Epoxy Injection and Grouting for Underwater Concrete Repair
By Ed Brannigan, Marine contractor. Reviewed by Dana Whitcombe, technical reviewer.
Marine Construction | Boat Lift Installation I remember a night dive on a mooring anchor block in Jacksonville Harbor, water maybe six feet of visibility, crack running the full face of the block. We'd hauled the pump down wired to a lift bag, and I just lay there watching epoxy bleed out where the crack opened up, no answer for it but to hit it with more pressure and hope the viscosity held. That was a bad night. The water was 52 degrees, and that epoxy was acting like cold honey. You learn quick that underwater concrete crack repair isn't about pumping goop into a hole; it's about getting the material to sit still long enough to bond. And that's as much about patience as it is about thixotropy. This is the part of the trade that's caught between heavy civil and marine. A lot of guys know how to inject on dry concrete, but add a foot of water head and the rules change. So here's how I've done it, mistakes included.
First, Read the Crack Like a Witness
You got to decide whether the crack is just surficial or carries load all the way through the section. Common method: drill a test hole or two at a steep angle, clean with air, and see if it's dry or weeping. If it weeps water, you've got a through-crack, and your injection plan changes entirely. Wet crack means you're fighting hydraulic pressure. A hairline crack less than 1/16 of an inch wide is one thing; a crack you can fit a flathead screwdriver into is another. The widest I've welded shut was 1/2 inch at an intake manifold on the Banana River. That one required a neat dry pack of rapid-set hydraulic cement before we ever tried to inject epoxy. For the standard job, the drill goes: inspect, clean, seal the surface, install ports, inject under pressure. But the underwater twist is that you're often sealing the surface with hydraulic cement that sets slowly, and you better test the set time before you hang on a curve. Call it 45 minutes to an hour for most fast-set plugging compounds, but it depends on the plant.
Picking the Right Epoxy: Wet and Drunk
The old-timers used to inject anything that came in a tube, and half the time it washed out. Today, you need a 100% solids, moisture-insensitive epoxy with a viscosity that works in the structure you're fixing. A high-viscosity paste isn't going to penetrate a hairline crack; a low-viscosity syrup will run all over the place in a wide crack. Check the manufacturer's data sheet for 'underwater application' or 'damp or wet surface' rating. We've had good luck with products like SealBoss Underwater Epoxy and Simpson Strong-Tie's Set+X for a few years now, but the spec is the thing. Get the pressure rating, the working time, and the cure time, and work backwards from your dive time. Cold water changes everything. Your fingers get clumsy, and the epoxy kicks off faster than the data sheet says because you're chasing your own bubbles. Keep a spare pump on deck, and make sure your diver has a comms unit so you can coordinate the pressure from above. For a crack that's actively leaking, you might need a hose or a 'quit' that gives you a few extra minutes. But don't rely on that. Keep the mix ratio exact with a calibrated scale, not eyeballing in the bucket. That's how you get strands of uncured goo in the water and a float on your hands.
Ports, Packers, and How to Press the Trigger
Once you've sealed the surface, you set injection ports. We use metallic packers that thread into holes drilled every 6 to 8 inches along the crack, never more than half the depth of the element apart. If the element is thin, just use surface flanges with a hydraulic cement saddle. The pump should be a positive displacement pump, hand-crank or hydraulic. For small jobs, a 10:1 or 20:1 ratio cartridge gun works, but for larger jobs, you need a pump that can hold pressure without the ram dogging. We use a Graco HSD-1 for a lot of our underwater work, but that's a heavy rig to lower on a line. Start injecting at the lowest port and work up. The pressure at the pump might read 300 psi, but underwater you're deadheading into a column of water. The real driving pressure at the crack face is that pump pressure minus the water head at the port. So if you're 30 feet down, subtract about 13 psi for the water head, give or take. Watch for epoxy to bleed out at the next port, then cap that one and move on. You'll know you're doing it right when the crack surface doesn't weep water anymore.
Cure, Inspect, and Know When to Quit
The epoxy's got to cure hard. For most products, that means 12 to 24 hours at full strength, but the product spec is the boss. We've set a temporary sandbag brace over a repair overnight, then pulled it in the morning. Don't let the contractor rush you. After the full cure, air dive and inspect the surface for spalls or leaks. And if you want to be certain, core a sample. We'll drill a 2-inch core through the crack and look at the penetration. If you got a nice continuous band of epoxy through the section, you're done. If you see it's mostly on the surface and dry inside, then you either didn't seal the perimeter or your injection pressure was too low. There's also a point where you accept a repair won't hold. If the crack is a live notch from a structural movement, epoxy won't fix it. You have to decide early if you're sealing water or welding structure. Don't pretend a crack injection is a structural fix for a moving joint. That's the work. It's messy, it's cold, and you don't get a second chance if the mix kicks in the line. But when you see the crack dry during a wet dive, you all can feel a little proud of the job.
Frequently Asked Questions
Can you really inject epoxy into a crack underwater? Yes, but only with a moisture-tolerant, 100% solids epoxy that's rated for underwater placement. The trick is the crack itself has to still be solid, and the surface has to be clean enough for the epoxy to grip. How long does underwater epoxy take to cure? Depends on the product and water temp. Most working epoxies develop enough strength in 12 hours to release pressure, but full cure can take 7 days. Check the data sheet. What causes epoxy injection to fail underwater? Usually it's a dirty crack, too low a pressure, or the wrong viscosity. Or the crack moves from structural settlement. You got to treat the cause, not the symptom. How deep do you have to drill for injection ports? Usually we drill to intersect the crack at a depth of at least half the slab or element thickness. In a concrete wall that's a foot thick, that's a 6-inch hole.