Wei Beilan
Wuhai Highway Maintenance Center
Abstract: asphalt pavement is the main pavement structure form in the mainland, with the rapid development of highway construction, the mainland highway has entered the stage of construction and maintenance, and many asphalt pavement began to enter the stage of maintenance and repair. How to use asphalt pavement waste materials to realize the recycling of materials is the focus of scientific maintenance and green maintenance. Foamed asphalt recycling technology has the characteristics of room temperature, environmental protection and high utilization rate, and has become an important technical means in asphalt pavement recycling technology. In view of the problems of many influencing factors of foamed asphalt and large variability of the performance of foamed asphalt mixture, combined with an asphalt pavement overhaul project in Wuhai City, the foaming temperature and water content of foamed asphalt were determined by carrying out indoor tests, and the feasibility of foamed asphalt recycled mixture was verified. Through the experiment, it can be seen that with the increase of temperature, the expansion rate and half-life of foamed asphalt will increase. As the moisture content increases, the half-life decreases and the expansion rate increases. The test determined that the optimal foaming temperature of foamed asphalt was 165 °C, and the optimal water content was 2.5%. Based on the dry splitting strength, wet splitting strength and splitting strength ratio tests on the properties of foamed asphalt recycled mixture, the mix ratio of foamed asphalt recycled mixture was determined, and the mix ratio was verified by rutting test, and the final foamed asphalt dosage was determined to be 2.2%, the moisture content was 5.4%, and the cement dosage was 1.5%.
Keywords: foamed bitumen; recycled mixes; Half-life; expansion rate; splitting strength ratio;
About author:WEI Beilan (1967-), female, from Wuhai, Inner Mongolia, senior engineer, research direction: road maintenance.
1 Introduction
Asphalt pavement has become the main pavement structure form of mainland grade roads due to its advantages of smooth and joint-free, low driving noise and convenient maintenance and repair. The mainland highway industry has entered the stage of paying equal attention to construction and maintenance, and how to maintain asphalt pavement in a green, environmentally friendly and cost-effective manner has become a hot spot in current research. There are two main technologies for the recycling of asphalt pavement materials: hot recycling and cold recycling. Cold recycling of asphalt pavement will not cause secondary aging of asphalt pavement, and the strength can also meet the requirements of the base or lower layer, which has become an important means of some low-grade road regeneration. Compared with the semi-flexible recycling technology of emulsified asphalt, foamed asphalt cold recycling pavement has the advantages of good deformation resistance, fatigue resistance, high early strength, fast construction speed, energy saving and emission reduction, and good engineering economy, and has been widely used in domestic highways, urban trunk roads, and local roads [1].
Scholars at home and abroad have carried out a lot of research work on foamed asphalt recycled mixture. At the beginning of the 20th century, a series of studies were carried out on foamed asphalt treatment of road surface materials. In the 50s of the 20th century, Professor Csanyi L H of Iowa State University in the United States used high-temperature water vapor to carry out foaming tests on asphalt [2]. Bowering et al. [3,4] used indoor health maintenance methods to simulate the on-site health conditions of early foamed asphalt mixtures, and analyzed the mechanical properties of the mixtures under different conditions such as different health temperatures and humidity. Domestic scholars have also carried out a lot of research on the strength molding mechanism, molding method and durability of foamed asphalt mixture. Li et al. [5] used scanning electron microscopy to observe the microscopic morphology of RAP and foamed asphalt mortar before and after mixing with cement, revealing the mechanism by which cement can improve the strength of the recycled mixture. The hydration of cement consumes the water in foamed asphalt, and when the cement content is too high, the brittleness of the mixture will be improved, and the cement content in the mixture is usually limited to 1.5%~2.5% [6]. Bo Xuefeng et al. [7,8,9] found that foamed asphalt mixture has good crack resistance and can effectively alleviate the occurrence of reflection cracks in asphalt layers. Huang Weidong et al. [10] evaluated the crack resistance of ultra-thin wear layer mixture by indirect tensile cracking method. Feng Decheng et al. [11] found that the test could truly reflect the actual low-temperature cracking of asphalt pavement in Jilin through the improved SCB test, and through the comparative analysis of fracture energy and fracture toughness, it was concluded that it was more reasonable to use fracture energy as the evaluation index of low-temperature performance of mixture.
Scholars at home and abroad have not only carried out a lot of research on foamed asphalt, but also carried out a lot of research on the strength formation mechanism, road performance and durability of foamed asphalt mixture. However, the foaming effect of foamed asphalt is affected by the quality of asphalt, foaming equipment, foaming temperature and moisture content, and should be tested and verified before construction. The performance of the recycled mixture also needs to be verified by field tests according to the different recycled materials, cement content, and the type of aggregate. Based on this, this paper carried out a study on the related properties of foamed bitumen and foamed asphalt mixture, so as to provide reference for similar research.
2 Materials and Methods
2.1 Bitumen
In this study, No. 70 matrix petroleum bitumen was used, and the basic performance indexes are shown in Table 1.
Table 1 Detection index of No. 70 matrix asphalt for test Download the original figure
2.2 Cement
The test uses ordinary Portland cement with a strength grade of 42.5, and the main test indicators are shown in Table 2, and the test results meet the requirements of the specification.
Table 2 Cement detection indicators Download the original image
2.3 RAP material
The milling material of asphalt pavement of a project in Inner Mongolia is selected, and the main detection indicators are shown in Table 3 and Table 4. Through the analysis of the test results, it can be seen that the performance of the milling material of the pavement basically meets the requirements of the specification, and can be used for the application of foamed asphalt cold recycling project.
Table 3 RAP detection metrics Download the original image
Table 4 RAP grading results Download the original image
2.4 Ores
The project needs to add an appropriate proportion of new aggregate to adjust the gradation, so that the gradation of the ore is within the range recommended by the specification. At the same time, the performance of the ore should also meet the requirements of the corresponding specifications. The test results of coarse aggregate and fine aggregate selected in the test are shown in Table 5 and Table 6, and the test results meet the requirements of coarse aggregate and fine aggregate in the Technical Specification for Highway Asphalt Pavement Recycling (JTG/T 5521-2019).
3. Analysis of foaming performance of foamed asphalt
3.1 Effect of temperature on the foaming effect of asphalt
The foaming effect of asphalt is an important factor affecting the performance of foamed asphalt cold recycling mixture, and the water consumption and foaming temperature affect the foaming effect of asphalt. In this paper, the effects of water consumption and temperature on the half-life and expansion rate of foamed bitumen were analyzed through experiments, and the optimal water consumption and foaming temperature were determined. The test selects 1.5%, 2.0%, 2.5%, 3.0% and 3.5% water consumption, and the foaming temperature is controlled at 145 °C, 155 °C, 165 °C and 175 °C respectively, and the half-life and expansion rate of the asphalt after foaming are detected, and the specific index test results are shown in Table 7 and Figure 1 and Figure 2.
Table 5 Coarse aggregate test results Download the original image
Table 6 Detection results of fine aggregates Download the original image
Table 7 Asphalt foaming results table Download the original image
Fig.1 Half-life variation of asphalt under different foaming temperature and water consumption Download the original map
Fig.2. Variation of asphalt expansion rate under different foaming temperature and water consumption Download the original figure
Through the analysis of Table 7, Fig. 1 and Fig. 2, it can be seen that under the same foaming temperature, with the increase of water consumption, the expansion rate of foamed asphalt increases, the volume of the foamed asphalt increases, the texture is more uniform, and the wrapping property is also enhanced, but its half-life shows a downward trend, that is, the volume decay of foamed asphalt is faster, which affects the use of foamed asphalt; When the water content remains unchanged, with the increase of temperature, the half-life and expansion rate of asphalt will increase, that is, the increase of temperature will improve the foaming effect of asphalt, but at 175 °C, its half-life will decrease, so the asphalt should choose the appropriate foaming temperature range.
3.2 Determination of the best foaming effect
In order to determine the optimal foaming temperature and water consumption of asphalt in the project application, the foaming characteristics of asphalt were drawn with the water consumption as the abscissa and the expansion rate and half-life as the ordinate, and the expansion rate and half-life were comprehensively analyzed. The optimal foaming conditions are to determine the foaming temperature and water consumption that will achieve the optimal foaming effect. Through the analysis of the influence of foaming temperature on foaming effect, the optimal foaming temperature of 165°C was selected, and the expansion rate and half-life variation of different water content of asphalt were plotted, as shown in Figure 3.
Fig.3. Determination of optimal moisture content Download the original image
As can be seen from Figure 3, the optimal moisture content is 2.5%, that is, the optimal foaming conditions of the asphalt are 165 °C, the moisture content is 2.5%, the half-life is 29.4s, and the expansion rate is 14.6 times.
4. Mix ratio design of foamed asphalt recycled mixture
4.1 Synthetic gradation of ores
Through full analysis and continuous trial of the gradation of milled material, gravel, stone chips and cement, it was finally determined that the mineral composition of the mixture was 70% milled material + 15% fine aggregate (0~3mm) + 13.5% coarse aggregate (4.75mm~26.5mm) + 1.5% cement, and the specific synthetic gradation is shown in Table 8, and the pass rate of each sieve hole of the synthetic gradation meets the requirements of the "Technical Specification for Highway Asphalt Pavement Regeneration" (JTG/T 5521-2019).
Table 8 Synthetic gradation table Download the original image
4.2 Determination of optimal moisture content
Referring to the method of "Highway Geotechnical Test Regulations" (JTG E40) T 0131, the compaction test of the synthetic ore was carried out to determine the maximum dry density and the best water content of the mixture. The compaction test results are shown in Table 9 and Figure 4.
Table 9 Changes in dry density and wet density Download the original image
Fig.4. Dry density variation Download the original image
According to Figure 4, the optimal moisture content of the mixture was determined to be 6.7%. In engineering practice, it is found that reducing the moisture content of about 20% on the basis of the maximum dry density corresponding to the compaction test may be more conducive to the dispersion of foamed asphalt and ensure the performance of the mixture. Therefore, the water withdrawal is 5.4%, and the maximum dry density is 2.073g/cm3.
4.3 Determination of the optimal amount of foamed bitumen
Based on the synthetic grading range of the foamed bibitumen cold recycling mixture and the asphalt content in the RAP, the estimated foamed bitumen usage is 2.2%. Taking the estimated amount of foamed bitumen as the median, 5 groups of mixtures were mixed at 0.3% intervals, and then the foamed asphalt mixture was formed into Marshall standard specimens (75 times compaction on each side). The sample is placed on the side together with the mold test in a blast oven at 60 °C to maintain to constant weight, and the health care time is generally not less than 40h, and the sample is directly placed on the side to cool for 12h after health and then demoulded; The dry splitting test and wet splitting test were carried out on the health specimens. The test results are shown in Table 10 and Figure 5.
Table 1-0 Cleavage strength change table Download the original image
Fig.5. Cleavage strength variation diagram Download the original image
According to the test results, when the dosage of foamed asphalt is 2.2%, the wet splitting strength is maximized, and the dry-wet splitting strength ratio also meets the requirements of the specification. Therefore, the design value of foamed bitumen dosage is 2.2%.
4.4 Mix performance verification
According to the requirements of the Technical Specification for Highway Asphalt Pavement Recycling (JTG/T 5521-2019), the rutting performance of the foamed asphalt mixture specimen needs to be verified. Using 2.2% foamed asphalt, 5.4% moisture content, 1.5% cement, according to the requirements of the specification, the 50mm thick cold recycling mixture rutting plate specimen is formed by wheel rolling method, and the specimen is quickly placed in a 60°C blast oven to dry to constant weight (generally about 48h) after the rolling is completed, and then the dynamic stability test is carried out. The holding time of the specimen before the test is 8h~10h. Turn on the testing machine to ensure that the temperature in the working room of the testing machine is 60°C±0.5°C. The amount of deformation at 45 min and 60 min is read to calculate the dynamic stability. According to the formula for calculating the dynamic stability of asphalt mixture, the rutting test results of foamed asphalt cold recycling mixture are obtained, as shown in Table 11. The test results show that the dynamic stability meets the requirements of the specification.
Table 1-1 Rutting test results Download the original image
5 Conclusion
Through the analysis of foamed asphalt foaming performance and the design and performance verification of foamed recycled asphalt mixture, the following conclusions can be obtained:
(1) The foaming performance of asphalt is affected by the moisture content and foaming temperature, the higher the moisture content, the higher the expansion rate, and the shorter the half-life; The higher the temperature, the expansion rate and the half-life increase, but the half-life decreases after a certain temperature.
(2) Through laboratory tests, it is determined that the optimal foaming temperature of the asphalt used in this project is 165 °C, and the optimal water content is 2.5%.
(3) Through the laboratory test, it is determined that the mineral composition of the foam recycled mixture is 70% milling material + 15% fine aggregate (0~3mm) + 13.5% coarse aggregate (4.75mm~26.5mm) + 1.5% cement, the asphalt dosage is 2.2%, and the moisture content is 5.4%; The performance of the mixture meets the requirements of the specification and can be used for engineering applications.
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