What is Pavement Design Life?

Pavement design life is the period for which engineers design a pavement structure to carry its forecast aircraft traffic.

FAA AC 150/5320-6G defines two lives. In the FAARFIELD design program, design life is the structural life. This is the number of load cycles a pavement carries before it fails structurally. Functional life is the time the pavement gives acceptable service. Indicators such as foreign object debris, skid resistance and roughness measure it. A pavement can keep much of its functional life after a structural failure.

Structural life and functional life

Structural failure has a different meaning for each pavement type (AC 150/5320-6G, section 3.10):

• Rigid pavement. Structural failure occurs when the concrete panels have extensive load-related cracking.
• Flexible pavement. Structural failure occurs when the subgrade is no longer protected from load damage.

Both types have the same FAA minimum design life. A pavement can still serve aircraft after it fails structurally, so the functional life can be longer than the structural life. Pavement design in FAARFIELD sets the thickness of each layer so that the structure lasts for the design life.

Design life values in FAA AC 150/5320-6G

• A new pavement on a federally funded FAA project: minimum 20 years.
• A rehabilitation project: minimum 10 years.
• An overlay: 20 years from the time of overlay. A shorter life is possible, to a minimum of 10 years. This needs an original pavement more than 15 years old and an overlay for functional rehabilitation.
• A phased project: a temporary pavement for 1 to 2 years can be enough.
• Large and medium hub airports: a longer life can be appropriate when the traffic forecast is accurate and the airport layout will not change.
• Smaller airports: the AC advises no more than 20 years, because future traffic and layout are not known.

The AC does not set a separate figure for flexible and rigid pavements. A life-cycle cost analysis (LCCA) can support a period other than 20 years. On a federally funded project, the engineer’s report must give the reason for the design period.

How design life enters the design

FAARFIELD multiplies the annual departures by the design life to get the traffic that the pavement must carry. The AC gives the formula N = (1 + r × L / 200) × NA × L. N is the total departures and NA is the annual departures. L is the design life in years and r is the growth rate in percent. The AC gives two examples. 250 annual departures with no growth over 20 years are 5,000 total departures. 225 annual departures at 1 percent growth over 20 years are 4,950.

The program then checks the structure with the cumulative damage factor (CDF). The CDF is a form of Miner’s rule. For each aircraft in the traffic mix, it divides the applied load repetitions by the repetitions that cause failure. The total CDF is the sum over all aircraft. A CDF of 1 means that the structural fatigue life is fully used. FAARFIELD adjusts the thickness until the CDF is 1 for the design traffic mix and the design life.

What decides whether a pavement reaches its design life

The design life is a design input, not a guarantee. The AC lists the factors that decide whether a pavement reaches it:

• The actual aircraft mix, compared with the traffic in the design analysis.
• The initial quality of materials and construction.
• Timely routine and preventive maintenance.

The AC records that airport pavements have historically performed well for 20 years (report DOT/FAA/AR-04/46). It also says that no pavement structure performs for the desired life without quality materials, proper installation and maintenance. Section 4.2 adds that a flexible pavement can last beyond the 20-year structural life. This needs quality materials, quality construction, timely crack sealing and surface treatments.

Maintenance extends the functional life. AC 150/5320-6G section 3.10.5 lists the work:

• Flexible pavement. Routine crack sealing, seal coats and small patches.
• Rigid pavement. Crack sealing, joint sealant repair or replacement, isolated panel replacement and partial depth spall repairs.
• Both types. Wear and weather can still require rehabilitation. This is a mill and overlay on a flexible pavement or surface diamond grinding on a rigid pavement.

AC 150/5380-7B says that routine maintenance is required to preserve the pavement for its design life. Examples are yearly crack sealing and daily inspection. The preventive maintenance term covers the treatments that slow deterioration, and the pavement rehabilitation term covers the work after the pavement reaches a poor condition.

Overloads and design life

Loads larger than the design load use up the life faster. Smaller loads use it up more slowly. ICAO Annex 14 states this in guidance in Attachment A (section 19.1), and AC 150/5335-5D repeats it. Except for massive overloading, a pavement does not fail suddenly or catastrophically.

ICAO suggests these criteria for overloads that do not justify a detailed analysis:

• Occasional movements by aircraft with an ACR up to 10 percent above the reported PCR should not harm a flexible or a rigid pavement.
• The annual number of overload movements should not exceed about 5 percent of the total annual movements, not counting light aircraft.

ICAO also advises that overload movements should not normally be permitted on a pavement that shows distress or failure. The authority should review the pavement condition regularly. The pavement strength term explains how the PCR is reported.

Life-cycle cost and the analysis period

The analysis period is not the design life. It is the time over which an LCCA compares alternative pavement sections. The AC requires that this period includes at least one rehabilitation of each alternative. The AC tells the analyst to base the salvage value on the remaining functional life of an alternative at the end of the analysis period. It also says that routine maintenance costs, such as incidental crack sealing, have a marginal effect on the net present value.

Remaining life and inspection

A Pavement Condition Index (PCI) survey rates a pavement from 0 (failed) to 100 (new). The rating uses the type, severity and quantity of distress. ASTM D5340 sets the method. One survey gives the condition now. Repeat surveys give the rate of deterioration, and a pavement management program can project it.

AC 150/5380-7B describes the usual pattern. A pavement performs well for most of its life. Then it reaches a critical condition and deteriorates rapidly. The number of years in good condition depends on construction type and quality, use, climate and maintenance. The AC says that it is hard to know when a pavement reaches the critical condition. It also says that preservation of a good pavement costs four to five times less than rehabilitation of a fair or poor pavement.

A survey of the surface has limits. The AC notes that a pavement with environmental cracks can also be in the early stage of load damage. Only testing detects that damage. For a concrete pavement, the structural condition index (SCI) counts only the load-related distresses of the PCI. The FAA defines an SCI of 80 as structural failure of a rigid pavement.

For the ACR-PCR system, the life period is the design life for a new pavement and the remaining life for a pavement in service. AC 150/5320-6G uses a related measure for an overlay of a rigid pavement. This is the percent CDFU, the estimated share of the fatigue life that traffic has used. It applies only when the existing pavement has an SCI of 100. The AC warns that a CDFU of 31 percent does not mean that 31 percent of the original structural design life is used. It means that the pavement received 31 percent of the traffic passes that the model predicts before the first structural defect. The pavement still has significant structural life at that point.

Design life on an airfield

The age of a pavement alone does not show how much life remains. The traffic mix, the construction quality, the climate and the maintenance change the result. Repeat condition surveys of the same sample units show the rate of deterioration.

TarmacView plans pavement survey flights and captures the photos today. Crack detection and the ASTM D5340 distress rating are in development and offered as early access. The blog post pavement design life and when an overlay is due shows how a series of surveys helps.