In this article, Ben du Preez, HMP’s Contracts Director, discusses how we repair spalling concrete.
Spalling concrete appears as patches where the surface has flaked, chipped, or broken away. It often results in delamination and exposure of the underlying aggregate. This occurs without immediate external influences, such as impact loads or fire.
One of the leading causes of concrete spalling is corrosion of reinforcement due to carbonation. Other factors include:
- Sulphate attack [Sulfates and concrete. | Concrete Society]
- Alkali-silica reaction [Alkali-silica reaction | Concrete Society]
- Freeze-thaw cycles [Damage due to freeze-thaw | Concrete Society]
Carbonation causes a significant decline in concrete alkalinity. When the pH value drops to about 9 or lower, the passivation layer around the rebar is compromised. Fresh concrete is highly alkaline, with a pH of about 12, which is sufficient to protect reinforcing steel from corrosion.
The high alkalinity of fresh concrete results from calcium hydroxide, a by-product of Ordinary Portland Cement hydration.
Moisture containing dissolved carbon dioxide penetrates the concrete matrix. This reaction converts calcium hydroxide into calcium carbonate (an insoluble salt). As the process continues, the calcium hydroxide content decreases, reducing alkalinity.
Carbonation-Induced Concrete Spalling
Some environments are more prone to carbonation-induced concrete spalling.
The agents of carbonation are present everywhere, but especially outdoors.
In non-defective, high-grade concrete, carbonation is a slow, gradual process. It can take many years for alkalinity levels to decline to pH 9 at the rebar depth.
However, this is not the case with low-grade, highly porous concrete. Defective concrete — with cracks, voids, or honeycombing — is also at high risk, regardless of grade. Refer to Honeycombing of concrete. | Concrete Society, Cracks in concrete | Concrete Society and Porous Concrete : GCCA.
Insufficient concrete cover is another major issue. For example, a 30 mm cover provides noticeably less protection against carbonation than a 40 mm cover. This due to the square root relationship in carbonation depth progression over time.
Controlling Carbonation-Induced Concrete Spalling
HMP deploys proven techniques to control carbonation-induced spalling, preventing the need for major, expensive, and disruptive repairs.
Where defects are present in existing concrete, we promptly repair them to prevent moisture ingress. This is achieved using high-quality repair materials, such as controlled-shrinkage fibre-reinforced mortar. Refer to Mapegrout Thixotropic | Mapei, Sikadur®-31 CF Normal | Structural Adhesive, and epidermix 318 | a.b.e.® Saint-Gobain.
Two-component epoxy resins with very low viscosity are injected into microcracks or applied to wet surfaces. Refer to EPOJET LV | Mapei, Sikadur®-52 ZA | Rigid Bonding | Epoxy Adhesive, and pds-dural-618lv-july-2023.pdf.
To seal cracks in screeds, a two-component solvent-free epoxy adhesive is used. Refer to EPORIP | Mapei, Sikadur®-30 | Structural Epoxy Adhesive, and epidermix 372 | a.b.e.® Saint-Gobain.
A two-component thixotropic epoxy adhesive is used for patch and crack repairs. Refer to ADESILEX PG2 TG | Mapei and Sikadur®-30 | Structural Epoxy Adhesive. We also use Epoxy Repair, Grout & Adhesives | a.b.e. Construction Chemicals, among other systems.
If the concrete cover is insufficient, we implement carbonation protection measures.
This can be achieved by treating affected areas with a two-component, flexible mortar system. They are made from cementitious binders, fine-grained selected aggregates, special additives, and synthetic polymers.
Refer to 1_02146_mapelastic-guard_en_9db6341200494268a2c3d1810c1ce58d.pdf and Euclid Chemical Africa | Concrete Solutions and Construction Products.
Structures in exposed environments may require protective coatings with proven anti-carbonation properties to extend service life. Refer to 1_02146_mapelastic-guard_en_9db6341200494268a2c3d1810c1ce58d.pdf, Sikagard®-550 W Elastic | Concrete Protection and Protective Coatings • abe.
Spalled Concrete Repair Processes
Our spalled concrete repair processes follow tried-and-tested methods.
These interventions are undertaken when spalling due to steel corrosion is visible, indicating that carbonation has reached an advanced stage.
We begin by removing loose and delaminated concrete until a sound substrate is reached. This may involve undercutting the corroded rebar by about 20 mm. Meanwhile, we ensure that we keep the prepared cavity shape as simple as possible. The edges of square or rectangular cavities are saw-cut perpendicular to the surface to a depth of about 12 mm. This is to avoid feather-edging the repair material.
The surface is then roughened to achieve an amplitude of about 6 mm. Residual dust, debris, and contaminants are removed to ensure sound adhesion. We typically use water-jetting at a minimum pressure of 250 MPa.
Corrosion is removed from the reinforcing steel using wire brushing, abrasive blasting, or needle scaling. The permitted limit for section reduction of the reinforcing steel is typically 20%. The rebar is then cleaned and coated with a suitable protective product. Refer to MAPEFER 1K | Mapei, Sika® FerroGard®-903 Plus | Corrosion Inhibitor, and Preparation of Reinforcing Steel | a.b.e.®.
In general, absorbent substrates must be pre-wetted to a saturated surface-dry (SSD) condition before applying cementitious mortars. This prevents excessive moisture loss from the mortar at the bond line, which could impair adhesion.
This step is not necessary when using polymer-based agents.
Spalled Concrete Repair Material Selection
Spalled concrete repair material selection depends on several factors:
- Placement method (e.g., hand-applied, cast in formwork, or shotcrete), based on repair volume and required physical strength.
- Required physical properties, such as compressive and flexural strength and compressive modulus.
- Mortar characteristics. This includes permitted application thickness, flow, shrinkage compensation, non-bleed properties, and strength gain rate – applied strictly per the manufacturer’s specifications.
HMP ensures uninterrupted curing protection for all cementitious repair materials, typically for seven days, after which formwork is stripped.
A high-quality protective coating is then applied to prolong the structure’s service life.
