# Reporting the Bearing Capacity of Airfield Pavements Using PCR Index

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## Abstract

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## 1. Introduction

_{AC}). For this reason, deflection records from FWD measurements were used for back-calculating the modulus of elasticity of the individual layers of the pavement. In addition, a sensitivity analysis was performed in order to investigate the impact of the variation of the thickness of the AC layers and of the E

_{AC}on PCR, in relation to the typical FAA material characteristics and the design assumptions. In order to achieve this goal, laboratory data were also considered, which supported the characterization of the AC layers derived from NDT data. For the analysis, the concept of the Cumulative Damage Factor (CDF) was used, as presented in the recent developments of the FAA airfield pavement design and evaluation principles [17].

## 2. Materials and Methods

#### 2.1. Reporting Systems

#### 2.1.1. Method ACN-PCN

#### 2.1.2. Method ACR-PCR

_{subgrade}) is equal to 1.0 for 36,500 coverages of the aircraft. The total thickness of the pavement that results from the above procedure corresponds to the reference thickness for ACR calculation. Using the above reference thickness, a DSWL is obtained which has a constant tire pressure of 1.50 MPa and produces a CDF

_{subgrade}equal to 1.0. The ACR is then defined as two times the DSWL (expressed in hundreds of kilograms).

_{subgrade}of the aircraft mix is determined and the aircraft with the highest contribution to the maximum CDF

_{subgrade}is considered as the critical aircraft (AC

_{(i)}). The number of departures of the critical aircraft are adjusted until the maximum aircraft CDF

_{subgrade}is equal to the total CDF

_{subgrade}of the aircraft mix. Then, the critical aircraft weight is modified in order to obtain a maximum CDF

_{subgrade}of 1.0 for this number of departures. This weight corresponds to the MAGW of the critical aircraft. The next step includes the determination of the ACR of the critical aircraft at its MAGW, which is considered to be equal to PCR

_{(i)}. In case this is the maximum ACR aircraft from the ones initially calculated, the PCR is considered to be equal to PCR

_{(i)}. Otherwise, the AC

_{(i)}is removed from the traffic mix and the above procedure is repeated, until the calculated PCR index equals the maximum ACR aircraft [16].

#### 2.2. Data Collection

#### 2.3. Sensitivity Analysis—CDF

_{i}: The CDF of each aircraft in the traffic mix.

_{i}: The number of aircraft passes.

_{i}(z): The damage contributed by a pass of aircraft i.

_{AC}) and the failure of the subgrade as well (CDF

_{subgrade}). In case CDF < 1, the pavement is not expected to fail due to the related mode of failure. In the present investigation, emphasis is given on the failure of the subgrade, since that index consists of the base for the determination of the PCR index. Especially for subgrade failure, for the estimation of the allowable coverages to failure the following failure models are used [17,22]:

_{z}is the vertical strain at the top of the subgrade.

## 3. Results

#### 3.1. Reporting the Bearing Capacity of Runway Pavement Using Design Thicknesses and Typical FAA Materials

_{design}), while the PCR index occurred 490/F/D/X/T (PCR

_{design}).

#### 3.2. Reporting the Bearing Capacity of Runway Pavement Using Insitu Thickness and Typical FAA Materials

_{design}and PCR

_{design}values, respectively.

^{2}= 0.98). Since the R

^{2}coefficient corresponds to the percentage of the variability in the PCR index that is explained by the regression line, the change in the PCR index can be described by the change in the PCN index. Therefore, it seems that the fit of the regression line to the data in question is excellent. This information could be useful for airport authorities for a preliminary estimation of PCR in the absence of detailed pavement evaluation techniques, during the transfer period until the full implementation of the ACR-PCR system.

#### 3.3. Reporting the Bearing Capacity of Runway Pavement Using Insitu Thicknesses and Materials

_{AC}), since the assumption of the typical FAA material (P-401 with E

_{AC}= 1378 MPa at 32 °C) was considered quite conservative for the mixes used in this area. It is noted that the corresponding mixes were expected to present E

_{AC}of about 3000 MPa, adjusted to the temperature of 32 °C. The results of the relevant analysis are shown in Figure 10. In the same Figure, the recorded temperature in the body of the AC layers is also presented, given that this parameter affects the behavior of the asphalt mix and consequently of the pavement.

_{Ac}values were normalized to a temperature of 32 °C using the conversion algorithm of Equation (6), based on international experience and practice [23].

_{ref}: Modulus of elasticity of AC layers to reference temperature (°C).

_{AC}: Modulus of elasticity of AC layers from back-analysis.

_{ref}: Reference temperature (°C).

_{AC}: Temperature at 1/3 of AC layer thickness.

_{AC}of the characteristic cross-sections was E

_{AC}= 3860 MPa with a standard deviation of 386 MPa, therefore the value E

_{AC}= 3475 MPa can be considered as a characteristic value of the sample, which differs significantly from the characteristics of typical P-401 FAA material.

_{AC}(insitu) greatly affects the PCR index which is used for classifying the bearing capacity of an airfield pavement. Moreover, the use of the E

_{AC}(insitu) instead of the typical P-401 FAA material, leads to an increase in the reported bearing capacity and consequently on the acceptance of the aircraft operations for the runway pavement. Based on the above it is apparent that all of the investigated pavement cross-sections can accept without weight restrictions the expected traffic fleet.

_{AC}, a laboratory determination of the stiffness measure ITSM (Indirect Tensile Stiffness Modulus) (ΕΝ 12697-26) [24] was carried out on the cores obtained. From the testing it occurred that the mean was E

_{AC}= 5418 MPa with a standard deviation of 1140 MPa. Therefore, the value E

_{AC}= 4278 MPa could be considered as a characteristic value of the sample coming from the laboratory testing. It is noted that this value approximates the value of E

_{AC}that has resulted from the back-calculation procedure.

#### 3.4. Sensitivity Analysis on PCR

_{AC}on the evaluation of an airfield pavement and on reporting its bearing capacity, a sensitivity analysis was carried. The main criterion was the CDF

_{subgrade}, since this index is the basis for PCR estimation.

_{AC}, three values were considered for the analysis: the value corresponding to the initial pavement design and the typical FAA material (E

_{AC}= 1378 MPa), the characteristic value based on the back-calculation procedure (E

_{AC}= 3475 MPa) and the characteristic value based on the results of laboratory testing (E

_{AC}= 4278 MPa). The rest of the pavement elements (base and subbase thickness and mechanical properties of materials) were taken into account based on the design cross-section. Consequently, the analysis focused on the combined effect of the characteristics of the AC layers on the behavior of the pavement.

_{AC}= 3475 MPa) leads to a sufficient bearing capacity of the pavement for the considered traffic. However, the choice of a more conservative approach regarding the thickness of the asphalt layers in combination with the consideration of the E

_{AC}leads to high values of the CDF

_{subgrade}index.

## 4. Discussion

## 5. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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Geophone | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 |
---|---|---|---|---|---|---|---|---|---|

Distance from center (mm) | 0 | 200 | 300 | 450 | 600 | 900 | 1200 | 1500 | 1800 |

Aircraft | Gear Configuration | Aircraft Weight (ton) | Annual Departures |
---|---|---|---|

A320-200 | Dual | 78.400 | 2000 |

B757-300 | Dual Tandem | 124.058 | 2000 |

C-130 | Single Tandem | 70.307 | 1000 |

F-16C | Single | 19.187 | 4000 |

Aircraft | ACN (D) | ACR (D) |
---|---|---|

A320-200 | 50.2 | 444.00 |

B757-300 | 58.1 | 516.54 |

C-130 | 37.6 | 340.32 |

F-16C | 18.3 | 175.21 |

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**MDPI and ACS Style**

Armeni, A.; Loizos, A.
Reporting the Bearing Capacity of Airfield Pavements Using PCR Index. *NDT* **2024**, *2*, 16-31.
https://doi.org/10.3390/ndt2010002

**AMA Style**

Armeni A, Loizos A.
Reporting the Bearing Capacity of Airfield Pavements Using PCR Index. *NDT*. 2024; 2(1):16-31.
https://doi.org/10.3390/ndt2010002

**Chicago/Turabian Style**

Armeni, Angeliki, and Andreas Loizos.
2024. "Reporting the Bearing Capacity of Airfield Pavements Using PCR Index" *NDT* 2, no. 1: 16-31.
https://doi.org/10.3390/ndt2010002