Youngguk Seo

Dynamic Modulus Testing of Asphalt Concrete in Indirect Tension Mode

This study presents the results from an analytical and experimental study on the dynamic modulus testing of hot-mix asphalt using the indirect tension (IDT) mode. An analytical solution for the dynamic modulus in the IDT mode was developed with the use of linear viscoelasticity. To verify the analytical solution, temperature and frequency sweep tests were conducted on 12 asphalt mixtures commonly used in North Carolina; both axial compression and IDT test methods were used. A modified dynamic modulus test protocol is introduced to reduce the required testing time by using more frequencies and fewer temperatures on the basis of the time-temperature superposition principle. A comparison of results from the axial compression and IDT test methods shows that the dynamic modulus master curves and shift factors derived from the two methods are in good agreement. It also was found that Poissons ratio was a weak function of the loading frequency; its effect on the phase angle master curve is discussed.

Application of digital image correlation method to mechanical testing of asphalt-aggregate mixtures

The digital image correlation (DIC) method, a noncontact, full-field displacement measurement technique, has been applied to mechanical testing of asphalt concrete. A single couple charged device camera acquires images of an area of interest from a specimen in the undeformed and deformed states. These images are correlated to determine deformations, and advanced mathematical procedures are applied to these deformations to calculate strains. To verify the DIC measurements, vertical displacements for the middle and bottom sections of a specimen subjected to monotonic tension are compared with conventional linear variable differential transformer measurements. A series of DIC images captured during the monotonic and cyclic tests visualizes the evolution of the failure zone (i.e., the fracture process zone) at the crack tip. Also, it is demonstrated that the full-field measurement and post-processing nature of DIC allows a more accurate determination of the stress-strain behavior of the fracture process zone. The applicability of this method to a cylindrical specimen with a curved surface is also investigated by testing a 75-mm-diameter cylindrical specimen. Finally, the DIC method is extended to cyclic testing of asphalt mixtures with the aid of a synchronized image acquisition technique.

Development of a Geothermal Snow Melting System for Highway Overlays and Its Performance Validations

Geothermal energy has been applied to a snow melting system for highway overlays. This paper described practical steps for the design, construction, and validation of the system. These steps were carried out both in a laboratory and in the field. In the laboratory, a series of heat conductivity tests were carried out to understand the heat transfer characteristics in a pavement layer and to determine an optimum pipe pitch at 50 mm depth. Steel fiber reinforced concrete (SFRC) was proposed as the pavement material to make the system durable in overlays subjected to heavy traffic, and its resistance to construction cracks was tested in the field. In 2009, the first trial version of the geothermal snow melting system was built into a 50 mm thick SFRC overlay. Since then, its snow melting performance has been monitored and evaluated under different snowfall accumulations and intensities. Given the design variables, it took about 30 to 60 minutes to remove snow and ice even under heavy snowfalls. Finally, the construction and operation costs of the developed system were calculated and compared with those of other snow melting methods. It is expected that the developed system will be installed on tunnel entrances and bridge decks on highways once its durability is verified under heavy axle loads.

A Full Scale In Situ Evaluation of Strain Characteristics at Highway Flexible Pavement Sections

The objective of this study was to evaluate the strain characteristics in asphalt pavements with respect to five in situ factors: Layer thickness, vehicle speed, axle weight, pavement temperature, and lateral distance between loading tire and sensor (i.e., loading offset). Two sets of moving load tests have been performed at three asphalt sections (A14, A5, and A8) selected from the Korea Highway Corporation Test Road. Design variables for those sections included base thickness (80, 180, and 280 mm) and base type (BB1, BB3, and aggregate). A three-axle (single-tandem) dump truck was adopted as a loading source, and steel plates were added to simulate heavy axle loads. Each pavement section was loaded at different speeds ranging from 20 to 80 km/h and was tested in different seasons from 2003 to 2004. In addition, the effect of loading offset was investigated with seven loading courses in one of the sections in 2006. This study first demonstrated that the rank between longitudinal and transverse strains can be changed as pavement ages under different temperatures. With multiple regression analysis on 123 data collected, a relation between field testing variables and the maximum strains in both directions was created to further understand statistical contributions of individual variables to stain anisotropy characteristics. As a result, both pavement temperature and layer thickness were found to be the most significant field variables.

Validation of Material-Level Performance Models: Using the Third-Scale Model Mobile Loading Simulator

A mechanistic pavement analysis with laboratory fatigue cracking and rutting models was validated with the response and performance measured from asphalt pavements. Asphalt pavements with different air void contents were tested using the third-scale Model Mobile Loading Simulator (MMLS3). The fatigue life prediction algorithm adopts a cumulative damage analysis; the permanent deformation prediction algorithm uses a sublayering method. These algorithms, which are similar to the ones adopted in the NCHRP 1–37A Mechanistic–Empirical Pavement Design Guide (MEPDG), account for the loading rate and temperature variation along the depth of the pavements. The major difference between the algorithms used in this study and the ones in the MEPDG is that the difference in loading frequencies between the laboratory test method and the MMLS3 test was accounted for in this study using the time–temperature superposition principle with growing damage. The predictions of fatigue life and permanent deformation growth in the MMLS3 tests revealed that the proposed algorithms do a reasonable job in predicting these parameters, although improved predictions may be achieved by adopting more fundamental models. It is expected that the resulting alliance between the accelerated pavement test, laboratory material level test, and performance models can serve as a cornerstone for the successful estimation of the service life of in situ pavements.

A Mechanistic Approach to Determine Price Reduction Factors for Density-Deficient Asphalt Pavements

This paper presents the research undertaken for the development of price reduction factors for density-deficient asphalt pavements. Performance characteristics included in this study are fatigue cracking and rutting. Air void models for the dynamic modulus, fatigue cracking, and rutting were developed using laboratory test data. The results from the material level performance tests and the third-scale Model Mobile Loading Simulator (MMLS3) tests allowed the calculation of the PRF values. It was found that the PRF values are not sensitive to the testing methodology used; rather, they are significantly different depending upon which performance characteristic is used (i.e., fatigue cracking vs. rutting). Pavement performance prediction methodologies based on the cumulative damage analysis were developed that predict the fatigue life and permanent deformation growth of the asphalt pavement under the MMLS3 loading. These methodologies are based on material level performance models, multilayered elastic analysis, and the time-temperature superposition principle to account for the differences between the material level testing conditions and the MMLS3 testing conditions. It was found that the prediction methodologies yield reasonable predictions of fatigue life and permanent deformation growth of asphalt slabs under the MMLS3 loading. These pavement performance prediction methodologies were implemented into the computer program called AP 4 (Asphalt Pavement Performance Prediction Program). This program allows the determination of the service life for fatigue cracking and rutting based on the inputs of air void contents in all the HMA layers. Case studies of five density-deficient pavements were conducted, which resulted in reasonable price reductions.

Distress Evolution in Highway Flexible Pavements: A 5-Year Study at the Korea Highway Corporation Test Road

In Korea, a 7.7 km-long test road was constructed with an aim to better understand the behavior of pavements and to develop a mechanistic-empirical pavement design guide (MEPDG). This two lane highway consisted of asphalt and concrete pavements that were further divided into diverse sections: 33 asphalt sections and 25 concrete sections. Construction of the test road began in 1997 and ended in 2002. It opened to traffic in March 2004. So far, many field tests have been carried out to evaluate the response and performance of pavements. At asphalt sections, the first attempt was made to correlate design parameters with the performance of pavements, and this is the focus of this paper. Performance was characterized by the evolution of failures, such as rutting, cracking, and longitudinal road profile. Design parameters included surface type, base type, base thickness, and anti-frost layer. Field surveys were made with an automatic road analyzer for the 2002–2007 period, and trends in pavement failures were analyzed with respect to individual design parameters. Also, a highway present condition index (HPCI) was adopted to assess the overall performance of individual sections. Findings of this study are being used to validate pavement performance models incorporated into the MEPDG that is scheduled to be released in 2010.

Determination of Price Reduction Factors for Density-Deficient Asphalt Pavements

This paper presents the research undertaken for the development of price reduction factors for density-deficient asphalt pavements. Performance characteristics included in this study are fatigue cracking and rutting. The following laboratory tests were performed on two North Carolina Superpave mixtures with varying air void contents: (1) axial compression dynamic modulus tests for modulus determination; (2) indirect tension tests for fatigue performance evaluation; (3) triaxial repeated load permanent deformation tests for rutting evaluation; and (4) accelerated pavement tests on laboratory pavement slabs for fatigue and rutting evaluation using the third-scale Model Mobile Loading Simulator (MMLS3). Air void models for the dynamic modulus, fatigue cracking, and rutting were developed using the laboratory test data. These models and the results from the MMLS3 testing were used to develop the price reduction factors for density-deficient asphalt mixtures. In order to determine the effect of deficient density of hot-mix asphalt (HMA) on the performance of asphalt pavement as a system, a computer program called AP4 (Asphalt Pavement Performance Prediction Program) was developed. The algorithm adopted in AP4 for the damage calculation is based on the incremental damage concept and is very similar to that used in the National Cooperative Highway Research Program (NCHRP) 1-37A Mechanistic-Empirical Pavement Design Guide. This program allows the determination of the service life for fatigue cracking and rutting based on the inputs of air void contents in all the HMA layers. Case studies of five density-deficient pavements were conducted, and the price reduction factors were determined.

Fatigue Crack Assessment of Asphalt Concrete Pavement on a Single Span Highway Bridge Subjected to a Moving Truck

Characterization of fatigue crack in asphalt concrete (AC) pavement placed on the bridge is quite complex because it involves not only viscoelastic behavior of AC mixture but also dynamic interaction between bridge and traffic. In this study, a new approach that accounts for these material and structural aspects of the problem has been proposed to evaluate the fatigue crack potential of AC pavement on the bridge. A highway bridge surfaced with the 13-mm stone mastic asphalt (SMA) pavement that was subjected to a moving two-axle dump truck was selected as a simulation target and its equation of motion was derived using the Lagrange’s formulation. Given the range of testing variables, the maximum center deflections of the bridge were calculated and converted into the peak strains at the bottom of the SMA pavement. A simplified linear viscoelastic continuum damage model (S-VECD) was then implemented for the strains ranging from the largest peak strain to the specified allowable strain to predict the fatigue life of the SMA. The outputs showed that the fatigue life of the SMA got shorter as strain increases, and that hard SMA failed earlier than soft SMA. It was also found that SMA’s temperature was more influential than vehicle’s speed in causing fatigue cracks.

Laboratory Evaluation of CCM Parameters for Mode-I Cracks in HMAs

A series of experimental testing, data acquisition and analysis procedure to determine substantial fracture parameters for openning mode (mode-I) cracks in hot mix asphalt (HMA) has been developed. The cohesive crack model (CCM) has been adopted for the identification and evaluation of fracture parameters: Tensile strength; Fracture Energy; Critical Crack Opening Displacement; and Characteristics Crack Opening Displacement. The concept of pseudo fracture energy has been introduced to better account for nonlinear and time dependent fracture in HMAs. Behavioral trend between pseudo fracture energy and reduced strain rate agrees with that of fracture energy determined from LVDT measurements. At low temperatures, the fracture parameters remain constant irrespective of reduced strain rates. This implies that the Linear Elastic Fracture Mechanics (LEFM) could be applied for low temperature cracking in asphalt concrete. Finally, a better understanding of cracking mechanism relating to strain localization in a fracture process zone (FPZ) was achieved. Further study on characteristics of FPZ due to other cracking modes is necessary to bnefit the pavement performance model.