The Hot Mix Asphalt (HMA) industry in developing countries (in general) and in Pakistan (in particular) is facing a substantial challenge to meet the increasing transportation demands in terms of traffic volume and loads.
High ambient temperatures and truck loads further complicate the problem and accelerate premature pavement failure in Pakistan. In order to meet the challenge, the HMA industry and the transportation authorities must embark on critical endeavors to enhance the design of HMA and to improve the selection of the materials used in highway construction. These require better understanding of the impact of design methodologies and material characteristics on the long-term pavement performance.
Zaynab (2017) evaluated the performance of asphalt mix by using Marshall and Superpave techniques, after adding filler content. The objective of an asphalt concrete mix design method is to determine the proper proportions of aggregates and asphalt to produce an economical mix that meets the performance requirements of the pavement. Namir (2009) conducted his research on the compaction method and determined the performance of Hot Mix Asphalt (HMA) using conventional and Superpave methods. Superpave is an acronym for Superior Performing Asphalt Pavements. Superpave is a new, comprehensive asphalt mix design and analysis system, a product of the Strategic Highway Research Program (SHRP).
The Superpave mix design method was designed to replace the Hveem and Marshall methods. The volumetric analysis common to the Hveem and Marshall methods provides the basis for the Superpave mix design method. The Superpave system ties asphalt binder and aggregate selection into the mix design process, and considers traffic and climate as well.
The compaction devices from the Hveem and Marshall procedures have been replaced by a gyratory compactor and the compaction effort in mix design is tied to expected traffic. Under the Strategic Highway Research Program (SHRP), an initiative was undertaken to improve materials selection and mixture design by developing: • A new mix design method that accounts for traffic loading and environmental conditions.• A new method of asphalt binder evaluation.
• New methods of mixture analysis.Safwan (2016) told that when SHRP was completed in 1993 it introduced these three developments and called them the Superior Performing Asphalt Pavement System (Superpave). Although the new methods of mixture performance testing have not yet been established, the mix design method is well-established. Superpave is a performance-related asphalt binder and mixture specification.
Superpave is not just a computer software package, nor just a binder specification, nor just a mixture design and analysis tool. Superpave is a system which is inclusive of all these parts.Zumrawi (2016) compared Superpave mix design with Marshall Mix design using local materials of Sudan. Superpave was created to make the best use of asphalt paving technology and to present a system that would optimize asphalt mixture resistance to permanent deformation, fatigue cracking and low temperature cracking. The key parts of the process are the Performance Graded (PG) system for specifying the properties of the asphalt binder and the volumetric and densification characteristics determined by the Superpave Gyratory Compactor (SGC).
The system was developed and calibrated for a wide range of applications. Israa (2012) conducted a research on the comparison of both mix design techniques and evaluated the performance of HMA against ambient temperature and high traffic volume. The high traffic intensity in terms of commercial vehicles, the serious overloading of trucks and significant variation in daily and seasonal temperature of the pavement have been responsible for early development of distress like rutting, fatigue and thermal cracking on bituminous surfacing. One of the advantages of the Marshall Mix Design method is that the performance of the mixes can be expected for local materials and environmental impact. The Superpave mix design method differs from the Marshall Mix design methods by using performance-based and performance-related criteria to design the proper asphalt mix.
This allows a direct relationship to be drawn between the lab and field performance of the asphalt mix. 2. Problem statementIn developing countries the dramatic growth in vehicular traffic have augmented axle loads and increased tire pressure on the pavements resulting in rutting and cracking. (et al, 2013). Khosla (2013) told that compaction of asphaltic concrete mixtures in flexible pavements plays a major role in the performance of these pavements. Israa (2012) linked volumetric characteristics and Mix properties with the compaction method. It was added that mix density and air voids are highly dependent on the degree and the method of compaction. These properties in turn affect pavement performance indicators, such as rutting and fatigue cracking.
Imran (2014) worked on the failure criteria of flexible pavements and told that the cause of failure of pavement is volumetric mix design which causes cracking due to low temperature and liquid-binder specification. Superpave mix design method is designed to replace Marshall Method of mix design. In Superpave design, asphalt pavements are designed to resist thermal cracking and rutting under extreme temperatures.
New procedures in the Superpave methodology have been developed to address these problems more effectively through the use of physical tests conducted under laboratory conditions by duplicating more accurate pavement loading and failure conditions in the field.This research is conducted with the aim to use the existing conventional methods of Asphalt mix design as well as the Superpave mix design to evaluate the physical, engineering and rheological properties of the various constituent materials of the HMA mixes and to analyze the impacts of these properties on the long-term pavement performance in lieu of the local traffic and environmental conditions in Pakistan. This will help in reducing the chances of fatigue or rutting failures in flexible pavements.3. ObjectivesThe objectives of this research study are:? To compare conventional method with Superpave technique and to measure its effectiveness under heavy axle loadings and atmospheric conditions.
? To evaluate the performances of both methods, using locally available materials, on the basis of different laboratory tests. ? To study the impact of gradation bands, aggregate requirements, volumetric properties and design gyration levels on the performance of HMA. 4. Literature reviewPanneer (2016) during the study of transition from Marshall to Superpave Mix Design evaluated the performance of both methods.
The Marshall method of mix design had been used for many years and those pavements have performed well, however, with increased traffic and heavier axle loads, it was decided that an improved method of design was needed. The Superpave mix design method was developed to fill this need. A Superpave design system implemented at three levels.
The level one method relied totally on volumetric analysis to determine mix proportions. The other levels of Superpave analyses require complex equipment and have not been implemented. The purpose of any asphalt mix design method is to determine the optimum proportions of aggregate and asphalt cement to be used in an asphalt pavement mix.
Two empirical mix designs methods are traditionally used. These are Marshall and Hveem Methods. Superpave method developed by the Strategic Highway Research Program (SHRP), is being considered for full implementation as a design method.
The main advantage of Superpave over currently used mix design methods is that it is performance-based method that implies a direct relationship between Laboratory analysis and field performance after construction. Other design methods are empirical and therefore cannot accurately predict how a pavement will perform after construction. Juaidah (2014) evaluated permanent deformation of Superpave and Marshall Mix using SPT Dynamic Modulus Test. The low VMA (Voids in Mineral Aggregates) of Superpave mixes can generally be contributed to the increased compactive effort by Superpave gyratory compactor. This has led to the increased use of coarser asphalt mixes (gradations near the lower control points).
The inference made was the minimum VMA requirements in Superpave volumetric mix design for these coarse mixes are the same as those developed for the dense mixes designed by the Marshall method. Di Mino (2007) conducted a research on the volumetric mix-design of porous asphalt. The Superpave mix-design of asphalt mixtures using the gyratory compactor is a procedure used to define the proportion of materials on the basis of specific volumetric parameters.
In Superpave, the number of gyrations, Ndesign, is important because it represents the compactive effort that produces a test sample with voids properties similar to those that the same mix would experience in the field. Aravind (2009) evaluated the hot recycled mix asphalt for the determination of its optimal proportioning under Superpave mix design technique. Increased traffic, axle loads and tire pressures, coupled with limited financial resources have resulted in commonly occurring overstressed asphalt pavements. These conditions have forced asphalt engineers and researchers to reconsider the current mix design approaches. The proper selection of the aggregates and the asphalt binder can improve pavement performance, depending upon the environmental and traffic conditions to which the pavement is exposed. However, the asphalt concrete mix will not perform as required if the proper compaction procedure is not followed.
Alani (2010) selected Fifteen Hveem mix designs from around the state that are often used in their region were used as the basis of this study. The 15 selected mix designs vary in binder Performance Grading (PG-grade), binder type (unmodified, rubber, and polymer), aggregate gradation and mineralogy, and Reclaimed Asphalt Pavement (RAP) percentage. Based on the Hveem mix designs, Superpave volumetric mix designs were developed for each mix and comparisons were made between mixes developed from both methods. Specifically, the mixes were evaluated to meet the draft Caltrans Superpave volumetric mix design specification which includes the design air-void content, percent VMA, percent VFA (Voids Filled with Asphalt binder) and dust proportion as major design components.Zaynab (2017) evaluated the performance of different aggregate-asphalt binder blends. The Superpave method, like other mix design methods, creates several trial aggregate-asphalt binder blends, each with different asphalt binder content. Then, by evaluating each trial blend’s performance, optimum asphalt binder content can be selected.
Cooley (2007) said that the trial blends must contain a range of asphalt contents both above and below the optimum asphalt content. Therefore, the first step in sample preparation is to estimate optimum asphalt content. Trial blend asphalt contents are then determined from this estimate. The Superpave gyratory compactor was developed to improve mix design’s ability to simulate actual field compaction particle orientation with laboratory equipment.Al-Mistarehi (2014) did a research on calculating design number of gyrations.
Each sample is heated to the anticipated mixing temperature, aged for a short time (up to 4 hours) and compacted with the gyratory compactor, a device that applies pressure to a sample through a hydraulically or mechanically operated load. Mixing and compaction temperatures are chosen according to asphalt binder properties so that compaction occurs at the same viscosity level for different mixes. Key parameters of the gyratory compactor are: ? Sample size = 150 mm (6-inch) diameter cylinder and 115 mm (4.5 inches) in height.
? Load = Flat and circular with a diameter of 149.5 mm (5.89 inches) and area of 175.5 cm2 (27.24in2).? Compaction pressure = Typically 600 KPa (87 psi).? Number of blows = varies.
? Simulation method = the load is applied to the sample top and covers almost the entire sample top area. The sample is inclined at 1.25o and rotates at 30 revolutions per minute as the load is continuously applied. This helps achieve a sample particle orientation that is somewhat like that achieved in the field after roller compaction.Panneer (2016) studied the effect of compaction method on the performance of HMA. Superpave Gyratory Compactor was used to find the design number of gyrations.
The Superpave gyratory compactor establishes three different gyration numbers:1. Ninitial. The number of gyrations used as a measure of mixture compactability during construction. Mixes that compact too quickly (air voids at Ninitial are too low) may be tender during construction and unstable when subjected to traffic. Often, this is a good indication of aggregate quality – HMA with excess natural sand will frequently fail the Ninitial requirement.
A mixture designed for greater than or equal to 3 million ESALs with 4 percent air voids at Ndesign should have at least 11 percent air voids at Ninitial.2. Ndesign. This is the design number of gyrations required to produce a sample with the same density as that expected in the field after the indicated amount of traffic. A mix with 4 percent air voids at Ndesign is desired in mix design.3. Nmax.
The number of gyrations required to produce a laboratory density that should never be exceeded in the field. If the air voids at Nmax are too low, then the field mixture may compact too much under traffic resulting in excessively low air voids and potential rutting. The air void content at Nmax should never be below 2 percent air voids.Ahmad (2014) used local materials to find their suitability for performance based evaluation and performed different tests to determine their resistance against rutting.
The local material satisfies the Superpave consensus and source aggregate properties criteria and is therefore suitable for use in the Superpave system. Superpave-designed mixtures are more superior and least susceptible to permanent deformation compared to Marshall-designed mixtures based on pavement performance tests. The Simple Performance Test (SPT) dynamic modulus test has the potential to replace the resilient modulus test, wheel tracking test, dynamic creep test to evaluate rutting deformation. Rutting can be better performed using the SPT dynamic modulus test and most of the correlations between these tests are moderate to strong which indicates that the SPT dynamic modulus test is viable and reliable in predicting rutting performance. As such, a large amount of specimen fabrication can be minimized to be used for different testing methods.
Therefore, the dynamic modulus test is highly recommended for Superpave mixture characterization under tropical climatic conditions since this test provides full characterization of the mix over a broad range of temperatures and loading frequencies.To investigate the performance characteristics of Superpave and Marshall Method design HMA mixtures in tropical climatic conditions. Laboratory tests were conducted to evaluate the rutting (permanent deformation) and resilient modulus of different Superpave and Marshall mixes. In addition, dynamic modulus tests by means of the Simple Performance Test (SPT) were also conducted. The relationships between the SPT dynamic modulus test and other performance test results were also examined.
It was found that the Superpave-mix design showed far superior performance compared to the Marshall-mix design based on all types of testing in this study. Since the dynamic modulus test provides full characterization of the mix over a broad range of temperatures and loading frequencies, this test is highly recommended for Superpave mixture characterization under tropical climatic conditions. Ibrahim (2012) conducted in Taiwan to compare the volumetric and mechanical performance properties of Superpave mixtures and Typical Taiwan Mixtures (TTM) using the Marshall method. It was found that the binder contents of the Superpave-designed mixtures are lower than the TTM Marshall-designed mixtures. It was found that the Superpave mix design procedure recommended, for the local environmental and loading conditions, lower asphalt content than that predicted by Marshall Mix design procedure. In addition, it was found that using the presently recommended local aggregate gradation for heavy traffic in the Superpave design method gave dust proportion higher than the maximum specified limit by the Superpave procedure. High dust proportion will usually lead to brittleness of the mixes. Therefore, shifting to the Superpave design procedure might help in solving the bleeding problem and some of the distresses common in the local asphalt structures.
Aziz (2012) studied the volumetric characteristics of Hot Mix Asphalt (HMA) samples which were prepared using Superpave Gyratory Compactor (SGC). Superpave uses volumetric analysis for the mix design and follows three major steps in the testing and analysis process. The unique feature of Superpave system is that it is a performance-based specification. The tests and analysis have direct relationships to field performance. The Superpave mix design procedure involves selecting of asphalt and aggregate materials that meet the Superpave specifications and then conducting a volumetric analysis of mix specimen compacted with the Superpave gyratory compactor.Zumrawi (2016) did a research on the compaction method of HMA samples preparation. During this research, the impact of using Gyratory compactor on the performance of HMA was studied.
The Gyratory Testing Machine (GTM) developed by US Corps of Engineers. This machine has the capability to compact HMA mixtures using a kneading process that simulates the action of rollers during construction. The GTM was operated at vertical pressure of 8.2 kg/cm2, which was approximately equal to the truck tyre inflation pressures, the gyration angle degree and 300 revolutions. The GTM can be used for achieving the ultimate density is obtained in the actual field.
It was observed that the design asphalt content for Superpave and Marshall Mixes were found to be 5.3 and 5.5 percent (by weight of mix) respectively. It was concluded that, higher density in Superpave is due to Superpave Gyration Compactor compactive effort. From the study it was concluded that, performance based Superpave mixes performed better than Marshall Mixes.5. Research MethodologyThe following research methodology will be adopted in this study:Materials such as aggregates will be collected from Sargodha and Margalla quarries and bitumen will be brought from refineries like Attock, Parco and National refinery. The properties of aggregates and bitumen will be found by various tests.
? Comprehensive literature review will be conducted.? Aggregates will be tested for gradation, hardness, angularity and strength using the following tests;• Abrasion Test will be performed to test the suitability of road stones to resist the abrading action due to traffic.• The specific gravity test of the aggregates is conducted to measure of strength or quality of the material.
Stones having low specific gravity are generally weaker.• The problem of stripping is experienced when the bituminous pavement layer is subjected to prolonged soaking under water. Stripping problem is more predominant in bituminous mixes which are permeable to water. For the purpose stripping test is performed.? Asphalt Binder will be tested for determining different properties like Softening point, penetration, viscosity, thermal cracking, fatigue analysis, aging etc.
• The point at which the bitumen flows and changes its state is the softening point. Softening Point test is conducted to find the softening point of binder.• Penetration test is one of the indirect methods to determine the consistency of bitumen.
Bitumen is allotted a grade number on the basis of this test.• Dynamic shear rheometer test (DSR) will be performed for the analysis of fatigue and rutting failure in asphalt samples for Unaged, RTFO aged and PAV aged binder conditions.• Bending beam rheometer test (BBR) will be conducted for low temperature cracking of the binder.
• Rotation viscometer test will be performed to determine the viscosity of asphalt binder.• RTFO (Rolling Thin Film Oven) test is performed for short term aging of the binder and PAV (Pressure Aging Vessel) test is conducted for long term aging of asphalt binder. ? Hot Mix Asphalt (HMA) will be evaluated on the basis of the following tests;• Dynamic Modulus Test is performed to find the stiffness characteristics of HMA which is the function of loading rate and temperature.• The indirect tensile strength test (IDT) is performed to determine the tensile properties of the bituminous mixture which can further be related to the cracking properties of the pavement. Low temperature cracking, fatigue and rutting are the three major distress mechanisms.• Resilient Modulus (MR) Test is conducted to determine the resilient modulus of HMA which is used to check how pavements respond to traffic loading.
? After performing these laboratory tests, results from both conventional and Superpave techniques will be compiled, compared and evaluated on the basis of their performances. Figure-1 presents the flow chart of the research methodology to be adopted in this research.