Mechanistic and Volumetric Properties of Asphalt Mixtures with RAP

This research examines how the addition of RAP changes the volumetric and mechanistic properties of asphalt mixtures. A Superpave 19 mm mixture containing 0% RAP was used as the control for evaluating properties of mixes containing 15%, 25%, and 40% RAP. Two types of RAP were evaluated: a processed RAP and an unprocessed RAP (grindings). Testing included dynamic modulus in tension and compression, creep compliance in compression, and creep flow in compression. Using the time-temperature superposition principle, dynamic modulus and creep compliance master curves were constructed to describe the behavior of each mix over a range of temperatures. The VMA and VFA of the RAP mixtures increased at the 25% and 40% levels, and there was also an influence of pre-heating time on the volumetric properties. The dynamic modulus of the processed RAP mixtures increased from the control to 15% RAP level, but the 25% and 40% RAP mixtures had dynamic modulus curves similar to the control mixture in both tension and compression. The creep compliance curves showed similar trends. A combination of gradation, asphalt content, and volumetric properties is likely the cause of these trends. INTRODUCTION The use of recycled asphalt pavement (RAP) material is increasing as local, state and federal transportation agencies make more efficient use of their resources. RAP material is generated when old, damaged pavement materials are milled and crushed for addition as a component to new mixtures placed in the pavement structure. Historically, old pavement material was removed and disposed of in landfills. As landfilling these materials has become less practical and more expensive and the availability of quality virgin materials declines, the addition of RAP to pavement mixtures has become more and more prevalent. Recycling of pavement material can be done as an in-place process or a central plant process. The in-place process combines the reclamation, mixing, laydown, and compaction procedures into a single paving train in the field. In-place recycled materials are typically used for base or binder courses and are typically overlaid with a surface course. The central plant process involves stockpiling RAP at the asphalt plant, which is then mixed with virgin materials at the plant and trucked to the construction site for laydown and compaction. Currently, the state of New Hampshire allows up to 30% RAP from a known source or 15% RAP from an unknown source to be used in a mixture. These values were selected based on general guidelines developed at the national level for the use of RAP in HMA (1,2,3), however, the actual effect of the RAP on the mixture properties and field performance of these mixtures is unknown. The addition of RAP to an asphalt mixture changes the mechanistic properties (i.e., strength, durability) of the mixture and affects its performance (i.e., resistance to cracking and deformation) in the field. The mechanistic properties change as a result of the aged binder introduced to the mixture as part of the RAP. The binder in the RAP will have a different chemical composition and different properties than the virgin binder added during the mixing process. These two binders will mix to some extent, changing the properties of the mixture containing RAP from one that contains only virgin material. A study by Huang, et al (4) showed that the addition of RAP increased mixture stiffness, measured by the indirect tension and semi­ circular bending tests. As the pavement industry moves towards more mechanistic based pavement design and analysis methods such as the American Association of State Highway and 3 Daniel and Lachance Transportation Officials (AASHTO) Design Guide and proposed Simple Performance Test (5), it is essential to evaluate the effect of RAP on the properties of asphalt mixtures. The objective of this research study was to determine how the volumetric properties, dynamic modulus, and creep of mixtures change with the addition of RAP. Two RAP sources were used to study the change in volumetric properties and one RAP source was used for dynamic modulus and creep testing. A control mixture containing only virgin materials (0% RAP) was tested along with mixtures containing 15%, 25%, and 40% RAP. MATERIALS AND MIX DESIGN The materials for the asphalt mixtures tested in this study were obtained from Hooksett Crushed Stone in Hooksett NH, a division of PIKE, with the assistance of the New Hampshire Department of Transportation (NHDOT) Bureau of Materials and Research. A 19 mm Superpave gradation designed for low volume roads (0.3-3 ESALs) was used with an unmodified PG 58-28 binder. Two types of RAP were used in this study; a processed RAP and an unprocessed RAP, or grindings. The processed RAP contains a mix of recycled asphalt pavement, portland cement concrete, and sometimes slight amounts of organic material with an asphalt content of 3.6%. The extracted processed RAP binder has a grade of PG 94-14. The unprocessed RAP, hereinafter referred to as grindings, is material milled from a pavement surface and contains recycled asphalt pavement with an asphalt content of 4.9%. The extracted grindings binder has a grade of PG 82-22. Mix Design The mixes are designed based on a NHDOT approved 19 mm Superpave mixes containing 15% RAP performed by PIKE. The existing mix designs were verified at UNH and are used for fabricating the 15% RAP specimens. For the remaining mixtures (control, 25% RAP, 40% RAP), the stockpile percentages were adjusted to achieve an overall mixture gradation similar to the original 15% RAP gradation. The relative proportions of blast rock and sand stockpiles were held constant for the different mixtures to maintain the same relative structure (particle angularity, type of material) for the virgin material in the mixture. The gradations for the processed and grindings RAP mixtures are shown in Figures 1a and 1b, respectively. For the processed RAP mixtures, the increasing percentages of RAP cause the gradations to become finer at the smaller sieve sizes, with the 40% RAP mixture going into the restricted zone. All of the other gradations fall on the coarse side of the restricted zone. The three mixtures containing the grindings produce slightly finer gradations at the #30 sieve size, but are essentially the same as the control mix. The asphalt contents and volumetric properties of the mixtures are shown in Table 1. Mixing and compaction temperatures were kept constant for all of the RAP percentages; the RAP material was preheated at the mixing temperature. SPECIMEN FABRICATION AND TESTING Specimens were fabricated for both tension and compression testing, using the processed RAP only. The loose mixture was compacted into cylindrical specimens 150 mm in diameter and approximately 180 mm tall using a Superpave Gyratory Compactor (SGC). The final test specimens were cut and cored from the gyratory cylinders thus producing specimens 100 mm in diameter and 150 mm tall for the compression tests and 75 mm in diameter and 150 mm tall for the tension tests. These specimens have the most consistent air void distribution in both the vertical and radial directions based on the study by Chehab et al. (6). These are the specimen 4 Daniel and Lachance geometries currently recommended for the simple performance test (5) and used in constitutive modeling of asphalt concrete in tension and compression (7,8,9). Specimens were tested using a closed-loop servo hydraulic Instron testing system with computer control and data acquisition. An environmental chamber was used to control the testing temperature. The specimens were conditioned to the test temperature for several hours; the specimen temperature was monitored using a dummy specimen with an embedded thermocouple that had been subject to the same temperature history as the test specimen. Four LVDTs were mounted on the specimen to measure deformations over the middle 100 mm of the specimen height. For the compression tests, frictionless membranes were placed between the specimen ends and loading plates to allow the specimen to expand radially during loading, preventing a barreling effect due to restrained ends. The tension test specimens were glued to end plates that are then rigidly connected to the loading frame. The compression and tension test set-ups are shown in Figures 2a and 2b, respectively. Complex modulus and creep tests were performed on the specimens. Complex Modulus Testing The complex modulus test measures the response of the material to cyclic loading at different frequencies (usually ranging from 0.1-30 Hz) in the undamaged state. Asphalt concrete is a viscoelastic material, meaning that its response to a particular load depends on the magnitude of the load, the rate of application, and the duration of the load. Therefore, it is important to evaluate how the material responds to different frequencies or rates of loading, which correspond to the different traffic speeds a pavement could experience in the field. The complex modulus test consists of applying a sinusoidal load history to the specimen at different frequencies. The load amplitude is adjusted based on the material stiffness, temperature, and frequency to keep the strain response within the linear viscoelastic range. The dynamic modulus, |E*|, at each frequency is calculated by dividing the steady state stress amplitude (σamp) by the strain amplitude (εamp) as follows: σ amp E * = (1) ε amp Static Creep Compliance The creep test measures the time-dependent deformation of the material under a static load. A constant load is applied and the strain response is measured during this test. The deformation or strain response can be divided into three zones: 1. Primary zone – where the strain rate decreases with loading time; 2. Secondary zone – where the strain rate remains constant with loading time; and 3. Tertiary zone – where the strain rate increases with loading time. The creep compliance, a visco