Concrete Spalling Repair Guide: Thixotropic Repair Mortar, 6-Step SOP, Class 25 vs 40

What Is "Spalling"?

Spalling refers to the delamination, flaking, and cracking of concrete. Many older buildings in Hong Kong suffer from this problem — chunks of concrete fall from ceilings, beam soffits, and external walls, exposing heavily rusted reinforcement underneath.

Spalling repair uses dedicated repair mortar to restore the damaged concrete. Note that this refers to purpose-made repair mortar, not ordinary Portland cement — the difference between the two is significant, as explained below.

Two Core Properties of Repair Mortar

The key differences between professional repair mortar and ordinary cement come down to two critical material properties:

1. Thixotropic — Stays Put Even on Ceilings

After mixing, the mortar has a thick, paste-like consistency that does not flow when applied with a trowel. Even when applied to the underside of a ceiling (overhead position), it will not sag or fall off. It can be built up to 25–40 mm in a single pass without slumping. This property makes repair mortar particularly suited for ceilings, beam soffits, column faces, and other vertical or overhead surfaces.

By contrast, ordinary cement slurry or grout is fluid and simply cannot stay in place on overhead surfaces — it runs off immediately upon application.

2. Shrinkage-Compensated — No Cracking After Hardening

The material undergoes slight expansion during hardening, which offsets shrinkage caused by moisture evaporation, ensuring dimensional stability after curing. The result: no shrinkage, no cracking, with a tight bond to the existing concrete that effectively prevents moisture from reaching the reinforcement.

Repair Mortar vs Ordinary Cement Sand Mix: Which One Is Right?

Many people instinctively think "just mix up some OPC and sand to patch it," but the two are worlds apart:

Property	Repair Mortar	Ordinary Cement Mix (OPC)
Alkalinity	Very high and sustained, with special additives that enhance electrochemical protection of reinforcement	Initially high alkalinity, but drops over time due to carbonation
Density	Contains polymers — extremely dense structure that resists penetration by moisture and chloride ions	Higher porosity, allowing moisture to penetrate easily and accelerate rebar corrosion
Bond Strength	Strong — excellent adhesion without the need for SBR additive	Weak — tends to fall off ceilings, prone to debonding at old/new concrete interfaces
Shrinkage	Shrinkage-compensated, crack-free	High shrinkage — hairline cracks commonly appear at edges after drying
Thixotropy	Yes — can be applied to ceilings and vertical surfaces without sagging or falling	None — too fluid for overhead application

In short: patching concrete with ordinary cement leads to early cracking, debonding, and failure — and you end up having to redo the job. Repair mortar is purpose-made for this type of work.

Chemical Protection: Why High Alkalinity Matters

Repair mortar is more than just a filler — it also provides chemical protection for the reinforcement. To understand why, you first need to know how rebar corrodes:

Normal concrete is highly alkaline (pH around 12–13). This alkaline environment forms a passivation layer on the rebar surface that prevents rusting. Over time, however, atmospheric CO₂ gradually penetrates the concrete and lowers its alkalinity — a process called carbonation. Once the pH drops below the critical threshold, the passivation layer breaks down, rebar begins to rust and expand, the concrete is forced apart, and spalling occurs.

The high alkalinity of repair mortar directly addresses this:

• Neutralises the carbonated layer: When you apply the high-alkalinity mortar onto the old concrete, alkalis migrate into the surrounding concrete, raising the pH back up
• Restores the passivation layer: The stable, high-pH environment helps re-establish the passivation layer on the rebar, fundamentally inhibiting further corrosion
• Blocks harmful agents: The polymer-modified formulation creates an extremely dense structure that effectively blocks CO₂, moisture, and chloride ions from re-entering

This is the fundamental difference between repair mortar and ordinary cement — the alkalinity of ordinary cement declines with carbonation over time, leaving rebar unprotected.

When Do You Need Repair Mortar?

1. Ceiling Concrete Spalling (Slab Spalling)

This is the most typical scenario for spalling repair. Concrete on the underside of slabs delaminates due to rebar corrosion and expansion, with loose pieces at risk of falling. This is where the thixotropic property is essential — ordinary cement slurry simply cannot stay in place overhead, whereas repair mortar's thick consistency lets it adhere securely to the ceiling soffit.

2. Beam Soffit Repair

Like ceilings, beam soffits are overhead positions, but typically involve deeper structural repairs. The high bond strength of repair mortar ensures a tight interface between old and new concrete, supporting the weight of the material without debonding.

3. Column and Vertical Surface Repair

Column spalling usually results from insufficient concrete cover and carbonation. Repair mortar has excellent build-up property, allowing tradespeople to apply the required thickness on vertical surfaces in a single pass without sagging or slumping.

4. External Wall and Cantilever Repair

External wall repairs involve working at height, where safety and speed are paramount. The low permeability of repair mortar effectively blocks atmospheric CO₂ and moisture, preventing further rebar corrosion. Its thixotropic property also makes application straightforward on scaffolding or gondolas.

6-Step Spalling Repair SOP

Below is the standard spalling repair procedure. Each step has its purpose — do not skip any:

Step 1: Hacking — Thorough Removal

Method: Remove all loose, hollow-sounding concrete down to sound concrete. Tap each area with a hammer — any spot that produces a hollow "dong" sound must be removed.

Key point: All edges must be cut to a square edge with a minimum depth of 10 mm. Never hack at an angle or create feather edges — thin edges will inevitably crack and debond later.

Step 2: Rebar Cleaning

Method: Clean exposed rebar to Sa 2.5 grade, i.e. down to a bright, silvery metallic finish.

Key point: Also hack behind the rebar (approx. 10–15 mm clearance) to ensure the primer and mortar can wrap around 360 degrees without leaving blind spots. If corrosion behind the rebar is left untreated, it will eventually cause a halo effect (翻發).

Step 3: Rebar Protection — Zinc-Rich Primer

Method: Apply zinc-rich primer to the cleaned rebar surface.

How it works: Once dry, the primer forms a dense metallic zinc layer that provides three levels of protection:

• Barrier effect: The dense zinc layer physically blocks moisture (H₂O), oxygen (O₂), and chloride ions (Cl⁻) from reaching the rebar
• Cathodic protection: Zinc acts as a "sacrificial anode" — when both zinc and steel are exposed to moisture, zinc oxidises preferentially, protecting the rebar until the zinc is fully consumed
• Potential equalisation: Suppresses the potential difference at the old/new concrete interface, preventing a halo effect (翻發) — where fixing one area causes adjacent areas to fail

Recommended products: Fosroc Nitoprime Zincrich, Sika Armatec 110 EpoCem, or equivalent zinc-rich primers. Wah Hing generally keeps these in stock.

Step 4: Bonding Primer

Method: Apply a dedicated bonding primer to the cleaned concrete surface.

Key point: You must strictly follow the wet-on-wet method — apply the mortar while the primer is still tacky and not fully dried. If you wait until the primer has fully cured before applying mortar, it will actually cause delamination and severely reduce bond strength.

Recommended products: Unicell Primer, Fosroc Nitobond AR, or equivalent bonding primers. Wah Hing generally keeps these in stock.

Step 5: Mortar Application

Method: First press the repair mortar into the repair area by hand to expel air and ensure full contact with the substrate, then finish with a trowel.

Key points:

• Press firmly during application — do not leave any voids, as trapped moisture would undo all your work
• Each layer should not exceed 40 mm. If the repair is deeper, apply in multiple layers
• When applying in layers, the first layer must be scored (keying) — i.e. scratch grooves into the surface with a trowel to provide mechanical key for the next layer

Recommended products: Unicell 40, Unicell 25, Renderoc HB40, Renderoc HB25, Optimix RM740, Optimix RM725, SikaRep 25 HB, Mapei HB25, Mapei HB40, or equivalent repair mortars. Wah Hing generally keeps these in stock.

Step 6: Curing

Method: Keep the repaired area moist after completion.

Key point: Either spray with water or apply a curing compound. The purpose of curing is to prevent the mortar surface from drying out too quickly, which would cause drying shrinkage cracks. Duration depends on weather and ambient humidity.

Class 25 or Class 40? How to Choose

Repair mortars are graded by compressive strength. The two most common grades are Class 25 and Class 40:

• Class 25 (25 MPa): For general repairs — suitable for most non-structural work such as ceiling plaster delamination, external wall surface repairs, etc.
• Class 40 (40 MPa): For structural repairs — suitable for load-bearing elements such as beams, columns, floor slabs, and other locations requiring higher strength

Simple rule of thumb: if the repair involves a load-bearing structural element, or the project specification calls for it, use Class 40. For general maintenance, Class 25 is sufficient. If in doubt, consult your engineer or structural consultant.

Recommended Products

Below are products related to spalling repair. Wah Hing generally keeps these in stock:

Zinc-Rich Primers (Rebar Protection)

• Fosroc Nitoprime Zincrich — by Fosroc, solvent-based zinc-rich primer
• Fosroc Nitoprime Zincrich WB — by Fosroc, water-based zinc-rich primer
• Sika Armatec 110 EpoCem — by Sika, epoxy-based anti-corrosion coating

Bonding Primers

• Unicell Primer — by Sika Ronacrete, for use with Unicell mortars
• Fosroc Nitobond AR — by Fosroc, for use with Renderoc mortars
• Fosroc Primer 7 — by Fosroc, epoxy-based primer

Repair Mortar — Class 40 (Structural Grade)

• Unicell 40 — by Sika Ronacrete, PCCS-RM certified
• Renderoc HB40 — by Fosroc, high-strength thixotropic mortar
• Optimix RM740 — by Optimix, PCCS-RM certified
• Mapei HB40 — by Mapei, high-strength repair mortar
• SikaRep 40 HB — by Sika, thixotropic repair mortar

Repair Mortar — Class 25 (General Maintenance Grade)

• Unicell 25 — by Sika Ronacrete, PCCS-RM certified, top choice for old building maintenance
• Renderoc HB25 — by Fosroc, good value
• Optimix RM725 — by Optimix, PCCS-RM certified
• SikaRep 25 HB — by Sika, thixotropic repair mortar
• Mapei HB25 — by Mapei, suitable for general maintenance

→ Browse all spalling repair mortars

Frequently Asked Questions

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• Spalling vs Cement: Why You Shouldn't Use OPC for Concrete Repair