Flat-jack test for brick masonry wall

The flat-jack test is a minor-destructive in-situ test to find the compressive stress, modulus of elasticity and strength of existing masonry. A flat jack is a thin steel bag filled with oil that acts as a hydraulic pressure cell.

Test set-up

 wall surface
  o     reference pins (demec points)
 ----- ----- ----- -----
  o  [  flat jack in slot ]  o      <- horizontal bed joint slot
 ----- ----- ----- -----
        |
      pump + pressure gauge

Procedure (single-jack test: in-situ vertical stress)

1. Fix 4 to 6 pairs of measuring points (pins) across the mortar joint, above and below the line of the slot; measure initial distance $d_0$ with a mechanical gauge.
2. Cut a slot in the mortar bed joint with a masonry saw or by drilling; stress is released and the distance reduces to $d_1<d_0$.
3. Insert the flat jack into the slot and increase pressure slowly with a hydraulic pump until the pins return to the original distance $d_0$. Note the cancellation pressure $p$.
4. Compute the in-situ stress in the wall.

$$\sigma_m = K_m\,K_a\,p$$

with $K_m$ = calibration factor of the jack (about 0.8 to 0.9) and $K_a$ = ratio of area of jack to area of slot (about 0.9 to 1.0).

Double flat-jack test (modulus and strength)

1. Cut two parallel slots 400 to 500 mm apart (4 to 5 courses) in the same vertical line and install a jack in each.
2. Increase pressure in steps; measure the vertical deformation of the masonry between the jacks, giving the stress-strain curve and modulus of elasticity $E=\Delta\sigma/\Delta\varepsilon$.
3. Continue to the first cracking to get the compressive strength.

  ====[ jack 2 ]====   slot
   |                 |
   | masonry panel   |   stress and strain measured
   |                 |
  ====[ jack 1 ]====   slot

Uses: determine the actual stress state in old buildings (e.g. heritage structures), compressive strength and elastic modulus for seismic assessment, with little damage (the slots are refilled with mortar).