Strain Capacity of Strain-Hardening Ultra-High-Performance Concrete with Steel Fibers

The maximum post-cracking tensile strength (sigma(pc)) recorded in numerous investigations of ultra-high-performance fiber-reinforced concrete (UHP-FRC) remains mostly below 15 MPa, and the corresponding strain (epsilon(pc)) below 4/1000. Both values are significantly reduced when the specimen size increases, as is needed for real structural applications. Test data on spc and epc from close to 100 series of direct tensile tests carried out in more than 20 investigations are analyzed. Factors influencing the strain capacity are identified. However, independently of the numerous parameters encountered, two observations emerged beyond all others: 1) the higher the post-cracking tensile strength (whichever way it is achieved), the higher the corresponding tensile strain; and 2) fibers mechanically deformed and/or with slip-hardening bond characteristics lead to an increase in strain capacity. A rational explanation for these observations is provided. The authors believe that achieving a large strain (epsilon(pc)) at maximum stress is paramount for the successful applications of ultra-high-performance concrete in concrete structures not only for strength but, more critically, for ductility and energy absorption capacity improvements.