Influence of ambient temperature on early age concrete behaviour of anchorage zones

Many anchorage zone failures of post-tensioned concrete beams and slabs have occured in the recent past, during stressing stage, prompting urgent attention to investigate the performance of anchorage zone concrete at early age. The strength properties of early age concretes at the time of tendon stressing is significantly influenced by the fluctuation of ambient temperatures. The existing design equation for allowable bearing stress is a function of concrete compressive strength at the time of post-tensioning. This paper reports parametric studies based on finite element modelling to investigate the effects of variable temperatures on concrete strength. The model was validated against experimental results. Allowable bearing stresses were calculated from compressive strength results between 1 and 7 days. These allowable bearing stresses can then be compared with average bearing stresses from the post-tensioning load to evaluate the bearing capacity of the local anchorage zone. Based on the illustrative example presented, it is shown that the values of allowable bearing stress can be exceeded at the final stressing stage. Inadequate compressive strength at early age can contribute to failures of anchorage.