High cycle tensile fatigue behavior of steel rebar reinforced - UHPFRC at high R-ratio

• A highly correlated S-N diagram and a consolidated fatigue endurance limit are obtained for steel rebar reinforced-UHPFRC elements under a high R-ratio, representative of structural applications. • Fatigue deformation concentrates in the zones with low UHPFRC fiber orientation coefficients, where rebar strain has larger value and increase rate. • The fiber orientation of UHPFRC determines the locus of crack localization and of fatigue fracture of steel rebars, thus final fracture of the elements. This paper investigates the high cycle tensile fatigue behavior of steel rebar reinforced − UHPFRC elements, at a fatigue load ratio, i.e., R-ratio of 0.3, representative for structural applications. Prior to testing, magnetoscopy is conducted on each specimen to determine the local fiber orientation and volume inside UHPFRC. During testing, global specimen deformation is recorded by displacement transducers; specimen surface is monitored by digital image correlation; and strain along rebars inside the specimen is measured by fiber-optic sensors. Based on the test results, an S-N diagram with a high regression coefficient is obtained. Hereby, the normalized fatigue force S is defined as the ratio between the maximum fatigue force and the estimated specimen ultimate tensile resistance. The fatigue endurance limit is identified as being about S = 0.40. It is found that fatigue deformation of the specimen mainly occurs in the zones with low fiber orientation coefficient μ 0 , y of UHPFRC ( μ 0 , y decreases when average angle between fiber axis and principle tensile direction changes from 0° to 90°), where the strain along steel rebars also have their higher value and increase rates during fatigue testing. The lowest UHPFRC fiber orientation determines the locus of crack localization and of fatigue fracture of steel rebars, thus final fracture of the elements.