Presentation on theme: "Background: Hot mix asphalt (HMA) is a mixture of aggregate and asphalt binder. The asphalt is a petroleum based substance and, since the oil embargo of."— Presentation transcript:
Background: Hot mix asphalt (HMA) is a mixture of aggregate and asphalt binder. The asphalt is a petroleum based substance and, since the oil embargo of the 1970’s, efforts have been underway to use recycled road materials when formulating mixes for new roadways. Currently, most roads in the state of New Hampshire are paved with asphalt cements that contain 10 to 15% reclaimed asphalt pavement, or RAP. According to the NHDOT, in 2010 162,135 tons of RAP were diverted from landfills and put to use in area roadways. This resulted in a cost savings of 3.8 million dollars. NH is also one of the few states that currently allows RAP contents higher than 15% in all layers of new pavement. RAP is created when old road pavement is removed, milled and sorted into fine and coarse grain components. These materials are combined in specific ratios with virgin aggregate and a new asphalt binder to produce new asphalt concrete. The addition of this recycled material conserves petroleum resources and reduces disposal of hazardous material. However, inclusion of RAP can change the properties and performance of the new pavement, so it is important to determine the best ratio of RAP to virgin material in order to obtain maximum performance and life expectancy for a given set of conditions. Two important factors that determine an asphalt pavement’s life expectancy are its viscoelasticity and its ability to resist moisture. Once a pavement has aged it becomes stiffer and this can lead to increased cracking of the roadway. When RAP is added to a new mix, the asphalt binder adhered to the RAP aggregate has already aged, resulting in a product that has an increased stiffness from the outset. Moisture also has an effect on pavement life and properties. When water enters the asphalt it reduces the cohesion within the binder and the adhesion between the binder and the aggregate. This softens the pavement and can cause stripping of asphalt binder from the aggregate material. This can lead to the development of potholes, cracking or raveling. The intent of this study is to look at the combined effects of aging and moisture on asphalt concretes composed of all virgin material and those with high RAP contents to determine if there is a difference in material properties that could result in changes to pavement performance. Purpose: To compare the effects of moisture on aged HMA with 0% RAP with aged HMA with a 40% RAP content. Materials: NHDOT Type E 12.5 mm Superpave surface course mixture with PG grade 64-28 base binder. The design was for less than 3 million ESAL’s. One mix contained 0% RAP while the other contained 40% RAP. A total asphalt content of 5.7% was used for all mixtures and gradations were also constant. Methods: Six 100 mm diameter by 150 mm high cylindrical specimens of virgin HMA and six specimens with the same dimensions of HMA with 40% RAP were created from Pike Industries NH mix. Three samples of each specimen type were immediately moisture conditioned by vacuum saturating them to between 70-80%, wrapping them in plastic and placing them in a -18 o C freezer for 16 hours. They were then placed in a 60 o C water bath for 24 hours. (AASHTO T-283-03) The remaining six specimens were aged for 8 days in an 85 o C oven, then moisture conditioned using the previously described procedure. After conditioning, all samples were tested for dynamic modulus, which is a measure of stiffness. Dynamic modulus is determined by applying a stress, or load, to the sample and measuring the resulting strain, (AASHTO TP62-03). This was done at six different frequencies (0.1,0.5,1.0,5.0, 10, 25 Hz) and four different temperatures (4.4 o C, 21.1 o C, 37.8 o C, 54.4 o C). Dynamic modulus results for the moisture conditioned specimens were compared to previously obtained results from unconditioned aged and unaged specimens. ￼ Acknowledgements: Special thanks to Dr. Jo Daniel for her guidance and expertise, Sean Tarbox for his collaboration and invaluable assistance, and Dr Daniel’s other graduate students: Mike Elwardany, Marcelo Medeiros, Kelly Barry, and Justin Lowe for their sharing their knowledge and ideas. Thank you also to the National Science Foundation and the Leitzel Center at University of New Hampshire for making this opportunity possible. The Effects of Moisture on Aged Asphalt with High RAP Concentrations Barbara Reid, Sean Tarbox (Masters Candidate CiE), Dr. Jo Daniel (advisor) Dynamic Modulus of Unaged Specimens Dynamic Modulus for Aged Specimens Average Dynamic Modulus Master Curves Asphalt is heated and weighed before molding. Specimens are molded to the correct dimensions in the gyratory compactor. Molded specimens numbered and allowed to cool Cored specimens are numbered, dried and measured They are vacuum saturated to 70-80 %, frozen at - 18 o C for 16 hours then placed in a 60 o C water bath Nuts are placed for LVDT attachment Dynamic modulus is measured at four temperatures and six frequencies for each sample Results: Conclusions: As expected, the 40% RAP HMA was stiffer than the virgin mix in both the unaged and aged conditions. The increase in stiffness due to aging appears to be smaller for 40% RAP mixtures. This again is consistent with expectations since some of the binder in the 40% RAP has already undergone aging. Moisture conditioning seemed to have little effect on the performance of either of the unaged samples, however there was a notable effect on the aged samples. The aged virgin material showed softening as a result of moisture conditioning, which is the expected outcome of moisture infiltration. The moisture conditioned aged 40% RAP samples indicate an increased stiffening of the material. This result could be anomalous due to the small sample size and warrants further investigation.