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Concrete In Australia : June 2013
Concrete in Australia Vol 39 No 2 41 e deck rests on 10 spans comprising of eight girders each, which are supported by nine piers and two abutments. Each girder is 1.4 m high, 0.5 m wide and 25 m long. e girders are heavily reinforced with a cage and 24 prestressed 15.2 mm diameter high tensile steel tendons. e bridge is crossing a tidal river and is subject to intermittent flooding, as shown in Figure 2 and Figure 3. e design called for a 100 year service life. 5.1 Durability issues With the piers and the girders exposed to splash by saltwater and occasional immersion, the exposure condition of the bridge was conservatively classified as exposure condition "C" in accordance to AS5001.5. Exposure condition "C" requires a minimum concrete cover of 70 mm cover using >50 MPa concrete when standard form work, and standard compaction is used or a minimum 50 mm cover if rigid formwork and intense compaction is used as in a precast situation. During the design it was identified that critical elements of the structure, such as the prestressed girders, headstocks and abutments cannot be provided with sufficient concrete cover to satisfy the design standards for durability. Further, intense compaction was not possible due to the high steel density. e options to achieve the design life without cathodic protection/prevention were considered. After construction, periodic inspection and maintenance of the bridge would be required, which should include visual inspections as well as concrete testing. is would assist identifying any deterioration at an early stage or even prior to such damage occurring, eg by identifying that chloride levels approach the critical threshold to induce reinforcement corrosion. At this point in time, counter measures could be imposed such as the retrofit of cathodic protection. Coatings and additives could slow down the process of chloride diffusion, but would require regular maintenance with multiple coating applications necessary over the design life. It was indentified that the feasibility of this option was severely restricted by access and cost. 5.2 Cathodic prevention or cathodic protection After the durability assessment, the options to retrofit the bridge with cathodic protection were assessed. It was concluded that a retrofit of the bridge with a cathodic protection system would not be possible due to the following challenges, which are also diagrammatically shown in Figure 4: • e construction of the bridge involved eight parallel girders per span, preventing access to the sides of the internal girders after construction to install ribbon anodes. • High density of steel made the installation of internal anodes in drilled holes impossible. Table 3. Advantages and disadvantages of installation options. Cathodic prevention Installation of CP components at time of construction Provision of continuity and retrofit of CP Protection from day one of operation. Option to quickly complete and energise the system if found necessary at relative low cost. Higher capital investment at the time of construction. Higher capital investment at the time of construction. Expensive retrofit required -- pending access, retrofit may not be possible. No cutting of slots for anode ribbons required. No cutting of slots for anode ribbons required. Cutting of slots for anode ribbons or drilling of holes for discrete anodes is required. Ongoing monitoring costs from day one of operation. Ongoing monitoring and testing is required to indentify the right time for energisation. Ongoing monitoring and testing is required to indentify the right time of retrofit. Lower level of durability design can be tolerated, eg lower cover or mix than otherwise required. Higher risk since corrosion of pre/post tensioned steel tendons may not be detected (small diameter tendons may fail without cracking or spalling). Higher risk since corrosion of pre/post tensioned steel tendons may not be detected (small diameter tendons may fail without cracking or spalling). Operational life of the CP system is longer. Operational life of the CP system is longer. System will require a shorter life. Lower running and maintenance costs due to fewer repairs and lower average current density required over the design life. Lower system cost due to lower current density and fewer anodes required. Corrosion monitoring must be conducted throughout system life to identify when to energise cathodic protection. Concrete repair costs can be avoided. Concrete repair costs in addition to installation costs if retrofitted after corrosion has initiated.