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This report presents the results of a study on asphalt pavement (RAP) mixtures. The report includes a literature review on RAP and a summary of past research. The research focuses on the use of resilient modulus and complex modulus testing to compare mixtures compacted with only virgin materials to those compacted with varying amounts of RAP. The addition of RAP makes the mixture stiffer, as evidenced by an increase in resilient modulus and complex modulus measurements, according to the research. The addition of RAP also decreases the mixture phase angle, which corresponds to an increase in the elastic properties and a decrease in the viscous mixture properties. The report includes recommendations regarding the proper temperatures and loading frequencies for use in the complex modulus test. It also gives the percentage of RAP and the respective asphalt binder grade necessary to yield the stiffness similar to a virgin mixture.

A laboratory-based linear loading pavement test stand, the Minnesota Accelerated Loading Facility (Minne-ALF) simulates the passage of heavy traffic loads moving at speeds up to 65 kph (40 mph) over small, full-scale pavement test slabs. Hydraulic actuators control a rocker beam, which simulates loads. Researchers simulated the passage of 40-kN (9-kip) single-wheel loads at a rate of 172,000 per day, although wheel loads up to 100 kN (22 kip) can be simulated at varying speeds. Full-axle simulations are possible with frame modifications. Concrete slabs were cast and dowels were installed in slots across cracks/joints. Test variables included joint face texture, repair backfill material, and dowel material and length. Test outputs included measurements of load transfer efficiency and differential deflection across the joint/crack. The effect of joint/crack face texture was great when the joint/crack remained tight. Load carrying performance was improved using Speed Crete 2028 in place of 3U18 concrete backfill with similar joint and dowel bars. Load transfer was unaffected by the use of stainless steel-clad dowel bars in lieu of epoxy-coated dowel bars. Researchers recommend additional testing to examine the effects of dowel length and dowel materials.

This study characterizes asphalt emulsions that are typically used in cold in-place recycling applications. A simple approach was presented that treated the cured residue as asphalt binder and applied the standard Superpave specifications to the material. A literature review examined methods that have historically been used to produce, characterize, and apply asphalt emulsions. Four emulsions were tested in this project. The emulsions were cured two ways, the first being allowed to sit overnight in a pan at room temperature, and the second being a modified rolling thin-film oven approach. Air cured samples were also aged in the pressurized aging vessel. These residues were then tested with the bending beam rheometer and direct tension tester (DTT) at low temperatures and with the dynamic shear rheometer (DSR) at high and intermediate temperatures. AASHTO MP1 specifications were applied in order to characterize the emulsions by performance grade. Following this, AASHTO MP1a specifications were followed in order to find the critical cracking temperature of the emulsions. Master curves were constructed from the DSR tests of complex shear modulus vs. frequency. Finally, a sample mix design was presented using these emulsions and an empirical equation to predict the dynamic modulus of the mixture. Results showed that the air-curing method of recovering asphalt emulsion residue produced more conservative results and is the recommended curing method. MP1 specifications provided a straightforward approach to characterizing the emulsion residues, but showed that the critical cracking temperature obtained by the intersection of thermal stress and strength curves was substantially higher than the limiting temperature obtained in MP1 specification. More research is needed to determine the reasons for the low DTT strength values for these emulsions.

Soil water retention refers to the relationship between the amount of soil water and the energy with which it is held. This relationship is important for characterizing water movement through granular materials. In this project, we generated soil moisture retention data of 18 non-recycled and 7 recycled materials used in pavement construction. The results showed that water retention of non-recycled materials was nearly similar. The major differences among the curves were in the inflection points (air entry values) and in the water contents either near saturation or at 15,300 cm of suction. Using this database, we also developed Pedo-transfer functions that can predict (1) water retention or (2) the parameters of functions that describe water retention from easily measurable properties of the pavement materials. Water retention of concrete with and without shingles was only slightly different. This is partially because shingle chips imbedded in the concrete were large. Traditionally, the influence of matric suction has not been directly considered in pavement design. The water retention data in this report will be helpful in developing resistance factors for Minnesota Flexible Pavement Design Program either through physical modeling or through statistical relationships between design criteria and the water contents.

Several tests are used for characterizing unbound granular materials for pavement applications. The California Bearing Ratio (CBR), resilient modulus (MR), Dynamic Cone Penetrometer (DCP) tests are three of the most common tests used for this purpose. The objective of this research is twofold. The first is to develop numerical models for these three tests. The second is to investigate relationship between basic material properties, boundary conditions, and test results, ultimately, to develop a physics-based correlation between these tests. A 3-D discrete element method (DEM) based model is adapted to simulate these tests. Good agreement is observed between the results of the simulations and sample numerical and experimental studies on granular materials. The DEM code is used to determine effects of aggregate shape, coefficient of friction, gradation, stiffness and other details on test results. The model is also used to investigate statistics of inter-particle interaction between the granular particles.

The objective of the research was to determine the strength and deformation characteristics of base material produced from recycled asphalt pavement (RAP) and aggregate. Various samples with different ratios of RAP and aggregate base were mixed (% RAP/aggregate): 0/100, 25/75, 50/50, 75/25. Laboratory compaction testing and field monitoring indicated that gyratory compacted specimens were closer to the densities measured in the field. Resilient modulus (MR) tests were generally conducted following the National Cooperative Highway Research Program 1-28A test protocol. MR increased with increase of confining pressure, but MR showed little change with deviator stress. The specimens with 65% optimum moisture contents were stiffer than the specimens with 100% optimum moisture contents at all confining pressures. Cyclic triaxial tests were conducted at two deviator stresses, 35% and 50% of the estimated peak stress, to evaluate recoverable and permanent deformation behavior from initial loading to 5000 cycles. The specimens with RAP exhibited at least two times greater permanent deformation than the 100% aggregate material. As %RAP increased, more permanent deformation occurred. In summary, the base material produced with various %RAP content performed at a similar level to 100% aggregate in terms of MR and strength when properly compacted.

This Technical Summary pertains to the LRRB-produced Report 2009-21, “Mechanistic Modeling of Unbound Granular Materials,” published June 2009.