UFC 3-260-03
15 Apr 01
CHAPTER 2
EVALUATION CONCEPTS
1. RELATION OF DESIGN TO EVALUATION. The design of a pavement requires selecting materials
with the necessary strength and placing them at the proper thickness, density, and depth, so that the
pavement will be capable of carrying an anticipated number of passes of a given load. Because of vari-
ations in materials and placement conditions, the as-constructed pavement may have strengths and
thicknesses of layers greater or less than those required in the design process. Also, with time, usage,
and environmental impacts, the elements of a pavement contributing to its strength can be subject to
some change. Thus, an evaluation determines the physical properties of a pavement as actually built or in
its current condition and establishes therefrom the traffic/load-supporting capacity of the pavement.
2. CONCEPTS. The primary function of a pavement is to spread and distribute the wheel loads placed
on it. Each airfield or landing strip has its own natural soil and environmental conditions, and the in situ
soils must ultimately sustain the stresses resulting from loads applied to the pavement. Since the
strengths of native soils can vary widely from site to site, the ability to support loads also varies widely.
Except in special cases, aircraft tire loads cannot be satisfactorily sustained directly on the native soils.
a. Pavement Structure. Pavement design and evaluation are concerned with determining the
capability of the pavement structure to reduce the load intensity to a magnitude the airfield site soils can
sustain. The larger the load on the surface or the higher the contact pressure, the stronger the pavement
structure must be to distribute load and reduce load intensity (pressure or stress) to that which the native
soil can accept. Layered flexible pavements distribute load by broadening the effective area supporting
the load, from the tire contact area on the surface to a wider area on the base, to a still wider area on the
subbase, and so on. Each layer must be of a quality to sustain the load intensity or stress it must accept,
and each must be thick enough to broaden or distribute the load and reduce intensity to that which its
supporting layer can sustain. Rigid pavements are stiffer and have a "beam action" or flexural capability
that spreads or distributes load more widely, so these pavements can be much thinner than flexible pave-
ments. However, thickness, flexural strength, and other quality aspects must be assessed during the
evaluation process.
b. Loadings. Early aircraft were primarily supported on two main landing gear wheels, referred to
as "single" wheels. With the large increases in aircraft gross weights, landing gears have changed to twin
(2 per strut) wheel loadings, to twin-tandem (4-wheel) loadings, and to more complex (16 and 24 main
gear wheels, extra "belly" gear) wheel support systems. The two main wheels of single-wheel aircraft are
generally spaced far enough apart that there is no significant overlap of the distributed loads for even very
thick pavement structures protecting weak subgrades. For twin wheels, however, and closely spaced
tandem wheels or complex wheel groups, the patterns of distributed surface loadings at and near the
bottom of pavement structures overlap so that the intensities (pressures or stresses) combine between
adjacent wheels. This combining effect of load intensities is greater as the adjacent wheels become
closer.
c. Tire Pressure. The intensity of stress at a given point in a flexible pavement is affected by the
tire contact pressure, which, for large aircraft tires, is roughly equivalent to the inflation pressures. The
major difference in stress intensities caused by variation in tire pressure occurs near the surface; conse-
quently, the pavement surfacing and upper base-course layers are most seriously affected by high tire
pressures.
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