GENERAL REMARKS

It is recognised in Section 12.1 that prestressing tendons (either in the form of wire or strands of wire) reinforce the weaknesses of concrete in an active manner. Because of this, considerable concentrated forces are exerted at the extremities of a prestressed beam. At the end zones, these forces in pretensioned beams translate into intensive bond stresses in the steel concrete interface. In post-tensioned beams, they induce acute lateral tensile stresses and the anchor heads (see Figure 12.4(3)a) create high bearing stresses on the concrete ends.

These stresses need to be fully considered and carefully designed for, to prevent cracking and even premature failure in the end zones. A properly reinforced end zone is referred to as an end block.

The nature and distribution of the bond stress in the end zones of a pretensioned beam are given in Section 16.2, which also includes the design method recommended in AS 3600-2009 (the Standard). Section 16.3 identifies the three types of stresses induced by a post-tensioned anchorage system. These are the bursting stress and the spalling stress, both of which are tensile, and orthogonal or transverse to the axis of the post-tensioned tendon. There is also the bearing (compressive) stress on the concrete behind the steel anchor head. The design for the bursting, spalling and bearing stresses is discussed in Section 16.4. Finally, the distribution and detailing of the end-block reinforcement are presented in Section 16.5.