Push shear test of masonry

The push shear test is an in-situ, minor-destructive test to find the shear strength of the bed joints of existing masonry, i.e. the cohesion and friction of the unit-mortar interface.

Procedure

1. Select a brick in a plain region of the wall (not near openings). Remove the adjacent brick at one end to make room for the jack, and rake out the head (vertical) joint at the other end to free the test brick.
2. Place a small hydraulic jack and load cell in the gap, with a steel plate bearing on the end of the test brick.
3. Push the brick horizontally (parallel to the wall) at a slow, steady rate until it slides along its bed joints. Record the load $F$ at first movement (sliding).
4. Estimate the vertical compressive stress $\sigma_v$ on the joints from the load above (or from a flat-jack test).
5. Repeat on several bricks and take the average.

 wall elevation
 +----+----+----+----+
 |    |  jack |    |    <- removed brick position
 +----+--[F]->[brick]--+   brick pushed along bed joint
 |    |    |    |    |
 +----+----+----+----+

Calculation

$$\tau = \frac{F}{2A_b},\qquad \tau = c + \mu\sigma_v$$

where $F$ is the sliding load, $A_b$ the bed-joint area of the brick (length $\times$ thickness; there are two joints, above and below), $c$ the cohesion and $\mu$ the coefficient of friction (about 0.4 to 0.65, from tests). The cohesion $c=\tau-\mu\sigma_v$ is used in the shear design of the wall.

Merits: quick, cheap and nearly non-destructive (only one brick is replaced). Demerits: scatter in the results, sensitivity to local mortar quality, and it gives the strength of the joint only.