Quantification of microstructural damage in asphalt

This research is concerned with quantifying damage in asphalt mixtures at the micro level. X ray Computer Tomography (CT) a non destructive technique along with image analysis has been used to study the internal microstructural properties of asphalt. During laboratory testing of asphalt mixtures, it has been observed that specimens lose strength without any visible cracks. UK asphalt mixtures have been tested in uniaxial compression and tension compression fatigue tests and scanned in X ray CT. In the uniaxial compression test, specimens have been tested at three different strain rates. Both monotonic and cyclic tests have been conducted at three different temperatures. Testing has been carried out both continuously and with rest periods at selected stages. The specimens were scanned in X ray before starting the tests and also during the testing at on selected stages until failure. X ray machine operation was optimized to achieve good quality of images of different types of asphalt samples. The 2D images of the specimens were collected from the X ray CT and were stacked to regenerate into 3D images of the asphalt samples. Techniques for adjusting the threshold grey values of the images and analysing the X ray images for different parameters have been developed. The images have been analysed to evaluate the microstructure of the asphalt specimens internally and non destructively. Air voids content is considered as the parameter that represents the change in microdamage during the application of loading cycles. Moisture damage in asphalt mixtures was studied from X ray CT. Two types of mixture were investigated, one with acid aggregate and one with basic aggregate, with three different ranges of void content. Dry specimens and specimens saturated in the laboratory were scanned in X ray CT to study the internal connected air voids which cause the permeability to moisture in an asphalt mixture and result in moisture damage. Damage due to combined moisture and ageing was studied from X ray images. From the analysis of X ray images, it was observed that a non uniform increase in air voids occurred both along the height and across the diameter of the specimens tested in monotonic compression and tension compression fatigue. This may perhaps be due to the heterogeneous nature of asphalt. New voids developed along with a size increase and joining together of existing voids. Using continuum damage mechanics, the data from both the mechanical testing and from X ray computer tomography was compared. For specimens tested in fatigue, damage parameters were determined for a damage model. The dissipated pseudo strain energy approach was applied to the test data and the parameters for the damage model were determined. A modified model with a new parameter of adhesion between binder and aggregate was used for data analysis. Results from X ray computer tomography and from the fatigue damage model were compared. In the case of specimens tested for moisture damage and ageing, the retained saturation was determined from X ray image analysis and was related to the stiffness of asphalt mixtures. Asphalt mixtures containing basic aggregate were found to have a high retained stiffness value after moisture and ageing tests compare to mixtures containing acidic aggregate. The stiffness values for the retained saturation were determined and it is observed that in the case of mixtures containing acidic aggregate, the retained stiffness decrease with the increase in retained saturation

Laboratory mix design of asphalt mixture containing reclaimed material

This paper presents a study on the production of asphalt test specimens in the laboratory containing reclaimed asphalt. The mixtures considered were stone mastic asphalt concrete mixtures containing up to 30% of reclaimed asphalt. Specimens were compacted to the reference density obtained from the Marshall mix design. Gyration compaction method was used for preparing specimens for the experimental programme, while coring and cutting methods and X-ray computed tomography (CT) were used to investigate the change in properties within the specimens and to validate the selected methodology.The study concluded that gyratory compaction is suitable to produce homogeneous test specimens also for mixtures containing high amount of reclaimed asphalt. Nevertheless, preliminary trials for each material are mandatory, as well as final coring and trimming of the specimens due to side effects

Quantification of microstructural damage in asphalt

This research is concerned with quantifying damage in asphalt mixtures at the micro level. X ray Computer Tomography (CT) a non destructive technique along with image analysis has been used to study the internal microstructural properties of asphalt. During laboratory testing of asphalt mixtures, it has been observed that specimens lose strength without any visible cracks. UK asphalt mixtures have been tested in uniaxial compression and tension compression fatigue tests and scanned in X ray CT. In the uniaxial compression test, specimens have been tested at three different strain rates. Both monotonic and cyclic tests have been conducted at three different temperatures. Testing has been carried out both continuously and with rest periods at selected stages. The specimens were scanned in X ray before starting the tests and also during the testing at on selected stages until failure. X ray machine operation was optimized to achieve good quality of images of different types of asphalt samples. The 2D images of the specimens were collected from the X ray CT and were stacked to regenerate into 3D images of the asphalt samples. Techniques for adjusting the threshold grey values of the images and analysing the X ray images for different parameters have been developed. The images have been analysed to evaluate the microstructure of the asphalt specimens internally and non destructively. Air voids content is considered as the parameter that represents the change in microdamage during the application of loading cycles. Moisture damage in asphalt mixtures was studied from X ray CT. Two types of mixture were investigated, one with acid aggregate and one with basic aggregate, with three different ranges of void content. Dry specimens and specimens saturated in the laboratory were scanned in X ray CT to study the internal connected air voids which cause the permeability to moisture in an asphalt mixture and result in moisture damage. Damage due to combined moisture and ageing was studied from X ray images. From the analysis of X ray images, it was observed that a non uniform increase in air voids occurred both along the height and across the diameter of the specimens tested in monotonic compression and tension compression fatigue. This may perhaps be due to the heterogeneous nature of asphalt. New voids developed along with a size increase and joining together of existing voids. Using continuum damage mechanics, the data from both the mechanical testing and from X ray computer tomography was compared. For specimens tested in fatigue, damage parameters were determined for a damage model. The dissipated pseudo strain energy approach was applied to the test data and the parameters for the damage model were determined. A modified model with a new parameter of adhesion between binder and aggregate was used for data analysis. Results from X ray computer tomography and from the fatigue damage model were compared. In the case of specimens tested for moisture damage and ageing, the retained saturation was determined from X ray image analysis and was related to the stiffness of asphalt mixtures. Asphalt mixtures containing basic aggregate were found to have a high retained stiffness value after moisture and ageing tests compare to mixtures containing acidic aggregate. The stiffness values for the retained saturation were determined and it is observed that in the case of mixtures containing acidic aggregate, the retained stiffness decrease with the increase in retained saturation.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Numerical modelling of mode-I cracking in asphalt concrete

This paper presents a numerical model to simulate mode-I fracture in asphalt concrete. The phase field method is used to simulate diffuse damage in asphalt concrete. In addition to the displacement degrees of freedom, a scalar damage degree of freedom is added at each node of a finite element. Evolution of the damage variable is governed by a crack potential which regularizes a sharp crack over a finite volume. The phase field method does not suffer from mesh dependency problem like the continuum damage models and therefore preferred for damage analysis. A single edge notch beam is analysed to show the performance of the model. The damage pattern as well as load versus crack mouth opening displacement graph is compared with the experimental data. The numerical results show good agreement with the experimental observations

MOISTURE DAMAGE IN ASPHALT MIXTURES FROM SATS TEST AND X-RAY CT

This study investigated damage in asphalt due to moisture by using the modified SATS (saturation aging tensile stiffness) test along with image analysis. Specimens made from granite aggregate and limestone aggregate and of high grade (hard) bitumen were tested in the modified SATS test. The samples were scanned within an X-ray CT before the testing using SATS, after the initial saturation and at the completion of SATS test. Bitumen was extracted from tested specimens and scanned separately in a high resolution Nanotom. The extracted bitumen was tested in a DSR for Shear properties. The specimens made from granite (acidic) and limestone (basic) aggregate were found to have similar permeability and was high for specimens having high design void content. Specimens made from basic aggregate had higher stiffness value than the specimens made from acidic aggregate. The Master curve of aged binder is different to that of virgin binder (bitumen). The microstructure of binder observed was different for aged and virgin binder

Ageing and Driving Performance in the UK Using Normal Mixture Model Cluster Analysis Technique

This research addresses the issue of an individual's ability to drive and especially of those individuals that are questionably demented (dementia is suspected) or are in a state of very mild dementia and are therefore the most difficult to identify. A methodology has been developed for categorization of drivers by considering three driving performance indices/parameters simultaneously. This novel approach precluded the previous technique whereby only a single driving performance index (an omnibus approach without the ability to discriminate between normal driving behavior and risky driving habits primarily due to cognitive decline) is considered. Driving performance of 28 young and 28 old drivers was gauged by 24 driving performance parameters through two designed drives on driving simulator. Normal Mixture Model Cluster Analysis was used in the performance-based categorization of drivers. It was found that out of a total of 56 drivers, 8 turned out to be "poor drivers". Results from neuropsychological/cognitive tests showed on average lower cognitive performance for the "poor drivers" group. This methodology will preclude the need for measurement of driving skills through driving instructors

The use of neuropsychological tests to study the effects of aging on driving performance in the UK

Research was conducted to identify a series of neuropsychological tests to assess the ability to drive. Driving performance of young and old UK drivers was modeled through multiple linear regression and univariate logistic regression tools. The UFOV3 test (i.e. the third subtest of the UFOV test) had comparatively high discriminating ability in separating poor-drivers from not-poor-drivers, with 92.86% of the drivers correctly classified; the UFOV3 test resulted in a Sensitivity of 62.5%. Age and a composite cognitive measure were also found to be sound discriminators of poor-drivers and not-poor-drivers with 91.07% and 89.28% of the drivers correctly classified respectively; both age and the composite cognitive measure resulted in a Sensitivity of 50%. It was found that the commonly recommended Clock Drawing Test and the Trail Making-B test were insignificant predictors of driving ability. Results suggest that for a score greater than 220 on the UFOV3 test, the driver may be further evaluated by a driving specialist to ascertain questionable driving behavior. Also, drivers above the age of 77 were more susceptible to exhibiting unusual driving behavior; if such drivers have UFOV3 scores greater than 220 it would be more appropriate to evaluate driving behavior through a driving specialist.</p

LABORATORY MIX DESIGN OF ASPHALT CONTAINING RE-CLAIMED MATERIAL

This paper presents a study on the production of asphalt test specimens in the laboratory containing reclaimed asphalt. The mixtures considered were stone mastic asphalt concrete mixtures containing up to 30% of reclaimed asphalt. Specimens were compacted to the reference density obtained from the Marshall mix design. Gyration compaction method was used for preparing specimens for the experimental programme, while coring and cutting methods and X-ray computed tomography were used to investigate the change in properties within the specimens and to validate the selected methodology. The study concluded that gyratory compaction is suitable to produce homogeneous test specimens also for mixtures containing high amount of reclaimed asphalt. Nevertheless, preliminary trials for each material are mandatory, as well as final coring and trimming of the specimens due to side effects

Performance of hot-mix asphalt using polymer-modified bitumen and marble dust as a filler

Marble dust waste powder generated by the marble cutting industry has a high alkalinity. In this research, the use of marble dust (MD) as a mineral filler substitute in hot mixed asphalt (HMA) was evaluated. The Marshall mix design was used to determine the optimum bitumen content (OBC) for all of the mixtures. For each of the four MD contents, i.e., 0, 2%, 4%, and 6% by weight of the total aggregates, four different bitumen percentages were used. The results of the Marshall stability test showed that the optimum filler content was 4% MD. Samples were prepared with 0 MD in the control mix and varying percentages of MD as an alternate filler. In addition, MD aided in increasing the Marshall stability, rutting resistance, and permanent deformation and reduced the fatigue life of asphalt mixtures. As the percentage of MD increases, the rutting resistance and stiffness at high temperatures both increase. As the percentage of MD increases, the fatigue life reduces. Rut resistance in high-temperature conditions can be improved by using MD in HMA as a partial substitute for stone dust (SD). In areas where extensive MD waste is present, MD can be incorporated into HMA mixtures instead of conventional fillers

Effect of mixing time on reclaimed asphalt mixtures: An investigation by means of imaging techniques

This paper presents an application of imaging techniques to investigate the distribution of reclaimed asphalt and air voids within gyratory compacted specimen. Stone Mastic Asphalt specimens containing reclaimed asphalt, were produced at five different laboratories participating in the collaborative project Re-Road: University of Nottingham, Belgium Road Research Centre, Danish Road Directorate, TU Braunschweig and Laboratoire Centrale de Ponts et Chaussees (now IFSTTAR). This study applies the X-ray Computed Tomography and Optical Microscopy techniques to view the internal structure of the specimen with particular focus on the air voids and reclaimed asphalt distributions within the specimen. The observations revealed that with doubling the mixing time the reclaimed asphalt is well mixed with the fresh materials within the gyratory compacted specimen and also the air voids distribution along the specimen height result to be more consistent