The recycling of asphalt pavement can save a lot of raw materials, such as asphalt, sand and gravel. At the same time, it is beneficial to waste treatment and environmental protection. According to the development and application of asphalt recycling technology at home and abroad, this paper introduces several common methods of recycling construction of old asphalt pavement, focusing on the characteristics and construction technology of on-site hot recycling technology which is widely used at present. Key words: highway asphalt pavement recycling technology Recycling asphalt pavement is a set of technology that mixes the old asphalt pavement with recycling agent, new asphalt material and new aggregate in a certain proportion after excavation, recycling, crushing and screening, so that it can meet certain road performance and use it to resurface the pavement. At present, China's highway construction is developing rapidly, and the annual investment scale has exceeded 200 billion yuan. After the 1990s, expressway, which was built one after another, entered the period of large and medium-sized repair, and a large amount of asphalt mixture was abandoned, which on the one hand caused environmental pollution, on the other hand, it was also a waste of resources for China, which was extremely short of high-quality asphalt. Moreover, a large number of new stones are used, and mining stone mines will lead to serious ecological environment damage such as forest vegetation reduction and soil erosion. According to the design life of asphalt (15-20 years), from now on, 12% of asphalt pavement needs to be renovated every year, and the waste of old asphalt will reach 2.2 million tons every year. If it can be used, it can save 350 million yuan in material cost every year, and this figure is increasing at the rate of 15% every year. After 10 year, the old asphalt mixture produced by the asphalt pavement maintenance will reach100000 tons, and the annual recycling can save the material cost at that time1500 million yuan. Otherwise, these huge asphalt concrete layers can only be discarded in vain after excavation, which not only wastes resources, but also causes serious pollution to the environment. Therefore, the research and popularization of asphalt recycling technology and the development of related special equipment are of great significance to reducing project cost, protecting ecological environment and highway construction in China. With the increasing maintenance of high-grade asphalt pavement in China, it is necessary to strengthen the theoretical research of asphalt pavement recycling technology, develop suitable recycling agents and mechanical equipment, and lay a foundation for the wide application of old material recycling in practical projects. 1 overview of research at home and abroad. The research on asphalt pavement recycling abroad first started from the United States 19 15, but the research on this technology was neglected due to the large-scale construction of expressway in the future. 1973 After the oil crisis broke out, the United States began to pay attention to this technology and conducted extensive research nationwide. By the end of 1980s, the amount of recycled asphalt mixture in the United States accounted for almost half of all road asphalt mixtures, and the development of recycled agent, the design of recycled mixture and the research of construction equipment were also deepening. The recycling of asphalt pavement is a common practice in the United States, and its recovery rate is as high as 80% at present. Western European countries also attach great importance to this technology. The Federal Republic of Germany is the first country to apply recycled materials to pavement maintenance in expressway. 1978 All the waste asphalt pavement materials were recovered. Almost all towns in Finland organize the collection and storage of old pavement materials. French also began to popularize and apply this technology in pavement restoration projects of expressway and some heavy traffic roads. 2 Performance of old asphalt pavement materials Asphalt concrete pavement adopts asphalt material with strong adhesion as binder, which greatly enhances the adhesion between mineral aggregate, improves the strength and stability of mixture, and improves the service quality and durability of pavement. Asphalt concrete pavement has the advantages of smooth surface, no water seepage, comfortable driving and low noise, so it has been widely used. However, it is often influenced by weather, temperature, driving and materials, and the reason of pavement structure design, which inevitably produces various diseases, and these diseases have adverse effects on driving speed, pavement service life, passenger comfort and traffic safety. The aging of asphalt pavement is mainly the aging of asphalt and the refinement of aggregate. Under the wheel load, asphalt pavement bears compressive stress, shear stress and tensile stress. At the same time, asphalt pavement exposed to nature for a long time will be affected by various natural factors such as oxygen, sunshine, temperature, water and wind, which will cause physical and chemical changes in the properties of asphalt and aggregate in the mixture, and finally show the decline in the quality of asphalt mixture. The damage of asphalt concrete pavement can be divided into two types: one is structural damage, including the whole or partial damage of pavement structure, which makes the pavement unable to bear the predetermined load; The other is functional damage, which may not be accompanied by structural damage, but it no longer has the predetermined function due to the decline of ride comfort and anti-skid performance, thus affecting the driving quality. 3 asphalt pavement recycling construction method The construction of recycled asphalt pavement can be divided into hot construction and cold construction according to temperature. According to the construction technology, hot construction can be divided into on-site hot recycling method and factory-mixed hot recycling method. 3. 1 On-site cold recycling method On-site cold recycling method is to mill and crush the pavement mixture on the original pavement with a high-power pavement milling mixer, then add stabilizer, cement, water (or emulsified asphalt) and aggregate for on-site mixing at the same time, then mix it on-site with a pavement mixer, and finally roll it into shape. This method is mainly used in cold construction, and the newly added binder is emulsified asphalt. This method has low requirements for facilities and low production cost, but at the same time the quality of recycled pavement is not very good. At present, this method is rarely used, mainly for low-grade roads or paving grass roots, and is mostly used for the reconstruction of rural roads abroad. 3.2 In-place hot recycling method In-place hot recycling is the process of repairing damaged pavement in place. It softens the pavement by heating, scoops up the pavement garbage, mixes it with asphalt binder, and sometimes needs to add some new aggregates. Then the recycled materials are re-paved on the original road surface. Generally, a large-scale "asphalt pavement thermal regeneration combined unit" is used to bake the asphalt pavement to soften it, and then the old asphalt layer is collected and transported to the double horizontal shaft continuous mixer in the unit, and new aggregate and new asphalt are added, and then the mixture is discharged to the paver of the unit for paving, tamping and ironing, and then rolled by a roller to pave a new pavement. On-site thermal regeneration can be completed by a single operation, combining raw materials with the pavement to be repaired. Or it can be done in two stages, that is, the recycled material is compacted again first, and then a wear-resistant layer is laid on it. This method is simple and convenient to construct, and it is mostly used in road sections with good bearing capacity of base and cracked surface due to fatigue, especially for roads with less aging but poor flatness. 3.3 Factory Hot Recycling Method Factory hot recycling method is a practical, flexible, simple and quality-guaranteed asphalt pavement recycling technology, which centrally treats recycled asphalt mixture in the factory. The factory hot recycling method is to dig the old asphalt pavement and transport it back to the mixing station, and then concentrate on rolling it. According to the quality requirements of different grades of pavement, mix design is carried out to determine the addition ratio of old asphalt mixture, and the recycled agent, new asphalt material and new aggregate are re-mixed into new mixture in a blender according to a certain proportion, so as to obtain excellent recycled asphalt concrete and pave recycled asphalt pavement. By using this method, it is convenient and effective to reinforce the excavation base and even some sections of subgrade. The asphalt layer can be resurfaced according to the different technical requirements of the lower layer, the middle layer and the upper layer (wear layer), and the addition ratio of the old asphalt recycled material can be determined, just like the new road construction. 4 Characteristics of Hot-in-Place Recycling Among the above methods, the hot-in-place recycling technology of asphalt pavement is a high-grade technology with international level. It is economical, efficient, rapid, environmentally friendly and economical to use continuous operations such as on-site heating, scarifying, stirring, paving and compacting to form a new pavement at one time, which has remarkable economic and social benefits. This maintenance method can be adopted when cracks, flooding, abrasion, rutting, pits and other diseases appear on the surface of asphalt pavement or the road performance declines, and the damage degree of pavement has not yet spread to the base. Advanced on-site hot recycling units can be used to heat the old pavement on site, rake up the old materials, add appropriate new asphalt mixture and recycling agent for in-machine hot mixing, and then pave, iron and roll, which can quickly open the traffic. According to the data provided by Ying Da Company, compared with the traditional method, its "Road King" on-site hot regeneration repair method can save 5/6 of the repair time,12 of the workers and12 of the new asphalt mixture. Therefore, the in-place hot recycling method of asphalt pavement has been widely used in asphalt pavement maintenance in recent years. The main features of asphalt pavement on-site thermal regeneration method are: (1) All projects that are directly resurfaced or filled after milling can be regenerated by thermal regeneration method, and the old pavement mixture can be regenerated on-site, without garbage disposal technology and garbage dump. (2) Aggregate can be kept intact, the composition and performance of asphalt can be preserved, and old materials can be used 100%. The traditional factory recycling method can only use 40-50% old materials, and the new equipment can produce higher quality asphalt, which has the same life cycle as the new asphalt mixture. (3) It is not limited by the large traffic flow, and compared with the previous maintenance methods, it has less influence on the traffic and residents along the way, and can be opened to traffic after the construction. (4) The vibration and noise generated by construction are less than other construction methods, which is beneficial to environmental protection. (5) This maintenance method takes the pavement surface as the construction object, and is suitable for the pavement with good bearing capacity at the grass-roots level, which is cracked, rutted and damaged due to surface fatigue. When the damage spreads below the grassroots, it is not applicable in principle, and the grassroots must be treated first. In-situ thermal regeneration usually cannot repair structural damage. (6) Hot-in-place recycling can't repair the expansion cracks at a deeper position below the asphalt layer, but it can reach the position with the maximum depth of 50mm, and in some cases it can reach a deeper recycling depth. (7) Before the on-site hot recycling method is implemented, a large number of cold mix patches and spray patches must be removed from the pavement. (8) This method is to heat the old pavement on the pavement, which is easily affected by special temperature, so it is generally not suitable for construction in cold season, and the efficiency will be reduced in cold weather and rainy days. 5 Practical application of on-site thermal regeneration method The main equipment of on-site thermal regeneration construction is the thermal regeneration unit, and the key part is the heating plate, which provides efficient radiant heat energy and heats the old pavement to a certain depth in a short time; Do not overheat, so the asphalt will age and lose the meaning of regeneration. The equipment that can continuously regenerate the whole lane in situ generally has the functions of pavement preheating, hot milling, adding new materials, mixing and paving. In addition, according to the size of the repair area, it is best for the heating plate to have partition function. ROADMIXKRM2000RS field recycling machine purchased by Jiangsu Jinghu Company can be used for single-layer recycling and double-layer recycling. Equipped with two road surface preheaters, the surface temperature of the road surface can reach 160℃ in front of the milling drum and 50℃ 60mm below the road surface. Single-layer recycling operation refers to that new materials and hot milled old materials are fully stirred in the mixer, and then enter the storage bin between the mixer and the screed, and a new pavement is formed by paving and vibrating the screed, and finally the compaction is completed by a vibratory roller or a hydrostatic roller. The double-layer recycling operation is to recycle the mixture into a single layer as the bottom layer, and then spread new materials through the second screed to form a 1-2 cm wear layer. Because the bottom layer and wear layer are both hot paved and vibrated by in-situ recycling machinery, and then finally formed by compaction machinery, the repair quality and smoothness of pavement can be well controlled. After a certain period of service in a provincial highway in Huai 'an City, Jiangsu Province, there are many small longitudinal and transverse cracks on the pavement. After the on-site thermal regeneration treatment of the upper asphalt, the pavement basically reached the required performance index and achieved good results. From June to August, 2003, the asphalt pavement in Shanghai section of Huning expressway was repaired by thermal regeneration in a large area, covering an area of 200,000 square meters. The regeneration equipment is Remixer4500 thermal regeneration unit produced by Wirtgen Company, and it can also carry out various thermal regeneration constructions such as shaping and finishing, remixing and remixing finishing on the old asphalt pavement. According to the test, the performance index of recycled asphalt in this project fully meets the relevant provisions of the current industry standards. On this basis, the high-temperature stability of hot recycled asphalt concrete has been greatly improved, which verifies the relevant foreign data. The BA 10000F mobile asphalt recycling equipment used in Changzhou City, Jiangsu Province is mainly designed for common diseases in daily maintenance of asphalt pavement. It weighs only 5 tons and is easy to move. When in use, the crushed asphalt concrete is added into the hopper and sent to the drum together with additives for stirring and heating. After a heating period of 15 minutes, the recycled hot mixture can be obtained, and the operation can be completed by using ordinary paving and compaction processes, which not only saves money. According to a great deal of application experience, the thickness, type, traffic load, prophase maintenance, existing conditions and ambient temperature of the pavement must be considered when the in-place hot recycling method is adopted. Among them, the uniform and full integration of new asphalt, recycling agent and old mixture is the key issue, and mixing machinery and equipment should be fully considered in the design and construction process. Conclusion To sum up, asphalt recycling technology can make full use of old materials, and by choosing the appropriate ratio of old and new materials and mixing ratio, recycled mixture with quite good quality can be regenerated. Preliminary research and practice show that this recycled mixture can be used for asphalt pavement of high-grade highway. At the same time, considerable economic and social benefits can be obtained by using asphalt recycling technology. References [1] Lv Weimin, Yan Jiaji, Asphalt Pavement Recycling Technology, People's Communications Press, 1989 [2] Gui Xiheng, Xu Xiaorong, Huang Xiu, Recycling of Waste Asphalt, China Highway, 2003, 1 1 [3]