Pavement analysis and design : current perspective

Present practice of pavement design involves considerations of three aspects: structural design, functional design and drainage design and they are explained briefly in the following:

Structural design

In structural design the stresses due traffic loading and temperature are estimated, and the thickness of the pavement is designed in such a way that these developed stresses/ strains are below the allowable values. The current practice of pavement design, more popularly, is known as Mechanistic-Empirical pavement design and is followed by a number of organizations around the world (Asphalt Institute 1999, Shell 1978, Austroads 1992, NCHRP 2005, IRC 2001). It is mechanistic pavement design because it uses stress/ strain of a pavement structure using mechanics based principle, and, as well, it is empirical because the expected life for a given stress/ strain level is estimated from empirical relationships obtained from laboratory or field performance studies. The pavement design approach is not governed by the maximum amount of load that the pavement can sustain, rather, it estimates the number of standard load repetitions that can cause failure.

Estimation of pavement stress/strain

Stress/ strain due to load

• For pavement design purpose, the stress/ strain value of a pavement structure is obtained from structural analysis of the pavement (Ioannides et al. 1998). The stress/ strain values at any point of a pavement structure can be estimated when the elastic moduli, Poisson's ratio and the thicknesses of the individual layers are known. The strain values can also measured using strain gauges.
• Any analysis procedure involves idealization regarding the structure; similarly, measuring strain involves measurement errors - hence the true value of stress/ strain is never known.
• A concrete pavement slab, in general, has finite dimensions, and thus the analysis approach of concrete pavement becomes different than the analysis of bituminous pavement. For bituminous pavement, in general, the pavement is assumed as infinite in both the directions, whereas for concrete pavement, in general, it is analysed as discrete slabs connected by joints. The concrete pavement is also assumed to have bending moment carrying capacity, whereas flexible pavement is assumed to have no moment carrying capacity.

Stress/ strain due to temperature

• The change of temperature causes the pavement to expand or contract. The restriction of free movement causes temperature stresses.
• There exists temperature variation across the depth of the pavement - this causes warping stresses.
• The temperature stress varies across the corner, interior and edge of the concrete slab, also at different times of the day. The most critical combination of load and temperature stress is used as design criteria.
• The temperature stress in bituminous pavement is insignificant. Hence, temperatire stress, is not considered in pavement design. However, temperature affects the elastic modulus of the bituminous layer, which needs to be duly considered in pavement design.

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