THE Nga Awa Purua geothermal power station at Rotokawa, on the banks of the Waikato River, north of Wairakei, is currently being commissioned. It has the capacity to produce 132MW of electricity, sufficient to power 140,000 homes.
The power station is a joint venture project between Mighty River Power and Tauhara North No. 2 Trust.
Hawkins Construction was contracted to build the station structure including the cooling tower basin and cold well. Construction began with initial earthworks back in May 2008.
Cooling towers play an important role in the geothermal power generation process. Super heated steam used to drive the generator turbines must be condensed and cooled to a specific temperature prior to injecting back into the ground.
The concrete cooling tower basin and cold well presented a number of challenges for the design team from BECA and Hawkins Construction. These included:
• Concrete had to have a degree of resistance to the acidic, geothermal environment.
• Design called for minimal control joints.
• Concrete needed to be crack free.
• Dry quickly to minimise delays in the application of the protective coating.
• Tight dense surface.
Sample floor panels were cast on site using various finishing techniques. Once completed and cured, Sika (NZ) Ltd technical staff and Sika approved contractor, Man Construction Ltd carried out an extensive series of tests to evaluate and optimise the concrete and surface finish.
Specialist concrete contractors, BBR Contech and Conslab Ltd were employed to place and post tension the floor slabs. As the basin covered an area of approximately 2000 square metres the floor slab was cast in three sections. The concrete walls were precast off site then put in place and grouted into position.
A protective epoxy coating on the concrete was essential to ensure long term durability of the structure. Geothermal water is acidic and corrosive to concrete because of the hydrogen sulphide content but the effects of MIC (microbiologically influenced corrosion) from bacteria in the water is also a lesser known factor to be taken into account. Sikagard -62 was the product of choice because of its exceptional chemical resistance, previous use on geothermal projects and very good rating in a published article Evaluation of Coatings and Mortars for protection of concrete cooling tower structures from Microbiologically influenced Corrosion on Geothermal Power Plants, prepared by the Materials and Chemical Science Division, Department of Applied Science, Brookhaven National Laboratory, New York.
The concrete was prepared and pinholes, bugholes and defects were bagged using Sikagard -720 EpoCem epoxy cement repair mortar and temporary moisture barrier. Sikagard -62 heavy duty protective coating was then applied to all internal concrete surfaces of the cooling basin and cold well. Quality of the coating application was constantly monitored by Man Construction. Sika provided quality assurance support throughout the project and carried out final testing and evaluation as stages were completed. When complete, all coating work was independently assessed and reviewed by Opus
Sika products were also used to complete the lining work to ensure complete watertightness of the structure. These included:
• Minor concrete repairs using Sikadur 31.
• Sikaflex TS Plus – a control joint sealant specifically designed for joints in waste water facilities.
• Sikadur Combiflex – a high performance joint sealing system,
• Sika Grout 212 for grouting precast walls into the floor slabs.
Sika is an international leader in construction technology. Whenever there is a need to build or maintain a structure, Sika is on hand with the right concepts and technologies.
The emphasis for Sika is on providing integrated system solutions to meet every requirement – from admixtures for enhancing concrete to sealing and waterproofing of structures, concrete repair and protection, epoxy flooring systems or engineering applications for tunnels.
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