Short-Term Laboratory Conditioning of Asphalt Mixtures (2015)

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1 S U M M A R Y The stiffening of asphalt mixtures over time due to oxidation, molecular agglomeration, and other chemical processes is referred to as aging. Aging occurs due to the heating of the binder during production and construction in the short term and to continued oxidation during its service life. The ability to simulate aging in asphalt binders and mixtures produced in the laboratory has been studied extensively, and procedures have been adopted for use in binder specifications and mixture design. In the past, the aging of asphalt mixtures has been assumed to be relatively consistent, with laboratory procedures resulting in the same degree of aging as would take place during production or during long-term aging in the field, depending upon the particular protocol. While the comparison of stiffening in laboratory- produced and plant-produced mixtures was never an exact match, there was an acceptance that the laboratory aging was representative of field aging. However, this occurred in an environment in which the amount of recycled materials was relatively low, polymer-modified asphalts were not common, and mixing temperatures were mostly in a consistent range. Recent changes in asphalt mixture components, mixture processing, and plant design have left many paving technologists questioning the validity of current mix design methods in adequately assessing the volumetric needs of asphalt mixtures and the physical character- istics required to meet performance expectations. While a variety of other research studies either attempted or are attempting to address many of these issues, this project considered the impact of climate, aggregate type (to reflect differences in asphalt absorption), asphalt type and source, recycled material inclusion (reclaimed asphalt pavement [RAP] and recycled asphalt shingle [RAS]), warm mix asphalt (WMA) technology, plant type, and production temperature on the volumetric and performance characteristics of asphalt mixtures during construction and over an initial period of performance. The objectives of NCHRP Project 9-52 were to (1) develop a laboratory short-term aging protocol to simulate the aging and asphalt absorption of an asphalt mixture as it is produced in a plant and then loaded into a truck for transport and (2) develop a laboratory long-term aging protocol to simulate the aging of the asphalt mixture through its initial period of performance. Accordingly, the project was divided into two phases: Phase I evaluated short- term aging protocols for hot mix asphalt (HMA) and WMA to simulate plant aging, and Phase II evaluated long-term aging protocols to simulate the aging of the asphalt mixture 1 to 2 years after construction. In addition to developing these mixture protocols to simulate the short-term and long-term aging occurring in the field, the research study sought to discern the effects of the following variables on aging: (1) WMA technology, (2) aggregate asphalt absorption, (3) plant temperature, (4) plant type, (5) presence of recycled materials, and (6) asphalt source. Nine field sites in eight states (Figure S-1) were constructed to provide the materials nec- essary to complete this research. Each field site had one or more of the previously listed Short-Term Laboratory Conditioning of Asphalt Mixtures

2variables as its primary study component. Mineral aggregates and binders were sampled ahead of production and construction in order to fabricate laboratory-mixed, laboratory- compacted (LMLC) specimens to replicate conditions at mixture design; mixtures produced by the asphalt plant were sampled and compacted on or near the job site to provide plant- mixed, plant-compacted (PMPC) specimens; and roadway cores were obtained immediately after construction and, if possible, at intervals up to 2 years after construction. Guidance on laboratory short-term oven aging (STOA) protocols for preparing LMLC specimens was obtained from the results of NCHRP Project 9-49, “Moisture Susceptibility of Warm Mix Asphalt” (Epps Martin et al. 2014). From the various STOA protocols investi- gated, the best match in mixture stiffness between LMLC specimens and PMPC specimens or cores at construction was obtained by conditioning loose mixtures for 2 hours at 240°F (116°C) and for 2 hours at 275°F (135°C) prior to compaction for WMA and HMA, respec- tively. These short-term aging protocols were adopted for this research effort. Two common long-term oven aging (LTOA) protocols on compacted LMLC specimens were evaluated: (1) 5 days at 185°F (85°C) per AASHTO R 35 and (2) 2 weeks at 140°F (60°C), which was included in NCHRP Project 9-49 (Epps Martin et al. 2014). On a limited basis, an additional LTOA protocol of 3 days at 185°F (85°C) was also evaluated. The effects of both short-term and long-term aging on mixtures were primarily evaluated using the resilient modulus (MR) test per ASTM D7369 at 77°F (25°C) and the Hamburg wheel-tracking test per AASHTO T 324 at 122°F (50°C). In addition, a limited amount of dynamic modulus (E*) testing was conducted in accordance with AASHTO TP 79-13. The MR test was effective for evaluating the effects of aging because (1) the binder properties govern the MR stiffness and (2) it can be used directly to compare the stiffness of cores and LMLC and PMPC specimens. The STOA protocols developed under NCHRP Project 9-49 were validated across the wide range of mixture components and production parameters represented in this study. As seen in Figure S-2, there is an agreement in the MR stiffness values at 77°F (25°C) between LMLC samples and PMPC specimens, which indicates the laboratory samples achieved a degree of aging similar to that caused by heating and mixing in the asphalt plants. Factors affecting the short-term aging of asphalt mixtures included asphalt source, recy- cled material inclusion, aggregate absorption, and WMA technology. The effect of asphalt source on the magnitude of aging was demonstrated with the materials from the field site in West Texas. The presence of RAP and RAS in mixtures was found to significantly increase Figure S-1. Locations of field sites used in NCHRP Project 9-52.

3 the stiffness of the material through the plant. Increased aggregate absorption was found to actually lower the initial stiffness of mixtures due to increased effective asphalt content dic- tated by volumetric mixture design requirements. WMA technologies produced lower stiff- nesses in plant mixtures, primarily because of the presence of the technologies combined with lower production temperatures. Factors having no significant effects on the short-term aging of asphalt mixtures were plant type and plant production temperature, which was varied by as much as 30°F (17°C) within HMA and WMA. The modeling of LTOA of asphalt mixtures is more challenging as the number of variables affecting the degree of aging increases. For instance, mixture parameters such as total air voids, the interconnectivity of air voids, asphalt binder film thickness, and asphalt source interact in complex ways with the field in-service temperature and time. To capture aging in the field, the use of cumulative degree-days (CDD) was proposed as a field metric that allowed the analysis to account for both in-service temperature (i.e., climate) and time. As defined herein, the CDD is the sum of the daily high temperature above freezing for all the days being considered from the time of construction to the time of core sampling. Thus, mixtures placed in two different climates, say New Mexico and Iowa, could be considered to age differently over the same period. Aging taking place in the field can be quantified by a property ratio that is simply a certain property of the mixture at the time of sampling in the field relative to that same property at the initial stage (i.e., at construction). This allows consideration of the rate of aging by tracking the property ratio with the CDD. Asphalt mixtures with a higher property ratio demonstrate a higher rate of aging over a given period. As with STOA, MR proved to be an adequate measure for evaluating the effect of field aging. Seven of the sites from Phase I were included in the Phase II experiment. The MR ratio for all field sites included in Phase II of the work is plotted against CDD in Figure S-3. As might be expected, a great deal of aging occurs early in the pavement’s life, and then the rate of aging decreases as time passes. As previously discussed, two LTOA protocols were selected for extensive evaluation in this project, and an additional protocol was tried on a limited basis. As shown in Figure S-4, for the Florida and Indiana field sites, the equivalent CDD values for various LTOA protocols determined based on MR ratio results showed that the 5 days at 185°F (85°C) LTOA protocol stiffened mixtures more than the 2 weeks at 140°F (60°C) LTOA protocol, and the 3 days at 185°F (85°C) LTOA protocol was very similar to the longer 2 weeks protocol. Thus, the 5 days at 185°F (85°C) protocol represented a higher number of CDD at a given site compared to the other two protocols. One of the questions this study attempted to answer was, “When does the stiffness of WMA equal that of HMA in the field?” Of the field sites included in Phase II of this project, Figure S-2. Comparison of resilient modulus of LMLC and PMPC samples. 0 200 400 600 800 1000 0 PM PC - M R (k si) LMLC - MR (ksi) TX I NM CT WY SD IA IN FL TX II 1000800600400200

4three different behaviors were exhibited for the aging of WMA compared to HMA. In one instance, the aging of WMA proceeded along a stiffening path parallel to that of HMA. For four of the field sites, WMA started at a lower stiffness but then intersected the HMA stiff- ness at CDD values between 16,000 and 28,000. Finally, in two field sites, the stiffness of the WMA mixtures was equivalent to the stiffness of the HMA mixtures over time. In terms of factors affecting the long-term aging of the asphalt mixtures, it was found that the inclusion of recycled materials, aggregate absorption, and WMA technology were all sig- nificant factors, while plant type and production temperature were not as significant, which was consistent with the short-term aging study. The presence of recycled mixtures produced a lower rate of aging, as might be expected since aging has already occurred in the recycled materials. Mixtures with aggregates having higher asphalt absorption showed a higher rate of aging than those with lower asphalt absorption. WMA mixtures demonstrated a high rate of aging over the first 1 or 2 years of field service. In summary, this project was successful in validating the proposed STOA protocol sug- gested for preparing WMA and HMA mixtures in the laboratory compared to plant-mixed, laboratory-compacted mixtures over a wide range of mixture components and production parameters. Certain LTOA protocols were developed to simulate aging in the field for a period of 1 to 2 years, depending upon the in-service climate. It is recommended that these field sites be further monitored to understand the long-term behavior of WMA and HMA mixtures. 0.5 1.0 1.5 2.0 2.5 3.0 0 M R St iff ne ss R at io Cumulative Degree-Days (F-days) Texas I New Mexico Wyoming South Dakota Iowa Indiana Florida 40000300002000010000 Figure S-3. Performance ratio versus cumulative degree-days for Phase II field sites. 0 3000 6000 9000 12000 IN FL C um ul at iv e D eg re e- D ay s Field Site 2w@60C 3d@85C 5d@85C Figure S-4. Cumulative degree-days for Phase II field sites.