Geogrid reinforcement can improve the performance of pavements by stiffening the aggregate base material and decreasing pavement deformations. Understanding the effects of cyclic loading on the modulus of geogrid-reinforced base materials would help engineers better anticipate actual increases in the modulus of aggregate base materials under given traffic loads. The objective of this laboratory research was to investigate the effects of cyclic loading on the resilient modulus, the modulus to peak axial stress, the elastic modulus, and the modulus at 2 percent strain of geogrid-reinforced aggregate base materials. The scope of the research included two aggregate base materials (Wells Draw and Springville) having different particle-size distributions and particle angularity. Geogrid-reinforced and unreinforced specimens were subjected to conditioning periods consisting of cyclic loading ranging from 10 to 10,000 cycles. Immediately following cyclic loading, all specimens were tested using the quick shear portion of the American Association of State Highway and Transportation Officials T 307 (Determining the Resilient Modulus of Soils and Aggregate Materials). Specimen preparation involved material weigh-outs, compaction, and membrane applications. Specimen testing in the loading machine consisted of two testing portions, including cyclic loading and quick shear testing. The cyclic loading data were used to calculate the resilient modulus on 200-cycle intervals throughout the duration of the conditioning period. The quick shear data were used to calculate the peak axial stress, the modulus to peak axial stress, the elastic modulus and the modulus at 2 percent strain. For the Wells Draw material, the resilient modulus increases by 11 percent for the specimens with geogrid and increases by 8 percent for the specimens without geogrid as the number of load cycles increases from 1,000 to 10,000. For the Springville material, the resilient modulus increases by 2 percent for the specimens with geogrid and increases by 3 percent for the specimens without geogrid as the number of load cycles increases from 1,000 to 10,000. As with other studies, the results do not show a consistent or significant effect of geogrid reinforcement on the resilient modulus of the tested materials. The modulus at 2 percent strain has the most potential for consistently showing improvements to aggregate base materials due to both cyclic loading and geogrid reinforcement. For the Wells Draw and Springville materials, the modulus at 2 percent strain increases by 31 and 9 percent, respectively, as the number of load cycles increases from 10 to 10,000. Additionally, for the Wells Draw and Springville materials, the modulus at 2 percent strain of the specimens with geogrid is 23 and 46 percent, respectively, greater than that of the specimens without geogrid. The results show a consistent and significant positive effect of geogrid reinforcement on modulus at 2 percent strain of the tested materials. According to the modulus at 2 percent strain results, a sufficient conditioning period appears to occur at 5,000 cycles for the Wells Draw material and 10,000 cycles for the Springville material.

The installation of geogrid as a means of extending the service life of a roadway or reducing the required base course thickness of a pavement structure has become increasingly popular. The realization of these benefits depends largely on the degree to which the geogrid reinforcement leads to an increase in the stiffness of the aggregate base course layer. The objective of this research was to investigate the structural capacity of geogrid-reinforced aggregate base materials in flexible pavements through full-scale testing. The scope involved field testing at two sites in northern Utah that each included five different geogrid-reinforced sections and five accompanying unreinforced control sections. Five different geogrid types were utilized to ensure that the experimentation was representative of the geogrid products available in the industry at the time of the study. At each of the two field sites, 10 test sections were established, and several field tests were conducted during and following construction of the two pavements to characterize the in-situ structural properties of the subgrade, base, and hot mix asphalt layers of each geogrid-reinforced and unreinforced test section. The procedures involved nuclear density gauge, soil stiffness gauge, Clegg impact soil tester, dynamic cone penetrometer (DCP), portable falling-weight deflectometer, and falling-weight deflectometer testing of each test section. Samples of the subgrade and base materials were also obtained from both field sites for laboratory testing, which included dry and washed sieve analyses, Atterberg limits testing, and material classification. An analysis of covariance (ANOCOVA) was conducted on the results of each field test to determine if the structural capacity of the geogrid-reinforced sections was different than that of the accompanying unreinforced control sections.Among the 24 ANOCOVA models developed for the two field sites, only four indicated that geogrid presence was statistically significant. Of these four models, three indicated that the presence of geogrid reinforcement led to higher values of the given measurement of structural capacity compared to the unreinforced condition; however, in none of the cases was the difference practically important as defined in this research and would therefore not result in a different input in the pavement design process. Notably, in all three of these models, the same testing procedure, namely the DCP, was used for the testing. A measurable increase in the structural capacity of the reinforced layer may not be immediately observable using standard pavement testing procedures. Further field research is recommended to investigate the duration of the required conditioning period and also the extent of the zone of influence of geogrid reinforcement in aggregate base courses.