Randy West

Improved Mix Design, Evaluation, and Materials Management Practices for Hot Mix Asphalt with High Reclaimed Asphalt Pavement Content

The objectives of this project were to (1) develop a mix design and evaluation procedure that provides satisfactory long-term performance for asphalt mixtures containing high reclaimed asphalt pavement (RAP) contents—in the range of 25 to 50% or greater—and (2) propose changes to existing American Association of State Highway and Transportation Officials (AASHTO) standards to adapt them to the design of high RAP content mixtures. The project team conducted a comprehensive laboratory experiment to answer basic questions about preparing and characterizing RAP materials for mix designs. A series of mix designs was then prepared with materials from four different parts of the United States with different RAP contents and different virgin binders. Those mix designs were evaluated against standard Superpave criteria and a set of performance-related tests to further assess the mix designs for their susceptibility to common forms of distress, particularly fatigue cracking, low-temperature cracking, and moisture damage. A concurrent effort developed a set of best practices for RAP management in field production and construction from information obtained through a literature review, surveys of current practices in the industry, discussions with numerous contractor quality control (QC) personnel, and analysis of contractor stockpile QC data from across the United States The research found that only minor, though important, revisions to the current AASHTO standards for asphalt mix design, AASHTO R 35 (Superpave Volumetric Design for Hot Mix Asphalt) and M 323 (Superpave Volumetric Mix Design), were needed to adapt them for the successful design of high RAP content asphalt mixtures. As expected, high RAP contents substantially increased the dynamic modulus of the asphalt mixtures as well as their rutting resistance as measured by the confined flow number test. Tensile strength ratios of high RAP content mixtures as measured by AASHTO T 283 were comparable to those of control mixtures without RAP, indicating similar moisture damage susceptibilities. As might be expected, compared to control mixtures without RAP, the high RAP content mixtures generally had lower fracture energies at test temperatures used to evaluate susceptibility to fatigue and low-temperature cracking. This finding suggests that careful attention should be given to the selection of the performance grade of the virgin binder used in high RAP content mixtures to minimize any long-term risk of cracking distress.

Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt

This report identifies improved test methods for determining laboratory mixing and compaction temperatures of modified and unmodified asphalt binders. The report will be of immediate interest to materials engineers in state highway agencies and the hot-mix asphalt (HMA) construction industry.

Testing of Moderate and High Reclaimed Asphalt Pavement Content Mixes: Laboratory and Accelerated Field Performance Testing at the National Center for Asphalt Technology Test Track

This paper describes the performance of test sections containing moderate and high levels of reclaimed asphalt pavement (RAP) at the National Center for Asphalt Technology test track. The test sections included two with 20% RAP, four with 45% RAP, and a control with no RAP. Each mixture contained the same component aggregates and RAP. One of the 20% RAP mixes contained PG 67-22 binder, and the other contained PG 76-22 binder. Different binders in the 45% RAP mixes included PG 52-28, PG 67-22, PG 76-22, and PG 76-22 plus 1.5% Sasobit. All sections performed well for rutting and raveling. Low-severity longitudinal wheelpath cracking was documented in two sections. The 45% RAP section with PG 76-22 plus Sasobit had moderate cracking, which appears to reflect cracking from the underlying pavement. The 20% RAP section with PG 76-22 had less cracking. Other sections had no cracking. Laboratory tests included asphalt pavement analyzer (APA) rutting tests, dynamic modulus, bending beam fatigue, and energy ratio. The APA results corresponded to the effective stiffness of the binder in the mixes. Master curves of dynamic moduli showed the effects of the virgin binder grade on the stiffness of the mixtures. Beam fatigue tests indicated that the 45% RAP mixes have lower fatigue lives compared with those of the other mixes, but this is because of a lower effective volume of asphalt in these mixes.

Strategies for Design and Construction of High-Reflectance Asphalt Pavements

The occurrence of higher air and surface temperatures in urban areas is known as the urban heat island (UHI) effect. Reducing the UHI effect may decrease summer energy use and improve human and ecological health. The Leadership in Energy and Environmental Design certification system has awarded up to three points for construction projects that provide any combination of the following cool pavement strategies for up to 75% of the site landscape: (a) shading hard surfaces on the site with landscape features, (b) using high-reflectance materials with a minimum solar reflectance index (SRI) of 29, and (c) utilizing an open-graded pavement or porous pavement system. Although a guide to the design and construction of porous asphalt pavements has existed for some time, such a guide is not readily available for high-reflective asphalt pavements. The objective of this study is to identify and validate high-reflectance asphalt materials and pavement surface treatments that are suitable for use in parking lots and other large paved surfaces, have a minimum SRI of 29, and are economical. In this study, six technologies exhibited SRI values of 29 or greater: E-Krete microsurfacing, Street-Bond coating, synthetic binder, Densiphalt, and chip and sand seals using light-colored aggregates. Another technology, surface gritting using light-colored aggregate, most likely would have exhibited SRI values of at least 29 if the aggregate had adhered properly to the asphalt mat.

Effect of Tire Rubber Grinding Method on Asphalt-Rubber Binder Characteristics

The results of a study carried out to evaluate the effect of rubber grinding processes on the properties and characteristics of the resulting asphalt-rubber binder are presented. Several ambient and cryogenic ground tire rubber (GTR) materials were evaluated using measurements of surface areas and bulk densities. The rubber materials were then, respectively, mixed with an AC-30 asphalt; the resulting blends were tested to determine the corresponding viscosity, settlement during storage, and potential for binder draindown. The findings indicate that the asphalt-rubber binders produced with rubber from the different grinding processes have measurable differences in properties and storage characteristics that are critical to the performance of the binder in open-graded mixtures. The wet-ground rubber material had substantially lower bulk densities and larger surface areas than rubber resulting from other grinding methods. GTR materials with greater specific surface areas and more irregularly shaped particles produced asphalt-rubber binders with higher viscosities. Binders with the cryogenic ground rubber had the greatest amount of settlement and the least resistance to draindown.

Analysis of Hot-Mix Asphalt Lab Compactability Using Lab Compaction Parameters and Mix Characteristics

The compactability of hot-mix asphalt (HMA) mixtures is often used to describe how easy or difficult it is to compact a mixture on a roadway. Several asphalt researchers have proposed the use of laboratory-measured parameters for mixtures and their components as indicators of HMA compactability and resistance to permanent deformation. In this study, laboratory parameters used to represent field compactability of HMA mixtures, such as the compaction energy index and Bailey method ratios, were compared with the compaction slope, the locking point, and the number of gyrations required to reach 92% of the theoretical maximum specific gravity. Once the relationships among these parameters were established, basic mix parameters such as gradation, aggregate shapes, binder grade, and mix volumetric properties were used to explain why some mixtures are more compactable than others in the gyratory compactor. The results suggested that any of the parameters mentioned above can be used to describe compactability in the laboratory. Gradation type, aggregate type, and aggregate size were the most significant variables that can be used to explain the compactability of specimens compacted with the Superpave® gyratory compactor.

Use of Data from Specific Pavement Studies Experiment 5 in the Long-Term Pavement Performance Program to Compare Virgin and Recycled Asphalt Pavements

The Specific Pavement Studies Experiment 5 (SPS-5) in the Long-Term Pavement Performance program was designed to study the effects of overlay rehabilitation type on typical distress measures. The rehabilitation treatments compared overlay thickness, overlay type, and surface preparation before rehabilitation. The thicknesses used were 50- and 125-mm overlays. The overlay types were virgin asphalt mix and recycled asphalt that contained approximately 30% reclaimed asphalt pavement (RAP). Surface preparation consisted of either milling or not milling the existing pavement before rehabilitation. Eighteen states and provinces in North America built SPS-5 projects between 1989 and 1998. Seven distress parameters from these test pavements were analyzed, including international roughness index (IRI), rutting, fatigue cracking, longitudinal cracking, transverse cracking, block cracking, and raveling. Analyses were conducted to determine which factors affected overlay performance as measured with the above parameters. Further statistical testing compared the performance of the virgin mix sections directly with equivalent sections that contained 30% RAP. Overlays with mixes that contained 30% RAP were found to perform as well as overlays with virgin mixes in terms of IRI, rutting, block cracking, and raveling. Thicker overlays improved pavement performance, except for rutting. Milling before rehabilitation decreased IRI, fatigue cracking, and transverse cracking but increased rutting.

Evaluating Georgia's Compaction Requirements for Stone Matrix Asphalt Mixtures

This study determined a compaction level for stone matrix asphalt (SMA) mixes with the Superpave® gyratory compactor (SGC) that would match a 50-blow Marshall compaction using aggregates and mix designs common in Georgia. SMA mix designs were prepared with five aggregate sources using a 50-blow Marshall compaction and using 50, 75, and 100 gyrations with an SGC. Optimum asphalt contents from the mix designs were compared. Aggregate breakdown from each of the compactions was analyzed. Laboratory rutting tests were conducted on each mix design by using the asphalt pavement analyzer (APA). Results of the laboratory mix designs indicated that generally 35 gyrations in the SGC provided the same density as Marshall compaction. At 50 gyrations, the optimum asphalt contents for the SMA mixtures were 0.1% to 0.5% less than when the Marshall hammer was used; however, four of the five mix designs with 50 gyrations met Georgia Department of Transportation (DOT) specifications. Aggregate breakdown was slightly less with the SGC than with the Marshall hammer. APA tests showed that the mix designs were not sensitive to asphalt content, and all passed Georgia DOT requirements. Testing with plant-produced SMA mixtures confirmed that 34 gyrations in the SGC yielded specimen densities equivalent to the Marshall compaction. For the plant-produced mixtures, compaction in the SGC caused less aggregate breakdown than with the Marshall hammer. The field mixtures also performed well in APA testing. On the basis of results from the laboratory and field mixes, 50 gyrations are recommended with the SGC to replace 50-blow Marshall compaction for SMA mix design in Georgia.

Evaluation of the Effect of Sampling Location on Laboratory-Measured Quality Control Parameters

The sampling of hot-mix asphalt (HMA) is an important part of the quality control testing process. The quality of the sample can have a significant effect on the test results. When a sample of HMA is taken, the technicians goal should be to obtain a sample that is representative of the mixture that will be placed on the pavement and is not segregated or contaminated by foreign materials. Several possible choices exist for sampling locations. Samples may be taken from the back of the haul truck at the plant with either a shovel or a robotic sampling device. Samples can be taken at the paving site directly behind the spreader after the mix has been released from the screed but before the first roller has passed over it. Samples can also be taken with cores being cut from the compacted roadway. Each of these methods has its advantages and disadvantages. For this study, samples of two mixes were taken at various locations (truck by shovel, truck by robotic device, behind the spreader, and cores) for a period...

Flow Number Test and Assessment of AASHTO TP 79-13 Rutting Criteria: Comparison of Rutting Performance of Hot-Mix and Warm-Mix Asphalt Mixtures

The flow number (FN) test was recommended in NCHRP Project 9-19 as a simple performance test for rutting evaluation of asphalt mixtures. The test showed good correlation with rutting performance of mixtures from WesTrack, MnROAD, and FHWAs accelerated loading facility. Despite this fact, no standard protocol was recommended for temperature and required stress level. Subsequent NCHRP studies allowed the development of a provisional standard. AASHTO TP 79-13 includes test parameters for stress and temperature, specimen conditioning, and minimum FN criteria that were established for hot-mix asphalt (HMA) and for warm-mix asphalt (WMA) on the basis of traffic level. In NCHRP Project 9-47A, the rutting potential of WMA mixtures was compared with that of HMA mixtures by using the FN test and the rutting criteria included in the AASHTO TP 79-13 were also evaluated. The analysis included results of samples produced by using field and lab mixtures. Thirteen mixes using 10 WMA technologies and eight corresponding HMA mixes were included. The FN test results for plant-produced WMA mixes were found to be statistically lower than those for corresponding HMA mixes in more than two-thirds of the comparisons. The study also found that the FN criteria recommended for both HMA and WMA seemed appropriate for evaluating plant-produced mixes. Another finding from the study was that FN results from lab-produced WMA mixtures were consistently lower than FN values from field mixtures; this result suggests that adjustments to the specimen conditioning requirements should be considered.