Nondestructive approach to evaluate strain changes in prestressed concrete by saw-cutting

Prestressed concrete members are designed to generate concrete pre-compressive states through the prestressing force. After casting, prestress losses reduce the prestressing force, whose evaluation requires indirect determinations for existing unmonitored members. Pioneer determinations have been made by applying non-destructive methods such as saw-cutting, with limitations related to the use of electric-resistance strain gauges. As an alternative measurement approach, this paper presents a new specific methodology for evaluating concrete strain changes based solely on the use of mechanical strain gauges. The feasibility of the methodology is experimentally assessed through redundant determinations to characterise the behaviour of the Isolated Concrete Block (ICB) generated by saw-cutting a post-tensioned concrete beam. The approach offers a consistent derivation of concrete strain changes and a clear identification of the decompression state. For the selected key parameters of 120 mm distance between the pair of cuts and progressive discretised cut depths until 50 mm, the ICB presented a clear evolutive behaviour and showed full prestress release between 30 and 35 mm cut depth. The robustness of the experimental results obtained from different measurement base lengths demonstrates the viability of the conceived methodology for future applications as a basis for assessing the residual prestressing force.