Post-tensioning ~ this method is usually employed where stressing
is to be carried out on site after casting an in-situ component or
where a series of precast concrete units are to be joined together
to form the required member. It can also be used where curved
tendons are to be used to overcome negative bending moments. In
post-tensioning the concrete is cast around ducts or sheathing in
which the tendons are to be housed. Stressing is carried out after
the concrete has cured by means of hydraulic jacks operating from
one or both ends of the member. The anchorages (see next page)
which form part of the complete component prevent the stressed
tendon from regaining its original length thus maintaining the
precompression or prestress. After stressing the annular space in
the tendon ducts should be filled with grout to prevent corrosion
of the tendons due to any entrapped moisture and to assist in
stress distribution. Due to the high local stresses at the anchorage
positions it is usual for a reinforcing spiral to be included in the
design.Prestressed Concrete