Nima Roohi Sefidmazgi

Internal structure characterization of asphalt mixtures for rutting performance using imaging analysis

Characterization of the asphalt concrete microstructure using two-dimensional (2-D) imaging techniques is an economically efficient approach. However, the features that have been captured and quantified using 2-D imaging in most published research have been limited to simplistic analyses of aggregate structure. The present research focused on introducing a more elaborate method of characterization of internal structure, and proposing new indices to relate to and explain rutting resistance performance of asphalt mixtures. The aggregate internal structure provides the skeleton of the asphalt concrete, which plays an important role in rutting resistance. It is shown that this structure can be captured using a combination of image analysis indices developed in this research, namely: number of aggregate-on-aggregate contact points, contact length/area, and contact plane orientation. These parameters are defined for both the total aggregates and for the effective load bearing aggregate structure, referred to as the ‘skeleton’ in this study. Software developed in a previous study and significantly modified for this paper, is used to process digital images of a set of asphalt mixtures with different gradations, binder contents, types of modification, and compaction efforts. The results demonstrate a correlation between the internal structure indices and the mixture rutting performance. Additionally, the indices were successfully used to capture the effect of compaction effort, gradation quality, and binder modification on the mixture internal structure.

Aggregate structure characterisation of asphalt mixtures using two-dimensional image analysis

In current practice of mixture design, volumetric properties such as voids and binder content along with mechanical properties such as modulus or rutting resistance are used as the main quality indicators. Visualisation is an important tool that has not been widely used in asphalt mixtures. As part of the Reunion Internationale des Laboratoires et Experts des Materiaux activities, the aggregate structure has been identified as a possible important mixture characteristic in need of measuring and quantifying. This paper is a report on part of this effort. Software for processing and analysing two-dimensional images of asphalt concrete mixtures to provide information about the aggregate structure within a mix was developed. Images with accompanying volumetrics and gradation information can be processed with the software and a virtual sieve analysis of aggregates within the image is performed to verify a match with known measured gradations. Once images were successfully processed, analysis is performed to determine the number of contact points between aggregates as well as radial distribution and orientation of each aggregate. Segregation of aggregates within each specimen was also determined. Mixtures with a broad range of variables were compacted in the laboratory, using a number of compaction methods of various countries. In addition, mixtures with various nominal maximum aggregate sizes, aggregate type (limestone or gravel) and design ESALs (E-3 or E-10) were compacted in the US gyratory compactor, using two pressures (600 and 300 kPa) and two temperature levels (120°C and 60°C). Results indicate that the aggregate structure is affected by compaction methods and conditions although volumetrics are very similar. The results show that a fresh look at evaluating the aggregate structure within mixtures is required.

Effect of particle mobility on aggregate structure formation in asphalt mixtures

During compaction of asphalt mixtures, aggregate structure starts building up by proximity and direct contact of aggregates. In the previous studies, it has been shown that the aggregate structure directly affects the service performance. However, the mechanisms of the aggregate structure formation are not clearly understood. This study is focused on the mechanisms affecting aggregate mobility during compaction and the effect of material properties on the aggregate structure formation. At the initial stages of compaction, there is a relatively thick layer of mastic (i.e. mix of binder and filler) between aggregates, which allows for a shearing mobility in the mix, if the mastic viscosity is sufficiently low. However, as compaction proceeds, the mastic layer at proximity zone of aggregates becomes thinner due to high stress intensity and the higher viscosity of thin mastic film or the aggregates dry contact effect increases the shearing resistance against compaction (i.e. mix becomes locked). In this study, mixes are compacted at different temperatures using one base binder and three different modified binders. The quality of the aggregate structure and packing throughout the compaction is characterised using two-dimensional imaging of mixture sections and the total aggregate on aggregate proximity length is measured as an indication of the aggregate-packing level. It is shown that for mixtures to obtain the maximum packing, the compaction temperature should be picked based on mastic viscosity. The viscosity of mastic should be low enough for lubrication, but high enough to provide sufficient film thickness at proximity zones and prevent locking of mixture at the early stages of compaction.

Application of Diffusion Mechanism: Degree of Blending Between Fresh and Recycled Asphalt Pavement Binder in Dynamic Shear Rheometer

There are many concerns about the blending between virgin and aged binders in reclaimed asphalt pavement (RAP). Determining the extent of the blending and identifying the main factors affecting the blending between two binders have gained great interest in recent years. Viscosity as a function of temperature, exposure time, and film thickness have been mentioned as the main factors controlling the blending between two binders that are in contact. The in-contact blending between two binders is hypothesized to be governed by a diffusion phenomenon in which material is transferred through a medium because of the Brownian motion of molecules. This study aimed to estimate the rate of diffusion of virgin binder (as the diffusing matter) into the RAP binder, which controls the blending of two binders during exposure. The effect of temperature, which was considered as the main factor influencing the diffusion rate, was evaluated at varying levels. Ficks law calculations and dynamic shear rheometer (DSR) measurements were fitted to estimate the diffusion rate. The results showed a high dependency of diffusion on the temperature (viscosity) of binders. The blending development was also found to be a function of the diffusion rate and exposure time. A DSR test method was applied to RAP mortars (mix of RAP fine particles and fresh binder) to verify the effect of time and temperature on the blending level between fresh and actual RAP binders. The results of mortar testing verified the factors affecting the diffusion phenomenon.

Exploring the feasibility of evaluating asphalt pavement surface macro-texture using image-based texture analysis method

Pavement surface texture significantly affects tyre–pavement friction and noise characteristics. The traditional methods for evaluating pavement surface texture result in a single index called mean profile depth (MPD). Although this index can reflect the overall texture properties, it cannot reveal the range and distribution of pavement surface texture, which play a critical role in prediction of tyre–pavement interaction characteristics. In this paper, a cost-effective and relatively precise image-based texture analysis method (ITAM) was developed based on digital image processing and spectral analysis technologies. Mixture sample produced using surperpave gyratory compactor (SGC) is cut into three sections which are scanned using a standard commercial scanner. Mixture surface profile is then identified from the scanned cut section images by applying a series of image analysis technics. Afterwards, a discrete Fourier transform is applied on the mixture surface profiles to calculate the texture distribution indicators through the ITAM software. Additionally, the traditional texture indicator (MPD) is derived. Previous researchers have shown that the stationary laser profilometer (SLP) serves as an effective method to characterise pavement texture properties as this method correlates well with traditional texture testing methods. In this study, the ITAM analysis results are verified by comparing with those from the SLP method. It is shown that ITAM results correlate well with SLP and therefore considered as an effective method to characterise pavement surface texture properties. The results indicate that this method is a promising and powerful tool for future application in mixture designs to estimate texture as related to noise and friction.

Mechanisms of Failure in Uniaxial Repeated Creep Test and the Relationship to Aggregate Packing

Rutting performance characterization of asphalt mixtures has attracted lots of attentions after the Strategic Highway Research Program (SHRP). The main reason is the lack of a stability or strength test in the Superpave volumetric criteria. Different test methods and analysis have been proposed with a goal of accurately as well as simply predicting the rutting performance of designed mixtures. A uniaxial repeated creep and recovery test is recommended in NCHRP 456 report (i.e. Flow number test) as a performance characterization test method, and it is currently being used by researchers extensively. This study is focused on comparing the mechanisms and fundamental properties that control the performance of asphalt mixtures throughout the uniaxial flow number testing. It is shown that aggregate packing, as measured using an image analysis method, is the key property affecting deformation characteristics in uniaxial testing. Additionally, it is shown that the main cause of tertiary flow of mixtures in the flow number test is structural instability and bulging (dilation) of aggregate skeleton. Aggregate skeleton discontinuities at outer layers of samples are observed after failure in the tertiary zone. It is observed that mixtures with better aggregate packing showed a better rutting performance due to lower stress level within the binder phase due to aggregate skeleton serving as the main stress path, and the aggregates in proximity or contact showed a supporting structure, which delays the tertiary flow in material, and reduces the rate of permanent deformation. Based on these observations, it is shown that applying confinement has a very significant effect in preventing tertiary flow conditions by maintaining the aggregate packing, thus improving rutting resistance of asphalt mixtures.

Establishment of Relationship Between Pavement Surface Friction and Mixture Design Properties

In recent years, pavement designers have been increasingly challenged to achieve ideal surface friction properties to enhance driver safety through mixture design manipulation and material selection. A clear understanding of the relationship between pavement mixture design properties and surface friction is needed. In this study, the surface laser profilometer device was used to measure surface texture by scanning the surface of laboratory-compacted and field core samples, from which a surface texture profile was derived. Surface texture data were analyzed to calculate the mean profile depth. Statistical analysis and neural networks modeling were used to find the relationship between the mean profile depth results estimated from laser profilometer measurements and mixture design properties (i.e., aggregate size, gradation curve shape, binder content, air voids, and volumetric properties). Promising trends were observed in the data that could be used as guidance by mixture designers to optimize mixture design properties, such as to achieve better pavement surface friction and hence enhanced driver safety. Relationships between field-measured friction (mainly friction number) and laboratory-measured texture parameters were developed. Friction number values measured with the locked-wheel tire test in the field were taken from the MnROAD test track database and used for analysis. The study shows a correlation between friction number and mean profile depth that is of potential applicability to pavement designers for achieving target field friction with the aid of laboratory measurement and analysis. Such relationships between laboratory measurements and field friction could lead to better control of pavement surface friction.