1 Limitations of current Marshall asphalt mixture design method
This method was put forward by BruceMarshall, Mississippi Highway Bureau, and its characteristic is to pay attention to the stability/flow value, compactness/porosity characteristics of asphalt mixture. The purpose of this analysis is to obtain the porosity suitable for the durability of HMA mixture. However, Marshall compaction method will inevitably cause aggregate breakage, which will affect the final test results of the specimen, such as porosity and fuel consumption.
Marshall design index is mainly developed for dense graded conventional asphalt mixture. For some polymer modified asphalt, such as SBS modified asphalt, the flow value is over 40, but it can still be used. Although the technical index of the new code has been improved, it still needs further study.
According to the research of many scholars, Markov impact compaction can not simulate the compaction characteristics of the mixture formed on the actual pavement. Production indicators, such as stability and flow value, cannot reflect the shear strength of HMA. Therefore, it has nothing to do with pavement damage such as rutting, fatigue and low temperature cracking, and it is impossible to prevent early pavement damage. Therefore, this empirical method gradually shows its limitations.
2. Overpaving concept of asphalt mixture volume design method
In order to overcome some shortcomings of current asphalt mixture design methods and establish a direct relationship between asphalt mixture performance and asphalt pavement performance, the United States put forward a new asphalt mixture design method-Superpave asphalt mixture volume design method after six years' efforts of 1987~ 1993. Superpave mixture design system focuses on the method of material selection and mixture design according to the climate and design traffic volume where the project is located. This method requires that the influence of temperature on the pavement during the service period should be fully considered when designing asphalt pavement, and that the asphalt pavement can meet the requirements of high temperature performance at the highest design temperature without excessive rutting. At the lowest design temperature of pavement, it can meet the requirements of low-temperature performance and avoid or reduce low-temperature cracking; Control fatigue crack in normal temperature range. For asphalt binder, the aging of asphalt mixture in mixing station and paving process is simulated by rotating film oven test. Pressure aging container is used to simulate the aging of asphalt in the process of pavement use. For aggregate, when designing aggregate gradation of asphalt mixture, the concepts of control points and restricted areas are used to define and optimize the experimental gradation design. Asphalt mixture specimens are prepared by rotary compaction machine. In the process of compaction, the relationship between the number of rotary compaction and the height of the specimen is recorded, so as to evaluate the volume characteristics of asphalt mixture.
3 SAC is in China.
Academician Sha Qinglin once wrote an article (Highway No.6, August, 2003) detailing that the development direction of dense hot-mixed mineral aggregate gradation is coarse aggregate gradation. SAC series is a coarse aggregate graded asphalt concrete independently developed by China. From the completion of Zhengding Experimental Road 1988+065438+ at the beginning of October, it was applied to expressways over 2000Km, and from 16, it was applied to expressways over 2000 km through unremitting efforts and painstaking research.
4 Comparison between 4Superpave and SAC
The two most remarkable differences between Superpave and SAC are: First, the instruments and equipment are different. Superpave uses a rotary compactor, and SAC is mainly a Markov compactor. The second is mineral aggregate gradation. Although Superpave put forward the maximum density line, the determination of gradation depends on experience, and there is no gradation design method related to structural state, so SAC is not.
One of the main contents of Superpeave is to provide the method of mineral aggregate gradation composition and concrete mineral aggregate gradation. Superpave no longer directly uses Fuller formula to calculate gradation curve, but still draws a maximum density line according to this formula, and on this basis, seven control points and a restricted area are set up above and below this line as the basis for designing mineral aggregate gradation or mineral aggregate particle composition curve. As shown in figure-10.45 power series curve.
Superpeave does not advocate continuous gradation similar to the diagonal on the diagram, but requires that the gradation curve not only passes between the seven control points on the diagram, but also does not enter the restricted area (later, some experiments of Tongji University proved that the performance of asphalt concrete graded by mineral aggregate in the restricted area is even better than that of asphalt concrete without mineral aggregate gradation in the restricted area. In fact, the United States has long used the restricted area. This grading curve is no longer continuous grading, and it is usually required to be below the restricted area at the grading curve in the United States, which is called coarse aggregate grading. The publication of Superpave mineral aggregate gradation composition is actually equivalent to giving up the traditional continuous gradation that has been used in the United States for nearly a hundred years. It should be said that Superpave mineral aggregate gradation design method has no clear principles and belongs to experience.
The design principle of SAC mineral aggregate gradation is that coarse aggregate is used to form the skeleton, and fine aggregate is used to fill the pores in the skeleton. The asphalt concrete designed in this way is not only low in dense air rate, but also difficult to penetrate water, and has strong resistance to permanent deformation at high temperature. The grading of SAC mineral aggregate is divided into three parts. The boundary between coarse aggregate and fine aggregate is 4.75mm, that is, coarse aggregate is greater than 4.75mm, fine aggregate is 4.75~0.075mm, and filler is less than 0.075mm. Usually, three control points are used. The first control point is the throughput of the nominal maximum particle size, such as 95%, 97.5% or 100%. The second control point is the throughput of 4.75mm network, such as 30%, 35% or 40%. The third control point is the throughput of 0.075mm mesh, such as 4 ~ 65438. For mineral aggregate energy matching, instead of giving a wide range of gradation as before, the mineral aggregate gradation curve is determined by calculation formula. The basic calculation formula is as follows:
Pdi=A(di/Dmax)B
Where: PDI- throughput of mesh size DI;
Dmax-the maximum particle size of each part of mineral aggregate (mm);
Di- a mesh size (mm);
A, b-coefficient;
Another notable feature of SAC mineral aggregate gradation design is that after the mineral aggregate gradation is determined, it needs to be tested by dry-pressing void ratio VCADRC of coarse aggregate and void ratio VCAAC of coarse aggregate in asphalt concrete. So as to establish a hierarchical design method related to structural performance.
VCADRC method is used to detect mineral aggregate gradation of raw materials (including asphalt). Adjust and restore the mixture ratio and corresponding mineral aggregate gradation that meet the application requirements of the target mixture ratio. Its basic principle is to take the porosity VCADR of air-dried coarse aggregate with different densities (such as dry tamping density, general density and loose density) and the available porosity after deducting the reserved porosity Va (expressed by VCADRU), that is, VCADR-Va as the standard, and the sum of the volume ratios of skeleton dense structure, fine aggregate, filler and asphalt (referred to as asphalt mortar volume for short, expressed by B and B Vuormaa) just fills VCADRU.
VCAAC method is used to test and adjust the mineral aggregate gradation of asphalt concrete specimens, and finally the mixture ratio and corresponding mineral aggregate gradation required for production are obtained. Its basic principle is that the gaps between coarse aggregate skeletons in asphalt concrete with dense skeleton structure are just filled by the sum of the volume ratios of fine aggregate, filler and asphalt (which can be called the volume of asphalt mortar, expressed by B and B Vuormaa) and the reserved void ratio VA. Through experiments, the design method of mineral aggregate gradation related to structural state is established, so that coarse aggregate can be truly embedded and extruded. This is a breakthrough in SAC design.
5 Can 5Superpave and SAC be organically combined?
Since SAC and Superpave have their own advantages and disadvantages, can they be perfectly unified, such as using SAC's volume grading design concept, Superpave's rotary compactor, PG grading of binder, performance evaluation of mixture and material selection method.
To sum up, a new design method of mixture is put forward, which I call Chinese Superpave. The mixture design process is as follows:
1 material selection
According to the temperature conditions of the project site, asphalt binder is selected, and aggregate is selected according to the traffic volume standard.
2 aggregate gradation selection
According to SAC mineral aggregate gradation design method, aggregate gradation is selected, and FHWA0.45 power diagram is still used to control gradation composition.
3 determine the asphalt content
According to the estimated initial asphalt consumption; SGC specimens are composed of 0.5% and 1.0% of the initial asphalt content, and the asphalt content is selected according to Superpave grade.
4 Verify the volume property and water sensitivity of the selected mixture.
Rutting test and water damage test were carried out according to Technical Specification for Highway Asphalt Pavement Construction (JTGF40-2004). The aging of the mixture shall be conducted according to the short-term aging test of PP2, and the water sensitivity shall be conducted according to T283 (or it may be conducted according to the methods of JTJ 052-2000 and T0729-2000, and the porosity shall be the same as T283, with Va=7%).
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