Propagation of Compaction Quality from Subgrade to Base Layer in Pavement Structures

Density has long been the main quality control (QC) and acceptance (QA) indicator in pavement foundation construction; however, the nuclear density gauge (NDG) method has limitations in assessing large sections. These spot measurements are limited in depth and area and may be biased by outside factors. Deflection-based testing is a more promising alternative as the depth of influence is greater; however, it, too, is a spot test limited by the plate diameter. Intelligent compaction (IC), with an even deeper depth of influence, can cover the entire area and provide information almost in real time. An integrated approach that considers the density, deflection modulus, and IC stiffness is essential for a complete QC or QA evaluation. This paper focuses on the overall compaction of a pavement foundation by comparing NDG density and lightweight deflectometer (LWD) modulus with IC stiffness. Twelve lifts between two accelerated pavement loading test pits were analyzed in both the longitudinal and transverse paths of an IC-equipped soil compactor to show the limitations of current QA standards and how vital sublayer compaction control affects the upper lifts. NDG shows little to no variability across lifts, while LWD measurements show the opposite, consistent with IC. IC highlights that poor compaction in the subgrade or subbase layer propagates throughout the structure, leading to suboptimal compaction in the layers above, even if the density is consistent. These findings highlight the limitations in accepting density and how deflection and IC stiffness can help state highway officials ensure a more uniform product that will last longer.