Concrete Cracks: Types, Causes and Repair Methods

• Ahmet Toka
• 7 August 2026
• Structural Strengthening
• 4 min read

Not every crack in concrete is a sign of danger; but every crack carries information. A shrinkage crack tells you about the drying behaviour of the concrete, a diagonal crack about shear stress, and a crack running along the line of the reinforcement about corrosion that has already begun. Read correctly, a crack is the clearest signal a structure gives about itself.

In this article we explain how to tell the types of crack apart, which ones require urgent action, how to measure whether a crack is moving, and the correct repair method for each type.

Filling a crack is not the same as solving it. Unless the reason it opened is found first, the same crack reappears in the same place.

Ahmet Toka, General Manager

Types of Crack and What They Tell You

• Plastic shrinkage cracks: These form while the concrete is still fresh, as a result of the surface drying too quickly. They are short, irregular and superficial. They generally carry no structural risk but they make the surface permeable.
• Drying shrinkage cracks: These form through loss of volume after the concrete has hardened. On slabs they appear as a network. They do not move and are resolved by filling.
• Structural (load-induced) cracks: Seen vertically at mid-span of a beam, diagonally in the support regions and horizontally in a column. Their position and direction indicate the type of stress. These require assessment.
• Corrosion cracks: They run along the line of the reinforcement, usually accompanied by rust staining and spalling of the cover. They form as the reinforcement rusts and increases in volume; they are progressive and always require action.
• Settlement cracks: These are related to ground movement. They are generally wide, diagonal and continuous through a storey. A repair carried out without solving the ground problem will not be permanent.
• Thermal cracks: These open and close with temperature differences. Because they move, they require elastic solutions.

Which Cracks Are Urgent?

The signs looked at when assessing the urgency of a crack are as follows:

1. Is it getting wider? A crack that grows over time indicates movement that is still going on.
2. Is reinforcement visible? If the cover has spalled and the reinforcement is exposed, corrosion has begun; waiting leads to loss of section.
3. Is water coming through? Active water both accelerates corrosion and starts freeze–thaw damage.
4. Where is it? Column and beam ends, support regions and wall-to-slab junctions are critical areas.
5. Which way does it run? Diagonal cracks indicate shear stress, while horizontal cracks in a column indicate axial or flexural stress.
6. Is it in more than one member? Cracks spread throughout the same storey point to the behaviour of the system rather than to a problem in an individual member.

If any of these signs is present, assessment must come before superficial repair; if necessary it is addressed within the scope of structural strengthening it is addressed.

Is the Crack Moving? How Is It Measured?

The repair method is chosen according to whether the crack moves. There are practical ways of establishing this.

• Plaster tell-tale: A thin strip of plaster is placed over the crack and the date written on it. If the strip cracks, there is movement. It is the simplest and most widespread method.
• Crack gauge (tell-tale): A two-part transparent gauge is fixed to either side of the crack; opening and shear are read off in millimetres.
• Periodic measurement: The crack width is measured at specified points with a crack card and recorded with the date.
• Seasonal monitoring: Thermal movement reads differently in summer and in winter; monitoring over at least two seasons is valuable for telling them apart.

The monitoring period is generally a few months. That may seem long, but the cost of carrying out a rigid repair on a moving crack is far higher.

Repair Methods

The right method is chosen according to the type of crack and the objective.

Epoxy Injection (Structural Repair)

Used on dry, non-moving cracks to restore the integrity of the section. When cured it reaches a strength higher than the concrete and the member behaves monolithically again. It is not used on cracks with water coming through. Details: crack repair.

Polyurethane Injection (Water Cut-off)

Used on cracks with active water; it reacts with the water and expands to close the path. It provides no structural strength; its purpose is watertightness. Details: injection applications.

Elastic Joint Solution

On moving cracks, the crack is treated as a joint: a channel is cut, a backer rod is inserted and an elastic sealant is applied. In this way the movement is not restrained, only watertightness is provided. Details: joint sealing applications.

Section Repair and Passivation

With corrosion cracks the approach is different: loose concrete is removed, the reinforcement is cleaned and derusted, a protective (passivating) primer is applied and the section is reinstated with repair mortar. The surface is then protected against carbonation and chlorides with a concrete protection coating. Where the damage is extensive, concrete damage repair and concrete maintenance and repair processes come into play.

After Repair: Removing the Cause

The permanence of a repair depends on the cause of the crack being removed. This is usually a separate item of work from the repair, and if it is skipped the same crack returns.

• If it is water-related: The waterproofing is renewed, the drainage improved and any plumbing leak repaired.
• If it is load-related: The capacity of the member is increased; carbon fibre, steel or jacketing comes into play.
• If it is settlement-related: Ground improvement or foundation strengthening is required; merely filling the crack achieves nothing.
• If it is thermal: The movement is accepted and an elastic solution applied; a joint is also added where necessary.
• If it is corrosion-related: After repair, the ingress of chlorides and carbon dioxide is slowed with a surface protection coating.

Observing the area at intervals after repair and keeping records is the simplest verification of whether the solution is working.