Experimental detection of the residual prestressing level in pre-tensioned and post-tensioned reinforced concrete beams by means of non-destructive tests

Object of the paper is the execution of an experimental campaign conducted on six prestressed concrete beams (PC) and different levels of prestressing. In the first phase, three beams with straight adherent cables were casted, with the same eccentricity and three different levels of prestressing. For each beam, static tests were carried out until failure, and for each load step several non-destructive tests were carried out, namely dynamic free vibration tests and ultrasonic tests. The beams were instrumented with strain gauges and LVDTs to measure deformation and displacements during load cycles. The same tests were then repeated on three further beams with straight post-tensioned cables, with the possibility of controlling the level of prestressing force. In this case, each of the three beams had a different eccentricity. The variation of non-destructive parameters (such as dynamic characteristics and properties of ultrasonic waves) as a function of the degree of prestressing is evaluated, as well as the increase in the structural damage of the element. In particular, dynamic response data were analyzed in both frequency domain and time domain. Ultrasonic response data were also analyzed, not only in terms of velocity but also through attenuation of the ultrasonic signal. The aim of the study is to improve the techniques currently used for the detection of damage of PC elements with non-destructive tests, by defining a new protocol for the evaluation of residual prestressing force through the use of non-destructive tests. This will allow a more detailed assessment of the condition of the structure with a limited amount of data.