Magnesite floors were common in the 1960s, mainly in Sydney, as a slab topping or an underlay to carpet and other forms of floor covering. 

Made from calcined (or burnt) magnesite and various organic and inorganic fillers such as wood, sawdust, ground silica and talc, mixed together with a solution of magnesium chloride, magnesite flooring is extremely rich in chlorides, resulting in problems of sweating, diffusion of chloride ions into the concrete substrate and metal corrosion, usually seen as lumps appearing below the carpet or cracks in the floor tiles.

Repairing a magnesite floor

The first and most important step after detecting the problem is to find out the extent of the problem and to confirm the cause of the spalling. This can be done by measuring the chloride content in the concrete and the cover to reinforcement in selected locations where the magnesite can be partially removed. This information will assist in confirming the level of chloride in the vicinity of the steel reinforcement and provide an overall idea of the percentage of steel reinforcement located in chloride contaminated concrete. 

Tests such as potential mapping to determine the corrosion activity of reinforcement in the sound concrete areas of the slab can only be undertaken if the magnesite topping is fully removed.

Options for repair

Option 1 involves removal of magnesite topping and all chloride contaminated concrete and replacing with fresh concrete.

Option 2 is to remove the magnesite topping and all spalled and delaminated concrete, carry out cathodic protection repair and install an impressed current cathodic protection system to the floor slab using mixed metal oxide (MMO) titanium mesh or ribbon anode.

Option 3 would be the removal of magnesite topping and all spalled and delaminated concrete, carry out conventional repair work (breakout behind reinforcement), and install a sacrificial anode system around the perimeter of the repaired areas as well as in the remaining sound areas of the slab.

Option 4 would require the removal of the magnesite topping and all spalled and delaminated concrete, carrying out conventional repair work, installing a sacrificial anode system around the perimeter of the repaired areas and applying a concrete sealer to prevent any further ingress of moisture and oxygen to the steel reinforcement.

Which option to select?

Only a thorough investigation by a professional corrosion engineering company can determine the extent of existing and potential deterioration and the most appropriate option for repair.

Option 1 is the preferred option under all circumstances as it will remove the risk for any further deterioration; however, in most cases this option would not be applicable due to structural consideration and cost.

Option 2 would require continuous monitoring of the floor slab and also require a professional cathodic protection company to design and install the system and undertake the monitoring of the system in order to limit any potential overprotection that may not be suitable in this application. Cathodic protection can be applied to selected areas of the slab where there is a high risk of corrosion, such as kitchens, bathrooms and along the perimeter of the premises where there is a possibility of moisture ingress. This solution is not suitable for small applications. 

Option 3 is relatively expensive and may provide corrosion protection for up to 10 years.

Option 4 is a cost effective option but it would be very difficult to estimate the time of corrosion protection that can be offered.

Therefore, it is essential an expert’s advice is obtained and a thorough assessment of the slab condition is undertaken prior to determining the repair option. 

Remedial Technology Pty Ltd provides initial professional free advice regarding all magnesite floor problems.