Written and reviewed by the Apolodor LTD concrete repair team

Concrete water tanks, reservoirs, sumps and attenuation structures are designed to hold water reliably for decades. When they start to leak, the consequences range from wasted water and higher pumping costs to undermined foundations, damage to neighbouring structures and, for drinking water, potential contamination.

This guide explains how leaks in concrete tanks are found and repaired, and what extra care is needed when the tank holds potable water. It also sets out how we approach tank repairs at Apolodor LTD.

Quick answer: Leaking concrete water tanks are repaired by draining and cleaning the tank, locating leaks through inspection and testing, injecting or sealing cracks, repairing failed joints and penetrations, reinstating damaged concrete and, where needed, applying a lining. For drinking water tanks, every material in contact with the water must be suitably approved, for example through WRAS or the relevant regulatory scheme.

Which Concrete Tanks Commonly Leak?

• Drinking water storage tanks and service reservoirs
• Fire water and sprinkler tanks
• Rainwater harvesting and attenuation tanks
• Swimming pools and water features
• Process and effluent tanks on industrial sites
• Pump chambers and sumps

Liquid-retaining structures are designed to tighter crack width limits than ordinary concrete, typically following BS EN 1992-3, because even small cracks can leak.

Why Do Concrete Tanks Leak?

• Cracks from shrinkage, thermal movement or settlement
• Failed joints, where waterstops or sealants are damaged or were poorly installed
• Pipe penetrations that were never fully sealed
• Honeycombing and poor compaction, especially at wall bases and around pipes, as described in our guide to identifying and repairing honeycombing in concrete
• Chemical attack from soft water, treatment chemicals or aggressive effluents
• Reinforcement corrosion in older tanks

Step 1: Drain, Clean and Inspect

The tank is drained and cleaned to expose the concrete. A close visual inspection identifies cracks, damp patches, deposits, failed joints and surface deterioration. Work inside tanks usually requires confined space procedures.

Step 2: Locate the Leaks

Where leaks are not obvious, they can be located by:

• Looking for damp patches or deposits on the outside face, where it is accessible
• Level drop testing over a set period
• Testing sections in isolation
• Dye or tracer methods

Step 3: Repair Cracks

Crack repair depends on whether the crack is moving. Dormant cracks are often injected with epoxy resin to restore continuity, while active or wet cracks are usually injected with polyurethane resin that stays flexible and reacts with water. Our guide to crack injection techniques in concrete repair explains how the method is chosen, and our guide on whether a concrete crack is active or dormant explains how to tell the difference. We choose the injection resin crack by crack, because using one product throughout is a common reason for tank repairs leaking again.

Step 4: Repair Joints and Penetrations

Failed joints are cut out and resealed, sometimes with surface-applied joint bands. Leaking penetrations are sealed with resin injection or proprietary sealing systems.

Step 5: Reinstate Damaged Concrete

Spalled, honeycombed or chemically damaged concrete is broken out and reinstated with suitable repair mortars, ensuring good bond and compatibility with the original structure.

Step 6: Apply a Lining Where Needed

Linings provide an extra barrier or protect the concrete against aggressive water. Options include cementitious crystalline or polymer-modified coatings and resin linings. Choosing between them follows similar principles to those in our guide to cementitious and liquid waterproofing systems.

What Changes for Drinking Water Tanks?

For tanks storing drinking water, every material in contact with the water, including repair mortars, resins, sealants and linings, must be suitable for that use. In the UK, products used in premises are commonly approved through the Water Regulations Advisory Scheme (WRAS), while materials used in public water supply are subject to approval under the drinking water regulations. After repairs, tanks are cleaned, disinfected and tested before returning to service.

Step 7: Test and Monitor

Once repairs have cured, the tank is filled and monitored for level drop over an agreed period to confirm watertightness.

How Apolodor LTD Approaches Tank Repairs

We start every tank repair by finding the actual leak paths, not by lining the whole tank and hoping for the best. Cracks, joints and penetrations are each repaired with the method that suits them, and the tank is tested before it goes back into service.

We also plan the work around the client’s need for the tank. Where it supports fire protection, production or water supply, we agree the drain-down, repair and refill sequence in advance so the time out of service is kept as short as possible.

Where Apolodor LTD Works

We repair tanks, sumps and water-retaining structures through our concrete repair services in:

• London
• Manchester
• Birmingham
• Liverpool
• Leeds
• Nottingham
• Bristol
• Brighton
• Cardiff
• Plymouth
• Luton
• Reading
• Norwich
• Swindon
• Portsmouth
• Oxford
• Ipswich
• Maidstone
• Cambridge
• Southampton
• Slough
• Warrington
• Sheffield
• Leicester
• Coventry
• Milton Keynes
• Northampton
• Derby
• Stoke-on-Trent
• Wolverhampton
• Hull
• Exeter
• Gloucester
• Sunderland
• York
• Peterborough
• Chelmsford
• Newcastle
• Bradford
• Bournemouth
• Swansea
• Newport
• Middlesbrough
• Preston
• Blackpool
• Bolton
• Stockport
• Huddersfield
• Lincoln
• Worcester
• Colchester
• Canterbury

Key Takeaways

• Cracks, joints and penetrations are the most common leak paths
• Moving cracks need flexible repairs, while dormant cracks can be bonded
• Drinking water tanks require suitably approved materials
• Tanks should be tested for watertightness after repair
• Regular inspection stops small leaks becoming major failures

Frequently Asked Questions

Can a concrete tank be repaired while it is full?

Some leaks can be sealed by injection from the outside while the tank stays in use, but most permanent repairs require the tank to be drained.

Do small leaks matter?

Yes. Even small leaks can wash fines out of the surrounding ground, corrode reinforcement and grow over time.

How long does a tank repair take?

It depends on size, access and the extent of repairs, along with curing times for linings and any disinfection requirements.

Talk to Our Team

Apolodor LTD repairs leaking concrete tanks, sumps and water-retaining structures across the UK. Call 07808 709670 or contact us here.