Dangerously damaged hand rails with exposed rusting metalwork Elastomeric polymer membrance on a city high-rise mitigates the effects of exposure from the steel. All the corroded material is then removed and the steel treated or replaced, after which specialist repair concrete mortars are applied and the surface made good. A modern development is for the repair mortars to be polymer modified to improve adhesion and resist further ingress of contaminants. Coatings are commonly used in combination with patch repairs to reduce further entry of carbonation or chlorides. These “patch repairs” that remove the contaminated concrete from the deteriorating sections often do not address this hidden corrosion and result in accelerated deterioration to the surrounding areas, commonly failing again within three - five years. Godson added, “One of the limitations of patch repairs is that you have to remove large quantities of sound concrete to solve the problem, causing significant noise and disruption to the building occupants.” The main alternative to patch repair is Cathodic Protection. One type, Impressed Current Cathodic Protection (ICCP), is a technique whereby a small, permanent current is passed through the concrete to the reinforcement in order to virtually stop the corrosion of the steel. The main benefit of ICCP is that the extent of removal and repair of concrete is vastly reduced, with only the spalled and delaminated concrete required to be repaired. Once installed, the ongoing corrosion can be controlled for the long term, eliminating future spalling and deterioration even in severely chloride or carbonation contaminated concrete. The selection of anode systems is the most vital design consideration for a durable and efficient ICCP system. Incorrect selection and placement of the anode system can result in poor performance and vastly reduced life of the installation. According to Godson, cathodic protection is relatively simple in theory. “Insert anodes into the concrete at set spacing attached to the positive terminal of a DC power supply and connect the negative terminal to the reinforcing steel. ICCP systems commonly operate at 2 to 5 Volts DC,” he said. “The drawback is that you need lots of cables and permanent power supplies which results in this technology being mainly restricted to civil structures such as wharves and bridges with very rare applications to buildings.” A relatively recent development has been Hybrid CPwhich utilises zinc anodes installed in drilled holes with the anodes powered for an initial period of around 10 days. The high initial CP current totally passivates the steel reinforcement, migrating chloride away from the bars and restoring an alkaline (high pH) environment in the concrete. Following the initial impressed current phase, the temporary power supply and cables are removed, with the anodes then connected to the reinforcement via locally placed junction boxes to provide ongoing galvanic protection. This relatively low galvanic current maintains the ongoing passive condition at the reinforcement and prevents further concrete damage. Hybrid CP systems are usually designed to give a 30 year or longer design life. Hybrid CP offers all the advantages of ICCP, including corrosion control and reduced concrete removal, without the high cost and maintenance of power supplies, cables and control systems. Areas and structures that were previously difficult and uneconomical to treat with ICCP can now be protected using Hybrid CP technology. This includes small scale and remote structures including those situated in nonpowered sites such as bridges, marine dolphins and culverts. In the case of building repairs, Hybrid CP offers significant advantages over ICCP by eliminating the need for unsightly and costly cabling and power supplies. The ACA is a not-for-profit, membership Association which disseminates information on corrosion and its prevention or control, by providing training, seminars, conferences, publications and other activities. The industry association was formed in 1955 and represents companies, organisations and individuals involved in the fight against corrosion and promotes cooperation between academic, industrial, commercial and governmental organisations. Hybrid anode installation, typically with Anodes installed in 30mm diameter holes, typically spaced at approximately 400mm, with titanium connector wires. Repair mortar then completely covers the hybrid system components www.engineeringnews.co.nz 29
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