30 June 2001
The beam center point deflection and applied dynamic load are measured immediately after
200 load repetitions for calculation of extreme fiber strain ,. The test is continued at the constant
stress level until the specimen fractures. The apparatus and procedures described have been found
suitable for flexural fatigue tests at temperatures ranging from 4.4 to 38 degrees Celsius (40 to
100 degrees Fahrenheit) and for extreme fiber stress levels up to 3.1 MPa (450 psi). Extreme fiber
stress levels for flexural fatigue tests at any temperature should not exceed that which causes
specimen fracture before at least 1,000 load repetitions are applied.
(3) A set of 8 to 12 fatigue tests should be run for each temperature to adequately
describe the relationship between extreme fiber strain and the number of load repetitions to
fracture. The extreme fiber stress should be varied such that the resulting number of load
repetitions to fracture ranges from 1,000 to 1,000,000.
f. Report and Presentation of Results. The report of flexural fatigue test results should
include the following:
(1) Density of test specimens.
(2) Number of load repetitions to fracture, Nf.
(3) Specimen temperature.
(4) Extreme fiber stress, F.
The flexural fatigue relationship is plotted in Figure M-2.
M-2. PROVISIONAL FATIGUE DATA FOR BITUMINOUS CONCRETE. Use of the graph shown in
Figure M-3 to determine a limiting strain value for bituminous concrete involves first determining a
value for the elastic modulus of the bituminous concrete. Using this value and the design
pavement service life in terms of load repetitions the limiting tensile strain in the bituminous
concrete can be read from the ordinate of the graph.