What is spalling in concrete and how to prevent it?

Spalling is a type of damage that affects concrete surfaces and structures. It is characterised by the flaking or chipping of the concrete surface, which can range from minor cosmetic issues to major structural problems. The term “spall” is used to describe the breaking up of a material into fragments, often due to subsurface fissures that cause a portion of the surface to peel away. Spalling can compromise the structural integrity of a building and pose safety risks if not addressed. Therefore, it is important to identify and repair spalling as soon as it is detected.

See also: All about segregation of concrete

Spalling: Types

Spalling can broadly be classified into three different types:

Aggregate spalling

This type of spalling is characterised by the loss of the concrete’s coarse aggregate (gravel or crushed stone) from the surface. It occurs when the bond between the aggregate and the concrete matrix is weakened, often as a result of freeze-thaw cycles, corrosion, or chemical attack.

Corner spalling (or sloughing off)

This type of spalling occurs at corners and edges, where the concrete is more susceptible to stress and damage. Corner spalling is often a result of improper reinforcing or a lack of proper waterproofing measures, which can cause water to seep into the concrete and freeze, causing expansion and damage to the surface.

Explosive spalling

This type of spalling is the result of high-stress conditions, such as a sudden impact, fire, or thermal shock. Explosive spalling occurs when the internal pressure within the concrete exceeds its strength, causing pieces of the concrete to burst and flake away from the surface.

Spalling: Causes

Source: Pinterest

There are several causes of concrete spalling, including:

Freeze-thaw cycles

When water infiltrates the concrete and then freezes, it can cause the concrete to expand and contract. This repeated expansion and contraction can cause the surface of the concrete to break away and form spalls.

Corrosion of reinforcing steel

Steel is used to reinforce concrete structures and give them added strength. However, if the reinforcing steel begins to corrode, it can cause the concrete to crack and break away.

Shrinkage cracking

As concrete dries and hardens, it shrinks. This shrinkage can cause small cracks to form in the concrete, which can then become larger and cause the surface of the concrete to break away.

Chemical attack

Certain chemicals can react with the concrete and cause it to break down and form spalls. This can include chemicals used in construction, such as acids, or environmental factors, such as salt or pollutants.

Improper mixing

If the concrete mixture is not properly mixed or contains too much water, it can lead to a weaker concrete structure that is more prone to spalling.

Which areas are at a high risk of spalling?

Several areas are at a higher risk of experiencing spalling than others. These include:

• Exposed areas: Spalling is more likely to occur in areas where concrete is exposed to the elements, such as outdoor structures, bridges, and parking garages.

• Areas with freeze-thaw cycles: Concrete structures in areas with harsh weather conditions that experience frequent freeze-thaw cycles are at a higher risk of spalling.

• Areas with high levels of moisture: Excessive moisture can lead to corrosion of reinforcing steel and eventually cause spalling.

• Areas with high levels of pollution: Exposure to pollutants and chemicals can cause a chemical attack, which can weaken the concrete and increase the risk of spalling.

• Structures with poor design or construction practices: Structures that were built using substandard materials or without proper attention to design details are at a higher risk of experiencing spalling.

It’s important to note that even concrete structures in low-risk areas can still experience spalling if not properly maintained or if the right preventative measures are not taken during the design and construction phases.

How to prevent spalling?

Several measures can be taken to prevent concrete spalling. Some of the measures include:

• Adequate cover depth

The thickness of the concrete cover over the reinforcing steel should be sufficient to prevent corrosion. 

• Proper reinforcement

Reinforcing steel should be placed properly to prevent corrosion and improve the overall strength of the concrete.

• Waterproofing measures

Proper waterproofing measures should be taken to prevent water from penetrating the concrete, which can cause freeze-thaw cycles and other forms of corrosion.

• Durable materials

High-quality concrete and reinforcing steel should be used to ensure that the structure is durable and less likely to experience spalling.

• Good design and construction practices

Good design and construction practices, such as proper curing and protection of the concrete during construction, can also help prevent spalling.

• Regular maintenance

Regular inspections and maintenance of concrete structures can help identify and address spalling before it becomes a major issue.

Spalling: How to repair it?

There are several options for repairing spalling in concrete: 

• Shotcrete repair

This involves applying a spray of concrete onto the damaged surface using a high-pressure hose. The new concrete is then smoothed and moulded to match the original surface. 

• Overlays and toppings

This involves applying a new layer of concrete over the damaged surface, either to restore the original appearance or to give it a new, decorative look. 

• Patching and resurfacing

This involves filling in small, isolated areas of spalling with patching material and then resurfacing the entire surface. 

• Replacement

This involves removing the damaged concrete and replacing it with new, fresh concrete. This is often necessary when the spalling is extensive or has damaged the reinforcing steel. 

• Injection grouting

This involves injecting a specialised grouting material into the damaged concrete to reinforce and stabilise it.

Choosing the right repair option will depend on the extent of the damage, the location of the spalling, and the desired outcome. It’s recommended to consult a professional contractor or engineer to assess the damage and recommend the best course of action.

FAQs