Research on Roadway Performance and Distress at Low Temperature

This research project investigated the performance and damage characteristics of Nebraska roadways at low-temperature conditions. To meet the research objective, laboratory tests were incorporated with mechanistic numerical modeling. The three most common pavement structures in Nebraska were selected and modeled considering local environmental conditions and pavement materials with and without truck loading. Cracking of asphalt overlay was predicted and analyzed by conducting finite element simulations incorporated with cohesive zone fracture. Parametric analyses were also conducted by varying pavement geometries and material properties, which could lead to helping pavement designers understand the mechanical sensitivity of design variables on the overall responses and performance characteristics of pavement structures. This better understanding is expected to provide NDOR engineers with more scientific insights into how to select paving materials in a more appropriate way and to advance the current structural pavement design practices

Rate- and Temperature-Dependent Fracture Characteristics of Asphaltic Paving Mixtures

Cracking in asphaltic pavement layers causes primary failure of the roadway structure, and the fracture resistance and characteristics of asphalt mixtures significantly influence the service life of asphaltic roadways. A better understanding of the fracture process is considered a necessary step to the proper development of design-analysis procedures for asphaltic mixtures and pavement structures. However, such effort involves many challenges because of the complex nature of asphaltic materials. In this study, experiments were conducted using uniaxial compressive specimens to characterize the linear viscoelastic properties and semi-circular bending (SCB) specimens to characterize fracture behavior of a typical dense-graded asphalt paving mixture subjected to various loading rates and at different temperatures. The SCB fracture test was also incorporated with a digital image correlation (DIC) system and finite-element model simulations including material viscoelasticity and cohesive-zone fracture to effectively capture local fracture processes and resulting fracture properties. The test results and model simulations clearly demonstrate that: (1) the rate- and temperature-dependent fracture characteristics need to be identified at the local fracture process zone, and (2) the rate- and temperature-dependent fracture properties are necessary in the structural design of asphaltic pavements with which a wide range of strain rates and service temperatures is usually associated

Impact of Truck Loading on Design and Analysis of Asphaltic Pavement Structures- Phase III

This study investigated the impact of the realistic constitutive material behavior of asphalt layer (both nonlinear inelastic and fracture) for the prediction of pavement performance. To this end, this study utilized a cohesive zone model to consider the fracture behavior of asphalt mixtures at an intermediate temperature condition. The semi-circular bend (SCB) fracture test was conducted to characterize the fracture properties of asphalt mixtures. Fracture properties were then used to simulate mechanical responses of pavement structures. In addition, Schapery’s nonlinear viscoelastic constitutive model was implemented into the commercial finite element software ABAQUS via a user defined subroutine (user material, or UMAT) to analyze asphalt pavement subjected to heavy truck loads. Extensive creep-recovery tests were conducted at various stress levels and multiple service temperatures to obtain the stress- and temperaturedependent viscoelastic material properties of asphalt mixtures. Utilizing the derived viscoelastic and fracture properties and the UMAT code, a typical pavement structure was modeled that simulated the effect of material nonlinearity and damage due to repeated heavy truck loads. Twodimensional finite element simulations of the pavement structure demonstrated significant differences between the cases: linear viscoelastic and nonlinear viscoelastic modeling with and without fracture in the prediction of pavement performance. The differences between the cases were considered significant, and should be addressed during the process of performance-based pavement design. This research demonstrates the importance of accurate and more realistic characterizations of pavement materials

Moisture Damage Characterization of Warm-Mix Asphalt Mixtures Based on Laboratory-Field Evaluation

This study presents laboratory evaluation integrated with field performance to examine two widely used warm-mix asphalt (WMA) approaches—foaming and emulsion technology. For a more realistic evaluation of the WMA approaches, trial pavement sections of the WMA mixtures and their counterpart hot-mix asphalt (HMA) mixtures were implemented in Antelope County, Nebraska. Field-mixed loose mixtures collected at the time of paving were transported to the laboratories to conduct various experimental evaluations of the individual mixtures. Among the laboratory tests, three (two conventional and one newly attempted) were performed to characterize moisture damage potential which is the primary focus of this study. From the laboratory test results, WMA mixtures showed greater susceptibility to moisture conditioning than the HMA mixtures, and this trend was identical from multiple moisture damage parameters including the strength ratio and the critical fracture energy ratio. Early-stage field performance data collected for three years after placement presented satisfactory rutting-cracking performance from both the WMA and HMA sections, which generally agrees with laboratory evaluations. Although the field performance data indicated that both the WMA and HMA show similar good performance, careful observation of field performance over a period of years is necessary since moisture damage can be accelerated after rutting or cracking as a later-stage pavement distress

Moisture Sensitivity of Hot Mix Asphalt (HMA) Mixtures in Nebraska – Phase II

As a consequential effort to the previous NDOR research project (P564) on moisture damage, this report presents outcomes from this project incorporated with the previous project. Performance changes and fundamental material characteristics associated with moisture damage due to various anti-stripping additives in asphalt mixtures are studied through various experimental approaches and a numerical simulation. Three additives (i.e., one reference additive, hydrated lime, and two alternative additives: fly ash and cement) are investigated by adding them into two types of mixes (SP2 for low-traffic-volume roadways and SP5 for high-traffic-volume roadways) where two different asphalt binders (PG 64-22 for the SP2 mix and PG 70-28 for the SP5) are used. Two asphalt concrete mixture scale performance tests, the AASHTO T-283 and the APA under water, and two local-scale mixture constituent tests, the boiling water test (ASTM D 3625) and the pull-off test, are conducted to characterize the effects of binderspecific anti-stripping additives on the binder-aggregate bonding potential in mixtures. The pull-off tensile strength tests are then numerically modeled through the finite element technique incorporated with the cohesive zone modeling approach to seek more fundamental scientific insights into the effect of each anti-stripping additive on the overall moisture damage resistance. Results from laboratory tests and numerical simulations indicate that the SP5 mixtures, where high-quality aggregates and polymermodified binder are used, are fairly self-resistant to moisture damage without treating any anti-stripping additive and do not show any visible sensitivity among additives, whereas the effects of additives and their sensitivity are significant in the SP2 mixes that use the unmodified binder PG 64-22 and low-quality aggregates. With the limited amount of test data, hydrated lime seems to perform slightly better than other additives, particularly with longer moisture-conditioning time. Fly ash contributes to reducing moisture damage by improving binder-aggregate interfacial properties, which are validated from the integrated experimental-computational evaluation

Layer Moduli of Nebraska Pavements for the New Mechanistic-Empirical Pavement Design Guide (MEPDG)

As a step-wise implementation effort of the Mechanistic-Empirical Pavement Design Guide (MEPDG) for the design and analysis of Nebraska flexible pavement systems, this research developed a database of layer moduli — dynamic modulus, creep compliance, and resilient modulus — of various pavement materials used in Nebraska. The database includes all three design input levels. Direct laboratory tests of the representative Nebraska pavement materials were conducted for Level 1 design inputs, and surrogate methods, such as the use of Witczak’s predictive equations and the use of default resilient moduli based on soil classification data, were evaluated to include Level 2 and/or Level 3 design inputs. Test results and layer modulus values are summarized in the appendices. Modulus values characterized for each design level were then put into the MEPDG software to investigate level-dependent performance sensitivity of typical asphalt pavements. The MEPDG performance simulation results then revealed any insights into the applicability of different modulus input levels for the design of typical Nebraska pavements. Significant results and findings are presented in this report

Impact of Truck Loading on Design and Analysis of Asphaltic Pavement Structures- Phase II

In this study, Schapery’s nonlinear viscoelastic constitutive model is implemented into the commercial finite element (FE) software ABAQUS via user defined subroutine (user material, or UMAT) to analyze asphalt pavement subjected to heavy truck loads. Then, extensive creep-recovery tests are conducted at various stress levels and at two temperatures (30oC and 40oC) to obtain the stress- and temperature-dependent viscoelastic material properties of hot mix asphalt (HMA) mixtures. With the viscoelastic material properties characterized and the UMAT code, a typical pavement structure subjected to repeated heavy truck loads is modeled with the consideration of the effect of material nonlinearity with a realistic tire loading configuration. Three-dimensional finite element simulations of the pavement structure present significant differences between the linear viscoelastic approach and the nonlinear viscoelastic modeling in the prediction of pavement performance with respect to rutting and fatigue cracking. The differences between the two approaches are considered significant and should be addressed in the process of performance-based pavement design. This also implies the importance of proper and more realistic characterization of pavement materials

Nanoindentation Test Integrated with Numerical Simulation to Characterize Mechanical Properties of Rock Materials

It is important to determine the mechanical properties of rock materials accurately from the viewpoint of the design, analysis, and modeling of various transportation infrastructure systems. Conventional methods have some drawbacks, including relatively inaccurate measurements, cumbersome testing-analysis processes, and high variability in measurements. A nanoindentation test integrated with a numerical modeling technique has been validated in other fields as an efficient and accurate tool for the characterization of the key mechanical properties of various irregularly shaped materials, such as the rock materials in this study. This paper presents an integrated experimental-numerical effort based on the nanoindentation measurement and finite-element modeling of a representative rock material, limestone. The experimental efforts, including specimen fabrication and laboratory tests, are presented, and the corresponding analyses of test results combined with the finite-element technique and linear interpolation to evaluate the property measurements are discussed. The elastic properties estimated from the nanoindentation test are similar to the simulation results, demonstrating the validity of the test method and modeling approach. The success of the proposed approach should facilitate the better design of mixtures and structures based on the more accurate characterization of the core material properties

Implementation of Warm-Mix Asphalt Mixtures in Nebraska Pavements

The primary objective of this research is to evaluate the feasibility of several WMA mixtures as potential asphalt paving mixtures for Nebraska pavements. To that end, three well-known WMA additives (i.e., Sasobit, Evotherm, and Advera synthetic zeolite) were evaluated. For a more realistic evaluation of the WMA approaches, trial pavement sections of the WMA mixtures and their HMA counterparts were implemented in Antelope County, Nebraska. More than one ton of field-mixed loose mixtures was collected at the time of paving and was transported to the NDOR and UNL laboratories to conduct comprehensive laboratory evaluations and pavement performance predictions of the individual mixtures involved. Various key laboratory tests were conducted to identify mixture properties and performance characteristics. These laboratory test results were then incorporated into other available data and the MEPDG software to predict the long-term field performance of the WMA and HMA trial sections. Pavement performance predictions from the MEPDG were also compared to two-year actual field performance data that have annually been monitored by the NDOR pavement management team. The WMA additives evaluated in this study did not significantly affect the viscoelastic stiffness characteristics of the asphalt mixtures. WMA mixtures generally presented better rut resistance than their HMA counterparts, and the WMA with Sasobit increased the rut resistance significantly, which agrees with other similar studies. However, two laboratory tests—the AASHTO T283 test and semi-circular bend fracture test with moisture conditioning—to assess moisture damage susceptibility demonstrated identical results indicating greater moisture damage potential of WMA mixtures. MEPDG results simulating 20-year field performance presented insignificant pavement distresses with no major performance difference between WMA and HMA, and this has been confirmed by actual field performance data. Although only two-year field performance is available to date, both the WMA and HMA have performed well. No cracking or other failure modes have been observed in the trial sections. The rut depth and the roughness of WMA and HMA sections were similar