A study on FRP-confined concrete in presence of different preload levels

The application of external confinement of concrete columns by using Fiber Reinforced Polymers (FRPs) composites has been widely studied and recognized as an effective technology. The most part of the experimental studies that were conducted are related to columns that were confined without a pre-loading condition, which is the most common situation that is met in the field. In order to increase the knowledge, this research illustrates and discusses the experimental results that were obtained by testing concrete cylinders that were confined with Carbon FRP (CFRP) unidirectional sheets at five different pre-loading levels. The concrete cylinders were prepared and poured at the same time in order to minimize difference in the starting properties of the concrete core. Five specimens were prepared for each pre-load level. Thus, 25 concrete specimens having a diameter of 100 mm diameter and height of 200 mm have been tested. In detail, 5 coupons were unconfined (“U”); 5 were CFRP-confined with null pre-load level (“P0”); and three sets of 5 specimens, named “P20”, “P50” and “P80”, were FRP-confined during the application of a pre-load level equal to 20%, 50% and 80% of the ultimate unconfined concrete compressive strength, respectively. The experimental results revealed the different mechanical response depending of the pre-damage state of the cylinders corresponding to different pre-load levels. Until a pre-load level of 20% the effects can be considered almost negligible, while for higher pre-load levels (50% and 80%) it is remarkable to consider the loss of mechanical properties respect to an ideal un-loaded configuration. The paper will discuss all the aspects related to the experimental results, also showing an analytical procedure to take into account the effects of the pre-loading conditions.