Design to Traffic/Resistance to Fatigue
Fatigue design is made using either methods based on Odemark’s formula or on analytical calculation methods (e. g. a multi-layer linear elastic design program). In both, the seasonal variation can only be considered by choosing the material properties for each (especially the unbound) layer so as to represent weighted mean values for the whole year. The “worst” thawing and moist period of the year (the so-called spring bearing capacity period) has a very large influence on the mean value using this weighting approach. Calculations are also made using a single temperature (+20 °C).
The obligatory use of winter tyres (most of them studded) during four to five months each winter causes most of the rutting in Finnish medium and high volume roads. The design against rutting is based on empirical information between rutting speed and amount of traffic, asphalt type, binder and aggregate properties. No seasonal variation is considered, but it is well known that moist/wet conditions at the road surface will increase the rate of rutting.
At the moment only discussions about the possibility of using several, varying – length, time periods (each associated with individual material parameters – depending on moisture, temperature and density) have been carried out. During the analyses mentioned above the vertical deformation (the elastic recoverable strain) is also checked to to ensure that it remains less than the maximum allowable limit for each layer material.
As a brief summary it can be said that seasonal variation is only taken in to account by choosing parameter values based on the most dominating period, the spring thaw period.
The new AASHTO Mechanistic-Empirical Pavement Design Guide (MEPDG) (AASHTO, 1998 & 2004) integrates climatic factors, materials properties, and traffic loadings to predict pavement performance. The Enhanced Integrated Climatic Model (EICM) that is used in the new AASHTO MEPDG integrates three models:
• The two-dimensional drainage infiltration model (ID Model), developed at Texas A&M University, that evaluates the destination of the rainfall on the pavement.
• The Climatic-Materials-Structure Model (CMS Model) developed at the University of Illinois. A one dimensional finite difference engine is used to calculate coupled heat-moisture flows in pavement structures and predict pavement temperature.
• The CRREL frost heave and thaw settlement model developed at the United States Army Cold Region Research and Engineering Laboratory (US Army CR – REL) which computes temperature and moisture flow at different temperatures and predicts the depth of frost and thaw penetration.
The EICM is based on the Integrated Climatic Model, developed for the Federal Highway Administration in 1989, containing several improvements. It replaced the Gardner equation for the soil-water characteristic curve (SWCC) with the equations proposed by Fredlund and Xing (1994). It also provides better estimates of the saturated permeability and specific gravity of soils, given known soil index properties such as grain-size distribution (percent passing sieve no. 200 sieve (75 ^m) and effective grain size with 60% passing by weight) and Plasticity Index (PI). The unsaturated permeability prediction based on the SWCC and proposed by Fredlund et al. is also incorporated in the model (Fredlund et al., 1994).
The model uses actual climatic data (hourly or monthly) and predicts the following parameters throughout the entire pavement/subgrade profile:
• Resilient modulus adjustment factors
• Pore water pressure
• Water content
• Frost and thaw depths
• Frost heave and
• Drainage performance.
The model evaluates the expected changes in moisture condition from the initial or reference condition (generally, near optimum moisture condition and maximum dry density) as the subgrade and unbound materials reach equilibrium moisture condition. The model also evaluates the seasonal changes in moisture condition, and consequently the changes in resilient modulus, Mr. In addition the model calculates the effect of freezing, and thawing and recovery of Mr and uses these Mr values for calculation of critical pavement response parameters and damage at various points within the pavement system.