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MODIFIED MECHANISTIC-EMPIRICAL AIRFIELD UNBONDED CONCRETE OVERLAY TRAFFIC PREDICTION MODEL
ABSTRACT
Unbonded concrete overlays, one rehabilitation technique applied to rigid pavements, offers benefits by taking advantage of the existing rigid pavement structure. By leaving the original rigid pavement in place, the underlying structure is undisturbed; the existing rigid pavement structure can thus be considered in overlay design, typically resulting in a thinner overlay. In addition, the application of an interlayer between the overlay and the original rigid pavement may reduce pre-overlay repairs. Current procedures for airfield unbonded concrete overlay design are documented in advisory circular AC 150/5320-6E, published in 2009, accompanied by the design software “FAARFIELD.” Due to the lack of comprehensive unbonded concrete overlay experimental data, unbonded concrete overlay design has been considered in the same way as new rigid pavement design, despite the differences in conditions.
This thesis utilizes full-scale accelerated testing data from the National Airport Pavement
Test Facility to verify both the mechanistic model (stress computation) and empirical model (fatigue relationship) used in the current unbonded overlay design procedure. It is proposed that the mechanistic model be refined by including crack density, in addition to the current Structural
Condition Index (SCI). Backcalculation theory is found to affect E-Ratio determination; the ERatio from one theory should not directly be used in a model based on another theory. In addition, E-Ratio predicted from a model developed using data for unbonded concrete overlays is larger than E-Ratio predicted from a model developed based on slab test data, especially at low values of SCI. It is suggested that the cracked slab model, developed on the basis of WES data (slab data), should not be used for unbonded concrete overlay E-Ratio prediction in FAARFIELD.
For the empirical model, it is concluded that the design factor of an unbonded concrete overlay should consider the interactions between design factors of the overlay and underlying slabs, along with the conditions of the underlying slabs. The method of determination of SCI for a test item has significant effect on prediction of allowable coverages. The difference between predicted and observed coverages for the same SCI is much larger when SCI is determined based on partial slabs within a test item, as compared to when SCI is determined based on all slabs within a test item. Test item SCI, which includes all slabs within a test item for SCI calculation, should be used for failure model development when data from full-scale experiments is used. Based on recomputed test item SCI values for CC2, the proposed rigid pavement traffic prediction model provides a better statistical fit than the current model.
TABLE OF CONTENTS
LIST OF FIGURES ………………………………………………………………………………………………vi
LIST OF TABLES………………………………………………………………………………………………..xi
ACKNOWLEDGEMENTS ……………………………………………………………………………………xiii
Chapter 1 INTRODUCTION …………………………………………………………………………………1
1.1BACKGROUND………………………………………………………………………………………1
1.2PROBLEM STATEMENT…………………………………………………………………………4
1.3RESEARCH HYPOTHESIS AND OBJECTIVE……………………………………………6
1.4SCOPE……………………………………………………………………………………………………8
Chapter 2 LITERATURE REVIEW………………………………………………………………………..9
2.1 AIRFIELD RIGID AND OVERLAY PAVEMENT DESIGN HISTORY…………..9
2.2 AIRFIELD FULL-SCALE ACCELERATED TESTING…………………………………16
2.2.1 Lockbourne No. 1 Experiment……………………………………………………………162.2.2 Lockbourne No. 2 Experiment……………………………………………………………18
2.2.3 Sharonville Experiment…………………………………………………………………….18
2.2.4 National Airport Pavement Test Facility………………………………………………19
2.3 FULL-SCALE ACCELERATED TESTING DATA ANALYSIS……………………..26
2.3.1 Heavy Weight Deflectometer and Backcalculation ………………………………..26
2.3.2 Application of FWD (HWD) ……………………………………………………………..29
2.3.3 Instrumentation Response Collection and Analysis at NAPTF …………………33
2.4 MODEL DEVELOPMENT WITH FULL-SCALE TESTING DATA………………..36
Chapter 3 CC4 DATA COLLECTION AND BASIC ANALYSIS ……………………………….41
3.1 HWD DATA ANALYSIS ………………………………………………………………………….41
3.1.1 BAKFAA Backcalculation ………………………………………………………………..423.1.2 Load Transfer Efficiency Analysis………………………………………………………45
3.1.3 Impulse Stiffness Modulus (ISM)……………………………………………………….48
3.2 DISTRESS SURVEY AND STRUCTURAL CONDITION INDEX (SCI)
CALCULATION…………………………………………………………………………………….50
3.3 INSTRUMENTATION RESPONSES………………………………………………………….58
Chapter 4 CRACKED SLAB MODEL ASSESSMENT AND MODIFICATION FOR
UNBONDED CONCRETE OVERLAYS…………………………………………………………..77
4.1 MATERIAL CHARACTERIZATION FOR A DAMAGED PCC SLAB……………77
4.2 CRACKED SLAB CHARACTERIZATION APPROACH SELECTION AND
DETERMINATION OF PREDICTORS ……………………………………………………..79
4.3 MODIFIED CRACKED SLAB MODEL DEVELOPMENT…………………………….86
4.4 MODIFIED CRACKED SLAB MODEL VERIFICATION WITH OTHER
FULL-SCALE EXPERIMENTAL DATA …………………………………………………..94
4.5 NUMERICAL COMPARISON BETWEEN MODELS …………………………………..98
4.5.1 Modified Cracked Slab Model based on ILLI-BACK and CC4 Data…………98
4.5.2 Numerical Comparison between Models Performance Using WES Data……103
Chapter 5 TRAFFIC PREDICTION MODEL DEVELOPMENT…………………………………107
5.1 CC4 COVERAGES CONVERSION ……………………………………………………………108
5.2 CRACK DENSITY PERFORMANCE MODEL…………………………………………….110
5.3 DETERMINATION OF STRESSES FOR CC4 WITH FAARFIELD………………..115
5.4 CC2 DATA RE-CONSIDERATION……………………………………………………………120
5.5 TRAFFIC PREDICTION MODEL DEVELOPMENT…………………………………….125
Chapter 6 SUMMARY AND CONCLUSION…………………………………………………………..139
6.1 SUMMARY…………………………………………………………………………………………….139
6.2 INDIVIDUAL FINDINGS …………………………………………………………………………142
6.3 CONCLUSIONS………………………………………………………………………………………143
6.4 RECCOMANDATIONS ……………………………………………………………………………144
6.5 SIGNIFICANCE ………………………………………………………………………………………147
REFERENCES…………………………………………………………………………………………………….151
Appendix A WES EXPERIMENT LAYOUTS AND DATA……………………………………….156
Appendix B WES EXPERIMENT LAYOUTS AND DATA……………………………………….162
Appendix C CC4 LOADING CONFIGURATION SPECIFIED IN FAARFIELD FOR
COVERAGE CONVERSION AND ANALYSIS………………………………………………..165
Chapter 1
INTRODUCTION
1.1 BACKGROUND
Airfield pavement overlays are often utilized to ameliorate the problems of existing pavements. For example, an existing pavement that is approaching its design life or is expected to serve heavier airplanes or more airplane traffic than that originally designed might be improved with a structural overlay. Unbonded concrete overlays (UBOL), which are one type of pavement overlays, are an ideal treatment to extend the design life of an existing portland cement concrete pavement. An unbonded concrete overlay refers to a new portland cement concrete pavement placed over an existing portland cement concrete (PCC) pavement, typically with a bondbreaking interlayer between the PCC layers. The application of an unbonded concrete overlay can regain the loss of structural capacity of an existing PCC pavement due to damage, correct the rideability and extend the structural life of an existing PCC pavement (ACPA, 2007).
AC 150/5320-6E “Airport Pavement Design and Evaluation” is the current airfield pavement design guide, published by the Federal Aviation Administration (FAA) in September 2009. UFC 3-260-02C “Pavement Design for Airfields,” published in 2002, is the current military airfield pavement design guide. Both include the design of unbonded concrete overlays. Despite the fact that these two design guides use different mechanistic-empirical approaches to compute the required airfield unbonded concrete overlay thickness, the concepts in both design guides are identical and can be categorized into two parts: Mechanistic Stress Calculation and Empirical Fatigue Analysis. While this dissertation focuses on the FAA design guide, and utilizes data from a project funded by the FAA under Cooperative Agreement Number 01-G-002 with the Innovative Pavement Research Foundation, the military airfield pavement design may also benefit from this research since the core design concepts are the same.
The FAA unbonded concrete overlay design procedure in AC 150/5320-6E is also incorporated into the accompanying design software FAARFIELD (FAA Rigid and Flexible
Iterative Elastic Layered Design). The free edge tensile stress is calculated by the finite element method, and then reduced by 25 percent to account for load transfer at the joints. For the stress computation, the material properties must be defined in advance. Since the unbonded concrete overlay is a new PCC pavement overlaid on an existing PCC pavement with a bond breaker in between for separation, it is critical to characterize the existing, usually damaged, PCC pavement in a rational way for stress computation.
There are two potential approaches that have been used to characterize damaged layers for mechanistic calculation of responses, either by adjusting the thickness or by reducing the elastic modulus to characterize the existing PCC pavement with its accumulated damage. Because the concrete slab thickness can be obtained with confidence and will be typically unchanged, adjusting the elastic modulus for characterization of the existing PCC pavement seems to be the more rational approach; it was thus proposed by Rollings (Rollings, 1988). Rollings introduced the Structural Condition Index (SCI), a rating based upon the observed surface structural distress, as quantified using standard pavement rating procedures (ASTM D 5340, 2004). Utilizing nondestructive deflection testing data, Rollings then proposed and developed a regression model relating the ratio between backcalculated damaged PCC modulus and initial intact PCC modulus to the corresponding SCI of the same slab. The model is termed the “Cracked Slab Model” and is integrated into FAARFIELD.
After the stress is computed mechanistically, an empirical fatigue analysis is used to predict the allowable number of coverages of the designed traffic mix. “Design factor” is defined as the ratio between flexural strength of the PCC slab and computed tensile stress at the bottom of the same slab. The design factor is related to coverages at SCI values of 100 (C0) and 0 (CF). Then, the coverages between SCI values of 0 and 100 can be interpolated linearly. The design factor is related to coverages rather than directly to aircraft passes.
When the gear of an airplane moves over a specific point on the pavement, the specific point is said to receive a full-load application (one coverage). If all airplanes are assumed to have an identical gear configuration and to always move in the exact same path on the runway or taxiway, the coverages and passes would be identical. However, such assumptions are far from reality for airfield pavements (although often a simplification with merit for highway pavements). To convert passes into coverages, or vice versa, a parameter termed the “Pass-to-Coverage Ratio” (P/C) is introduced. The number of required passes for a specific point to receive one coverage is defined as “Pass-to-Coverage Ratio,” and the ratio must be derived mathematically for each airplane because the gear configurations are different for different types of airplanes, and also consider typical traffic wander patterns. Airfield pavement design typically considers departures only, to characterize one full aircraft operation (take-off and landing), due to the significantly higher aircraft weights on departure. Consequently, one departure is considered to equal one pass.
The relationship between design factors and coverages has been developed and improved from rigid pavement accelerated traffic tests since 1940. Due to the lack of traffic data from fullscale UBOL experiments, the model developed based on traffic data from full-scale rigid pavement experiments has been applied directly to unbonded concrete overlay design, with the presumption that the same predicted maximum stress in the overlay would produce the same fatigue relationship as in an single-layer concrete pavement.
To fulfill the need to better understand the performance of unbonded concrete overlay airfield pavements, the Innovative Pavement Research Foundation (IPRF), in cooperation with the FAA, undertook a series of research projects with the objectives of improving understanding of the influence of design parameters for airfield unbonded concrete overlays, and providing inputs for a new design methodology. Two phases of full-scale accelerated testing were conducted at the FAA’s National Airport Pavement Test Facility (NAPTF) at the William J.
Hughes Technical Center. The first phase, the Baseline Experiment, was initiated in September 2005 with new unbonded overlay construction, consisting of a new and intact underlying PCC pavement, a nominal one-inch asphalt concrete interlayer as a bond breaker, and the new PCC overlay. The overlay and the interlayer were then removed. Construction of the second phase, the SCI Validation Study, began in 2007 with new overlay slabs constructed on the damaged underlying slabs left in place from the Baseline Experiment. Testing at the NAPTF included loading to pavement failure, nondestructive testing, coring, extensive instrumentation monitoring and distress surveys. The total cycle of testing, including both the Baseline Experiment and SCI Validation Study, is called Construction Cycle 4 (CC4). More detailed information about CC4 is provided in section 2.2.4.
Significant improvements of both aviation and military airfield rigid pavement designs have recently been made, and the design guides moved greatly towards the mechanistic-empirical stage after Rollings’ report in 1988. Despite the wide application of his models, it is worthy to reinvestigate his model with data collected from CC4. Since the performance data from a fullscale unbonded concrete overlay experiment is now available for the first time, it is valuable to see how his models compare to those observations, and what improvements or adjustments should be made.
1.2 PROBLEM STATEMENT
The models incorporated in the current FAA unbonded concrete overlay design procedure were developed on the basis of data obtained from full-scale rigid pavement tests. The availability of similar accelerated testing data on unbonded overlays allows several key questions to be posed and investigated.
Will the performance of an unbonded concrete overlay be the same as the performance of a new rigid pavement? A significant difference between a new rigid pavement and an unbonded concrete overlay is that the underlying material of an unbonded concrete overlay is stiffer than that of a new rigid pavement (PCC versus granular or stabilized materials). As a result, if the unbonded concrete overlay and new rigid pavement have the same thickness of surface PCC slabs, the performance of an unbonded concrete overlay would be expected to last longer under the same traffic conditions. The benefit of a strong underlying condition was addressed and confirmed by the FAA from observations from Construction Cycle 2 (CC2), for new rigid pavement design, and has been incorporated into the current fatigue analysis procedures, in AC 150/5320-6E.
A second question to be addressed is that of how the E-ratio prediction from the cracked slab model compares to the results from CC4. E-ratio, which is defined as the ratio between backcalculated damaged modulus and initial intact modulus of a slab, can be predicted based on SCI of the same slab. When Rollings developed the cracked slab model, he used a layeredelastic-theory based program to estimate the elastic modulus of the slab-on-subgrade structure, utilizing only the maximum deflection information rather than backcalculating iteratively using the full set of deflections from multiple sensors. The reason was that the discontinuity of the deflection basin due to the cracks in the slab caused convergence and repeatability problems in the backcalculation.
Furthermore, the structure tested by Rollings was relatively simple, consisting only of existing PCC slabs and subgrade. He assumed that the modulus of the subgrade was constant, based on observations of subgrade conditions in the beginning and in the end of the experiment (Rollings, 1988). As a result, the only unknown parameter was the modulus of the PCC surface slab layer, which could be calculated based on the maximum deflection from FWD. There are advantages of calculating the modulus of PCC surface layer based on the maximum deflection from FWD. First, the change in the elastic modulus is directly related to the structural deterioration within the slab due to the crack. Second, the backcalculation time is greatly reduced by using only maximum deflection instead of using all deflection at multiple locations. Last but not least, the backcalculation result is free from convergence problems due to the effects of the cracks.
Despite the advantages stated above, there is a significant limitation in the data used to develop the Rollings cracked slab model. The cracked slab model was based only on test results from slab-on-subgrade structure, but the model has been applied directly to unbonded concrete overlay design. For an in-situ unbonded concrete overlay, replicating his steps is not possible. First, the structure of an unbonded concrete overlay is far more complicated than that of a slabon-subgrade. For example, the structure of the unbonded concrete overlays in CC4, based upon typical in-service pavement structures, consisted of the PCC overlay, an asphalt interlayer, the PCC underlying pavement, a granular base layer, and a medium-strength subgrade. From backcalculation results based on deflection information using seven sensors, it was found that the modulus of each layer apparently varied with time. Deconstruction observations and testing confirmed that the condition, and thus effective moduli, of some underlying layers decreased with time. As a result, it is impossible to use only the maximum deflection or to assume moduli of all layers except the PCC overlay are constant throughout loading until the time of surface cracking.
Another limitation is that Rollings calculated SCI based on separate distress surveys of each individual slab, assuming that half of the slabs in a sample unit would experience that distress, for the six slabs in the experiment. In the field, SCI is calculated based on sample units consisting of multiple slabs with a minimum number of twelve. As a result, using SCI values obtained from field conditions in the cracked slab model might yield a different predicted E-ratio, which will affect the stress calculation for unbonded concrete overlay design. The test items in the CC4 tests each contained twelve slabs, allowing for the minimum sample unit size, although some limitations remain due to the test layout and loading configurations.
Based on the fact that the cracked slab model will affect the fatigue analysis for traffic prediction, this study will reinvestigate the cracked slab model first, and then subsequently compare the fatigue analysis results to CC4 observations.
1.3 RESEARCH HYPOTHESIS AND OBJECTIVE
Three main hypotheses are made based on the problem statement:
- The performance of a new rigid pavement is not the same as the performance of an unbonded concrete overlay due to the underlying support effect. Furthermore, factors in addition to the current design factor (DFOL) should be included in the UBOL traffic prediction model.
- Different definitions of sample unit for a test item in SCI calculation will lead to differences in predicted coverages for the same SCI value of a test item.
- SCI should not be the primary parameter in the cracked slab model. The influence of factors other than SCI will be explicated by utilizing statistical analysis on the CC4 data.
The objectives of this dissertation are as follows:
- Modified cracked slab models that are suitable for PCC overlays and PCC underlying pavements within an in-situ pavement will be developed based on the original concept of Rollings. As a result, the modified models could be easily incorporated into both civil and military unbonded overlay design procedures.
- A traffic prediction model based on the CC4 data will be developed for the unbonded concrete overlay design procedure. A design factor for unbonded concrete overlays, which is expected to differ from that for a new rigid pavement, will be proposed. The modified traffic model is expected to be easily incorporated into both civil and military unbonded overlay design procedures, if desired, by the appropriate agencies.
A simple flowchart showing the FAA airfield pavement design procedure and how the objectives would be incorporated is provided in figure 1.1.
Figure 1.1 The FAA Unbonded Concrete Overlay Design Methodology, with Areas of Research Focus Indicated
1.4 SCOPE
This dissertation is focused on improving the current FAA unbonded concrete overlay design procedure with data collected from two phases of full-scale accelerated testing conducted at the National Airport Pavement Test Facility (NAPTF) for Construction Cycle 4: the Baseline Experiment and the SCI Validation Study. For each phase, three cross-sections were constructed and loadings with two types of aircraft gear configurations were applied. With the combination of cross-sections and gear loadings, there were six test items in each phase.
Distress surveys were regularly conducted to monitor the deterioration condition of the test items. Distress maps from each distress survey were documented in a spreadsheet file, as well as the loading pass information. SCI for each slab or for each unit sample (test item) is calculated based on the information in the distress maps. A total of 53 overlay distress surveys and three underlying pavement distress surveys from throughout CC4 were analyzed.
Heavy weight deflectometer (HWD) testing was conducted by the FAA at requested intervals. Load transfer calculation and backcalculation is performed on the available HWD data. There are approximately ten sets of HWD data available from each phase for backcalculation, plus four sets of HWD data from the Baseline Experiment and eight sets of HWD data from the SCI Validation Study for load transfer calculations.
Instrumentation responses (strain gauge responses) were recorded with each pass of loading. It is time-consuming to extract all instrumentation responses for analysis. As a result, the instrumentation responses are extracted with an interval of 500 passes in the Baseline Experiment and an interval of 2000 passes in the SCI Validation Study.
In addition to the CC4 data, data from CC2 was requested for model verification and comparison purposes. Thirty-one distress surveys from two cross-sections (four test items) are used for SCI calculation. Traffic data for the same test items are used for fatigue model analysis.
The data collected from CC2 and CC4 are used to verify and/or improve two important portions of the design methodology—the cracked slab model for determining modulus input to the finite element program used for stress prediction, and the fatigue or traffic prediction model. The study is limited to the data collected from the full-scale testing, and to the unbonded concrete overlay modules of the program.
