Everything You Need to Know About Spalling Concrete (2024)

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24 April 2024 - 15:35, by , in Structural strengthening, No comments

What Is Spalling Concrete?

Spalling concrete refers to the deterioration seen on concrete surfaces, often characterized by visible chipping, flaking, and cracks. This condition generally arises due to the rebar inside the concrete corroding. When the steel rebar rusts, it expands, placing increased pressure on the surrounding concrete.

Over time, this pressure causes the concrete to crack and chip away, revealing the corroded steel. Factors like moisture penetration, poor concrete cover, and environmental exposure accelerate this process.

Effective prevention and repair methods involve using quality materials, ensuring proper concrete cover, and employing protective coatings or inhibitors to shield the rebar from corrosive elements.That being said, it is important to look for the warning signs and the causes of spalling concrete.

What Causes Spalling Concrete?

Spalling concrete slabs are usually caused by carbonation and extreme moisture. Nevertheless, a concrete surface can break off in various ways. Some of them are:

  • Corrosion of Steel Bar
    As mentioned, earlier, as water permeates the concrete surface, it causes the steel bar to rust and occupies more space than its original rebar, which results in pressure buildup and consequent spalling.
  • Freeze-Thaw Cycle
    This is a weathering erosion process, commonly in cold areas where ice can form. Whenever temperatures drop, water can freeze into ice when it seeps into the concrete. Ice may crack when it expands. The ice melts when the temperature rises, causing further damage to the interior. Concrete slabs spall as a result. As the temperature in Singapore is not cold enough to create freezing conditions, this does not happen here.
  • An Alkali-Silica Reaction
    This happens as a reaction between the reactive silica in concrete aggregates and alkaline cement pastes, which results in increased pressure in the concrete, swelling, and, eventually, spalling.

How to Spot Concrete Slabs?

In order to detect spalling concrete slabs early, keep an eye out for flaking, chipping, or missing large chunks of concrete from the surface, as well as exposed coarse rocky concrete aggregate. Spalling, however, can be difficult to detect by physical signs alone. Chemical and physical processes within concrete cause concrete to spall. Mixing methods and environmental factors can also contribute to concrete spalling. For this reason, it is best to consult a professional if you see any of the visual signs mentioned above.

What Is Carbonation? How Does It Cause Concrete Spalling?

Carbonation in concrete refers to a chemical reaction that occurs when carbon dioxide (CO₂) from the atmosphere reacts with calcium hydroxide (hydrated lime) present in the concrete. This reaction forms calcium carbonate, which is an insoluble salt.

Initially, fresh concrete has a high pH of around 12 due to the presence of calcium hydroxide, providing a protective alkaline environment that prevents corrosion of steel rebars embedded within it. Over time, as carbonation progresses, it reduces the pH of the concrete. When the pH drops to around 9 or lower, the passivation layer on the steel rebars breaks down, leaving them vulnerable to corrosion.

The process of carbonation is accelerated by the presence of moisture and CO₂, particularly in open, exposed environments. Factors such as low-quality, high-porosity concrete and the presence of defects like honeycombs, voids, and cracks can further expedite this process. Spalling in concrete occurs when the steel rebars within it begin to rust due to the loss of alkalinity and passivation.

As the steel corrodes, it expands, exerting pressure on the surrounding concrete, which eventually leads to cracking, flaking, and spalling of the concrete surface. This not only weakens the structural integrity of the concrete but also significantly reduces its lifespan.

How to Slow Down or Control Carbonation in Concrete?

To control or slow down the process of carbonation in concrete and thereby prevent the corrosion of steel rebars and spalling of concrete, several effective measures can be implemented. These measures aim to enhance the durability of concrete structures, particularly those exposed to regular wetting and drying cycles. Here’s how carbonation can be controlled:

  1. Use High-Grade, Low Water-to-Cement Ratio Concrete: Employing high-grade, low water-to-cement ratio (low-porosity) concrete is crucial for structures exposed to environmental elements. Low-porosity concrete limits the penetration of carbon dioxide into the concrete, reducing the rate of carbonation.

  2. Adherence to Good Concrete Practices: Proper placement, compacting, and curing of concrete are essential. Even a well-designed mix can fail to achieve its intended durability if not correctly handled and processed. Ensuring thorough compaction and adequate curing prevents defects and enhances the concrete’s resistance to environmental factors.

  3. Prompt Repair of Defects: Any detected defects in the hardened concrete, such as honeycombs, voids, or cracks, should be immediately repaired. Products like MAPEGROUT THIXOTROPIC, EPOJET, and Triflex Easy Repair Set Concrete are recommended for patching and crack repairs. These repairs prevent easy moisture ingress, which is a significant factor in accelerating carbonation.

  4. Restoration of Adequate Concrete Cover: If the concrete cover over the rebars is found to be below the designed thickness, corrective measures should be taken to restore the intended level of protection against carbonation. This might include applying a render like MAPELASTIC GUARD, Sikagard and  which has tested protective properties.

  5. Application of Protective Coatings: Regardless of the concrete grade or the thickness of the cover over the rebars, applying protective coatings with proven anti-carbonation properties can significantly extend the life of concrete structures. This is particularly important for structures located in harsh environmental conditions.

How to Repair Spalling Concrete?

Below are the steps on how to repair spalling concrete in your home:

  1. The corroded steel bar will be exposed after removing the spalled concrete with care.
  2. Corroded steel bars should be cleaned by removing all the rust. A two-coat anti-rust paint should then be applied.
  3. The affected surface should be treated with a bonding agent. The patch should then be made from polymer modified cement mortar.
  4. Match the rest of the surface to the repaired area.

Even so, it is still advisable to seek professional assistance since this type of issue can be extremely challenging to fix and is considered dangerous. Because of this, there is a high risk of further damage to your house if it is not done correctly.

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Struggling with spalling concrete? Secure your property’s safety and aesthetics with Waterproofing.com.sg. Our expert solutions restore and strengthen your concrete effectively. Visit us now for reliable service!

About author:
Nelson is a renowned author and construction engineering expert, specializing in waterproofing and roofing solutions. Graduating with a Master’s degree in construction engineering from the National University of Singapore (NUS), he applies his extensive practical experience and deep theoretical knowledge to author comprehensive guides on effective flooring practices. When he’s not writing or consulting, Nelson enjoys exploring new architectural designs and spending time with his family in Singapore.

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