Adam J. Hand

Laboratory and Field Evaluations of Foamed Warm-Mix Asphalt Projects

Warm-mix asphalt (WMA) is much like hot-mix asphalt (HMA), but it is produced at lower plant temperatures than conventional HMA. Key benefits of the reduced WMA production temperature include the reduction of fuel consumption and of emissions. Granite Construction performed two WMA paving demonstration projects from its Indio, California, facility in early 2008. Both projects were paved with WMA produced with the free water method (Astec Double Barrel Green). The objectives of these demonstrations were (a) to demonstrate that WMA with reclaimed asphalt pavement could be produced and placed at lower temperatures while yielding mix properties and field compaction similar to those of conventional HMA and (b) to construct field test sections so that WMA and HMA performance could be compared side by side. These objectives were accomplished by producing and placing the WMA and by completing an in-depth sampling and testing program to compare the WMA and HMA paved on similar test sections and produced with similar methods (the only exception was production temperatures). The initial field performance of the WMA and HMA has been similar, and the long-term performance will be monitored. The WMA demonstration objectives were achieved, with the WMA exhibiting mix properties and field compaction similar to those of the HMA, with slightly lower initial stiffness, as expected. The potential rutting concern with WMA has not been an issue in this arid Southern California climate, and the sections placed on the haul road into and out of the Indio plant have been exposed to significant truck traffic.

Nevada's Approach to Pavement Management

The Nevada Department of Transportation has a fully implemented pavement management system (PMS) at the network level and an extensive pavement evaluation system at the project level. Both systems use actual field performance data that are collected annually from pavement sections throughout the state. The PMS is being further developed to incorporate performance modeling, life-cycle cost analysis, and network optimization processes. Once fully developed the system will allow the engineer to select the best rehabilitation and maintenance alternative that can carry the pavement over the entire analysis period. The project-level evaluation process includes the use of nondestructive testing data and condition surveys from the PMS. The overall systems are summarized at both levels, and the individual steps that are involved within each system are described.

Gradation Effects on Hot-Mix Asphalt Performance

The effect of gradation on hot-mix asphalt (HMA) performance has long been a contentious issue. One objective of the National Pooled Fund Study No. 176 was to evaluate the impact of gradation on the rutting performance of HMA. To this end, 21 Superpave mixtures were designed that used a range of materials, nominal maximum size aggregates, and gradations typical of those used throughout the United States. Tests that included both laboratory and prototype-scale loading were used to evaluate the permanent deformation characteristics of the mixtures. Analysis of the data revealed that adequate performance could be obtained with mixture gradations plotting above (ARZ), through (TRZ), and below (BRZ) the restricted zone. Laboratory tests suggested that ARZ and TRZ gradations might provide better deformation resistance than BRZ gradations. However, prototype-scale accelerated pavement testing did not show any clear trends in performance relative to gradation alone with respect to the restricted zone. This means that the restricted zone alone is not adequate to characterize gradation to ensure acceptable rutting performance and should therefore be omitted from Superpave specifications.

An object-oriented framework for finite element pavement analysis

In this study, we developed an object-oriented (OO) framework with interactive graphics to assist pavement studies using finite element analysis (FEA). FEA has been proven to be effective in studying various pavement failure problems; however, it is time consuming and error prone to manually generate the load sequences where non-regular tire footprints, non-uniform tire-pavement contact stresses, and transverse wheel wander distributions are used. After FEA, extracting the deformations for failure analysis is necessary but tedious. The OO framework developed in this study handles the preprocessing and postprocessing tasks for the FEA of pavements. It has a graphical user interface and is platform independent. It was successfully used in developing a new criterion for characterizing pavement failures that involved approximately four hundred different FEA simulations.

IMPACT OF GRADATION RELATIVE TO SUPERPAVE RESTRICTED ZONE ON HOT-MIX ASPHALT PERFORMANCE

Significant controversy has revolved around the Superpave gradation specifications, in particular the restricted zone, since the completion of the Strategic Highway Research Program (SHRP). Historically, dense-graded mixtures that encroach on the restricted zone had provided good performance prior to SHRP. However, current Superpave guidelines recommend that gradations passing through the restricted zone not be used. A synopsis of recent research related specifically to the impact of the Superpave restricted zone on performance of hotmix asphalt (HMA) is provided. The evolution and purpose of the Superpave restricted zone are presented, along with findings of both recently completed and ongoing research. Studies involving laboratory and full-scale accelerated performance tests of mixtures with gradations plotting above (ARZ), through (TRZ), and below the restricted zone were considered. The research reviewed clearly suggests that good performance can be achieved with fine-graded (ARZ and TRZ) mixtures and that no relationship exists between the Superpave restricted zone and HMA rutting or fatigue performance. It is suggested on the basis of this research that the restricted zone recommendation be eliminated from the Superpave volumetric mixture design specifications.

DEVELOPMENT OF PERFORMANCE MODELS BASED ON DEPARTMENT OF TRANSPORTATION PAVEMENT MANAGEMENT SYSTEM DATA

The Nevada Department of Transportation (NDOT) is developing a network optimization system (NOS). The objective of the system is to evaluate various alternatives and to recommend the most cost-effective rehabilitation treatment to be used on the various sections of the state highway system. The NOS consists of several subsystems, including performance models, life-cycle cost analysis, and network optimization. Performance models have been developed for the rehabilitation treatments most commonly used by NDOT: overlay and mill/overlay. The performance models were developed on the basis of the pavement management system data that have been collected by NDOT for the last 15 years. The models were generated through statistical analyses that related the present serviceability index to age, traffic, environment, and structural and materials properties. First cost and annual maintenance cost figures were established to be used in the life-cycle cost analysis. The annual maintenance costs proved to be the most difficult figures to estimate due to the extremely large variability. After several attempts, the fixed period cumulative cost approach was identified as the best alternative to reducing the variability of the annual maintenance costs. In order to accomplish true network optimization, the entire NDOT highway system was subdivided into project segments (total number of segments is in excess of 800). Several databases were formed to house the data corresponding to each project segment. The NOS will use the performance models in conjunction with the supporting databases to conduct a fully automated optimization analysis.

Field and Laboratory Performance of Superpave Mixtures in Nevada

The Nevada Department of Transportation (NDOT) has strongly supported Superpaver-related research and implementation efforts since the completion of the Strategic Highway Research Program. However, it is one of the state highway agencies that has not implemented use of the Superpave mix design system. Instead of rapidly implementing Super-pave in the mid-1990s, NDOT chose to build field test sections composed of Superpave and the mixtures conventionally used at the time (Hveem) and placed side-by-side to evaluate the Superpave system relative to the system in use. Extensive laboratory evaluations were conducted on the materials and mixtures placed on each section, and field performance was monitored over time. This study presents a synopsis of the field and laboratory performance information gathered over the past 6 years. Planned NDOT evaluations of Superpave mixtures based on the findings of this effort to date are outlined.

EVALUATION OF LIGHTWEIGHT NON-CONTACT PROFILERS

Pavement smoothness was recently identified as the most significant factor the motoring public uses to judge the quality of roadways. A new generation of lightweight profilers has recently been developed with a potential of providing nearly instantaneous smoothness measurements that provide contractors with a tool to identify and address process control issues promptly and cost effectively. The new technology was evaluated to assess the repeatability and reproducibility of the devices, as well as the potential for their use in Indiana. A successful field test was conducted, with four ASTM Class I lightweight profilers performing five replicate measurements at each of six sites; three hot mix asphalt and three portland cement concrete sites. The precision of the devices was determined in accordance with ASTM standards and also revealed good repeatability but poor reproducibility. Smoothness specifications of other states were reviewed in light of their application to lightweight profilers and a draft smoothness specification based on lightweight profilers was developed for INDOT.

Coarse superpave mixture sensitivity

Asphalt-aggregate mixture sensitivity has long been recognized for its potential negative impact on performance if variables including the asphalt content (AC) and the percent that passes the 0.075-mm sieve (p0.075) are not carefully controlled during construction. Typical production standard deviations for AC and p0.075 are 0.3 and 0.9 percent, respectively. This mixture sensitivity analysis shows that variations of this magnitude can result in significant changes in coarse Superpave mixture volumetrics. Three coarse mixtures designed for the WesTrack project were examined, including the original coarse mixture and the replacement mixture. The coarse mixture was most sensitive to both AC and p0.075, contradicting the relatively worse performance of the replacement mixture. The replacement mixture was relatively insensitive to p0.075, and the third mixture was extremely sensitive to reductions in p0.075. Results also suggest that mixture sensitivity may affect optimum AC by up to 1.0 percent. When measured sensitivities were compared with those predicted from models calibrated in the field, the potential for extreme sensitivity was demonstrated by errors in predicted changes in air voids that ranged from 0 to 98 percent because of small changes in AC or p0.075. These findings are disturbing when one considers the current industry trend toward the use of coarse Superpave mixtures. The conclusions drawn warrant a recommendation that mixture sensitivity analyses are needed in the Superpave volumetric mix design process. A mixture sensitivity analysis that is useful for identification of sensitive mixtures and that serves as an excellent field quality management tool is presented.

Second Phase Study of Changes in In-Service Asphalt (Vol 1: Main Report)

In response to the need to improve asphalt pavement performance, the Strategic Highway Research Program (SHRP) was initiated in 1987. Products from SHRP included performance-based specifications for asphalt binders and performance-based mix design tests and criteria for asphalt mixtures. The Superpave asphalt binder specification addresses performance of in-service pavements for given environmental and traffic conditions. During the 1985 paving season in Indiana, the original asphalt binder and HMA (truck mix samples) from a number of pavements were sampled and retained. Cores were also taken from these pavements after seven or eight years of service. Condition surveys were performed on these pavements, prior to coring. The study objectives were directed toward characterizing the original and extracted/recovered asphalt binders from the field samples using Superpave test methods and equipment. Taking this approach provided an early opportunity to examine a number of features of the Superpave system, including short (RTFO) and long-term (PAVE) aging. Results of this research documented the difference in stiffness between RTFO aged and asphalts extracted and recovered from loose mixtures; in addition to, the difference in stiffness between PAV aged and in-service aged asphalts, in Indiana. Additionally, asphalt stiffness was related to pavement performance. A poor relationship was found between the stiffness of RTFO aged asphalts and rutting. On the other hand, a good relationship was found between stiffness of in-service asphalts and cracking. Collected weather data from seven counties indicated agreement between calculated PG grades for these counties and recommended Superpave PG grade, based on the environmental conditions. Based on the results of this research, recommendations were made to further investigate the short-and long-term binder aging to better address variability of these procedures, to address storage conditions sometimes unavoidable for field samples, and to validate PG grades requirements for Indiana.