The measurement of asphalt materials modulus is now routinely carried out in Australia and elsewhere using the Indirect Tensile Test (ITT) method. Likewise asphalt fatigue testing is routinely carried out in Australia using the beam flexure test method (Austroads, 2006a) to derive flexural modulus and fatigue performance parameters.
The stiffness of asphalt and corresponding response to load is significantly affected by the pavement temperature in service. Figure 8 and the predictive models illustrate the moduli for dense graded asphalt over the range of operating conditions and is compared against the composite resin modified asphalt product Rigiphalte. It is observed as would be expected the latter is both stiffer and less affected by temperature increase.
Traffic speed (km/h)
Temperature (°C) 0-5 km/hr 10-20 km/hr 50 km/hr
10 12,500 15,000 16,300
The designer should be aware of the range in operating conditions and then select representative values for design considering as well the typical environmental variations. Refer to Dickinson (1981) for further details of observed temperature fluctuations by season and depth in asphalt in Australia.
The weighted Mean Annual Pavement Temperature (wMAPT) approach has proven to be reasonable and the following relationship to Mean Annual Air Temperature (MAAT) is derived from the Austroads pavement design guide. Essentially the wMAPT is the notional pavement temperature at which the design traffic causes the same damage as the segmented traffic over the temperature spectrum.
wMAPT = 1.3 MAAT + 5 (oC)
DYNAMIC MODULUS E* V TEMPERATURE FREQUENCY 10 Hz
1000
Figure 8: Comparison of the unconfined dynamic modulus of asphalt and Rigiphalte over the typical operational temperature range. ...
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With the improved dynamic modulus characterisation available from the SPT a more rigorous analysis is possible. On conclusion of the HIPAVE spectral damage
analysis the user may reduce the traffic to a selected number of passages of a single extreme load case and determine damage. The dynamic modulus of the asphalt and traffic spectrum can then be manually input to represent the full temperature and traffic spectrum. The damage at each temperature and traffic spectrum is estimated and the cumulative damage summed and compared with the damage calculated at wMAPT. In future development of HIPAVE the spectral damage related to
temperature may be automated.
While the fatigue data is not used as a specification parameter it’s application over many years has established confidence in conservative nature of the asphalt fatigue models used in design practice (refer to the Shell method following). The flexure test is of most value in the evaluation of alternative binders, with the limitation being that the relationship between field and laboratory performance is uncertain and has not had substantial empirical validation in the Australian environment.
It is known that the fatigue performance of asphalt in the field is considerably greater than in the laboratory (at given tensile strain). This is thought to be primarily due to the effects of the healing of micro-cracks in the bitumen binder in warm conditions during rest periods between loads. The Strategic Highways Research Program (SHRP) from their comparison of laboratory (NLAB) and field (NFIELD) asphalt fatigue suggests Shift Factor (SF) of 10 to 14 for 85% and 50% design reliability i.e.
(NFIELD) = SF. (NLAB)
One of the limitations of the laboratory asphalt fatigue test is that it is a continuous cyclical test at a low temperature in order to complete testing within a reasonable timeframe. These test conditions do not allow healing of the micro-cracks.
Consequently caution is advised in the interpretation of fatigue in mixes with Polymer Modified Binder (PMB) because research suggests the healing of binder may be inhibited by the polymer components.
Considering the magnitude of many industrial pavement projects the cost of specific materials characterisation is warranted although it must be understood that the relationship between the laboratory and field performance data is not yet well
calibrated. Notwithstanding, it is valuable to use the laboratory test data as a point of verification of the input parameters used in the design process. In time, these data bases will be established and will provide valuable insight into performance.
At the initial pavement design stage the use of predictive models for stiffness and fatigue performance are considered adequate. A number of approaches of greater or lesser complexity are available and their use is preferable to simply adopting typical values. The application of the predictive methods gives the designer a better feel for the critical mix parameters. One such method based on the Shell Pavement Design Guide is available in an Microsoft Excel spreadsheet that may be downloaded from www.mincad.com.au/hdipdg .
of temperature and load frequency (refer to appendix X) expected in the field. For each material a master curve is developed to enable the designer to input asphalt properties that enable the estimation of damage across the full climatic spectrum.
This facilitates the move away from the simplifying weighted Mean Annual Pavement Temperature (wMAPT) approach often used for road pavement design. While this approach has served us well over decades and seems to be appropriate for conventional binders it does not adequately treat modified binders because of the consequent changes in temperature sensitivity.
Interestingly for airport applications (where similar load magnitude to ports are applied) the US Department of Army & Air Force Technical Manuals (Nov. 1989) TM 5-825-8-1 and AFM 88-6, respectively, state that 75 – 125 mm asphalt thickness generally suffices, over a thick granular pavement, provided that: “it must be
assumed that if the minimum thickness of asphalt is used as specified in TM 5-825-2 / AFM 88-6 Chapter 2, then fatigue cracking will not be considered. Thus, for a conventional pavement, the design problem is one of determining the thickness of pavement required to protect the subgrade, with adequate controls in place for the granular components (i.e. material, quality, density, susbsurface moisture control etc).
This compares with the empirical performance observation of Australian ports where 150 mm asphalt on unbound granular base materials has given good performance over decades and fatigue cracking in the wheelpaths has generally not been observed The empirical evidence suggests the pavement thickness required to protect the subgrade provides sufficiently strong support to protect the asphalt from fatigue, with adequate controls in place for the granular components (i.e. material, quality, density, subsurface moisture control etc).
The designer is cautioned about the reliability of the analysis of thin layers – particularly wearing surfaces. In the design models it is assumed the layers are homogeneous, the tyre contact stress is uniform and normal to the surface. In practice it is difficult to compact thin asphalt layers so their properties will be different to similar materials placed at greater depth; tyre stress is far from uniform and often has a considerable shear force component due to the tyre properties and
acceleration.
It is suggested that the analysis of layers of thickness < 50% of the model tyre contact radius be treated with caution. In highway conditions this relates to layer thickness < 40 mm; in heavy duty applications 80 mm is probably more appropriate.
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