Airport Runways: What's The Standard Thickness And Why?

The thickness of an airport runway depends on the type of construction and the expected load. There are three general types of runway construction: concrete, asphaltic concrete, and asphalt. Concrete runways are the thickest, at up to six feet, followed by asphaltic concrete, which can also be up to six feet thick. Asphalt runways are generally laid over a bed of crushed stone and are usually over four feet thick. The thickness of the pavement, including the subgrade, can vary from 10 to 48 inches for heavy-duty commercial aircraft.

Concrete vs asphalt

The choice between concrete and asphalt for airport runways depends on various factors, including climate, traffic, and raw material access. While asphalt is more commonly used, concrete runways are also prevalent. Here is a detailed comparison between the two:

Composition

Concrete is formed using aggregates, which are rocks available in coarse (larger) and fine (smaller) sizes. Aggregates make up 60-75% of a concrete mix and are bound by a paste that constitutes the remaining 25-40%. This paste primarily consists of water (14-21%) and cement (7-15%), with 8% air that provides workability to the freshly mixed concrete. On the other hand, asphalt also contains aggregates but uses a different binding agent—a petroleum-based hydrocarbon mixture, resulting in a darker brown or black appearance.

Installation and Maintenance

Concrete is generally more expensive and time-consuming to install and repair than asphalt. It is also more prone to cracking, which further increases maintenance costs. However, concrete is more environmentally friendly due to asphalt's petroleum-based binder. Additionally, concrete does not deform in hot conditions, unlike asphalt, which is more flexible.

Usage

The decision to use concrete or asphalt depends on several factors. Concrete is often used in areas with heavy aircraft traffic and where the climate is hot, as it can withstand higher temperatures without deforming. Asphalt, being more flexible, is favoured in colder climates to resist freeze-thaw conditions. Additionally, asphalt is preferred for its higher friction, making it suitable for runways with frequent rainfall.

Examples

Many major airports, such as Charlotte, Louisville, and Sea-Tac, use concrete runways. Airports like Frankfurt and Dubai have asphalt runways, and some, like New York JFK, have a mix of both surfaces. Smaller airports and those with limited budgets may opt for asphalt due to its lower initial cost and quicker installation.

Composite pavements

The thickness of a runway is an important consideration, and composite pavements are generally designed to be more than 2.5 meters thick. This thickness is achieved through the construction of four distinct layers, each serving a specific purpose. The first layer, known as the subgrade, is the base layer and is composed of native soil that is meticulously compressed layer by layer to create a stable foundation. This is the thickest of all the layers.

The second layer, known as the graded granite aggregate base, is positioned above the subgrade and includes a specific mix of coarse to medium-grained materials such as sand, gravel, and crushed stone. These materials are combined in precise proportions to enhance the foundation's stability and drainage capabilities.

The third layer is the cement-treated base (CTB), which combines granular soil, aggregates, cement, and water to form a stabilized layer that adds significant strength and durability to the runway structure.

Finally, the topmost layer is the hot mix asphalt concrete, which constitutes the runway's surface. This layer is crucial for handling thermal expansion and contraction and providing good friction for landing and takeoff activities. It consists of asphalt mixed with aggregates, creating a smooth, flexible, and durable surface.

Subbase and base layers

The subbase and base layers of an airport runway are crucial components that provide a stable foundation for the entire structure. These layers are typically made of crushed stone, gravel, or other aggregates, and their primary functions include load distribution, drainage enhancement, and surface settlement prevention.

The subbase layer, also known as the subgrade, is the thickest layer in the runway construction. It is meticulously compressed layer by layer, using native soil, to create a stable and robust foundation. This layer is essential in bearing the weight of aircraft and preventing the runway surface from deforming over time.

Positioned above the subbase layer is the base layer, also known as the graded granite aggregate base. This layer consists of a specific mix of coarse to medium-grained materials, including sand, gravel, and crushed stone. These materials are combined in precise proportions to enhance the foundation's stability and improve drainage. This layer adds strength and durability to the overall runway structure.

The preparation and compaction of these layers are crucial to ensuring the stability and longevity of the runway. The subbase and base layers work together to distribute the aircraft's weight evenly, preventing the formation of weak spots that could compromise the runway's integrity.

By incorporating these layers, the runway can handle heavier aircraft loads and frequent usage without compromising its structural integrity. These layers also play a vital role in drainage, preventing water accumulation that could weaken the runway structure over time.

The subbase and base layers are carefully constructed to meet the specific requirements of the airport and ensure the runway can withstand the demands of modern air travel. These layers form the backbone of the runway, providing a solid foundation for the surface layers and contributing to the overall safety and functionality of the airfield.

Runway maintenance

Procedures and Practices

• Regular inspections: The integrity of runway surfaces is assured through regular inspections, which help identify issues and determine the need for renewal of the top surface.
• Renewal of the top surface: The wearing surface needs to be renewed periodically, and the interval between renewals depends on the type of surface and weather conditions. For example, asphalt can be damaged by snow and high temperatures.
• Rubber removal: Rubber left by aircraft during takeoff and landing reduces the adherence of the runway, affecting stopping distances. This is usually removed using high-pressure water.
• Visual aid maintenance: The approach light systems and other visual aids need to be cleaned regularly using methods such as water and sodium bicarbonate or dry ice. The quality of these lights is also checked, and replacements are made as needed.
• Marking maintenance: Runway markings, such as paint, may need to be redone periodically.
• Pavement evaluation: Trucks equipped with sensors are used to evaluate the structural condition of the runway and create a map of its condition.
• Foreign object detection: Airports use fire trucks or sweepers to regularly check the runway for foreign objects, debris, grass, dirt, and pebbles.
• Crack repair and joint resealing: Visual inspections are conducted daily to look for cracks, and repairs are made as needed to prevent long-term water infiltration damage.
• Grass cutting: The grass around the runway is kept at a specific height to reduce debris that could be blown onto the runway and to minimize the presence of birds and other animals.
• Water evacuation: All water evacuation paths are kept clear to prevent water buildup, which can increase stopping distance and cause freezing issues.

Challenges and Strategies

One of the main challenges in runway maintenance is performing repairs and resurfacing without impacting airport traffic. Strategies to mitigate this include:

• Night maintenance: Many maintenance operations are conducted at night when there is less traffic, with some tasks divided into perfectly timed shifts between 11 PM and 5 AM.
• Partial closures: For longer maintenance projects, airports may close a portion of the runway while keeping the rest operational.
• Short maintenance operations: Instead of closing the runway for days, airports opt for multiple short maintenance operations to minimize disruptions.
• Slab replacement: Some airports, like Orly (ORY) in France, replace one slab of concrete on their runway every night, allowing the entire strip to be renewed gradually without disrupting traffic.

Importance of Maintenance

Proper runway maintenance is crucial for several reasons:

• Safety: Maintaining the runway helps prevent accidents and incidents, such as those caused by a lack of flight crew awareness of temporary closures or reduced friction due to rubber buildup.
• Efficiency: Regular maintenance can help reduce delays caused by closed runways, contributing to on-time departures and an improved passenger experience.
• Cost savings: Preventive and predictive maintenance can reduce the risk of equipment failure and lower lifecycle costs for the airport.
• Customer satisfaction: Effective runway maintenance, along with connected airport technology, can positively influence the overall passenger experience.

Runway construction

The thickness of a runway varies. Typically, runways are made of concrete or asphalt, with concrete being more common in the US. A standard thickness for reinforced concrete runways is around 4 feet, but this can vary depending on the weight-bearing capacity and specific construction parameters of the runway.

During construction, temporary closures of runways are necessary to ensure the safety of construction crews and aircraft. These closures can impact the assignment of aircraft to open runways by the FAA. To reduce the impact on surrounding communities, construction activities are planned to occur during the day, and runway usage may be adjusted to accommodate air traffic volumes with minimal disruption.

Additionally, the subgrade and soil conditions beneath the pavement play a crucial role in runway performance. Soft ground can create challenges for long-term performance, requiring solutions like Tensar geogrids to provide support and control differential settlement.

Frequently asked questions