Design and Characterization of Cold Recycled Foamed Asphalt Mixtures with High RAP Contents
Type of DegreePhD Dissertation
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Cold recycling with foamed asphalt has been considered as a sustainable pavement rehabilitation technology. Although different mix design methods have been proposed, there is no widely accepted standard. The effects of mineral additives on the performances of cold recycled mixes are not well understood in either laboratory or field curing conditions. Moreover, there is a lack of guidance on how to evaluate cold recycled mix resistance to permanent deformation and cracking using laboratory testing methods. To address these research needs, this study included three tasks, which are (1) improvement of the mix design method, (2) investigation of the effects of mineral additives, and (3) evaluation of different laboratory testing methods. In the mix design study, the compactive effort based on the Superpave gyratory compactor (SGC) was determined by matching laboratory-compacted densities with the field-compacted densities. Accordingly, an SGC-based approach was developed to determine the optimum water content (OWC) of reclaimed asphalt pavement (RAP) material which is the water content needed to achieve the maximum dry density. However, the OWC does not yield the maximum indirect tensile strength (ITS) which is the primary mix design objective. To maximize the ITS, a multiple linear regression model for determining the optimum total water content (OTWC) was developed and validated. For the mineral additive study, recycled mixtures were prepared with cement, hydrated lime, fly ash, and asphalt plant baghouse fines. Results showed that the mixtures with cement had the highest ITS and the best resistance to moisture damage in both laboratory and simulated-field curing conditions compared with mixtures containing the other mineral additives. A curing experiment showed that mixtures cured in the typical laboratory condition (40°C for 3 days) had similar rankings regarding the dry or wet ITS results as those cured in a 100-day simulated-field condition. In the laboratory testing study, dynamic modulus (|E*|) of small-scale specimens was used to assess the undamaged viscoelastic properties of cold recycled mixes. However, this method may not be ready for implementing in pavement M-E design. Resistance of cold recycled mixtures to permanent deformation was characterized by the strain accumulation rate in the steady state (kst) and the strain at the beginning of the steady state (ɛst) from Flow Number (FN) test. The Flexibility Index (FI) from the Illinois Flexibility Index Test (I-FIT) and the proposed Fracture Work Factor (FWF) based on ITS test data was used to evaluate the cracking resistance. The variabilities of these test results for cold recycled mixes were higher than those of typical HMA materials. These laboratory testing results showed the RAP gradation has a statistically significant effect on the ITS, |E*|, FI, and FWF results. It may also impact the results from FN test (kst and ɛst). The cold recycled mixes with finer RAP had higher ITS, |E*|, and the FN test results but lower FI results compared to those with coarser RAP. The effect of virgin binder was only significant on the ITS result and may also have impact on the results from FN test (kst and ɛst). It is worth mentioning that the conclusions from this study were based only on test results of the limited materials. Further research using more materials and validation with field conditions and performance are needed.