Pouring concrete underwater requires specialized techniques to prevent the cement from washing away before it can set. The core challenge is simple: when fresh concrete contacts flowing water, the cement paste separates from the aggregates, and you lose both strength and material. Every method used for underwater concrete placement solves this problem in a different way, either by shielding the concrete from water contact during placement, by modifying the concrete mix itself, or both.
Why Regular Concrete Fails Underwater
When standard concrete is dropped into water, the cement particles disperse into the surrounding water instead of binding with the aggregates. This process, called washout, strips away the paste that holds everything together. The result is weak, honeycombed concrete with large voids and poor structural integrity. Underwater concrete typically retains only 40 to 90 percent of the compressive strength it would have achieved in dry conditions, depending on the placement method and mix design. Washout loss correlates directly with strength reduction, so minimizing cement loss during placement is the single most important factor in getting a strong final product.
Water movement makes things worse. Research shows that concrete placed in currents faster than 0.5 meters per second (roughly 1.6 feet per second) suffers significant aggregate segregation and cement dispersion. At lower velocities, between 0.2 and 0.5 m/s, well-designed underwater mixes can still achieve over 65 percent of their dry-cast strength. At the slowest currents, that number climbs above 90 percent. If you’re working in moving water, slowing or diverting the current before placing concrete makes a measurable difference in the finished product.
The Tremie Method
The tremie is the most widely used technique for placing concrete underwater. It works by feeding concrete through a long, sealed pipe (the tremie) that extends from above the water surface down to the placement area. The bottom of the pipe stays buried in the fresh concrete at all times, so the new material pushes outward from within the mass rather than falling through open water. This keeps the cement paste from ever contacting the surrounding water directly.
Setting up a tremie pour starts with lowering the pipe to the bottom and plugging its end with a “go-devil” or similar seal. Concrete is loaded into a hopper at the top, the seal is released, and the weight of the concrete column pushes material out the bottom. As the pour progresses, the pipe is slowly raised, but the discharge end must stay embedded in the growing mound of concrete. If the pipe pulls free and water enters, the seal is broken and washout begins. Maintaining a continuous flow is critical. Stopping and restarting a tremie pour creates a weak cold joint where the old and new concrete meet.
Tremie concrete needs to be highly fluid so it flows outward from the pipe under its own weight. You can’t vibrate or work it once it’s placed. Slump values are typically much higher than for conventional concrete, and the mix usually includes extra cement and fine material to improve flow while keeping the paste cohesive enough to resist washout.
Concrete Pumping Underwater
Concrete pumps work on a similar principle to tremies but use mechanical pressure to push concrete through a pipeline to the placement point. The discharge end of the pump line stays submerged in the fresh concrete, just like a tremie pipe. Pumping offers more control over placement rate and works well for reaching locations where a vertical tremie setup isn’t practical, such as filling forms with complex shapes or placing concrete horizontally under an existing structure.
The concrete mix for pumped underwater placement needs to be cohesive enough to hold together during transport through the line, fluid enough to flow through the pump without clogging, and resistant to washout once it exits. This is a demanding set of requirements, and getting the mix design right is essential.
Preplaced Aggregate Method
This technique flips the usual process. Instead of mixing everything together and placing it underwater, you first fill the formwork with coarse aggregate (stone), then inject grout into the voids between the stones. The aggregate acts as a skeleton, and the grout fills the gaps and bonds everything together.
The aggregate is typically a uniform size, commonly in the 19 to 26.5 millimeter range (roughly 3/4 to 1 inch), to create consistent void spaces that grout can flow through. The maximum aggregate size should be less than one-quarter of the narrowest dimension of the form. Grout can be injected using pumps or, in simpler applications, by gravity feed through pipes inserted into the aggregate mass. The injection point starts at the bottom, and grout rises upward through the stone, displacing water as it goes.
Preplaced aggregate concrete works especially well for underwater repairs, filling cavities, and encasing structural elements. Because the stone is already locked in position before grouting begins, there’s no risk of aggregate segregation. The method also produces very dense concrete with minimal voids, since the grout is forced into every available space.
Anti-Washout Admixtures
Anti-washout admixtures (AWAs) are chemical additives mixed into the concrete that dramatically increase the cohesion of the cement paste, making it resist dispersal in water. They work by creating a three-dimensional molecular network within the concrete that holds the paste together even when it contacts flowing water. The technology dates back to the 1970s in Germany, where researchers first added cellulose-based thickeners to conventional concrete and successfully placed it underwater.
The three most common types of AWAs are cellulose derivatives, polysaccharides (such as welan gum and xanthan gum), and acrylic-based polymers like polyacrylamide. All of them increase the viscosity of the mixing water, which keeps the cement particles suspended and bonded rather than drifting away. The typical dosage is 1 to 1.5 percent by weight of cement. Going higher than 3 percent can actually backfire, causing the molecular network to break down when combined with other common admixtures, which increases washout rather than preventing it.
AWAs don’t replace careful placement techniques. They’re used alongside tremie or pump methods to provide an extra layer of protection against cement loss. In situations where some water contact is unavoidable, such as placement in moving water or in forms that are difficult to seal, AWAs can mean the difference between concrete that holds together and concrete that washes away.
Cofferdams and Dewatered Enclosures
When practical, the simplest approach is to keep water away from the concrete entirely. A cofferdam is a temporary enclosure, typically made of steel sheet piles driven into the riverbed or seabed, that walls off the work area. Water is pumped out, and concrete is placed in dry or near-dry conditions. This produces the strongest results because there’s no washout at all, but it’s also the most expensive and labor-intensive option. Cofferdams are common for bridge piers, dam repairs, and other large structural projects where the cost is justified by the need for full-strength concrete.
For smaller jobs, sandbag enclosures or portable barriers can reduce water flow enough to allow placement with minimal washout, even if they don’t create a fully dry workspace.
Pouring in Saltwater vs. Freshwater
Saltwater actually accelerates concrete’s early chemical reactions. The chlorides in seawater speed up the hydration process, shorten setting time, and boost early strength. In the first week, seawater concrete can outperform freshwater concrete in strength tests. After that initial period, however, the trend reverses. Long-term compressive strength in seawater concrete ends up 7 to 10 percent lower than concrete mixed and cured with fresh water.
The bigger concern with saltwater is corrosion. Chloride ions penetrate concrete over time and attack steel reinforcement from within. For any reinforced concrete placed in a marine environment, the rebar needs significantly more concrete cover (the thickness of concrete between the steel and the outside surface) than it would in freshwater or dry conditions. Using coated or stainless steel reinforcement, or fiber-reinforced polymer bars instead of traditional steel, further extends the life of marine concrete structures.
Practical Steps for a Successful Pour
Regardless of which method you use, several principles apply to every underwater concrete placement:
- Minimize water movement. If you can slow or stop water flow at the placement site, do it. Even a modest current degrades the final product. Aim for velocities well below 0.5 m/s.
- Keep the concrete sealed from water during placement. Whether you’re using a tremie, pump, or grout injection, the fresh concrete should never fall freely through open water. Every moment of exposure means cement loss.
- Use a cohesive mix. Underwater concrete mixes need higher cement content, more fine aggregates, and often an anti-washout admixture. The goal is a mix that flows easily but clings to itself.
- Pour continuously. Interruptions create cold joints, which are weak planes where layers of concrete meet but don’t bond properly. Plan your pour so materials, equipment, and crew can keep going without breaks.
- Don’t disturb the concrete after placement. Unlike dry concrete work, you typically cannot vibrate or rework underwater concrete. The mix needs to be self-leveling and self-compacting from the start.
- Overfill slightly. The top layer of underwater concrete, called laitance, is always the weakest because it has the most water exposure. Plan to place more concrete than you need and remove or ignore the weak surface layer.