What is Fatigue Testing in Pavement Engineering?

What is fatigue testing?

Fatigue testing is a laboratory test that applies a repeated load to a pavement material and counts the load cycles until the material fails. Each wheel pass is one load cycle.

Fatigue cracking in the field has the same cause. FAA AC 150/5380-6C describes alligator (fatigue) cracking as cracks caused by fatigue failure of the asphalt surface under repeated traffic loading. The cracks begin at the bottom of the asphalt, where tensile stress and strain under a wheel load are highest. They then grow up to the surface.

The asphalt beam fatigue test

The best-known test for asphalt is AASHTO T 321, a four-point bending test on a beam. The FHWA laboratory manual gives these test conditions:

• The beam is about 380 mm long, 50 mm high and 63 mm wide. The test uses three replicate beams.
• The test holds the beam at 20 degrees C and loads it at 5 to 10 Hz.
• The machine holds a constant strain between 250 and 750 microstrain in each cycle. The load needed for that strain falls as the beam gets damaged.
• The test records the stiffness of the beam and multiplies it by the number of cycles (S x n). The test stops when that product is 15 percent below its peak.

The result is the number of cycles to failure at each strain level. A higher strain gives fewer cycles. Engineers must adjust laboratory results before they use them for field design. NCAT notes that bending beam tests generally underestimate field fatigue life, and that engineers apply a shift factor from 4 to 100.

NCAT also names 70 microstrain as a commonly used fatigue threshold for flexible pavement design. It is a value based on laboratory tests at one temperature, not a property of every mixture.

Fatigue in airfield pavement design

FAARFIELD, the design program in FAA AC 150/5320-6G, uses fatigue as a design limit. For flexible pavement it uses two predictors of pavement life. One is the horizontal strain at the bottom of the asphalt layers. The other is the vertical strain at the top of the subgrade. It expresses fatigue failure as a cumulative damage factor (CDF), a Miner’s rule model. The CDF is the sum of applied load repetitions divided by the allowable repetitions to failure for each aircraft. The thickness is adjusted until CDF = 1 for the design life.

For concrete, AC 150/5320-6G names flexural strength as the critical strength and refers to ASTM C78 to determine it. ASTM C78 is a static strength test, not a fatigue test.

What an inspection sees

A laboratory fatigue test does not replace a field inspection. The inspection finds where fatigue cracking has already appeared. PAVER records it as distress 41, alligator or fatigue cracking, and rates it by severity as part of the pavement condition index . AC 150/5380-6C also links fatigue to potholes: when fatigue cracks interlock and the loose pieces are picked out by wheel loads, a pothole can form.