Accounting for the Presence of a Base Layer by Adjusting the Subgrade Modulus, <i>k</i>

This paper challenges the popular narrative whereby the contribution of a base layer under a concrete pavement slab can be accommodated by augmenting the subgrade modulus, k . Following a close reading of the primary sources, it is demonstrated that the roots of this concept hail to a tentative suggestion in 1955 by Yoder, whose idea was catapulted to prominence by its immediate adoption by the Portland Cement Association. Subsequent implementations by the American Association of State Highway Officials, the Federal Aviation Administration, and agencies in the U.S. Department of Defense have resulted in the widespread and uncritical proliferation of Yoder’s proposal, even though Yoder himself never acknowledged it again. It is submitted in this paper that generating “bump-the- k -value” charts is a misguided violation of the immiscibility principle that proscribes mingling the dense liquid subgrade idealization by Westergaard with that of the elastic solid analog by Burmister. Mathematical relationships are derived and used to prove the interdependence of all available charts of this type, despite any differences in appearance. It is concluded that pavement engineers are well advised to abandon this methodology for accommodating base layers and to pursue alternatives grounded on fundamental principles, thereby exploiting computational tools that are currently available.