###### Abstract  
As a preventive maintenance treatment, the ultra-thin porous friction course (UPFC) has been widely recognized and used in road maintenance because of its excellent performance and cost effectiveness. The Marshall compaction method (MCM) has been adopted to design UPFC mixtures worldwide, particularly in China. However, there are few studies concerning the field compaction properties of MCM-designed UPFCs. The laboratory test results of this study from simulating on-site compaction showed that all UPFC specimens with thicknesses of less than 20 mm barely achieved the target compaction thickness, and all UPFC specimens with different thicknesses failed to meet the air void (AV) requirements of UPFC mixes designed using the MCM. According to the results of a virtual compaction test, and using the discrete element method, the strong force chains were strengthened as the UPFC thickness decreased inside the specimen, making it difficult to evenly diffuse and transfer inside the specimen and resulting in insufficient compaction of the UPFC. Furthermore, it was demonstrated that the MCM-designed UPFC specimens showed significant differences in the AV distributions along the vertical and lateral directions from those of the UPFC specimens that simulated field compaction. The UPFCs designed using the MCM had a poor correlation with field compaction. air void distribution; discrete element method; field compaction characteristics; ultra-thin porous friction course; X-ray computed tomography; compaction method +
Footnote †: journal: Medical Engineering, Tongji University, Shanghai 201804, China  
cold-mixed ultra-thin asphalt overlay was proposed [9]. Another study recommended a porous ultra-thin overlay (PUTO) technology [10], and its service performance was evaluated [11]. The compaction characteristics of UPFCs have also been the focus of researchers. It is well known that compared to ordinary asphalt layers, the thickness of a UPFC is only 1-2 cm, resulting in different compaction requirements, temperature, and other conditions in the process of construction [12]. Owing to its minimal thickness, heat is easily lost, leading to a rapid temperature drop rate during the paving process, which has an adverse effect on compaction [13]. Insufficient compaction leads to air void (AV) content in the UPFC over the design range, which impairs the moisture sensitivity, fatigue life, and road performance at different temperatures of the UPFC, making it prone to aggregate peeling, loosening, and other problems [14]. [PERSON] et al. studied the field compaction parameters of a UPFC using laboratory compaction tests based on the energy equivalent principle [13]. Similarly, a study by [PERSON] et al. investigated the influence of Marshall compaction efforts on the compaction characteristics of UPFCs [15]. It was found that the target mineral gradation and the traffic level are key factors in selecting the suitable compaction efforts of a UPFC mix. [PERSON] et al. focused on the AV characteristics in PUTO using X-ray CT technology [16]. The aggregate size and compaction thickness of the UPFC mix affected the particle flow during the compaction process, thereby affecting the AV characteristics and determining the compaction effect. Similarly, [PERSON] et al. investigated the influence of densification on the road performance of UPFC mixes [17]. Density control of the UPFC during construction is suggested to ensure a balance between mix durability and functionality. Furthermore, the internal AV structures were analyzed between laboratory samples and UPFC field cores and it was found that significant differences existed [18]. According to ground-penetrating radar data, [PERSON] et al. studied the thickness and density in the field compaction process of UPFCs and proposed a corresponding prediction algorithm [19].