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Generation and Control of Aggregation in Asphalt Pavement Paving Author: Sun Jian, Jiangxi Provincial High-Grade Highway Administration Abstract: The causes and consequences of grading aggregation and temperature aggregation in asphalt mixtures are analyzed and discussed. It is pointed out that improper design of the paving machine and construction methods are the main reasons for grading aggregation, while the transportation process using dump trucks is the primary cause of temperature aggregation; corresponding control measures are proposed for each issue. Keywords: paving, segregation, generation, control. 1 Introduction: As demands for the smoothness of highways and driving comfort increase, the task of laying stabilizing materials, which was previously carried out using graders and road mixers starting from the subbase layer, is now done by pavers. This approach ensures the highest level of road smoothness and quality. Due to the widespread use of pavers, controlling segregation that occurs during the paving process has become a very important issue. Based on years of experience in construction, I would like to share some insights on the occurrence and control of segregation. 2 Formation of segregation during paving. Segregation in mixtures includes gradation segregation and temperature segregation. Gradation segregation refers to the phenomenon in which, under external forces, larger and smaller particles of aggregates gather separately. In a mixture, the inertial mass of aggregates varies depending on their particle size. When the mixture is subjected to external forces (mechanical forces), the larger particles move at a faster rate than the smaller particles, which leads to the separation of these particles of different sizes and thus to segregation. Temperature segregation refers to the phenomenon where, during the paving of asphalt concrete surfaces, the asphalt mixture, which has been thoroughly mixed and has a uniform temperature, develops significant temperature differences after going through a certain process. 2.1 Generation of longitudinal segregation due to gradation segregation: Numerous tests and paving practices have shown that longitudinal segregation is caused by the structure of the spiral distributor, as shown in Figure 1. Since the material laid by the paver is conveyed from the center to the sides, in this open space of the spiral feed channel, a centrally driven sprocket box is necessary due to structural requirements; moreover, multiple extended supports are required at both ends. These supports cause discontinuities in the spiral shape, and the flow of material increases resistance. Within this space, larger and smaller particles of material separate from each other, resulting in multiple zones of segregation. The occurrence of lateral segregation in grade segregation of level 2.2 is not closely related to the characteristics of the paver itself; it is mainly caused by the methods of operation, as shown in Figure 2. Figure 2: During one working cycle of the paver, a conical pile is formed when material is stored in the mixer station’s storage bin, which causes segregation within that bin. When the mixer station’s unloader discharges the material into the truck, another conical pile is formed, leading to segregation in the truck’s hopper. Once the truck dispenses the material into the paver’s hopper, yet another conical pile is created, resulting in further segregation within the paver’s hopper. After one working cycle is completed, the delivery truck leaves and the paver retracts its hopper; at this point, all the larger-sized materials remaining are sent to the spiral distributor, thereby forming one after another horizontal band of segregation. 2.3 Generation of temperature segregation The transportation and unloading processes of dump trucks are the main causes of temperature segregation in asphalt mixtures. While the dump truck is transporting the material to the paver, the asphalt mixture exchanges heat with the air through the walls of the container. Undoubtedly, the temperature of the mixture near the walls and the top of the container drops significantly. Due to its low thermal conductivity, heat spreads from the center of the mixture to the surrounding areas at a relatively slow pace. Therefore, when the truck arrives at the paving site, the temperature of the material on the four walls of the cargo compartment will be **lower than the temperature of the material in the middle, resulting in a temperature difference. During unloading, the material in the middle of the truck bed, due to its high temperature and low viscosity, is unloaded first into the spreading hopper, while the material near the walls of the truck bed ends up on the sides and top of the hopper last. The material at high temperature is the first to be spread on the base layer, whereas the material at lower temperature is spread later; this process further increases the unevenness of the temperature within the asphalt mixture, leading to more severe temperature segregation. This phenomenon of segregation occurs with each shipment and unloading. 3 Control of segregation: Although segregation is inevitable during the paving process, we can take certain measures to effectively control it and reduce the occurrence of separation. Asphalt concrete pavements must be constructed by paying close attention to every stage, from mix design, aggregate processing and storage, mix mixing, transportation, to paving, and all work must be carried out in strict accordance with technical standards and specifications. 3.1 Control of longitudinal segregation: During the actual paving process, it is controlled mainly through three methods. First, high-performance pavers are used to ensure that the scraper feeders and spiral distributors operate steadily and continuously, so that the mixture is evenly distributed across the width of the screed, minimizing segregation within the mixture. Second, by adjusting the ultrasonic level sensor, the material is kept submerged more than 2/3 of the way up the spiral blades, allowing the material to move from the center toward the edges in a unified manner rather than rolling around, which prevents separation between larger and smaller particles. Third, the gap of the spiral guide plate is adjusted appropriately in a random manner, along with multiple adjustments to the spiral itself; reverse blades are also installed at both ends of the spiral distributor, enabling secondary mixing of the mixture at those ends during distribution. This helps reduce the accumulation of coarse and fine particles, thereby effectively suppressing segregation. 3.2 Control of lateral segregation The most effective measures to address lateral segregation are as follows: First, it is advisable to use large-tonnage trucks for transporting the mixture, as these trucks result in less segregation. Second, when loading, the dump truck should move while loading, following a sequential order of filling the front, back, and middle parts of the truck, thereby reducing segregation. Third, when unloading from the storage bin, it is not necessary to empty the bin completely each time; since concentric circular segregation always occurs during loading, the last portion of material unloaded usually consists mainly of coarse aggregates. Fourth, the dump truck should unload quickly, allowing the entire mass of material to fall downward to minimize segregation. Fifth, after each truck has finished unloading, the material in the screw conveyor should not be completely emptied; the hopper should be retracted, the remaining material sent to the scraper conveyor, and then the hopper lowered so that new material can be promptly fed into the paver. This ensures thorough mixing of old and new material during transportation by the scraper and screw conveyors, effectively suppressing segregation. 3.3 Control of temperature segregation Currently, two main methods are used to control temperature segregation. First, a heavy stirring screw mechanism is used to reduce or eliminate temperature segregation. If designed as a screw conveyor with variable diameter and pitch, the mixture can enter the paving area only when the pitch is wide; when the pitch is narrow, the mixture is remixed, and this process thoroughly recombines the cold coarse material with the heated finished material, eliminating temperature segregation and gradation segregation that occur during transportation. Secondly, a new construction machine called an “asphalt mixture transporter” is used, whose main function is to remix the asphalt mixture before it is laid. At the same time, it prevents the material transport vehicles from colliding with the paver. Eliminate gradation segregation and temperature segregation that occur during transportation. 3.4 Other measures to control segregation: (1) The production capacity of the mixing plant should be matched to the paving capacity of the paver; otherwise, the paver will operate intermittently, and segregation of the mixture will become inevitable. (2) Strictly control the maximum particle size and gradation of the mixture. The larger the particle size in the aggregates, the greater their inertial mass; as a result, during mixing, transportation, and paving, it becomes easier for large and small particles to separate from each other, leading to an unproportional grading. This increases the likelihood of segregation between large and small particles. Therefore, it is essential to strictly control the content of the largest particle sizes during the crushing, screening, and grading of aggregates, in order to ensure accurate grading. (3) For wide-width operations (width greater than 7m), the method of using two machines in parallel for trapezoidal joint heat welding is adopted. 4 Conclusion: The segregation of the mixture leads to a deterioration in the mechanical properties of the paved layer and the formation of undesirable structures, thereby reducing the service life of the road surface. With the current construction methods, both grading segregation and temperature segregation of materials cannot be eliminated. Therefore, taking various measures to control and reduce the segregation of the mixture is key to ensuring the smoothness and quality of highway pavements. References: He Tingji et al. Handbook of Road Construction Machinery. Beijing: People’s Transport Publishing House, 1998. Li Baoyun, Zhang Decen. Several Difficulties in the Use of Multi-functional Pavers. Road Construction Machinery and Construction Mechanization, 2001, No. 5. Shen Jinan. Issues Related to the Uniformity and Segregation of Mixtures. Highway Traffic Science and Technology, 2001, No. 12