Pavement Strength – Load-Bearing Capacity of Paved Surfaces in Airport Infrastructure

What Is Pavement Strength?

Pavement strength is the quantified ability of a paved surface—such as a runway, taxiway, or apron—to support applied aircraft loads without experiencing excessive distress, structural failure, or accelerated deterioration. This value is central to airport engineering and operational planning, as aircraft impose significant concentrated loads through their landing gear, which can exceed hundreds of thousands of kilograms per wheel. The strength of airport pavements is measured not only by the surface material but also by underlying layers and the supporting subgrade soil, which together form a structural system. To ensure safety, pavements are engineered and continuously evaluated to accommodate the current and anticipated fleet mix, environmental conditions, and operational demands.

The importance of pavement strength is reflected in its standardized assessment and reporting, enabling safe aircraft operations and efficient airport management. It is expressed using indices such as the Pavement Classification Number (PCN) or Pavement Classification Rating (PCR), which allow for direct comparison of aircraft load effects (ACN or ACR) with pavement capacity. These indices are critical for operational decision-making, including determining which aircraft can safely use specific runways, taxiways, and aprons. The concept also underpins regulatory compliance with ICAO and FAA requirements, ensuring that airports publish accurate pavement strength data in Aeronautical Information Publications (AIPs) and airport master records.

Pavement strength directly impacts airport capacity, as insufficiently strong pavements can restrict the operation of heavier or more frequent aircraft, leading to rerouting, operational delays, or costly pavement upgrades. The measurement process involves technical evaluation of pavement materials, thicknesses, subgrade conditions, and anticipated traffic loads. Engineering standards such as ICAO Doc 9157, FAA AC 150/5335-5D, and ASTM D5340 inform these evaluations. The resulting data supports not only aircraft operations but also asset management, maintenance planning, and long-term infrastructure investment.

How Is Pavement Strength Used in Airport Infrastructure?

Pavement strength data is foundational in the management and operation of airport infrastructure. Engineers, planners, and airport operators rely on accurate strength assessments to ensure that aircraft operations remain within safe limits. This data underpins several critical functions:

• Aircraft Operations: Before authorizing a specific aircraft to use a runway, taxiway, or apron, airport authorities must verify that the aircraft’s load effect—represented by its ACR or ACN—does not exceed the pavement’s reported PCR or PCN. This check prevents structural damage, rutting, cracking, or more catastrophic failures, especially with new generation widebody aircraft or those with high tire pressures.
• Operational Routing and Parking: Pavement strength governs not only runway usage but also taxi routes and parking positions. Airports may have sections of varying strength, and routes are planned to avoid overloading weaker pavement areas. For example, heavy aircraft may be routed along specific taxiways or assigned to particular stands designed for higher loads.
• Regulatory Compliance: ICAO Annex 14 and FAA regulations require airports to report pavement strength data in a standardized format. This information is published in the Aeronautical Information Publication (AIP) and entered into the Airport Master Record (AMR), ensuring pilots, dispatchers, and airlines have access to current capacity data for flight planning.
• Pavement Management and Maintenance: Pavement strength data informs maintenance scheduling, rehabilitation planning, and asset management. By monitoring changes in strength over time—due to aging, weather, or increased traffic—airports can plan preventive interventions, extend pavement life, and optimize maintenance budgets.
• Design and Upgrade Decisions: When planning new construction or major rehabilitation, pavement strength requirements are determined based on projected aircraft types and frequencies. The design process uses traffic forecasts and structural analysis to ensure new or upgraded pavements meet or exceed required strength levels for the intended operational life.

The integration of pavement strength data into airport operations and planning is facilitated by digital asset management systems and spatial mapping tools, which provide real-time access to pavement condition and strength across the airfield. This capability enables dynamic allocation of stands, rapid response to operational changes, and informed decision-making during unusual events (e.g., emergency landings, temporary overloads).

Key Concepts and Definitions

Aircraft Classification Rating (ACR)

The Aircraft Classification Rating (ACR) is a standardized numerical index representing the relative pavement loading effect of a specific aircraft configuration. Derived from the aircraft’s maximum ramp weight, gear type and arrangement, and tire pressure, the ACR quantifies how much structural demand the aircraft places on a pavement system. The ACR is calculated using internationally recognized methods that factor in the pavement type (flexible or rigid) and the subgrade strength category.

The ACR value enables direct comparison to the pavement’s load-carrying capacity (PCR or PCN). If an aircraft’s ACR is less than or equal to the pavement’s PCR/PCN (and tire pressure does not exceed limits), unrestricted operations are permitted. The ACR is provided by aircraft manufacturers in the Aircraft Flight Manual (AFM) or through regulatory data sheets and is based on the worst-case loading scenario—typically the maximum operating or ramp weight, with the most critical gear configuration and full fuel.

ACR values are determined by applying the relevant calculation procedures outlined in ICAO Doc 9157 Part 3 and FAA AC 150/5335-5D. These procedures take into account the depth and composition of the pavement, the distribution of wheel loads, and the properties of the subgrade soil. The ACR is recalculated for each pavement type and subgrade category, as the structural response varies between flexible and rigid surfaces and with changes in subgrade strength.

In practical terms, the ACR provides a single, easily referenced value for flight operations, airport compatibility studies, and regulatory compliance. It simplifies the complex interaction between aircraft gear loads and pavement structures into a manageable comparison, ensuring that airports and airlines can efficiently assess operational feasibility and plan for new aircraft introductions.

Pavement Classification Rating (PCR)

The Pavement Classification Rating (PCR) is the numerical value that expresses the load-carrying capacity of an airport pavement, taking into account its construction, condition, subgrade support, and projected traffic. The PCR is a cornerstone of the ACR-PCR pavement strength reporting system, which has replaced the older ACN-PCN system as the international standard under ICAO and FAA guidance.

The determination of PCR involves either empirical methods based on actual aircraft operations (the Using Aircraft Method) or detailed structural analysis (the Technical Evaluation Method). The PCR represents the highest ACR value of aircraft that can operate on the pavement without restriction, considering both static and dynamic loading, environmental effects, and cumulative damage over the pavement’s design life.

PCR is reported in a standardized five-part code that includes the numerical value, pavement type, subgrade category, tire pressure limitation, and evaluation method. This code provides all necessary information for pilots, dispatchers, and airport operators to assess the suitability of a pavement for specific aircraft operations.

The PCR is not static; it is subject to periodic reassessment as operational loads change, pavement ages, or maintenance activities are performed. Accurate PCR values are essential for safety, operational efficiency, and regulatory compliance. Airports must update PCR ratings following major rehabilitation, significant traffic changes, or observed deterioration.

By providing a clear, standardized measure of pavement capacity, PCR facilitates the safe integration of new aircraft types, supports efficient resource allocation, and underpins the long-term sustainability of airport infrastructure.

Aircraft Classification Number (ACN)

The Aircraft Classification Number (ACN) is a legacy ICAO term synonymous with ACR, representing the aircraft’s relative effect on a pavement of specified type and subgrade strength. The ACN-PCN system was established by ICAO to standardize pavement strength reporting worldwide and allowed for the direct comparison of aircraft load effects (ACN) to pavement capacity (PCN).

The ACN is calculated using methods prescribed in ICAO Doc 9157, which consider the aircraft’s maximum weight, gear configuration, and tire pressure, along with standardized pavement and subgrade models. The ACN is determined for four subgrade categories (A–D) and two pavement types (flexible and rigid), resulting in multiple ACN values for a single aircraft.

Although the ACN-PCN system remains in use in some jurisdictions, it is being phased out in favor of the more technically robust ACR-PCR system, which incorporates updated evaluation methods and broader data inputs. The transition to ACR-PCR reflects advances in pavement engineering, aircraft design, and load modeling, as well as harmonization efforts between ICAO and the FAA.

ACN values, like ACR, are published by aircraft manufacturers and regulatory authorities and are included in flight manuals, airport planning documents, and aeronautical publications. Operators are responsible for ensuring that aircraft ACN values do not exceed reported PCN limits for intended operations.

Pavement Classification Number (PCN)

The Pavement Classification Number (PCN) is the internationally recognized numerical value expressing the load-bearing capacity of an airport pavement for unrestricted aircraft operations. The PCN is reported as a five-part code that details not only the numerical strength but also the pavement type, underlying subgrade category, maximum allowable tire pressure, and the method used in the evaluation.

The PCN serves as a universal “language” for pavement strength, enabling direct comparison with the Aircraft Classification Number (ACN) or Aircraft Classification Rating (ACR) of different aircraft. This comparison ensures that only compatible aircraft operate on a given pavement, thus preserving structural integrity and minimizing maintenance costs.

PCN values are determined through either technical evaluation—using detailed structural analysis and traffic projections—or by reference to aircraft that have historically operated without pavement distress (the Using Aircraft Method). The reporting method is indicated in the PCN code. For example, a PCN of 56/R/B/W/T indicates a rigid pavement with medium subgrade strength, no tire pressure restriction, and evaluation by technical analysis.

Airports are required by ICAO Annex 14 and FAA regulations to publish PCN values for all airfield pavements in the Aeronautical Information Publication (AIP) and other official records. These values are essential for flight planning, airport compatibility studies, and regulatory oversight. As new aircraft types are introduced or pavement conditions change, PCN values must be reassessed and updated to ensure ongoing operational safety.

Subgrade Category

The subgrade category is a classification of the support strength provided by the soil or material beneath the pavement structure. The subgrade’s ability to bear load is a critical factor in pavement design and performance, as it directly influences the distribution of applied aircraft loads and the risk of structural failure.

Subgrade categories are denoted by letters A (high), B (medium), C (low), and D (ultra-low), and are based on standardized measures such as elastic modulus (E) or California Bearing Ratio (CBR) for flexible pavements. The category is selected by evaluating site-specific geotechnical data—obtained through boreholes, laboratory testing, and in-situ evaluations—or by referencing established soil maps and historical records.

For rigid pavements, the subgrade category is based solely on elastic modulus, as the concrete slab distributes loads more widely than flexible pavements. For flexible pavements, both modulus and CBR are used, reflecting the greater sensitivity to subgrade deformation.

Accurate subgrade classification is essential for realistic pavement strength assessments and long-term performance predictions. Misclassification can lead to premature pavement distress, increased maintenance costs, and operational restrictions.

Pavement Types: Rigid and Flexible

Airport pavements are categorized as either rigid or flexible, based on their structural behavior and material composition:

Flexible Pavement (F):
Flexible pavements are constructed with layers of asphalt concrete (hot-mix asphalt, HMA), aggregate base, and subbase materials. These layers work together to distribute aircraft loads through bearing action, with the highest stresses occurring at the surface and decreasing with depth. Flexible pavements are designed to accommodate minor deformations and recover from load-induced strains, but they are sensitive to subgrade strength and moisture conditions. Maintenance typically involves periodic resurfacing and crack sealing.

Rigid Pavement (R):
Rigid pavements consist primarily of thick slabs of Portland cement concrete (PCC), often reinforced with steel mesh or bars. The concrete slab acts as a structural beam, distributing loads over a much wider area than flexible pavements. This reduces the stress on the subgrade and allows rigid pavements to perform well even over weaker soils. Rigid pavements are less affected by water infiltration and temperature variations but are susceptible to cracking, joint deterioration, and slab movement. Maintenance includes joint repairs, slab replacements, and surface grinding.

The choice between rigid and flexible pavement depends on expected traffic loads, subgrade conditions, climate, life-cycle costs, and maintenance considerations. Many airports employ both types, using rigid pavement for areas with high wheel loads (e.g., aprons, intersections) and flexible pavement for less critical sections.

Standardized Methods for Determining Pavement Strength

Rigorous, standardized methods ensure the consistency and reliability of pavement strength data at airports worldwide. These methods are codified in ICAO and FAA guidance, notably ICAO Doc 9157 and FAA AC 150/5335-5D, and provide two main evaluation approaches:

Using Aircraft Method

The Using Aircraft Method estimates the pavement strength (PCR/PCN) based on the operational experience of aircraft that regularly use the pavement without visible distress. This empirical approach is especially useful when detailed pavement structure data are lacking or when a quick assessment is needed after changes in traffic patterns or maintenance activities.

Procedure:

1. Compile a complete list of all aircraft types that operate routinely on the pavement section, including their maximum ramp weights and frequency of operations.
2. Obtain the current ACR/ACN values for each aircraft, corresponding to the relevant pavement type and subgrade category.
3. Identify the “critical” aircraft—the one with the highest ACR/ACN among those with regular (not infrequent or abnormal) operations.
4. Assign the PCR/PCN value to match the ACR/ACN of the critical aircraft, ensuring that all regularly operating aircraft can continue without restriction.
5. Complete the five-part reporting code, specifying pavement type, subgrade, tire pressure limit, and evaluation method (U for Using Aircraft).

This method assumes that the pavement has supported these aircraft without damage, but it does not account for cumulative damage or future changes