The design of Superpave compared with Marshall design method

In order to compare and analyze Superpave and Marshall design method for asphalt mixture. Through the analysis about gradation selection, predicition asphalt content, the determination method of theoretical maximum density, the compaction methods of specimens, the determination method of optimum asphalt content and the test method of mixture performance of these two design methods. The following conclusions have been come to Superpave design method is better than Marshall design method for asphalt mixture. Keyword Superpave Marshall design method gradatiom compaction methods optimum asphalt


FOREWORD
At present, the common asphalt mixture design method is Marshall (Marshell) method and the Superpave method.Marshall method first by Bruce Marshall (Brue Marshall) is put forward, in 1948 the U.S. army corps of engineers of this method is improved, and add some test performance test, standard development into the mix proportion design of asphalt mixture; Superpave method is 1987 ~ 1992, the United States SHRP plan of asphalt project research results, this system puts forward a new set of design based on performance related to traffic and climate on the basis of material selection and mixture design method.
The Marshall developed on the basis of experience and have some certain limitation at the field of method of sample shaping and test indexes.And the Superpave is not popularized in china because of the stiff price.There have some different between the grading choose, the method of sample shaping and OAC design.So most technician care for that whether have some point and relationship between two methods.

COMPARE WITH THE METHODS
(1) the grading choose Marshall method is fit of the design of asphalt of continuity dense grading.In the past the grading choose usually use the mid-value that recommend by norm.The present norm was required to confirm the "engineering design gradation scope" by engineering practice, and according to the highway classification, climate and traffic condition to choose the coarse type (C) or thin type (F).Usually the gradation curve must continuousness, smooth and has large compactness.It is need to analyze whether form the skeleton structure .
Superpave gradation selection is through (area) to implement the control points and limits, locus of control is the requirement that aggregate gradation shall not exceed that of the specified range, respectively in nominal maximum size screen, medium sieve (2.36 mm) and minimum screen (0.75 mm).Its purpose is that to limit the amount of sand and provide enough VMA.Restricted area is located in the biggest density level wiring between the medium sieve and 0.3 mm sieve.There have same question of compaction if the asphalt beyond the restricted and lack of ability to resist permanent deformation during using [3] .And then, it is cause VMA too small to affect the asphalt usage when the gradation beyond the restricted by Superpave.Therefore, Superpave should make design gradation in between control points and avoid the limits when choose the gradating, and this grading are usually formed the skeleton structure.
(2) estimate the content of cementing agent Marshall use this formula to estimate the asphalt usage: This way can quickly to count the resolute.But there have some reasons to cannot confirm the estimate proportion of asphalt was very access the really one.The reasons are that there have much error because the data that from the constructed project is not fully reliable [4].
The formula about estimate the asphalt usage in the Superpave is : Through the formula it is can be found that the superpave about the method of asphalt usage is conclude the volume based on the mixture.The Pb need a initial value is from the experience.And the coefficient C choose by hydroscopicity of aggregate.The more small the value, the more big the hydroscopicity, and common the rage is 0.5~0.8.
There have to setting parameter is Pb and C if measure the density of aggregate and cementing agent because the void ratio Va is 4% in the Superpave.The void ratio of Superpave is 4%, but marshall is a rage.Marshall will confirm a aim void ratio when count the asphalt-aggregate ratio, and the aim void ratio is confirm by experience.
(3) confirm the max theory density ratio The Marshall using the vacuum test method for determining the biggest theory relative density for the modification of ordinary asphalt mixture, and for the modified asphalt and SMA mixture calculated using the following formula: Superpave have hear tester (SST) and indirect tensile test and instrument (IDT) to test and prediction the mixture performance.These two instruments were improve the forecast methods for performance test.But it was too expensive and the research are rarely.Superpave about water sensitivity of the test mixture, is not based on performance test, and there are two purposes: first, determine whether the combination of asphalt cement and aggregate is sensitive to water; Second, the effect of measuring antistripping agent.Superpave mixture water sensitivity of the test method is similar to the current specification of freeze-thaw splitting test [3].
Due to the shear test and indirect tensile test method and standard is perfecting, when the domestic scientific research units in the Superpave mixture design, they usually to test method to performance verification of the mixture by Marshall.Although it is not fully reflect the Superpave mixture excellent road performance, it yet be regarded as a kind of auxiliary Superpave mixture design method.

CONCLUSION
1 when gradation selection, Superpave used the concept about restricted area and restricted, and the design of the grading was better than Marshall generally with skeleton structure.
2 Superpave is more reasonable than experience of the Marshall method because it is based on the volume of the mixture properties for Forecast for cement content 3 To determination the theoretical maximum density of mixture, the Marshall method use the vacuum test for ordinary asphalt mixture, and for modified asphalt mixture was used by calculation method.But Superpave are all made of the measurement method.It is due to the many shortcomings of measurement method for modified asphalt mixture, that use the Marshall to test the max theoretical density .
4 Superpave conclude the traffic parameter for moding.At the same time to conclude the situation of pavement.Ttherefore the design of mixture density than the Marshall method design.

References
the percent of cementing agent; W is the quality of aggregate, g; be