Discussion on Temperature Control of Asphalt Mixing Equipment in the Production of Drainage Asphalt Mixture

Abstract: In view of the characteristics of the asphalt mixture used in the drainage asphalt pavement, the causes of temperature control deviations during the production process of the asphalt mixture were analyzed. The temperature control requirements and setting principles were established. Based on these principles, the temperature control specifications for asphalt, aggregate and drainage asphalt mixture were given. The production of drainage asphalt mixture was adjusted and optimized in line with the performance of the equipment, achieving the goal of accurately controlling the temperature.

Key words: Mixing equipment, drainage, asphalt mixture, temperature control

Drainage asphalt pavement is a new type of pavement that has developed in developed countries since the 1980s. It uses drainage asphalt mixture with a large void ratio (generally between 17% and 22%) as the upper layer of the pavement (with an impermeable layer laid beneath it). Due to the porous feature of this surface layer structure, the accumulated water on the road surface can seep directly into the road surface structure and be quickly discharged without passing through drainage facilities. This reduces the occurrence of hazards such as water splashing and spraying. Moreover, due to the skeleton pore structure of the drainage asphalt road surface, the friction coefficient of the road surface and the road surface's resistance to rutting are increased, further enhancing driving safety.

Since the drainage asphalt pavement highlights its drainage function, the most important quality inspection and evaluation index for its drainage asphalt mixture is the void ratio. Under the given production mix ratio, temperature has the greatest impact on void ratio: If the temperature of the drainage mixture produced by the asphalt mixing equipment is too high, it is easy for the asphalt within the material to flow, forming lumps and blocking the void passage. When the temperature is low, on-site construction work becomes extremely difficult. It can be said that in the construction of drainage asphalt pavement, the production of asphalt mixture in the asphalt mixing plant is the key, and the temperature control of asphalt mixture is the core and difficulty of the entire production operation. Therefore, the production of drainage mixtures must be carried out under strict temperature control requirements.

Performance Analysis of Asphalt Mixing Equipment and Principles of Temperature Control

1.1 Performance analysis of equipment and pavement materials

The production capacity of drainage asphalt pavement mixture is reduced by approximately 60% compared to that of dense-graded asphalt mixture. The main reasons for the reduction are as follows:

(1) The drainage asphalt mixture has a large amount of single-particle coarse aggregates, and the waiting time of the asphalt mixing equipment and the storage capacity of the hot material silo increase accordingly.

(2) To prevent flowing, the set temperature for the production of drainage asphalt mixture is lower than the mixing temperature usually obtained by using the viscosity-temperature curve. To achieve uniform coating of aggregates, the mixing time is prolonged.

(3) The use of fibrous materials as additives increases production time due to material measurement, input, etc.

Given the characteristic that the production capacity of asphalt mixing equipment is lower than that for other asphalt mixtures when manufacturing asphalt drainage pavement, in order to produce asphalt mixtures that meet quality requirements, it is necessary to make necessary adjustments to the equipment. By adjusting the set parameters of the asphalt mixing equipment, it can be made more suitable for the production of drainage asphalt mixtures, and efforts should be made to reduce consumption. Improve the operational efficiency of equipment and enhance production efficiency.

1.2 Temperature setting Principles for the production of drainage asphalt Mixtures

On-site modification is completed in the stirring device of the equipment using granular fiber additives. In the case of forming high-viscosity modified asphalt mixtures, the temperature setting principles of the temperature control system for asphalt mixing equipment:

(1) The temperature for heating and drying aggregates in front of the aggregate elevator should be 185±5℃, so as to ensure that the standard temperature of the mixed material does not exceed the range of 180±5℃.

(2) Ensure the operation and construction and take into account the inferiority of asphalt (the temperature of asphalt mixture at the construction site is >160℃);

(3) According to the external environmental conditions, adjust the temperature of the mixture appropriately within the permitted range (when the ambient temperature is less than 10℃, the temperature standard is set at 185±5℃).

The production of drainage asphalt mixture by asphalt mixing equipment is based on appropriate temperature control and quality management. The control system of the asphalt mixing equipment adopts a computer as the PLG universal automatic control device. The temperature control is achieved by comparing the sensor with the set value and providing feedback, which can fully automatically realize temperature control.

2. Asphalt temperature control

Asphalt temperature control mainly includes two aspects: base asphalt temperature control and on-site modified high-viscosity modified asphalt temperature control.

The storage temperature of base asphalt is mainly determined by its physical and chemical performance indicators. The temperature of the asphalt storage tank is indirectly obtained by heating the heat transfer oil in the combustion furnace. The storage temperature generally varies depending on the physical properties of the base asphalt. An appropriate storage temperature can ensure that the performance of the asphalt is not affected during long-term storage. Usually, the storage temperature for base asphalt is set within the range of 110-130℃. When the mixing equipment is in normal operation, the asphalt temperature should reach the operating temperature before production is allowed. The operating temperature is set according to the viscosity curve of the base asphalt and obtained by the automatic control system of the heating temperature of the combustion furnace. The general operating temperature range for base asphalt is 150 to 170 degrees Celsius.

Since high-viscosity modified asphalt is formed by the base asphalt entering the mixing pot at the service temperature and simultaneously mixing with high-temperature aggregates and fiber additives, and taking advantage of the high temperature of the hot aggregates on-site, the temperature control of high-viscosity modified asphalt is mainly determined by the service temperature of the base asphalt and the temperature of the hot aggregates. Its temperature range is 160 to 175 degrees Celsius.

The temperature control of asphalt in the production of drainage asphalt mixture is not much different from that in the production of ordinary asphalt mixture. The heat transfer oil heating and oil temperature control system of the asphalt mixing equipment can conveniently achieve the automatic adjustment of asphalt temperature.

3. Temperature control of aggregates

3.1 Aggregate "Curtain Theory"

Compared with the production of ordinary dense mixtures, the aggregates of the drainage mixture are discontinuous, and the formed aggregate curtain is also not dense. The asphalt mixing equipment is equipped with an infrared temperature sensing and automatic control device at the outlet of the dryer. Thus, when encountering a broken gradation aggregate curtain, the temperature displayed by the automatic temperature control system will deviate from the actual aggregate temperature, and the temperature sensing control system will be unable to measure the true aggregate temperature.

For this reason, under normal circumstances, when asphalt mixing equipment is producing drainage asphalt mixture, excessive fine aggregates are added. Besides consuming the relatively excess drying capacity, the main reason is to build a dense material curtain by adding fine aggregates, so that the automatic temperature control system of the asphalt mixing equipment can truly reflect the temperature of the aggregates after heating.

Traditional fine fillers act as cooling agents, increasing the feed rate of materials, absorbing excess heat energy, and the excess materials overflow from the overflow pipe, resulting in waste of fuel and materials. Even if the materials can be reused, their properties will change when they are heated.

(2) It can be known from the research during the production process that when the particle size of the aggregate increases and the mixture becomes coarser, the temperature displayed by the infrared temperature sensing system tends to be larger than the actual measured material temperature. When no fine materials are added, the infrared sensing temperature differs from the actual material temperature by approximately 10 to 20 degrees Celsius compared with ordinary mixtures.

Therefore, it is possible to grasp the variation relationship between the particle size of aggregates and the display of the infrared temperature sensing system of the equipment based on this, and explore the realization of automatic and true measurement and control of hot aggregates by the equipment temperature control system without adding fine materials. When the various particle sizes of cold aggregates are relatively stable, the hot aggregates of the drainage asphalt mixture form a stable discontinuous material curtain. Thus, the relative temperature difference between the infrared sensing temperature and the actual material temperature can be utilized for correction, achieving automatic and accurate measurement and control. The temperature measurement and control error is no more than ±5℃, fully meeting the temperature difference control requirements for drainage asphalt mixtures.

(3) To achieve stable temperature control, in the production of drainage asphalt mixtures, it is also necessary to have a certain understanding of the dryness and wetness of raw materials. Increasing the moisture content of the material can reduce the drying capacity of the burner by raising the moisture content. However, there is a problem that it is more difficult to stabilize the temperature control when the moisture content is uneven.

The burner effectively adjusts the drying capacity to match the actual moisture content. The uniformity of moisture content has a significant impact on the drying capacity of aggregates. Every 1% reduction in moisture content. Production capacity has decreased by approximately 10%. Therefore, if the moisture content is uneven, it is easy to cause the heating temperature of the aggregates to be too high or too low, which is difficult to control. Furthermore, the uniformity of particle size of various aggregates has an impact. If the uniformity of bone particle size is unstable, it is particularly easy to form an unstable, discontinuous and intermittent batching curtain, making it more difficult for infrared temperature measurement to reflect the true temperature.

The proposal and application of the material curtain theory have overcome a series of defects caused by excessive fine aggregates, making it possible for asphalt mixing equipment to produce drainage asphalt mixtures. It has achieved accurate temperature control when draining asphalt mixtures under the condition of relatively stable aggregate particle size and little change in moisture content, which is a major breakthrough in temperature control of mixing equipment.

3.2 Technical transformation of the burner system

To effectively address the issue of matching the drying capacity with the production of drainage asphalt mixture and to further reduce the drying capacity of the original system, the burner system was modified and adjusted.

(1) Reduce the size of the fuel injector (from φ4 to φ3.5), decrease the fuel intake per unit time, and thereby lower its overall drying capacity.

(2) Adjust the fuel pressure and air intake volume to match the appropriate air-fuel ratio and improve the combustion performance;

(3) The parameters of the burner system were optimized to keep the pressure of the fuel system entering the fuel injection port at 25 to 30 mpa, reducing the fuel consumption to 4.5 to 5.0kg/t and improving the economic performance of the equipment.

Overall, the issue of matching the drying capacity of the asphalt mixing equipment with the reduced production capacity during the production of drainage asphalt mixtures has been resolved.

3.3 Optimization of the production cycle

In the PLC microcomputer automatic control system of the equipment, there are many internal parameters that affect the operation of the equipment, determining the stability of the equipment operation and the accuracy of the weighing system, and influencing the production cycle.

Optimize the internal parameters of the asphalt mixing equipment. Among them, the factors that have an impact on the cycle period are: the stabilization time of aggregate weighing, the stabilization time of powder weighing, the stabilization time of asphalt weighing, and the opening time of the mixing pot unloading, etc.

(2) Asphalt spraying method. The spraying type is more likely to evenly coat the aggregates than the flowing type, and the mixing time can be relatively shortened.

(3) Improve the way additives are conveyed. The method of directly adding granular fibers by feeding on the upper side of the mixing device is adopted. Cellulose is directly put into the mixing device by hand in a quantitative container at the same time as the coarse aggregate is discharged. The cellulose and mineral materials are dry-mixed for 15 to 18 seconds, and then asphalt is sprayed for mixing. The wet mixing time is 40 seconds, and the total mixing time is reduced from the original 64 seconds to 58 seconds, which can ensure that the stabilizer is fully opened and mixed evenly. It is more convenient, reliable and time-saving than the traditional forced air conveying method.

After reasonable optimization, the production cycle of each batch is relatively shortened, the feed volume is increased, the contradiction of relatively excessive drying capacity is improved, and the equipment operation is also more reliable and stable.

4. Temperature control of the mixture

To know the temperature of the finished mixture produced by the asphalt mixing equipment in a timely and accurate manner, an additional material temperature detection device and display instrument are set up at the finished material outlet of the mixing device of the equipment, making the feedback of the material temperature more timely, accurate and convenient.

The storage time of the drained asphalt finished mixture in the finished product storage silo should not be too long. Because if it is left for a long time, it is easy for the asphalt in the mixture to flow due to high temperature. In addition, the coarse temperature of the aggregate is easy to dissipate, which will affect the performance of the material and is also prone to adhesion to the silo wall. Therefore, in principle, the storage time of the drained asphalt finished mixture in the silo should not exceed two hours.

In cold seasons, when the temperature is below 5℃ or above 5℃ but the wind is strong and the asphalt mixing equipment cannot be used for production, a certain value higher than the normal situation should be adopted. To prevent the high-temperature flow and deterioration of asphalt, the set temperature should not exceed the necessary mixture temperature of 195℃.

The construction method directly determines the quality of the project. Temperature control is particularly important in the construction of drainage asphalt mixture. When general asphalt mixing equipment is used to produce drainage asphalt mixtures, in order to seek technical improvements and enhancements in the production of drainage asphalt mixtures, the production and construction process, changes in temperature control, relevant parameters for technical improvements, and data affecting temperature control should be recorded and organized. The record of the construction of drainage asphalt mixture is not only beneficial to the continuous improvement of the temperature control of production technology, but also provides valuable basic data for the standardization of temperature management in the construction of drainage asphalt mixture in the future.

5 Conclusion

Through the analysis in this article, the temperature control principles for the production of drainage asphalt mixture can be known. Based on these principles, the operation parameters of the equipment can be optimized. At the same time, it is determined which one or several factors affect the temperature control of the drainage asphalt mixture during its production and storage processes, and then corresponding temperature control measures can be taken on the asphalt mixing equipment in response to these factors. By reducing the production capacity of the equipment (to 40% to 70% of normal production); The control temperature of asphalt (with little difference when producing general asphalt mixtures); The control temperature of aggregates (not by adding excessive fine aggregates, but by using empirical formulas to achieve accurate temperature measurement); Renovate the combustion drying system to match the reduction in production capacity and lower the drying capacity of the equipment. A series of technical measures, such as temperature control of asphalt mixture, have provided a practical basis for achieving accurate temperature control. Of course, there are still some areas that need improvement in the production of asphalt mixtures. For instance, the law of temperature control deviation being affected by aggregate particle size still needs to be further summarized in production practice. The phenomenon of flame roasting on the inner wall of the drying cylinder is prone to occur after the transformation of the drying system, and the problem of accelerated wear caused by the long mixing time of aggregates in the mixing device. Therefore, the production of drainage asphalt mixtures by asphalt mixing equipment still needs to be coordinated with the further optimization of the equipment's structural performance.

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