Judging Concrete Underwater by Sound
By Ed Brannigan, Marine contractor. Reviewed by Dana Whitcombe, technical reviewer.
Marine Construction | Boat Lift Installation Late November 2011. I was on a barge off Fernandina Beach, waiting for a load of concrete to pour a tie-back for a seawall. The water was flat and green, and the only sound was the generator chugging and the seagulls going after baitfish. The ready-mix dispatcher called to say they were running late, and I remember thinking, good. Because the part I hated about those big underwater pours isn't the waiting. It's that you can't see the concrete after it leaves the pipe. You have to listen. I've spent thirty years building seawalls, docks, and pile foundations along the Gulf and Atlantic. The first time I did an underwater tremie pour, I was the gofer, and the super told me to 'listen for the tone to change.' I didn't know what that meant. I've learned. This is how to judge concrete underwater by sound — the stuff they don't write in the spec book.
The Tremie Pipe Rumble
When we place concrete by tremie, we lower a pipe to the bottom and pump concrete down it. The concrete pushes up around the pipe, and water never touches it if we do it right. I listen to the pipe itself. A good, consistent pour makes a low rumble, like a freight train a mile off. The pipe vibrates in your gloves. That rumble means the concrete is moving down as a mass, not breaking apart. If the rumble gets high-pitched, that's fine for a moment — it means the head of concrete is thinning. If it stays high and watery, you're in trouble. The concrete is segregating or you've been pulling the pipe too fast. You'll hear a change before you see anything at the surface.
The Suck You Never Want to Hear
There's a sound that makes any diver's stomach drop. It's a slurp, or a gulp — like a straw finding the bottom of a cup. It means the tremie pipe has lost its seal. Water has run into the pipe, and that column of concrete is now contaminated. I heard it once on a job in Mobile while we were placing a pier shaft. The diver was three feet under and I heard that suck, and we had to abandon that shaft and chip it out. Cost us two days. You learn that sound by listening to what goes down the pipe. If you hear a slurp, stop the pour immediately. Don't 'wait one more bucket.' There's no fixing it from the top. You pull the pipe, check the seal, and re-sink it. Or you accept the contaminated section and drill it out later.
Tapping Forms for Voids
Once the form is stripped — or on big pours, while it's still in place — we go back with a hammer and listen for voids. A solid mass of concrete against a form rings: a clear, sharp tap when you strike a steel form. On a wood form, it's a dull thud that still feels solid, like hitting a plank with a mallet. But a void? It thuds with a hollow ring, like a drum. You can hear the difference acoustically even under water. The trick is to develop a consistent strike. I use a two-pound maul, and I hit flat, not with a glancing blow. I tap a grid pattern — every foot. I also carry a length of rebar and hold it against the form while tapping the other end with a hammer. The rebar transmits the sound directly to your ear if you press your ear to the other end. This works better than trying to hear through the water.
The Diver's Ear and the Split-Sound
When a diver taps from inside, you often get two sounds: the initial strike and a secondary echo. If the second sound comes back quick and sharp, the concrete's solid. If it lags, or you hear a 'ping' like a bell, there's a cavity behind the concrete. This is called a 'split-sound.' You need to mark it and grout it after. I always tell my divers to practice on known good sections. Before we pour, tap the empty steel form — you'll hear the ring of the empty space. After the pour, tap a section you're sure of. Build the memory in your ear. It's like learning a language.
Frequently Asked Questions
Can I use a hydrophone or electronic stethoscope to hear underwater concrete better? Yes, but I've found a simple metal rod and your ear on the handle works just as well on most jobs. Electronics add complexity and cost. If you're recording for QA, that's different. But for everyday judgment, the rod and hammer are hard to beat. What if the concrete is behind a wood form? Does sound work? Wood forms absorb sound more than steel. You'll hear a lower frequency, but the same principle applies. Tap a known spot first. Compare. If you hear a distinct hollow, it's a void. How big does a void have to be to hear it? With practice, you can catch a void an inch deep and a foot across. It's not exact. But if you're at the 'can I hear it' stage, you've already got a problem. Use ground-penetrating radar or pachometer if you need numbers.