Design parameters

The design parameters can be primarily divided into three categories, material, traffic and environmental parameters and are discussed in the following.

Material parameters

The elastic modulus, Poisson's ratio, fatigue life, modulus of rupture etc. are the engineering parameters used for the structural design of the concrete pavement. The input parameters are either found out experimentally or estimated from various recommendations provided in the design guidelines (ACI 2001, ASTM 2003, IS-456). Statistically suitable design value is to be adopted if there are variations in the input parameters (Chakroborty and Das 2003).
The modulus of subgrade reaction (k) is generally used for characterization of subgrade strength. k in idealized model represents the spring constant of a dense liquid foundation. The k value is obtained by performing plate load test on the subgrade. The IRC:58 (2002) provides a table with suggested k values when CBR values of subgrade are known. Concrete slab, in general, are constructed on bound, stabilized, or unbound sub-base layer, and not directly on subgrade. The sub-base layer provided below the concrete pavement serves certain purposes (Austroads 2004), such as,

1. It provides uniform support to the concrete slab.
2. It limits pumping at joints and slab edges.
3. It controls shrinkage of concrete slab or swelling of subgrade soil

The effective k value that includes the sub-base layer and the subgrade should be used for design of thickness of the concrete slab. This effective k value can be obtained by performing plate load test on the constructed sub-base, or by computational means. Various guidelines (IRC:58 2002, AASHTO 1993, PCA 1984, Austroads 2004) suggest suitable values of effective k when the type and thickness of the sub-base layer is known.

Traffic parameters

The concept of axle load, wheel configuration, traffic volume, lane traffic, traffic growth rate, and calculation of axle load repetitions have already been introduced in the other web course on Transportation Engineering-I.
Interestingly, vehicle damage factor (VDF) (also known as Truck Factor (MS-1 1999)) is not considered in concrete pavement design. This is due to the fact that typically for concrete pavement design considers the individual damages created by individual axle load groups are considered. To take care of the variability of traffic axle load measurement, and unpredicted heavy truck load, a factor called the load safety factor (LSF) is generally used in concrete pavement design and is multiplied with the total equivalent standard axle load repetitions (Austroads 2004, IRC:58 2002).

Environmental parameters

The temperature and the subgrade moisture are the environmental parameters that affect the concrete pavement design. The temperature variation and the temperature differential induces temperature stresses to the concrete pavement. Temperature issues in concrete pavement has been discussed in the lecture 'analysis of concrete pavement' . The subgrade moisture level affects the subgrade strength and also it induces curling stresses due to moisture gradient.