Shongtao Dai
Minnesota Department of Transportation
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Featured researches published by Shongtao Dai.
Transportation Research Record | 2002
Mihai O. Marasteanu; Shongtao Dai; Joseph F. Labuz; Xue Li
There has been a sustained effort in applying fracture mechanics concepts to crack formation and propagation in bituminous pavement materials. Adequate fracture resistance is an essential requirement for asphalt pavements built in the northern part of the United States and Canada, for which the prevailing failure mode is cracking due to low-temperature shrinkage stresses. The current Superpave® specifications address this issue mainly through the use of strength tests on unnotched (smooth boundary) specimens. However, recent studies have shown the limitations of this approach and have suggested that fracture mechanics concepts, based on tests performed on notched samples, should be used instead. Research in progress at the University of Minnesota investigates the use of fracture mechanics principles to determine the low-temperature fracture properties of asphalt mixtures. A testing protocol is presented that makes it possible to obtain multiple measurements of fracture toughness as a function of crack propagation based on the compliance method to measure crack length. An increase in fracture toughness with crack length is observed, which is consistent with the behavior displayed by other brittle materials. The plateau of the curves may be representative of the asphalt concrete resistance to fracture, because the initial values can be significantly influenced by the presence of the inelastic zone at the crack tip.
Transportation Research Record | 2007
Woosung Kim; Joseph F. Labuz; Shongtao Dai
Twenty resilient modulus (MR) tests were conducted for specimens with different ratios of recycled asphalt pavement (RAP) and aggregate to investigate the effect on material stiffness. Specimens were prepared by a gyratory compactor instead of a vibratory hammer, because the density of a gyratory-compacted specimen was closer to the field density. Moisture content and density were estimated before and during the tests according to the NCHRP 1–28A protocol requirements. MR data were evaluated by quality control–quality assurance (QC/QA) criteria such as the angle of rotation, signal-to-noise ratio, and coefficient of variance, and about 95% of the sequences passed the QC/QA criteria. Specimens with 65% optimum moisture content were stiffer than the specimens with 100% optimum moisture content at all confining pressures. The 50% aggregate–50% RAP specimens developed stiffness equivalent to 100% aggregate specimens at lower confining pressures; at higher confinement, the RAP specimens were stiffer. However, from the tracking of axial displacement during the conditioning sequence, it appeared that the specimens with RAP exhibited greater permanent deformation than the 100% aggregate material.
Transportation Research Record | 2002
Shongtao Dai; John Zollars
Laboratory remolded subgrade soil samples have been widely used to study subgrade resilient modulus (M r ). But physical conditions, such as moisture content and density, of such specimens may not represent in situ conditions very well. Therefore, AASHTO and the Long-Term Pavement Performance program have recommended that undisturbed, thin-walled tube samples be used to study subgrade resilient behavior. The Minnesota Department of Transportation (MnDOT) is developing mechanistic-empirical pavement design approaches through the Minnesota Road Research project and has realized the importance of M r in the design approaches. Currently, MnDOT is making an effort to study the M r of unbound pavement materials through laboratory experiments. Under a research project at MnDOT, several thin-walled tube samples of sub-grade soil were obtained from six different pavement sections at the Minnesota Road Research project. Repeated loading triaxial tests were conducted on the soil specimens to determine the M r at the MnDOT laboratory. Also, some soil properties, such as resistance, R-value, and plasticity index, were obtained. R-value is an indicative value of performance when soil is placed in the subgrade of a road subjected to traffic. Two constitutive models (the Uzan-Witczak universal model and the devia-tor stress model) were applied to describe the M r . The objectives of the research were to compare these two well-known constitutive models in describing subgrade soil resilient behavior and to study the effects of material properties on the M r . From the specimens tested, the experimental results showed that the universal model described the subgrade M r slightly better than the deviator stress model, and the coefficients in these two constitutive models were found to have correlation to material properties. Also, no well-defined relationships between the R-value and the coefficients in the constitutive models were observed from the results of the tested specimens.
Transportation Research Record | 2006
Lev Khazanovich; Clara Celauro; Bruce Chadbourn; John Zollars; Shongtao Dai
The characterization of unbound materials in the mechanistic-empirical pavement design guide (MEPDG), also known as the 2002 design guide, is reviewed, and this characterization is applied to Minnesota subgrades. The main emphasis is on the collection of k1, k2-, and k3-parameters for Minnesota fine-grained soils and the procedure for the interpretation of the resilient modulus test to provide an input to the multilayer elastic theory (MLET) analysis (Level 2 input). This is an important aspect of adaptation of the MEPDG, because the guide recommends measurement of resilient moduli from laboratory testing, but the procedure does not specify how to interpret the test data to obtain an input for an MLET analysis. The resilient modulus test results from 23 samples collected from several Minnesota locations were used to provide information for the nonlinear finite element program and multilayer elastic theory program. The obtained elastic moduli were compared with the MEPDG recommended ranges for subgrade mod...
Transportation Research Record | 1996
Joseph F. Labuz; Shongtao Dai; K. R. Shah
Failure of such materials as dense or overconsolidated soil, rock, and concrete is associated with a zone of localized deformation, where the failure plane develops by a coalescence of damage in the form of pores and microcracks. As damage initiates and propagates, transient elastic waves are emitted from the sudden release of energy. These microseismic events are called acoustic emission (AE). The acoustic sources can be located through the first arrival of the waveform, the P-wave component, recorded by an appropriate data acquisition system. The source location can help identify the eventual failure plane well before it is visible. In this study, an acoustic emission system with eight channels of recording was used to monitor the failure process in rock specimens under uniaxial compression and four-point bend loading. Failure was controlled by conducting the experiments in a closed-loop, servo-hydraulic load frame. The sources were located using a nonlinear regression technique. From calibration studie...
Transportation Research Record | 2001
Brent Theroux; Joseph F. Labuz; Shongtao Dai
An earth pressure cell (EPC) is a device designed to provide an estimate of normal stress in soil. The practice of designing and manufacturing stress-measurement devices revolves around the study of the interaction between the measuring device and the host material. However, distribution of normal stress is not necessarily uniform across a given surface. Consequently, output from an EPC may be different under soil-loading conditions than under fluid pressure. In addition, depending on the design, as the cell deflects, an arching-type phenomenon may develop. A study was conducted to devise a scheme for calibration of EPCs and to recommend a procedure for field installation. A new testing device was designed to permit the application of uniaxial soil pressure to the EPC by using various types of soil and load configurations. Sensitivities computed from soil calibrations varied from those determined from fluid calibrations by as much as 30 percent. A field installation procedure was developed from model tests. In the laboratory, a thin-walled steel cylinder with a geotextile bottom was filled with uniform silica sand in a medium-dense state, and the EPC was placed within the sand. The entire apparatus (EPC, cylinder, and sand) was carried into the field and installed in the desired locations. The steel cylinder was then removed, leaving the cell, sand, and geotextile behind. Preliminary field data indicate that the soil calibration and placement procedure provide reasonably accurate measurements of the change in vertical stress.
Transportation Research Record | 2016
Eshan V. Dave; Chelsea Hoplin; Benjamin Helmer; Jay Dailey; David Van Deusen; Jerry Geib; Shongtao Dai; Luke Johanneck
Asphalt pavements in colder climates encounter significantly shortened service lives because of excessive transverse cracking. This paper presents the results for 26 pavement sections in Minnesota that were studied to evaluate the effects of asphalt mix designs on pavement cracking performance. The field performance is presented with various cracking measures and compared with mix design aspects such as amount of asphalt binder, binder grade, and amount of recycling. The disk-shaped compact tension (DCT) fracture energies measured on the field cored samples are also compared with cracking performance. In this study, asphalt pavement sections from several locations were evaluated to encompass various types of asphalt mixtures and asphalt construction types that were commonly used in Minnesota. The amount of transverse cracking for each section was converted into a newly proposed cracking performance measure that accounted for the amount, rate, and timing of cracking. The comparisons between asphalt mixture attributes and cracking performance measures showed that the amounts of total asphalt binder and recycled asphalt binder may not be sufficient. Performance testing, in addition to currently used controls (mix volumetrics and constituent properties), is recommended to ensure good cracking performance. The DCT fracture energy results for companion sections show that mixtures with higher fracture energies exhibit lower amounts of transverse cracking.
Transportation Research Record | 2013
Luke Johanneck; Shongtao Dai
This paper details the construction and analysis of three stabilized full-depth reclamation (SFDR) sections (Cells 2, 3, and 4) constructed at the Minnesota Road Research Facility on I-94 in 2008. Three test sections with different ratios of pulverized asphalt concrete to granular base were constructed, and the performance of full-depth reclaimed pavements stabilized with engineered emulsion was studied. Emulsion content and base structure varied between test sections. Each test section was designed for 3.5 million equivalent single-axle loads for a period of 5 years. As of June 30, 2012, the sections had been subjected to approximately 2.2 million such loads. Responses were measured with strain gauges embedded at the bottom of the hot-mix asphalt and SFDR layers in each test section. The strain gauges indicated that the bottom of both the hot-mix asphalt and the SFDR layers was subject to horizontal tensile strain from falling weight deflectometer testing and heavy vehicle loading. Pavement performance for rutting, cracking, and international roughness index was measured periodically. The results indicated that all three cells were performing well. The only crack in the three cells was in Cell 3; the roughness index values were well within the acceptable range and rutting, although progressing, was still acceptable. The paper concludes with modeled responses and performance predictions from DARWinME and BISAR. Model predictions indicate that an SFDR layer will provide greater structural benefits and increased performance than will similar structures with unstabilized full-depth reclaimed or granular base layers.
Transportation Research Record | 2018
Mirkat Oshone; Debaroti Ghosh; Eshan V. Dave; Jo Sias Daniel; Joseph M. Voels; Shongtao Dai
To address asphalt pavement thermal cracking, researchers have developed performance-based evaluation tools for asphalt mixtures. A minimum fracture energy obtained from a disc-shaped compact tension test and Black space parameters determined by the stiffness and relaxation properties of asphalt mixtures are two such methods to ensure good thermal cracking resistance. Mix specifiers and producers strive to meet the requirements set by these performance-based criteria by adjusting their mix designs. However, there is a lack of information and consensus on the effect of mix design variables (such as binder grade and mix volumetrics) on thermal cracking performance of mixtures as it relates to fracture energy and Black space location. This study strives to fill this gap by quantifying the effect of: (1) recycled asphalt content, (2) effective binder content, (3) air voids, (4) asphalt film thickness, (5) voids in mineral aggregates, and (6) PG low and high temperature grades on thermal cracking resistance. A large dataset, 90 mixtures from the Minnesota Department of Transportation and 81 mixtures from University of New Hampshire database, was used for the study. The results indicate a strong correlation between binder related properties (binder content, asphalt film thickness, PG spread) and fracture energy. The correlation coefficients obtained from this study for PG spread, effective binder content, and air void can be confidently employed to achieve targeted fracture energy thresholds. The same can be achieved for the Glower-Rowe parameter at 15ºC by employing the correlation coefficients obtained for PG low temperature, virgin asphalt content, and voids in the mineral aggregate.
Transportation Research Record | 2018
Kyle Hoegh; Shongtao Dai; Trevor Steiner; Lev Khazanovich
The compaction of asphalt concrete significantly affects long-term pavement performance. Although coring provides a relatively accurate way of assessing in-place density at specific locations, the coverage of the assessment is limited, especially at longitudinal joint locations. This can be particularly problematic because it is difficult to identify problematic locations that are likely to fail prematurely using current compaction assessment methods. Ground penetrating radar (GPR) provides an attractive nondestructive testing alternative for evaluation of compaction quality, especially with recent significant improvements in the GPR technology for this specific application. However, assessment of the air void content of the asphalt mix from the GPR-measured dielectric constant of the surface requires conversion of dielectric variation to air void content variation, which is the subject of this paper. An alternative to the commonly used model is proposed, leading to more justifiable predictions for low values of dielectric constants. The proposed model was used to interpret data from a 7-mi long asphalt overlay construction project. The results of the interpretation as compared with the results obtained with the conventional model show an improvement on the stability of the prediction at low air void contents, especially when core calibration data are limited and uncertainty is considered. These results are promising in the direction of reducing field cores necessary to have a stable model providing continuous compaction assessment of new asphalt pavement construction.