The segregation of mixture includes gradation segregation and temperature segregation. Grading segregation refers to the phenomenon that large-size aggregate and small-size aggregate gather separately under the action of external force. The inertia of aggregate in the mixture varies with the particle size. When the mixture is subjected to external force (machinery), the movement rate of large-size aggregate is necessarily higher than that of small-size aggregate, so the separation of large-size aggregate and small-size aggregate produces segregation. Temperature segregation refers to the phenomenon that the temperature of asphalt mixture with uniform temperature varies greatly after a certain process when paving concrete surface.

(A) the generation of vertical segregation grading segregation

A large number of tests and paving practices show that the longitudinal segregation phenomenon is caused by the structure of spiral distributor. Because the materials laid by the paver are transported from the middle to both sides, there must be a sprocket box driven by the middle and a plurality of extension brackets at both ends in the open space of the spiral feeding trough due to the structural needs. These supports cause the spiral discontinuity, and at the same time, the material flow increases the resistance, and the material flow forms the separation of large particle materials and small particle materials in this space, resulting in multiple segregation.

(B) Graded segregation horizontal segregation

The occurrence of lateral segregation has little to do with the paver itself, but is mainly caused by the operation method. In a working cycle of paver, when materials are stored in the silo of the mixture mixing station, a conical pile is formed, which causes segregation in the silo for the first time, and when the unloader of the mixing station unloads the materials onto the truck, it also causes secondary segregation in the hopper of the big truck. When the truck unloads the material into the hopper of the paver, the conical pile is formed again, and segregation is formed again in the hopper of the paver. After a working cycle is completed, the unloading truck leaves and the paver puts the hopper away. At this time, all the large particles left at the end are sent to the spiral distributor, forming one lateral isolation zone after another.

(C) the generation of temperature segregation

The transportation and unloading process of dump truck is the main reason for temperature segregation of asphalt mixture. When the dump truck transports materials to the paver, the asphalt mixture will exchange heat with the air through the carriage, and the temperature of the mixture near the carriage wall and the top floor will decrease. Because of the low thermal conductivity, the speed of heat transfer from the center of the mixture pile to the surrounding is quite slow, so when the truck arrives at the paving site, the temperature of the materials in the four walls of the carriage will be lower than that in the middle of the materials, resulting in temperature difference. In the process of unloading, the materials in the middle of the carriage are first unloaded into the hopper of the paver because of their high temperature and low viscosity, while the materials close to the carriage wall always end at both sides and the top of the hopper. The materials with high temperature are laid on the base first, and the mixture with low temperature is delayed until the last paving. This process further aggravates the temperature inhomogeneity of asphalt mixture, and the temperature segregation is more serious. This segregation phenomenon will occur during the transportation and unloading of each vehicle.

Second, control segregation.

Although segregation is inevitable during paving, we have taken some measures to effectively control segregation and reduce its occurrence. Asphalt concrete pavement must be constructed in strict accordance with technical standards and specifications from the aspects of mixture design, aggregate processing, accumulation, mixing, transportation and paving.

(1) In the actual paving process, it is mainly controlled by three ways: one is to choose a high-performance paver to keep the scraper conveyor and the spiral distributor working stably and continuously, so that the mixture is always evenly distributed along the width direction of the screed to minimize the segregation of the mixture; Secondly, by adjusting the ultrasonic material level device, the material submerges more than 2/3 of the spiral blade, so that the whole material moves from the middle to both sides instead of tumbling constantly, thus avoiding the separation of large and small materials; Thirdly, the gap between spiral guide plates is randomly and moderately adjusted, the spiral is adjusted in multiple stages, and reverse blades are modified at both ends of the spiral distributor, so that the mixture forms secondary mixing at both ends during distribution, which reduces the accumulation of coarse and fine particles and effectively inhibits segregation.

(2) controlling lateral segregation

The most effective measures to solve the phenomenon of lateral segregation are: first, the mixture should be transported by large-tonnage trucks, which will form less segregation; Second, the dump truck should be moved when loading, and the material segregation should be reduced according to the loading order of the front, back and middle of the carriage; Third, when unloading from the storage bin, don't unload all the materials in the bin every time, because the storage bin always forms concentric segregation when loading, and most of the last material in the unloading process is coarse aggregate; Fourth, the dump truck should discharge quickly, so that the whole material can be discharged downward to reduce the segregation of the material; Fifth, when each truck has finished unloading, don't scatter materials in the spiral chute, put away the hopper, send the remaining materials to the scraper conveyor, put down the hopper, and unload the new materials into the paver in time, so that the scraper and the spiral conveyor can fully mix the old and new materials, effectively restraining the segregation phenomenon.

(3) Control of temperature segregation

At present, there are two main methods to control temperature segregation: one is to reduce or eliminate temperature segregation by re-stirring screw mechanism, such as designing a variable diameter and pitch screw conveyor, and the mixture can only enter paving when the pitch is wide; When the pitch is narrowed, the mixture is remixed. In this process, cold coarse materials and hot fine materials are completely mixed again, eliminating temperature segregation and gradation segregation during transportation. The second is to use a new type of construction machinery called "asphalt mixture transfer vehicle", whose main function is to stir the asphalt mixture again before paving. At the same time, the collision between truck and paver is avoided, and the gradation segregation and temperature segregation produced during transportation are eliminated.

(4) Other measures to control segregation.

The production capacity of the mixing station should match the paving capacity of the paver, otherwise it will cause intermittent operation of the paver and segregation of the mixture is inevitable.

Strictly control the maximum particle size and gradation of the mixture. The larger the particle size in aggregate, the greater the moment of inertia, and the easier it is for the mixture to separate between large and small particles during mixing, transportation and paving. Gradation is out of proportion, and large and small particles are easier to separate. Therefore, the content of maximum particle size should be strictly controlled in the crushing, screening and grading of mixture to ensure the accuracy of grading.

The segregation of mixture leads to the deterioration of mechanical properties of pavement and the appearance of bad structure, thus reducing the service life of pavement. At present, the construction technology, material gradation segregation and temperature segregation cannot be eliminated, so it is the key to ensure the smoothness and quality of expressway pavement to take various measures to control and reduce the segregation of mixture.

The above are collected and sorted by Zhong Da Consulting Company.

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