Caratterizzazione del comportamento fessurativo di calcestruzzi soggetti a ritiro vincolato mediante "Ring Test"

During recent years it has been observed an increasing attention to the shrinkage of concrete by considering it as an action that cannot be neglected in structural design. In fact, the distance between the joints of the buildings or walls depends significantly on the shrinkage of the material. In design of floors, the shrinkage is the main cause of cracking that has generated numerous disputes between customers and manufacturers. Shrinkage makes cracks when some restraint prevents the free deformation of concrete. Therefore, the risk of shrinkage cracking should be evaluated through restrained shrinkage tests since free shrinkage tests are not representative. National technical codes and international experimental investigations of drying shrinkage of concrete, do not provide direct information on the risk of cracking when the material is subjected to boundary conditions. In fact, the reference standard method for the evaluation of the cracking risk is based on the so-called "ring test" (ASTM C1581) which, however, mainly detects the time of incipient cracking in concrete but does not provide direct measures of the cracking risk or indications on the behavior of post-cracking material failure. The present work describes the development of a test method which aims to detect the post-cracking behavior of the material in terms of crack opening displacement with time, in addition to the time of initiation of cracking. The method developed has also proved suitable to appreciate significant differences in performance of concretes characterized by different potential risks of cracking