With the economic development in recent years, the traffic volume has increased sharply, and a large number of heavy vehicles have passed, and the road load has exceeded its design load level, resulting in different degrees of damage to local sections. The author consulted a large number of materials, taking an asphalt pavement reconstruction project as an example, the original asphalt pavement appeared common road diseases such as cracks, net cracks, looseness, potholes and subsidence, which not only affected people's production and life, but also seriously affected traffic safety and smooth roads. Therefore, in order to solve this problem, the project has been overhauled, and it is planned to use 8cm asphalt concrete +20cm water-stabilized base +20cm cold recycled mixture base to repair the pavement structure.

Second, the project overview

The road reconstruction route is 7.5km 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.

Third, the construction preparation

1. Investigate the concrete structure of the old road in detail. The old asphalt concrete pavement can be said to be a semi-rigid structure. Its cold recycling technology is mainly to crush the asphalt concrete of the old road, then add a proper amount of cement and mix it evenly, and then roll it. 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.

2. Preparation of machines and tools

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.

3. Specific requirements for materials used

(1) After the surface of the old pavement concrete is crushed, the obtained mixture is used as aggregate and filler for the cold recycled structure in site. If the aggregate is greater than 5mm, it needs to be controlled between 40% and 75%. If the quality standard cannot be reached, aggregate or filler needs to 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, and drinking water can also be used directly.

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.

Four, 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 preliminary tests on the 300-meter section, screen the old road materials, carry out compaction tests, and determine various quality indicators. The final design of the main construction technology is as follows:

1. Roll the old pavement and mix.

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.

2. The 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 when loose laying 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.

3. Plastic work after the old road is broken.

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.

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 specific consumption is 25t/ 1 000m. 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%. The cement dosage is calibrated by grid, and the grid size is 160cm× 140cm, and the dosage of 22cm deep cement is 50kg. Because of this project, 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.

Verb (abbreviation of verb) 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 industry in China. 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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