Flat-jack and push-shear tests

Flat-jack test

A flat jack is a thin rectangular steel bag (about 5 mm thick) which expands under hydraulic pressure. It measures the in-situ stress and deformability of masonry.

1. Single-jack test (stress): two pairs of measuring pins are fixed above and below a horizontal bed-joint line and the distance between them is measured with a mechanical gauge. A slot is cut in the bed joint with a saw; the stress is released and the pins move closer. The jack is placed in the slot and pressurised until the distance returns to its original value.
2. Double-jack test (modulus and strength): two parallel slots are cut about 4 to 5 courses apart and jacks fitted in both; the masonry between them is loaded like a compression specimen to get a stress-strain curve.

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

where $p$ is the cancellation pressure, $K_m$ the jack calibration constant (0.7 to 0.9) and $K_a$ the ratio of jack area to slot area.

 pin  +----------+  pin
  o   |==========|   o    measure d before / after cutting
 --- [  flat jack  ] ---  <- slot in bed joint
  o   |==========|   o
      +----------+
      pump -> pressure p

Push-shear test

An in-situ test for the shear strength of bed joints: one brick is freed by removing a head joint and a jack pushes it along its bed joints until it slides. Shear strength $\tau = F/(2A_b)$ and, with the vertical stress $\sigma_v$ (from the flat-jack test), cohesion $c=\tau-\mu\sigma_v$.

 +----+----+----+----+
 |    |[jack]->[brick]|   F pushes brick horizontally
 +----+----+----+----+