UFC 3-260-02

30 June 2001

layer thicknesses. For each material in the assumed section, the modulus of elasticity (E) and Poisson's

ratio () are determined. The design flexural strength (R) of the concrete is also determined. The

aircraft parameters are defined beginning with the first aircraft (AC1) in the list of aircraft. The

parameters required for the response model are tire contact area, tire loading, number of tires, and tire

spacing. Traffic volume is expressed in terms of coverages. The elastic parameters for the materials,

the layer thicknesses, and the aircraft parameters for the first aircraft are input into the response model

(JULEA computer code) to calculate the tensile stress (F1) in the concrete resulting from loading the first

aircraft. The computed stress is used along with the concrete design strength to compute a design

factor for the first aircraft (DF1). The design factor is input into the performance model to determine the

allowable traffic (N1) in terms of coverages for the first aircraft on the assumed pavement section. The

damage caused by the first aircraft is computed by dividing the applied traffic by the allowable traffic, i.e.,

n1/N1 . The damage caused by the first aircraft is then added to the damage caused by subsequent

aircraft. After computing the damage for the first aircraft, the procedure is repeated for other aircraft.

After completing the damage computations for all aircraft, the computed cumulative damage is

compared with unity. If the assumed section gives a computed cumulative damage substantially

different from unity, then a new section is assumed and the procedure repeated for all aircraft. After

computing the damage for two sections, a plot of log damage as a function of pavement thickness can

be used to estimate the required thickness and used as the assumed section for the next iteration. By

updating the plot, the thickness yielding a cumulative damage approximately equal to unity can quickly

be established.

4.

MATERIAL CHARACTERIZATION.

a. Portland-Cement Concrete (PCC).

(1) General. The effects of repeated load on PCC modulus of elasticity are not considered

because of the complexity of the relationship between modulus of elasticity and repeated loads and the

apparently small magnitude of change caused by traffic. There may be some decrease in modulus

because of repeated loads or exposure, but conversely, there should be some increase because of the

effects of long-term hydration. The net result is that the computation of the modulus of elasticity from the

stress-strain relationship obtained from the initial loading of a PCC specimen is considered adequate for

characterizing the material for the life of a pavement.

(a) Poisson's ratio for PCC normally receives very little attention. The range of statically

determined Poisson's ratio is only about 0.11 to 0.21, and the average of dynamically determined values

was about 0.24. Added factors are the difficulty of measurement and relatively small influence that

varying Poisson's ratio within a reasonable range has on the computed response. No procedures are

recommended for determining Poisson's ratio for PCC. It is recommended that a value of 0.15 be used

for all PCC.

(b) The magnitude of stress that can be sustained by PCC before cracking is a function of

the number of repetitions of the stress. This stress magnitude decreases as the number of stress

repetitions increases. The number of stress repetitions of a given magnitude that a material can sustain

is dependent on numerous factors, such as age, mix proportions, type of aggregate, rate of loading,

range of loading, etc. The most important, however, is the static strength of the material. The stress in

the slabs is due primarily to bending, and a flexural test is considered the most appropriate for

characterizing PCC.

(2) Modulus of elasticity and flexural strength. The modulus of elasticity Ef and flexural

strength R of PCC will be determined from static flexural tests of beams having a cross-sectional area

19-2

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