The d-Rebar is based on a miniature long base length inductive strain gauge, small enough to be recessed into a 3.2mm groove in the bolt.
What is an inductive strain gauge?
A miniature inductive displacement sensor comprising a precision coil and a high permeability core attached to stainless steel rod. The ends of the displacement sensor are secured to the bolt at a specified distance apart referred to as the baselength (BL in Fig 2). This baselength may vary between 300mm and 2000mm so comprising a long baselength strain gauge.
Fig 1. Both ends of the inductive strain gauge are positioned within the 3.2mm groove.
Fig 2: The baselength (BL) is the distance between the ends of the gauge which are attached to the rebar using micro-drilled holes.
Fig 3: Staggered versus stacked configuration
d-Rebar technology can be applied to any rigid bolt or bar. Either a pair of strain gauges (for end anchored bolts spun in resin) or an array of strain gauges can be specified. For applications which require enhanced axial load resolution the strain gauge array should be staggered (Fig3 upper). If bending moments need to be more accurately resolved then a stacked configuration may be preferable (Fig 3. lower). A theoretical comparison of both is presented in a related technical note.
The specification of the strain gauge locations should be based on an understanding of the load distribution along a fully grouted rock bolt. At the collar of the borehole the load depends on the stiffness (i.e. bearing conditions) of the faceplate. Along the length of the bolt the load increases in the bolt along the “pick-up” length and reaches a peak at the neutral point. The load decreases along the remainder of the bolt length, the “anchor length”, and is zero at the free end. In the case of a stiff bearing surface for the plate and a rapid decrease in the displacement magnitude around the excavation then the neutral point may actually occur at the faceplate.
Every instrument is subjected to a multipoint calibration. The calibration coefficients are written into microcontroller memory, and a piecewise linear function is used to enhance linearity the output signal and applied strain.
Zero-ing the d-Rebar
Prior to installation the initial readings should be zeroed. The loads can be zeroed at any time by leaving the instrument plugged into the d-Reader readout unit for approximately 1 minute with the bolt oriented upright to avoid sag ( bending of the bolt under its own weight is enough to change the values). A second set of readings should be taken when the bolt has been grouted into the borehole.
Installing instrumentation with a rock drill can be challenging even for the toughest design. Care must be taken to ensure that the drill is well aligned with the borehole.
- Instrumented rebar that can be spun in resin
- Available with 2 to 6 strain gauges in two diametrically opposed slotted channels.
- Readout head encapsulated in stainless steel tube for protection.
- Microcontroller provides output directly in m
- Each bolt is individually calibrated.
- Immunity to hostile environment
- High survivability to shock and vibration
- Easy to install and maintain
- Low cost peripheral devices for data-logging (d-Logger) and data upload using TCP/IP (IoT GateWay)
- Wireless mesh using d-Mesh.
YieldPoint’s low cost d-Reader readout unit or the new d-TAB. Both provide the temperature and displacement data directly in oC and mm.
Automated data retrieval:
Clusters of instruments (4 per Logger) that are monitored using YieldPoint’s d4Logger can be wireless networked using:
- dMesh – a 900MHz wireless mesh solution.
- IoT GateWay if Ethernet is available