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How Do I Install and Set Up the LVDTs on an ASTM C469 Compressometer/Extensometer?

The ASTM C469 compressometer/extensometer uses Linear Variable Displacement Transducers (LVDTs) to measure specimen deformation during testing. When configured for both Modulus of Elasticity (MOE) and Poisson’s Ratio, two LVDTs are used:                                                                                                                                                            Channel 1 – MOE: Measures axial deformation.                                                              Channel 2 – Poisson’s Ratio: Measures lateral deformation.                                                                                                                                                                                                                    This guide covers the physical installation of the LVDTs, mounting the cage to the specimen, and determining the gauge length and correction factor settings required in ForneyVault.

Important: Install and position the LVDTs before testing so that each LVDT has sufficient travel in both directions. An LVDT should not be fully compressed or extended when the specimen is unloaded. 

Install the MOE LVDT – Channel 1

The MOE LVDT measures the axial deformation of the specimen.

  1. Locate the vertical LVDT mounting position on the compressometer/extensometer.
  2. Install the LVDT vertically into the mounting bracket.
  3. Secure the body of the LVDT in the bracket.
  4. Position the LVDT so its measuring tip contacts the corresponding moving portion of the cage.
  5. Adjust the LVDT so the plunger is partially compressed and has sufficient travel available during the test.
  6. Make sure the LVDT and linkage can move freely without binding.

The MOE LVDT will be connected to Channel 1 on the machine.

Install the Poisson’s Ratio LVDT – Channel 2

The second LVDT measures lateral deformation, or the change in specimen diameter during loading.

  1. Locate the horizontal LVDT mounting position at the split center yoke.
  2. Install the LVDT horizontally in the mounting bracket.
  3. Position the measuring tip against the corresponding moving portion of the center yoke.
  4. Adjust the LVDT so the plunger is mostly compressed and has sufficient travel.
  5. Verify that the split center yoke can move freely without the LVDT or mounting hardware restricting its movement.

The Poisson’s Ratio LVDT will be connected to Channel 2 on the machine.

Install the Compressometer/Extensometer on the Specimen

Before installing the cage, back out the specimen contact screws enough to allow the cylinder to fit freely inside the assembly.

  1. Place the concrete cylinder upright on a flat surface.
  2. Position the compressometer/extensometer over the cylinder.
  3. Center the cylinder within the cage.
  4. Support the bottom ring on three equal-height blocks while positioning the cage. This helps keep the assembly level during installation.
  5. Hand-tighten the three lower-ring contact screws evenly against the specimen.
  6. Hand-tighten the two upper-ring contact screws.
  7. Hand-tighten the two center-yoke contact screws.
  8. Remove the temporary supports once the cage is secured.

The contact screws should securely hold the assembly in position without being excessively tightened against the specimen.

After installation, verify that the cage is centered and that the upper and center yokes can move as designed.

Check LVDT Operation

Before placing the specimen into the compression machine:

  1. Verify that neither LVDT is fully compressed or fully extended.
  2. Check that the LVDT plungers move freely.
  3. Verify that the cables have enough slack and are not pulling against the LVDTs or cage.
  4. Gently move the axial mechanism and verify a response from Channel 1.
  5. Gently move the center split yoke and verify a response from Channel 2.
  6. Return the assembly to its unloaded position before zeroing the channels.

If an LVDT is close to the end of its available travel, reposition it before testing.

Determine the Gauge Length and Correction Factor

Once the LVDTs are installed, the gauge length and correction factor must be configured for each channel.

Forney uses the physical dimensions of the compressometer/extensometer to determine these settings.

Channel 1 – MOE

For axial deformation:

Gauge Length = C

The MOE correction factor is:

Correction Factor = A ÷ (A + B)

For typical Forney accessories:

Specimen Gauge Length Correction Factor
4 × 8 in. 5.25 in. 0.500
6 × 12 in. 8.00 in. 0.500
Channel 2 – Poisson’s Ratio

For lateral deformation:

Gauge Length = D

Where D is the specimen diameter.

The Poisson’s Ratio correction factor is:

Correction Factor = E ÷ (E + F)

For typical Forney accessories:

Specimen Gauge Length Correction Factor
4 × 8 in. 4.00 in. 0.414
6 × 12 in. 6.00 in. 0.444

These are typical values for Forney accessories. If the dimensions of the cage differ, measure the actual A, B, C, D, E, and F dimensions and calculate the appropriate correction factors rather than assuming the typical values.

For additional information and diagrams showing these measurements, see:

Determining Gauge Length and Correction Factor Settings for ASTM C469 MOE Test

Before Running the Test

With the cage installed on the specimen:

  • Confirm Channel 1 is connected to the axial/MOE LVDT.
  • Confirm Channel 2 is connected to the lateral/Poisson’s Ratio LVDT.
  • Confirm the appropriate gauge length and correction factor for each channel.
  • Verify both LVDTs have sufficient available travel.
  • Check that the cage and LVDTs move freely without binding.
  • Place the specimen and cage assembly into the testing machine.
  • Connect the LVDTs without allowing the cables to interfere with the cage or machine.
  • Zero the displacement channels with the specimen unloaded and the cage completely installed.

The specimen is then ready for the ASTM C469 test.