The test specimens prepared from the laboratory mixtures described earlier in subsection 4.1.3 were also subjected to the dynamic modulus (|E*|) and uniaxial T/C fatigue tests only. The flow number (FN) and SCB tests were not conducted here either because the results of these tests from the field cores gave clear and reliable conclusions. Concentration here is given to the resistance of asphalt concrete mixtures to fatigue damage.
4.5.1 Dynamic modulus (|E*|) test results
The raw data collected from the |E*| test of the laboratory mixtures (L-Mar-Pen, L- Spav-Pen and L-Spav-22E) were used to develop their master curves using the same sigmoidal fitting function used before in this study. The objective of this test was to assess the stiffness and the temperature susceptibility of different AC mixtures mixed, compacted and prepared in the laboratory. Table 35 and Figure 71 show the shift parameters and the constructed master curves of each laboratory mixture, respectively.
Table 35. Shift parameters of master curves for the laboratory mixtures.
Mix Code δ α β γ A b c
1 L-Mar-Pen 1.006 3.386 -1.151 0.564 0.0004 -0.1396 2.678
2 L-Spav-Pen 1.110 3.242 -1.155 0.617 0.0004 -0.1430 2.661
137
Figure 71. Dynamic modulus master curves for the laboratory mixture specimens.
According to the developed master curves of the laboratory mixtures, the dynamic modulus (|E*|) values for all of them are high at low reduced-time values. In addition, the use
of PMB in a Superpave laboratory mixture flattened its master curve and reduced the effect of temperature and frequency on the stiffness significantly.
Figure 71 also shows that the master curves of the laboratory mixtures with unmodified bitumen (L-Mar-Pen and L-Spav-Pen) are very similar.
4.5.2 Uniaxial T/C fatigue test results
After the dynamic modulus (|E*|) test, the laboratory mixtures (L-Mar-Pen, L-Spav-Pen and L-Spav-22E) were tested under controlled-strain uniaxial T/C fatigue test. The test was performed under the same conditions assumed for the field cores and field mixtures (20 °C and 10 Hz). Similarly, each laboratory specimen was tested under strain amplitude L1 (55 µε) and then strain amplitude L2 (130 µε), and both strain amplitudes were high enough to exceed the LVE region and cause fatigue damage. Most of the tested specimens with 60-70 Pen bitumen failed early, before the end of the first fatigue test, and did not complete the 200,000 cycles. Therefore, it was decided to analyse each uniaxial T/C fatigue test independently. The dynamic modulus and phase angle values from the first cycle interval (N = 10) in each fatigue test data
138 were considered as the LVE dynamic modulus (|𝐸∗|
𝐿𝑉𝐸) and the LVE phase angle (𝜑𝐿𝑉𝐸),
respectively.
Based on the raw data obtained from the fatigue test conducted on the laboratory mixtures, the target strain amplitude (55 µε or 130 µε) was not achieved but remained constant during both tests (L1 and L2). Figure 72 shows the average strain amplitude applied on the tested specimens during fatigue tests (L1 and L2). This will surely affect the reliability of the results if it is not considered using a suitable fatigue characterisation approach.
Figure 72. Average strain amplitude applied to the tested laboratory mixtures.
The major outputs of each fatigue test were dynamic modulus (|E*|) and phase angle (φ) against number of cycles. The results for each specimen of the laboratory mixtures tested under strain amplitude L1 are presented in Figure 73.
139
Figure 73. Dynamic modulus and phase angle results for the laboratory mixtures tested under strain amplitude L1 test.
According to Figure 73, the specimens of each laboratory mixture showed a similar reduction rate of modulus and increment of phase angle but different fatigue lives, especially in the mixtures with unmodified bitumen. In addition, damage happened in the laboratory mixtures during this first fatigue test (L1) and affected the specimen in the second fatigue test (L2). However, this is not revealed by the dynamic modulus or phase angle curves. Therefore,
140 these results alone are not enough to evaluate the performance of these mixtures against fatigue cracking. A fatigue characterisation approach should be implemented in order to identify the fatigue life of each laboratory mixture and compare them.
The results for each specimen of the laboratory mixtures tested under strain amplitude L2 are presented in Figure 74.
Figure 74. Dynamic modulus and phase angle results for the laboratory mixtures tested under strain amplitude L2 test.
141 Similar to the results of the fatigue test under strain L1, the specimens of each laboratory mixture showed a close reduction rate of dynamic modulus and increment of phase angle but different fatigue lives. All specimens of L-Mar-Pen and L-Spav-Pen mixtures failed early in the second test under strain amplitude of 130 µε, as shown in Figure 74, and this generally shows the superiority of the use of PMB in a Superpave mixture (L-Spav-22E). Since the traditional interpretation (E* vs. N) was giving undependable results and conclusions, an advanced fatigue characterisation approach was used to analyse the uniaxial T/C fatigue test data. This is described in Chapter 5.
4.6 Conclusions
After the investigation of asphalt pavement structures against different distresses and deteriorations in Chapter 2, it was important to assess the performance on the mixture level. Therefore, and with the aim of assessing the performance of different AC mixtures against rutting, fracture, temperature susceptibility and fatigue damage, several field cores, field mixtures and laboratory mixtures were tested and evaluated in this chapter. The conducted tests were useful to characterise and assess the performance of the mixtures against several major distresses. The following are the main findings based on the results of these laboratory tests:
All flow number values obtained from the field cores are high, which indicates that these AC mixtures are expected to have high resistance to rutting.
Resistance to rutting for asphalt concrete mixtures was mainly affected by the bitumen grade, aggregate source and aggregate gradation. The high rut- resistance of asphalt mixtures can be achieved by a well-designed mixture with the use of polymer-modified bitumen and Gabbro aggregate
The fracture toughness (K) and maximum tensile stress (σmax) values of all field
cores’ mixtures are high, which reflects good performance against fracture cracking.
142
Results of the SCB test revealed that fracture cracking is not a major problem in the field cores’ mixtures.
The dynamic modulus master curves of all tested mixtures were mainly affected by the bitumen type/grade and aggregate type. In addition, the use of polymer- modified bitumen flattened the master curve of the asphalt concrete mixtures and reduced the temperature and frequency susceptibility on the stiffness and rut-resistance.
According to the fatigue raw data of all tested mixtures, the target strain amplitudes (L1 and L2) were not achieved all the time but remained constant during both fatigue tests. This will surely affect the reliability of the results if it is not considered using suitable characterisation and analysis approaches.
The replicate specimens of each field and laboratory mixture showed a similar reduction rate of dynamic modulus and increment of phase angle opposite to the field cores’ specimens, but different fatigue lives.
The preliminary results and the traditional interpretation of the data of the uniaxial T/C fatigue test could not be used directly to assess the performance against fatigue cracking. A suitable fatigue characterisation approach is needed. It is obvious from this part of the study that rutting and fracture cracking are not major distresses for mixtures in Qatar, especially if they are designed following the Superpave mix design with the appropriate modified bitumen content and Gabbro aggregate. However, fatigue damage is a main distress for pavement materials in Qatar, and its characterisation should be investigated in depth. The traditional method (E* vs. N) to interpret fatigue tests data was not
useful to characterise and evaluate mixtures against fatigue damage. Therefore, two advanced fatigue characterisation approaches were selected to be performed on the raw data obtained
143 from the T/C fatigue test of the tested specimens prepared from different mixtures. These two approaches are presented in detail and their results are discussed thoroughly in the following chapter.
144
5
Fatigue Damage Characterisation of Asphalt Concrete Mixtures
The uniaxial tension-compression fatigue tests conducted on the field cores, field mixtures and laboratory mixtures were discussed in the previous chapter using the |E*| and φ results obtained from the fatigue tests. However, these results were not enough to evaluate the performance of the mixtures against fatigue cracking accurately. Therefore, in this chapter, the dissipated energy (DE) approach and the viscoelastic continuum damage (VECD) approach, in their latest form, are introduced in detail and then implemented to analyse the fatigue test data, assess the different AC mixtures and predict their fatigue lives.