Concrete Crack Filler: How to Choose the Right Repair

A concrete crack filler is any material used to close a crack in concrete: a flexible sealant, a rigid epoxy, an expanding polyurethane or a cementitious mortar. Each does a different job. Choosing one is less about the product and more about the crack: why it formed, whether it is still moving, and whether it matters structurally.

This article sets out how a structural engineer reads a crack before filling it, the main filler types and where each belongs, how to prepare and apply them, and the cracks that should never simply be filled. It complements our wider guides on cracking and repair rather than replacing a site assessment.

Why concrete cracks

All concrete cracks to some degree. It is strong in compression and weak in tension, and reinforcement is designed on the assumption that fine cracks will open. The common causes include:

Plastic shrinkage in the first hours, when the surface dries faster than it bleeds.
Drying shrinkage over weeks and months as water leaves the concrete.
Thermal movement, from early-age heat of hydration or from daily and seasonal temperature changes, especially on roofs.
Loading, producing flexural and shear cracks in beams and slabs.
Settlement of foundations or the ground beneath slabs.
Reinforcement corrosion, where rust expands and splits the cover.

The pattern, width, location and orientation of a crack usually point to its cause. Our guide to the types of concrete cracks shows how to recognise each.

Active or dormant: the first question

The most important distinction when choosing a concrete crack filler is whether the crack is still moving.

A dormant crack formed once and has stopped, for example a drying shrinkage crack in an old slab. It can be filled with a rigid material that restores continuity.
An active crack opens and closes, with temperature, load or ongoing settlement. Fill it with something rigid and either the filler cracks again or a new crack forms beside it. Active cracks need a flexible sealant, or a joint, or the cause must be stopped first.

Telling them apart takes observation. Tell-tale gauges, glass or plastic crack monitors, or simple pencil marks with dated measurements across the crack show whether it is growing. Cracks that widen during hot afternoons and close at night are thermally active; cracks that widen progressively over months suggest settlement or overload and need an engineer.

Types of concrete crack filler

Flexible sealants

Polyurethane, silicone, polysulphide and hybrid polymer sealants stay elastic after curing and accommodate movement. They are used for active, non-structural cracks and for joints. They work best when the crack is routed to a regular groove, with a bond-breaker or backer rod at the base so the sealant bonds to two sides only and can stretch.

Epoxy resins

Low-viscosity epoxy, injected under pressure, bonds the crack faces together and can restore much of the original tensile capacity across a dormant crack. This is the usual structural crack repair. Epoxy is rigid, so it suits only cracks that are not expected to move. It also needs dry or near-dry conditions to bond reliably.

Polyurethane injection resins

Hydrophobic and hydrophilic polyurethane resins react with water and expand to fill and seal leaking cracks. They stop water ingress in basements, tanks and roof slabs, but they do not restore structural strength. Our sibling article on crack injection with epoxy and PU explains the injection process in detail.

Cementitious and polymer-modified mortars

For wide, dormant cracks and surface damage, cement-based or polymer-modified repair mortars are packed or trowelled in. They are compatible with the concrete, but they are rigid and will crack again if the crack moves.

Gravity-fed resins and sealers

Very low-viscosity resins poured over horizontal surfaces penetrate fine cracks by gravity. They suit networks of fine shrinkage cracks on decks and slabs.

Preparing and applying the filler

A concrete crack filler is only as good as its bond and its placement.

Clean the crack. Remove dust, debris, loose material and contamination by brushing, compressed air or vacuum. Oil and old sealants must be removed.
Prepare the geometry. For sealants, rout a groove of the width and depth the product needs; for injection, seal the surface and install ports along the crack.
Control moisture. Epoxies want dry faces; polyurethane injection resins can be used against water; cementitious materials want a damp surface without standing water.
Fill completely. Injection proceeds from one port to the next until resin appears at the adjacent port, ensuring the full depth is filled. Sealants are tooled to press them against the sides.
Cure and finish. Protect the repair from traffic, sun and rain for the curing time, then remove surface seals and ports where needed.

On structural repairs, cores through injected cracks can confirm penetration, which is why engineers specify testing on significant jobs.

Cracks you should not simply fill

Some cracks are a warning, and filling them hides the evidence. Have an engineer look before any filler goes in when you see:

Cracks wider than a few millimetres, or growing over time.
Diagonal cracks near beam supports or around openings, which can indicate shear.
Cracks along the line of reinforcement with rust staining or spalling.
Stepped, tapered or displaced cracks in walls and slabs, pointing to foundation settlement.
Cracks in columns, transfer beams, cantilevers or other critical elements.
Cracks accompanied by sagging, doors and windows sticking, or ponding on slabs.

In these cases, the crack is a symptom of corrosion, overload, inadequate design or movement in the ground. The right sequence is to diagnose, address the cause, and then repair, which may involve injection, patching or strengthening as part of a wider concrete repair scheme.

Singapore context

Singapore’s hot, humid climate and heavy rain make water-tightness the main reason cracks are sealed on roofs, external walls, planter boxes and basements. Thermal cracking on exposed roof slabs is common, and those cracks are often active, so a flexible system is usually better than rigid filling.

For buildings under BCA’s Periodic Structural Inspection regime, the appointed Professional Engineer records significant cracking and may recommend investigation or repair. Repairs that affect structural members, or that are part of a strengthening scheme, are designed by a PE and may need BCA approval; filling non-structural shrinkage cracks does not.

Frequently asked questions

What is the best concrete crack filler?

There is no single best product. For dormant structural cracks, injected epoxy restores bond; for leaking cracks, polyurethane resin stops water; for moving cracks and joints, a flexible sealant works; and for wide, shallow damage a repair mortar is suitable. The crack’s cause and movement decide the choice.

Can I fill a crack in concrete myself?

Fine, dormant, non-structural cracks in paths, driveways or floor slabs can be filled with a suitable sealant or mortar by a careful homeowner. Wide, growing, diagonal or rust-stained cracks, and cracks in beams, columns or walls, should be assessed by an engineer first.

Why does a filled crack reappear?

Usually because the crack is still moving and was filled with a rigid material, or because the cause, such as settlement, thermal movement or corrosion, was not addressed. Poor cleaning and bond also allow fillers to debond and reopen.

How wide a crack is too wide to just fill?

As a rule of thumb, hairline and fine cracks under about a millimetre are usually cosmetic, while cracks of several millimetres or more, or any crack that is growing, need assessment. Width alone is not enough; location, pattern and movement matter as much.

Seeing cracks you are not sure about? Talk to our consultancy or get in touch for a diagnosis before you fill them.