Strain gauge measurement as a tool for assessment of post-tensioning in building structures below a historic brickwork vaulted arch

Abstract

The effectiveness of post-tensioning applied to concrete structures is accurately described in literature and normative documents. However when applying this post-tensioning to an underground structure, this effectivenessmay be doubted, since the surrounding soil mass introduces opposing friction and disturbance of the uplifting forces. In order to assess this operation on the new foundation of a historic brickwork arch, strain gauge measurements are carried out, applying long grid length gauges to top and lower surface of the foundation slab. The conditions for these measurements are very difficult due to various reasons inherent to large civil engineering projects. The results indicate that a longitudinal as well as a transverse disturbance of the effectiveness is present. Nevertheless the average measured values correspond satisfactorily to the calculated values at the moment of measurement. Since the causes of this disturbance will mostly disappear in the final stage of construction, as the soil will be excavated under the foundation, the final values of post-tensioning effectiveness will comply with the design. The non-destructive testing method presented in this paper has made it possible to justify the design and prove that no additional post-tensioning operations would be needed in successive construction phases.