1. General
This document sets out the options for the repair of reinforced concrete floor slabs affected by corrosion caused by magnesite floor topping.
2. Background information
Magnesite floor topping was commonly installed in the 1970s, mainly in residential buildings in Sydney, as a slab topping or an underlay to carpet and other forms of floor covering.
The topping material is made from calcined (or burnt) magnesite and various organic and inorganic fillers such as wood, sawdust, ground silica and talc, which are mixed with a solution of magnesium chloride, and thus magnesite flooring is extremely rich in chlorides. Over time, the topping can cause problems with diffusion of chloride ions into the concrete slab leading to corrosion of the reinforcing steel. The corrosion problems in the slab usually appear as lumping below the carpet or cracks in the floor tiles.
3. Testing of magnesite affected floor slabs
After detecting the problem, the first step is to fully remove the magnesite topping from the slab. As part of the removal process, it is required to test the topping for asbestos content.
Once the floor slab is exposed, testing is required to ascertain the extent of the problem and to confirm the level of chloride ingress with relation to rebar locations in the slab.
In most cases in residential buildings, spalling and delamination in the slab are evident near windows and in the vicinity of bathroom and kitchen doors. The reason for this is the relatively high probability of water/moisture ingress in those areas. It is common to find fewer problems in the bedrooms and corridors as the likely ingress of water and moisture in those locations is minimal. Internal floors which may have been subjected to water flooding in the past may have a higher risk of corrosion and concrete spalling.
To identify the corrosion status of the slab, the following test methods are required:
Concrete Test method: |
1. Visual inspection |
2. Delamination testing |
3. Measurement of the chloride content in the slab at depth |
4. Carbonation testing |
5. Measurement of the cover to reinforcement |
6. Rebar continuity testing |
7. Potential mapping |
8. Resistivity testing |
9. Physical inspection of condition of rebar at breakout locations |
The analysis of the combined data obtained from the tests above will assist in determining the extent of concrete deterioration, the risk of corrosion in the sound concrete areas, and the repair solutions that can be considered.
4. Options for repair
All repair options require full removal of magnesite topping from the floor slabs.
4.1 Option 1 – Removal of all chloride contaminated concrete and replacement with new concrete
This involves the physical removal of all chloride contaminated concrete from the floors (jack hammering), and the disposal of all chloride contaminated concrete. This work may require propping of the floor slabs from below during the demolition process. The units impacted by the repair work must be vacated until the work has been fully completed.
The full removal of chloride contaminated concrete is the only 100% risk-free option under all circumstances. This option will fully eliminate the risk of any future chloride induced corrosion and concrete spalling. However, in the majority of cases for residential buildings, this option is considered not viable due to its highly destructive nature, structural considerations, disruption to residents and its high cost.
4.2 Option 2 – Global electrochemical corrosion protection systems (desalination, ICCP, SACP)
(limited applicability for residential units)
These options for remediation of the concrete slab include the application of chloride extraction (desalination), impressed current cathodic protection (ICCP), or sacrificial anode cathodic protection (SACP). It is important to note that these options are normally applied for the repair of chloride induced corrosion on bridges and wharves located in marine environments. For indoor applications in residential buildings, where concrete resistivity may be relatively high in enclosed environments, these options can only be considered under some circumstances, however performing a trial or pilot installation is required to verify their applicability and effectiveness prior to full implementation.
For these options to be considered viable, there are two major requirements: continuity of the embedded rebar, and concrete resistivity level suitable for electrochemical application. Resistivity of concrete impacts on the capacity of an electrochemical protection system to deliver the protection current. Chloride extraction can operate with relatively high resistivity concrete. ICCP can operate at medium ranges of concrete resistivity. SACP systems can only operate in relatively low resistivity concrete.
4.2.1 Desalination (chloride extraction)
In this option, repair work is performed without the need for breakout behind the rebar, all defective areas of the floor are repaired with low resistivity mortar, and the desalination process (chloride extraction) is performed to the entire floor slab to extract the chloride from around the steel reinforcement.
This is a technically sound solution; however, its implementation requires full access to the floor slab for at least 10-12 weeks for the treatment. The concrete resistivity of the slab must be suitable for this application, and the process may not fully extract all chloride from around the rebar. After desalination, the risk of renewed corrosion is relatively low, however in Australia, this repair method is uncommon and has never been implemented for the repair of magnesite floor in a residential building.
For further information on desalination: https://www.remedialtechnology.com.au/chloride-extraction
4.2.2 Impressed Current Cathodic Protection (ICCP)
In this option, repair work is performed without the need for breakout behind the rebar, all defective areas of the floor are repaired with low resistivity mortar, and the cathodic protection (ICCP) system is installed to the entire floor slab. This system requires the installation of a permanent power supply unit and full monitoring for the life of the system.
This is a technically sound solution for ongoing corrosion prevention. The concrete resistivity of the slab must be suitable for this application, and the ICCP system must be monitored for its entire life. The risk for renewed corrosion is low and the ICCP system can be installed and monitored based on Australian Standard AS 2832.5.
ICCP systems must not operate without guaranteed routine maintenance and monitoring program for the life of the system. To eliminate the risk which could be associated with the operation of the system without ongoing monitoring, ICCP is not recommended for residential units for internal flooring.
For further information on ICCP, refer to: https://www.remedialtechnology.com.au/cathodic-protection
4.2.3 Global Sacrificial Anode Cathodic Protection (SACP)
In this option, repair work is required to be performed behind the reinforcement and would require breakout behind the rebar (20 mm), all defective areas of the floor are repaired with low resistivity mortar and the SACP is installed for the entire floor.
The life of the system is limited (10-15 years) as the anode material (such as zinc) is consumed to provide corrosion protection. The concrete resistivity must be relatively low for this type of system to operate. The level of protection is reduced over time and overall, this type of systems cannot meet the protection criteria for cathodic protection based on the applicable Australian Standards.
It is suitable to consider SACP systems as an additional prevention measure. A trial application is required to verify the performance of the proposed system for a specific application as galvanic systems may not operate at all in high resistivity concrete. Subject to the type of galvanic anodes, these systems can be costly, and their use can only be justified if an adequate level of corrosion protection in accordance with the applicable cathodic protection standards can be achieved.
4.3 Option 3 – Concrete repair in conjunction with localised galvanic anodes and a moisture barrier
(applicable for floor slabs with existing corrosion damage and spalling)
In this option, the repair work requires breakout behind the rebar (20 mm) and all defective areas of the floor to be repaired with polymer modified repair mortar.
There is risk associated with this option with renewed localised corrosion only in the non-repaired areas of the floor. This risk is reduced by the application moisture barrier; however, it is not eliminated as the corrosion may have been already initiated.
All areas of spalling are fully repaired by conventional patch repair eliminating risk of corrosion in the repaired areas. Galvanic anodes are installed at the perimeter of the repaired areas and are encapsulated in low resistivity mortar. This application will reduce the risk of possible formation of incipient anode corrosion in the good concrete areas around the patches. Additional galvanic anodes can be installed in drilled holes in the concrete in high corrosion risk areas for corrosion prevention purposes. Following the repair and anode installation at the border of the repairs or/and in localised high-risk corrosion areas, a moisture barrier is installed over the entire floor to eliminate further ingress of moisture and oxygen.
This option is very practical and is normally the most suitable treatment for magnesite flooring with concrete damage. The galvanic anodes are not considered to be providing primary corrosion protection measure but rather as an additional low-cost prevention measure. The installation of the galvanic anodes in high-risk areas must be evaluated based on the level of concrete resistivity. In high concrete resistivity, there may not be any benefit of using galvanic anodes.
For further information refer to https://www.remedialtechnology.com.au/galvanic-systems
4.4 Option 4 – Application of a moisture barrier
(applicable for floors with no existing corrosion damage and spalling)
This option is commonly implemented for magnesite-affected floors where there is no evidence of any concrete spalling or major corrosion activity in the slab.
This method is based on the prevention of any additional ingress of moisture and oxygen into the concrete slab which can cause steel corrosion.
The risk associated with this option is renewed isolated corrosion in localised areas where the corrosion has already initiated and there is sufficient moisture in the concrete for the corrosion cell to propagate regardless of the moisture barrier.
This option is the optimum and most cost-effective option that can be considered for floors with high concrete resistivity as any additional preventive measure using localised galvanic anodes may not be effective or viable.
5. Summary
For chloride induced corrosion caused by magnesite, the only 100% risk free solution is to fully remove all chloride contaminated concrete from the entire floor slab, clean the steel (to ‘as new’ condition) or replace all steel located in the chloride contaminated concrete, and pour a new slab with chloride free concrete (Option 1). This option is usually not considered viable for residential buildings due to practical, structural constraints, and cost considerations.
For the global electrochemical protection solutions (Option 2), if considered, these methods must be trialed and verified prior to full implementation on magnesite floors. It is important to note that these treatments are normally applied for structures located in marine environments and not to magnesite floors in residential units.
Options 3 and 4 may not fully eliminate the risk of future renewed corrosion in isolated locations in floor slabs, however, these options are considered the most practical and cost-effective methods to manage corrosion risk related to magnesite floor repairs.
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