CIA CPN29-1996

In the last forty years, prestressing systems have developed very large tensile forces in the cables which apply very large compressive forces within the concrete. In a post-tensioned system, the anchorage component is usually an external steel bearing plate or an embedded steel casting, with suitable lock-off device which transfers prestressing force to the concrete by direct bearing. In a pretensioned system, the transfer of these forces is by bond along individual cables at their ends. It is in this highly stressed zone of concrete immediately behind the anchorage, called the anchorage zone, that a complex three-dimensional interaction of stresses occurs as the load is transformed to a two-dimensional direct stress at some distance remote from anchorage. As this concrete is highly stressed, careful design and detailing of this anchorage zone is required. Many failures have occurred as a result of poor detailing of reinforcement in the anchorage zone or the stressing of the prestressing cable prior to the concrete gaining the required strength. This Current Practice Note provides information on the causes of these failures and information and examples of good detailing of reinforcement.