# Using Imaging Techniques to Analyze the Microstructure of
## 1 Introduction  
Asphalt concrete (AC) is a paving material widely used in North America and worldwide. There are more than 4 million kilometres of paved roads in North America, and more than 93% of them are asphalt roads. More than 55% of North American asphalt roads are in poor or fair condition and require major maintenance or rehabilitation which costs millions of dollars annually from the budget of transportation agencies in North America [1].  
Understanding the failure mechanism of AC requires consideration of the heterogeneous nature of the mixture. Mechanistic models for analyzing AC mixtures focus on macroscopic behaviour [2]. These models were developed based on continuum mechanics theory, which assumes that AC is a homogeneous mixture. Its geometric and physical characteristics (such as aggregate shape, spatial position, aggregate texture, and gradation) are ignored in this theory. This assumption is made due to the difficulty in quantifying and measuring AC microstructures and the randomness of the aggregate distribution in the mixture. However, the internal geometry and the physical properties of AC mixture components can significantly affect the overall performance of pavement structures [3].  
Microstructure characteristics have an impact on the design of AC with satisfying performance [4]. Most previous studies analyzed the microstructure of asphalt mixtures using experimental tests such as the ignition method to estimate asphalt binder content, the wash gradation method to estimate the aggregate gradation, and stranded test methods to estimate the air voids percent in compacted AC mixture sample [5; 6; 7]). These experimental tests are time-consuming and labor-intensive and involve high variability in results.  
The need for fast, user-friendly, and practical methods of analyzing the microstructure of AC mixtures has become the concern of many researchers. In the mid-1990s, imaging techniques were used as an effective method of analyzing the microstructure of AC mixtures. Imaging techniques provide a non-destructive analysis of the AC mixtures, which allows for repeated measurements of the same sample without altering or damaging it. The non-destructive nature of this testing allows the same sample to be monitored after exposure to environmental conditioning and evolution of its internal structure. The high resolution and accuracy of image-based analysis provide detailed information on particle size, shape, distribution, and interconnectivity, along with the detection of anomalies or defects.  
Imaging techniques allow researchers to gain a deeper understanding of the microstructural characteristics and behaviour of asphalt mixtures. This knowledge is essential for the development of more effective mix designs and the identification of factors affecting mixtures' durability and performance. The potential for automation and standardization of image-based analysis enhances comparability and reproducibility across studies or laboratories. While imaging techniques should be used in conjunction with traditional tests, they offer valuable insights and advantages for initial screenings, quality control, and research in asphalt mixture analysis.  
This paper provides a review of the use of digital camera, scanning electron microscope (SEM), and X-ray computed tomography (CT) scanning in the analysis of the micro-structure of AC mixtures. The paper provides guidance for researchers and practitioners on the use of these imaging techniques to analysis AC mixtures, as well as selection of the appropriate methodology and analysis methods. The paper contributes to the advancement of knowledge and understanding in the field and fosters further innovation in asphalt mixtures analysis and design.