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Introduction -- Mechanistic-Empirical Pavement Design Guide and AASHTOWare Pavement ME Design (TM) Software Overview -- Survey of Agency Pavement Design Practices -- Common Elements of Agency Implementation Plans -- Case Examples of Agency Implementation -- Conclusions.
The Virginia Department of Transportation (VDOT) currently follows pavement design procedures for all new and rehabilitated pavements based on the 1993 AASHTO Guide for Design of Pavement Structures. VDOT's Materials Division is in the process of implementing the Mechanistic-Empirical Pavement Design Guide (MEPDG) procedure via AASHTOWare Pavement ME Design software. The MEPDG uses mechanical properties of pavement materials for pavement structural design. The mechanistic-empirical design process presents a major change in pavement design from the 1993 AASHTO design guide. It calculates pavement responses through mechanistic analysis based on inputs such as traffic, climate, and materials properties to predict the pavement damage or distress over time for both asphalt and concrete pavements. The purpose of this study was to evaluate the mechanical properties of cement-treated aggregate (CTA) and recommend values for use in AASHTOWare Pavement ME Design software. The field construction of CTA was monitored, and samples were collected for laboratory determination of the compressive strength, modulus of elasticity, and modulus of rupture. Tests with the falling weight deflectometer were conducted to back-calculate the CTA modulus of elasticity, and field cores were collected for testing compressive strength and modulus of elasticity. CTA gained strength with increases in cement content, and the increase in strength and the strength level depended on the aggregate properties, such as the resilient modulus of unbound aggregate. All measured properties were highly variable. VDOT would need to implement a strength-based CTA design to be able to use the required mechanical properties of CTA in the MEPDG system. The study recommends using a target design 7-day compressive strength of 600 to 800 psi. Such strength corresponds well with VDOT's current pavement design practice in accordance with the 1993 AASHTO design guide. CTA mechanical properties were suggested based on this target strength. Most of the default values presented in the MEPDG are considered reasonable. In addition, the values recommended for use in the MEPDG are 1.5 million psi for modulus of elasticity and 200 psi for modulus of rupture.
The Virginia Department of Transportation (VDOT) currently follows pavement design procedures for all new and rehabilitated pavements based on the 1993 AASHTO Guide for Design of Pavement Structures. VDOT's Materials Division is in the process of implementing the Mechanistic-Empirical Pavement Design Guide (MEPDG) procedure via AASHTOWare Pavement ME Design software. The MEPDG uses mechanical properties of pavement materials for pavement structural design. The mechanistic-empirical design process presents a major change in pavement design from the 1993 AASHTO design guide. It calculates pavement responses through mechanistic analysis based on inputs such as traffic, climate, and materials properties to predict the pavement damage or distress over time for both asphalt and concrete pavements. The purpose of this study was to evaluate the mechanical properties of cement-treated aggregate (CTA) and recommend values for use in AASHTOWare Pavement ME Design software. The field construction of CTA was monitored, and samples were collected for laboratory determination of the compressive strength, modulus of elasticity, and modulus of rupture. Tests with the falling weight deflectometer were conducted to back-calculate the CTA modulus of elasticity, and field cores were collected for testing compressive strength and modulus of elasticity. CTA gained strength with increases in cement content, and the increase in strength and the strength level depended on the aggregate properties, such as the resilient modulus of unbound aggregate. All measured properties were highly variable. VDOT would need to implement a strength-based CTA design to be able to use the required mechanical properties of CTA in the MEPDG system. The study recommends using a target design 7-day compressive strength of 600 to 800 psi. Such strength corresponds well with VDOT's current pavement design practice in accordance with the 1993 AASHTO design guide. CTA mechanical properties were suggested based on this target strength. Most of the default values presented in the MEPDG are considered reasonable. In addition, the values recommended for use in the MEPDG are 1.5 million psi for modulus of elasticity and 200 psi for modulus of rupture.
The proposed Mechanistic-Empirical Pavement Design Guide (MEPDG) procedure is an improved methodology for pavement design and evaluation of paving materials. Since this new procedure depends heavily on the characterization of the fundamental engineering properties of paving materials, a thorough material characterization of mixes used in Virginia is needed to use the MEPDG to design new and rehabilitated flexible pavements. The primary objective of this project was to perform a full hot-mix asphalt (HMA) characterization in accordance with the procedure established by the proposed MEPDG to support its implementation in Virginia. This objective was achieved by testing a sample of surface, intermediate, and base mixes. The project examined the dynamic modulus, the main HMA material property required by the MEPDG, as well as creep compliance and tensile strength, which are needed to predict thermal cracking. In addition, resilient modulus tests, which are not required by the MEPDG, were also performed on the different mixes to investigate possible correlations between this test and the dynamic modulus. Loose samples for 11 mixes (4 base, 4 intermediate, and 3 surface mixes) were collected from different plants across Virginia. Representative samples underwent testing for maximum theoretical specific gravity, asphalt content using the ignition oven method, and gradation of the reclaimed aggregate. Specimens for the various tests were then prepared using the Superpave gyratory compactor with a target voids in total mix (VTM) of 7% ± 1% (after coring and/or cutting). The investigation confirmed that the dynamic modulus test is an effective test for determining the mechanical behavior of HMA at different temperatures and loading frequencies. The test results showed that the dynamic modulus is sensitive to the mix constituents (aggregate type, asphalt content, percentage of recycled asphalt pavement, etc.) and that even mixes of the same type (SM-9.5A, IM-19.0A, and BM 25.0) had different measured dynamic modulus values because they had different constituents. The level 2 dynamic modulus prediction equation reasonably estimated the measured dynamic modulus; however, it did not capture some of the differences between the mixes captured by the measured data. Unfortunately, the indirect tension strength and creep tests needed for the low-temperature cracking model did not produce very repeatable results; this could be due to the type of extensometers used for the test. Based on the results of the investigation, it is recommended that the Virginia Department of Transportation use level 1 input data to characterize the dynamic modulus of the HMA for projects of significant impact. The dynamic modulus test is easy to perform and gives a full characterization of the asphalt mixture. Level 2 data (based on the default prediction equation) could be used for smaller projects pending further investigation of the revised prediction equation incorporated in the new MEPDG software/guide. In addition, a sensitivity analysis is recommended to quantify the effect of changing the dynamic modulus on the asphalt pavement design. Since low-temperature cracking is not a widespread problem in Virginia, use of level 2 or 3 indirect tensile creep and strength data is recommended at this stage.
Characterization of pavement materials in the three hierarchical design levels of the proposed mechanistic-empirical pavement design (MEPD) guide involves application of the dynamic modulus technique for asphalt concrete and the resilient modulus for unbound materials. This approach, if adequately implemented, is expected to improve the road design processes. The advance design level recommends using actual laboratory test data of the dynamic and resilient modulus determined under simulated environmental and traffic loading conditions. To circumvent the need for conducting the mechanical test in lower design levels, predictive equations and correlations established with physical properties are used to estimate the mechanistic properties needed as input to the design software. This paper examines the simplifications incorporated in the model using results of dynamic and resilient modulus tests performed at the National Research Council Canada (NRC). For the covering abstract of this conference see ITRD number E211426.
This guide provides guidance to calibrate the Mechanistic-Empirical Pavement Design Guide (MEPDG) software to local conditions, policies, and materials. It provides the highway community with a state-of-the-practice tool for the design of new and rehabilitated pavement structures, based on mechanistic-empirical (M-E) principles. The design procedure calculates pavement responses (stresses, strains, and deflections) and uses those responses to compute incremental damage over time. The procedure empirically relates the cumulative damage to observed pavement distresses.