Long-term strains measured in reinforced concrete building

Abstract

Strains measured over a period of 17 years are reported for a reinforced concrete building. Vibrating wire strain gauges were embedded in the ribs and beams of the first floor of a three-storey telephone exchange in south-west England. Groups of gauges were located at mid-span of two 170 mm thick ribs, at mid-span of a 470 mm thick beam, and near the support column and edge beam of the 470 mm thick beam. Gauges in each group were positioned at different depths in the direction of the expected stress and transversely, so that shrinkage and load-induced strains could be calculated. Concrete temperatures and relative humidities were measured so that allowance could be made for the small variations of thermal strain and changes in drying conditions. Elastic modulus, weight loss and strain measurements were undertaken on site-stored and laboratory-stored prisms cast from the in situ concrete. n e embedded vibrating wire strain gauges enabled the strains to be measured sensitively and reliably over the 17 year period of observation. It took about 10 years for drying shrinkage to reach approximate equilibrium, but some additional shrinkage was then observed owing to change in use of the building. There was only approximate agreement between the measured shrinkage strains and those estimted from British Standard BS 81l0: Part 2: 1985. The normal pattern of flexural strains in the floor was superimposed by a tensile strain component that seemed to arise from the restraint of drying shrinkage.