Why is the CBR value utilized for Flexible Pavement Design and the Modulus of Sub-Grade Reaction value for Rigid Pavement Design?

Rigid Vs Flexible Pavement

Why is the CBR value utilized for Flexible Pavement Design and the Modulus of Sub-Grade Reaction value for Rigid Pavement Design?

According to the AASHTO Pavement Guide 1993, CBR is not used directly in the design of Flexible Pavement. It is used to compute the Resilience Modulus (MR), also known as the Dynamic Elastic Modulus of Sub-Grade, Granular Sub-Base, and Agg. Base Course layers. CBR is utilized because it is a simpler test to complete than directly determining MR value from testing. As a result, CBR-MR correlations are employed to calculate MR values for layers other than asphalt. Various methods, such as Van der Poel's Nomograph, can be used to estimate the MR value of an asphalt layer. Because the MR value of asphalt varies from 150,000 psi to 450,000 psi mostly due to temperature, it must be determined independently.

While designing flexible pavement using the AASHTO 1993 Guide, layers are considered non-linear elastic layers; thus, the Elastic Modulus of layers is the fundamental design parameter. However, we know that Asphalt is a viscoelastic material, whereas Sub-Grade deviates from elastic behavior by exhibiting persistent deformation under load, i.e., Rutting. Mechanistic methods were employed to describe this inelastic behavior, culminating in the production of the AASHTO Mechanistic Empirical Pavement Design Guide (MEPDG) 2002.

Why couldn't rigid pavement be designed similarly to flexible pavement? The reason is because PCC has an Elastic Modulus that is roughly ten times greater than Asphalt (3,122,000 psi for fc'=3000 psi). Because of this, the PCC slab behaves rigidly in comparison to the layer’s underneath it (thus the name "Rigid Pavement"). Consider the slab to be resting on elastic springs to mimic this behavior better precisely. The spring value of subgrade is represented by the modulus of subgrade reaction (k). The deflection of the slab is proportional to the deflection of the spring. The slab is further examined for tensile cracking using the concrete's Modulus of Rupture. It is also understandable that Rigid Pavement will display nonlinear behavior due to crack growth. This behavior has been modeled into the new AASHTO MEPDG as well (2002).

So, because the design technique and underlying assumptions for both types of pavements differ, different material properties are used in the design of both types of pavements.