UFC 3-260-02
30 June 2001
allowable strain. Where the grouping cannot be reduced to a single group, then the concept of the
cumulative damage must be used in the design process.
4. MATERIAL CHARACTERIZATION. Characterization of the pavement materials requires the
quantification of the material stiffness as defined by the resilient modulus of elasticity and Poisson's ratio
and, for selected pavement components, a fatigue strength as defined by a failure criterion. Inasmuch
as possible, repeated load laboratory tests designed to simulate aircraft loading are used to determine
the resilient stiffness of the materials. For some materials, such as unbound granular bases and
subbases, an empirically based procedure was judged a better approach for obtaining usable material
parameters. Failure criteria have been provided; thus, fatigue testing will not be necessary. In general,
the use of layered elastic design procedures does not negate the material requirements set forth in
Chapters 7, 8, and 9. In particular, the gradation, strength, and durability requirements as stated must
be maintained.
a. Modulus of Elasticity.
(1) Bituminous mixtures. The term "bituminous mixtures" refers to a compacted mixture of
bitumen and aggregate designed in accordance with standard practice. The modulus for these materials
is determined by use of the repetitive triaxial test. The procedure for preparation of the sample is given
in Appendix F with the procedure for the conduct of the repetitive triaxial test given in Appendix G.
(a) The stiffness of the bituminous mixtures will be greatly affected by both the rate of
loading and by temperature. For runway design, a loading rate of 10 hertz is recommended. For
taxiway and apron design, a loading rate of 2 hertz is suggested. These loading rates are appropriate
for aircraft speeds of over 45 meters/second (100 miles/hour) on runways and less than 9 meters/second
(20 miles/hour) on taxiways and aprons. Specimens should be tested at temperatures of 44, 21, and
38 degrees Celsius (40, 70, and 100 degrees Fahrenheit) so that a modulus-temperature relationship
can be established. If temperature data indicate greater extremes than 4.4 and 38 degrees Celsius (40
and 100 degrees Fahrenheit), tests should be conducted at these extreme ranges if possible. The
modulus value to be used for each strain computation would be the value applicable for the specific
pavement temperature determined from the climatic data.
(b) An indirect method of obtaining an estimated modulus value for bituminous concrete is
presented in detail in Appendix H. Use of this method requires that the ring-and-ball softening point and
the penetration of the bitumen as well as the volume concentration of the aggregate and percent air
voids of the compacted mixture be determined.
(2) Unbound granular base- and subbase-course materials. The terms "unbound granular
base-course material" and "unbound granular subbase-course material" as used herein refer to materials
meeting grading requirements and other requirements for base and subbase for airfield pavements,
respectively. These materials are characterized by use of a chart in which the modulus is a function of
the underlying layer and the layer thickness. The chart and the procedure for use of the chart are given
in Appendix I.
(3) Stabilized material. The term "stabilized material" as used herein refers to soil treated with
such agents as bitumen, portland cement, slaked or hydrated lime, and fly ash or a combination of such
agents to obtain a substantial increase in the strength of the material. Stabilization with portland cement,
lime, fly ash, or other agent that causes a chemical cementation to occur shall be referred to as chemical
stabilization. Chemically treated soils having unconfined compressive strengths greater than the
minimum strength specified for subbases are considered to be stabilized materials and should be tested
11-5
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