ASPHALT MIXTURE DESIGN CONCEPTS TO DEVELOP AGGREGATE INTERLOCK (original) (raw)
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International Journal of Pavement Engineering, 2019
Aggregate size distribution is an important parameter in asphalt mixture design and performance. The main objective of this study is to develop a framework to define the aggregate structure of asphalt mixtures when fine and coarse aggregate stockpiles are blended. To develop this framework, an analytical model for binary mixtures is proposed. The model considers the effect of size ratio and air volume between the particles on the aggregate structure and packing density of binary mixtures. Based on this model, three aggregate structures, namely coarse pack (CP), dense pack (DP) and fine pack (FP), are defined. The model is validated using a series of 3D discrete element simulation. Furthermore, the simulation of multi-sized aggregate blends using two representative sizes for fine and coarse stockpiles was carried out to apply the proposed analytical model to actual aggregate blends. In order to assess how well the model applies to asphalt mixtures, compaction parameters including compaction slope (CS), initial density (N ini), locking point and compaction energy index (CEI) were analyzed. The numerical simulations verify the proposed analytical model can satisfactorily determine the particle structure of binary and multi-sized asphalt mixture gradations and can be used to better design asphalt mixtures for improved performance. ARTICLE HISTORY
Aggregate Packing Characteristics of Asphalt Mixtures
2019
Voids in the mineral aggregate (VMA), as a main volumetric design parameter in the Superpave mixture design method, is an important factor to ensure asphalt mixture durability and rutting performance. Moreover, an asphalt mixture’s aggregate skeleton, related to VMA, is another important factor that affects critical asphalt mixture properties such as durability, workability, permeability, rutting, and cracking resistance. The objective of this study is to evaluate the effects of aggregate size distribution and shape parameters on aggregate packing characteristics (volumetric and compaction properties) of asphalt mixtures. Three tasks were undertaken to reach this goal. The first task was to propose an analytical approach for estimating changes in voids in the mineral aggregate (VMA) due to gradation variation and determining the relevant aggregate skeleton characteristics of asphalt mixtures using the linear-mixture packing model, an analytical packing model that considers the mecha...
Relationship between Aggregate Packing Characteristics and Compactability of Hot-Mix Asphalt Mixes
Airfield and Highway Pavements 2019, 2019
Compaction of Hot-Mix Asphalt (HMA) is an essential process in the construction of asphalt pavement as it is critical to the performance of the road infrastructure. The compaction process enables the asphalt mixtures to achieve stability, provide resistance to deformation (rutting) against traffic loading, reduce permeability of the layer and improve its durability. One of the factors affecting compactability (or workability) of the HMA is its internal structure. The research work presented in this paper is aimed at investigating the influence of the internal structure of HMA based on aggregate gradation and packing characteristics on the compactability. Three coarse-graded HMA mixes were used to evaluate parameters that describe aggregate packing characteristics, including the gravel to sand ratio, gradation shape factor, as well as the traditional and revised rational Bailey ratios. HMA specimens were compacted using a Superpave Gyratory Compactor (SGC), and the data were analyzed to determine five parameters of HMA compactability. The HMA compactability parameters were then correlated to the aggregate packing parameters. Overall, the parameters of HMA compactability exhibited a strong correlation with the aggregate packing parameters. The outcomes of the study not only provide a better understanding of the influence of aggregate gradation and packing characteristics on HMA compactability, but also identify essential input parameters to be considered when developing models to simulate HMA compaction.
EFFECT OF AGGREGATE GRADATION ON THE STIFFNESS OF ASPHALT MIXTURES
Aggregate gradation plays an important role in the behaviour of asphalt mixtures. Packing of aggregate is a very important factor that will be affected by changing the aggregate gradation. Many researchers have investigated different ways of describing packing both theoretically and practically. Bailey ratios have recently been used to understand the volumetric properties of mixtures. In this paper, the Bailey ratios have been used, and two further ratios have also been introduced to allow the asphalt mixture gradation to be fully understood. Thirteen different aggregate gradations have been chosen within the 14 mm asphalt concrete specification to investigate the effect of particle size distribution on the stiffness of the mixture. It was found that variation in aggregate gradation has a significant effect on asphalt stiffness, even within specification limits, and a reasonable correlation between the set of ratios investigated and the Indirect Tensile Stiffness.
A New Framework for Determining the Gradation of Asphalt Mixtures
Australian Journal of Modernity in Road and Pavement Engineering
Strength and rutting resistance as well as durability are the main quality measures for any asphalt mix. While the strength and resistance of the asphalt mixture to permanent deformation are controlled by the aggregate interlock and aggregate packing, the durability of asphalt mixture depends on adequate volumetric characteristics and film thickness of asphalt mixture. Accordingly, providing an adequate interlock between aggregates and the aggregate packing is of high importance in designing a new asphalt mixture in order to produce desirable volumetric characteristics in asphalt mixture. A new framework for asphalt mixture gradation has been developed and designed following an intensive review of the various currently used approaches. The new framework considers all the controlling factors in the aggregate gradation design for optimum gradation of the asphalt mixtures. Satisfactory interlocking between aggregate particles, proper skeleton within the asphalt mixture, and acceptable ...
— Stone Mastic Asphalt (SMA) mixtures rely on stone-to-stone contacts among particles to resist applied forces, and permanent deformation. Aggregates in SMA should resist degradation (fracture and abrasion) under high stresses at the contact points. Current practices for asphalt mix design and acceptance testing rely on volumetric properties. Vital to the calculation of mix volumetric properties are specific gravity measurements of the mixture and the aggregate in the mixture. For the Motorways wearing course,SMA stone grid must fulfil the mineralogical-petrographic condition to be on rock of igneous and/or metamorphic origin but of silicate composition, specific weight and LA method on resistance to fragmentation. During the construction of the Kosovo motorway for wearing course was used the SMA as asphaltlayer for providing longer lifetime to the road construction. The super-pave mix design for SMA wearing course has been composed considering the available stone with high mineralogical and petrographic composition. The stone used for the wearing course on this motorway has resistance to crushing of LA =18 which is below the standard criteria for heavy traffic roads and motorways wearing courses. Thespecific weight of used stone was 3100 kg/m3which is about 15% heavier than standard weight. For this specific stone were prepared special super-pave design mix with binder content 4.5% which was well below than typical SMA composition of 6.0−7.0% binder rather of mixture was 3100 kg/m 3. In this paperwork it is described the design mix of SMA composed with relatively high specific weight and their impacts on asphalt mix design properties used in Kosovo Motorway.
Construction and Building Materials, 2018
Voids in the mineral aggregate (VMA), as a main design parameter in the Superpave mixture design method, is an important factor to ensure asphalt mixture durability and rutting performance. Moreover, an asphalt mixture's aggregate skeleton, related to VMA, is another important factor that affects critical asphalt mixture properties such as durability, workability, permeability, rutting, and cracking resistance. The main objectives of this research are to propose an analytical approach for estimating changes in VMA due to gradation variation and determining the relevant aggregate skeleton characteristics of asphalt mixtures using the linear-mixture packing model, an analytical packing model that considers the mechanisms of particle packing, filling and occupation. Application of the linear-mixture packing model to estimate the VMA of asphalt mixtures shows there is a high correlation between laboratory measured and model estimated values. Additionally, the model defines a new variable, the central particle size of asphalt mixtures that characterizes an asphalt mixture's aggregate skeleton. Finally, the proposed analytical model shows a significant potential to be used in the early stages of asphalt mixture design to determine the effect of aggregate gradation changes on VMA and to predict mixture rutting performance.
Powder Technology, 2020
In this study, the effect of coarse aggregate shape characteristics on the compactability and microstructural properties of asphalt mixtures was virtually investigated using a discreet element method (DEM). Results reveal there is a statistically significant correlation between coarse aggregate flatness, elongation, roundness, and sphericity as shape descriptors and initial asphalt mixture density, as a compaction parameter. Analyses indicate that among all particle shape descriptors, only roundness and regularity had a statistically significant relation with compaction slope, and as the amount of roundness and regularity increase, the compaction slope decreases. Additionally, flatness and elongation values greater than 0.75 result in lower average coordination number values compared to those values less than 0.75. Results also indicate that although the probability density function (PDF) of low contact force highly depends on flatness, elongation and sphericity of particles, the PDF of high and very high contact forces is independent of the particle shape.
DEVELOPMENT OF A MIXTURE DESIGN PROCEDURE FOR STONE MATRIX ASPHALT (SMA)
2000
Stone Matrix Asphalt (SMA) has been used successfully in Europe for over 20 years to provide better rutting resistance and to resist studded tire wear. Since 1991, the use of SMA has increased steadily in the United States. At present, some states routinely use SMA even though a standard mixture design procedure is not available. A mixture design procedure that provides guidance on material properties, aggregate gradation, determination of optimum asphalt content, and mixture properties is needed. This paper presents a mixture design procedure for SMA mixtures developed by the National Center for Asphalt Technology. Data for the development of the procedure was collected from a laboratory study conducted with various samples of aggregates, fillers, asphalt binders, and stabilizing additives. Compacted mixtures were tested to evaluate the effects of aggregate structure, asphalt binder, and binder-fine aggregate mortar. Specific conclusions from this study were: 1) The Los Angeles abrasion loss showed good correlation with aggregate breakdown; 2) it appeared that the 3:1 or 2:1 flat and elongated particles provided much better classification for the various aggregates than a 5:1 ratio; 3) the flat and elongated particle ratio showed excellent correlation with aggregate breakdown; 4) in a SMA mix, the percent passing the 4.75 mm sieve must be below 30 percent to ensure proper stone-on-stone contact; 5) the percent passing the 0.02 mm sieve did not show a correlation with mortar stiffness. However, the dry compacted volume, as obtained from the Penn State test method, did show a good correlation with mortar stiffness and can be utilized to characterize the shape of fillers. Generally, a more angular filler tends to produce a higher air voids result in this test; 6) In-place results from about 86 projects showed that very little rutting has occurred in SMA pavements constructed in the United States since 1991. However, for the pavements with air voids falling below the 3 percent range, some rutting was observed; 7) a VMA significantly lower than specified VMA can be obtained due to aggregate breakdown. Hence, the mix designer must consider aggregate type, compactor type and compactive effort along with the gradation in meeting the required VMA criteria. Specifying a minimum asphalt content can result in different requirements for aggregates with different specific gravity; 8) Fifty blows of Marshall hammer were found to be approximately equal to 100 revolutions of the Superpave gyratory compactor in terms of resultant density. The Superpave gyratory compacter was found to produce less aggregate breakdown than the Marshall hammer; 9) Fiber stabilizers were found to be more effective in reducing draindown than polymer stabilizers. However, mixes modified with polymer showed better resistance to rutting in laboratory wheel tracking tests.
Influence of aggregates angularity on the locking point of asphalt mixtures
Road Materials and Pavement Design, 2019
Aggregate angularity plays a critical role in the performance of asphalt mixtures. Current asphalt mix design methods do not specify compaction effort according to aggregate angularity or locking points. This study utilised two methods of compaction, gyratory (Superpave Gyratory Compactor) and impact (Marshall Hammer), to investigate the influence of aggregate angularity on the locking point of asphalt mixtures. For gyratory compaction, densification curves were used to determine the gyratory locking point. For impact compaction, an accelerometer was connected to the Marshall Hammer to register asphalt response, by which the impact locking point was determined. Five mixtures were designed and produced in the laboratory with different aggregate angularities. The mixtures were tested for their locking points and then related to aggregate angularities. Results show that locking point could be determined for most of the mixtures in the study by the gyration and impact methods, except for the mixture composed of uncrushed, round aggregates compacted with the Marshall Hammer. The locking point of asphalt mixtures was highly related to aggregate angularity. A higher aggregate angularity resulted in a lower locking point. Mixtures produced with only crushed aggregates showed locking points much lower than mixtures made with both crushed and uncrushed aggregates, while mixtures made with only uncrushed aggregates had much higher locking points than those made with both aggregates. The findings from this study indicate that compaction effort in asphalt mix design should be specified according to the locking point of the mixtures.