Concrete cracks are a common feature of both new and ageing buildings, but not all cracks indicate the same level of risk. Some remain unchanged for decades, while others continue to move and deteriorate over time. Before deciding on any repair, it is critical to establish whether a crack is active or dormant. This single assessment often determines whether a repair performs for years or fails prematurely.

Many recurring concrete issues arise not because the repair material was inadequate, but because the crack’s behaviour was never properly understood in the first place.

Why Crack Activity Matters

An active crack is still responding to forces acting on the structure, while a dormant crack has stabilised and is no longer moving. Treating both in the same way is one of the most common causes of failed concrete repairs.

If a crack is still active, rigid materials are likely to fracture as movement continues. If a crack is dormant, using flexible or structural systems may be unnecessary and costly. Correctly identifying crack behaviour ensures the repair strategy matches the real condition of the concrete rather than assumptions based on appearance alone.

What Is an Active Concrete Crack?

An active crack is one that continues to widen, lengthen, or shift position over time. Movement may be gradual or seasonal, but it has not stopped.

Common causes include foundation settlement, changes in structural loading, thermal expansion and contraction, and moisture-related mechanisms such as reinforcement corrosion. These issues are frequently encountered in UK buildings, particularly in exposed slabs, basements, and older reinforced concrete structures.

Active cracking often indicates that underlying stresses remain unresolved and should always be investigated before repair work is specified.

What Is a Dormant Concrete Crack?

Dormant cracks result from movement that has already occurred and has since stabilised. While the crack remains visible, it does not change measurably over time.

Typical causes include early-age shrinkage during curing, thermal movement shortly after construction, or historic settlement that has already completed. Although dormant cracks are generally less structurally concerning, they can still permit water ingress and accelerate deterioration if left untreated.

How to Tell If a Concrete Crack Is Active or Dormant

Monitor the crack over time

The most reliable indicator of crack activity is change. Mark the crack at fixed reference points and measure its width periodically. Even minor variations can indicate ongoing movement.

If no measurable change occurs over a reasonable monitoring period, the crack may be considered dormant, although seasonal temperature and moisture effects should always be taken into account.

Use crack monitoring tell-tales

On higher-risk structures or commercial properties, simple visual checks are often insufficient. Crack monitoring gauges are fixed across the crack and record movement in multiple directions. Any recorded displacement confirms the crack remains active and rules out rigid repair solutions.

Look for secondary warning signs

Active cracks are often accompanied by other indicators, including repeated cracking after repair, fine debris along the crack line, damp staining, or rust marks from corroding reinforcement. These signs usually point to ongoing processes rather than a one-off historic event.

Consider crack location and orientation

Crack position provides valuable context. Vertical cracks can indicate settlement, diagonal cracks often suggest structural movement, and horizontal cracks in reinforced concrete may point to corrosion or restraint. Cracks affecting load-bearing elements should always be treated with caution.

Why Concrete Crack Repairs Commonly Fail

Concrete repairs most often fail when materials are selected before crack behaviour is properly understood. Whether a crack requires flexible injection, rigid structural repair, or surface protection depends on how it behaves over time, not simply how it looks. This is why understanding the different ways concrete repairs are carried out is essential before specifying any remedial work.

How UK Weather Influences Crack Behaviour

In the UK, environmental exposure plays a major role in crack activity. Repeated wet–dry cycles, freeze–thaw exposure, and seasonal temperature changes can cause cracks that appear dormant during warmer months to become active again in winter. This is particularly common in external slabs, balconies, basements, and retaining walls, and strongly influences how concrete performs over time in UK climates.

Repair Approaches Based on Crack Behaviour

Dormant cracks are typically treated using resin injection, cementitious repair mortars, or surface protection systems designed to prevent water ingress and further deterioration.

Active cracks usually require flexible systems such as polyurethane injection or crack-bridging coatings that can accommodate movement. In some cases, additional structural measures may be required to address the underlying cause of cracking.

Choosing the wrong approach is one of the main reasons repairs need to be repeated.

When Professional Assessment Makes Sense

Professional input is recommended where a crack is widening, allowing water ingress, affecting structural elements, or located within a commercial or public building.

This assessment process forms part of professional concrete repairs across the UK,
where durability and long-term performance are prioritised over cosmetic fixes. In urban environments, additional factors such as traffic vibration, basement construction, and nearby redevelopment often influence crack movement, which is why experience with concrete repairs in the capital is particularly important.

Get Advice Before You Repair

Correctly identifying whether a concrete crack is active or dormant reduces risk, prevents repeat failures, and protects the long-term integrity of the structure.

If there is any uncertainty over whether a crack is still moving, obtaining an early professional opinion is usually far more cost-effective than repeating failed repairs later. You can also speak directly with our team by calling 07808 709670.

Understanding the crack comes first. The repair should always follow.