The average bond stress in longitudinal steel bar can be calculated from the strain difference of two measure points on same longitudinal steel bar.
Taking strains of two gauge measure positions on the same longitudinal steel bar and same loading cycle into formula (1), the corresponding average bond stress can be obtained.
(1) In the initial loading stage, the average bond stress is very low, and the bond stress distributions in positive loading and negative loading are symmetrical.
After the bond stress reached its peak value, the increasing degree in the load-bond stress curves begins to decrease and the average bond stress begins to reduce which means that slips occur on longitudinal steel bars.
Average Bond Stress. In order to quantitatively study the development and failure process of bond stress on longitudinal steel bar in the three specimens, the corresponding average bond stresses of longitudinal steel bars in the full crack stage, the ultimate stage, and the failure stage are shown, respectively, in Table 4, concluded as follows:
where: [[lambda].sub.1] is the expected pull-out length ratio; [[lambda].sub.2] is the fibre orientation factor; [[lambda].sub.3] is the group reduction factor associated with the number of fibres pulling out per unit area; [tau] is the average bond stress
of a single fibre embedded in the concrete; [V.sub.f] is the fibre volume percentage; [beta] is the bond factor, accordingly to Campione (2008), can be assumed being equal to 0.5 for round fibres, 0.75--for crimped fibres, 1.0--for hooked fibres; [l.sub.f] and [b.sub.f] are the length and the diameter of fibre, respectively.