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Comprehensive Guide to Spalling Concrete: Causes, Prevention, Repair, and Protection

Spalling concrete is when parts of a concrete surface break off and peel away on their own, without obvious causes like heavy impacts or fires.

What Exactly is Spalling Concrete? And What is The Cause of It?

Spalling can occur due to various factors.

  1. Corrosion of Rebars: The steel reinforcement bars inside the concrete corrode and expand, leading to the concrete cracking and separating.
  2. Concrete Degradation from Sulphate Exposure: Exposure to sulphates, which can be found in soil or water, causes the concrete to deteriorate.
  3. Alkali-Silica Reaction: A chemical reaction between alkali in cement and silica in aggregates causes the concrete to expand and crack.
  4. Freezing and Thawing Cycles: Repeated cycles of freezing and thawing water within the concrete cause it to crack and eventually break apart.

This article discusses the issue of concrete spalling, which occurs when the embedded steel reinforcements corrode and expand.

What is Carbonation? How Does it Contribute to Concrete Spalling?


The pH of fresh concrete is typically around 12, which is highly alkaline and helps protect the steel rebars inside it from corroding by providing a passivation layer. However, as time passes, this alkalinity decreases, and when the pH drops to about 9 or lower, this protective passivation is lost.

A process known as carbonation leads to the reduction in alkalinity in concrete. Fresh concrete is highly alkaline due to calcium hydroxide (also known as hydrated lime), which forms as a by-product when OPC cement hydrates. When moisture that contains dissolved carbon dioxide penetrates the concrete, it reacts and transforms the calcium hydroxide into calcium carbonate, an insoluble salt. As this transformation continues, the content of calcium hydroxide in the concrete diminishes, along with its alkalinity.

From the description of the carbonation process, several key points emerge:

  1. Moisture and carbon dioxide, the catalysts for carbonation, are ubiquitous but are particularly prevalent in open, exposed areas.
  2. In well-made, high-quality concrete, carbonation occurs slowly and gradually. It can take a significant amount of time for the alkalinity to decrease to a pH of 9 at the level of the rebars.
  3. Various factors can accelerate the rate of carbonation. These include using low-quality, highly porous concrete, and having defects such as honeycombs, voids, and cracks, regardless of the concrete’s grade.
  4. Engineers design reinforced concrete with a specific thickness of concrete cover over the rebars based on design requirements. For instance, a concrete cover of 30 mm offers 25% less protection against carbonation for the rebars compared to a cover of 40 mm. Therefore, construction errors that lead to inadequate concrete cover need to be corrected to maintain the structure’s durability.

How Can Carbonation Be Controlled?

It’s evident that without protective steps, carbonation leads to rebar corrosion, subsequently causing spalling in concrete. To extend the lifespan of structures and avoid costly, intrusive repairs, controlling the carbonation process is crucial.

From the considerations mentioned in point (2), we can pinpoint essential strategies to achieve this:

Opt for high-quality, low w/c (low-porosity) concrete in structures exposed to frequent wetting and drying. Ensuring proper placement, compaction, and curing of concrete is vital; the mix design alone does not ensure a defect-free outcome. When flaws such as honeycombs, voids, or cracks are found in hardened concrete, they should be immediately addressed to prevent moisture from easily entering the structure.

Should the concrete cover be insufficient compared to the design specifications, steps should be implemented to reinstate the intended level of protection against carbonation, such as applying a tested protective render. In addition, for any concrete quality or cover thickness, particularly in exposed settings, the application of protective coatings with proven anti-carbonation qualities may be considered by engineers to prolong the structure’s durability.

Repair of Spalled Concrete and Protection Measures

When spalling due to steel corrosion becomes visible on a concrete structure, carbonation has likely reached an advanced stage, necessitating a systematic, multi-step repair approach. LeFong Building services are able to use mapei’s products to repair your spalling concrete.

Essential Repair Steps for a Typical Repair Situation

  1. Breaking out the spalling concrete: Start by removing loose, delaminated concrete until only sound concrete remains. This includes undercutting the corroded steel reinforcements by about 20 mm to ensure thorough repair. The cavity should be kept simple, usually square or rectangular, and the edges saw-cut perpendicular to the surface to a depth of 12 mm to prevent feather-edging the repair material.
  2. Substrate preparation and cleaning: The final surface texture should be rough, with an amplitude of approximately 6 mm. It is crucial to remove all residual dust, debris, and contaminants that might hinder proper bonding, which can be achieved by water-jetting at a minimum pressure of 250 MPa.
  3. Treatment of exposed reinforcements: It is vital to remove all corrosion from the reinforcing steel using methods like wire brushing, abrasive blasting, or needle scaling. If the reinforcing steel’s cross-sectional area has significantly diminished, consulting a structural engineer about the permissible limit of reduction (typically 20%) and potential strengthening measures is necessary. Following cleaning, apply a protective coating to the steel (e.g., MAPEFER 1K, a re-alkalising protective coating for rebars).
  4. Substrate pre-wetting: Generally, absorbent substrates such as concrete should be pre-wetted to a saturated, surface dry (SSD) condition before applying cementitious mortars. This practice helps prevent excessive moisture loss from the mortar at the bond line, which could weaken adhesion. Note that SSD conditions are not required when using a polymer bonding agent; always follow the manufacturer’s instructions in these cases.
  5. Material selection and re-profiling: Choose the appropriate cementitious mortar for re-profiling based on several factors:
    • Method of placement (hand-patching, formwork casting, or shotcreting), which depends on the repair volume and the mortar’s required physical strengths.The desired physical properties such as compressive and flexural strengths and compressive modulus.Special resistive properties needed, like water permeability and chloride ion diffusion.Characteristics of the mortar, including permissible application thickness, flow, shrinkage compensation, non-bleed, and rate of strength gain.
    Once selected, the material should be applied following the manufacturer’s instructions and the engineer’s approved method statement. Relevant products include the MAPEGROUT series for hand-patched mortars, MAPEFILL for formwork casting, and MAPEGROUT GUNITE for shotcreting repairs.
  6. Curing and protection of repair works: Ensure all cementitious repair materials are provided with uninterrupted curing protection for the recommended period, usually seven days. Formwork should only be stripped after the manufacturer’s specified minimum waiting period.
  7. Protective coatings: Given the degradation experienced by the structure over its service period, engineers often specify protective treatments to enhance durability. These treatments can provide resistance to carbonation, chloride ion diffusion, and water absorption. Coloured or pigmented coatings are also available to improve the aesthetic appearance of the structure.

Key Summary

AspectDetails
Definition and CausesSpalling concrete occurs when parts of the concrete surface break off, often without direct impact or fire. Causes include corrosion of rebars, sulphate exposure, alkali-silica reaction, and freezing and thawing cycles.
What is Carbonation?Carbonation reduces concrete’s alkalinity from around pH 12 to below pH 9, affecting the passivation layer that protects steel rebars from corrosion. It results from the reaction of calcium hydroxide in concrete with atmospheric CO2 and moisture, converting it to calcium carbonate, which decreases alkalinity. Factors accelerating carbonation include low-quality porous concrete, environmental exposure, and structural defects like honeycombs and cracks.
Control MeasuresTo control carbonation and extend the lifespan of structures: use high-quality, low water-to-cement ratio concrete; ensure proper concrete placement, compaction, and curing; address any concrete flaws promptly; apply protective coatings; and adjust concrete cover thickness as necessary for adequate protection.
Repair Steps1. Remove spalled and loose concrete, ensuring sound substrate remains.
2. Clean the substrate and prepare it for repair.
3. Treat exposed reinforcements to remove corrosion and apply protective coatings.
4. Pre-wet substrates to maintain proper moisture levels during repair.
5. Select and apply the appropriate cementitious mortar for re-profiling.
6. Ensure uninterrupted curing and protect the repair site.
7. Apply protective coatings for enhanced durability.