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Pavement Structural Analysis August 2004 Report No. 15953-2/1 ARA-ERES Consultants study, applies the cumulative damage concept in the prediction of these two modes of distress. Use of the cumulative damage concept permits accounting, in a rational manner, for damage caused by each load application. Seasonal and other variations in material properties and modulus of each layer with different loads can be considered in these predictions of damage. Evaluations of design life for candidate pavement structures are based on computations of damage caused by each truck type and load (or an 18-kip ESAL) for different seasons of the year, and summing the results to obtain the total damage to the pavement structure. For this design study, however, dynamic modulus tests results were unavailable for all HMA mixtures planned for use along I-96. As noted above, the dynamic modulus for the HMA mixtures were calculated using the Witczak regression equation included in the new M-E Pavement Design Guide software. An equivalent annual modulus for similar HMA mixtures was used in the design study. The equivalent annual modulus values are included in Appendix C. 4.1 Pavement Structural Evaluation Assumptions The pavement layer thickness and material types were based on mechanistic-empirical techniques, in accordance with the following assumptions and design features. • The pavement structural response model used to calculate pavement responses for the simplistic M-E analysis method was based on elastic layer theory - EVERSTRS. • The proposed pavement structure was evaluated using the design criteria for fatigue cracking (limiting the tensile strain at the bottom of the HMA layers), HMA rutting (limiting the vertical strain at the mid-depth of each HMA layer), and subgrade distortion (limiting the vertical strain at the top of the foundation soil). • Structural design life = 20 years. • Tire load = 4,500 lbs. (20 kN) per tire. • Tire pressure = 120 psi (827 kPa). • Sand Subbase; Assumed to be non-frost susceptible and determined from repeated load resilient modulus tests included in the LTPP database, figure 7; Poisson’s ratio = 0.40. • Aggregate Base (21AA-MOD); Determined from repeated load resilient modulus tests included in the LTPP database, figure 8; Poisson’s ratio = 0.35. • The combined equivalent annual elastic layer modulus for the HMA surface, leveling, and base mixtures = 892,000 psi (127.6 MPa) for the simplistic M-E analysis method, refer to Appendix C; Poisson’s ratio = 0.30. For the new M- E Pavement Design Guide, the dynamic modulus default values for a Superpave mix with a PG 70-22 asphalt was used (figures 9-11). The asphalt grades included as defaults in the new M-E Design Guide software are only for the standard grades. 14
Pavement Structural Analysis August 2004 Report No. 15953-2/1 ARA-ERES Consultants 4.2 Evaluation Criteria The objective of this study was to evaluate the flexible pavement structure shown in figure 1 using M-E criteria over two analysis periods: 20 and 40 years. The failure of a pavement system under the cumulative damage concept is assumed to occur when the damage index reaches a fixed amount, generally 1.0. It should be understood that a damage index of 1.0 does not necessarily imply a functional failure, but is instead that level of damage selected as sufficient to warrant maintenance and/or rehabilitation. Failure of flexible pavements is defined as alligator cracking over 10 to 20 percent of the area subjected to wheels or one-half inch (12.7 mm) of foundation rutting. For this study, a damage index of 1.0 means the pavement has been subjected to a sufficient number of wheel loads (Nf) to cause 10 to 20 percent alligator cracking of moderate to high severity or 0.5 inches (12.7 mm) of foundation distortion. In addition, a value of 0.40 inches (10 mm) of distortion (rutting, ∆HMA) in the HMA layers was also used in the evaluation. These values of 10 to 20 percent cracking and 0.5 inches (12.7 mm) of foundation distortion were selected, because previous studies of in-service pavements have indicated that these levels will usually trigger some type of pavement rehabilitation. 4.3 Simplistic M-E Analysis Method Fatigue Cracking Evaluation Two fatigue cracking models were used with the simplistic M-E analysis method. Equation 1 and figure 13 were used to determine the allowable number of load applications for fatigue cracking analysis of the pavement structure. Where: −3. 291 −0. 854 ( Fatigue) = 0. 00432( C)( ) ( E) N t f ε (1) ( ) M C = 10 (2) ⎛ Vbe M = 4. 84 ⎜ ⎝Va + Vbe ⎞ − 0. 69 ⎟ ⎠ (3) εt = Tensile strain at the bottom of the HMA layer, in./in. E = HMA elastic or dynamic modulus, psi. C = Correction factor to account for volumetric properties Vbe = Percent effective asphalt content by volume in HMA mixture, % Va = Percent air voids in the HMA mixture, % Equation 1 is based on 20 percent fatigue cracking, and is the equation embedded in the new M-E Pavement Design Guide software, but without the global calibration factors. The global calibration factors were not used because they were determined based on the cracking predictions using the modulus values of each layer within specific time intervals and seasons. The simplistic M-E method uses equivalent annual modulus values for each 15
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