Slip-Hardening Bond: A Key to the Success of Ultra High Performance FRC Composites

Bond is recognized as a fundamental causal parameter in the mechanics of fiber reinforced composites. Its paramount function is to transmit forces between fibers and matrix and vice-versa. This paper focuses on “slip-hardening bond” which can be achieved in cement composites with some fibers, as compared to commonly observed slip-softening bond or constant bond. In particular: 1) it describes how bond is characterized from a pull-out test or a pull-through test on a single fiber; 2) it gives examples of fibers with bond stress versus slip exhibiting slip-hardening behavior; and 3) it clarifies how slip-hardening response can be likely achieved. Slip-hardening bond is an extremely important characteristic which should be evaluated at the onset of design; it implies that, at the composite level, once a crack is formed in the matrix, the fibers bridging the crack provide increasing resistance to crack opening. This is likely to encourage multiple cracking in the composite and helps lead to composites with strain-hardening behavior in tension, as well as large composite strains at failure. Slip-hardening bond is considered critical for the further development and success of high performance and ultra-high performance fiber reinforced cement composites [1, 6].