Concrete is one of the toughest building materials around, but even a well-laid slab can start to flake, pit, or crumble over time. This surface deterioration is called spalling, and it's more common in New Zealand than many homeowners realise — especially in cooler southern regions and coastal areas. Understanding what causes it is the first step to fixing it properly and preventing it from coming back.
What Is Concrete Spalling?
Spalling refers to the flaking, chipping, or peeling of the surface layer of concrete. It can look like shallow pitting, rough patches, or larger chunks breaking away. Left untreated, spalling exposes the aggregate beneath and — more seriously — can allow moisture to reach the steel reinforcing inside, leading to rust, cracking, and structural problems.
The Most Common Causes of Concrete Spalling
1. Freeze-Thaw Cycles
In colder parts of New Zealand — think Central Otago, the Southern Alps, and elevated areas of the North Island — water can seep into the tiny pores of concrete and freeze overnight. When water freezes, it expands, and that expansion puts enormous pressure on the concrete from within. Repeated freeze-thaw cycles gradually break apart the surface, causing it to flake away. This is one of the most destructive forces concrete faces in NZ's alpine and inland climates.
2. Poor Finishing Technique
One of the most preventable causes of spalling is overworking the surface during placement. If a concreter adds water to the mix to make it easier to work with, or trowels the surface while bleed water is still rising, it weakens the top layer. This creates a thin, weak skin that is prone to flaking — sometimes within the first year. BRANZ research consistently highlights poor workmanship during placement as a leading cause of early concrete deterioration.
3. Deicing Salts and Chemical Attack
While road salt is less common in NZ than in North America or Europe, coastal properties are exposed to salt-laden air, which can penetrate concrete and attack the reinforcing steel. Fertilisers, pool chemicals, and some cleaning products can also cause chemical deterioration of the surface. If you're near the coast, this is worth taking seriously.
4. Carbonation and Corrosion of Reinforcing Steel
Over time, carbon dioxide from the atmosphere reacts with the alkaline compounds in concrete in a process called carbonation. Once carbonation reaches the steel reinforcing bars (rebar), the steel begins to rust. Rust occupies more volume than steel, so it pushes outward — cracking and spalling the concrete above it. You'll often see rust staining alongside this type of spalling. This is a structural concern and needs professional assessment.
5. Insufficient Concrete Cover
The NZ Building Code and standards like NZS 3109 specify minimum cover depths — the thickness of concrete over reinforcing steel — to protect against corrosion. If the cover is too thin (due to poor placement or rebar shifting during the pour), the steel is more vulnerable to moisture and carbonation, accelerating spalling.
6. Low-Quality Mix or Incorrect Water-to-Cement Ratio
A concrete mix that contains too much water produces a weaker, more porous slab. Excess water evaporates and leaves behind tiny capillaries that allow moisture to penetrate easily. A low water-to-cement ratio is critical for durability — this is why buying cheap ready-mix or adding water on site is a false economy.
7. Inadequate Curing
Curing is the process of keeping fresh concrete moist so it gains strength properly. If concrete dries out too quickly — common on hot, windy NZ summer days — the surface becomes weak and prone to dusting and flaking. Proper curing for the right duration (as recommended by BRANZ) is essential for a durable finish.
How Serious Is Spalling?
Not all spalling is equal. Here's a quick guide:
| Severity | Appearance | Action Required |
|---|---|---|
| Surface dusting / minor pitting | Shallow, no exposed aggregate | DIY resurfacing may be suitable |
| Moderate flaking | Exposed aggregate, rough patches | Repair mortar or resurfacer |
| Deep spalling with rust staining | Exposed rebar, cracking | Professional repair needed |
| Structural spalling | Large sections breaking away | Engineer assessment required |
Can You Repair Spalled Concrete Yourself?
For minor surface spalling with no exposed steel, a homeowner can often apply a concrete resurfacer or repair mortar. Products from Ardex, Sika, and Mapei are available through trade suppliers and some hardware stores. The key steps are:
- Clean the surface thoroughly — remove all loose material, dust, and contaminants
- Prime the area if required by the product instructions
- Apply the repair mortar in layers, following the manufacturer's guidelines
- Cure the repair properly, especially in hot or windy conditions
Products and materials are available at Bunnings and Mitre 10, though for larger repairs, PlaceMakers can be a good source of trade-grade materials.
How to Prevent Spalling in New Zealand
- Use a quality concrete mix with the correct water-to-cement ratio for your exposure conditions
- Ensure adequate steel cover as per NZS 3109
- Avoid overworking or adding water to the surface during finishing
- Cure properly — use curing compounds or wet hessian for the recommended period
- Apply a quality concrete sealer to driveways and paths, especially in coastal or frost-prone areas
- Avoid harsh chemicals on concrete surfaces
When to Call a Professional
If you notice rust staining, cracking alongside the spalling, or large sections of concrete breaking away, it's time to call a qualified concreter. Spalling that has reached the reinforcing steel is a structural issue — not just a cosmetic one — and needs proper diagnosis and repair. A licensed professional can assess whether the slab can be repaired or needs replacement, and ensure any work meets the NZ Building Code. You can find experienced local concreters on tradedirectory.co.nz.