Reading the Concrete You Can't Core

A mass-concrete dam is almost all interior. You can walk its galleries and inspect its faces, but the concrete that actually carries the reservoir, lift line after lift line, is metres deep and will never be seen directly. Coring reaches it, but a core is a pencil line through a mountain: it tells you about one spot and nothing about the metre beside it. So the question that governs an ageing dam, how sound is the concrete inside it, is one that visual inspection cannot answer. The answer comes from sound itself. Drive an ultrasonic or seismic pulse through concrete and it travels faster the denser and sounder the concrete is. Where there is a crack, a void or deterioration, the pulse slows or detours around it, and the velocity drops. The speed of sound becomes a reading of quality.

Seminoe Dam, a concrete arch dam in south-central Wyoming completed in 1939, had been quietly failing that test for decades. Alkali-aggregate reaction, the slow internal swelling that cracks concrete from within, had been expanding and weakening it, but the faces showed only the symptoms: cracking and staining over the upper part of the downstream face. How deep the damage ran, and how the quality varied from the upstream face to the downstream, no inspection could say. So between 1999 and 2000 a U.S. Bureau of Reclamation team did something more ambitious than coring. They mapped the inside with sound.

A total of approximately 13,700 seismic waveforms were acquired across three upstream-downstream cross sections during 9 days of data acquisition.

U.S. Bureau of Reclamation, Dam Safety Office report DSO-02-03.

Impacting and recording at points across the crest and both faces, they timed how fast a seismic pulse crossed the dam along thousands of different ray paths, then inverted all those travel times into a velocity image of each cross section, a tomogram, the same idea as a medical CT scan. What it showed was a dam with a soft upper skin. Velocities ran from around 4.7 kilometres a second, the speed through strong, competent concrete, down to under 2.1 in the worst zones, concentrated in the upper nine to twelve metres. Over the eighteen years to 1998, the average compressive strength of that concrete had fallen about twenty-three percent.

The areas with the lowest P-wave velocities are interpreted as regions with the most severely deteriorated concrete.

U.S. Bureau of Reclamation, Dam Safety Office report DSO-02-03.

And it was not a guess. The velocity pattern matched what the cores, the fracture mapping and the deformation surveys had each found at their own scattered points; the tomography tied them together into a continuous picture of where the good concrete ended and the bad began. That is the discipline that makes NDT defensible. You still take a few cores, but you use them to calibrate the velocity-to-quality relationship, not to guess at the ninety-nine percent of the dam that no core will ever touch.

The lesson: A dam's faces tell you where it is cracking. They do not tell you how deep the damage runs, or how the concrete varies from one face to the other. Non-destructive testing is how you read the interior without destroying it, and the ultrasonic pulse velocity is the workhorse. But remember what the number is. Velocity tracks density and stiffness directly, and strength only indirectly, so a reading is a verdict on quality, not a substitute for a calibrated, cored strength. Read the whole dam with sound, confirm the verdict with a few cores, and never mistake a sound-looking face for sound concrete behind it.

Read more: NDT Tests for Dam Concrete: An Owner's Guide →