ICAR Technical Reports
Permanent URI for this collectionhttps://hdl.handle.net/2152/32838
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Browsing ICAR Technical Reports by Subject "Base course (Pavements)"
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Item Characterization of Unbound Granular Layers in Flexible Pavements(2001-12) Adu-Osei, AlexThe mathematical characterization of unbound granular materials should ideally be based on the behavior of the individual constituent elements and their interaction. Until particulate mechanics are developed to a level where it can easily be applied to characterize unbound granular materials, a nonlinear and cross-anisotropic model must be used to characterize the behavior of granular materials in pavements. Existing pavement design and analysis methods have generally taken a very conservative view of the relative strength properties of granular materials used as base and subbase layers in conventional flexible pavements. The mechanical properties of unbound granular layers in flexible pavements are important to the overall structural integrity of the pavement structure. Linear elastic analysis can be used with reasonable confidence for pavements with full depth asphalt layers, but it is inappropriate for unsurfaced or thinly surfaced flexible pavements unless the nonlinear behavior of unbound granular materials are properly taken into account. Work done by several researchers suggest that incorporating a cross-anisotropic elastic model significantly improves isotropic models and drastically reduces the tensile stresses computed within granular layers. This is due to the fact that the behavior of granular materials depends on particle arrangement. The laboratory determination of cross-anisotropic properties of granular materials has been a difficult task for researchers. In this study, a new laboratory testing protocol has been developed based on the theories of elasticity to determine cross-anisotropic properties of granular materials. The testing protocol is efficient and precise. The test is also an excellent tool for comparative analysis of compacted materials. The behavior of four unbound granular materials was studied. The resilient responses of the materials obey the Uzan type nonlinear model. It was observed that under low stress levels accumulation of permanent strain could stabilize in granular layers. However, at high stress levels, permanent strain will continuously accumulate. A finite element program was modified to incorporate the cross-anisotropic material model. Pavement sections were analyzed with the finite element program. It was observed that cross-anisotropic modeling eliminates the presence of tension zones predicted by isotropic resilient models. Deflection bowls predicted by nonlinear resilient models agree with field deflection bowls.Item Field Validation of the Cross-Anisotropic Behavior of Unbound Aggregate Bases(2001-03) Tutumluer, Erol; Adu-Osei, Alex; Little, Dallas N.; Lytton, Robert L.The ICAR Research Project 502 has focused on determining structural considerations of unbound aggregate pavement layers for a proper representation in the new AASHTO Pavement Design Guide 2002. The research team developed models for the resilient and permanent deformation behavior from the results of triaxial tests conducted at the Texas Transportation Institute (TTI) and at the University of Illinois. The studies have mainly indicated that the unbound aggregate base (UAB) material should be modeled as nonlinear and cross-anisotropic to account for stress sensitivity and the significant differences between vertical and horizontal moduli and Poisson's ratios. UABs were constructed at the TTI Riverside research facility and tested for response and performance using the one-third scale model of the Texas Mobile Loading Simulator. The resilient responses of the test sections were modeled. The nonlinear cross-anisotropic material models used in the base layer predicted vertical deflections that are close to field deflections in the analyzed TTI pavements. Field validation data were also collected from a full-scale pavement test study conducted at Georgia Tech. The test sections had extensive instrumentation and the pavement response variables, such as stresses, strains, and deformations, were measured in all pavement layers including the UABs. The validation of the anisotropic modeling approach was accomplished by analyzing these test sections using GT-PAVE finite element program, predicting UAB responses, and comparing them to the measured ones. Laboratory testing of the aggregate samples was conducted at the University of Illinois and the characterization models were developed for the stress sensitive, cross-anisotropic aggregate behavior. With nonlinear anisotropic modeling of the UAB, the resilient behavior of pavement test sections was successfully predicted at the same time for a number of response variables. In addition, the stress sensitive, cross-anisotropic representation of the base was shown to greatly reduce the horizontal tension computed in the granular base when compared to a linear isotropic representation.Item Increased Single-Lift Thicknesses for Unbound Aggregate Base Courses(1998-10) Allen, John J.; Bueno, Jaime L.; Kalinski, Michael E.; Myers, Michael L.; Stokoe, Kenneth H.A study was conducted to evaluate the feasibility of compacting unbound aggregate base courses in thicker lifts than currently permitted by state departments of transportation (DOTs). At present, the majority of states allow a maximum lift thickness of 8 inches or less. This project constructed and tested full-scale test sections using a variety of material types. Two test pads were constructed in an aggregate quarry in Texas utilizing crushed limestone. Three crushed granite test sections were built as part of a road widening project in Georgia, and two test pads were constructed of uncrushed and partially crushed gravel with loess fines at a gravel production facility near Memphis, Tennessee. Single-lift thicknesses varied from 6 inches to 21 inches. Moisture contents and densities were evaluated using the Nuclear Density Gauge (NDG). Nondestructive seismic testing, using the Spectral-Analysis-of-Surface-Waves (SASW) technique, was used to evaluate stiffness profiles within the compacted lifts. Cyclic plate load tests were accomplished by means of the Rolling Dynamic Deflectometer (RDD), modified for this static application. Results showed that compaction targets could be attained for lifts up to 21 inches thick. Density and stiffness results for 13-inch thick lifts in the Georgia tests were equal to, or better than, the results for the base placed in two lifts, a 7-inch lift followed by a 6-inch lift. Higher moisture contents during compaction yielded lower shear wave velocity and Young’s modulus values. Seismic results show that the upper 3 inches of the final test pads had lower stiffness values, presumably from lower effective stresses near the surface and possibly from some disturbance caused by the compaction equipment. This zone of lower stiffness and slightly less compaction is less evident in the density measurements.Item Prediction of Working Load Displacements Under Plate Loading Tests from Seismic Stiffness Measurements(1998-10) Myers, Michael L.; Stokoe, Kenneth H.; Allen, John J.A study was conducted to evaluate the feasibility of compacting unbound base courses in thicker lifts than currently permitted by state departments of transportation (DOTs). At present, the majority of states allow a maximum lift thickness of 8 inches or less. This project constructed and tested full-scale test section using a variety of material types. Two test pads were constructed in an aggregate quarry in Texas utilizing crushed limestone. Three crushed granite test sections were built as part of a road widening project in Georgia, and two test pads were constructed of uncrushed and partially crushed gravel with loess fines at a gravel production facility near Memphis, Tennessee. Single-lift thicknesses varied from 6 inches to 21 inches. Moisture contents and densities were evaluated using the Nuclear Density Gauge (NDG). Nondestructive seismic testing, using the Spectral-Analysis-of Surface-Waves (SASW) technique, was used to evaluate stiffness profiles within the compacted lifts. Plate load tests were conducted on the surface of the crushed limestone test pads by means of the Rolling Dynamic Deflectometer specially modified for this fixed site application. Low frequency cyclic loads were applied to determine axial stiffness under transient working loads of varying magnitude. The base courses were tested at to moisture contents. The results were evaluated and compared with small strain seismic tests result. Strain amplitudes in the plate load tests led to a 5% to 25% reduction in measured stiffness as compared to the seismic results.Item Structural Characteristics of Unbound Aggregate Bases to Meet AASHTO 2002 Design Requirements: Interim Report(2001-07) Adu-Osei, Alex; Little, Dallas N.; Lytton, Robert L.This report gives the results of a study of the properties of unbound aggregate base materials using both laboratory testing data from full scale field tests in Illinois, Georgia, and Texas, and a model of cross-anisotropic elastic materials to characterize the behavior of the base materials under traffic loads. Using the cross-anisotropic model, the stress distribution in a base course is more realistic than that developed when the aggregate base is considered to be linear and isotropic. The stress distribution based on cross-anisotropic analysis is not only more correct, but it is also more favorable to the unbound aggregate in that significant tensile stresses are found not to occur. The analogy is presented in this report that the response of the aggregate base to the load is as if the stress distribution directly under the wheel load due to anisotropy acts as a moving column under the wheel in which the aggregate essentially produces its own confinement and does not enter into tension. Other findings in this report include the following: 1) The unbound aggregate base material should be modeled as non-linear and cross-anisotropic to account for stress-sensitivity and the significant differences between vertical and horizontal moduli and Poisson’s ratios. 2) The ICAR laboratory testing protocol is efficient and precise and should be considered as a candidate to model the unbound aggregate base. The protocol uses three stress regimes and ten stress levels within each regime to determine stress sensitivity and cross-anisotropy. A system identification method is used to select the five material properties based on the tests results necessary to properly characterize the aggregate base and to satisfy the requirements of elastic work potential theory. 3) The Fast Industrial Process Controls cell is efficient and should be used to characterize unbound aggregate bases. The ratio of the diameter to the specimen height is 1:1. While testing of such sample sizes is discouraged in the literature, improvements made to the IPC cell minimize frictional development between the sample and loading platens resulting in minimal constraint at the sample ends. This is verified in the report based on comparative triaxial testing and finite element analysis. 4) The ICAR testing protocol is an excellent tool for both unbound aggregate characterization and comparative analysis of materials. A compaction study on two very different aggregates (uncrushed river gravel and crushed limestone) was performed in which the aggregates were subjected to impact, kneading gyratory compaction. The difference in the tendency of the compaction techniques to produce varying levels of particle orientation (which affects anisotropy) was evident in the degree of anisotropy measured.Item A Study on the Feasibility of Compacting Unbound Graded Aggregate Base Courses in Thicker Lifts Than Presently Allowed by State Departments of Transportation(1998-10) Bueno, Jaime Luis; Stokoe, Kenneth H.; Allen, John J.A study was conducted to evaluate the feasibility of compacting unbound aggregates base courses in thicker lifts than currently permitted by state departments of transportation (DOTs). At present, the majority of states allow a maximum lift thickness of 8 inches or less. This project constructed and tested full-scale test sections using a variety of materials types. Two test pads were constructed in an aggregate quarry in Texas utilizing crushed limestone, and three crushed granite test sections were built as part of a road widening project in Georgia. Single-lift thickness varied from 6 inches to 21 inches. Moisture contents and densities were evaluated using the Nuclear Density Gauge (NDG). Nondestructive seismic testing, using the Spectral-Analysis-of Surface-Waves (SASW) techniques, was used to evaluate stiffness profiles within the compacted lifts. Results showed that compaction targets could be attained for lifts up to 21 inches thick. Density and stiffness results for 13-inch thick lifts in the Georgia tests were equal to, or better than, the results for the base placed in two lifts, a 7-inch lift followed by a 6-inch lift. Higher moisture contents during compaction yielded lower shear wave velocity and Young’s modulus values. Seismic results show that the upper 3 inches of the final test pads had lower stiffness values, presumably from lower effective stresses near the surface and possibly from some disturbance caused by the compaction equipment. This zone of lower stiffness and slightly less compaction is less evident in the density measurements.