Fatigue Characteristics for Airport Pavements

Faculty Investigator(s): Samuel H. Carpenter

Overview:

The newest developments in damage assessment of hot mix asphalt (HMA) performance involve the assessment of fracture and the generation of damage in the mixtures. The previous work conducted for the FAA on this project has validated the dissipated energy approach, proposed by others, but never successfully applied. The new approach has clearly shown that the determination of damage per load cycle (Ratio of Dissipated Energy Change, RDEC) unifies the relation between load cycles to failure and the loading mode.

Thicker airport HMA pavements are said to relate best to the loading mode of constant stress conditions in the laboratory flexural fatigue test. This load mode is the basis of the fatigue relationship adapted for the current LEDFAA fatigue design approach recommended for airport HMA pavements. The current research has clearly demonstrated that an energy based approach eliminates the need to separately consider different loading modes. This approach provided the ability to predict constant stress behavior from standard constant strain flexural fatigue tests.

The RDEC approach has validated the existence of a Fatigue Endurance Limit (FEL) as a strain level below which there will be minimal or no accumulation of damage, producing a maximum thickness for the HMA. For airport pavements, which are by nature thicker to protect the subgrade from the heavy aircraft wheel loads, the typical HMA thicknesses could already be thicker than are necessary for fatigue considerations. Another validated performance characteristic is the impact of rest period between loads producing a fatigue life extension. For an airport pavement which has a specific pattern of aircraft usage, this rest period effect could be factored in to a thickness design if validated for sufficient number of mixtures.

Limited testing has been conducted on healing due to rest periods between loads, but what has been completed on this project clearly shows the potential for a fatigue life extension in excess of 10 to 40 for rest periods typical of airport conditions. More testing on different mixtures/binders is required to validate this, and develop the framework for including this in the design approach to go from lab to field.

Objectives:

This project will conduct flexural fatigue testing (four point, standard controlled strain) on a variety of different p-401 mixtures taken from field construction projects around the United States. This would be coordinated with FAA personnel to ensure a good cross section of mixtures was obtained. The standard preparation of mixtures as used in this project would be followed, and used for further validation of the preliminary design equation that has been developed. This will provide a more diverse set of mixtures, especially designed per FAA regulations to be included in the model, expanding its validity.

Additional testing would involve the healing test which applies a rest period after each load cycle. This testing produces a curve that indicates life extension for different rest periods. This can then be included as an input when designing HMA pavements when load spacing is known, or assumed. The inclusion of different mixtures/binders will establish just how variable this healing effect can be. This preliminary testing will illustrate if more testing is required because of the different impact of binders on the healing effect. It is expected that polymer binders may show more effect, but it is not known if different polymers of different degrees of polymer modification have an effect.

These findings will be integrated into the preliminary design equation.

Analysis of NAPTF Trafficking Dynamic Response Data For Pavement Deformation Behavior

Faculty Investigator(s): Dr. Erol Tutumluer

Overview

The National Airport Pavement Test Facility (NAPTF) was constructed to generate full-scale testing/trafficking data to support the investigation of the performance of airport pavements subjected to complex gear loading configurations of new generation aircraft. Two gear configurations, a six-wheel trident landing gear (B777) in one lane and a four-wheel dual-tandem landing gear (B747) in the other lane were tested simultaneously with an applied transverse wander pattern consisting of a fixed sequence of 66 vehicle passes (33 traveling East and 33 traveling West), arranged in nine equally spaced wander positions (or tracks) at intervals of 10.25 inches. Sensor installation included Multi-Depth Deflection (MDD's) and pressure cells to capture pavement responses under traffic loading. Rutting was monitored throughout the traffic test program by transverse surface profile (TSP) measurements, rolling inclinometer and straightedge rut depth measurements, and individual layer rut data collected using MDD's.

Individual pavement dynamic response data were collected due to passing of each gear for various combinations of applied load magnitudes, traffic directions, and wander positions. To minimize the interaction of gear loads at the subgrade level, the 6-wheel B777 type and the 4-whell B747 type gears moved in phase, with both gears moving left and right together rather than towards and away from each other.

Hayhoe et al. (2004) highlighted some of the complicated trends observed in the NAPTF pavement deformation behavior as follows: “The net accumulated unrecovered (permanent) deformation in the pavement structure over a complete wander cycle is shown to be a small fraction of the range of the unrecovered deformations occurring during the wander cycle over individual back and forth load applications. That is, the sum of the upward and downward unrecovered displacements almost cancel, leaving the structure in approximately the same configuration at the end of a wander cycle as at the start. The unrecovered displacements are about the same magnitude as the recovered (elastic) displacements, with the relative magnitudes depending on the transverse position of the load relative to the transverse position of the measurement… One consequence of this conclusion is that typical laboratory measurements of permanent deformation in unbound pavement materials with repeated loading may not be representative of behavior under traffic...”

For successful planning, construction, and testing of the currently studied and future NAPTF pavement test sections, the FAA’s Center of Excellence for Airport Technology (CEAT) established at the University of Illinois is in a unique position to continue to support the ongoing activities dealing with analyses of trafficking response data and property evaluations of the subgrade soils and base/subbase course materials by fully utilizing the facilities provided at the Advanced Transportation and Research Laboratory (ATREL) at the University of Illinois. Fiscal Year 2007 (FY07) research will be aimed at utilizing the NAPTF trafficking dynamic response database for a detailed analysis and better understanding of the pavement deformation behavior (both recovered and unrecovered deformations). Trends in flexible pavement layer deformation behavior will be studied for the NAPTF granular base/subbase and subgrade soil layers.

Objectives

The overall objective in this new FAA CEAT project will be to analyze the NAPTF trafficking dynamic response database, as well as the response tests conducted in association with the traffic tests, for a better understanding of the CC1 and CC3 flexible pavement test section deformation trends (both recovered and unrecovered deformations). To achieve this objective, specific goals will be to:

• (1) Investigate deformation trends with respect to the various combinations of applied
  • load magnitudes and loading sequences (application order and stress history effects);
  • trafficking speeds (load duration effects);
  • traffic directions (shear stress reversals);
  • gear spacing and gear/wheel interaction;
  • wander positions and wander sequences (order of 66 loadings);
• (2) Based on the previously proposed test procedure (Kim and Tutumluer, 2005), fully develop and validate models to evaluate and predict potential rutting in variable thickness unbound base/subbase courses due to realistic full scale aircraft gear loading.

Enhancements of ICON Software: Environmental Condition Modeling and Design Implementation

Faculty Investigator(s): David A. Lange

Overview

ICON (Illinois CONcrete) software is a finite element based code to analyze and predict slab curling. This tool developed from previous work accounts for complexities of aging concrete properties and gradient properties through the thickness of a pavement. The analysis model is capable to capture the impact of change in hygro-thermal properties, and predict impact on stress and strain field in a concrete. It is also a powerful tool to perform parametric studies to evaluate the sensitivity of stress and deformation to various material properties, environmental conditions, slab configurations, and other factors that contribute to the curling problem. While ICON is stand-alone software, enhancements including the environmental condition modeling and pavement design implementation can be added as plugins to expand the applicability of this tool.

Objectives

The proposed project for 2007 will include an additional component to initiate a partnership with Prof. Kim Kurtis and Quintin Watkins (Ph.D. student) at the Georgia Institute of Technology. Quintin is employed at Atlanta’s Hartsfield-Jackson International Airport.

We will focus on two main enhancements of ICON: environmental condition modeling and pavement design implementation. In the current ICON software, the environmental condition for the hygrothermal modeling uses discrete temperature and moisture profiles with times from the field measurement. A mathematical model converting ambient condition to temperature and moisture gradients will be a useful plug-in for broader applications. For the design implementation, we will continue the evaluation of the sensitivity of design parameters that are used in current pavement design procedure. In addition, a partnership with Prof. Kim Kurtis and Quintin Watkins (Ph.D. student) at Georgia Institute of Technology will provide a unique opportunity for additional field testing that will serve as additional validation of ICON. This additional test program creates an opportunity for a UIUC-GaTech-NAPTF collaboration that contributes additional test data and analysis which will enhance our confidence in the ICON results, and contribute knowledge to understanding how ICON can be implemented to serve FAA design methodology.

Evaluative Study of Non-FWD Non-Destructive Testing and Evaluation (NDTE) Technologies for Airport Pavement Maintenance and Acceptance Activities

Faculty Investigator(s): John S. Popovics and Imad Al-Qadi

Overview:

In-situ airport pavement system characteristics need to be quantified for quality control, to effectively assess its condition, and to monitor its performance. Among the most important characteristics to monitor are layer moduli, densities, surface roughness and profile, as well as pavement and layer thicknesses. Pavement layer thickness directly affects the structural capacity and, consequently, the service life of the pavement system. For example, any changes in the aforementioned characteristics of the pavement system would result in a reduction in its service life.

Despite recent advances in nondestructive testing and evaluation (NDTE) technologies and computation power, applied procedures for quality control/quality assurance of new airport pavements and monitoring the condition of existing airport pavements have changed little over the past three decades. For example, agencies depend heavily on the testing results of hot-mix asphalt (HMA) cores, obtained from the field, to evaluate the pavement condition. However, recent developments in NDTE methods allow the evaluation of individual pavement layers continuously and rapidly with acceptable reliability.

Although engineers usually express their interest in NDTE techniques, they have not always welcomed such methods in practice. This reluctance stems from the fact that NDTE methods have not delivered the expected or promised accuracy in some cases, especially where the methods were applied inappropriately. This experience led engineers to avoid NDTE methods, even for appropriate and beneficial applications. To increase the confidence in NDTE methods, they should be appropriately used and their application optimized. Some of the methods are likely to be better suited for network pavement condition monitoring (i.e. be sufficiently fast but perhaps less accurate) and others, more accurate and slow, better suited for project evaluation (i.e. QC/QA application, which may impact payment based on performance-based specifications). While QC/QA parameters are well known, NDTE measuring techniques that are reliable, cost effective, and require minimum training are still not well defined for specific inspection cases.

Objectives:

The ultimate objectives of this research are the following: (i) to determine the effectiveness and practicality of selected NDTE technologies for maintenance, evaluation, quality control and acceptance of airport pavements; and (ii) to recommend appropriate technologies to the FAA based on the field evaluation results. The proposed work is essentially comprised of two phases: the first phase identifies new and existing NDTE technologies for potential application, and the second phase quantitatively evaluates in the field the identified promising NDTE methods for effective airport pavement implementation. Among the technologies likely to be evaluated are ground-penetrating radar (GPR); the individual mechanical wave methods offered by the Portable Seismic Pavement Analyzer (PSPA) unit: sonic and ultrasonic body wave and surface wave measurement, surface wave analysis such as spectral analysis of surface waves (SASW), multi-channel analysis of surface waves (MASW), and the impact-echo (IE) method. The equipment for the identified methods will be made available to the project by the FAA as needed. The focus of the NDTE work is predictions of pavement layer thickness, modulus, and density. Other techniques that can be evaluated, given that are made available by FAA, are surface profile surface distress detection. Other performance characteristics will also be considered, per discussion between the research team and the FAA technical panel.

Alternative Fatigue Cracking Modes On Airfield Rigid Pavements

Faculty Investigator: Jeffery Roesler

Overview:

Airfield rigid pavement thickness design has always been based on the critical tensile bending stress at the bottom of the slab whether it was at the edge or interior location. Recent observations from full-scale rigid pavement tests at the FAA’s NAPTF (Brill et al. 2005 ; Brill 2006; Guo 2006) and Toulouse, France (Fabre et al. 2005) have shown top-down cracking can occur under certain combined loading and pavement geometry situations. Over the past year, two-dimensional finite element analyses of individual gears and the main landing gears on a limited number of aircraft have been conducted (Roesler et al. 2007). The gears have been systematically positioned on slabs (e.g., see Figure 1 for B777 main gear positions) of varying geometric properties and have produced high top of the slab tensile stresses (see Table 1). The numerical results have shown that the ratio between the top and bottom of the slab tensile stresses were significantly higher for the full gear analysis relative to the individual gear analysis (Roesler et al. 2007). Furthermore, the 2-D finite element analysis has shown consideration of the full aircraft gear is necessary if the top tensile stresses are going to be accurately predicted. These preliminary analyses have indicated that top-down cracking may occur only if the concrete strength at the top of the slab is less than the tensile strength at bottom of the slab.

Objectives:

The objective of the research project is to identify potential alternative design cases that capture the critical tensile stress for top-down cracking given no initial curling state in the concrete slab. This fiscal year research will focus on including in the analysis additional aircraft types, slab geometry and materials (L/l), and a more comprehensive 3-D finite element analysis of the critical loadings with NIKE3D. Full-scale rigid pavement distress locations from the NAPTF and Toulouse, France will be used to verify the numerical modeling results due to the observation of top-down fatigue cracks from these field experiments.

Development of New Methodologies For Mechanist Design of Asphalt Overlays

Faculty Investigator(s): William G. Buttlar

Overview:

Very often, asphalt overlays are used to restore smoothness, structure, and waterproofing benefits to existing airfield pavements. However, the controlling factor for overlay lifespan is often reflective cracking in the new overlay caused by stress concentrations in the vicinity of joints and cracks in the underlying pavement. Reflective cracking can lead to roughness, spalling, and moisture infiltration at joints and cracks, which can greatly reduce the life expectancy of the overlay. Advanced spalling can significantly increase FOD potential.

Objectives:

While significant progress has recently been made in the development of mechanistic analysis tools for pavements with asphalt overlays, there is still work to be done to simplify the analysis tools and testing requirements to make them usable by practitioners. Such models would combine the relative strengths of mechanics-based models, i.e., physical correctness, and those of empirical approaches, i.e., the efficiency of the solution. This project includes tasks which will extract the most useful elements from current mechanistic and empirical models and combine them into a new mechanistic-empirical analysis and design tool for asphalt overlays. While the focus will be on asphalt overlays placed on PCC pavements, it is envisioned that the tool could be readily adapted to asphalt overlays placed on existing flexible pavements.