Magnesite flooring—A challenge for Building Owners

Magnesite flooring—A challenge for Building Owners

The risk posed by Magnesite

Magnesite toppings were a popular floor underlay material used between the 1950s to the mid-70s. It had the appearance of cork and had good soundproofing qualities, it was ready to take carpet, parquet, vinyl or tiled finishes. The concrete slab beneath had a fairly rough finish, and the magnesite was an economical and popular floor topping. By the mid-70s it was realised that this “wonder” material had a most undesirable property, it contained a high level of soluble chlorides which can cause corrosion of reinforcement, which is essential to the concrete beneath, and it quickly went out of fashion.

In the ensuing years, storms, equipment malfunctions and occupant carelessness have led to water reaching the magnesite. The windows and doors were not as good as their modern counterparts, and in stormy conditions they leaked. Also, over half a century of use, doors and windows have been left open in stormy conditions, water has been spilt, it has overflowed from kitchens and bathrooms due to equipment failure, waterproofing membrane failure or simply carelessness. Water has gained access to the magnesite, particularly in high-risk areas.

When the magnesite became wet, the chlorides were leached into the underlying concrete, and with time the risk of corrosion has increased. Once chlorides reached the reinforcing steel in critical amounts, corrosion commenced, and in the wettest locations, advanced corrosion has caused spalling, the shelling off of concrete due to the expansive corrosion of reinforcing steel.

The Owner’s dilemma

For many years the traditional reaction to corrosion induced expansion of the concrete (spalling) was to simply remove the loose concrete, excavate till rust free reinforcing steel was found, clean and prepare the reinforcing steel and apply a concrete patch. All that these local fixes achieved was to apply a “band-aid” to the worst spots, the areas of the most advanced corrosion, and at the same time, increase the risk of corrosion immediately adjacent to the repaired patch.

Because chloride salts attract moisture and dissolve, just as kitchen salt does when left out in Sydney’s humid summer, chloride migration from magnesite into the concrete slab has occurred everywhere, even in relatively dry areas. Chlorides have been drawn from apparently dry magnesite into concrete developing the conditions necessary for corrosion. Hence building owners have slabs with some degree of chloride contamination everywhere. The worst areas of local corrosion are observable and can be identified visually or with a hammer “drummy” test, but significant corrosion risks are concealed elsewhere where chloride movement is slower, and corrosion is developing more slowly.

If the risk of chloride related corrosion is left unattended and the magnesite is left on floors, with time, the top layer of reinforcing steel in slabs will start to rust and the cost of remediation will be maximised. Unfortunately, this is the strategy that many Owners Corporations adopt because it is the easiest, it looks OK so it must be OK, but not so. The chlorides in the magnesite continue to migrate down into the drier concrete, and once they get to the steel, corrosion will eventually start. In 5, 10 or 20 years, the cost of remediation will be greatly increased. Early action is responsible and pays dividends. Leaving the problems for the future can be seen as being negligent, leaving greater costs for future owners.

Magnesite flooring—A challenge for Building Owners

State-of-the-art technology—Assessment—BCRC is at the forefront

Over the past 40 years, senior members of the BCRC team have been developing and using nondestructive tools (NDT) and modelling for the assessment of structures. They have pioneered techniques to detect corrosion, measure strength and model deterioration mechanisms, all essential for assessing risk, the time to corrosion initiation and the long-term impact on serviceability.

State-of-the-art technology—Cathodic Protection—a BCRC area of expertise

Where chlorides have penetrated the concrete slab to a critical degree, the risk of future corrosion is high unless the chloride contaminated concrete is removed or cathodic protection (CP) is applied. Demolition and reconstruction, the traditional solution in the local industry is noisy, dusty and expensive, and may require temporary propping. CP, while not well understood by many engineers, is much faster, less noisy, less dusty and cheaper.

BCRC can help find the right solution for you

To find the right solution, it is critical to understand the requirement of each client. BCRC’s approach systematically addresses these to ensure the solution selected is technically sound, cost effective and best in terms of meeting owner needs.

BCRC have been the consultants on inspection and repair of many buildings. Their team leaders are internationally recognised in the concrete repair field. Key objectives sought are:

Magnesite flooring—A challenge for Building Owners

  • an early understanding of long-term risks and remediation options;
  • rapid installation to minimise inconvenience and disruption to unit holders;
  • least cost solutions;
  • elimination of ongoing corrosion;
  • structural reinstatement; and
  • re-levelling of the floor to existing levels

The principle components of the overall systems developed are:

  • NDT and laboratory testing to accurately identify the extent of the problem;
  • modelling to accurately assess the severity and time frame of risk development;
  • design including the inclusion of CP to eliminate the risk of any future corrosion;
  • external strengthening using carbon fibres;
  • the use of concrete repair materials which are compatible with CP; and
  • the use of toppings that provide long term durability, rapid installation, minimum disturbance and favourable acoustic properties

Concrete Repairs – The use of a CP system removes the need to breakout the undamaged concrete. Only fractured concrete is removed and that is relatively easy, and hence the use of noisy breakers that reverberate through the building is minimised. The quantity of replacement concrete in repairs is minimised, making the system faster and environmentally friendly. Carbon fibres can be incorporated into the repairs to strengthen the slab where reinforcement corrosion is excessive. This is an alternative to breaking out concrete and replacing steel.

BCRC services – BCRC directors have led the use of CP for concrete structures in Australia for over 30 years. We can just prepare the specification for remedial works, but more often we oversee the project taking care of contractor selection and administration as well as undertaking quality assurance testing at key points. Our contractors prefer this new faster, cleaner way of working.