Surface Layer Displacement
Linear Variable Differential Transducer - DT
Description
A "DT" sensor measures surface layer (HMA, PCC, or COMPOSITE) displacement due to dynamic loads. The Linear Variable Differential Transducer (DT) sensor is an electro-mechanical device that produces an electrical output proportional to the displacement of a separate, moveable core. Displacements are measured relative to a fix reference point (a buried stainless steel or invar rod).
Manufacturer
Several models of DT sensors have been used: Schaevitz model HCD-500 or Macro GHSAR 750-250.
Location and Quantity
DT sensors were originally mounted to an expansion ring within holes cored into hot-mix asphalt and concrete surface layers. Most displacement reference points were 12 foot long steel rods driven into the subgrade below the sensor, and jacketed by a PVC tube to prevent frost heave action. Other length reference rods were installed to monitor displacement at various layers within the pavement structure.
Current Status
These sensors were installed full time into the original tests cells and performed well for approximately 5 years. Once maintenance of the core hole areas became too demanding, the sensors were removed and installed as needed for load testing sessions. Newer DT sensors are only installed during load test events. 24
MnROAD Database Table
Data continues to be collected during various load response testing sessions, and some has been processed and loaded into the MnROAD database. Data is located in database tables named: MNR.DYNAMIC_LOAD_TEST_VALUES and MNR.DYNAMIC_LOAD TEST DESCRIPTIONS.
Piezo-Accelerometer - PA
Description
A "PA" sensor measures the vertical acceleration of a PCC pavement surface subjected to a moving vehicle. The vertical deformation of the pavement surface can then be calculated from measured acceleration using double integration analysis. A Piezo-Accelerometer (PA) sensor is a small, cylindrical, acceleration device with a magnetic base. A series of six of these sensors were magnetically attached to lag bolts installed into recessed slots formed during the paving of the PCC slabs. The PA sensors were only attached to the pavement during special load testing sessions.
Manufacturer
Kistler Model 8628B50 Piezo-Accelerometers.
Location and Quantity
PA sensor attachment locations were formed using blockouts during the paving of the PCC slabs. They were located to allow for measuring joint efficiencies under traffic loads.
PA sensor attachment locations were originally installed in 7 MnROAD PCC test cells.
Current Status
These sensors have been installed and tested only a few times since 1994. It is doubtful the original connection wires running to the attachment locations are still functional.
MnROAD Database Table
Limited data has been collected from this type of sensor. Data is not readily accessible at this time, but any future data will be input into the MnROAD database table: MNR.PA_VALUES.