Repairs to Zeekoevlei wastewater treatment works

ZeekoeThe Zeekoevlei Nature Reserve on the Cape Flats is the home of the False Bay Ecology Park (FBEP) Environmental Centre and has a strict environmental protection policy pertaining to any construction works that occur in the area. When Sika was called on to assist with concrete repairs to the anaerobic digesters at the Cape Flats wastewater treatment works, the company was vigilant in ensuring there was no seepage or leakage into the environment and thus no incidents of chemical contamination in the nature reserve while the project was under way.

The City of Cape Town retained consulting engineers Ninham Shand from Africon, together with Sika and contractors, Empa Construction, to undertake the repairs required to extend the lifespan of the wastewater treatment facility.

The three existing post-tensioned structures were built 31 years ago. Although the digester walls were of high quality, high strength concrete and in fairly good condition, the treatment works are in an area close to the sea. As a result of the high chloride, coastal environment, the steel reinforcement had corroded, creating cracks and spalling in the ‘covercrete’ on the outside walls. The internal walls were being attacked by a combination of aggressive gases and condensate, particularly at soffit level, which in turn created exposed aggregate finish, large scale loss of cover and consequent spalling.

As roof rehabilitation was not an option due to structural instability, the solution was to demolish and rebuild the concrete roof. The demolition presented the biggest challenge because of the roof’s high compressive strength.

The spalled concrete external walls were repaired using Sika’s advanced concrete repair systems.

SikaTop Armatec-110 Epocem was used as a bonding agent and anti-corrosion coating as it is suitable for bonding new concrete to old (wet to dry). It has excellent adhesion qualities and acts as a barrier against penetration of water and chlorides, so was ideally suited to this project. Sika MonoTop-615HB, a high-build repair and re-profiling mortar, was used for thick layer concrete repairs and Sika MonoTop-612, a hand-placed fibre-reinforced repair mortar, was applied on overhead and vertical surfaces. SikaGrout-212, a high performance, expansive cementitious grout, was used to grout anchors in the concrete and to fill cavities, gaps and voids.

Once the cementitious repairs were completed, Sikadur-31 CF was used with Sikadur-52 as a crack injection system for the walls. Sika FerroGard-903+ was then applied as the migratory corrosion inhibitor to help ‘repassivate’ the existing steel.

A protective crack-bridging coating was applied on the external facades, using Sikalastic-150. This elastic, cement-based waterproofing coating offers both chloride and carbonation protection.

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