TM 5-822-13/AFJMAN 32-1018
APPENDIX C
PROCEDURES FOR DETERMINING THE FATIGUE LIFE OF BITUMINOUS CONCRETE
C-l. laboratory Test Method.
a. General. A laboratory procedure for determining the fatigue life of bituminous concrete paving mixtures
containing aggregate with maximum sizes up to 1 inches is described in this appendix. The fatigue life of a
simply supported beam specimen subjected to third-point loadings applied during controlled stress-mode flex-
ural fatigue tests is determined.
b. Definitions. The following symbols are used in the description of this procedure:
(1)
= initial extreme fiber strain (tensile and compressive), inches per inch.
Extreme fiber strain of simply supported beam specimens subjected to third-point loadings, which produces
uniaxial bending stresses, is calculated from
12td
=
(3L2 - 4a2)
where
t = specimen depth, inches
d = dynamic deflection of beam center, inches
L = reaction span length, inches
a = L/3, inches
c. Test equipment.
(1) The repeated flexure apparatus is shown in figure C-l. It accommodates beam specimens 15 inches
long with widths and depths not exceeding 3 inches. A 3,000-pound-capacity electrohydraulic testing machine
capable of applying repeated tension-compression loads in the form of haversine waves for 0.1-second durations
with 0.4-second rest periods is used for flexural fatigue tests. Any dynamic testing machine or pneumatic pres-
sure system with similar loading capabilities is also suitable. Third-point loading, i.e., loads applied at distances
of L/3 from the reaction points, produces an approximately constant bending moment over the center 4 inches of
a 15 inch-long beam specimen with widths and depths not exceeding 3 inches. A sufficient load, approximately 10
percent of the load deflecting the beam upward, is applied in the opposite direction, forcing the beam to return
to its original horizontal position and holding it at that position during the rest period. Adjustable stop nuts
installed on the flexure apparatus loading rod present the beam from bending below the initial horizontal posi-
tion during the rest period.
(2) The dynamic deflection of the beam's center is measured with a Linear Variable Differential Trans-
former (LVDT). An LVDT that has been found suitable for this purpose is the Sheavitz type 100 M-L. The LVDT
core is attached to a nut bonded with epoxy cement to the center of the specimen. Outputs of the LVDT and the
electrohydraulic testing machine's load cell, through which loads are applied and controlled, can be fed to any
suitable recorder. The repeated flexure apparatus is enclosed in a controlled-temperature cabinet capable of
controlling temperatures within degree F. A Missimer's model 100 x 500 carbon dioxide plug-in tempera-
ture conditioner has been found to provide suitable temperature control.
d. Specimen preparation. Beam specimens 15 inches long with 3-inch depths and 3-inch widths are pre-
pared according to ASTM D 3202. If there is undue movement of the mixture under the compactor foot during
beam compaction, the temperature, foot pressure, and number of tamping blows should be reduced. Similar
modifications to compaction procedures should be made if specimens with less density are desired. A diamond-
blade masonry saw is used to cut 3-inch or slightly less deep by 3-inch or slightly less wide test specimens from
the 15-inch-long beams. Specimens with suitable dimensions can also be cut from pavement samples. The widths
and depths of the specimens are measured to the nearest 0.01 inch at the center and at 2 inches from both sides
of the center. Mean values are determined and used for subsequent calculations.
e. Test procedures.
(1) Repeated flexure apparatus loading clamps are adjusted to the same level as the reaction clamps. The
specimen is clamped in the fixture using a jig to position the centers of the two loading clamps 2 inches from the
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