Strength Rating System to protect Airport Asphalt Surfaces ..... the stress distribution and peak stress imparted on the pavement surface (De Beer et al. 2002; Yoo ...
This is an unformatted pre-publication version of: White, G 2017, ‘Limitations and potential improvement of the aircraft pavement strength rating system’, International Journal of Pavement Engineering, vol. 18, no. 12, pp. 1111-1121.
Limitations and Potential Improvement of the Aircraft Pavement Strength Rating System to protect Airport Asphalt Surfaces Greg White School of Science and Engineering, University of the Sunshine Coast, Sippy Downs, Queensland, Australia
Acknowledgements Historical information regarding the development of the ACN-PCN system and the incremental increase in tyre pressures and wheel loads was provided by Bruce Rodway. Abstract A pavement strength rating system is internationally adopted in order to protect aircraft pavements from inadvertent overload. The system has two elements. The primary element is designed to protect the pavement against subgrade rutting and the second is intended to protect asphalt pavement surfaces. The surface-protection element is arbitrary and empirical, placing category-based limits on aircraft tyre pressures. In 2008 increases in the tyre pressure limits were proposed by aircraft manufacturers and these were approved in 2013. The research reported in this paper assesses the impact of tyre pressure and individual wheel load increases on calculated flexible pavement stress indicators, as well as identifying an improved surface layer protection element. Stresses were calculated near the surface, at the surface layer interface and at the subgrade. Tyre pressure and wheel load combinations included current (18 t and 1.35 MPa), imminent (33 t and 1.75 MPa) and future (40 t and 2.15 MPa) aircraft. Surface layer stress increased significantly (20-30%) with increases in both tyre pressure and wheel load. The subgrade stress increased near-equally (97%) with wheel load but was insensitive ( 99%) with calculated subgrade stress (Figure 5).
If the ACN-PCN system remains unchanged, aircraft tyre pressures and individual wheel loads may continue to increase in an unlimited manner without necessarily affecting calculated ACNs. This is achievable by simply spacing the wheels further apart. For the majority of the asphalt surfaced runways in the world, with a Category W (unlimited) tyre pressure limit, no pavement concession is triggered, regardless the tyre pressure. Aircraft ACN does not reflect the significant impact of increased tyre pressure and individual wheel loads on surface layer stress indicators. This reflects the intentional focus of the ACN-PCN system on the protection of pavement subgrade. 4.6
Implications for Airport Owners
The importance of the limitations of the ACN-PCN system is reinforced by the practical availability of more stress resistant pavements to airport owners. An airport owner faced with the introduction of aircraft with a significantly higher ACN has expensive, but readily available, remedies. A structural asphalt overlay provides a significant increase in strength and justifies raising the PCN. New pavements constructed at the airport are simply designed thicker or with stiffer base and sub-base materials to provide the necessary increase in subgrade protection. The ACN of the new aircraft triggered the upgrade and airlines usually provide a financial contribution to the work.
In contrast, an airport owner in a hot climate faced with the introduction of an aircraft with higher tyre pressure and individual wheel load, but a comparable ACN, has a narrower range of options. A surface with greater shear stress resistance and improved interface performance is required. Airport quality asphalt is already as highly performing as practicable within the limits of currently available materials. The only remedy available to the airport owner is to rebuild runways using concrete pavements.
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The ACN of the new aircraft did not trigger this work. If the ACN did not increase and a Category W (unlimited) tyre pressure rating was in place, no Pavement Concession was required. The justification for airlines to contribute to airport upgrade costs is less clear when there is no need for Pavement Concessions. In light of the increased tyre pressure category limits, only airports servicing high tyre pressure military jet aircraft (with low wheel loads) would be expected to have a Category W tyre pressure rating. This is particularly important for airports located in hot climates, where the temperature-dependence of bituminous binders reduces asphalt shear resistance to a greater extent during extended hot and sunny conditions. 5.
Potential Improvement
The current surface protection element of the ACN-PCN system compares the aircraft tyre pressure to the limit/category nominated by the airport owner. Wheel loads are not considered. For example, two aircraft with tyre pressure 1.6 MPa, one with a wheel load of 12 t and one with 32 t, are considered equal in terms of requiring a surface-related Pavement Concession. The analysis presented above indicated that the aircraft with the 30 t wheel load results in significantly greater surface layer shear stress. Shear stress is an indicator of risk associated with asphalt rutting, interface delamination and surface layer shoving. It follows that improvement of the ACN-PCN system would result from incorporation of a surface protection element that reflects both the tyre pressure and the wheel load impacts.
It is recommended that the current ACN-PCN tyre pressure limit rating be replaced by a Surface Classification Number (SCN). The SCN would be nominated by the airport owner and compared against an aircraft Wheel Classification Number (WCN). Like the ACN, the WCN is exact and provides no discretion. SCN would be selected based on experience, surface material testing or empirical performance. Asphalt surface material and age, longitudinal runway grade, typical prevailing wind strength and the distance between touch down points and exit taxiways all impact the selection of SCN. Guidance on the selection of SCN and the granting of Surface Concessions (similar to Pavement Concessions) is required. A specific aircraft WCN is determined based on categorical tyre pressure and wheel load (Table 8) similar to the categorical subgrade support for ACN calculation. As an example, for the two aircraft with equal tyre pressure detailed above, the WCNs are 17 and 25, for the 12 t and 32 t wheel loads, respectively. At an airport with SCN 20, only the aircraft with 12 t wheel load is permitted to operate without a Surface Concession. Refinement of the preliminary WCN values (presented in Table 8) is required to ensure that different combinations of wheel load and tyre pressure that result in similar critical stress states are assigned the same WCN rating. Systems should only be as complicated as they need to be in order to affect their aim. Incorporating a wheel load and tyre pressure dependent surface protection element into the ACN-PCN increases the system complexity. However, this is necessary if the system is to provide a robust indication of the impact of both individual wheel load and tyre pressure on asphalt surfaces. 6.
Conclusions
The ACN-PCN system is not able to protect asphalt surfaces from increasing tyre pressures and individual wheel loads. This is more broadly a limitation of current routine pavement design
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capabilities which do not actively include the surface layer distress modes now being observed in the field. The limitations of the ACN-PCN system should not be seen as a failure of the system or its developers. The system was intentionally developed with a focus on the protection of pavement subgrade.
Further increases in aircraft tyre pressures or individual wheel loads are expected to impact adversely on asphalt surfaced runway performance. Airports located in hot climates are most susceptible to surface layer distress, including those associated with higher tyre pressures and individual wheel loads. The importance of these issues is reinforced by the limited availability of remedies to increased surface layer distress. The increase in ACN-PCN tyre pressure limits proposed in 2008 has now been approved with the Category X tyre pressure limit now 1.75 MPa. This covers all current commercial aircraft. With the exception of airports catering for regular military jet aircraft, it is recommended that no airport publishes a tyre pressure rating above Category X (1.75 MPa). It is also recommended that future increases in actual tyre pressures and aircraft wheel loads be curbed until additional full scale testing is performed to address these deficiencies. The ongoing efforts to incorporate additional asphalt surface failure modes into routine pavement design must be given high priority. In the meantime, it is recommended that the tyre pressure element of the current ACN-PCN system be replaced by the WCNSCN to reflect the combined impact of both the tyre pressure and the wheel load on shear stresses in the surface layer. The preliminary system presented in this paper must be refined. Different combinations of tyre pressure and wheel load that result in comparable critical surface-layer shear stress conditions must be assigned the same WCN. Further development is required, including provision of guidance for airport owners regarding the selection of SCN at their airport. 7.
References
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Horak, E, Maina, J & Emery, S 2009b, ‘A case study: quantification and modeling of asphalt overlay delamination on an airport pavement’, Proceedings Eight International Conference on the Bearing Capacity of Roads, Railways and Airfields, Urbana-Champaign, USA, 29 June - 2 July, pp. 1475-1483. Kim, M, Tutumluer, E & Kwon, J 2009, ‘Nonlinear pavement foundation modeling for threedimensional finite-element analysis of flexible pavements’, International Journal of Geomechanics, vol. 9, no. 5, pp. 195-208. Loizos, A & Charonitis, G 2004, ‘Bearing capacity and structural classification of flexible airport pavements’, Journal of Transportation Engineering, vol. 130, no. 1,pp. 34-42. Maina, JW, Denneman, E & De Beer, M 2008, ‘Introduction of new road pavement response modelling software by means of benchmarking’, Proceedings 27th Annual South African Transportation Conference, Pretoria, South Africa, 7-11 July. Maina, JW & Matsui, K 2004, ‘Development of software for elastic analysis of pavement structure due to vertical and horizontal surface loadings’, Proceedings 83rd Meeting of the Transport Research Board, Washington, DC, USA, 11-15 January, Transport Research Board. Maina, JW, Ozawa, Y & Matsui, K 2012, ‘Linear elastic analysis of pavement structure under non-circular loading’, Road Materials and Pavement Design, vol. 13, no. 3, pp 403-421. Perdomo, D & Button, JW 1991, Identifying and Correcting Rut-Susceptible Asphalt Mixtures, Research Report FHWA/TX-91/1121-2F, Federal Highways Administration, February. Rodway, B 2009, ‘Asphalt deformation due to high tyre pressure’, FAA Airport Pavement Working Group Annual Meeting, Atlantic City, New Jersey, USA, 21-23 July, Federal Aviation Administration. Roginski, MJ 2007, ‘Effects of aircraft tire pressures on flexible pavements’, Proceedings Advanced Characterisation of Pavement and Soil Engineering Materials, Athens, Greece, 20-22 June, Taylor and Francis, pp. 1473-1481. Roginski, MJ 2013, ‘ICAO update – status of high tyre pressure revision to Annex 14’, presented to the FAA Working Group Meeting, Atlantic City, New Jersey, USA 15-17 April, Federal Aviation Administration. Shell 2015, The Shell Bitumen Handbook, 6th edn, ICE Publishing, Italy. Shepson, O 2009, ‘Boeing and Airbus tire pressure test programs’, presented to ALACPA Airport Pavement Seminar and FAA Workshop, Sao Paulo, Brazil, 26-30 October. Uzan, J 1999, ‘Granular material characterization for mechanistic pavement design’, Journal of Transportation Engineering, vol. 125, no. 2, pp. 108-113. White, GW 2007, Thickness Design of Flexible Aircraft Pavements, A Thesis submitted to the Faculty of Built Environment and Engineering, Queensland University of Technology, for the Award of Master of Engineering, July. White, G 2014a, ‘Cyclic shear deformation of asphalt at Melbourne Airport’, Proceedings 2014 Worldwide Airport Pavement Technology Transfer Conference, Galloway, New Jersey, USA, 57 August, Federal Aviation Administration.
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White, G 2014b, ‘New airport pavement technologies from the USA’, Proceedings 2014 Australian Airport Association National Conference, Gold Coast, Australia, 23-27 November, Australian Airports Association. White, G 2015, ‘Shear Stresses in an Asphalt Surface under Various Aircraft Braking Conditions’, International Journal of Pavement Research and Technology, submitted but not yet published. White, G & Rodway, B 2014, ‘Distress and maintenance of grooved runway surfaces, Proceedings Airfield Engineering and Maintenance Summit, Furama Riverfront, Singapore, 2528 March. Witczak, MW & Uzan, J 1988, The Universal Airport Pavement Design System, Report I of IV : Granular Material Characterization, University of Maryland, College Park, Maryland, USA. Yoo, PJ, Al-Qadi, IL, Elseifi, MA & Janajreh, I 2006, ‘Flexible pavement responses to different loading amplitudes considering layer interface conditions and lateral shear forces’, International Journal of Pavement Engineering, vol. 7, no. 1, pp. 73-86.
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TABLES Table 1. ACN-PCN Subgrade Categories Category
Representative CBR
CBR Range
A
15
Greater than 13
B
10
8-13
C
6
4-8
D
3
Less than 4
Category
Original Tyre Pressure Limits
Revised Tyre Pressure Limits
W
Unlimited
Unlimited
X
1.50 MPa
1.75 MPa
Y
1.10 MPa
1.25 MPa
Z
0.50 MPa
0.50 MPa
Table 2. Tyre Pressure Category Limits
Table 3. Representative Tyre Pressures and Wheel Loads Representation
Base Aircraft
Max. Wheel Load
Vertical force
Horizontal force
Tyre Pressure
1958 FAA policy
DC8-50
19 tonnes
170 kN
65 kN
1.35 MPa
Present Typical Aircraft
B767-300
18 tonnes
160 kN
52 kN
1.35 MPa
New X Category Limit
A350-900
33 tonnes
290 kN
94 kN
1.75 MPa
Possible Future Aircraft
Unknown
40 tonnes
350 kN
114 kN
2.15 MPa
Table 4. Distance in front of the Centre of the Tyre for Surface Layer and Interface Evaluation Points Vertical Force
Tyre Pressure 1.35 MPa
1.75 MPa
2.15 MPa
160 kN
200 mm
175 mm
155 mm
290 kN
265 mm
235 mm
210 mm
350 kN
290 mm
255 mm
230 mm
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Table 5. Calculated Indicators of Stress at Evaluation Points Tyre Pressure (MPa)
Vertical Force (kN)
Horizontal Force (kN)
Surface (kPa)
Interface (kPa)
Subgrade (kPa)
1.35
160
52
1207
873
11.1
1.35
160
0
825
733
11.1
1.35
290
94
1415
984
19.9
1.35
290
0
994
833
19.9
135
350
114
1487
1013
23.9
1.35
350
0
1051
859
23.9
1.75
160
52
1450
1069
11.1
1.75
160
0
969
896
11.1
1.75
290
94
1712
1214
20.0
1.75
290
0
1194
1024
20.0
1.75
350
114
1802
1263
24.0
1.75
350
0
1257
1069
24.0
2.15
160
52
1634
1253
11.1
2.15
160
0
1057
1051
11.1
2.15
290
94
1877
1441
20.0
2.15
290
0
1352
1215
20.0
2.15
350
114
2091
1498
24.1
2.15
350
0
1440
1265
24.1
Note: Stress indicators were octahedral shear stress at the surface, horizontal shear stress at the interface and vertical stress at the subgrade. Table 6. Calculated ACN for each Subgrade Category Tyre Pressure
Vertical Force
1350 1350
Single Wheel
Dual-Tandem Gear
A
B
C
D
A
B
C
160
33
32
32
32
30
31
34
43
290
61
60
60
60
64
70
81
110
1350
350
74
72
72
72
80
88
107
141
1750
160
34
33
33
32
31
32
35
44
1750
290
64
61
61
60
65
71
82
110
1750
350
77
74
73
72
83
89
108
141
2150
160
35
34
33
32
32
32
35
44
2150
290
66
64
62
61
68
71
82
110
2150
350
79
76
74
73
85
91
108
142
Table 7. Increase in Calculated Indicators of Pavement Damage under Braking Response
Min
Average
Maximum
Near-surface OSS
39%
45%
55%
Interface Shear Stress
18%
19%
19%
Subgrade Vertical Stress
0%
0%
0%
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D
Table 8. Preliminary Wheel Classification Number (WCN) Determination Wheel Load (t)
Tyre Pressure (MPa) < 0.75
0.75-1.00
1.00-1.25
1.25-1.50
1.50-1.75
1.75-2.00
> 2.00
40
17
20
23
26
29
32
35
19
FIGURES
Figure 1. Surface Layer Evaluation Points
Figure 2. Impact of Tyre Pressure on Indicators of Pavement Damage
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Figure 3. Impact of Wheel Load on Indicators of Pavement Damage
Figure 4. Impact of Tyre Pressure and Wheel Load on ACN
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Figure 5. Correlation between ACN and Indicators of Pavement Damage
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