## 1 Introduction  
Asphalt mixtures are used in over 90% of highway construction in China. Currently, asphalt pavement generally has a short service life, and there is a gap in the design life. Aggregates account for more than 90% of the total mass of asphalt mixtures. Mineral aggregate, a vitally important road material, forms a good load-bearing capacity after gradation combination and sufficient compaction and can be used directly in the paving of the subgrade or subbase of road structures [1]. At the same time, multiphase composites are structurally characterized by a coarse aggregate skeleton and fine aggregate filling through binders such as asphalt and cement [2]. These materials have long dominated the pavement structure. Using aggregate properties to improve their mixes' mechanical properties and service life has been an important area of research in road engineering. Most of the existing mix design methods use macroscopic bulk indicators (bulk density, void ratio, effective asphalt content, etc.) and optimize the gradation composition by mechanical empirical methods [3]. However, the reliability and accuracy of their test results lack scientific and effective verification and evaluation methods. A mixture design method based on fine mechanics has been developed recently to break through the traditional empirical and continuum mechanics design concept [4]. The interaction mechanism of aggregates, mortar and other components from the micro-factors of materials was explored by mechanical means to reveal the inner law of the mechanical properties change of the mixture for a more excellent mixture structure design [5]. Currently, researchers in various countries have carried out macroscopic and fine-scale studies in mechanical property tests and pavement structural response of pavement materials such as asphalt mixes [6; 7] and cement concrete [8] and have successfully applied various mechanical analysis tools such as the analytical method [9], finite element method [10; 11], and discrete element method [12]--all of which can further demonstrate the feasibility of the mix design of materials. As early as the 1990s, researchers of the SHRP program in the United States realized that micro-factors of materials had a huge influence on the mechanical properties of the mixes and qualified the geometric characteristics of the aggregates (pinch content, angularity, fracture surface, etc.) [13]. In recent years, pavement workers have mostly analyzed aggregate shapes through two-dimensional digital images, yet their properties' complexity and variability are not limited to flat surfaces. The anisotropy of physical mechanics should also consider the variability of three-dimensional morphology [14]. With the introduction of X-ray computed tomography (X-ray CT) scanning technology, the computer performance and digital modeling of pavement engineering tend to mature, providing an effective way to accurately describe aggregates' properties and structural characteristics and their mixes. Therefore, this paper discusses the current research progress and main technical challenges in aggregate visualization by integrating existing research and related results. This paper reviews the techniques and numerical simulations for visualizing the morphological characteristics of road aggregates, especially asphalt mixtures. The development status of digital image acquisition equipment is summarized and introduced, including dynamic digital imaging techniques, static digital imaging techniques, and the influence of morphological characteristics of road aggregate on its fine structure and properties. The progress of visualizing the morphological characteristics of road aggregate is discussed. In addition, the evaluation methods and evaluation indexes from the shape, angularity and texture of aggregates are summarized and generalized systematically. In terms of numerical model reconstruction of aggregates, the modeling procedures of real aggregate models and random generation models are introduced, respectively.