Introduction
EN 12697-24:2004 specifies the methods for characterizing the fatigue of bituminous mixtures using alternative tests, including bending tests and direct and indirect tensile tests. The tests are performed on compacted bituminous material under a sinusoidal loading or other controlled loading, using different types of specimens and supports. The procedure is used: a) to rank bituminous mixtures on the basis of resistance to fatigue; b) as a guide to relative performance in the pavement; c) to obtain data for estimating the structural behavior of the road; and d) to judge test data according to specifications for bituminous mixtures. Because this European Standard does not impose a particular type of testing device, the precise choice of the test conditions depends on the possibilities and the working range of the device used. For the choice of specific test conditions, the requirements of the product standards for bituminous mixtures need to be respected. Results obtained from different test methods or using different failure criteria are not assured to be comparable.4PB Testing Setup
- 50 KN EHF-U Servopulser system
- 4PB testing tool (Figure 2)
- QF-40 Hydraulic power supply
- 4830 Controller
- Windows software for 4830 controller
- 4PB Fatigue calculation files
Testing method
According to named standard this test is applied after the stiffness test (Stiffness test is a subject of No. SCA_300_065)
- based on principle of 4-point bending test,
- test sample: beams of 450 mm length, 50 mm x 50 mm or 70 mm x 70 mm,
- LVDT controlled deflection sine wave, maximum bending strain should be chosen in three different levels, so that fatigue lives are between 1E4 and 2E6 cycles. Test frequency 30 Hz.
- Definition of life time: when load peak values become half of values from first captured cycle in “constant deflection” loading mode.
Test conditions
- EN standard distinguishes between two
possible loading modes: “constant deflection”
and “constant force”. This method is made for
“constant deflection” loading mode.
2. Maximum amplitude for the bending strain of
105.19 µm, mean value 0 µm. Deflection is
controlled by LVDT sensor built in 4PB tool.
3. Sinusoidal loading wave of 30 Hz is applied.
4. Tests are performed at 20 ºC.
5. Force, Stroke and 4PB deflection are sampled
in 1000 cycles, from cycle 100 to 2.000.000.
6. Dynamic characteristic values are automatically
calculated by 4830 software and automatically
exported to CSV format.
7. Standard allows to stop the test when force
drops to half of initial amplitude. Initial
amplitude can be determined from the peak
graph (Figure 3) and half initial amplitude set
into the software, to stop the test at set value.
Results and postprocessing
Figure 5 shows Result window for one sample.
Calculated stiffness and phase angle are shown in
regard to the cycle number. A separate sheet is
created for each sample and a Result sheet with
final results (Figure 4).
The Results worksheet contains a table which
summarizes the individual results and additional
table for calculating the linear regression over the
logarithms of the result for the fatigue line, together
with the correlation coefficient of the regression
and the residual standard deviation. With the
regression results, the strain level for a fatigue life
at defined number of cycles (106) is calculated.
Fatigue line for the 4PB calculation is shown in
Fatigue Line worksheet, Figure 6. It contains graph
with the fatigue line and the individual measured
data points and the strain level for the fatigue life
of 106 cycles.
Summary
During the fatigue tests, the complex modulus continuously reduces and the phase angle continuously increases as shown in the Figure 5. The test can be terminated when the complex modulus reaches 50% of it original value. Generally three phases can be determined on the graph (Figure 5 and 7). With 2 million cycles done at 105.19 μm deflection, the third phase is not expressed. lt can be observed that the specimens initial complex modulus defined at 100 cycles was 6.703 MPa and 3.820 MPa at the last cycle. Curve on Figure 6 shows relation between fatigue failure criteria and strain amplitude. This fatigue data is expressed using the Whöler type fatigue line which was drawn by making a linear regression between the decimal logarithm of N and E: Log (Ni) = A0 + A1 Log (E) 2Reference
- EN 12697-24:2004, Bituminous mixtures –test methods for hot mix asphalt part 24: resistance to fatigue.
- Asphalt testing package v 1.1, Shimadzu Europa GmbH, 2019
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