Stress redistribution of concrete prisms due to creep and shrinkage: long-term observations and analysis

In 1979, 16 concrete prisms with dimensions 140x150x4000 mm were casted in the Magnel Laboratory for Concrete Research. These prisms differ by the amount of passive reinforcement and by the applied loading level. Four reinforcement ratios were considered, i.e. 0%, 1.5%, 3% and 6%. For each reinforcement ratio a prism was subjected to an axial load corresponding to a concrete stress of 0, 5, 10 or 15 MPa. The combination of both parameters results in a total of 16 specimens. The compressive stress was applied to the prisms by means of post-tensioned unbonded strands at an age of 28 days. The stress level was kept constant during the first 12 years of the experiment by re-adjusting the force in the strands when the deviation exceeded 2% of the initial value. Afterwards, no re-adjusting of the strands took place. Significant redistribution of the stresses between the concrete and the steel can be expected due to the creep and shrinkage of concrete. This redistribution is larger for the prisms with a higher reinforcement ratio, resulting in lower creep and shrinkage strains. The time-dependent response of these prisms was modelled taking into account the prestress losses and stress redistribution between the concrete and the steel. For reasons of numerical efficiency, the compliance function was approximated by a Dirichtlet series using continuous retardation spectra. A comparison between the predicted results and the measurements is given for two prisms