When an aircraft touches down on a runway at high speed, the friction between its tires and the pavement surface is the single most critical factor determining stopping distance and directional control. Over thousands of landing cycles, rubber deposits accumulate on runway surfaces, steadily degrading this essential friction. Construction professionals involved in airport infrastructure projects need a working knowledge of surface restoration techniques such as shotblasting to maintain safe operational conditions. This article examines the science behind runway friction loss, the shotblasting method in detail, a real-world case study from San Antonio International Airport, and the measurement protocols used to verify results. For contractors evaluating the precision of their site measurement tools, understanding the difference between Digital Levels Vs Bubble Levels Which Tool Is right for specific applications can improve the accuracy of pavement surveys and quality control checks before and after surface treatment.
The Problem of Rubber Buildup on Airport Runways
Every time an aircraft lands, its tires skid briefly on the pavement at the moment of touchdown. This skidding action wears off microscopic particles of rubber, which adhere to the runway surface. Over time, these deposits build up into a smooth, continuous layer that dramatically reduces the coefficient of friction between tires and pavement.
How Rubber Contamination Affects Braking Performance
The physics of tire-pavement interaction depends on two scales of surface texture:
- Macrotexture refers to the large-scale surface irregularities that allow water to escape from beneath the tire during wet conditions. Adequate macrotexture prevents hydroplaning.
- Microtexture refers to the fine-scale roughness of the aggregate particles at the pavement surface. This provides the actual contact points that generate friction at the molecular level.
Rubber deposits fill in both macrotexture and microtexture features, smoothing the pavement surface. The result is a measurable loss of friction that, if left unchecked, leads to longer stopping distances, reduced directional control during crosswind landings, and increased risk of runway excursion incidents.
FAA Regulatory Requirements for Runway Friction
The Federal Aviation Administration (FAA) provides clear guidance on runway friction management through Advisory Circular 150/5320-12C. Airport operators must:
- Monitor friction levels on a routine schedule using FAA-approved Continuous Friction Measuring Equipment (CFME).
- Establish baseline friction values for each runway surface type.
- Identify action thresholds at which maintenance or restoration is required.
- Remove rubber buildup when friction levels fall below FAA minimums for safe aircraft operations.
- Verify the effectiveness of rubber removal through post-treatment friction testing.
These requirements are not optional. Airports that fail to maintain adequate friction levels risk FAA enforcement actions, increased insurance premiums, and liability exposure in the event of an incident.
Three Methods for Rubber Removal on Runways
Airport pavement engineers have three primary options for removing rubber deposits and restoring friction. Each method has distinct operational characteristics, cost profiles, and effectiveness levels.
Shotblasting: Mechanical Abrasion with Steel Media
Shotblasting is a mechanical process in which a high-velocity stream of steel pellets, or shot, is propelled onto the pavement surface. The impact of the shot abrades the surface layer, removing rubber deposits and simultaneously creating fresh texture in the pavement. This is the only method that improves both macrotexture and microtexture simultaneously.
Key advantages of shotblasting include:
- The steel media is contained within a closed-loop recovery system and recycled multiple times, minimizing waste.
- Removed material (rubber dust, pavement debris) is captured in the containment system for proper disposal at permitted sites.
- The process is environmentally friendly since no chemical solvents are introduced to the pavement or surrounding environment.
- Shotblasting creates additional surface texture that improves both water evacuation and tire contact.
High-Pressure Waterblasting
Waterblasting uses jets of water at extremely high pressure to strip rubber from the pavement surface. This method is effective at removing rubber but is generally less aggressive than shotblasting in terms of texture restoration. Waterblasting primarily removes surface contaminants without substantially improving macrotexture. It may be preferred in situations where pavement preservation is the primary concern and only light rubber removal is required.
Chemical Cleaning
Chemical cleaning involves applying solvents or chemical agents to the pavement that break down rubber deposits through chemical reaction. After a designated dwell time, the chemicals and dissolved rubber are washed away with lower-pressure water. This method does not alter the pavement surface texture at all, making it suitable only for situations where the existing texture is adequate and only the rubber layer needs removal. Chemical methods also introduce the additional concerns of runoff management, environmental permitting, and worker safety around chemical handling.
Case Study: San Antonio International Airport Runway 13R/31L
The San Antonio International Airport in Texas required retexturing of Runway 13R/31L, one of its primary operational runways. The airport authorities contracted Skidabrader, a specialist in surface abrasion services, to perform shotblasting across the affected pavement areas. The project provides an instructive example of how shotblasting is specified, executed, and verified in a real airport environment.
Project Execution and Third-Party Verification
San Antonio International Airport requires that all rubber removal contractors engage an independent third party to perform friction measurements before and after treatment. Skidabrader retained The Transtec Group, a pavement engineering firm with extensive experience in friction testing and data collection, to carry out the testing protocol.
Josh Jones, transportation market manager for Blastrac/Skidabrader, commented on the collaboration: “There was no question in the trust and professionalism you get working with Transtec. Transtec is at the forefront of the industry when it comes to data collection. Everyone within the firm has extensive knowledge of pavement design and the different remedies for pavement distress.”
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The GripTester CFME Device
Friction measurements were conducted using the GripTester, an FAA-approved CFME device listed in Advisory Circular 1150/5320-12C. The GripTester is a fixed-slip continuous friction measuring device that provides a friction reading, expressed as a GripNumber, approximately every three feet over the surface tested.
Operational characteristics of the GripTester include:
- Towed configuration: The device is towed behind a vehicle at the test speed, typically 40 mph for runway friction surveys.
- 14 percent slip ratio: The test tire rotates 14 percent slower than a free-rolling tire, generating a consistent drag force that quantifies friction.
- Self-wetting system: The device applies a 1 mm (0.04 inch) water film just in front of the test tire, simulating worst-case wet-pavement conditions.
- Multi-track capability: For runway testing, friction is measured at 10 feet and 20 feet on each side of the runway centerline to capture the full touchdown zone profile.
Results are reported in 500-foot intervals, allowing engineers to identify specific zones of rubber accumulation versus areas with adequate friction.
Friction Test Results and Performance Verification
Transtec conducted GripTester runs on Runway 13R/31L before shotblasting and again the day after completion. All runs were performed in both directions at 40 mph, 10 feet to the right of the runway centerline. Data collection began and ended at the first touchdown zone markings, ensuring the measurements captured the areas with the most severe rubber buildup.
Pre-Treatment Versus Post-Treatment Friction Levels
The friction results were evaluated against the thresholds specified in FAA Advisory Circular 150/5320-12C, Section 3-19. The following table summarizes the performance categories used and the outcomes observed.
| Friction Level Category | FAA GripNumber Threshold | Pre-Treatment Status | Post-Treatment Status |
|---|---|---|---|
| New Design/Construction Minimum | 0.52 | Below threshold | Achieved and exceeded |
| Maintenance/Planning Level | 0.42 | Below threshold | Restored above threshold |
| Alert Level (Immediate Action Required) | 0.32 | Below threshold in touchdown zones | Restored well above threshold |
The post-shotblast friction levels exceeded FAA required minimums for new design and construction, meaning the treated pavement performed as well as or better than a brand-new runway surface in terms of friction.
Why Shotblasting Succeeded Where Other Methods Might Not
The key differentiator in this project was shotblasting ability to restore both macrotexture and microtexture. The steel shot abrasion process physically removes a thin layer of the pavement surface, exposing fresh aggregate and creating new asperities at both scales. Waterblasting alone would have removed the rubber but left the underlying pavement texture largely unchanged. Chemical cleaning would have dissolved the rubber without affecting the surface at all.
For airport infrastructure projects involving runway approach zones, taxiways, and retaining walls, the engineering coordination required is substantial. The Fort Lauderdale Hollywood International Airport South Runway Construction retaining wall project demonstrates the complexity of structural elements that support runway pavement systems in modern airport environments.
Ongoing Friction Management Recommendations
The San Antonio project confirms that periodic shotblasting is an effective strategy for maintaining runway friction over the life of the pavement. Airports should implement a routine friction management program that includes:
- Quarterly friction surveys using FAA-approved CFME equipment, with baseline comparisons to detect trends before friction drops below action levels.
- Documentation of rubber accumulation patterns, which often concentrate in the first 3,000 feet of the touchdown zone and are more severe on runways serving heavy wide-body aircraft.
- Pre-treatment and post-treatment friction testing to verify the effectiveness of rubber removal and surface restoration.
- Selection of the appropriate removal method (shotblasting, waterblasting, or chemical cleaning) based on the type and severity of contamination and the condition of the underlying pavement.
- Coordination with airfield operations to schedule surface treatment during overnight or low-traffic periods to minimize disruption to flight schedules.
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Conclusion
Shotblasting is a proven, FAA-compliant method for restoring runway friction that has been degraded by rubber accumulation. The San Antonio International Airport case study demonstrates that properly executed shotblasting can restore friction levels above the thresholds required for new pavement construction, providing a margin of safety that exceeds regulatory minimums. For construction professionals working on airport pavement projects, understanding the capabilities and limitations of shotblasting relative to waterblasting and chemical cleaning is essential for specifying the appropriate treatment. With routine monitoring and timely intervention, airports can maintain the friction levels necessary for safe aircraft operations throughout the pavement service life.