k-value Rigid Pavements

Question

Chapter 4, Part 3 of the MEPDG (Mechanistic Empirical Pavement Design Guide) states

"The 'effective' dynamic k-value represents the compressibility of all pavement layers beneath the PCC slab and base course".

However, the Manual of Practice for the MEPDG states

"The effective k-value, therefore, essentially represents the compressibility of underlying layers (i.e. unbound base, subbase, and subgrade layers) upon which the upper bound layers and existing HMA or PCC layers are constructed."

I find these two definitions contradictory in regards of the inclusion or not of the base layer in the k-value estimation.

I have two questions:

1) Does the k-value represents all layers beneath the PCC slab or only all layers beneath the PCC slab and base?

2) Is the static backcalculated k-value from the FWD data the value I should enter directly as input in the Rigid Pavement Rehabilitation analysis in Pavement ME Design?

Answer 1

In relation to MEPDG/AASHTOWare Pavement ME Design, the resilient modulus values for all base (granular, asphalt, concrete or cement treated) and subgrade layers are used to calculate a single base modulus which represent all base layers and a subgrade dynamic k-value representing all the subgrade layers. These values are computed through backcalculation as mentioned in the NCHRP 1-37A main report and double letter appendices.

In short, the dynamic k-value is specific to the subgrade layers while the effective base modulus represents all base layers. This is why the computed base modulus is different than the resilient modulus specified for each individual base/subbase layer.

As for your second question, the value entered for a rigid rehabilitation design using FWD data should be the dynamic k-value which is approximately two times the static k-value as stated in the software 1-37A report.