1. Introduction
At present, with the rapid development of highway construction and the increasing traffic volume in China, the proportion of heavy and overloaded vehicles is increasing, and the asphalt pavement is obviously damaged in advance. According to the design life of asphalt pavement (10- 15) and actual use, from now on, about 12% of asphalt pavement needs to be renovated every year, and the waste of old asphalt mixture will reach 2.2 million tons. If it can be used reasonably, it can save more than 300 million yuan in material cost every year. Therefore, using cold recycling technology of asphalt pavement to turn waste asphalt mixture into treasure has become an important topic for road engineers and technicians in recent years.
2. Summary of cold recycling technology of asphalt pavement
Asphalt pavement recycling technology can be divided into four categories according to construction temperature and construction technology: plant-mixed hot recycling, on-site hot recycling, plant-mixed cold recycling and on-site cold recycling. Among them, cold recycling technology is a process of crushing old asphalt concrete pavement materials, adding some new aggregates or fine aggregates, emulsified asphalt (foamed asphalt), appropriate amount of water and certain additives (cement or lime) when necessary, and continuously milling, crushing, adding, mixing, paving and molding the materials at natural ambient temperature, and re-forming the structural layer. Recycled old asphalt mixture can be used as base or subbase of pavement or other semi-rigid base materials according to different highway grades. Because the old asphalt mixture is used as the base material, it can basically meet the requirements as long as it has certain strength, stiffness and water stability. In addition, cold recycling technology usually does not involve the recovery of the properties of old asphalt materials.
3. Cold recycling technology of asphalt pavement with different stabilizers.
At present, the most commonly used stabilizers for cold recycling of asphalt pavement are cement and emulsified asphalt, and foamed asphalt has gradually become a research hotspot. The different characteristics of these three stabilizers determine their unique design methods, construction techniques and quality control standards in the cold recycling process. Table 1 compares the advantages and disadvantages of the above three different cold regeneration stabilizers.
Table 1 comparison of advantages and disadvantages of three commonly used cold regeneration stabilizers
3. 1 Cold Recycling of Cement Stabilized Asphalt Pavement
When cement is used as stabilizer, there are two ways to add it, one is to mix solid powder cement with recycled materials, and the other is to mix it with recycled materials in the form of cement slurry.
When cement is used as stabilizer, the recycled structural layer is prone to shrinkage cracks. The following aspects should be considered to minimize the degree of shrinkage cracks:
1) cement content. When the amount of cement is high, it will shrink greatly. In order to control shrinkage cracks, the dosage of cement stabilizer is 2%~4%.
2) Properties of recycled old materials. Some materials are treated with cement, and the shrinkage is particularly large; Some materials have large volume changes and high plasticity index when water content changes. When the plasticity index of the material is greater than 10, cement should not be used alone for stability, and lime must be mixed with cement or used alone to reduce the plasticity of the material.
3) Water content during construction rolling. The degree of shrinkage cracking is directly proportional to the amount of water that disappears after rolling and drying, but low water content is easy to cause compaction. It is generally recommended to control the water content during construction rolling at a level lower than the optimal water content 1%~ 1.5%.
4) drying speed. Proper curing is carried out on the structural layer materials after cement treatment, so as to reduce the drying speed of the materials, thus reducing shrinkage and cracking. Generally, the cement stabilized structural layer must be watered for health preservation within 7 d after construction, or temporary seal coat and asphalt layer should be laid to avoid the structural layer from shrinking and cracking due to the rapid evaporation of water on its surface. If the temporary seal coat is not paved, the cement stabilized structure layer shall not be opened to traffic for a certain period of time.
3.2 Cold Recycling of Asphalt Pavement with Emulsified Asphalt as Stabilizer
Emulsified asphalt can be mixed with wet granular materials at room temperature to improve the strength of the materials. Therefore, emulsified asphalt is the most commonly used asphalt stabilizer. In general, mixing emulsified asphalt and cement can not only improve the water stability of recycled mixture, but also improve its early strength, but the addition of cement must be controlled below 2% of aggregate weight to avoid weakening the fatigue resistance of mixture.
In cold recycling, the relationship between water content and density is generally used as an index to control water content to ensure the rolling quality of structural layers. However, when emulsified asphalt is used as stabilizer, the water content must be replaced by the total liquid content, and the optimal total liquid content (OTFC) at the maximum density should be taken as the index. OTFC is the sum of water and emulsified asphalt in the mixture before demulsification. In practical engineering, if the water content of on-site paving materials is close to OTFC, adding emulsified asphalt will make the total liquid content of the materials exceed the saturation point. This situation is difficult to solve by reducing the amount of emulsified asphalt, and a small amount of cement (
In the mix design of emulsified asphalt recycled mixture, whether to add new aggregate should be considered according to the gradation of old asphalt pavement materials, and then different amounts of emulsified asphalt and water should be added to the mixed aggregate for experiments, and the optimum amount of emulsified asphalt and water should be determined by mechanical strength index.
3.3 Foam Asphalt Stabilized Asphalt Pavement Cold Recycling
When foamed asphalt is used as stabilizer, the gradation of the treated material must meet certain requirements, and the gradation composition of fine materials, especially the filler below 0.075 mm, has the greatest influence on the performance of foamed asphalt mixture, which is mainly due to the unique distribution of foamed asphalt in the mixture. In foamed asphalt mixture, foamed asphalt is only wrapped by fine aggregate to form mortar, and then coarse aggregate particles are bonded into a whole by mortar in a point connection way, unlike asphalt in common hot-mix asphalt mixture or emulsified asphalt mixture, which forms a uniform asphalt film on the surface of aggregate. Therefore, there must be enough fine materials in the mixture, and the passing rate of 0.075 mm is generally not less than 5% to ensure the effective dispersion of foamed asphalt.
Water content is an important parameter in the design of foamed asphalt mixture, which has two main functions, namely, ensuring the dispersion of foamed asphalt and the effective compaction of mixture. There are many studies on the reasonable water content of foamed asphalt mixture. At present, the water content corresponding to 65%~85% of the optimum compacted water content of aggregate OMC is mainly used in practical use.
In the mix design of foamed asphalt mixture, firstly, according to the gradation of old pavement materials, whether to add new aggregate or not is considered, and the reasonable foaming characteristics of asphalt are determined by different asphalt temperatures and different water consumption, and then the optimum water content of mixed aggregate is determined by compaction test, so as to determine the mixing water consumption of the mixture; Then the mixture is mixed with different amounts of foamed asphalt for mixing, molding and curing, and the optimum amount of foamed asphalt is determined through mechanical properties.
4. Prospect of cold recycling technology
From the point of view of saving energy and transportation cost, it is a more suitable asphalt pavement recycling process to use foamed asphalt for cold recycling in site. There is little research on foamed asphalt and its mixture in China. As an important content of asphalt material recycling research, the research and application of cold recycling technology of asphalt mixture need further attention. At the same time, for a long time, most roads in China use semi-rigid base (according to statistics, more than 90% of asphalt pavement base and subbase of high-grade highways in China use semi-rigid materials), and the application of recycled base can provide new flexible base materials completely different from semi-rigid base materials for high-grade highways. Strengthening the research on the recycling of asphalt pavement is of great value to environmental protection and pavement performance.
The above are collected and sorted by Zhong Da Consulting Company.
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