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Concrete In Australia : June 2008
A Pandora’s box at Maroubra Beach by Mark Joseph ACRA member Mahaffey Associates was engaged by the owners’ corporation of a building overlooking Maroubra Beach in Sydney’s eastern suburbs to carry out an investigation into cracks appearing at several locations in the fl oor of several home units. A unit owner had commenced renovation of a unit, had removed the old floor coverings and was hoping to re-level a sagging concrete floor before installing a new timber finished fl oor. The aim of the investigation was to determine the causes and extent of the cracking, which was visible both in the fl oor of one unit and in the ceiling of the unit below. The cracking appeared to be only around the edges of the walls and not in the centre of the floors. A program to investigate the cracking was prepared by Mahaffey. It included: • a covermeter survey, to determine the location and depth of steel reinforcement. • depth profi les of chloride contaminated and carbonated concrete. During the period of the investigation, Mahaffey recommended that a structural engineer be engaged to review the structural integrity of the fl oor. The structural engineer determined that much of the cracking was the result of improper placement of the reinforcement in the concrete at the time of construction. He concluded that the fl oor was not at risk of collapse provided that the reinforcement had not been compromised by corrosion and that any further corrosion would be prevented from spreading in the future. Mahaffey’s chemical test analysis of the concrete also showed that the concrete was fully carbonated and contained chloride ions well above the corrosion initiation threshold at the depth of the reinforcement. Accordingly, there was a signifi cant risk of ongoing reinforcement corrosion and potential catastrophic failure of the floor slabs. Therefore, based on the fi ndings of the site investigation and in conjunction with the recommendations of the structural engineer’s report and recommendations, Mahaffey prepared a specifi cation to carry out the necessary rectifi cation work. In principle, this work was required to: • restore the structural integrity of the area of fl oor slab that had been affected by advanced reinforcement corrosion. • provide corrosion protection for the steel reinforcement to the entire fl oor in the unit so that the overall structural integrity of the fl oor is maintained into the future. The structural integrity of damaged areas of the floor slab was restored using high-strength carbon-fibre reinforced strips that are produced by Sika Australia (also an ACRA member). These are lightweight and compact in size and are of very high tensile strength. The strips are epoxy fixed to the underside of the concrete element to provide additional support for the damaged area. These strips were installed in recesses in the ceiling for aesthetic reasons before the ceiling was repaired. As well, several options were considered byMahaffey to Concrete in Australia Vol 34 No 2 17 address and control any further corrosion into the future. However, the most cost effective option for the owners’ corporation was to implement the application of galvanic sacrifi cial anodes produced by Parchem (also a corporate ACRA member). These are proprietary galvanic units that act as sacrifi cial anodes to attract the corrosive chlorides and prevent any “incipient anode affect” from occurring. These units are applied in grids of tens, inserted into a pre-drilled hole in the fl oor, contacted to each other by a special wire, and finally connected at both ends to reinforcing bars. Once this process is completed and tested for continuity, the galvanic units can be then embedded in special cementitious mortar. The entire fl oor was covered with cementitious topping and the finishing timber fl oor was applied. The life expectancy of these galvanic units are up to 20 years, so the building’s owners will be well protected into the future. Mark Joseph is the managing director of MJ Engineering Projects, the Project Management division of Mahaffey & Associates.