Ultrasonic pulse velocity and tomography tests

Ultrasonic pulse velocity (UPV) test

A transmitter transducer generates an ultrasonic pulse (typically 50–150 kHz) at one point on the wall, and a receiver picks it up at another. The electronic unit measures the travel time $t$. The pulse velocity is

$$V = \frac{L}{t}$$

where $L$ is the path length between transducers.

Arrangement: direct (transducers on opposite faces), semi-direct (on adjacent faces), or indirect (same face). Direct is the most reliable. A couplant (grease or gel) is used for good contact.

  Transmitter ))))))))  path L  )))))))) Receiver
      [T]=========== wall ===========[R]
          \_______ time t _______/

Uses

• Compare quality: high velocity means dense, sound masonry; low velocity means voids, cracks, loose fill or damage.
• Estimate the dynamic elastic modulus: $E_d = \rho V^2\dfrac{(1+\nu)(1-2\nu)}{(1-\nu)}$.
• Locate flaws and check grout injection; estimate strength using calibrated correlations.

Limitations: results depend on moisture, mortar and unit properties, so the correlation to strength is poor; the pulse is scattered in heterogeneous masonry such as rubble stone, and the method needs access to both faces.

Elastic wave tomography

Many source and receiver points are placed around the wall section (or in a grid on both faces). The travel time of each ray is measured by hammer impact or ultrasonic transducers. A computer inversion (e.g. iterative ray tracing) divides the section into cells and finds the velocity in each cell so that the calculated travel times match the measured times. The result is a velocity map (image) showing the internal condition.

   S1  S2  S3  S4        Source points on one side
   \\\ ||| /// \\
   .-------------.       cell grid
   | 2.1  1.4 2.0|       velocity (km/s) in cells;
   | 2.2 [0.8]2.1|       low value [0.8] = void
   '-------------'
   R1  R2  R3  R4        Receivers

Uses: finding voids, cracks, delaminations, and the inner core quality of thick stone and multi-leaf walls, and checking the success of grouting.

Limitations: needs many readings and processing software; resolution is limited by wave length and number of rays; interpretation requires experience.

Both tests are non-destructive (no damage to the heritage fabric) and are used together with semi-destructive tests such as the flat-jack test for calibration.