Cold recycling technology can not only save resources, 100% use old asphalt mixture, but also prolong construction season, improve construction conditions and reduce environmental pollution. Compared with traditional methods, cold recycling can generally save about 40% ~ 50% of investment, and can also effectively solve the original pavement cracks, rutting and other diseases. Based on the international achievements, this paper discusses the structural methods of cold recycled asphalt pavement, which provides reference for the complete structural design of cold recycled asphalt pavement.
First, the cold recycling technology of asphalt pavement
1, Introduction of Cold Recycling Technology of Asphalt Pavement
Asphalt pavement recycling technology is a kind of pavement maintenance technology that properly treats the old asphalt pavement materials and adds a certain amount of cementing materials or recycling agents (some new aggregates can be added if necessary) to reuse the mixture. According to different processes, it can be divided into hot recycling technology and cold recycling technology. Thermal regeneration technology is used to restore the cohesiveness of aging asphalt, give full play to the role of binder and recover asphalt resources, so the only material that can be used for thermal regeneration is asphalt surface material.
Cold recycling technology mainly uses the original pavement material (as aggregate), so the cold recycling pavement material can be asphalt surface material or base material containing stable inorganic binder. Cold recycling at natural temperature, after asphalt pavement crushing, excavation, adding new materials, mixing, paving and compaction, re-forms the pavement structure layer, which is mainly used as the base or subbase of pavement, and can also be used as the surface layer of low-grade highway.
2. Research status of this technology.
The research on the recycling of waste asphalt pavement materials in the United States began in 19 15. However, due to the large-scale construction of new roads, the cost-performance ratio between recycled asphalt mixture and new asphalt mixture is quite different, and the key construction equipment can not meet the expectations, so this technology has not attracted enough attention.
Asphalt pavement regeneration in developed countries has formed a technical system. In the early days of China, old asphalt mixture was only used for light traffic roads, sidewalks or road mats. 1982 asphalt recycling test for more than 80 kilometers of large and medium-sized road projects in Shanxi Province. From 1983 to 1988, recycled asphalt pavement tests were carried out in Kunwan Road, Luo Kun Road and Guikun Road. 1998 10 Handan introduced WR2500, the world's most advanced recycling machine, and adopted on-site recycling technology to transform a section of road for the first time in China. In 2006, the cold recycling of asphalt pavement was used as the flexible foundation in the reconstruction and expansion of expressway in Huning, Jiangsu Province, and the effect was good. Jiangxi Changjiu expressway Maintenance Project will also use emulsified asphalt plant mixed cold recycling to regenerate the old asphalt pavement at the grass-roots level.
The success of recycling old asphalt pavement provides experience for recycling old asphalt pavement in China and lays the foundation for the next work. However, the research on asphalt pavement recycling technology in China is still in its infancy. Although it has been applied, it has not been widely promoted and the technology has not formed a system. There is no complete method for structural design of cold recycled asphalt pavement, and there is no relevant data and literature.
Second, the cold recycled asphalt pavement structure design
The strength of cold recycled asphalt mixture is gradually developed through the interaction between traffic load and nature, water evaporation and density increase. After pavement paving 1-2 years or more, the wear resistance of cold recycled asphalt mixture will decrease, leading to irreversible loose failure. Therefore, cold recycled asphalt mixture is generally not directly applied to pavement.
1, minimum surface thickness
The early wear resistance of cold recycled asphalt mixture is relatively low, and it is often loose and damaged due to traffic load and nature. Therefore, a high-quality surface layer is usually set on the structural layer of cold recycled asphalt mixture. However, how to determine the minimum thickness of the top layer of cold recycled asphalt mixture depends more on experience accumulation, which is difficult to solve fundamentally from theoretical calculation.
In the structural design method of cold recycled asphalt pavement, American Asphalt Association provides important reference data for the minimum thickness of the top layer of cold recycled asphalt mixture, as shown in Table 1. Among them, 80-kN ESAL is different from the current JTG D50 specification 100-kN BZZ- 100 in China, but it can be converted by an appropriate formula.
Note: a: 80-kn equivalent axial load; C: asphalt concrete or type I emulsified asphalt overlay treatment.
2. Mix proportion design of cold recycled asphalt mixture
The design operation of cold recycled asphalt mixture ratio includes: determining mixture gradation, testing raw materials, making and curing specimens, and determining the best asphalt-aggregate ratio and the best water content. Mix design is an important content of cold recycling of asphalt pavement, and it is the basis of determining the performance of cold recycling asphalt mixture through relevant indoor tests.
2.2. 1 raw material testing
Cold recycled asphalt pavement material is composed of crushed old asphalt pavement material, low viscosity asphalt, a small amount of new aggregate, a small amount of recycling agent, foamed asphalt or emulsified asphalt and a small amount of cement. Cement is usually a mixture of emulsified asphalt and crushed old asphalt pavement material at room temperature. The diversity of material composition determines that cold recycled mixture has complex heterogeneity. Therefore, it is necessary to test and analyze the raw materials of each component of cold recycled asphalt mixture to ensure that it meets various technical requirements.
2.2.2 Determine the gradation
American Asphalt Association divides cold recycled asphalt mixture into two categories: Grade A and Grade B. Grade A is the mixture whose aggregate gradation can meet the requirements of A, B, C and D in Table 2 (simple treatment, crushed stone rolling, natural storage yard or riverbank storage yard), and Grade B is the mixture whose aggregate gradation meets the requirements of E, F and G in Table 2 (sand or silt).
Table 2 Aggregate Gradation of Cold Recycled Mixture of American Asphalt Association
Sample forming method
For the laboratory test of emulsified asphalt cold recycled mixture specimen, please refer to JTJ 052 specification T 0702 "Method of Making Asphalt Mixture Specimen (Compaction Method)" and operate at room temperature (22 5℃). First, add a proper amount of water into the mineral aggregate and stir it evenly to make the aggregate surface completely wet; Then inject a proper amount of emulsified asphalt, and stir evenly within 65438±0min to make the mixture brown; Finally, the mixture is put into a test mold and compacted. It should be noted that the mixture should be fully compacted before demulsification of emulsified asphalt. After double-sided punching for 50 times, it should be placed horizontally for 24 hours with the test mold, and then double-sided punching for 25 times to demould. Finally, put the specimen in a ventilated oven at 60℃ for 72 hours, but pay attention to ensure that there is no water in it and it is completely dry.
2.2.4 Determine the optimum water content and asphalt-aggregate ratio.
The strength of the final recycled asphalt mixture is directly related to the compacted density, and the higher the density, the higher the strength. Proper amount of water makes emulsified asphalt evenly dispersed on the surface of aggregate, and lubricating aggregate is beneficial to the compaction of mixture. In the process of mixture compaction, if the water content is too small, emulsified asphalt is difficult to disperse, and the lubrication between aggregate particles is insufficient, so the mixture is difficult to compact. However, if there is too much water, the hydrodynamic pressure will increase during the compaction process, and it is difficult to compact the mixture. At the same time, it may also lead to the loss of emulsified asphalt, too long curing time and reduced strength. Therefore, when cold recycled asphalt mixture is compacted, the treatment method should be similar to soil compaction to achieve the best water content and maximum dry density of the mixture.
Determining the best asphalt-aggregate ratio is one of the main test purposes of cold recycling mix design laboratory.
At present, there is no uniform step for mix design of cold recycled mixture at home and abroad, and most relevant institutions establish design guidance methods based on test results and previous practical experience. Different from the hot mix asphalt mixture designed by Marshall test at 60℃, the tests to determine the optimum asphalt-aggregate ratio of cold recycled mixture are diversified, including splitting test, Marshall test, unconfined compressive strength test and other methods. The test temperature is not limited to 60℃, but also can be 25℃ or other methods. The state of the sample can be dry after curing or wet after soaking in water again. In addition, there are some methods to calculate the optimum asphalt-aggregate ratio only according to aggregate gradation without laboratory test, such as the method in MS-2 1 manual of American Asphalt Association. Compared with hot-mix asphalt mixture, the mix design steps of cold recycled mixture are usually simpler and rougher.
Third, the problems existing in the application of cold recycled asphalt mixture
1, degree of compaction
Under normal circumstances, there is a big gap between the maximum dry density of the laboratory and the actual construction, mainly as follows:
3. 1. 1 recycled materials have great variability.
Cold recycling technology, especially in-situ cold recycling technology, has an inherent feature: the gradation of milling materials has great variability. There will be some variability when the original pavement is heavily maintained. However, the maximum dry density of recycled materials will even change by more than 5%. Therefore, in most projects, the maximum dry density is measured where the field density is measured, and the standard density is used for each part. During the construction period, increase the frequency of indoor soil compaction test to eliminate this variability as much as possible.
3. 1.2 Influence of water content of field materials
The field density of recycled materials is greatly influenced by the water content of the materials. If the water content of the material is lower than the optimum water content, the compaction work can be increased to meet the compaction requirements; However, if the water content exceeds the optimum water content, the compactness of the recycled layer will be difficult to meet the compactness requirements, so it is necessary to strengthen the monitoring of water content in construction. If the water content of the original pavement exceeds the optimum water content, it needs to be milled and aired before circulating construction.
3. 1.3 compaction equipment and technology
The binder of cold recycled mixture has high viscosity, and the compaction requirement of cold recycled material is higher than that of ordinary material. Therefore, it is necessary to choose a large-tonnage roller to fully compact, improve the compaction level and reduce the compaction variability.
2, the use performance
The variability and uneven compaction of recycled materials also make the performance of recycled asphalt mixture very different. The strength of cold recycled asphalt mixture develops gradually with the evaporation of water and the compaction of vehicle load. How to evaluate the changing performance and integrate it into the pavement structure design is also a difficult problem.
Four. Concluding remarks
Cold recycled asphalt pavement can greatly reduce the maintenance cost, improve the maintenance quality, keep the pavement intact and shorten the construction process. At the same time, the recycling of old materials reduces the exploitation of new materials, and there is no problem of transportation and storage of old materials in cold recycling on site, so this is a green environmental protection technology.
The above are collected and sorted by Zhong Da Consulting Company.
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