UFC 3-260-03
15 Apr 01
(50,000 pounds) depending on the FWD model being used. The requirements for the FWD test equip-
ment and test procedures should be in accordance with ASTM D 4694. The FWD uses velocity trans-
ducers to measure the pavement response to the applied load. The number of velocity transducers
depends on the manufacturer and ranges from a minimum of four to a maximum of nine. Deflections are
obtained by integrating the surface velocity measured by the velocity transducers.
b. Testing. In this procedure, the response of a pavement system to an applied loading is charac-
terized using deflection basin measurements. Since the time required to measure the deflection basin at
each testing point is short (2 to 4 minutes), a large number of measurements can be made during the
normal evaluation period. The various pavement configurations (sections) and construction dates should
be considered in the selection of NDT test locations. Thus, a thorough study of as-built pavement draw-
ings is particularly helpful in determining the testing program.
(1) Test Locations. On runways and taxiways, deflection basin measurements should be made
every 31 meters (100 feet) on alternate sides of the center line along the main gear wheel paths. For
flexible pavements, the offset is usually 3 to 3.5 meters (10 to 12 feet) from the center line. For rigid
pavements, the tests should be performed at the center of the slab or largest unbroken piece. For apron
areas, deflection basin measurements should be conducted in a grid pattern at 31- to 61-meter (100- to
200-foot) spacings. Additional tests should be made where wide variations in pavement response values
are found. A minimum of three deflection basin measurements should be conducted on all pavement
features. Figure 3-1 shows NDT test locations for a typical airfield.
(2) Test Requirements. At each test location, the NDT equipment is positioned, a load is
applied, and the resulting surface deflections at offset distances are determined. The magnitude of the
loading will be largely dependent on the NDT equipment used, the type of aircraft for which the evaluation
is being performed, and the pavement structure. The modulus of subgrade and base-course materials
are dependent on the applied stress level. NDT loading should be conducted at force levels near the
single-wheel design load of the design aircraft. The decision to use the 305- or 457-millimeter (12- or
18-inch) load plate depends on the contact pressure produced by the design load. Tests should be per-
formed with the plate that produces similar contact pressures as the design load. Only one deflection
basin is required at each test location; however, for impulse devices, it is recommended that three repe-
titions be applied at a particular force level. The first loading is considered a seating load, and the results
are disregarded. The second and third loadings should produce similar results. Results from the final
loading should be used in the evaluation. If inconsistencies are observed in the third test sequence, the
second load point can be used.
(3) Joint Load Transfer. The ability of joints in PCC slabs to transfer load can be measured
with an NDT device in the configuration shown in figure 3-2. The ratio of deflections measured on each
side of the joint is defined as the deflection ratio and is related to joint efficiency or load transfer. Joint
efficiency tests should be performed on a transverse joint and the longitudinal joint nearest the wheel path
at a minimum of 20 percent of the NDT test locations where PCC joint locations can be determined. Joint
transfer tests should be performed early in the morning before the PCC slabs expand or a temperature
gradient develops. Expansion, warping, and curling of PCC slabs due to changes in temperature can
significantly affect the performance of joints. At low temperatures, the joint opening is presumably widest
with less frictional resistance between slabs, and the load-transfer efficiency will be at a minimum. As the
temperature rises, the joint tends to close or lock up, and the load-transfer capability approaches a
maximum. Reference point tests should be used to establish a relationship between air temperature and
the deflection ratio from NDT such that adjustments can be made to test results collected over a wide
range of temperatures. A reference slab should be selected within each feature to be tested on a given
day. Joint tests should be conducted on each reference slab at 1- to 2-hour intervals throughout the
testing period, or at closer intervals if the testing period is less than 4 hours on a given feature.
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