Design life

Design life is the number of years (or number of standard axle repetitions) for which the pavement is being designed. A pavement is expected to serve satisfactorily within the design life. For concrete pavements 20 to 40 years may be assumed as the design life (PCA 1984, IRC:58 2002).

Basic design principle

Though different approaches for concrete pavement design are suggested in various guidelines, the design principles tend to remain similar across different guidelines, for example, PCA (1994), Austroads (2004), NCHRP (2004), IRC:58 (2002) etc., except the AASHTO (1993) provisions, which is based on empirical approach. The basic steps involved in the design of concrete pavement method can be summarized as follows:

• The developed stresses due to load for a trial thickness of the concrete slab are calculated for various loading configuration and the critical one is chosen. The axle loads are generally divided into different axle load groups and the load stresses are calculated individually.
• The ratio between the load stress and Modulus of Rupture (MOR) is known as stress ratio . The stress ratio determines how many repetitions the pavement can sustain (i.e. allowable traffic repetitions ) for the individual axle load group. If the stress ratio is 0.55 or lower, it can withstand virtually infinite number of traffic repetitions (PCA 1984).
• The ratio between the allowable repetitions to the expected traffic repetitions is the damage fraction. The calculation process is repeated for various axle loads (sometimes, for various seasons, or various timings of the day), and the sum of individual damage fractions ( cumulative fatigue damage ) should be less than equal to one for pavement design being safe. If found unsafe, the trial thickness is changed and the design process is repeated.
• The design process may also include considerations for temperature stress, moisture stresses, and erosion distress.