Concrete is one of the most widely used construction materials due to its strength, durability, and versatility. However, even the strongest concrete can suffer from surface deterioration over time, especially in the form of spalling. If left unaddressed, spalling can compromise structural integrity, reduce service life, and negatively impact aesthetics. This article explores what concrete spalling is, why it occurs, and how it can be prevented and repaired effectively.
What Is Concrete Spalling?
Spalling refers to the breaking away or flaking off of the concrete surface. It often extends into the upper layers of embedded reinforcing steel. According to ACI 302.1R-4 , typical spalls may be 150 mm or more in diameter and 25 mm or deeper , although smaller spalls are also possible.
This condition affects a wide range of structures, including:
- Framed buildings
- Multi-storey car parks
- Bridges
- Jetties
- Tanks
- Retaining walls (bunds)
Spalling results in a patchy, pitted surface , exposing the coarse aggregate underneath. Prompt repair is essential to maintain the structure’s integrity and prevent further deterioration.
Effects of Concrete Spalling
When spalling occurs, several critical issues arise:
- Structural Integrity Compromise : The loss of concrete cover weakens the protective barrier around steel reinforcement, increasing the risk of corrosion.
- Reduced Load-Bearing Capacity : Damaged areas can lead to stress concentrations and potential failure under load.
- Aesthetic Degradation : Spalled surfaces appear unsightly, which is particularly problematic in visible architectural elements.
- Exposure of Internal Components : Reinforcing bars become vulnerable to moisture and oxygen, accelerating corrosion and worsening the damage.
Common Causes of Concrete Spalling
Understanding the root causes of spalling is crucial for effective prevention and repair. The primary causes include:
- Corrosion of Reinforcing Steel : Rust occupies more volume than steel, creating internal pressure that breaks the surrounding concrete.
- Fire Exposure : High temperatures cause free water in concrete to turn into steam, generating internal pressure that leads to explosive spalling.
- Freeze-Thaw Cycles : Water trapped in pores expands when frozen, causing micro-cracks that eventually lead to surface flaking.
- Alkali-Silica Reaction (ASR) : A chemical reaction between alkalis in cement and reactive silica in aggregates creates expansive gels that crack the concrete.
- Inadequate Cover Over Reinforcement : Thin layers of concrete over steel leave it exposed to environmental factors.
- Low-Quality Concrete Cover : Poor mix design or improper compaction results in weak zones prone to spalling.
- Improperly Constructed Joints : Inadequate joint design or execution leads to localized stresses and cracking.
- Bond Failure in Two-Course Construction : Differences in shrinkage or drying rates between layers cause delamination and spalling.
Mechanism of Spalling Due to Steel Corrosion
One of the most common mechanisms of spalling involves corrosion of embedded steel reinforcement . When moisture and oxygen reach the steel, rust begins to form. As rust accumulates, it expands up to six times the original volume , exerting pressure on the surrounding concrete. This pressure leads to cracking, delamination, and eventual spalling of the concrete surface.
Preventive Measures Against Concrete Spalling
Prevention is always more cost-effective and less disruptive than repair. Key preventive strategies include:
- Early-Entry Dry-Cutting for Floor Slabs : Helps control cracking during curing.
- Application of Water Sealants : Prevents water infiltration, which accelerates corrosion and freeze-thaw damage.
- Proper Concrete Placement Practices : Ensuring adequate cover depth and proper joint placement during pouring.
- Avoiding Excess Water in Mix : High water-cement ratios weaken concrete and increase porosity.
These practices help enhance the longevity and performance of concrete structures.
Repairing Concrete Spalling
The repair method depends on the severity of damage , type of structure , and location . Repairs should follow standards such as ACI Code and BS EN 1504 for optimal results.
Step 1: Assess Damage Severity
Begin by evaluating the extent of spalling to determine if surface patching or full-depth repair is required.
Step 2: Clean the Spalled Area
Thorough cleaning removes contaminants and ensures good bonding. Use a pressure washer for heavily soiled surfaces.
Step 3: Remove Loose and Damaged Concrete
Use a hammer and chisel, shot blasting, or grinding to remove all loose material. Aim to remove at least 3.81 cm (1.5 inches) of damaged concrete. Tap surrounding areas with a hammer; a hollow sound indicates deteriorated concrete needing removal. Cut rectangular sections to define the repair area clearly.
Finally, vacuum dust and debris from the cavity.
Step 4: Clean and Coat Corroded Steel Bars
Brush exposed rebar with a wire brush to remove rust and debris. Apply a protective coating to prevent future corrosion.
Step 5: Apply Repair Material Mixture
Choose appropriate repair materials based on depth and location:
- For spalls less than 1/3 of the slab thickness , apply a surface patch .
- For deeper damage, consider full-depth repair with new reinforcement.
Before applying the repair material:
- Roughen the surface to improve adhesion.
- Apply a bonding agent.
- Use Portland-cement-based or epoxy repair compounds.
- Match thermal expansion properties with existing concrete.
- Ensure ambient temperature is above 4.5°C during application.
For horizontal surfaces like driveways, use cementitious overlays . At joints, restore movement allowance to prevent recurrence.
Step 6: Finish the Surface
Use a margin trowel to smooth the surface and blend it with the surrounding area.
Step 7: Cure the Repair
Allow the material to cure properly to achieve the desired strength and durability.
Step 8: Apply Protective Coating or Paint
Once cured, apply a waterproof membrane , paint , or sealant to protect against future moisture intrusion.
Conclusion
Concrete spalling is a serious issue that can significantly affect the durability and safety of structures. While various factors contribute to its occurrence, timely detection and proper repair are key to extending the lifespan of concrete elements. Implementing preventive measures during construction and following standardized repair procedures can mitigate the risks associated with spalling.