Blog - Magnesite Induced Spalling in Strata Buildings

Between the 1960's and the 1990’s, a material called magnesite was commonly used as a self levelling floor screed for concrete floor slabs, particularly in residential strata buildings. This material predominately comprised of a magnesium chloride binder and wood chips. If mixed and bound correctly, this material was an ideal topping screed for concrete slabs as it provided good insulation properties and a smooth flat surface over the slabs, removing the necessity for a clean slab surface finish.

However, the use of magnesite was uncontrolled and often, construction workers would add extra magnesium chloride binder to the mix to produce a more workable mixture which was easier to level. Unfortunately this common practice resulted in many magnesite floor screeds having an abundance of magnesium chloride binder within the magnesite matrix. This means that, the chloride enriched floor screed comes into contact with water, free chlorides flush out of the floor screed into the concrete slab below, which over time causes reinforcement corrosion and subsequent concrete spalling. The most common sources of water inducing magnesite detriment are rain through open or leaking balcony doors or windows, fish tank accidents, bath overflows and dishwasher accidents.  

Magnesite topping screeds are now considered to be a threat to the structural integrity of a buildings fabric, which if left untreated after a single flood or other saturation event, could potentially result in complete structural failure through collapse.  

A further notable observation made through MJ Civil's involvement in a significant number of post saturation events is; that magnesite is incredibly difficult to dry out after it has been saturated. Our experience indicates that after days of intense drying with commercial heaters, only the top surface of the magnesite has dried, with a high moisture level still present at the underside of the topping where it is in contact with the slab. Furthermore to simply remove the magnesite after a flood or saturation event does little to address the Chlorides that have already been flushed into the concrete surface.

We approach a magnesite spalling project like any other, by firstly undertaking an initial inspection in order to identify the level of detriment in locations of significant structural concern. This can include spalling adjacent to the anchorage bars for cantilevered balconies or critical building supports such as columns, lift shafts or shear walls.

The findings of the inspection are compiled in an inspection report together with specific recommendations. Dependant on the scale of the observed problem this can include core samples to be extracted from specific locations and tested to determine the depth of chloride enrichment within the slab. Based on these results, we can produce a detailed scope of works for the slab remediation utilising our tailored repair specifications and preferred product selection. This is potentially a different approach to that taken by other consultants or contractors, who may be content to rectify only that which is currently visible. MJ Civil however, consider the potential cost of disruption to the residents coupled with damage to newly laid floor coverings and expensive alternative to the costs associated with an appropriate level of core investigations.

MJ Civil are industry leaders in the rectification of magnesite induced spalling and have a proven track record in remediation utilising our tailored repair specifications and products.

We have over 15 years experience with magnesite induced corrosion and have developed a number of tailored repair procedures which utilise a range of products with specific attributes designed to provide the most comprehensive and permanent remedial solutions. Furthermore, during the remedial works we will undertake a series of inspections of each excavation in order to certify the works.   

For further information or if you require a site inspection, please contact us.