Experimental study of concrete beams prestressed with basalt fiber reinforced polymers. Part II: Stress relaxation phenomenon

In a previous paper experimental results on the tests of prestressed concrete (PC) beams with basalt fiber reinforced polymer (BFRP) tendons having several degree of prestressing level were reported. In particular, details of experimental program from problem statement to manufacturing and testing performance was presented. This part of the general research provides theoretical and experimental analysis of stress relaxation phenomenon. Based on current development FRP have excellent resistance to creep, but for a prestressing tendon, relaxation is more important than creep, since relaxation reduces the available prestressing force. Estimation of the prestress loss over time requires that stress relaxation of the tensile element shall be known. Matrix resins and fibers are viscoelastic materials and thus exhibit stress relaxation. BFRP is not yet clearly included in design codes, thus none of the codes provide significant information in order to determine the prestress losses due stress relaxation and mechanical properties belong to scientist's interpretation and various prognosis. Taking proper advantage of FRP materials should involve getting away from the paradigm of replacing steel with FRR and toward the development of innovative reinforcing schemes which would make both FRP materials and concrete construction more cost effective. Due to lack of standard and experimental results the critical priority is to establish parameters for short-term and long-term performance of stress relaxation characteristics for new FRP reinforcing materials. Identified priorities reflected to this research work that there is a need to target specific, promising applications where BFRP reinforcing may find innovative use, such as internal and linear prestressing.