Design Gyrations Research Articles

A performance-related design for hot-mix asphalt (HMA) incorporating multiple design parameters

Traditionally, the mix design was majorly the trial and error process of optimising aggregate gradation and asphalt content. With the progress of concept and technology, the number of design gyrations (N des) has been gradually regarded as another important design variable. This research explored the potential advantage of mixes designed incorporating multiple design parameters. The paper is divided into two parts. Part 1 designed the varied aggregate gradations with different levels of N des to evaluate the effect of incorporating the N des variable in a traditional process. Part 2 further explored the possible combinations of multiple design parameters, involving binder type, N des, RAP addition and aggregate gradation. The result showed that by selecting a suitable level of N des, the performance of mixes with different aggregate gradations can be well balanced to a certain extent. Besides, the performance of HMAs may be largely improved and some marginal materials can be reutilised by selecting a suitable combination of multiple design parameters.

Laboratory Investigation and Machine Learning Modeling of Road Pavement Asphalt Mixtures Prepared with Construction and Demolition Waste and RAP

Due to the decreasing availability of virgin materials coupled with an increased awareness of environmental sustainability issues, many researchers have focused their efforts on investigating innovative technological solutions in the civil engineering domain. This paper aims to evaluate the suitability of construction and demolition waste (C and DW) and reclaimed asphalt pavement (RAP) reused within asphalt mixtures (AMs) prepared for the binder layer of road pavements. Both hot and cold mixing methodologies were investigated. The technical assessment was based on the volumetric and mechanical suitability, according to saturated surface dry voids (SSDV) and indirect tensile strength (ITS) tests carried out at 10 °C, respectively. Laboratory findings showed that all the hot AMs matched the desired target SSDV at the design gyrations number at different optimum bitumen content levels, alternatively showing a non-significant variation or a significant increase in ITS compared to conventional hot mix asphalt. Conversely, the cold AMs with cement and emulsion bitumen showed a greater volume of voids and moisture sensitivity, and lower temperature susceptibility compared to hot AMs, reaching, on average, 11% lower ITS when using coarse C and DW aggregates and 43% lower ITS when using filler from C and DW. These volumetric and mechanical properties were modeled by means of support vector machines and categorical boosting (CatBoost) machine learning algorithms. The results proved to be satisfactory, with CatBoost determination coefficients R2 referring to SSDV and ITS equal to 0.8678 and 0.9916, respectively. This allowed for the mechanical performance of these sustainable mixtures to be predicted with high accuracy and implemented within conventional mix design procedures.

A Study of Superpave Design Gyrations for High Traffic Surface Mixtures

The methodology of the research that was used to evaluate the comparative results of surface mixes with a nominal maximum aggregate size of 12.5mm is presented in this paper. Also presented are the recommended Ndes values for C-level and D-level mixes, which are designed to handle traffic levels of 3-30 Million and greater than 30 Million ESALs, respectively. In order to determine the amount of asphalt that was present, asphalt concrete mixes were fabricated utilizing the Superpave design process at Ndes levels of 50, 75, 100, and 125 gyrations. Using the Asphalt Mixture Performance Tester instrument, we were able to determine the dynamic modulus (E*) at the design asphalt content for a number of different gyration levels. The E* data and related binder properties were used as input in the AASHTO Darwin-ME software to anticipate the rutting and fatigue performance of the mixtures. This was accomplished by assuming a model pavement section and appropriate traffic levels. In order to determine which Ndes are most appropriate, relative performance indicators for rutting and fatigue have been developed and plotted against asphalt content. The Ndes value of 85 gyrations was found to be ideal for both surface mixes after extensive research.

A Sensitivity Study to Evaluate Intermediate Temperature Cracking Tests for Vermont Asphalt Mixes

The State of Vermont began implementing balanced mix design (BMD) in late 2019. Since 2020, both Hamburg wheel tracking test (HWTT) and Illinois flexibility index test (I-FIT) results were required to be included in the mix design submittal along with the traditional Superpave volumetric design. This paper presents a part of the ongoing benchmarking efforts in Vermont, among many other states around the country, and compares the sensitivity of the two popular intermediate temperature cracking tests [I-FIT and indirect tensile cracking test (IDEAL-CT)] to the design factors of mixes produced in Vermont. Both laboratory-produced and plant-produced mix data are evaluated, and the factors analyzed include mix type, design gyrations, production facility, binder grade, and binder supplier. We found that IDEAL-CT shows much higher sensitivity to the design factors than I-FIT according to the data presented in this study. Therefore, it is recommended that Vermont Agency of Transportation (VTrans) adopt IDEAL-CT as the intermediate temperature cracking test for BMD implementation rather than I-FIT, due to the higher sensitivity and simplicity of IDEAL-CT. Regression models are further developed for IDEAL-CT using selected mix properties to guide the mix design adjustment. Similar studies are highly recommended for other state agencies that are interested in implementing BMD.

Volumetric Mix Design Adjustments for Improving Asphalt Mixture Durability

State highway agencies (SHAs) are looking for guidance on how they can make changes to their Superpave volumetric mix design procedures to obtain higher design binder contents to improve the cracking resistance and durability of asphalt mixtures. To assist SHAs, this paper was prepared to: (a) synthesize most common adjustments made by SHAs to their Superpave volumetric mix design procedures to obtain higher design binder contents; and (b) verify the effectiveness of these adjustments through laboratory experiments. Based on a survey of SHAs, the three most common adjustments to the Superpave mix design system to obtain higher design binder contents include: lowered design gyrations (Ndesign), increased minimum requirements for voids in mineral aggregate (VMA), and lowered design air voids (including air voids regression). These adjustments were verified in a laboratory testing program which included a virgin fine-graded mixture and a 20% reclaimed asphalt pavement (RAP) coarse-graded mixture. The results suggest that lowering Ndesign may help increase the design binder content initially but mix designers can select design gradations to reduce VMA and binder content back to levels achieved with the higher Ndesign levels. The other common adjustments (i.e., increased minimum VMA and lowered design air voids) can yield higher design binder contents. However, these approaches may be ineffective if Gsb is not accurately measured during mix design or if VMA is not controlled during mix production.

Effects of nominal maximum aggregate size and compaction effort on the mechanical properties of hot-mix asphalt (HMA)

Considering the potential rutting distress and excellent fatigue performance of small nominal maximum aggregate size (NMAS) mixes, this study explored the function of number of design gyrations (Ndes) variable in the design, and attempted to balance the high-temperature and fatigue performance of these materials. Therefore, one experimental group (9.5 mm NMAS mixes under Ndes = 80 cycles) and three control groups (9.5 mm/13.2 mm/16 mm NMAS mixes under Ndes = 60 cycles) were designed. The uniaxial penetration shear test, the uniaxial compressive modulus test, the British pendulum test, the indirect tensile cracking test, and the indirect tensile fatigue test were then conducted. It can be found that through increasing the Ndes by 20 cycles, the rutting resistance of 9.5 mm NMAS mixes improved significantly, and can reach the same level as that of 13.2 mm NMAS mixes. Moreover, under this design, the 9.5 mm NMAS mixes can still perform a longer fatigue life than 13.2 mm NMAS mixes. In addition, increasing the Ndes also helps to improve the cracking resistance and dynamic modulus of these materials. Specifically, with the increase of Ndes by 20 cycles, the fracture energy and dynamic modulus of 9.5 mm NMAS mixes can improve by almost 516 J·m−2 and 1860 MPa (10 Hz and 21 °C), respectively. These results may indicate that selecting a suitable Ndes is helpful to balance the properties of small NMAS mixes.

Performance-based design of recycled hot-mix asphalt (HMA) incorporating compaction effort variable

Due to the remarkable ecological and economic benefits, more and more attention has been paid to the technology of reclaimed asphalt pavement (RAP). Currently, the potential risks of thermal and fatigue cracking still restrict the large-scale application of recycled materials. This study regards the number of design gyrations (Ndes) as a variable and uses a lower Ndes to design the recycled HMAs with higher RAP percentage, which attempts to compensate for the cracking problems by increasing the asphalt content. Five groups containing RAP content various from 0% to 80% were designed. Combined with the statistical analysis (i.e., Tukey pairwise comparison), the high-/low- temperature properties, fatigue life, dynamic modulus, and environmental impacts of this method were evaluated. It can be seen that recycled HMAs actually do not require as high compaction effort as the unrecycled ones (0% RAP). For the recycled HMAs containing 80% RAP, the Ndes of 60 cycles is sufficient for them to achieve the similar high-temperature resistance as the unrecycled ones (using Ndes = 100 cycles). In addition, the results indicate that this method can effectively improve the low-temperature and fatigue properties of recycled HMAs, even at a higher RAP percentage. Specifically, with the decrease of Ndes by 12%, the fracture energy and fatigue life of recycled HMAs can increase by almost 3–18% and 10–20%, respectively. Moreover, the method also has considerable effects on the environment. As the content of RAP increases by 20%, the energy consumption and CO2 emission can reduce by approximately 7–10% and 8–10%, respectively.

Effect of aggregate selection and design gyrations on the performance of polymer and devulcanized rubber modified mixtures

The objective of this study was to investigate the potential performance improvement of asphalt mixtures by changing the source of aggregates and the number of design gyrations during the mix-design process. Two surface course mixtures were produced in the laboratory using common Michigan’s aggregates and polymer modified binders. Their linear viscoelastic behavior was evaluated in the laboratory, together with the resistance to rutting, fatigue, and thermal cracking. The same characterization was performed on two mixtures designed with aggregates of better quality, a higher level of compaction (i.e., design gyrations), and the same binder type. Results were used for a direct comparison of thermal cracking susceptibility and for calibrating the rutting and fatigue cracking models of the Mechanistic-Empirical Pavement Design Guide (MEPDG). Results showed that the resistance to all the major pavement distresses improved sensibly. Moreover, it was observed that the presence of aggregate of better quality together with the application of higher compaction level did not affect the binder quantity of the enhanced mixtures (± 0.2%) which could be a major economic concern for State Departments. The results of this research could lead to a revision of the Superpave mix-design criteria currently adopted by the Department of Transportation.

Laboratory Performance Evaluation of Hot-Mix Asphalt Mixtures with Different Design Parameters

Aggregate gradation and asphalt type are traditional variables that affects mix design of Hot-Mix Asphalt (HMA). Recently, the number of design gyrations (Ndes) has been increasingly accepted as another variable parameter during the design process. Due to the growing shortage of high-quality raw materials, it is necessary to make full use of the combined roles between these design parameters, instead of solely relying on their individual effect, to improve the HMA properties. Therefore, this study comprehensively explored the effect of aggregate gradation, Ndes, and asphalt type on the performance of HMAs. Seven different combinations of aggregate gradation, Ndes, and asphalt type were evaluated. The volumetric indicators, uniaxial penetration shear test (UPST), unconfined compression test (UCT), low-temperature bending test (LBT), four-point bending test (FPBT), and dynamic modulus test (DMT) were used to assess the performance of HMAs designed by various parameter combinations. It was found that the contribution of adopting harder asphalt binder was able to make up for the high-temperature resistance loss caused by lower Ndes or coarser gradation. The dynamic modulus exhibited the similar phenomenon. By contrast, the harder asphalt binder led to the worse tenacity of HMAs at low temperature; however, the tenacity can be restored through using lower Ndes or coarser gradation by increasing asphalt content. In addition, the fatigue life of HMAs went up significantly by about 36 ~ 41%, when both Ndes and asphalt penetration grade decreased to one lower level.

Impact of Gyration Reduction and Design Specification Changes on Volumetric Properties of Virginia Dense-Graded Asphalt Mixtures

In 2015, the Virginia Department of Transportation (VDOT) proposed changes to their specification for asphalt mix design. The changes incorporated a reduction of design gyrations from 65 to 50; the addition of constraints on the No. 4 (4.75 mm) and No. 30 (600 µm) sieves; and adjustments to the production value for voids filled with asphalt and minimum voids in mineral aggregate. Before these modifications were fully adopted, a study was performed to assess the effect of the changes on mixture properties and laboratory performance. Eleven pairs of asphalt mixtures were evaluated; the pairs consisted of a typical VDOT 65-gyration mixture produced under the specification current at the time and a companion 50-gyration mixture designed and produced in accordance with the proposed specification. Mixtures were evaluated to determine the effect of the design parameters on volumetric properties, gradation, and permeability. The changes had very little effect on volumetric properties or gradation. Permeability effects were mixed, with 9.5 mm nominal maximum aggregate size mixtures requiring greater compaction to meet permeability requirements and 12.5 mm nominal maximum aggregate size mixtures showing improved permeability even at higher air-void contents. For the 50-gyration mixtures, core air voids were reduced, indicating the potential for increased durability attributable to improved densification in the field.

A Performance-based Evaluation of Superpave Design Gyrations for High Traffic Surface Mixes

The number of design gyrations (Ndes) is an extremely important parameter in the asphalt concrete mix design procedure using the Superpave method. The choice of Ndes is a function of total traffic during the pavement service life, expressed in ESALs. Asphalt concrete mixes for higher traffic levels are compacted to a higher Ndes as a denser mix resists rutting more effectively. However, this leads to a lower design asphalt content, which decreases fatigue performance of the mix. Therefore, a performance-oriented approach to determine Ndes was developed that optimizes mix performance with respect to both rutting and fatigue cracking.In this paper, the research methodology adopted to compare the relative performance of surface mixes with 12.5mm nominal maximum aggregate size is presented, along with recommended Ndes values for C and D-level mixes, designed to handle traffic levels of 3-30 Million and greater than 30 Million ESALs, respectively. Asphalt concrete mixes were designed at Ndes levels of 50, 75, 100 and 125 gyrations to determine asphalt content using Superpave design method. Dynamic modulus (E*) was measured at design asphalt content for different gyration levels using the Asphalt Mixture Performance Tester device. The E* data and corresponding binder properties were used as input in the AASHTO Darwin-ME software to predict rutting and fatigue performance of the mixes by assuming a model pavement section and appropriate traffic levels. Relative performance indicators were developed for both rutting and fatigue, and plotted against the asphalt content to determine optimum Ndes. For both surface mixes, the optimum Ndes value was determined to be 85 gyrations.

Performance Characterization of Asphalt Mixtures at High Asphalt Binder Replacement with Recycled Asphalt Shingles

Recycled materials can be used effectively in asphalt mixtures to replace virgin asphalt binder or virgin aggregates. Virgin material (asphalt binder or aggregate) can be replaced with reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in asphalt mixtures. The study presented in this paper examined the effect of high asphalt binder replacement (ABR) for an asphalt mixture with a low number of design gyrations (a low N-design) and RAP and RAS on performance indicators (e.g., permanent deformation, fracture, fatigue potentials, stiffness). A developed experimental program included complex modulus, fracture, overlay reflective cracking resistance, low-temperature cracking, wheel track permanent deformations, and push–pull fatigue tests. The ABR, combinations of RAS and RAP asphalt binder, and levels in the mix ranged from 43% to 64%. Potential permanent deformation resistance of the mixtures was improved in the presence of RAS. Fracture tests at low temperature revealed no significant difference between the specimens prepared at varying percentages of ABR. Fatigue potential of mixtures increased with an increase in RAS content and ABR. The specimens prepared with 2.5% RAS and PG 46-34 showed the best fatigue potential performance. The impact of asphalt binder bumping was highlighted by the results of all tests. The improvement in fatigue life and fracture energy was noticeable when the asphalt binder type was changed from PG 58-28 to PG 46-34 at the highest ABR level. The complex modulus test results can provide crucial information about the mix viscoelastic properties, such as relaxation potential and long-term stiffness. These results can be used, along with fracture test results, to evaluate mix brittleness at relatively high ABR levels.

Laboratory evaluation of the mechanical properties of plant-produced warm-mix asphalt mixtures

This paper presents a laboratory evaluation of plant-produced asphalt mixtures containing the control hot-mix asphalt (HMA) and four warm-mix asphalt (WMA) technologies: an organic additive (Sasobit®) and three foaming processes (Advera®, low-emission asphalt, and Gencor). These asphalt mixtures were produced using a single binder performance grade (PG 64-22) and 9.5-mm nominal maximum aggregate size Superpave mix design compacted to 75 design gyrations. Dynamic modulus (E*), flow number, and Hamburg wheel–track tests were utilised to evaluate the mechanical properties of the asphalt mixtures. The PG of the asphalt binders containing the WMA technologies was also verified. In addition, their rheological properties were evaluated using shear modulus master curves and multiple stress creep recovery tests. It was found that the asphalt binder containing Sasobit® technology increased the continuous high-temperature binder grade by 6°C and measured higher elastic properties. In general, the asphalt mixture prepared with Sasobit® technology and the control HMA mixture measured higher stiffness than those prepared with the other WMA technologies and measured the highest dynamic modulus rutting parameter and fatigue cracking parameter. In addition, these mixtures demonstrated increased resistance to rutting and moisture damage. Statistical analysis indicated that the binder rheological properties, WMA dosage rates, and production temperatures influenced the performance of the WMA mixtures.

Evaluation of Properties of Porous Friction Course Mixes for Different Gyration Levels

Porous frictions courses (PFCs) are characterized by high percent air voids content, and are widely used as pavement surface drainage layers. This paper presents details on the laboratory investigation performed on evaluation of properties of PFC mixes using the Superpave gyratory compactor. It also, provides a brief review of the latest specifications related to standard practices for mix design and the uses of these mixes adopted by various agencies. Major differences were observed in the design gyrations ( Ndesign ) and the design aggregate gradations. In this study, six gradations (G) were investigated with binder contents (BCs) ranging between 4.0 and 5.0% by mass of the total mix, for various gyration levels (N). The effect of N, G, and BC on the volumetric properties, unaged abrasion loss, permeability, and the permanent deformation characteristics of PFC mixes were investigated. The experimental results were statistically analyzed to identify the major influencing factors and their significance.

Verification of Superpave Number of Design Gyration Compaction Levels for Georgia

The objective of this project was to evaluate the performance of Georgia's mixes designed by using the Superpave® gyratory compactor. Mixes designed with numbers of design gyrations that are too high would be rut resistant but may be difficult to compact adequately in the field and may lack sufficient durability because of reduced asphalt binder content. The following conclusions were made on the basis of an evaluation of field projects and from laboratory volumetric and performance testing with various aggregate sources and gyration levels: (a) It is unlikely that Superpave mixtures will reach the design air voids of 4.0% during the life of the pavement. After nearly 5 years, the average air voids measured in the wheelpaths were 5.7% for Superpave projects and 3.8% for Marshall projects. On the basis of volumetric comparisons, 66 gyrations with the laboratory-produced mix gave approximately the same density achieved on the Superpave projects in the field. (b) The difference between 4-in.-diameter Marshall samples and 6-in.-diameter gyratory samples for moisture susceptibility was not statistically significant enough to assign a relationship between the tensile strength ratio (TSR) for samples with a diameter of 4 in. and the TSR for samples with a diameter of 6 in. (c) Although the asphalt binder content was an important factor in extending fatigue life, aggregate source and strain level were the most significant variables affecting fatigue results.

Achieving 4% Air Voids in Real Pavements with Superpave

The Superpave® mix design method has been used by the Colorado Department of Transportation since 1995. Initially, the number of design gyrations used was as recommended by SHRP research. However, the asphalt contents of these preliminary paving mixtures were lower than what had been historically successful for similar dense graded mixtures under similar conditions. This lower asphalt content began leading to premature moisture damage failures. Therefore, a study was initiated to determine how the pavements constructed according to the original Superpave compaction criteria densified under traffic. An experiment was designed to evaluate the in situ voids in 22 full-scale pavements over a 6-year period. Projects were selected on the basis of variation in traffic, environment, temperature, and elevation. Results of this research indicate that the original number of design gyrations recommended by SHRP for field-mixed–laboratory-compacted materials results in pavements that did not compact sufficiently under traffic, resulting in higher air voids than desired after 3 years service. An adjustment to the original SHRP design gyration levels was done in a subsequent study based on NCHRP research. The result of this revision has been a decrease in voids levels after 3 years of service with a corresponding improvement in performance. Further fine-tuning is needed and will be implemented in the future.

Optimum Number of Superpave Gyrations Based on Project Requirements

The Superpave® mix design method was developed as part of the SHRP Asphalt Research Program, which extended from 1987 to 1993. The concept behind the Superpave method was that of incorporating performance, environmental conditions, load factors, and material characterization into one design and evaluation process to improve the performance of the asphalt mixtures by reducing rutting, thermal cracking, and fatigue cracking. Several years have passed since the first Superpave mixtures were laid down in numerous states, including Texas. With the introduction in the design process of rolling wheel testers, such as the Hamburg wheel tracking device (HWTD), it has often been observed that although the mixtures perform well with regard to rutting resistance, some pavement sections begin to crack relatively early. One possible reason for this occurrence is that the Superpave volumetric mix design method (Level 1) and the performance requirements for tests such as the HWTD promote asphalt mixtures with relatively low asphalt contents. The mixtures have superior rutting resistance but often sacrifice cracking performance. One way to increase the cracking resistance of Superpave asphalt mixes is to reduce the number of design gyrations (Ndesign) currently used in the Superpave gyratory compactor. This paper introduces a simple method for selecting an optimum number of gyrations based on the performance requirements of the specific project with regard to simultaneous rutting and fatigue. The objective is to produce asphalt mixes that are more resistant to fatigue cracking without compromising their already established superior rutting performance.

Transitioning from Texas Gyratory Compactor to Superpave Gyratory Compactor

The Strategic Highway Research Program expended great effort in developing the Superpave® gyratory compactor (SGC) as a modern, technologically advanced tool for the design of hot-mix asphalt (HMA) paving mixtures and for preparation of realistic test specimens for laboratory evaluation. Since the widely accepted SGC was designed to produce specimens similar to actual pavement layers and since the current series of Texas Department of Transportation (TxDOT) HMA paving mixtures have demonstrated acceptable performance, TxDOT wished to replace the Texas gyratory compactor (TGC) with the SGC for the design of its current repertoire of dense-graded mixtures. In the first phase of this study, the authors developed a specification for the number of design gyrations with the SGC to achieve the same optimum asphalt content (OAC) as that obtained with the TGC. However, on the basis of past experience, review of the literature, and feedback from TxDOT, the design OAC was considered to be too low; mixtures that may be susceptible to fatigue cracking resulted. To mitigate this problem, the authors revised the specification for use of the SGC so that the resulting mixtures had an increased OAC. This revision was done by comparing the rutting resistance of HMA mixtures designed at different numbers of gyrations with the Hamburg wheel-tracking device and then selecting the lowest number of gyrations that did not compromise rutting resistance of the mixture. Results show that it is possible to select a considerably lower number of design gyrations with the SGC without a significant decrease in rutting performance of the mixtures.

Transitioning from Texas Gyratory Compactor to Superpave Gyratory Compactor

The Strategic Highway Research Program expended great effort in developing the Superpave® gyratory compactor (SGC) as a modern, technologically advanced tool for the design of hot-mix asphalt (HMA) paving mixtures and for preparation of realistic test specimens for laboratory evaluation. Since the widely accepted SGC was designed to produce specimens similar to actual pavement layers and since the current series of Texas Department of Transportation (TxDOT) HMA paving mixtures have demonstrated acceptable performance, TxDOT wished to replace the Texas gyratory compactor (TGC) with the SGC for the design of its current repertoire of dense-graded mixtures. In the first phase of this study, the authors developed a specification for the number of design gyrations with the SGC to achieve the same optimum asphalt content (OAC) as that obtained with the TGC. However, on the basis of past experience, review of the literature, and feedback from TxDOT, the design OAC was considered to be too low; mixtures that may be susceptible to fatigue cracking resulted. To mitigate this problem, the authors revised the specification for use of the SGC so that the resulting mixtures had an increased OAC. This revision was done by comparing the rutting resistance of HMA mixtures designed at different numbers of gyrations with the Hamburg wheel-tracking device and then selecting the lowest number of gyrations that did not compromise rutting resistance of the mixture. Results show that it is possible to select a considerably lower number of design gyrations with the SGC without a significant decrease in rutting performance of the mixtures.

Comparison of Superpave and Marshall Mix Performance in Alabama

The Alabama Department of Transportation specifies the Superpave® design system for most of its dense-graded HMA mixes. However, there is concern that the number of design gyrations ( NDesign) may be too high for specified traffic levels. Mixes designed with an NDesign level that is too high would be rut resistant but may be difficult to compact in the field and may suffer from durability problems such as premature cracking and raveling. The objective of this project is to evaluate the performance of Alabama's mixes designed according to Superpave criteria and compare them to the performance of Alabama's Marshall-designed mixes to determine whether current NDesign levels should be adjusted. The following conclusions are based on an evaluation of 25 Marshall and Superpave projects placed about the same time and serving under approximately the same traffic conditions: ( a) Generally, both Marshall and Superpave mixtures are performing quite well with little rutting and cracking after a period of about 4 years. ( b) It is likely that most Marshall and Superpave mixtures will not reach the design air voids of 4.0% during the life of the pavement. After 4 years, the average air voids measured in the wheelpaths was 5.3% for Marshall mixes and 5.9% for Superpave mixtures. ( c) It appears that the durability of Superpave mixtures can be improved, without adversely affecting the rutting resistance, by increasing the asphalt content. Practitioners, however, are cautioned to monitor rutting indicators closely during mixture design, production, and construction.