The cold recycling technology of asphalt pavement base of modern urban municipal roads is to use the existing resources of old roads, add some new mixed aggregates and stabilizers, and use cold recycling mixing equipment to regenerate the old asphalt pavement in natural environment to form a new flexible pavement base that meets the requirements of pavement strength and other indicators.
2. Application scope and advantages of cold recycling
The working principle of cold recycling of pavement base is to add additional materials such as cement and water according to the design requirements on the basis of the original old road, and use cold recycling equipment to complete the crushing, feeding, mixing, paving, leveling and rolling maintenance of the old road on site, so that its strength can meet the technical requirements of subgrade structure layer.
In-situ cold recycling can be used to repair the rut of the original pavement, repair during maintenance and block cracking of the pavement caused by load. During the cold recycling construction of asphalt pavement, the surface and base of the original road are mainly transformed into the base of the new road through construction, which leads to the need to build the surface of the new road based on this elevation. However, due to the relatively high standard of road reconstruction, recycled layer is often directly used as subbase. At the same time, it is necessary to continue to lay some semi-rigid base materials on the basis of subbase, which will lead to the pavement elevation after road reconstruction being much higher than the original road, and also lead to the changes of the structures along the road with the changes of the new pavement elevation during the pavement reconstruction design. However, compared with the traditional transformation methods, there are still advantages in the process of road transformation, mainly as follows:
2. 1 The process is relatively simple.
In the process of reconstruction, all the building materials of the original pavement are directly reused on the spot, which reduces the processing and transportation of materials, saves excavation and backfilling, and makes the process of road reconstruction much simpler than before.
2.2 The cost is relatively low
In the reconstruction process, because all the materials of the original pavement are reused, the cost can be reduced by 20% ~ 40% according to the thickness of the recycled layer, and the deeper the recycled layer is, the more the cost is reduced.
2.3 The production efficiency is obviously improved during the construction.
Because the cold recycling in the field is carried out at normal temperature, except for some serious pits and grooves, other pavements do not need any treatment. Moreover, the site cold recycling can ensure one-time passing, and the construction time is fully utilized, which will further improve the production efficiency in the whole construction process.
2.4 Conducive to the protection of environment and resources.
Because there is no need to continue mining new building materials in the whole road reconstruction process, there will be no problems such as transportation and dumping of old materials and wastes, which ensures that there will be no pollution phenomena such as dust and waste gas in the whole construction process, reasonably saves resources and protects the atmospheric ecological environment.
2.5 There is no need to interrupt traffic during construction.
Because the cold recycling process on site is very simple and few mechanical facilities are used, the traffic interference is very small. Therefore, if the traffic volume on the road is not large, the method of semi-opening and semi-construction can be adopted, which can not only ensure the normal transportation of traffic, but also continue the reconstruction and construction of the road.
3. Project overview
The route of the reconstruction project is 7.5 kilometers long. Asphalt cold recycling technology is adopted in the middle and lower layers of the pavement structure of this project. The pavement structure is: 8cmAC- 16C fine-grained modified asphalt concrete surface course, 20cm water-stabilized base course and 20cm cold recycling.
4. Construction preparation
4. 1 Investigate the old concrete structure in detail. The old asphalt concrete pavement can be said to be a semi-rigid structure. The cold recycling process is mainly formed by crushing the old asphalt concrete, adding a proper amount of cement, stirring evenly, and then rolling. Make sure it is in the best water content state before rolling.
(1) Check the deflection of the old road. On average, 40 ~ 50 points are detected every 1km in the lane, and the bearing capacity of the old road is detected and recorded in detail.
(2) Detect the thickness of asphalt layer, base and base material of old road. By cold recycling all the structural materials of the old road, sampling and measuring after crushing, the specific structural strength is studied. Through indoor grading test and mix proportion test, the point frequency is determined to be 3 points per1km, and then the maximum dry density and the best water content are tested according to different mix proportions.
(3) Analyze the soil quality of the old road structural materials, determine the admixture, and adopt portland cement with strength grade of 32.5.
4.2 Machine preparation
Before construction, all equipment and tools must be in place and put into normal use. The main construction machinery are: cold recycling machinery, three-wheeled roller, vibratory roller, cement transporter, sprinkler and grader.
4.3 Specific requirements for materials used
(1) After the concrete surface of the old road pavement is broken, the obtained mixture is used as aggregate and filler for the cold recycled structure in site. If the aggregate is greater than 5mm, the mass fraction should be controlled between 40% and 75%. If the quality standard cannot be reached, aggregate or filler should be added continuously.
(2) Use 32.5-grade portland cement to ensure that its initial setting time is over 4h.
(3) The water used in the construction process shall not contain any harmful substances, or can be directly used as drinking water.
4.4 Specific provisions on gradation and mixture ratio
(1) After the mixture is selected, it needs to be screened by the tester number, and then the actual gradation is determined.
(2) Determination of compressive strength: According to the determined gradation, it is necessary to carry out laboratory tests on the mixture of each number, and finally obtain the specific values of the maximum dry density of the test block with three cement contents, and determine the specific values of its optimal water content. Moreover, the test block needs to be moisturized for about 6 days according to the relevant temperature specifications, then soaked in water for 24 hours, and then the compressive strength test is carried out according to the relevant regulations. After obtaining the compressive strength value after 7 days, the cement content is determined according to the final result.
5. Cold recycled concrete construction technology
At present, cement is the most commonly used additive in the process of cold recycling of pavement, and the actual dosage of cement is usually controlled between 3% and 5%. It can be said that cold recycling is still a relatively new construction technology, and there is little accumulated practical experience at present. Therefore, in order to ensure the quality in the construction process, it is necessary to carry out initial inspection on the 300m-m section, screen the old road materials, carry out compaction tests and determine various quality indicators. Finally, the main construction technology design is as follows: 5. 1 Rolling and mixing the old pavement.
Wirtgen2500 cold mix recycling machine is mainly used in this construction process, with the maximum working width of 300cm and the maximum mixing depth of 40cm. It can be continuously mixed for a long time, with relatively high productivity and high control accuracy of pavement thickness. After determining the working depth, the control system and sensors can be used to determine the cutting depth, and then a more accurate cold regeneration thickness can be obtained. Semi-closed mode can be adopted in the construction process to avoid traffic interruption and ensure safety in the construction process.
5.2 Concrete Analysis Process of Pavement Mixture
When analyzing the mixture, it mainly includes determining the screening results of old materials, the maximum dry density of granular materials and the optimal water content of the mixture, as well as determining the length of the operation section. Laboratory personnel need to study the materials in the work site in a balanced way and carefully screen the mixture. After repeated compaction tests, it is determined that the maximum dry density is 2.25g/cm3, the optimum water content is 7.5%, and the thickness of loose paving is about 1.35. Then the strength test is carried out by using the cold regeneration delay, and the delay time is 4 h. In the construction process, the time from cement mixing to rolling should be strictly controlled within 4 hours, and the reasonable length of the working section should be determined to be 200m·m m.
5.3 The plastic work after the old road is crushed
After the mixing work is completed, the pavement needs to be initially leveled with a grader, and then rolled twice with a vibratory roller. Surveyors need to measure the elevation every 10m, then find out the datum points and connect them with lime. If the elevation does not meet the requirements, it needs to be leveled with a grader. The main purpose of this process is to ensure the smoothness of the road surface.
5.4 Construction technology and matters needing attention in the construction process
In the process of cement paving, the grid method can be used to control the cement consumption at about 4%, and the unit consumption is 25t/ 100m. The operation steps are as follows: according to the specific width of the cold recycling layer, determine the row number and interval distance of cement, then draw the corresponding grid, and finally evenly spread the cement.
(1) Determination of scrolling combination: In the scrolling process, it is necessary to follow the long-term strategy of first light and then heavy, and from slow to fast. At first, you can use YZ 18 vibratory roller to stabilize the road surface, and then use this vibratory roller to continuously roll it for four to five times. At rest, it can be done twice with a three-wheeled roller, and finally it can be rolled twice with a 20t tire roller. In the rolling process, it is necessary to stagger the width of the shaft at one time. When rolling, it should be rolled from the roadside to the center of the road in turn. After the final pressure is completed, it is necessary to ensure that the cold recycled layer is smooth and smooth, and that the surface of the recycled layer is oozing with cement slurry. The completion time of the final pressure should be controlled as far as possible within the time when the cement completes the final setting. After the completion of rolling, the compaction degree must be measured by sand filling method to ensure that the compaction degree meets the design requirements and relevant specifications.
(2) Reasonable control of construction time: During the construction process, it is necessary to control the average speed change of cold recycling machinery within 6m per minute, try to avoid unreasonable shutdown, and ensure that the operation time of each road section is controlled within 4.5h.
(3) Reasonable maintenance: After the pavement is rolled for four hours, it is necessary to cover the pavement with wet canvas or other materials for reasonable maintenance. In this process, it is necessary to ensure the supermarket degree of covering materials, and it can also be maintained by spraying water directly on the cold recycling layer.
Before this project adopts direct paving cement, the core part of the old road should be inspected in detail so as to have a specific understanding of the historical situation of the old road. Then, according to the specific conditions of different sections, aggregate is added or not, so that the gradation and strength of recycled subgrade can meet the requirements of inorganic aggregate base. It is recommended to use 32.5 ordinary portland cement for cold recycled cement, and the curing period is generally more than 7d. Through compaction test, it is determined that the dry density of the mixture is between 2.07 g/cm3 and 2.15g/cm3, and the cement metering is controlled between 4.5% and 5%. On-site grid calibration of cement dosage. The grid size is 60cm× 140cm, and the cement consumption at the depth of 22cm is 50kg, because the structural layers of each section of this project are different. Therefore, the degree of compaction is calculated point by point, and there is no uniform standard for maximum dry density and optimal water content. Through the test of the influence of cold regeneration delay time on strength, it is determined that the delay time is 4 hours, and the time from the beginning of adding cement to the completion of rolling is strictly controlled within 4 hours.
6. Conclusion
To sum up, cold recycling, as a new construction technology, will gain a firm foothold in the competition of highway construction technology with its unique construction technology and characteristics in today's rapid development of expressway. Compared with the traditional road construction methods, in-situ cold recycling technology can shorten the construction period, improve work efficiency, make full use of waste materials, greatly save construction costs and minimize traffic interference. The new cold recycling technology of this project has explored a new way for the old road reconstruction and accumulated new experience.
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