The use of linear polarisation corrosion rate measurements in aiding rehabilitation options for the deck slabs of a reinforced concrete underground car park

Abstract

It was proposed to add a further nine storeys to an office building for additional office accommodation. A four level car park was in use beneath the building to provide parking for some 200 cars. Satisfactory structural calculations and plans for structural alterations had been carried out for the additional loadings on the car park, but a survey revealed that the car park deck slabs were suffering serious deterioration due to the carriage of de-icing salts on the underside of cars. Deposition of these salts had caused the onset of severe reinforcement corrosion. An initial survey revealed that the concrete in the slabs was of very low quality, equivalent probably to a 20 Grade and cover to the steel was very variable and often low. This survey included a visual survey, half cell potential measurements, chemical analyses for cement and chloride content and a detailed petrographic appraisal of the concrete. The survey revealed high levels of chloride contamination in the slab surface over much of its area and high half cell potentials in most areas examined. The Client had requested remedial proposals and initial options for electrochemical desalination or cathodic protection were put forward, together with budget costings. The Client wished to delay repairs for several years to fit in with their maintenance budget and to avoid disruption at a difficult time. It was proposed that the appraisal be extended to establish the location and distribution of the steel reinforcement in the deck slabs at critical positions around column heads, the condition of the steel, including section loss due to corrosion and the corrosion rate by linear polarisation techniques. This would then enable a decision to be made on the safety of delaying repairs and any additional problems due to further spalling which might be anticipated. It was found that the corrosion rate was reasonably low at the time of the first survey and that it was considered quite feasible to delay repairs for up to three years as requested by the Client. Further monitoring over the first year was recommended to check on corrosion rates over the different seasons. The second survey revealed that both half cell potential and corrosion rates were variable and that corrosion rates doubled between readings taken at the end of the autumn and the end of the winter periods. This confirms the necessity not to rely on only a single set of data readings when undertaking this type of work.