FWD: AREA, subgrade modulus, effective SN and load transfer

AASHTO T 256 · ASTM D4694 · 1993 AASHTO Guide, Part III

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FWD: AREA, subgrade modulus, effective SN and load transfer

AASHTO T 256 · ASTM D4694 · 1993 AASHTO Guide, Part III

From one FWD drop: the basin’s AREA, the subgrade modulus, the effective pavement modulus and the effective structural number; or a joint’s load transfer.

Units US customary SI (metric)

Analysis

Load

Load, P, lbf

Plate radius, a, in.

Most FWDs have a 300 mm (11.8 in.) plate: radius 150 mm (5.91 in.).

Deflections

Center deflection, D₀, mils

Adjusted to the reference temperature for asphalt layers, as your agency does.

At 12 in. (305 mm), D₁₂, mils

At 24 in. (610 mm), D₂₄, mils

At 36 in. (914 mm), D₃₆, mils

Subgrade

Outer sensor offset, r, in.

A sensor far enough out to see only the subgrade, often the last one (60 in., 1,524 mm).

Deflection at r, d_r, mils

Structure

Pavement thickness above the subgrade, D, in.

All bound and unbound layers, from cores or records.

Joint

Specification

Required structural number, SN_f (optional)

Optional. The SN future traffic needs (the 1993 AASHTO flexible calculator gives it); the difference is the overlay’s share.

Example values: a 9,000 lb drop on a thick asphalt section. Enter the center deflection already adjusted to the reference temperature.

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