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Structural Coefficients of High Polymer Modified Asphalt Mixes Based on Mechanistic-Empirical Analyses and Full-Scale Pavement Testing
Civil and Environmental Engineering
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Asphalt concrete (AC) mixtures have been used as driving surfaces for flexibles pavements since the early 1900s. With the increase of highway traffic volume and axle loads, the introduction of modified asphalt binders provided transportation agencies an effective tool to design balanced asphalt mixtures that can resist conflicting distresses such as permanent deformation and fatigue cracking while maintaining good long-term durability (i.e., reduced moisture damage and aging). While polymer modified asphalt (PMA) mixtures, with 2-3% polymer content, have shown improved long-term performance, it is also believed that asphalt mixtures with high polymer (HP) content (i.e., >6% polymer content) may offer additional advantages in flexible pavements subjected to heavy and slow-moving traffic loads. The main objective of this study is to conduct an in-depth and comprehensive evaluation of asphalt mixtures in the state of Florida with a high polymer (HP) modified asphalt binder with approximately 7.5% Styrene-Butadiene-Styrene (SBS) polymer. The study combines the following five major aspects: (1) Literature Review: information and findings from the literature review on the performance of HP asphalt binders and mixtures in the laboratory and in the field were collected. In addition, attempts to determine a structural capacity for HP AC mixes using available data were executed. (2) Extensive laboratory evaluation of HP asphalt binder and mixtures: PMA and HP asphalt binders sampled from two different sources were evaluated in terms of long-term aging susceptibility to observe and quantify the influence of binder modification on the oxidative aging characteristics of these asphalt binders. Additionally, A total of 8 PMA and 8 HP AC mixes were manufactured and designed using PMA and HP asphalt binders and were evaluated in terms of engineering properties (i.e., stiffness) and performance characteristics (i.e., resistance to rutting, fatigue cracking, top-down cracking, and reflective cracking). (3) Advanced mechanistic analysis under heavy moving loads using 3D-MOVE: the developed properties and characteristics of PMA and HP mixtures were implemented in the 3D-MOVE model to determine the responses and performance of PMA and HP pavement sections under various loading conditions. Using the pavement responses from 3D-MOVE along with the performance models for the PMA and HP asphalt mixtures for rutting in AC and fatigue cracking, structural coefficients of the HP modified asphalt mixtures were determined using the fixed service life approach based on the fatigue performance life and verified against other distress modes (i.e., AC and total rutting, top-down cracking, and reflective cracking). (4) Full-scale pavement testing using PaveBox: the 11 feet width by 11 feet depth by 7 feet height PaveBox served as a full-scale laboratory tool to verify the structural coefficients developed and checked previously. (5) Advanced numerical modeling of PaveBox using FLAC3D (Fast Lagragian Analysis of Continua in 3-Dimensions): the three-dimensional explicit finite difference program was used to provide an advanced analysis of sections built-in the full-scale PaveBox experiment. The review of available literature led to the following findings and recommendations: • The reviewed laboratory studies indicated: a) Increasing the SBS polymer content from 0, 3, 6, to 7.5% continues to improve the performance properties of the asphalt binder and mixture, b) HP modification tends to slow down the oxidative aging of the asphalt binder, and c) HP asphalt binder should not be used to overcome the negative impact of RAP on the resistance of the AC mixture to various types of cracking. • The reviewed field projects indicated: a) HP AC mixes have been used over a wide range of applications from full depth AC layer to thin AC overlays under heavy traffic, b) HP AC mixes did not show any construction related issues, c) while early performance is encouraging, almost all HP field projects lack long-term performance information. • While several previous studies highlighted the positive impacts of the HP modification of asphalt binders and mixtures, there is still a serious lack of understanding on the structural value of the HP AC mix as expressed through the structural coefficient for the AASHTO 1993 Guide. The attempt by the research team to determine an aHP-AC based on the available information led to the conclusion that empirically-based aHP-AC can underestimate the structural value of the HP AC mix while determining the aHP-AC based on the mechanistic analysis of a singly failure mode (i.e., fatigue cracking) may overestimate the structural value of the HP AC mix. The laboratory evaluation of PMA and AC mixes and the advanced mechanistic analyses of PMA and HP flexible pavement structures led to the following findings and recommendations:• Overall, HP AC mixes showed better engineering property and performance characteristics when compared with the corresponding PMA control AC mixes which can be credited to the high polymer modification of the asphalt binder (i.e., HP binder). • The estimated initial fatigue-based structural coefficients ranged from 0.33 to 1.32. Using advanced statistical analyses and considering all factors and their interactions, an initial fatigue-based structural coefficient of 0.54 was determined for HP AC mixes. • The initial fatigue-based structural coefficient for HP AC mixes of 0.54 was verified for the following distresses; rutting in AC layer, shoving in AC layer, total rutting, top-down cracking, and reflective cracking. The verification process concluded that the structural coefficient of 0.54 for HP AC mixes would lead to the design of HP pavements that offer equal or better resistance to the various distresses as the designed PMA pavements with the structural coefficient of 0.44. This conclusion held valid for the design of both new and rehabilitation projects. • Based on the data generated in the execution of the experimental plan and the analyses presented, it was recommended that HP AC mixes be incorporated into the current FDOT Flexible Pavement Design Manual with a structural coefficient of 0.54.The following activities and analyses were completed under the full-scale PaveBox testing task:• Two full scale experiments were conducted in the Pave Box facility; experiment No.1 evaluated a flexible pavement with PMA AC layer and experiment No. 2 evaluated a flexible pavement with HP AC layer. The design thickness of the PMA AC layer was 4.25 inch (108 mm) based on a structural coefficient of 0.44 while the design thickness of the HP AC layer was reduced to 3.50 inch (89 mm) based on the recommended structural coefficient of 0.54. Both pavements had the same CAB and SG layers.• The full-scale pavements were instrumented to measure the responses to load in terms of surface deflections, tensile strains at the bottom of the AC layer, and vertical stresses in the CAB and SG layers. In addition, AC mixtures were sampled during construction and evaluated for their dynamic modulus, fatigue, and rutting characteristics. • The first analysis compared the measured pavement responses from the two pavements. In general, the reduced thickness of the HP AC layer resulted in higher vertical surface deflections, higher vertical stresses at the middle of the CAB layer, similar vertical stresses at 6 inch (152 mm) and 24 inch (610 mm) below the SG surface, and similar or lower tensile strains at the bottom of the AC layer. • The second analysis compared the responses of the two pavements calculated through mechanistic modeling. The mechanistic analysis showed the HP pavement generated; better fatigue and rutting performance in the AC layer, higher rut depths in the unbound layers but similar total rut depths. In general, the overall results of the full-scale testing in the PaveBox supported the aHP-AC selection of 0.54. A testing plan for the FDOT APT has been recommended to further validate the recommended structural coefficient for HP AC mixes. The main thrust of the APT plan is to identify unique cases where localized shear failure may occur in the CAB layer under the reduced thickness of the HP AC layer.