Asphalt mixture is the core material of pavement construction, its asphalt content and mineral aggregate gradation directly determine the high temperature stability, low temperature crack resistance, water stability and durability of pavement.Too much asphalt can easily lead to road flooding, rutting, encapsulation and other diseases, while insufficient asphalt can cause loose, peeling, seepage damage.Asphalt mixture extraction test is the core test item of indoor inspection of road engineering. Accurate determination of asphalt content and mineral aggregate gradation in mixture according to specifications is the key technical means to control asphalt mixture production quality, optimize mixture ratio design and verify mixing quality on construction site. It is widely used in quality inspection and acceptance of highway and municipal road engineering.

Test overview and scope of application

The core purpose of asphalt mixture extraction test is to accurately determine the actual asphalt content of asphalt mixture by solvent dissolution separation of asphalt binder and mineral aggregate in the mixture, and at the same time, screening test can be carried out on the extracted dry mineral aggregate to verify whether the mineral aggregate gradation meets the design requirements.At present, the mainstream detection method in the project is centrifugal separation method. The test is based on the Test Code for Asphalt and Asphalt Mixture in Highway Engineering (JTG E20-2011) and conforms to ASTM D2172, AASHTO T164 and other domestic and foreign general standards. It is suitable for asphalt content detection of various asphalt concrete materials such as hot mix asphalt mixture and warm mix asphalt mixture. It has the characteristics of fast detection speed, high accuracy and strong operability. It is a routine quality control test in the field laboratory.
The main application scenarios of this test include: asphalt mixture mix ratio verification, daily sampling inspection of production quality of mixing station, asphalt content detection of pavement core samples, completion acceptance inspection of engineering quality, etc., which can provide accurate data support for engineering quality evaluation and construction process optimization.

Test rationale

Centrifugation extraction test relies on the double principle of organic solvent dissolution and centrifugal force separation.Trichloroethylene is chosen as special solvent, which has strong solubility to asphalt and will not react with sand, mineral powder and other mineral ingredients.The crushed asphalt mixture sample is soaked in trichloroethylene, and the asphalt binder in the mixture is fully dissolved in the solvent to form an asphalt-solvent mixed solution, while solid particles such as mineral aggregate and mineral powder remain stable and insoluble.
After the mixed solution is transferred into centrifugal extraction equipment, strong centrifugal force is generated by high-speed centrifugal operation, so that liquid asphalt solvent is thrown out through the pores of special filter paper and flows into the recovery container. Mineral materials with large particle size and residual coarse particles are trapped in the centrifugal cup.Because the initial extraction solution will be mixed with trace fine ore powder, it is necessary to add solvent for repeated extraction until the outflow solvent is clear and transparent without turbid impurities to ensure that the asphalt is completely separated from the mineral aggregate.Finally, the asphalt content in the mixture can be calculated by weighing the sample mass before and after extraction and the mass of dried mineral aggregate, and the dry mineral aggregate can be screened to detect the gradation deviation of mineral aggregate.

Preparation of test instruments and reagents

Core test instruments

1. Centrifugal asphalt extractor: the whole machine is equipped with centrifugal separator, sealed top cover, solvent recovery device, speed control system, rated speed up to 3000~3600r/min, stable operation, good sealing, can effectively prevent solvent volatilization and fine ore loss, is the core equipment of the test.
2. Electronic balance: accuracy 0.001g, used for accurate weighing of mixture samples, dry mineral materials, filter paper and other test consumables, to meet the test requirements of high precision measurement.
3. Blast drying oven: constant temperature control 105℃±5℃, used to dry test minerals, remove solvent residue and moisture, ensure accurate weighing data.
4. Standard test sieve, glass rod, beaker, tweezers, desiccator and other auxiliary tools, used for sample crushing, transfer, cooling preservation and other operations.

Test reagents and consumables

The special solvent for the test is trichloroethylene, which has the advantages of fast dissolution speed, good asphalt solubility and strong stability. It is strictly prohibited to use gasoline, kerosene and other flammable and explosive solvents with poor solubility instead.The supporting consumables are special qualitative filter paper, which needs to be dried and weighed in advance. The centrifugal cup size can effectively intercept fine mineral powder and ensure smooth penetration of solvent.

Detailed test operation steps

Sample preparation and pretreatment

Select representative asphalt mixture samples, the sample weight is determined according to the maximum particle size of the mixture, and the samples of conventional AC-13 and AC-16 mixtures are 1000~1500g.Place the sample on a flat table, and gently crush the loose lumps manually to avoid destroying the original mineral particle size and eliminate obvious impurities.Put the crushed sample into a dry beaker and mark it with a number for later use.

Sample immersion dissolution

Add sufficient trichloroethylene solvent to the beaker containing the sample to completely immerse the mixture sample, and the solvent level is 2~3cm higher than the sample surface.Let stand for 30min, during which the mixture is gently stirred with glass rod every 10min to accelerate the dissolution of asphalt, so as to ensure that the asphalt in the mixture and on the surface of mineral aggregate is fully fused with the solvent without coating residue.For modified asphalt mixture, the soaking time can be appropriately prolonged to ensure complete dissolution.

Equipment assembly and sample transfer

Adjust centrifugal extractor in advance, check whether equipment tightness, speed control system and recovery pipeline are normal.Lay the special filter paper dried and weighed in advance on the edge of the centrifugal separator, and fasten the top cover of the equipment to prevent fine mineral powder from being lost during centrifugation.Slowly pour the completely dissolved mixture and solvent mixture in the beaker into the centrifuge cup, wash the inner wall of the beaker and glass rod with a small amount of clean solvent, and transfer all the residual mixture particles and asphalt solution into the centrifuge cup to avoid test error caused by sample loss.Place a sealed recovery bottle under the discharge port of the equipment to collect the asphalt solvent solution thrown out.

Centrifugal extraction operation

Start the extractor, start at low speed and speed up gradually, steadily increase the rotation speed to more than 3000r/min, continue centrifugal separation until no solution flows out of the discharge port, and then stop.After the first extraction is completed, an appropriate amount of trichloroethylene solvent is added again, and the soaking and centrifugation processes are repeated for 4 to 5 times.Observe the state of the solvent flowing out each time until the outflow solution is clear and pale yellow without turbid mineral powder impurities, and judge that the asphalt is completely extracted.

 Mineral drying and weighing

After extraction, take out all the minerals and filter paper in the centrifuge cup, put them into a blast drying oven at 105℃±5℃ for constant temperature drying for 4~6h, and thoroughly remove the residual solvent and moisture on the pores and surface of the minerals.

Common Test Errors and Quality Control Points

Main sources of error

1. Insufficient representativeness of sample: single sampling position, insufficient fragmentation of sample lumps, resulting in test data can not reflect the overall mixture quality.
2. Asphalt is not completely dissolved: insufficient soaking time, insufficient stirring, residual asphalt on the surface of mineral aggregate, resulting in low measured asphalt content.
3. Loss of fine ore powder: equipment sealing is not strict, filter paper is damaged, centrifugal speed is too high, resulting in fine ore powder flowing out with solvent, so that the weighing quality of mineral aggregate is too small, asphalt content calculation value is too high.
4. Solvent and moisture residue: mineral drying is not thorough, residual solvent, moisture increase the quality of mineral, resulting in test data deviation.
5. Weighing error: the balance is not calibrated, and the sample is not fully cooled, which affects the measurement accuracy.

Key points of quality control

1. Strictly control the sampling link, multi-point sampling and uniform mixing to ensure that the sample can truly represent the overall performance of the mixture and eliminate one-sided sampling.
2. Standardize the dissolution operation, strictly control the soaking time and stirring frequency, and appropriately extend the dissolution time of modified asphalt mixture to ensure that asphalt is completely separated from mineral aggregate.
3. Check the equipment thoroughly before the test, confirm that the filter paper is intact, the equipment is tightly sealed, the centrifugal speed is stable and controllable, and the mineral powder loss is avoided; repeatedly circulate the extraction until the solvent is completely clear.
4. Strictly abide by the drying and cooling process. After drying, mineral materials must be cooled to room temperature in the dryer before weighing, so as to eliminate the influence of temperature and humidity on data.
5. Regularly calibrate the test balance, drying box, extractor and other equipment to ensure that the accuracy of the instrument meets the specification requirements, strictly implement the parallel test system, and ensure the reliability of the data.

Significance and engineering application value of the experiment

Asphalt mixture extraction test is the core checkpoint to control the quality of pavement construction. Accurate asphalt content data is the core basis to judge whether the mixture ratio is compliant and whether the mixing process is stable.Through this test, problems such as asphalt content deviation and mineral aggregate gradation imbalance in the production process of mixing station can be found in time, mixing parameters can be adjusted in time, and early road diseases can be avoided from the source.At the same time, for the pavement with diseases after opening to traffic, it can be analyzed whether the diseases are caused by abnormal asphalt dosage and unqualified gradation through core sampling extraction test, which provides technical support for pavement maintenance and disease treatment.
Compared with other detection methods such as combustion method, centrifugal extraction test equipment has low cost, simple operation, stable detection accuracy, and meets the normal detection requirements of various site laboratories. It is an indispensable test method for quality control of asphalt mixture in road engineering.

Precautions for safety and environmental protection of test

Trichloroethylene is a volatile organic solvent with certain toxicity. The test must be carried out in a well-ventilated special laboratory. Operators shall wear anti-virus masks and protective gloves to avoid direct contact with skin and inhalation of volatile gases.Open fire is strictly prohibited during the test to prevent potential safety hazards caused by volatile gases of solvents.After the test, waste solvents and waste liquids shall be collected and treated uniformly, and dumping at will is strictly prohibited, and environmental protection construction inspection requirements shall be implemented; the test equipment shall be cleaned and dried in time to avoid solvent residue corrosion equipment.

Conclusion

Bituminous mixture extraction test seems to standardize the operation process, but each test detail will directly affect the accuracy of the test results.In the quality control of road engineering construction, it is necessary to strictly follow the test procedures, standardize the entire process of sample preparation, dissolution and extraction, drying and weighing, and data calculation, strictly control various test errors, and ensure that the test data is true, accurate, and effective.Through accurate extraction test, strictly control asphalt content and mineral aggregate grading quality of asphalt mixture, ensure the construction quality of asphalt pavement from raw material level, effectively improve road traffic stability and service life, and lay a solid foundation for road engineering quality improvement.