Evaluation of Airfield Pavement Strength

• Why Evaluate Airfield Pavement Strength?
• "Static" vs. "Dynamic" Aircraft Loadings
• Strength Evaluation Equipment and Methods
• Pavement Strength Reporting
• Allowable Aircraft Operations
• Aircaft Overloads Beyond Published Limits
• Pavement Strength Review and Update
• Strength Evaluation References

Why Evaluate Airfield Pavement Strength?

The bearing strength of airfield pavements must be measured, analyzed, evaluated and reported so that the operating weight of aircraft allowed to use the pavements can be controlled .... from heavy wide-bodied jets .... to relatively light aircraft.

 Pavements if overloaded beyond their original design strength can deteriorate very quickly leading to both FOD and roughness safety problems for aircraft. Cases of severe overloading may result in aircraft gear actually breaking through the pavement surface.

The stability (resistance to shoving or pushing) of the surface layer is also critical under heavy loads and high aircraft tire pressures. Surface stability must also be checked during the strength evaluation process.

"Static" Versus "Dynamic" Aircraft Loadings

Generally, pavement strength evaluation involves making a comparison of the structural loading effect of an aircraft on a pavement to the structural ability of the pavement to support the imposed load. The pavement bearing strength can be established using either design criteria or, more preferably, the results of field strength tests performed on the as-built pavement. If the aircraft gear loading effect exceeds the allowable pavement strength, then some degree of "overstressing" or "overload" will occur and a corresponding degree of pavement damage may result.

Gear loads are at their most critical when the aircraft is fully loaded with passengers, cargo and fuel and is stationary or moving slowly - such as when parked at the terminal apron gate, taxiing to a runway for takeoff or in the initial stages of the takeoff run. Dynamic (or impact loads) which are of much shorter duration are generally not as critical as stationary loads. The effect of dynamic loads imposed on the runway surface by a landing aircraft are considerably less than stationary loads. A landing aircraft has lost considerable weight from the in-flight consumption of fuel and applies about 50% of its weight to the surface during initial touchdown.

Strength Evaluation Equipment and Methods

Canadian aerodrome regulatory standards respecting pavement strength require that the "pavement strength shall be determined" by the airport operating authority. For the most part, the strength evaluation test methods, equipment and test frequency can be selected by the airport authority to meet site specific needs. A variety of strength testing equipment and methods (both static and dynamic) are available for strength measurement and evaluation - but, for best results, it is important to choose equipment and methods which are as consistent as possible with the original pavement strength design methods.

An established and industry recognized engineering method appropriate to the pavement construction type should be used to determine the structural capability of a pavement to support proposed aircraft loads and traffic levels. The strength determination method should rely on the results of in-situ pavement strength tests combined with a knowledge of the thicknesses and strength properties of the various material layers comprising the pavement structure.

As the owner and operator of many of the major airports in Canada from 1945-1994, Transport Canada measured the strength of airfields pavements using the static type repetitive plate load test. Custom built tanker-trailer units (such as those shown above) were used with on-board test equipment for surface load application and deflection measurement. Although, the plate load test program is no longer active, Transport Canada has some 50 years of strength testing data - data which remains valid and useful and which can be made available to airport site operators upon request.

The bearing strength of airfield pavements should be determined by a professional engineer or engineering consulting firm experienced in the measurement and analysis of the bearing strength of airfield pavements, in determining their ability to support aircraft loads, and in assessing the effect that aircraft loads are likely to have on the future structural performance and condition of the pavement.

Pavement Strength Reporting

Once the strength of an airfield pavement has been determined, regulatory standards require that the strength shall be reported in a standardized format that can be understood by both the Canadian and international aviation communities. Certified airports in Canada should have a Pavement Load Rating (PLR) chart which reports the strength of airside pavements in terms of the aircraft which can operate on them without causing premature pavement structural damage or failure.

Click on the thumbnail to see an example of a PLR Chart.

Two strength reporting systems are used - the Canadian Pavement Load Rating (PLR) system and the international ICAO Pavement Classification Number (PCN) system.

Under the PLR system, the pavement bearing strength is reported on a scale of 1 - 12, with 1 representing a weak pavement and 12 a very strong pavement. For flexible pavement systems (i.e. asphaltic concrete or gravel surfaces) a tire pressure restriction may also be published along with the PLR value.

The ICAO PCN pavement strength reporting system involves publishing a five (5) part strength code in the form of 51/F/D/W/T for flexible pavements or 62/R/B/W/T for rigid concrete pavements. Briefly, the first number is the reported PCN value on a scale of 1 to about 130, with 1 representing a weak pavement and 130 a very strong pavement. The second part of the code is either an "F" for flexible pavement systems or "R" for rigid pavement systems. The third part is a letter code A, B, C, or D indicating the subgrade/bearing strength, with A representing a high supporting strength and D a very low strength. The fourth part indicates the tire pressure limitation in MPa if applicable (0.5 MPa in the example above) - "W" indicates that no tire pressure restriction is in effect. The fifth and final part of the PCN code indicates the evaluation method used to determine the pavement strength - "T" if derived from an engineering study or "U" if based on satisfactory aircraft usage.

Allowable Aircraft Operations

Corresponding Aircraft Load Ratings (ALR's) and Aircraft Classification Numbers (ACN's) are published for most commercial aircraft operating today. As shown in the tables below, ALR and ACN values are published for both flexible and rigid pavements and at four (4) subgrade categories that span the range of subgrade and bearing support values normally encountered.

 

To determine whether a pavement is strong enough to support operations by a particular aircraft, it is a simple matter of checking to see that the ALR is equal to or less than the PLR or that the ACN is equal to or less than the PCN. Also, the aircraft tire pressure must not exceed any limits shown on the chart.

Aircraft Overloads Beyond Published Limits

The overloading of pavements can result from either loads that are too large, a substantially increased load application rate, or a combination of both. Loads larger than the original design or as-built evaluated load may shorten the pavement service life, while smaller loads will usually have a minimal effect on pavement deterioration.

Aircraft with load ratings greater than the reported pavement load ratings may still be allowed to use the pavement subject to the approval of the airport operating authority. However, the airport authority should only allow overload operations based on a detailed engineering study comparing the individual aircraft load to the structural capability of the pavement. The implications of allowing overload operations should be fully understood in terms of the accelerated structural deterioration and the reduction in pavement service life which may occur.

With the exception of massive overloading, pavements in their structural behavior are not subject to a particular limiting load above which they will suddenly or catastrophically fail. Behavior is such that a pavement can sustain a definable load for an expected number of repetitions during its service life. Occasional minor overloading can normally be accepted, when necessary, with only limited loss in pavement life expectancy and relatively small acceleration of pavement deterioration.

Pavement Strength Review And Update

The bearing strength of a pavement should be reviewed and re-determined when the structural composition and/or properties of the pavement change as a result of new or restorative construction (such as an overlay/reconstruction) or when significant change in the structural condition of the pavement occurs.

As a minimum, the bearing strength of a pavement should be reviewed, re-affirmed or re-determined as appropriate at least once every ten (10) years. As part of the review process, consideration should be given to retesting the strength of all or selected pavements at the airport.

If the review results indicate that pavement strength values have changed, the airport authority should make the appropriate revisions to the PLR/PCN codes reported on the PLR chart.

Strength Evaluation References

Complete information on airfield pavement strength measurement, evaluation and reporting procedures can be found in the Pavement Technical Documentation Series.

For further information on airport pavement strength values (both PLR and PCN), refer to Airport Pavement Strength Data.

Aircraft structural design/evaluation loading data and detailed ALR/ACN ratings for different subgrade strengths are available for download under the Aircraft Data link.