact resonance testing is a well-documented non-destructive testing method,and ithas been successfully appliedon asphalt mixturesto measure their elastic and viscoelastic properties. This research aims at extending the impact resonance testing methodology to characterization of temperature induced damage in asphalt mixtures and to investigate experimentally and numerically damage induced in asphalt mixtures due to thermomechanical mismatch between the masticand aggregate phases.In order to improve temperature control and thus accuracy of the resonance testing, an automated non-contact test procedure is developedwith a loudspeakerutilized as a source of excitation.The developed methodology has been evaluatedfor a range of asphalt concrete materialsand temperatures. The measurementsobtained from the new method have been verified by taking similar resonance frequency measurements usinganinstrumented impact hammer. Results from this work show that repeatable fundamental resonance frequency measurements can be performed onadisc shaped specimen in an automated manner without the need to open thethermal chamberthat is used to condition test specimens.Investigationsofmicro-damage in asphalt concrete due to differential thermal contraction during cooling cycles havebeen carried out experimentally by using the developedautomated non-contact resonance testingcombined withcyclic cooling. The results of the experimental work haveshown the initiation of low temperature micro-damage and a hysteretic behavior of stiffness modulus during thethermal cycles. Energy based micro-mechanical model is also utilized in order to characterize themicro-crackinitiation and growthin asphalt concrete due to cyclic low temperature variations.Results of this approach have indicated the initiation of micro-cracksat low temperatures as well as the decrease in their length with increase in temperature. In order to obtain a quantitative insight into the temperature induced damage formation, a micromechanical finite element model (FEM) of asphalt mixtureunder thermal loading is developed. The model is used to investigate the damage evolution during the thermal cycles as well as its effect on material’s stiffness. Four cases ofmastic-aggregate combinations aremodelledin order to investigate effects of aggregate gradation as well as of masticpropertieson the thermal damage evolution. Cohesive Zone Model (CZM) isused to define aggregate-masticinterface so that an initiation of micro-damage due to differential thermal contraction can be probedin terms of its effect on the overall stiffness modulus. It is observed numerically that during the thermal cycles, thermal damage is initiated at the aggregate-mastic interface due to the differential contraction of mastic. It is also shown that the modelling observations are in qualitative agreement with the experimental findings from the resonance testing. Accordingly, the proposed modelling approach is a viable tool for evaluation of theeffect of asphalt mixture design on its resistance to thermally induced damage.
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| 6. |
- Butt, Ali Azhar, 1984-, et al.
(författare)
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The Effect of Wax Modification on the Performance of Mastic Asphalt
- 2010
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Ingår i: International Journal of Pavement Research and Technology (IJPRT). - No.300, Jhongda Rd., Jhongli City, Taoyuan County 32001, Taiwan : Chinese Society of Pavement Engineering. - 1997-1400. ; 3:2, s. 86-95
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Tidskriftsartikel (refereegranskat)abstract
- The scope of this study is to evaluate the mechanical performance of the polymer modified mastic asphalt with 4% montan wax (Asphaltan A) additive. The impact of wax modification on binder, binder/filler mixtures and mastic asphalt was investigated in the laboratory. Wax modified binder properties were determined using dynamic mechanical analysis (DMA), Fourier transform infrared (FTIR) spectroscopy and conventional tests (softening point, penetration, elastic recovery, breaking point, viscosity and storage stability). The bending beam rheometer (BBR) was used for determining low temperature creep compliance and the tensile stress restrained specimen test (TSRST) for determining low temperature fracture. The fatigue cracking behavior of mastic asphalt was investigated using Superpave Indirect Tensile Test (IDT). Based on HMA Fracture Mechanics the influence of wax on the asphalt mixture resistance to fatigue and brittle cracking has been evaluated. The addition of wax to the polymer modified binder resulted in a viscosity reduction at higher temperatures, indicating a possible lower production and laying temperature as compared to asphalt without wax additive. DMA and BBR results showed some increase in stiffness and a more elastic response of the wax modified binder at medium and low temperatures. The TSRST fracture temperature was higher for the mastic asphalt containing wax, indicating a certain negative impact of wax modification.
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| 7. |
- Butt, Ali Azhar, 1984-, et al.
(författare)
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Using Life Cycle Assessment to Optimize Pavement Crack-Mitigation
- 2012
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Ingår i: Scarpas et al. (Eds.), 7th RILEM International Conference on Cracking in Pavements<em></em>. - Delft, The Netherlands : Springer Netherlands. ; , s. 299-306
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Konferensbidrag (refereegranskat)abstract
- Cracking is very common in areas having large variations in the daily temperatures and can cause large discomfort to the users. To improve the binder properties against cracking and rutting, researchers have studied for many years the behaviour of different binder additives such as polymers. It is quite complex, however, to decide on the benefits of a more expensive solution without looking at the long term performance. Life cycle assessment (LCA) studies can help to develop this long term perspective, linking performance to minimizing the overall energy consumption, use of resources and emissions. To demonstrate this, LCA of an unmodified and polymer modified asphalt pavement using a newly developed open LCA framework has been performed. It is shown how polymer modification for improved performance affects the energy consumption and emissions during the life cycle of a road. Furthermore, it is concluded that better understanding of the binder would lead to better optimized pavement design, hence reducing the energy consumption and emissions. A limit in terms of energy and emissions for the production of the polymer was also found which could help the polymer producers to improve their manufacturing processes, making them efficient enough to be beneficial from a pavement life cycle point of view.
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| 8. |
- Celma Cervera, Carlos, et al.
(författare)
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Contact-induced deformation and damage of rocks used in pavement materials
- 2017
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Ingår i: Materials & design. - : Elsevier. - 0264-1275 .- 1873-4197. ; 133, s. 255-265
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Tidskriftsartikel (refereegranskat)abstract
- Performance of stone-based construction materials, such as asphalt and unbound aggregate mixtures is defined to a great extent by the mechanics of the stone-to-stone interactions. Accordingly, the Discrete Element Method (DEM) is gaining popularity as a modelling tool to investigate the mechanical behavior of these materials. Contact and failure laws defining particles force-displacement relationships and the propensity of particles to break are crucial inputs for the DEM simulations. The present study aims at providing an experimental contact mechanics basis for the development of physically based stone-to-stone interaction laws. The attention is focused on investigating stone's force-displacement relationship and damage characteristics at pure normal loading for two stone materials used by the road industry. Experiments are performed at spherical contact profiles for cyclic and monotonically increasing loads. The emphasis lies on the evolution of contact compliance and accumulation of contact induced damage. The effect of surface roughness on the materials response is examined through comparative experiments performed on the specimens with different roughness values. Optical and environmental scanning electron microscopy (ESEM) observations of the contact induced damage at the material surface are presented and discussed in the context of contact mechanics. The implications of the reported experimental findings on the development of mechanics based contact and failure laws for the DEM modelling of stone-based construction materials are discussed.
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| 9. |
- Chen, Feng, 1987-, et al.
(författare)
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Experimental and numerical analysis of asphalt flow in a slump test
- 2019
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Ingår i: International Journal on Road Materials and Pavement Design. - : Taylor & Francis. - 1468-0629 .- 2164-7402. ; 20, s. S446-S461
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Tidskriftsartikel (refereegranskat)abstract
- The mechanical behaviour of uncompacted asphalt mixtures is still not well understood,threatening directly to the pavement practices such as control of mixture’s workability andsegregation. This situation may become even worse due to the gradually increasing complexityand advances in paving materials and technologies. This study adopts a slump flow testbased on concrete technology and a Discrete Element (DE)-based numerical tool to investigatethe mechanical behaviour of uncompacted asphalt mixture from a microstructural point ofview, particularly focusing on the bituminous binder effects. The combined experimental andnumerical analysis indicates that bitumen distinctly influences the contact interactions withinthe mixture and thus its macroscopic flow, which can be physically interpreted as a combinedeffect of lubricated friction and bonding force. Additional case studies demonstrate that the DEmodel is capable of simulating the flow response of asphalt mixtures under changed particlecontact conditions and driven force.
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| 10. |
- Das, Prabir, et al.
(författare)
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Importance of Thermal Contraction Coefficient in Low Temperature Cracking of Asphalt Concrete
- 2016
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Ingår i: Proceedings of the Fifty-Ninth Annual Conference of the Canadian Technical Asphalt Association (CTAA): Winnipeg, Manitoba.
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Konferensbidrag (refereegranskat)abstract
- A major distress mode in asphalt pavements is low temperature cracking, which results from the contraction and expansion of the asphalt pavement under extreme temperature changes. The potential for thermal cracking involves interplay between the environment, the road structure, and importantly the properties of the asphalt mixture. In the present study, the low temperature cracking performance of asphalt mixture has been investigated numerically and experimentally. A low temperature cracking model has been utilized, which was developed by integrating fracture energy threshold into an asphalt concrete thermal fracture model considering non-linear thermal contraction coefficients. Based on the asphalt concrete mixture viscoelastic properties, this enhanced model can predict thermally-induced stresses and fracture temperatures. It was observed that the thermal contraction coefficient in asphalt concrete is non-linear in the temperature range of interest for low temperature cracking. The implications of having non-linear thermal contraction coefficient were investigated numerically. From the analysis, it was found that this enhanced model can be utilized to evaluate the low temperature cracking performance of asphalt mixtures and rank them accordingly. Interestingly, non-linear thermal contraction coefficient gave much better prediction than the linear approach.
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