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51 Conclusions Based on the results of the research in this project, the fol- lowing conclusions are made. 1. Fatigue cracking, rutting/corrugations, depressions and frost heave in flexible pavements and cracking, pumping/ faulting, frost heave, and erosion in rigid pavements are distresses associated with poor performance of the recycled aggregates used in the unbound layers of these pavements. 2. Properties of recycled aggregates used in unbound base and subbase pavement layers that affect pavement per- formance include shear strength, stiffness, toughness, dura- bility, frost susceptibility, and permeability. Shear strength and stiffness (resilient modulus) have a much greater influ- ence on the performance of an unbound aggregate layer than the other properties. 3. The following tests relate to the performance of recycled materials used in unbound pavement layers: ⢠Screening tests for sieve analysis and the moisture-density relationship, ⢠The Micro-Deval test for toughness, ⢠Resilient modulus for stiffness, ⢠Static triaxial and repeated load at OMC and saturated for shear strength, and ⢠The tube suction test for frost susceptibility. Suggested Research Based on the work performed in this project, modifications of the repeated load and moisture content tests are recom- mended. The modification of the repeated load test will im- prove the application of the seating load and reduce the inter- laboratory variability. The modification of the moisture content test will allow air-drying at ambient temperature using a fan blowing across loosely spread aggregate placed on a tarp. In addition, ranges for selected test parameters were recom- mended for the use of RAP and RCP materials (or blends with virgin aggregate) in different climatic conditions and traffic levels. The recommended aggregate tests and the ranges of test parameters were based on laboratory test results. Further re- search is needed to confirm the validity of these tests and ranges under service conditions. A field validation plan is suggested to further validate the suitability of using the performance-related tests identified in this research as predictors of performance. The plan makes use of accelerated pavement testing (APT) and in-service pavement test sections. Accelerated Pavement Testing Many state DOTs use accelerated pavement testing to eval- uate potential construction materials, pavement designs, and other pavement-related features. During APT, wheel loads are applied to specially constructed or in-service pavements to determine pavement response and performance under a con- trolled and accelerated accumulation of damage in a short time. It is recommended to construct flexible and rigid pave- ment sections with unbound recycled aggregate layers to eval- uate the merits of the reported research findings. By varying the characteristics of recycled aggregates used for unbound pavement layers, the effects of various aggregate properties on the performance of unbound pavement layers can be assessed. This field performance can then be compared with the per- formance predicted using the methodologies developed in this research. The primary advantage of this approach is that the factors that affect pavement performance could be more closely con- trolled. This is particularly important if the test involves study- ing the effect of a single or a group of factors on pavement per- formance. The disadvantage of this approach is that long-term strength loss due to poor durability and frost effects cannot be fully assessed. C H A P T E R 6 Conclusions and Suggested Research
52 In-Service Test Pavements Testing in-service pavements is proposed to evaluate the recommended procedures in actual practice. This ap- proach assesses the adaptability of the recommended tests to state DOTsâ current methods of evaluating recycled ag- gregates and compares the test results. In this study, the performance of pavements incorporating unbound recy- cled material layers will be used to evaluate performance prediction accuracy. The study would involve identification of pavement proj- ects currently being designed that represent a range in traffic and climatic conditions. The recycled aggregate used in each project would be tested using the recommended procedures and compared with test results from current DOT evaluation procedures. Construction of the pavement project would be followed to document construction practices; performance of the test pavement would be monitored for future analysis. There are benefits in testing in-service pavements, but there are also major disadvantages. Testing the in-service pavement can provide the comfort of knowing that the pavement is ârealâ in all respects. However, in most cases, in-service pavements do not allow for good control of the factors that may affect pave- ment performance. Testing in-service pavement also limits, or at least makes more difficult, the use of instrumentation in- stalled in the pavement structure to measure response and per- formance. Also, it will generally take several years before per- formance data become available.
