Drescher, Andrew

At the Minnesota Road Research Project (Mn/ROAD), asphalt concrete mixtures were used to evaluate both warm and cold temperature material properties with selected text methods and a wide range of testing parameters. These parameters were selected to approximate different levels of environmental conditions, traffic speeds, traffic loads, and, in certain cases, confining pressures. The underlying theories used to calculate stress and strain from various loading configurations also were rigorously evaluated to determine the appropriateness of comparing results from one testing configuration to another. Mn/ROAD mixtures were evaluated as the first step in linking laboratory measurements and test method selection to live traffic pavement responses and performance. A comparison of axial and diametral testing using harmonic loading showed that experimental results did not agree with theory. That is, the complex deviatoric modulus determined for diametral testing should have been less than the Young's modulus determined from testing axially loaded samples. This was not the case. This suggested that a further examination of the sample instrumentation, testing variability, and the possibility of anisotropic mixture behavior due to particle orientation during compaction are needed to resolve these differences. Other findings indicated that the influence of load duration is minimized as the test temperature decreases, there was little influence in rest period times in repeated loading tests on modulus, and confining pressure only had a significant influence on modulus above about room temperature.

This report describes three separate studies that examine the deformability of shredded tire fill material: * The first study determined the response of shredded tires to cyclic loading. These tests were conducted in a load frame on both constrained and unconstrained samples and showed that the material became stiffer with increasing load. * The second study focused on the long-term creep settlements of constrained and unconstrained samples. The results indicate that creep exists for a period of up to two years after lading. * The third study investigated the possible anisotropic properties of the material. A larger modulus of elasticity was found in directions parallel to the layers than in the direction perpendicular to the layers. However, settlements computed using a representative anisotropic shredded tire fill material indicate that the current settlement analysis, based on isotropic shredded tire layers, predicts maximum settlements conservatively.

This report examines the diametral compression test, as described in ASTM D4123-82 (1987) and SHRP Protocol P07 (1993) procedures. The test helps determine the resilient modulus of asphalt concrete, and less frequently its Poisson's ratio, both mechanical parameters of an ideally elastic material. However, the actual behavior of asphalt concrete is not elastic, but viscoelastic. The viscoelastic behavior of asphalt concrete under traffic-induced loads can be described by the phase angle and the magnitude of the complex compliance or complex modulus. These can be determined from the diametral compression tests that subject the specimen to haversine load history, and from the viscoelastic data interpretation algorithms derived in the current research. To avoid inaccuracies in the data interpretation, the vertical deformation should be measured over a 1/4 diameter central sector of the cylinder by means, for example, of the in-house developed displacement gage. A series of tests on specimens with various asphalt binder viscosity verified the validity of the viscoelastic data interpretation. Specimens from Mn/ROAD materials showed the presence of viscoelastic properties even at temperatures well below freezing.

The use of shredded tires as a lightweight fill material over weak soil deposits is gaining popularity as a means of disposing a great quantity of an undesirable waste material in a beneficial manner. This report discusses the production, past applications, and properties of shredded tires with respect to their use in lightweight fills. Shredded tires have the advantages of low bulk density, high permeability, and insensitivity to the presence or state of moisture. However, they possess a high degree of compressibility, and they exhibit a degree of rebound atypical of materials normally used in lightweight fills (e.g., woodchips). Before using shredded tires in a fill, one must consider the layer thickness of the shredded tires, the amount of overburden to be placed on the tires, the type of pavement surface, and the volume of heavy traffic expected to use the roadway. Because of the orientation of shredded tires after placement, the importance of anisotropy as a future research topic is discussed.

It is estimated that the production of new roofing shingles generates approximately 1,000,000 tons of waste annually in the US., and about 36,000 tons of this waste is in the Twin Cities Metro Area of Minnesota. With another 8.5 million tons of waste materials which are similar to those used in asphalt concrete, it seems viable that their use in hot-mix would be an attractive alternative to disposing of them in landfills. This report presents the results of an effort to evaluate the use of roofing waste generated by manufacturers and from reconstruction projects. It was shown that up to 5%, by weight of mixture, manufacturing waste roofing shingles could be used in asphalt concrete with a minimum impact on the properties of the mixture. At a level of 7.5%, a noticeable softening of the mixture occurs, and this might be detrimental to pavement performance. The use of shingles from roof reconstruction projects resulted in the embrittlement of the mixture which may be undesirable for low temperature cracking of pavements. The manufactured shingle waste seems to work well in stone mastic asphalt mixtures.

Test rolling is a quality assurance test in which penetration of the wheels of a heavy vehicle into subgrade soils is used as a measure of the adequacy of compaction. Current criteria for acceptable test roller penetration are empirical. Two theoretical approaches for modeling test rolling are developed. One is analytic and the other is numerical, based on the finite element code ABAQUS. Both approaches relate wheel penetration to wheel geometry, wheel load, and soil strength parameters (friction angle and cohesion). Elastic soil properties are included in the numerical simulations but play a secondary role. The models accommodate both rigid and flexible wheels. Homogeneous and layered soil structures are considered. Scaled laboratory tests support the theoretical predictions, although full validation requires execution of extensive full scale field testing. The models developed can be used to enhance interpretation of test rolling data and assess the effects of test roller modifications.

This study deals with the experimental investigation of the effects of moisture and density on the elastic moduli and strength of four subgrade soils generally representing the range of road conditions in Minnesota. The testing approach involved i) reduced-scale simulation of field compaction, ii) field-type testing on prismatic soil volumes, and iii) element testing on cylindrical soil specimens. The field-type testing included: i) the GeoGauge, ii) the PRIMA 100 device, iii) the modified light weight deflectometer (LWD) device, iv) the portable vibratory deflectometer (PVD) and v) the Dynamic Cone Penetrometer (DCP). To compare the Young's modulus values stemming from the field-type and laboratory experiments, cylindrical specimens were extracted from the prismatic soil volumes and tested for the resilient modulus (Mr), small-strain Young's modulus using bender elements. The results reveal that both moisture and density have a measurable effect on the elastic modulus and strength of all four soils. On the element testing side, the small strain estimates from the bender element tests were in good agreement with the resilient modulus values. In the context of field testing, there was significant scatter of the estimated Young's moduli depending upon the particular testing device.

Resilient modulus, shear strength, dielectric permittivity, and shear and compressional wave speed values were determined for 36 soil specimens created from the six soil samples. These values show that the soils had larger stiffnesses at low moisture contents. It was also noted during testing that some non-uniformity was present within the axial displacement measurements; larger levels of non-uniformity were associated with low moisture contents, possibly due to more heterogeneous moisture distributions within these specimens. Lastly, the data collected during this study was used to recommend a relationship between granular materials' small strain modulus and their resilient modulus. This relationship was given in the form of a hyperbolic model that accurately represents the strain-dependent modulus reduction of the base and subgrade materials. This model will enable field instruments that test at small strains to estimate the resilient modulus of soil layers placed during construction.

In this study, the accuracy of the stiffness estimate from portable deflectometers is investigated, based upon the example of a particular device, PRIMA 100. The Beam Verification Tester (BVT) apparatus was developed at the University of Minnesota for the Minnesota Department of Transportation to: (i) verify the performance of the PRIMA device and (ii) to check the calibration factors of the sensors of the PRIMA device. The objective of such tests is to detect the potential occurrence of deterioration of the sensor's accuracy. Associated with the BVT apparatus, an enhanced setup for the portable device is examined. The inconsistency of the traditional data interpretation method using peak values of load and displacement time histories is pointed out by comparing the stiffness estimated from the PRIMA device against the known stiffness of the beam. An alternative method using Frequency Response Functions, spectral average, Single Degree of Freedom System analog, zero frequency estimates and curve fitting is proposed to extract the static stiffness from PRIMA measurements. Test results show good agreement between estimates based on the modified analysis and true beam stiffness. Implementation of both the alternative data interpretation method and the enhanced device setup to quality assurance field measurements are proposed.

This report works to identify potential mechanisms for the occurrence of top-down cracking, as well as to investigate stress patterns and concentrations due to surface load and preexisting transverse (thermal) cracks. It attempts to provide information on surface stresses that derives from both theory and experiments. In particular, contact mechanics solutions are analyzed to gain information on loads that are subsequently used in performing numerical evaluation of surface stresses. Examples of three-dimensional computations using the finite element code ABAQUS illustrate the analysis, and comparisons of stresses for uncracked and cracked pavements are made. The report concludes that the presence of a transverse crack in the asphalt concrete (AC) layer significantly increases the vertical stresses in the base. It also has a noticeable effect on the horizontal stresses in the AC layer.