How to Test and Repair Magnesite Floors

Many strata buildings face costly uncertainty when dealing with magnesite-contaminated floors. This article outlines a practical, risk-based testing and repair methodology that avoids unnecessary testing while ensuring long-term durability of reinforced concrete slabs.

Magnesite floor toppings were widely used in Sydney residential buildings during the 1960s and 1970s. Installed as slab toppings or acoustic underlays beneath carpet and tiles, they were considered a practical solution at the time. Decades later, however, these systems have proven to be a major cause of concrete durability issues.

Why Magnesite Floors Deteriorate

Magnesite flooring is manufactured from calcined magnesite combined with fillers such as sawdust, silica, and talc, and bound together using magnesium chloride. This composition makes the material highly chloride‑rich.

Over time, chloride ions penetrate the concrete slab, corroding the reinforcing steel. The damage often shows up as lumps beneath carpet or cracks in tiled surfaces.

Idenitfy the Severity of Concrete Deterioration

Effective identification must begin with the complete removal of the magnesite topping, exposing the concrete slab for assessment. Once exposed, slab conditions can generally be classified into three categories:

Category 1 – Severe deterioration	Severe spalling and delamination requiring major structural repair and possible propping.
Category 2 – Localised deterioration	Localised areas of spalling or delamination, commonly occurring near windows, bathrooms, and other wet areas.
Category 3 – No visible deterioration	Slabs that appear sound following magnesite removal, with no obvious cracking, delamination, or spalling.

Repair Area Identified by Delamination and Potential Mapping

Fast and Accurate Testing Methodology

For Category 1 slabs, detailed investigation is essential and is typically undertaken in conjunction with a structural engineering assessment.

For Category 2 and Category 3 slabs, a streamlined and targeted testing approach is sufficient. This includes:

• Comprehensive delamination testing (hammer testing to identify delaminated areas) across the slab for all sound concrete areas.

  
  

• Targeted concrete breakout at selected locations to expose reinforcement, verify rebar condition, verify continuity, and calibrate potential mapping/corrosion status.

• Global half-cell potential mapping to identify all areas of high corrosion activity.

This fast and targeted methodology accurately identifies defect areas without unnecessary procedures.

Magnesite Repair Process

A repair procedure for all spalled and defective areas typically involves:

Smart Zinc Magnesite Anode

1. Breaking out the defective concrete to expose the steel reinforcement.

   
   

2. Clean the steel reinforcement and apply zinc-rich primer.

   
   

3. Install Smart Zinc Magnesite anodes along the perimeter and within breakout areas.

   
   

4. Test and establish continuity for all exposed reinforcement within the breakout area.

   
   

5. Reinstating concrete using a low-resistivity, polymer-modified repair mortar.

Once all repairs are completed, the entire floor is sealed using a epoxy moisture barrier and overlay to provide a uniform substrate for a new floor.

Long-Term Benefits

This practical methodology provides an effective and efficient means of addressing magnesite-contaminated floors. It focuses on the direct identification and treatment of areas of active corrosion, eliminating unnecessary testing that does not influence repair outcomes.

Key benefits of this methodology include:

• Direct identification and treatment of all areas of active corrosion, ensuring repairs are targeted where they are required.

  
  

• Elimination of unnecessary chloride testing, which seldom impacts repair design or effectiveness.

  
  

• Rapid testing, typically completed within hours of magnesite removal.

  
  

• Full treatment behind reinforcing steel, with encapsulation using polymer-modified repair mortars.

  
  

• Significantly reduced risk of incipient anode formation adjacent to repaired areas by using Smart Zinc Anodes.

  
  

• Sealing of floor surfaces to prevent future ingress of moisture and oxygen, minimising the risk of ongoing corrosion in areas assessed as low risk through potential mapping.

  
  

• Restoration of the concrete slab to provide reliable, long-term durability.

  
  

By concentrating only on tests that directly influence repair outcomes, this approach avoids unnecessary extended and costly testing procedures while guaranteeing structural integrity and optimum long term protection.

Disclaimer

All magnesite floor testing and repair must be carried out by qualified corrosion engineers with expertise in electrochemical assessment.