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Concrete In Australia : June 2013
Concrete in Australia Vol 39 No 2 39 conditions upon the curing process etc. Damage may occur during transport and installation of precast elements leading to less than perfect results, even with onsite repairs being conducted. Furthermore, design parameters to achieve the desired durability cannot always be complied with, due to structural design requirements or actual exposure conditions being more severe than those explicitly covered by AS 5100.5. e end client may desire to maintain the asset beyond the 100 year design life. In these cases, the provision of cathodic prevention or protection can be considered as a long term option to prevent reinforcement corrosion. 3.0 WHAT IS CATHODIC PROTECTION/ PREVENTION 3.1 Cathodic protection Cathodic protection is an electrochemical method to protect steel or other metals in an electrolyte such as soil, water, or concrete from corrosion. A current is passed from the anode to the surface of the reinforcing steel. Where the current is taken up by the steel surface, an electrochemical reaction occurs, which forms alkalinity on the steel surface. is alkalinity reacts with steel causing it to passivate, thus preventing further corrosion. Protection can be achieved by connecting the steel to a more active metal such as zinc, aluminium or magnesium, thus forcing the steel to become the cathode in an electrochemical cell. is is referred to as galvanic, sacrificial or passive cathodic protection. Alternatively, a DC current may be supplied by an external source to an inert anode embedded in the concrete, which is called impressed current cathodic protection (ICCP). Figure 1 illustrates the principle of ICCP. Cathodic protection is routinely applied to reinforced concrete structures suffering from chloride induced corrosion. e effectiveness depends on sufficient current density being provided per area of steel surface. As per Australian standard AS2832.5, a current density of 20 mA per m2 of steel surface should be provided when designing cathodic protection systems for steel in concrete. Figure 1. Schematic of cathodic protection/prevention of reinforced concrete1. Arrows indicate current flow from anode to reinforcement. 3.2 Cathodic prevention Experience shows that when electrical current is provided immediately following construction, the steel surface within the concrete remains passive and the initiation of corrosion can be prevented. In contrast to cathodic protection where a current density of 20 mA per m2 of steel surface is required, cathodic prevention can be effective with current densities of 10-25% of this amount or 2-5 mA per m2 steel surface. Cathodic prevention is covered to limited extent by the Australian Standard for cathodic protection of steel reinforced concrete AS2832.5. e comparatively lower current demands required by cathodic prevention and ability to incorporate anodes into the structure prior to the pour can result in significant cost savings when compared to the retrofit of a cathodic protection system. 3.3 Typical anode materials Critical components of such a system are the anodes. A variety of anode types have been developed which can be utilised for cathodic protection/prevention of steel in concrete. ese are summarised in Table 1 and Table 2. 4.0 CATHODIC PREVENTION/PROTECTION IN NEW CONCRETE STRUCTURES Why consider cathodic prevention or provision for cathodic protection for new concrete structures? As indicated above, there are multiple possible reasons why the protection against reinforcement corrosion provided by the concrete may be insufficient. Examples include: • e required concrete cover cannot be provided for structural reasons. • e concrete cover required to achieve the design life would introduce shrinkage cracking problems. • e exposure of the concrete is very severe, eg splash zone or brine type solution in desalination plant outfall structures. • Quality control requirements to achieve the design life may not be achievable or guaranteed. • Inability to provide the required mix design or meet curing requirements, eg remote location or a hot site. • e asset owners expectations of layout, construction cost and maintenance-free design life are not compatible with current basic construction methodologies. For large infrastructure projects these reasons are usually identified during the durability planning stages and addressed if possible. However, if any concerns remain, the provision of cathodic prevention or the preparation for retrofit of cathodic protection provides a valid option. Cathodic protection/prevention systems can be installed during construction of the structure or be retrofitted. e reasons to install a cathodic prevention/protection system at the time of construction can be summarised in two groups: 1. Prevent any corrosion from commencing in the first place by energising the system at the time of construction. 2. Have the CP system ready to be energised when need is identified in the future. A cathodic prevention should be installed at the time of Ti mesh Rebars Rectifyer/ Control Unit Concrete overlay