**Table 3. Poisson's ratio for the material layers**.

*Poisson's ratio*

*Condition*

**Asphalt**

Layer temperature is less than 2.0C

0.30

Layer temperature is greater than or equal to 2.0C and less than or

0.35

equal to 1C

Layer temperature is greater than 1C and less than or equal to 8C

0.40

Layer temperature is greater than 8C

0.45

**Concrete**

0.15

Constant for all conditions

**Soil**

0.33

Thawed, volumetric ice content is less than 0.005

0.35

Frozen, volumetric ice content is greater than or equal to 0.005

ments within the same material type are combined

depth information related to the points where the

into a single layer if the modulus of the deeper

stresses and strains are to be computed, and 3) a

element is less than 20% different from the modu-

model number telling NELAPAV which form of

lus of the upper sublayer. A "weighted average"

the modulus equation to use for each material.

modulus of the two elements is then determined,

In the program, a 4082-kg (9000-lb) load was

with the weighting based on their relative lengths.

applied to a radius of 15.0 cm (5.91 in.), which

The modulus of the next lower finite element is

approximates the area of a standard set of dual

then compared with the modulus of the upper ele-

wheels or a falling weight deflectometer (FWD)

ment. The checking and combining process con-

testing plate. In all cases, stresses and strains were

tinues until an element modulus is outside of the

computed beneath the center of the load. For flex-

20% limitation or if a layer of a different material

ible pavements, the stress state at two points were

or a different frozen/unfrozen state is encountered.

analyzed: at the bottom of the pavement layer,

In this manner, a particular material layer in the

and at the top of the subgrade. For rigid pave-

pavement profile may be divided into several

ments, stress was computed only at the bottom of

sublayers. During the process of combining ele-

the pavement. In all cases the point of computa-

ments with similar modulus values, the thickness

tion was 0.01 in. from the interface between mate-

of each sublayer is also determined, as well as a

rials.

weighted average of its other properties such as

temperature, density and Poisson's ratio.

**NELAPAV**

Typically, the pavement profile that was di-

NELAPAV is an acronym for **N**onlinear **E**las-

vided into 99 finite elements for the FROST pro-

tic **L**ayer **A**nalysis for **PAV**ements. It computes

gram is combined by TRANSFORM into 5 to 20

stresses, strains, and displacements at any point in

sublayers with similar resilient modulus values.

an n-layered pavement system. The mainframe

During the winter and spring a larger number of

computer version of the program was developed

sublayers is more prevalent than in the summer

by Lynne Irwin of Cornell University and Gregor

months. TRANSFORM creates an additional "in-

Fellers of CRREL in 1980. The microcomputer

finite" layer beneath the modeled column for pass-

version was developed by Irwin and Daniel Speck

ing to NELAPAV, which has its properties set the

at Cornell University in 1984 and 1985 (Irwin and

same as those for the bottom modeled sublayer.

Speck 1986). The program is an adaptation of the

Additional items must be input to (or generated

Chevron Layered Elastic Systems program

from) TRANSFORM in order for it to create in-

(CHEVLAY).

put files for NELAPAV. They include 1) loading

Irwin and Speck (1986) describe the computa-

information such as the total load on a specified

tional approach used by NELAPAV and define

loaded radius, or load pressure, 2) location and

the following terms. The term *state *of a point in a

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