الفهرس 
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Abstract
In Egypt, many roads suffer from fatigue cracking as a result of overloading, improper mix design, material quality, and construction quality. Thus, effectiveness of geogrids in reinforcement of the cross section of flexible pavement systems for longer fatigue life was investigated. The study involved conducting routine as well as advanced triaxial dynamic loading laboratory testing for comprehensive material characterization. It also included testing five large-scale pavement sections in the laboratory along with finite element analysis (FEA) using the PLAXIS software. The routine tests were used to determine the basic material properties of the unbound granular base materials and subgrade soil used in the construction of five large-scale pavement systems in the laboratory. Shear strength parameters (apparent cohesion, C, and angle of internal friction,) were also determined for the subgrade and granular base materials through static triaxial tests. Repeated load triaxial tests were also performed in order to determine the stress dependent resilient modulus of the base and subgrade materials. The Asphalt Concrete (AC) mix for the wearing course layer was designed according to Marshall method. Five pavement sections consisted of a 5 cm asphalt layer (AC), 15 cm granular base layer, and a 30 cm clay subgrade were built inside a steel box in the laboratory and tested extensively. These pavement sections instrumented with strain gauges at different depth within the granular base layer. The base layer was reinforced with a single layer of RE540 uniaxial Tensar geogrid placed at four different positions within the base layer, one position at a time. These positions were 1) at the interface between the base and subgrade (B0), 2) 5 cm from the bottom of the base layer (B1/3h), 3) the middle of the base layer height (B1/2h), 4) and finally at the interface between the AC and base (Bh). The fifth pavement section did not include any reinforcement (control section). These pavement sections were loaded with static plate loading equipment and the results were compared with the control section (CS).During testing, the tensile strains at three different positions within the granular base layer and the total deformation at the surface of the AC layer were monitored with load. The testing results were analyzed and compared with the results of the finite element. Results from this study showed that geogrid can be used to improve the performance of flexible pavement systems. The position of the geogrid in the pavement system affects the performance significantly. The optimum position of the geogrid reinforcement to improve the pavement fatigue life was found to be directly underneath the AC layer then within 33 to 50% of the granular base layer height measured from the bottom of the base layer. FEA runs using Mohr-Coulomb model yielded relatively similar trends, to the laboratory results, between tensile strain and depth. However, the FEA underestimated the tensile strains. Finally, the TBR analysis showed significant extension in service life in terms of fatigue of the reinforced pavement systems compared to unreinforced systems.