Traffic load response
Biaxial Strain Gauge (BS)
Concrete Embedment Strain Gauge (CD)
Concrete Embedment Strain Gauge (CE)
Linear Variable Differential Transducer (DT)
Geokon Dynamic Strain Gauge (GD)
Bituminous Longitudinal Embedment or Transverse Embedment Strain Gauge (LE or TE)
Concrete Embedment Strain Gauge (MM)
Piezo-Accelerometer (PA)
Steel Strain Gauge (SS)
Environmental
Concrete Joint Opening Horizontal Clip (HC)
Concrete Strain Transducer (KB/XB)
Concrete Moisture (MH)
Thermocouple (TC)
Tiltmeter (TT/TL)
Concrete Vibrating Wire Strain gauge [short gauge length] (VG/XG)
Concrete Vibrating Wire Strain gauge [low modulus] (VM/XM)
Concrete Vibrating Wire Strain Gauge (VW/XV)
Instrumentation: Pavement & Surface Sensors
Pavement or surface layer sensors are installed on, embedded within, or placed immediately under the hot-mix asphalt or concrete layer of a pavement structure.
The sensors are linked by fiber optics to MnROAD’s computerized data collection system and the data is stored in a database at MnDOT Materials Research and Engineering Laboratory in Maplewood.
There are two categories of pavement sensors.
- Traffic load response: Sensors that measure the response resulting from dynamic forces applied by traffic.
- Environmental: Sensors that measure the response due to slower changing forces like temperature, moisture and frost depth, material expansion/contraction, shrinkage and creep.
All sensors are designated by a two-letter code.
Specification and performance history for an individual sensor type is available upon request.
Biaxial Strain Gauge - BS (BL/BT)
Function: Measures bituminous (hot-mix asphalt) strain response due to dynamic loads.
Description: The Biaxial Strain (BS) sensor consists of four electrical resistance strain gauges embedded in an asphalt mastic mix. Two of the strain gauges within a biaxial strain sensor are orientated in the longitudinal direction (with traffic) and designated as “BL”. The remaining two gauges are oriented in the transverse direction (90º to traffic) and designated as “BT”. Data is stored in the database under the BL and BT designations, rather than the BS designation. Physical dimensions of the biaxial strain sensor are: 152.4mm x 152.4mm x 12.7mm (6" x 6" x 1/2")
Manufacturer: BS sensors were designed by the Alberta Research Council.
Location and quantity: BS sensors are located at the bottom of the hot-mix asphalt surface layer. They are mounted on top of the base layer and paved over with the asphalt layer, to become embedded into the bottom of the asphalt layer.
There were originally 40 Biaxial Strain sensors located in Cell 26 of the Low Volume Roadway.
Sensor location tables:: BL (Excel spread sheet) BT (Excel spread sheet)
Current Status: Due to reconstruction of Cell 26 in 2000, these sensors were removed.
MnROAD Data Base tables: Data was collected during various load response testing sessions, but is currently not loaded into the MnROAD database. Data will eventually be loaded into database tables named: MN R.BL_VALUES, MN R.BT_VALUES.
Concrete Embedment Strain Gauge - CD
Function: Measures PCC strain response due to dynamic loads
Description: The Concrete Embedment (CD) strain sensor is an electrical resistance strain gauge embedded within a strip of glass-fiber reinforced epoxy, with transverse steel anchors at each end of the strip to form an H-shape.
Manufacturer: Dynatest model PAST-2PCC.
Location and quantity: CD sensors are embedded into the concrete surface layer near the top and bottom of the slab at various locations throughout a particular panel. There were originally 128 CD sensors located in 8 MnROAD PCC test cells .
Sensor location tables: CD (Excel spread sheet)
Current Status: These sensors ceased to function, and were all removed by coring in 2003.
MnROAD Data Base Tables: Data was collected during various load response testing sessions and can be found in MnROAD database tables: MNR.CD_VALUES
Concrete Embedment Strain Gauge - CE
Function: Measures PCC strain response due to dynamic loads.
Description: The Concrete Embedment (CE) strain sensor is an electrical resistance strain gauge hermetically sealed between two thin resin plates.
Manufacturer: Tokyo Sokki model PML-60.
Location and quantity: CE sensors are embedded into the concrete surface layer near the top and bottom of the slab at various locations throughout a particular panel. There were originally 436 CE sensors located in 14 MnROAD PCC test cells. Newer PCC test cells at the MnROAD facility have included the installation of Tokyo Sokki model PML-60 strain sensors.
Sensor location tables: CE (Excel Spread Sheet)
Current Status: These sensors performed very well, with a typically life span of approximately 6 years. Original (1993) sensors were removed by coring in 2003. Newer (2000) sensors remain in service.
MnROAD Data Base Tables: Data has been collected (and continues for newer test cells) during various load response testing sessions. Data can be found in MnROAD database table: MNR.CE_VALUES
Linear Variable Differential Transducer - DT
Function: Measures surface layer (asphalt or PCC) displacement due to dynamic loads.
Description: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 rod at MnROAD)
Photos: Sensor mount | 1 | 2 | 3 | 4
Manufacturer: Schaevitz model HCD-500.
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.
There were originally 119 DT sensors located in 10 hot-mix asphalt and 14 MnROAD PCC test cells.
Sensor location tables: DT (Excel Spread Sheet)
Current Status: These sensors performed very well for approximately 5 years, at which time, maintenance of the core hole areas became too demanding. Original (1993) sensors are removed and installed as needed for load testing sessions.
MnROAD Data Base Tables: Data was collected during various load response testing sessions, but is currently not loaded into the MnROAD database. Data will eventually be loaded into a database tables named: MNR.DT_VALUES
Geokon Dynamic Strain Gauge - GD
Function: Measures PCC strain response due to dynamic loads:
Description: The Geokon dynamic (GD) concrete embedment strain sensor consists of a full bridge gauged proving ring, coupled between two flanges.Photos: 1
Manufacturer: Geokon model 3900.
Location and quantity: GD sensors are embedded into the concrete surface layer near the top and bottom of the slab, in the wheelpath of one panel in Test Cell 60.
There are 2 GD sensors in test Cell 60.
Sensor location tables: GD (Excel Spread sheet)
Current Status: These sensors are currently in service (2006).
MnROAD Data Base Tables: Data has been collected and continues during various load response testing sessions. Data can be found in MnROAD database table: MNR.GD_VALUES.
Bituminous Longitudinal Embedment or Transverse Embedment Strain Gauge - LE or TE
Function: Measures bituminous (hot-mix asphalt) strain response due to dynamic loads.
Description: The Longitudinal Embedment (LE) or Transverse Embedment (TE) strain sensor is an electrical resistance strain gauge embedded within a strip of glass-fiber reinforced epoxy, with transverse steel anchors at each end of the strip to form an H-shape. The designation “LE” indicates a sensor mounted longitudinally, or in the direction of traffic. The designation “TE” indicates a sensor mounted transverse or 90º to the direction of traffic.
Manufacturer: Dynatest model PAST-2AC.
Location and quantity: LE and TE sensors are positioned at the bottom of the hot-mix asphalt surface layer and near wheelpaths.
There were originally 27 LE and 102 TE sensors located in 22 MnROAD hot-mix asphalt test cells. Newer asphalt test cells at the MnROAD facility have included the installation of Dynatest model PAST-2AC strain sensors.
Sensor location tables: LE -TE (Excel Spread Sheet)
Current Status: These sensors have performed very well and demonstrate a life span of approximately 2 years. Original (1993) sensors have either ceased to function, or have been removed due to reconstruction of a test cell. Newer sensors (Cell 31) remain in service.
MnROAD Data Base Tables: Data has been collected both automatically and during various load response testing sessions. Data can be found in MnROAD database tables: MNR.LE_VALUES, MNR.LE_ERRORS, MNR. TE_VALUES.
Concrete Embedment Strain Gauge - MM
Function: Measures PCC strain response due to dynamic loads.
Description: The Micro-Measurements Concrete Embedment (MM) strain gauge is an electrical resistance strain-sensing grid hermetically sealed within a polymer concrete casing.
Photos: 1
Manufacturer: Micro-Measurements models EGP-5-120 and EGP-5-350.
Location and quantity: MM sensors are embedded into the concrete surface layer near the top and bottom of the slab at various locations throughout a particular panel. There are 14 MM sensors located in newer MnROAD PCC test cells.
Sensor location tables: MM (Excel Spread sheet)
Current Status: Unknown.
MnROAD Data Base Tables: Data was collected during various load response testing sessions, but is currently not loaded into the MnROAD database. Data will eventually be loaded into a database table named: MNR.MM_VALUES.
Function: 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.
Description: Piezo-Accelerometer sensor is a small, cylindrical, acceleration device with a magnetic base. A series of six of these sensors are magnetically attached to lag bolts installed into recessed slots formed during the paving of the PCC slabs. The PA sensors are only attached to the pavement during special load testing sessions.
Manufacturer: Kistler Model 8628B50 Piezo-Accelerometers.
Location and quantity: Locations were formed using blockouts during the paving of the PCC slabs. They were located to allow for measuring joint efficiencies under traffic loads. Originally installed in 7 MnROAD PCC test cells
Sensor location tables: PA (Excel Spread Sheet)
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. Temporary wiring could be installed to enable testing at these locations again.
MnROAD Data Base Tables: Limited data has been collected from this type of sensor. Data is not readily accessible at this time, but future data will be input into the MnROAD database table: MNR.PA_VALUES
Function: Measures the strain applied to the steel dowel bars in PCC transverse joints dues to dynamic vehicle loads.
Description: Four weldable resistance strain gauges mounted to the top, bottom, and both sides of a dowel bar. As a dowel bar flexes, measured strain can be used to determine the efficiency of the bar to transfer load across a joint.
Manufacturer: Micromeasurements Group model LWK-06-W250b-350.
Location and quantity: SS sensors were originally mounted on the dowel bars prior to placement of the PCC. It was difficult to insure the joint was sawed in the correct location such that the dowel had equal embedment length across a joint. There were originally 180 SS sensors (45 dowels x 4 gauges per dowel), located in 7 MnROAD PCC test cells.
Sensor location tables: SS (Excel Spread Sheet)
Current Status: Performance of the SS sensors cannot be assessed, since they were connected and recorded a very limited number of times. None of the SS sensors remain functional at this time.
MnROAD Data Base Tables: Limited data has been collected from this type of sensor. Data is not readily accessible at this time, but future data will be input into the MnROAD database table: MNR.SS_VALUES
Concrete Joint Opening Horizontal Clip - HC
Function: Measures the opening and closing of a PCC transverse contraction joint due to environmental forces. Data is automatically collected once every 15 minutes, due to the slow rate of change of the movement.
Description: The Horizontal Clip (HC) sensor is a U-shaped strip of metal to which strain gauges are mounted. As the strip of metal is flexed, measured strain in the metal strip is converted to represent the displacement (separation) of the two legs of the device. Each leg of the sensor is mounted to span across a transverse PCC joint.
Manufacturer: Tokyo Sokki model PI-5.
Location and quantity: HC sensors were originally mounted within large holes, spanning across transverse joints, blocked out during the paving of the concrete slabs. Following installation of the sensors, a split steel cover, with a rubber disk underneath, was bolted to the surface to allow for movement of the joint and to provide protection to the sensor.
Sensor location tables: HC (Excel Spread Sheet)
Current Status: These sensors performed very well for approximately four years, at which time the original mounting systems began to fail (rusted brackets or sand packing from below). An improved mounting technique was created and used for more recent installations in newer PCC test cells. Original (1993) sensors have been removed, but newer (2000) installations are still functioning.
MnROAD Data Base Tables: Data has been collected automatically every 15 minutes over the life of each sensor. Data can be found in MnROAD database table: MNR.HC_VALUES
Concrete Strain Transducer - KB/XB
Function: Measures the strain in a PCC slab due to material shrinkage and environmental forces. A “XB” sensor, mounted within the KB sensor, is a thermocouple necessary for temperature correction of the measured strain. Data is automatically collected once every 15 minutes, due to the slow rate of change of the measured strain and temperature within a PCC slab
Description: The KB sensor is a strain transducer, with an active gauge length of 100 mm and a low modulus case, designed to measure strain in materials undergoing a transition from a compliant state to a hardened state (like new concrete). The sensor is self-temperature compensated, having a linear thermal expansion coefficient similar to concrete. The attached thermocouple (designated as XB) permits temperature measurement at the exact location of the sensor
Photos: 1
Manufacturer: Tokyo Sokki model KM-100BT Strain Transducer.
Location and quantity: KB/XB sensors are embedded into the concrete surface layer near the top and bottom of the slab, in the driving lane wheelpath, near a transverse joint and midpoint of one panel. There are 4 KB (and 4 XB) sensors located in MnROAD PCC test cell 60.
Sensor location tables: KB (Excel Spread Sheet)
Current Status: These sensors continue to perform after two years of service
MnROAD Data Base Tables: Data has been collected automatically every 15 minutes over the life of each sensor. Data can be found in MnROAD database tables: MNR.KB_VALUES, MNR.XB_VALUES
Function: Measures the relative humidity within a concrete pavement slab. Data is automatically collected once every 15 minutes
Description: The Moisture Humidity (MH) sensor is a custom made sensor consisting of a Sensirion SHT-75 humidity gauge placed within a plastic tube (ball-point pen case), sealed on one end (where the wires exit the tube), and covered by a piece of GORE membrane on the end where moisture enters the tube
Photos: 1
Manufacturer: Sensirion SHT75 Humidity VE50814 Series HPC GORE Membrane Vents
Location and quantity: MH sensors are mounted on a vertical “tree” within the concrete pavement slab. This mounting method allows vertical moisture content profiles to be determined, which are very important to the understanding of pavement response. A total of 20 MH sensors were located within slabs in Cells 60 and 62. A “tree” of 5 sensors (at various depths within the slab) was located both at the midpoint of a panel, and in the corner (near the shoulder) of the same panel.
Sensor location tables: MH (Excel Spead Sheet)
Current Status: These sensors seemed to perform well for the first year, however have ceased to function as of September 2006. The data collection equipment (a BasicX-24 circuit board) was not designed to be field worthy, and therefore it was difficult to keep the data consistently flowing. Each time an MH sensor failed (also not rugged enough for Minnesota climate), it would interrupt data flow from the remaining sensors.
MnROAD Data Base Tables: Data has been collected automatically every 15 minutes over the life of each sensor. Data can be found in MnROAD database table: MNR.MH_VALUES
Function: Measures the temperature of a material in which it is embedded. Data is automatically collected once every 15 minutes.
Description: The Thermocouple (TC) sensor is a pair of dissimilar metal alloy wires (copper and Constantine) connected together near the point of measurement. Thermocouples generate an open-circuit voltage, called the Seebeck voltage, which is proportional to the temperature difference between the hot (point of interest) end and a reference junction.
Photos: 1
Manufacturer: Omega Type-T thermocouple wire
Location and quantity: TC sensors within pavements are typically mounted on a vertical “tree.” This mounting method allows vertical temperature profiles to be determined, which are very important to the understanding of pavement response. Since TC sensors were mounted on “trees”, which often span both the surface and subsurface layers, it is difficult to characterize their quantities (originally at least 1000).
Sensor location tables: TC (Excel Spread Sheet)
Current Status: These sensors perform very well, with over 95% of the original TC sensors still functioning after 12 years of service.
MnROAD Data Base Tables: Data has been collected automatically every 15 minutes over the life of each sensor. Data can be found in MnROAD database table: MNR.TC_VALUES (current year), MNR.TC_VALUES_#### (where #### indicates one year of values)
Function: Measures the slope at a particular point in a PCC slab. Data is automatically collected once every 15 minutes, due to the slow rate of change of the movement.
Description: The Tiltmeter (TT/TL) sensor uses precision electrolytic transducers to detect angular motion in two perpendicular directions. The transducers operate on the fundamental principle that a bubble suspended in a liquid-filled case, is always bisected by a vertical gravity vector. As the transducer tilts, the case moves around the bubble, linearly changing the electrical resistance measured through the electrolyte. Internally, there are two sensors, one oriented longitudinally(TL) and one transversely(TT) to traffic.
Manufacturer: Applied Geomechanics model 756-1129 Mid-range Miniature Tilt Sensor.
Location and quantity: TT/TL sensors were mounted in one panel within test cell 39. The sensors were placed on top of a flat plate placed on the base, and paved over with the PCC, to become embedded near the bottom of the slab.
Sensor location tables: TT/TL (Excel Spread Sheet)
Current Status: Performance of the TT/TL sensor has been very good. All 15 sensors appear to continue working after 12 years of service.
MnROAD Data Base Tables: Data has been collected automatically every 15 minutes over the life of each sensor. Data can be found in MnROAD database tables: MNR.TL_VALUES, MNR.TT_VALUE
Concrete Vibrating Wire Strain Gauge [short gauge length] - VG/XG
Function: Measures the strain in a PCC slab due to material shrinkage and environmental forces. A “XG” sensor, mounted to the VG sensor, is a thermistor necessary for temperature correction of the measured strain. Data is automatically collected once every 15 minutes, due to the slow rate of change of the measured strain and temperature within a PCC slab
Description: The VG sensor is a small (active gauge length = 51 mm) vibrating wire stain gauge consisting of a taut wire that is anchored between two end flanges and surrounded by a protective tube. The wire is electro-mechanically excited, causing it to vibrate at its natural frequency. The resonant frequency of the wire changes with the tension. Thus the strain exerted on the flanges by movement in the PCC slab changes the frequency of vibration, which can be correlated to strain. The thermistor (XG) is used for temperature correction of the strain due to differences in coefficient of thermal expansion between the steel sensor and the concrete it is embedded into
Photos: 1
Manufacturer: Geokon 4202 Vibrating Wire Strain Gauges.
Location and quantity: VG/XG sensors are embedded into the concrete surface layer near the top and bottom of the slab in one outer panel of Test Cell 60 (near a transverse joint) There are 2 VG/XG sensors in test Cell 60.
Sensor location tables: VG/XG (Excel Spread Sheet)
Current Status: These sensors are currently in service (2006)
MnROAD Data Base Tables: Data has been collected automatically every 15 minutes over the life of each sensor. Data can be found in MnROAD database tables: MNR.VG_VALUES, MNR. XG_VALUES
Concrete Vibrating Wire Strain Gauge [low modulus] - VM/XM
Function: Measures the strain in a PCC slab due to material shrinkage and environmental forces. A “XM” sensor, mounted to the VM sensor, is a thermistor necessary for temperature correction of the measured strain. Data is automatically collected once every 15 minutes, due to the slow rate of change of the measured strain and temperature within a PCC slab.
Description: The VM sensor is a medium (active gauge length = 153 mm) vibrating wire stain gauge consisting of a taut wire that is anchored between two end flanges and surrounded by a protective tube. The wire is electro-mechanically excited, causing it to vibrate at its natural frequency. The resonant frequency of the wire changes with the tension. Thus the strain exerted on the flanges by movement in the PCC slab changes the frequency of vibration, which can be correlated to strain. Strain values from VW sensors are relative, in that one can only compare the change in value over time, or the change since the initial set of the concrete around the sensor. Initial VW values (at the time of concrete set) can be found in the database table MNR. VM_INITIAL_VALUES. The thermistor (XM) is used for temperature correction of the strain due to differences in coefficient of thermal expansion between the steel sensor and the concrete it is embedded into.
Photos: 1
Manufacturer: Geokon 4200L Vibrating Wire Strain Gauges.
Location and quantity: VW/XV sensors are embedded into the concrete surface layer near the top and bottom of the slab. There were two VM/XM sensors installed in Cell 60 in 2004.
Sensor location tables: XVVW/ (Excel Spread Sheet)
Current Status: These sensors have performed very well.
MnROAD Data Base Tables: Data has been collected automatically every 15 minutes over the life of each sensor. Data can be found in MnROAD database tables: MNR.VW_VALUES, MNR.XV_VALUES, MNR.VW_INITIAL_VALUES.
Concrete Vibrating Wire Strain Gauge [low modulus] - VW/XV
Function: Measures the strain in a PCC slab due to material shrinkage and environmental forces. A “XV” sensor, mounted to the VW sensor, is a thermistor necessary for temperature correction of the measured strain. Data is automatically collected once every 15 minutes, due to the slow rate of change of the measured strain and temperature within a PCC slab.
Description: The VW sensor is a modulus medium (active gauge length = 153 mm) vibrating wire stain gauge consisting of a taut wire that is anchored between two end flanges and surrounded by a protective tube. The wire is electro-mechanically excited, causing it to vibrate at its natural frequency. The resonant frequency of the wire changes with the tension. Thus the strain exerted on the flanges by movement in the PCC slab changes the frequency of vibration, which can be correlated to strain. Strain values from VW sensors are relative, in that one can only compare the change in value over time, or the change since the initial set of the concrete around the sensor. Initial VW values (at the time of concrete set) can be found in the database table MNR. VW_INITIAL_VALUES. The thermistor (XV) is used for temperature correction of the strain due to differences in coefficient of thermal expansion between the steel sensor and the concrete it is embedded into.
Photos: 1
Manufacturer: Geokon 4200 Vibrating Wire Strain Gauges.
Location and quantity: VW/XV sensors are embedded into the concrete surface layer near the top and bottom of the slab at nine locations throughout a particular panel. There were originally 162 VW (and 162 XV) sensors located in 9 MnROAD PCC test cells. Newer PCC test cells at the MnROAD facility have included the installation of Geokon 4200 Vibrating Wire Strain gauges
Sensor location tables: VW/XV (Excel Spread sheet)
Current Status: These sensors have performed very well, and most original VW/XV sensors are still operating after 12 years of service.
MnROAD Data Base Tables: Data has been collected automatically every 15 minutes over the life of each sensor. Data can be found in MnROAD database tables: MNR.VW_VALUES, MNR.XV_VALUES, MNR.VW_INITIAL_VALUES.