Quick Answer: Can You Repair Concrete Underwater?
Yes — but only with materials specifically engineered for submerged placement. Standard concrete, mortar, and grout will wash out and disintegrate when placed in water because the cement paste disperses before it can hydrate and harden. Underwater concrete repair requires anti-washout admixtures that keep the cementitious matrix cohesive, or epoxy injection systems that cure through chemical reaction regardless of water presence.
This guide covers the practical side of underwater concrete repair — the materials, methods, and decision-making that ACI 546.2R outlines in theory. As a manufacturer of both anti-washout grout and underwater-capable epoxy injection systems, we will walk you through what actually works.
Why Standard Concrete Fails Underwater
Portland cement cures through hydration — a chemical reaction between cement particles and water. In theory, water should help. In practice, placing standard concrete or grout in water causes three problems that destroy the material before it can cure:
Washout. Moving water strips cement paste away from the aggregate, leaving behind a weak, porous mass with a fraction of its intended strength. Even in still water, the placement process itself creates enough turbulence to cause washout. Segregation. Coarse aggregate sinks while cement paste floats and disperses, producing an inhomogeneous mass with no structural integrity. Dilution. The water-to-cement ratio increases uncontrollably at the placement interface, resulting in a weak, high-porosity layer exactly where bond strength matters most.The solution is not to keep water away from the concrete (though cofferdams achieve this when practical). The solution is to modify the material so it resists these failure modes.
Underwater Concrete Repair Materials
Anti-Washout Cementitious Grout
Anti-washout grout is the primary material for large-volume underwater concrete repair. It uses viscosity-modifying admixtures (VMAs) — typically welan gum, cellulose ethers, or similar polymers — that dramatically increase the cohesion of the fresh mix. The cement paste clings to the aggregate instead of dispersing, even in flowing water.
Our Underwater Grout XQ-GJ-UW is formulated specifically for submerged placement:
- Compressive strength (underwater cured): ≥ 40 MPa at 28 days
- Compressive strength (dry cured): ≥ 55 MPa at 28 days
- Anti-washout performance: Mass loss ≤ 5% (EN standard test)
- Flow: ≥ 250 mm (self-leveling under gravity)
- Pumpable distance: Up to 50 meters
- Expansion: 0.01–0.03% (non-shrink)
Underwater Epoxy Injection
For crack repair in submerged concrete, anti-washout grout is too viscous to penetrate fine cracks. This is where epoxy injection systems designed for wet conditions come in.
Our Injection Grouting Adhesive XQ-GJ-I is an ultra-low viscosity epoxy (200–500 mPa·s) that penetrates cracks as fine as 0.1 mm. Because it cures through polymerization rather than hydration, water presence does not affect the curing chemistry. It achieves 50 MPa compressive strength and 25 MPa tensile strength regardless of whether the crack is dry or water-filled.
For micro-cracks and fine voids (down to 0.05 mm), our Ultra-Fine Injection Grout XQ-GJ-UF offers even lower viscosity at 100–200 mPa·s.
How to use it: Install injection ports (packers) into drilled holes along the crack at 200–300 mm intervals. Seal the crack surface between ports with a rapid-set surface seal. Inject the epoxy through the ports under pressure (up to 2 MPa), starting from the lowest port and working upward. Each port is sealed when resin appears at the next port upstream, confirming full crack penetration.Underwater Repair Mortar
For patching spalls and surface defects on submerged structures, neither grout nor injection epoxy is ideal. Grout is too fluid for vertical surfaces, and injection epoxy is designed for cracks, not surface repair.
Polymer-modified repair mortars formulated with anti-washout admixtures can be hand-applied by divers to vertical and overhead surfaces underwater. These are specialty products — standard repair mortars, even polymer-modified ones, will wash out immediately.
Comparison: Underwater Repair Materials
| Property | Anti-Washout Grout (XQ-GJ-UW) | Epoxy Injection (XQ-GJ-I) | Ultra-Fine Injection (XQ-GJ-UF) |
|---|---|---|---|
| Compressive Strength | ≥ 40 MPa (underwater) | ≥ 50 MPa | ≥ 45 MPa |
| Application | Void filling, mass placement | Crack injection (≥ 0.1 mm) | Micro-crack injection (≥ 0.05 mm) |
| Viscosity | Self-leveling (≥ 250 mm flow) | 200–500 mPa·s | 100–200 mPa·s |
| Placement Method | Tremie or pump | Pressure injection (≤ 2 MPa) | Pressure injection (≤ 2 MPa) |
| Volume per Application | Large (cubic meters) | Small (liters per crack) | Very small (milliliters per crack) |
| Cure Mechanism | Hydration (cement-based) | Polymerization (epoxy) | Polymerization (epoxy) |
| Cost per Unit | $ | $$$ | $$$$ |
Repair Methods for Underwater Structures
1. Tremie Grouting (Large Voids and Foundations)
Tremie placement is the standard method for filling large voids, underpinning foundations, and grouting pile caps underwater. A pipe (the tremie) extends from the surface down to the repair area. Grout is pumped through the pipe, and the outlet remains embedded in the rising grout mass so that fresh material displaces water from below rather than falling through it.
Key rules for tremie placement:- Keep the tremie tip at least 300 mm below the grout surface at all times
- Start at the lowest point and fill upward
- Do not move the tremie horizontally through the fresh grout — this creates turbulence
- For large areas, use multiple tremie pipes rather than moving one pipe long distances
- Monitor grout level visually or with weighted depth gauges
2. Pressure Injection (Cracks and Delamination)
Pressure injection is used for sealing cracks and rebonding delaminated concrete in submerged structures. The process is the same as above-water crack injection, but the materials must be formulated for wet conditions.
Critical considerations for underwater injection:
- Surface seal must bond to wet concrete — standard surface seals may not adhere
- Injection pressure must overcome hydrostatic pressure at the crack depth, plus the resistance of water inside the crack
- Flushing — water inside the crack should be displaced by the resin, starting from the lowest injection port
3. Formed Repair with Pumped Grout
For restoring cross-sections of underwater structural elements (columns, beams, pile caps), a form or jacket is installed around the damaged area by divers. Anti-washout grout is then pumped into the form through injection ports at the base, filling the form from bottom to top.
This method combines the precision of formed repair with the practicality of underwater placement. The form confines the grout and prevents washout during initial hydration. After the grout hardens (typically 24–48 hours), the form can be removed.
4. Cofferdam (Dry Repair Underwater)
When the repair is critical and access permits, a cofferdam (temporary dam) can isolate the repair area from surrounding water. The water inside is pumped out, and conventional repair methods are used in dry conditions.
Cofferdams deliver the highest quality repair but are expensive, time-consuming to install, and only practical in relatively shallow water with manageable flow. They are the standard approach for bridge pier caps and dam gates where the repair must meet above-water specifications.
Common Applications
Dam repair. Erosion at the dam toe, apron cracking, and leakage through construction joints are addressed with anti-washout grout placed by tremie or pumped through embedded pipes. Crack injection seals active leaks through the dam body. Dock and wharf foundations. Timber, steel, and concrete pile caps deteriorate from wave action, biological attack, and chloride corrosion. Formed repairs with pumped anti-washout grout restore structural capacity without dewatering. Bridge pier scour protection. Scour removes foundation material from around bridge piers during flood events, exposing the foundation and reducing bearing capacity. Anti-washout grout fills the scour void and provides a stable base for riprap or other scour protection. Submerged pipeline supports. Undersea pipelines rest on concrete supports that settle and crack over time. Grouting the void beneath the support re-establishes full bearing contact.Cost Comparison: Underwater Repair Methods
| Method | Typical Cost per m² | Setup Complexity | Quality Level | Best For |
|---|---|---|---|---|
| Tremie grouting | $80-200 | Low-moderate | Good | Large voids, foundations |
| Epoxy crack injection | $150-400 per meter | Moderate | Excellent | Structural cracks |
| Formed repair + pumped grout | $300-600 | High (diver formwork) | Very good | Section restoration |
| Cofferdam + dry repair | $500-2,000 | Very high | Highest | Critical structural repairs |
| FRP sleeve wrapping | $400-800 | Moderate-high | Very good | Pile and column repair |
*Costs are indicative and vary significantly based on water depth, current conditions, accessibility, and project scale. Mobilization costs for dive teams typically add $5,000-15,000 per project regardless of method.*
The most cost-effective approach for most underwater concrete repair projects combines anti-washout grouting for large-volume void filling with epoxy injection for structural crack repair. Cofferdam construction is justified only when repair quality must meet above-water specifications and the water depth is manageable.
Related Guides
- Cementitious Grout vs Epoxy Grout: Types, Strength Data & Selection Guide
- Concrete Repair Mortar: Types, Strength Comparison & Selection Guide
- Epoxy Crack Injection for Concrete: Methods, Resin Types & Step-by-Step Guide
- CFRP Carbon Fiber Reinforcement for Bridge Strengthening
- Underwater FRP Sleeve Pile Repair: Bridge Pier Restoration Case Study — How FRP sleeve jacketing systems repair underwater bridge piers without cofferdam construction, using XINCHOR underwater epoxy grout.
- Resin Injection for Concrete Foundations — Foundation crack repair techniques applicable to below-water-table conditions.
- Two-Part Epoxy Adhesive Guide — Understanding the two-component epoxy chemistry used in underwater injection systems.
- View All XINCHOR Grout Materials — Underwater grout, epoxy grout, and injection grouting for submerged applications.
Frequently Asked Questions
Does cement cure underwater?
Yes. Portland cement cures through hydration — a reaction with water — so water presence does not prevent curing. The problem is not curing but placement: standard cement washes out before it can hydrate. Anti-washout admixtures solve this by keeping the cement paste cohesive during placement. Our underwater grout achieves 40 MPa compressive strength when cured fully submerged.
What is the best concrete mix for underwater use?
For structural underwater placement, anti-washout grout is superior to conventional concrete. It is pre-blended with optimized gradation, expansion agents, and anti-washout admixtures — just add water. Self-mixed underwater concrete using generic anti-washout admixtures is possible but much harder to control for consistent quality. For critical applications (dam repair, structural foundations), a manufactured anti-washout grout with certified performance data is strongly recommended.
Can epoxy be used for underwater concrete repair?
Yes — epoxy cures through polymerization, not hydration, so water does not interfere with the curing reaction. Epoxy injection is the standard method for sealing cracks in submerged concrete. However, epoxy is not practical for large-volume placement (it is too expensive and generates excessive exothermic heat in bulk). Use epoxy for crack injection and small-area bonding; use anti-washout cementitious grout for volume repairs.
How deep can underwater concrete repair be performed?
Anti-washout grout can be pumped to significant depths — our XQ-GJ-UW is pumpable up to 50 meters through standard hose. Epoxy injection has no inherent depth limit, as the injection pressure can be adjusted to overcome hydrostatic pressure. The practical limit is usually diver access and visibility rather than material performance. For deep-water applications beyond diver range, remotely operated vehicles (ROVs) equipped with injection tools are used.
What is anti-washout grout?
Anti-washout grout is a cementitious grouting material formulated with viscosity-modifying admixtures (VMAs) that prevent the cement paste from dispersing when placed in water. The VMAs — typically cellulose ethers or biopolymers — increase the cohesion of the fresh mix so it holds together instead of washing out. This allows the grout to be placed by tremie or pumping directly into water without cofferdams or dewatering. Quality is measured by the EN anti-washout test, which quantifies mass loss when the fresh material is immersed in water — our grout achieves less than 5% mass loss.
How long does underwater concrete take to set?
Anti-washout cementitious grout sets in approximately the same timeframe as above-water grout — initial set at 4 to 8 hours, with the structure protected from disturbance for at least 24 hours. Strength development is slightly slower underwater due to the lower curing temperature of submerged environments, but 28-day design strength (40 MPa for our XQ-GJ-UW) is reliably achieved. Epoxy injection systems typically reach full cure in 48 to 72 hours at 25°C, longer in cold water.
What concrete repair materials work underwater?
Three categories of materials work for underwater concrete repair: (1) Anti-washout cementitious grout for large-volume void filling and section restoration — these contain viscosity-modifying admixtures that prevent cement washout. (2) Epoxy injection resins for crack sealing — these cure through polymerization, not hydration, so water does not affect the curing chemistry. (3) Polymer-modified repair mortars with anti-washout admixtures for surface patching by divers. Standard concrete, standard mortar, and standard grout will all fail underwater because the cement paste disperses before it can hydrate.
How do you waterproof underwater concrete?
Waterproofing submerged concrete requires a different approach than above-water waterproofing. Membrane systems cannot be applied underwater. Instead, the concrete itself must be made watertight through crack injection (epoxy or polyurethane resin injected under pressure to seal all pathways), void grouting (anti-washout grout fills any cavities), and surface treatment (crystalline waterproofing admixtures that react with water to form insoluble crystals in concrete pores). For new underwater concrete placement, using anti-washout grout with integral waterproofing admixture provides the most reliable result.
Planning an underwater repair project? Contact XINCHOR with your project details — water depth, flow conditions, damage type, and repair volume — and our engineering team will recommend the right materials and provide placement guidance.