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.