Cracking and spalling developed to the post-tensioned base slabs of an aeration tank to an effluent treatment works approximately a year after construction and prior to the plant entering service. The tank has an overall length of 210 m and was 110m wide and was partitioned along its length into four bays. The base slab was divided into eight post-tensioned slabs, 110 m long and approximately 50 m wide, with a central conventionally reinforced infill strip to facilitate tensioning. The damage was principally around the movement joints between the central infill and adjacent post-tensioned slabs. Finer random cracking was also present elsewhere on the exposed upper surface of the slab. A comprehensive investigation involving non-destructive testing, finite element modeling and a review of construction records was carried out to systematically determine the cause and extent of the distress. A wide range of non-destructive test (NDT) methods was used to minimise damage to the structure, particularly at tendon locations, including radiography and strain relief. This paper describes the investigation and its findings and provides some important lessons for the design and construction of similar structures.
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