UFC 3-260-03
15 Apr 01
CHAPTER 5
EVALUATION OF FLEXIBLE PAVEMENT USING DIRECT SAMPLING
1. GENERAL. This chapter presents criteria for evaluating flexible pavements using data from direct
sampling. The data required for evaluation were presented in chapter 3. Computer programs are also
available for pavement evaluation and are discussed in chapter 9.
2. FACTORS LIMITING LOAD-CARRYING CAPACITY. The load-carrying capacity of a flexible pave-
ment is limited by the strength of its weakest component, either the bituminous pavement, base, sub-
base, or subgrade. The ability of a given subsurface layer to withstand the loads imposed on it depends
on the thickness and strength of material above it and its strength in its weakest condition. To be real-
istic, an evaluation must take into account possible future changes in moisture content and density as
well as the effects of freezing and thawing where pertinent.
3. SELECTION OF THICKNESS VALUES. The in-place thicknesses of the asphaltic concrete and
underlying layers are determined by actual measurement or from construction data. However, the mea-
sured thicknesses may be modified for use with the evaluation curves when the measured thickness
exceeds the required minimum thickness. Minimum thickness requirements are contained in TI 825-01/
AFM 32-1124 (I) NAVFAC DM 21.10. The excess thickness of asphalt is converted to an equivalent
thickness of base course and added to the existing base thickness. Then, any excess base-course
thickness is converted to an equivalent thickness of subbase and added to the subbase thickness. This
adjusted section is then used for evaluation. The equivalencies and their use are presented in
paragraph titled "Evaluation of Stabilized Layers."
4. SELECTION OF STRENGTH VALUES FOR SOIL LAYERS. The strengths of the subgrade and
overlying subbase and base courses are determined by means of CBR tests described in CRD-C 654,
ECP tests described in ASTM D 3441, or DCP tests described in FM 5-430-00-2 (AFJPAM 32-8013,
Vol 2). The quality of materials in the various layers of these courses can be determined by tests on the
materials in place, by laboratory tests on samples of the materials, and from construction data. The
CBR test results from an individual test pit will seldom be uniform, and the data must be carefully studied
to arrive at reasonable values for use in the evaluation. No rules or formulas can be given by which to
determine the number of values needed; rather, this is a matter of engineering judgment. A few guides
are mentioned in the following paragraphs that may assist in applying this judgment.
a. When the material is uniform, strength values should be determined at a minimum of five
locations.
b. When the uniformity of material and construction is not known, the number of test locations
should be sufficient to indicate that the values obtained are indeed representative of the area being
tested.
c. When materials and placement conditions are nonuniform, a relatively large number of test
locations will be required to obtain a representative value.
d. The study is usually accomplished by plotting test results on profiles or by arranging them in
tabular form to show the range of the data. In most cases, the value selected for use in the evaluation
should be on the conservative side. It should not be the lowest value in a range, but it should be a "low
average." When conditions are uniform, one method that may be used satisfactorily is that of taking the
lower quartile value from a cumulative distribution plot. Where conditions are not uniform, the following
example may be helpful.
5-1
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