Marshall vs Superpave Pavement Mix Design

Introduction

Asphalt pavements are the backbone of modern transportation infrastructure, providing durable and smooth surfaces for roads, highways, and airports. The design of asphalt pavements is a critical process that involves selecting appropriate materials and construction techniques to ensure longevity and performance under varying traffic and environmental conditions. Two prominent methods in pavement design are Marshall method and Superpave method of pavement design. Both methods have been developed to address specific challenges in pavement engineering and offer unique approaches to achieving optimal performance.

The Marshall Method

The Marshall method, named after Bruce Marshall, an engineer at the Mississippi Highway Department, was developed in the 1930s. It is one of the oldest and most widely used methods for designing asphalt concrete mixtures. It focuses on determining the optimum asphalt binder content for a given aggregate blend through a series of laboratory tests which include compaction, stability, flow, and density measurements conducted on compacted cylindrical specimens.

The Marshall Mix Design process includes the following steps: selection of aggregates, bitumen selection, sample preparation, and determining the optimum asphalt content. The desired aggregates are selected based on the physical properties, their size and gradation. The suitable asphalt binder is selected based on the requirement of the asphalt mixture. The aggregates and binder that are selected in the above process are mixed and samples are prepared at different asphalt content.

Samples should be compacted at a total of five bitumen content and for each binder content three trial samples are prepared. The samples that are compacted by dropping a Marshall Hammer, weighing 14kg and the height of fall being 450 mm, at different binder content for determining the optimum asphalt binder content, the asphalt binder content that corresponds to the desired air void content, typically 4% is selected as the optimum binder content. Each drop of hammer is known as a blow; the number of blows varies with different traffic considerations.

The air void content of the Marshall compacted samples are obtained and are placed for testing, the stability and flow values are obtained. These tests are conducted at 60oC.

The graphs are plotted to understand the relationship of binder content with the various parameters such as Density, Stability, Flow, Airvoids, Voids in Mineral Aggregates and voids filled with Asphalt.

The volumetrics properties of the asphalt binder is studied with the graph determining the properties at the optimum asphalt binder content using the graphs.

Design requirement comparison: The values are then compared with the design requirement values, if all the values comply to the design requirement, then the particular optimum binder content is selected.

The Marshall Mix Design method is widely used in the construction of highways and other pavement applications due to few key features.

Key characteristics of the Marshall method include:

The Marshall method of mix design gives an emphasis on the empirical relationships between aggregate gradation, and asphalt content. It has a relatively simple testing procedure which has been accepted widely by agencies in the world. The cost effectiveness of this method, due to its simple and economical equipment requirement, makes it an attractive option to the users. The Marshall method of mix design has proven itself in the past making it as a trusted approach.

However, there are few limitations to this method: The Marshall method uses an impact method of compaction, which some believe do not simulate the mixture densification effectively as in the field. The Marshall method compacts the mixture perpendicular to the loading axis which might not simulate proper densification. The mixes fail by shear in rutting, however this method doesn’t consider the shear strength of the HMA which makes it difficult to ensure rutting resistance. The method mostly focuses on determining the asphalt content while it cannot address the variation in traffic and climatic consideration.

In summary, while the Marshall Mix Design method is widely used and has its advantages, it also has limitations that may make it less suitable for certain applications or in comparison to other mix design methods like Superpave. Overall, the Marshall Mix Design method stands out for its cost-efficiency, adaptability to local materials, and established performance record, making it a preferred choice for designing asphalt mixtures.

The Superpave System

The Superpave (Superior Performing Asphalt Pavements) system emerged in the 1990s as a response to the shortcomings of traditional asphalt pavement designs. Developed by the Strategic Highway Research Program (SHRP) in the United States, Superpave is a performance-based approach that considers the properties of asphalt binders, aggregate characteristics, and traffic loading conditions. It is an enhanced method for identifying asphalt binders, mineral aggregates, designing of asphalt mixtures, and predicting the performance of the pavement.

The Superpave mix design involves the following steps: Selection of materials, design aggregate structure selection, estimation of design asphalt binder content, and assessment of the moisture sensitivity of the mix.

The Superpave mix design begins with selection of aggregates and binders. The aggregate selection process is critical for the performance of the asphalt mixes, and aggregate characteristics are a major factor in the performance of an asphalt mixture. The aggregate should satisfy the consensus properties and the source properties. Apart from that, the dust proportion is carefully adjusted in the gradation. The dust proportion /binder plays an important role in the mix design.

The gradation involves selection of aggregate distribution to achieve an optimum packing density, workability, and strength. A well-graded aggregate mixture enhances the interlocking effect between particles, creating a more stable matrix and improving the load-carrying capacity. Proper control of aggregate gradation is essential to ensure that the mixture meets the specified requirements and delivers the desired performance. The selected aggregate gradation and the binder are mixed using a gyratory compaction which provides shear to compact the mixes. A load of 600 KPa is applied to the samples to compact the samples. While applying the gyration at a rate of 30 gyration per minute, an angle of 1.160 is maintained. The compaction method is assumed to provide a similar effort as that on the field by the rollers.

The Superpave mix design process is divided into three levels based on traffic level: Level 1 (low traffic) where only volumetric properties are required, Level 2 (intermediate traffic) where Volumetric and some limited performance test are required, and Level 3 (high traffic) where volumetric and advanced high level performance test are required. Depending upon the traffic conditions the required mix design is done. The Superpave design consists of three gyration number Ninitial, NDesign, and Nmax. Ninitial number of gyrations to achieve 11% airvoid which simulates the air void of the mix that is just behind the screed. Ndesign is the number of gyrations to 4% airvoids which simulate the field density during its service life. Nmax is the final density that the pavement is expected to achieve by densification at the end of service life. These parameters are sensitive to gradation, binder content, mix temperature and the type of traffic.

Important feature of Superpave Mix Design

The Superpave mix design is based on performance characteristics of the pavement, which allows for a more accurate prediction of pavement performance. As binder testing is done, it offers better asphalt binder properties than other methods. The low temperature testing of the materials can be suitable for cold climatic regions. It considers the traffic volume and climate conditions, which are crucial factors in determining the performance of asphalt mixtures.

Limitations

The Superpave mix design is more complex than other methods which may make it more difficult to implement. It requires specialized equipment, such as the Superpave gyratory compactor, which can be expensive and less portable for low-traffic road projects. Very few field data is available that compares the Superpave with other mix design method, in turn making it difficult to compare the performances.

Conclusion

The Superpave Mix Design offers several advantages, including performance-based approach, improved binder properties, detailed low-temperature testing, integrated material selection, consideration of traffic and climate, and three levels of testing. However, it also has some disadvantages, such as complexity, equipment requirements, and limited comparison data.

Comparative Analysis between Marshall vs Superpave Mix Design

Material Selection and Characterization

Marshall Method relies on empirical relationships and experience-based guidelines for selecting asphalt binders and aggregates. Limited emphasis on binder properties.

Superpave utilizes advanced laboratory tests to characterize asphalt binders and aggregates, leading to more precise material selection and performance prediction.

Mix Design Procedure

Marshall Method employs a series of simple laboratory tests (e.g., compaction, stability, flow) to determine the optimum asphalt content.

Superpave integrates a comprehensive mix design process involving binder and aggregate tests, volumetric analysis, and performance modelling.

Performance Prediction

Marshall Method relies on historical data and empirical correlations to estimate pavement performance.

Superpave utilizes mechanistic-empirical models to predict pavement performance based on material properties, traffic loading, and environmental conditions.

Flexibility and Adaptability

Marshall Method has limited flexibility in accommodating varying traffic and climatic conditions and a relatively conservative approach.

Superpave offers greater flexibility and adaptability using performance models and advanced testing methods.

Key differences between Superpave and Marshall

• Design Philosophy: Superpave is a performance-based approach that considers traffic volume, climate, and pavement performance, while the Marshall method is more focused on aggregate selection and asphalt binder content determination.
• Compaction Devices: Superpave uses a gyratory compactor, while the Marshall method uses a Marshall drop hammer.
• Aggregate Specifications: Superpave specifies aggregate in three ways: gradation specifications, physical property requirements on aggregate angularity, and restrictions on aggregate gradation.
• Binder Selection: Superpave ties asphalt binder and aggregate selection into the mix design process, while the Marshall method primarily addresses the determination of asphalt binder content.
• Low-Temperature Testing: Superpave includes a more detailed low-temperature testing procedure, which is beneficial for regions with cold climates.
• Equipment Requirements: Superpave mix design requires specialized equipment, such as the Superpave gyratory compactor, which can be expensive and less portable for low-traffic road projects.

In summary, the Superpave method is a performance-based approach that considers traffic volume, climate, and pavement performance, while the Marshall method is more focused on aggregate selection and asphalt binder content determination. Superpave uses a gyratory compactor, while the Marshall method uses a Marshall drop hammer. Superpave specifies aggregate in three ways and includes a more detailed low-temperature testing procedure. Superpave requires fewer design gyrations and has different compaction requirements. Superpave mix design requires specialized equipment, while the Marshall method is known for its simplicity, economical equipment, and proven record.