James A. Musselman

SUITABILITY OF ASPHALT PAVEMENT ANALYZER FOR PREDICTING PAVEMENT RUTTING

Findings are summarized from an investigation performed to evaluate the suitability of a wheel-tracking device known as the asphalt pavement analyzer (APA) for assessing the rutting potential of asphalt mixes. The evaluation process consisted of correlating the APA’s predicted rutting with known field measurements. The correlation between beam and gyratory samples and the testing variability were also investigated. In addition, the APA test results were compared with those obtained using the Georgia loaded-wheel tester. The findings of this investigation indicated that the APA may be an effective tool to rank asphalt mixtures in terms of their respective rut performance. However, for each mixture type, the APA testing variability was significant between tests and between the three testing locations within each test. Differences in rut measurements of up to 4.7 and 6.3 mm were recorded for beam and gyratory samples, respectively. Therefore, using the APA as a clear pass-or-fail criterion for performance prediction purposes of asphalt mixtures may not be appropriate at this time. It should be noted that these findings are based on data collected on three mixes. Therefore, it is suggested that the APA testing variability (testing and testing locations within the device) be further assessed with a wider range of mixtures. The intent of such an assessment should not only be to correlate the APA results with field data but also to develop potential pass-or-fail limits and procedures.

Effects of Water Saturation Level on Resistance of Compacted Hot-Mix Asphalt Samples to Moisture-Induced Damage

The Florida Department of Transportation (FDOT) initiated monitoring of its first Superpave section on I-75 in Columbia County for stripping potential using AASHTO T 283, which specifies that all conditioned test samples be saturated to between 55 and 80 percent based on 7 ± 1 percent air voids. A fairly large saturation range is allowed because it was thought that mixtures may have different percentages of permeable air voids. However, the potential interaction between air void content and the level of saturation has not been fully investigated. It is also not clear whether test results from samples saturated to 55 percent are comparable with those of the same mixture saturated to 80 percent for a similar air void content. Therefore, although FDOT adopted AASHTO T 283, it also initiated a parallel study on the effects of different degrees of saturation on moisture damage. Findings are reported of both the ongoing monitoring of the I-75 project for potential stripping and the investigation of the effects of different levels of saturation on moisture susceptibility test results as determined using AASHTO T 283.

Nuclear Density Readings and Core Densities: A Comparative Study

Presented are the findings of an investigation performed to identify possible correlations between nuclear density gauge readings and core density results. The nuclear density data were collected on a Superpave section of I-95 in Brevard County, Florida. Core samples also were obtained from this section for laboratory density determination. Five gauge units [Troxler models 3401, 3440 (two units), 3450, and 4640] and three core density methods (Florida test method FM 1-T 166, ASTM D1188, and dimensional analysis) were considered. The relationships among the core density results were analyzed, then an investigation of the correlation among the different gauges used in this study was evaluated. Finally, the performance of each of the units with respect to the core density results was assessed. The findings indicated that the five nuclear gauge density units did not always produce similar results and did not consistently correlate with the core densities. In addition, the nuclear density testing variability differed not only from gauge to gauge but also from location to location within each gauge.

Superpave Field Implementation: Florida's Early Experience

Over the past 10 to 15 years, Interstate pavements in northern Florida have experienced a significant number of failures, primarily due to rutting. It was believed that the present fine-graded, 50-blow Marshall-designed mixes were inadequate to withstand current loading conditions. The newly developed Superpave system represented an opportunity to address several of Florida’s asphalt pavement problems. Thus, the Florida Department of Transportation made a concerted effort to implement Superpave technology in 1996. During this period, eight projects were changed from the traditional Marshall mix designs to Superpave. Although the new procedure offers potential for improved pavement performance, there has been very little experience nationally with its field application. Florida’s early experiences with the field implementation of Superpave are documented.

Development of the Florida Department of Transportation's Percent Within Limits Hot-Mix Asphalt Specification

The Florida Department of Transportation adopted a percent within limits approach in July 2002 for the acceptance and payment of all hot-mix asphalt. Contractor test data, after being verified by the department, are used to calculate payment. Acceptance and payment for dense-graded Superpave® mixtures are based on the following five asphalt material properties: roadway density, percent air voids, asphalt binder content, percent passing the No. 8 sieve, and percent passing the No. 200 sieve. Acceptance and payment for open-graded friction course mixtures are based on the following four asphalt material properties: asphalt binder content, percent passing the 3/8-in. sieve, percent passing the No. 4 sieve, and percent passing the No. 8 sieve. Contractor test data from recently completed construction projects were used to develop representative standard deviations of the asphalt material properties used for payment and acceptance. These standard deviations were then used to develop tolerance values for the pe...

Investigation of the CoreLok for Maximum, Aggregate, and Bulk Specific Gravity Tests

The Florida Department of Transportation uses long-established test procedures to determine the maximum specific gravity (Gmm) and bulk specific gravity (Gmb) of asphalt mixtures and the bulk specific gravity (Gsb) of aggregates. The CoreLok, a vacuum-sealing device that can be used to determine these properties, was evaluated by the department for these test procedures. With respect to the Gmm test procedure, for mixtures containing nonabsorptive granites, the CoreLok determined results equivalent to those of the departments test procedure. However, for mixtures containing absorptive limestones, the CoreLok determined higher Gmm values than did the departments test procedure. The apparent reason for the discrepancy is that the CoreLok does not determine a saturated surface dry condition of the sample. With respect to the aggregate specific gravity test procedures, the CoreLok provided test results equivalent to the departments test procedure for the nonabsorptive fine aggregates only. For the absorpti...

Field Conditioning of Superpave Asphalt Mixes

Failing volumetric properties (low air voids and voids in mineral aggregate) have been a common occurrence in the production of Superpave mixes in Florida and nationally. There are several potential causes of this problem including the effect of asphalt absorption as related to conditioning time. Research was done to determine (a) whether the 2-h force-draft oven conditioning adequately simulates actual roadway conditions and (b) how actual roadway conditions can best be simulated when the asphalt mixture is sampled and tested immediately following production at the plant. The results indicate the following: (a) 3 h of conditioning during design best correlated with roadway conditions; (b) a 1-h conditioning time at the asphalt plant resulted in maximum specific gravity values that best correlated with roadway conditions; (c) 1 h of conditioning at the asphalt plant increased the air voids of the mix by 0.5 percent for absorptive materials and 0.25 percent for nonabsorptive materials; and (d) conditioning had minimal effect on bulk density values.

EFFECTS OF AGGREGATE DEGRADATION ON AIR VOIDS OF STRUCTURAL ASPHALT MIXTURE IN FLORIDA

In an effort to further improve the rut resistance of asphalt pavements in Florida, the Florida Department of Transportation implemented specifications requiring that the production of asphalt mixes be stopped when the air-void content falls below a critical level. To address the problem of low air voids and rutting in north Florida, a proposal was made to reduce the maximum amount of material allowed to pass the 75-μm sieve (P-75μm) at design for asphalt mixtures containing north Florida limestone aggregates. A field study was then undertaken to determine whether this proposal would adequately resolve the problem of low air voids during production due to high P-75μm. The purpose of the study was to determine the amount of degradation to a typical north Florida limestone material and the subsequent effects that degradation has on air voids. The results indicate that although the north Florida limestone aggregates used in this study did degrade significantly, the asphalt contractor was, in general, able to control the amount of P-75μm material in the mix by wasting the baghouse fines. During production, the air voids were low on a number of samples. The source of these low air voids appears to be related to a combination of a high asphalt content in the mix as well as a high P-75μm content. The findings do not support the proposal to reduce the P-75μm content at design at this time. An unexpected finding of this study was that the bulk specific gravities of the commercial aggregate products were less than expected. The impact of this finding is that the voids in the mineral aggregate (VMA) of the mix at design would not meet minimum specification requirements. Although this could make it difficult for an asphalt mixture to have adequate air voids during production, the primary impact of a low VMA is that the pavement would have poor durability and would potentially become brittle and crack prematurely.

The Effect of Aging on the Cracking Resistance of Recycled Asphalt

Fatigue cracking is an important concern when a high percentage of Reclaimed Asphalt Pavement (RAP) is used in an asphalt mixture. The aging of the asphalt binder reduces its ductility and makes the pavement more susceptible to cracking. Rejuvenators are often added to high-RAP mixtures to enhance their performance. The aging of a rejuvenated binder is different from virgin asphalt. Therefore, the effect of aging on a recycled asphalt mixture can be different from its effect on a new one. This study evaluated the cracking resistance of 100% recycled asphalt binders and mixtures and investigated the effect of aging on this performance parameter. The cracking resistance of the binder samples was tested by a Bending Beam Rheometer. An accelerated pavement weathering system was used to age the asphalt mixtures and their cracking resistance was evaluated by the Texas Overlay Test. The results from binder and mixture tests mutually indicated that rejuvenated asphalt has a significantly better cracking resistance than virgin asphalt. Rejuvenated mixtures generally aged more rapidly, and the rate of aging was different for different rejuvenators.

Evaluation of Asphalt Mixture with High Percentage of Reclaimed Asphalt Pavement in Florida

This study examined the long-term performance and life span of mixture designs with a high percentage of reclaimed asphalt pavement (RAP) (≥30%) used on higher-tonnage (>5,000 tons) projects. The pavement performance of mixtures containing high RAP percentages and mixtures containing no RAP was compared for the period 1991 to 1999. The pavements analyzed contained a lower structural layer that contained RAP and an upper layer that contained either an open-graded or dense-graded non-RAP friction course. Several databases were consulted to obtain the necessary information regarding tonnage, mixture designs, percentage of RAP, project information, traffic volumes, pavement performance, and life span. A trend showing that the time needed for the pavement to reach a deficient state decreased as the percentage of RAP increased was evident when the data were examined without accounting for the volume of traffic. When traffic volume was accounted for and projects ≥5,000 tons were isolated, a trend of decreasing performance with increasing amounts of RAP was seen. However, in the range analyzed (30% to 50% RAP), all mixtures containing RAP performed better than the mixtures containing no RAP. Consideration of the type of non-RAP friction course placed over the RAP mixtures showed that as the amount of RAP increased, pavement performance decreased at the same rate, regardless of the type of friction course. Although this trend may be correct, the implication that RAP mixtures overlaid with an open-graded friction course have a longer life span than RAP mixtures overlaid with a dense-graded friction course may be the result of factors that are not correctly reflected in this data set.