Concrete Crack Injection: Epoxy vs Polyurethane Resin

Cracks in concrete walls, floors and other structures should not automatically be filled with a general-purpose repair product.

The correct concrete crack injection method depends on several factors, including what caused the crack, whether it is still moving, whether water is entering and whether the repair needs to restore structural strength.

Epoxy resin and polyurethane resin are both commonly used for concrete crack injection, but they perform different roles. Epoxy is generally associated with rigid structural repairs, while polyurethane is often used to seal cracks affected by water ingress or minor movement.

A professional assessment is essential before deciding which material and repair method to use.

What Is Concrete Crack Injection?

Concrete crack injection is a specialist repair process in which a liquid resin is introduced into a crack under controlled pressure.

Injection points or ports are normally positioned along the crack. The chosen resin is then injected so that it travels into the crack and fills the void within the concrete.

Depending on the material used, the repair may:

• Bond the cracked sections of concrete together
• Restore continuity within a structural component
• Seal a path through which water is entering
• Fill fine cracks that cannot be repaired effectively from the surface
• Reduce the risk of further moisture reaching embedded reinforcement

Crack injection is used in structures including concrete walls, floors, basements, retaining walls, car parks, bridges and commercial buildings.

However, injection should not be treated as an automatic solution for every crack. The cause and significance of the damage must be investigated first.

Read our guide to cracks in concrete walls and floors for more information about common warning signs and possible causes.

Why Do Cracks Develop in Concrete?

Concrete may crack for several reasons. Some cracks are relatively minor, while others can indicate movement, deterioration or structural stress.

Possible causes include:

• Drying shrinkage as concrete cures
• Thermal expansion and contraction
• Settlement or movement of the building
• Excessive loads
• Inadequate reinforcement
• Poorly positioned construction joints
• Corrosion of steel reinforcement
• Freeze-thaw damage
• Chemical attack
• Water pressure
• Defects in the original construction
• Movement between different building materials

Before any resin is injected, it is important to establish whether the crack is static or active.

A static crack is no longer expected to move significantly. An active crack may continue to open, close or change due to structural movement, temperature variations or changes in loading.

Injecting a rigid resin into an active crack may not provide a lasting repair because continued movement can cause the concrete or the repair material to crack again.

What Is Epoxy Crack Injection?

Epoxy crack injection uses a low-viscosity resin that can penetrate narrow cracks and cure to form a hard, rigid bond.

Where the crack is suitable and the surrounding concrete is sound, epoxy injection may help restore structural continuity by bonding the separated sections together.

Epoxy injection is commonly considered for:

• Static cracks in reinforced concrete
• Structural concrete walls and floors
• Beams, columns and slabs
• Fine cracks requiring a strong bond
• Repairs where restoring structural integrity is important
• Dry or suitably prepared cracks

The exact application will depend on the product specification, the width and depth of the crack and the condition of the concrete.

Advantages of Epoxy Resin Injection

The potential benefits include:

• High bond strength
• Ability to penetrate fine cracks
• A rigid repair once cured
• Restoration of continuity through suitable concrete
• Minimal disturbance compared with removing and replacing larger sections

Epoxy injection can be an effective structural repair, but only when the crack is stable and the surrounding concrete can receive the resin properly.

Our epoxy crack injection project in Bath provides an example of this type of specialist repair.

When Might Epoxy Injection Be Unsuitable?

Epoxy is a rigid material. It may therefore be unsuitable where the crack is expected to continue moving.

It may also not be the right choice when water is actively flowing through the crack, unless the water ingress is first controlled and the selected system is specifically suitable for the conditions.

Epoxy injection may not solve the problem where:

• The crack is still moving
• The concrete is weak, loose or contaminated
• Water is flowing heavily through the crack
• The underlying cause has not been corrected
• Reinforcement is badly corroded
• Sections of concrete have become detached
• The crack is part of a wider structural problem

In these situations, further investigation and additional repair work may be required.

What Is Polyurethane Crack Injection?

Polyurethane resin injection is commonly used to seal cracks and joints through which water is entering.

Some polyurethane resins react when they come into contact with moisture, expanding to fill the crack and help stop the flow of water. Other formulations cure into a more flexible material that can accommodate a limited amount of movement.

Polyurethane injection may be used for:

• Leaking basement walls
• Water entering through concrete cracks
• Retaining walls
• Construction joints
• Service penetrations
• Cracks subject to minor movement
• Below-ground concrete structures
• Voids or gaps allowing water ingress

The type of polyurethane resin must be selected carefully. Different products offer different levels of expansion, flexibility and reaction time.

Advantages of Polyurethane Resin Injection

Depending on the selected system, potential benefits include:

• Ability to seal active water leaks
• Reaction with moisture
• Expansion into irregular cracks and voids
• Flexibility after curing
• Suitability for some moving cracks
• Rapid control of water ingress

Polyurethane injection can be particularly useful when the main objective is to prevent water passing through a crack.

Our article about using resin injection to solve water ingress explains how injection systems can be used as part of a specialist waterproofing repair.

Does Polyurethane Injection Repair Structural Damage?

Polyurethane resin should not automatically be considered a structural repair.

Its main purpose is normally to seal cracks, fill gaps or stop water ingress. Although it may provide a durable water-sealing solution, it does not necessarily restore the original structural strength of the concrete.

Where a crack is structurally significant, additional measures may be required. These could include epoxy injection, reinforcement repairs, concrete replacement or another engineered repair system.

This is one of the main reasons a crack should be assessed before treatment. Stopping the visible leak does not necessarily resolve the structural cause.

Epoxy vs Polyurethane Crack Injection

The main difference between the two materials is the purpose of the repair.

Epoxy Resin

Epoxy is generally:

• Hard and rigid when cured
• Used for suitable static cracks
• Associated with structural bonding
• Applied where strength and continuity are priorities
• Best suited to sound, properly prepared concrete

Polyurethane Resin

Polyurethane is generally:

• More flexible than epoxy
• Used to control water ingress
• Suitable for some damp or wet conditions
• Able to expand into cracks and voids
• Used where limited movement may occur

The choice should not be based simply on which product is cheaper or quicker to apply.

A surveyor or repair specialist should consider the crack’s cause, depth, width, movement, moisture conditions and structural importance before specifying a system.

Can Both Resins Be Used in the Same Repair?

In some situations, more than one injection material or repair stage may be required.

For example, a polyurethane resin may initially be used to control active water ingress. Once the area is dry and stable, further investigation may determine whether an additional structural repair is necessary.

This does not mean that both materials are always required. The specification should be based on the particular structure and the objectives of the repair.

Combining products without a clear repair design can lead to poor results or make it more difficult to assess the crack later.

How Is Crack Injection Carried Out?

The exact procedure depends on the structure and the chosen repair system, but the work may include:

1. Inspecting and assessing the crack
2. Removing loose material and surface contamination
3. Cleaning and preparing the concrete
4. Installing injection ports at suitable intervals
5. Sealing the visible surface of the crack where required
6. Injecting the resin under controlled pressure
7. Monitoring the movement of resin between ports
8. Allowing the material to cure
9. Removing the ports and making good the surface
10. Inspecting the completed repair

Careful control of injection pressure is important. Excessive pressure could extend the crack, damage weak concrete or force resin into unintended areas.

The installer must also understand the working time and behaviour of the product being used.

Why Surface Filling Is Often Not Enough

A crack may extend much deeper into the concrete than can be seen from the surface.

Applying filler or mortar over the visible line may improve its appearance, but it may not seal the full depth of the crack or stop water travelling through the structure.

Surface-only repairs may fail where:

• Water pressure is present behind the concrete
• The crack extends through the wall or slab
• The crack continues to move
• The surface has not been prepared properly
• The original cause remains active
• The repair material is not compatible with the concrete

Injection allows a suitable resin to enter the crack rather than simply covering it.

When Should a Concrete Crack Be Professionally Assessed?

Arrange a professional assessment if:

• The crack is increasing in width or length
• One side of the crack is higher than the other
• Water is entering through the crack
• Rust staining is visible
• Concrete is flaking or breaking away
• Reinforcing steel is exposed
• The crack affects a retaining wall or basement
• Several cracks have appeared in the same area
• The structure has moved or settled
• A previous repair has failed
• You are unsure whether the damage is structural

Cracks in load-bearing walls, beams, columns or other important structural elements may also need assessment by a structural engineer.

What Should a Concrete Repair Survey Include?

A professional concrete survey should consider more than the visible width of the crack.

The inspection may examine:

• The location and direction of the crack
• Its width, depth and length
• Whether it passes through the structure
• Signs of continuing movement
• Moisture or active water ingress
• The condition of the surrounding concrete
• Possible reinforcement corrosion
• Previous repair attempts
• Loading and structural use
• Environmental exposure
• The likely cause of the damage

Further investigation may be recommended before a repair is specified.

Specialist Concrete Crack Injection Services

Shield Waterproofing & Preservation provides specialist surveys and repair solutions for cracks in concrete structures.

Depending on the findings of the inspection, the recommended work may include:

• Epoxy resin injection
• Polyurethane resin injection
• Sealing active water leaks
• Concrete crack and joint repairs
• Repairs to damaged concrete
• Waterproofing around below-ground structures
• Further structural investigation where required

Our team works on residential, commercial and historic properties across the UK.

Arrange a Concrete Repair Survey

The correct resin injection system depends on why the crack has formed and what the repair needs to achieve.

Epoxy may be suitable where a static structural crack needs to be bonded. Polyurethane may be more appropriate where the main problem is water ingress or limited movement.

Before filling or injecting a concrete crack, arrange a professional inspection to establish its cause and significance.

Contact Shield Waterproofing & Preservation to arrange a concrete repair survey and receive clear recommendations for an appropriate repair solution.