Elastic wave tomography and push shear tests

Elastic wave (sonic) tomography

Principle: elastic (stress) waves travel faster in sound, dense masonry and slower through voids, cracks and moisture. The travel time between many source and receiver positions is measured and processed to give a velocity image (a tomogram) of the wall interior.

Procedure

1. Mark a grid of points on both faces of the wall (e.g. every 0.25 to 0.5 m).
2. A small instrumented hammer (or piezo transmitter) at one point generates a pulse; accelerometers or receivers at other points record the arrival time $t$.
3. Velocity along each path $v = d/t$ ($d$ path length). Many crossing paths are collected.
4. Software (inversion, e.g. SIRT) divides the wall into cells and solves for a velocity in each cell, giving a colour map.
5. Interpretation: low-velocity zones are voids, cracks or moist or weak masonry; high velocity means sound masonry. Used to detect internal damage, repairs and leaf separation without damaging the wall.

 Transmitter (hammer)        Receivers
  T o ----------> o R1       Wall section grid
    o ----------> o R2       paths cross the wall;
    o ----------> o R3       slow zone = void or crack

Push shear test

An in-situ test of the shear strength of bed joints. A brick is freed by removing the vertical joints on both sides (one brick removed to fit a jack), then pushed horizontally with a hydraulic jack until it slides. Shear strength $\tau=F/(2A_b)$ and cohesion $c = \tau-\mu\sigma_v$, with $\mu\approx0.4$ to $0.65$ and $\sigma_v$ the vertical compressive stress on the joint.

 +----+----+----+
 |    |[jack]->[brick]|    F slides the brick
 +----+----+----+