FATIGUE AND HEALING PERFORMANCE OF BITUMINOUS
BINDERS AND MIXTURES FOR ROAD PAVEMENT – RESULTS OF
RE-ROAD PROJECT
W. Van den bergh 1, C. McNally 2, A. De Keersmaecker 1 and E. Fallon2
1 Applied Engineering Laboratory for Sustainable Materials, Infrastructure and Building,
Artesis University College of Antwerp, Paardenmarkt 92, 2000 Antwerp, Belgium – e-mail: wim.vandenbergh@artesis.be
2
School of Civil, Structural & Environmental Engineering, University College Dublin, Newstead, Belfield, Dublin 4, Ireland – e -mail: ciaran.mcnally@ucd.ie
Keywords: bitumen, mastic, asphalt, fatigue, healing, mechanical dynamic testing. ABSTRACT
An asphalt mixture is believed to be a partial self-healing material. For asphalt pavement design, this characteristic is taken into account as a shift healing factor. Healing during rest periods will result in a longer structural design life for the mixture in situ, compared to the fatigue life determined with a continuous sinusoidal test in the laboratory.
For the FP7 Re-Road project, an extensive research programme was completed in order to increase the recyclability of asphalt mixtures, in particular asphalt mixtures with polymer modified binder. One research task dealt about performance modelling of asphalt pavement containing Reclaimed Asphalt (RA). For this, fatigue and healing tests were performed on virgin and aged binders, mastics and asphalt mixtures. Bituminous binders and mastics (virgin and aged) were tested using a Dynamic Shear Rheometer (DSR) in oscillation model. The fatigue and healing properties of the asphalt mixtures were determined by means of a dynamic test on compacted asphalt samples. The binders, mastic and asphalt test results show a significant increment in fatigue life when rest periods are taken into account between fatigue cycles.
1. INTRODUCTION
An asphalt mixture is believed to be a partial self-healing material. For asphalt pavement design this characteristic is taken into account as a healing shift factor, representing the effect of rest periods between loadings on the fatigue life. The healing shift factor is mostly determined on asphalt mixtures by applying continuous and discontinuous fatigue tests. However, a standard test method is absent. An overall validated value is still not accepted since the performance of asphalt mixtures differ by bitumen origin, composition, production, climate and traffic history. One can only compare healing properties of different mixtures when identical test conditions (test procedure, temperature, frequency and loading) are selected. That’s why a wide range of healing factors are reported: from 1 to more than 100.
For the FP7 Re-Road project an extensive research programme was completed in order to increase the recyclability of asphalt mixtures, in particular asphalt mixtures with polymer modified binder. One of the work packages (WP5) dealt about performance modelling of asphalt pavement containing RA. In this WP5, fatigue and
healing tests were performed on virgin and aged binders, mastics and asphalt mixtures.
2. MATERIALS
The healing and fatigue mechanism can be considered in the asphalt mixture or in its components. Most fatigue and healing tests are performed in binder or asphalt. A compromise is to test mastic or mortar; this is a mixture of the bituminous binder with fine aggregates and filler. The effect of rest periods can be measured on virgin and aged cylindrical mortar specimens [1].
For the asphalt mixture composition, three asphalt mixtures of Stone Mastic Asphalt (SMA) were considered: the reference material (Material I) SMA 11 S with a polymer modified binder and two mixtures with respective 15 m% (Material II) and 30 m% (Material III) Reclaimed Asphalt.
In this study, the fatigue and healing was also evaluated on the binder and the mastic. An asphalt mixture can be considered as a mixture of aggregates with dimensions larger than 0.125 mm, coated with a thin binder film about 5 µm and bound by bituminous mastic. The mastic is defined as the mixture of filler and aggregates with dimensions smaller than 0.125 mm, and the binder. The binder quantity in the mastic is the volume of the binder in the asphalt mixture, decreased with the volume of the binder, that coats the aggregates larger than 0.125 mm with a binder film of 5 µm thickness. For the binder and mastic specimens, the same partial content of aged binder was taken into account as included in the asphalt mixtures e.g. the binder II+RA consists of 36 m% aged binder from RA and 64 m% virgin binder.
3. FATIGUE AND HEALING TESTS ON BINDER AND MASTIC
Bituminous binders and mastics were tested using a dynamic shear rheometer (DSR) in oscillation mode. In this setting, the sample is placed between two parallel plates (diameter 8 mm, gap 2 mm). A Fatigue test is defined as a constant stress-controlled loading signal on the sample until failure. Because of the limitations of DSR-loading signals, a specific healing procedure is developed as a continuous loop of two intervals: a fatigue period during 3 seconds (30 cycles)
In Figure 1, an example is given of the value of G* (complex shear modulus) in function of time during a healing test (30 cycles at 10 Hz) loading and 9 seconds rest of a binder of Material III. The results show that during loading cycles, the G*-value decreases and during unloading the cycles G*-value increases. The recovery of the complex modulus during rest periods is clearly shown.
The fatigue life criterion is defined as the top in the G*x number of cycles curve, expressed as the number of loading cycles. The effect of rest periods is not only observed in an increment of G*-value, but also as the extension of the fatigue life. In Figure 1 (b), the healing curve and fatigue curve are compared in function of the sample stress: at an equal sample stress rest periods will increase fatigue life or, for an equal fatigue life, rest periods will allow higher sample stresses. The healing shift factor is very high for all binders. Since there is no common applied sample stress between fatigue and healing curve, the factor cannot be quantified in function of sample stress. Quantification can be made as follows: for Binder I at an equal fatigue life (1000 and 10000 cycles) the implementation of rest periods allows increasing the sample stress with 230 kPa.
Figure 1: (a) Healing test, (b) fatigue (blue) and healing (red) curve of binder I For all binder and mastic types, rest periods increase fatigue life. For Mastic I, rest periods have a positive effect on fatigue life (factor 4.3 – 6.0). When Mastic III is taken, using the binder from RA, a slight increased fatigue life is noticed and healing factor 7.4 (only two tests).
4. FATIGUE AND HEALING TESTS ON ASPHALT
Tests were conducted on SMA samples with RA contents of 0% (Mix I), 15% (Mix II) and 30% (Mix III). The SMA has a maximum particle size of 11.2mm and was compacted using a roller compactor. Slabs with dimension 400mm x 305mm x 100mm were manufactured and the target bulk density was 2391 kg/m3, corresponding to an air voids content of 3%.
After allowing the compacted specimen slabs to cool to ambient temperature for 24 hours, six cylinders of nominal 100mm diameter were cored from the slab. A test specimen of nominal thickness 40 mm was then sliced from the middle of each cylinder using a circular saw.
The specimens were stored in a temperature controlled cabinet, which also contained the testing apparatus, at the test temperature of 10°C. The indirect tensile fatigue and healing tests were conducted using a Cooper Technology Servo-Pneumatic Universal Testing Machine NU-10 apparatus. The specimen was placed in a holding rig with loading strips in the vertical plane. A load was applied to the specimen in the vertical axis, while deformation was measured in the horizontal axis over the centre 40mm of the specimen.
The testing apparatus was controlled using the Cooper Technology Universal Software. This allowed customised fatigue testing profiles to be used and two such profiles were used. For the fatigue test, a loading duration of 0.1s and a test frequency of 10 Hz was used. A haversine waveform was employed and the stress level adjusted to give failure within a reasonable timeframe. For the healing test, a rest period of 0.1 seconds between each loading pulse was introduced to the test profile. The results of the testing programme are presented in Figure 2.
2,20E+07 2,40E+07 2,60E+07 2,80E+07 3,00E+07 3,20E+07 3,40E+07 0 3 6 9 12 15 18 21 24 27 30 33 36 G* [ P a ] Time [s] BII+RA HEA
FAT HEA FAT HEA FAT HEA
y = 4E+38x-12,09 R² = 0,8555 y = 8E+48x -15,06 R² = 0,8969 100 1000 10000 100000 1000000 400 600 800 1000 1200 F a ti g u e l ife
Sample Stress [kPa] Binder I FAT
Binder I HEA
Figure 2: Influence of RA content on fatigue life of asphalt mixtures
It can be seen that the materials tested had a very high resistance to fatigue and that significant numbers of load cycles were required to induce failure. The influence of RA content is very clear in that the addition of RA to the mixture resulted in the fatigue lines moving upwards. Similar observations can be made for the material tested using rest periods. It should however be noted that the number of tests conducted was low due to the long time required per test (several weeks for the more resilient materials). A follow on study using materials less resistant to fatigue is currently being implemented.
5. CONCLUSIONS
The implementation of rest periods after a loading interval will lead to an extended fatigue life and, during the rest periods, an increase of G* is demonstrated. A DSR allows the possibility of recording this process. It is possible to monitor the healing process during the individual rest periods as during the whole test. The improved fatigue resistance associated with using RA in asphalt mixtures was demonstrated; this will be quantified in later research.
ACKNOWLEDGMENTS
The authors would like to acknowledge the coordinator Bjorn Kalman and other members of the Re-Road project (FP7-project 2008-2013, grant 218747) for providing the materials and funding.
REFERENCES
[1] Van den bergh, W.: The Effect of Ageing on the Fatigue and Healing Properties of Bituminous Mortars, PhD. Thesis, TU Delft, The Netherlands, 2011.
