Underwater, conventional concrete washes out and loses its fines before it sets. That's why anti-washout mixes, specific mortars and placement methods that avoid contact with free water are used. The part that most affects the result isn't the mix, but surface preparation.
Underwater concrete repair is one of those specialities that never make it into the commercial diving brochures, yet it underpins a large part of marine and river civil works. Quays, bridge piers, dykes, gates and foundations degrade, and fixing them almost never means draining the water.
It's also a discipline where the diver stops being just a pair of hands and becomes the executor of a construction procedure. What's done badly underwater doesn't show up until years later, when the patch comes away.
Why conventional concrete doesn't work submerged
Concrete is a mix of coarse aggregate, fine aggregate, cement and water. Its strength depends on the cement paste coating and binding every grain. If that paste is lost, what's left is a skeleton of stones with no cohesion and no structural capacity at all.
That is exactly what happens when conventional concrete falls through a layer of water. The water passes through it, carries away the suspended cement and also washes out the finest aggregate particles. The result is a segregated material, with laitance on top, coarse aggregate below and unpredictable strength.
What exactly is lost
Two things are lost that can't be recovered. The first is cement, meaning binding capacity. The second is the continuity of the matrix: even if enough cement remains overall, the distribution stops being uniform and rich and poor zones appear within the same element.
There's also a side effect that's felt on site. The washed-out cement clouds the water around the work area and leaves the diver working blind within seconds, making it harder to check what's being done at exactly the moment it matters most.
Specific mixes: anti-washout concrete and repair mortars
The industry's answer to this problem is twofold: modify the mix and modify the placement method. Neither is enough on its own.
Hormigones antilavado
Anti-washout concretes include admixtures that increase the paste's viscosity and internal cohesion. In practical terms, the mix behaves as a mass that resists breaking apart when water passes through it, instead of dissolving.
The problem is that cohesion and workability pull in opposite directions: a highly cohesive mix tends to be hard to pump and place. That's why these concretes combine the cohesion admixture with superplasticisers, aiming for a material that flows well but doesn't wash out.
Repair mortars
When the volume is small and what needs filling is a void, honeycombing or lost cover over reinforcement, repair mortar is used rather than concrete. These are fine-grained, high-bond, controlled-shrinkage products formulated for application in wet conditions.
Some are applied by hand or with a trowel, others are injected or pumped into formwork. Both cement-based and epoxy-based versions exist, and the choice depends on how aggressive the environment is, the thickness to be made up, and whether the area stays permanently submerged or not.
Placement methods
The golden rule is simple: fresh concrete must never pass through free water. All the methods used in submerged work are variations on that idea.
Tubo tremie
It's the classic method for large volumes. A pipe is lowered to the bottom of the space being concreted and the concrete is poured through it, always keeping the pipe's mouth embedded in the mass already placed. The new concrete pushes the earlier concrete from within and displaces the water upward, without mixing with it.
There's an obvious critical point in the operation: if the pipe's mouth becomes exposed, water gets in, washout occurs and the continuity of the element is compromised. That's why pouring is continuous and the pipe is raised in a controlled way, with the diver or a ROV checking the level of the mass.
Bombeo directo
With anti-washout concrete and a pumping hose, the material can be placed directly at the work point, keeping the nozzle buried in the mass. It's more agile than the tremie method for moderate volumes and reaches points a rigid pipe can't fit into.
In exchange, it demands fine coordination between the diver directing the nozzle and the pump operator on surface. A sudden start or a long stoppage with a loaded hose causes problems, so communication through the umbilical stops being a formality and becomes part of the procedure.
Bags and fabric formwork
To repair piles, piers and elements with more or less regular geometry, technical fabric formwork is used, wrapped tightly around the element and filled by pumping. The fabric retains the paste and lets some water escape, which improves the water-cement ratio of the material placed.
The advantage is logistical: it's light, transported rolled up, and a small team can place it without a crane. The limitation is that the result depends heavily on how tightly the fabric fits the substrate and on the seams and closures holding up under pumping pressure.
Grout injection and preplaced aggregate
The preplaced aggregate method, known as prepacked aggregate, reverses the usual order. The formwork is first filled with clean, well-graded coarse aggregate, and then a cement grout is injected from the lowest point, rising and filling the gaps between the stones as it displaces the water.
It's a useful technique in spaces with awkward geometry and for large-volume fills, because the aggregate doesn't wash out and the grout, injected from below, doesn't pass through free water either. It requires strict control of grout fluidity and the sequence of injection points.
Grout injection on its own, without preplaced aggregate, is used to seal cracks, fill voids under slabs and consolidate the contact between a structure and the ground.
Surface preparation: the part that decides the result
All of the above is the visible part of the work. The part that determines whether the repair lasts comes before it and is far less visible: preparing the substrate.
Limpieza
All sorts of things grow on submerged concrete. Algae, calcareous fouling, mussels, fine sediment. None of that allows bonding, so it has to be removed down to sound concrete. It's done with a high-pressure washer, brushes, scrapers and pneumatic or hydraulic tools depending on the case. It's slow, unglamorous work, but it determines everything that follows.
Hydrodemolition
There are two ways to remove deteriorated concrete. Mechanical breaking is fast but microcracks the material left behind and can damage the reinforcement. Hydrodemolition, which uses a very high-pressure water jet, selectively removes degraded concrete while leaving sound concrete intact, also leaving a rough, clean surface that's an excellent substrate for the new material.
It's an operation with its own very serious risks: the jet cuts flesh and generates considerable reaction force. It's carried out under specific procedures, with the equipment positioned and the diver out of the firing line, and it's part of the equipment specialised kit that not every company has.
Corroded reinforcement
When corrosion has reached the steel, simply covering it over guarantees the problem keeps advancing under the patch. The correct procedure is to fully expose the reinforcement around its perimeter, clean it back to bare steel, assess the section loss and decide whether it needs recovering with lapped new bars or whether protecting and recoating is enough.
The chloride-contaminated concrete around the area also has to be removed, because if it's left in contact the repair can accelerate corrosion at the edges. On exposed marine structures, cathodic protection is often considered as an added measure too, and its performance is then tracked through periodic inspections of the kind described in the underwater civil works inspection.
Encofrados sumergidos
Underwater formwork has to solve two things at once: giving shape and preventing the paste from leaking out. Metal or phenolic plywood panels, precast moulds, fabric formwork and hybrid systems are all used.
The tricky points are sealing the contact with the existing structure, solved with compressible-material joints or pre-set seals, and anchoring, because fresh concrete exerts a pressure the diver can't counter by hand. Placement is often supported by underwater platforms and scaffolding that allow working with stability and both hands free.
An escape route for the displaced water and air also has to be provided. Formwork without vents at the high points leaves pockets that stay hollow and later show up as honeycombing exactly where it's least welcome.
Quality control
Checking can't just mean looking. The mix is controlled on surface, with consistency tests and cast specimens, and the result is checked on site through detailed visual inspection, tapping to detect hollow areas and, when the project requires it, coring to test strength and bond.
All of that is documented. In marine civil works, traceability of the process carries as much weight as the result, and a large part of the value an experienced team brings is recording properly what was done, with photos, video and signed reports.
Where this work comes up
Typical applications are piers and abutments of bridges over water, quays and pontoons, dykes and breakwaters, foundations and pile caps, gates and hydraulic structures, and in general any concrete attacked by the marine environment. It's a market tied to public works and infrastructure upkeep, with demand that's steadier and less cyclical than the offshore energy sector's.
A large part of this activity is concentrated in port areas, where it coexists with the rest of port and marine works and where specialised local companies are usually based. The listing of companies in the sector gives an idea of the kind of company that carries out these contracts.
What profile it calls for
This work rewards a particular kind of diver: patient, methodical and able to follow a construction procedure without improvising. The skill that sets someone apart isn't depth, because the work is almost always at modest depths, but manual dexterity in reduced visibility and understanding why each step is where it is.
The way in is the same as for the rest of the trade: a qualification recognised in the market where you want to work, according to the map of certifications, and the pathway described in the guide to training as a commercial diver. From there, the specialisation is picked up on site, working alongside teams that already have it mastered.
In closing
Concreting underwater isn't dry concreting made harder: it's a different procedure, with its own mixes and methods designed so the material never comes into contact with free water. Understanding that logic avoids most of the big mistakes.
And the lesson every experienced team repeats is that the mix matters less than it seems. Excellent concrete over a dirty surface or badly treated reinforcement still ends up coming away. The quality of an underwater repair is decided in the hours of cleaning and preparation that nobody sees.
Frequently asked questions
Can you pour concrete directly underwater?
Not with conventional concrete, because the water washes away the cement paste and the fine particles and the mix loses cohesion and strength. Underwater concreting is done with specific anti-washout mixes and with methods that stop fresh concrete falling freely through the water.
What is anti-washout concrete?
It's a concrete dosed with admixtures that increase the mix's internal cohesion, so the paste is held in place and resists being washed away by water. It's usually combined with superplasticisers to keep it flowable, since the mix needs to be both highly workable and highly cohesive.
What is the tremie pipe method?
It involves pouring concrete through a pipe whose mouth always stays embedded in the mass already poured. That way, fresh concrete never passes through free water: it pushes from within and displaces the water upward, which keeps the whole mass homogeneous.
Do you have to be a diver to work in underwater structural repair?
Yes, whenever the work needs hands underwater: cleaning, placing formwork, guiding the tremie, treating reinforcement and checking the result. It requires a recognised commercial diving certification and, in practice, prior experience in civil or marine works.
How long does an underwater concrete repair last?
It depends on the scope, the conditions and the quality of preparation, so there's no general figure. What is constant is that a badly prepared repair fails at the interface with the old concrete long before the new material itself fails.
Published 4 September 2026 · By Arthur More, professional saturation diver
Photo: ogwen · CC BY via Openverse
This article is part of our in-depth guide to underwater welding: the pay, the training and the myth — the complete picture, with sources.
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