Dielectric Profiling System
Asphalt pavement compaction quality control and quality assurance decisions are traditionally based on coring samples that are destructive and represent significantly less than 1 percent of the in-service pavement. Dielectric profiling systems (DPS) are emerging as a tool that can be used for continuous assessment of asphalt pavement compaction quality through measuring the pavement dielectric constant.
MnDOT started to evaluate DPS in 2015 as a part of SHRP2 implementation assistant program. A great effort has been spent on understanding the equipment though calibration with material of known frequency independent dielectric properties, Fresnel zone (footprint) calculations, and field experiments. The precision and accuracy evaluation using the controlled material and as well as initial field trials with comparisons to cores have let to both equipment stability improvements as well as demonstration of the ability of DPS to give a comprehensive characterization of the compaction effort. Full scale testing has been conducted by MnDOT on over 10 construction projects giving continuous assessment of the joint and mat compaction.
These evaluations included special projects including assessments of the effect of material changes such as increasing binder content, adding evotherm compaction aid, reducing aggregate size. The projects also include effects of adding a roller compactor to the project, changing roller compaction patterns, and echelon paving for hot joints. MnDOT and other states have shown the method to be effective in providing a continuous assessment of both joint and mat compaction. However, to date, destructive field cores are still required to relate the measured dielectric constant to the pavement density.
To address this limitation, MnDOT has developed an innovative method to measure laboratory compacted (gyrated) asphalt mixture specimens (pucks), instead of field cores, to convert the measured dielectric constant to the asphalt mixture density. This is accomplished through measuring the dielectric constant on laboratory compacted specimens using a time-of-flight based calculation and relating the measured dielectric constant to the specimen density by compacting the specimens to different levels. The new method of calibrating without field cores has been shown to effectively predict field density. Puck generated prediction curves from multiple production mix days of paving within one pavement project agreed well with static dielectric measurements using field cores to validate the puck predicted relationship. This method was also confirmed with a comparison of production mix puck predictions vs field cores on another paving project in 2018.
The core-free method agrees well to the conventional field coring method used to convert dielectric constant to air void content, mitigating the need for destructive field coring. This shows the potential of the DPS method for evaluating achieved compaction in the field with continuous coverage and limited field coring. The accuracy of the coreless calibration method is planned for validation on multiple construction projects in the 2019 construction season. This coreless calibration also opens the door to more timely field density assessments that can be used to adjust mix and construction compaction methods “on-the-fly” to ensure optimal strategies are used throughout the project based on DPS real-time feedback. This creates an opportunity for improvements in achieved field compaction that can extend pavement life significantly.