Civil Engineering Reference
In-Depth Information
of design rules often leads to excessively heavy reinforcement. It is not uncommon to
see square links of 300 mm side made of 25 mm bars. It needs to be remembered that
the force is transferred from concrete to reinforcement by bond or by direct bearing in
hooks and bends, and it is most improbable that such large bars can develop their full
strength. A more thoughtful design and careful analysis would allow the use of smaller
bars that bond better with the concrete.
Prestress anchors are frequently situated at the ends of a bridge deck, where there
are several other sets of reinforcement, such as shear links for the beams, shear and
bending reinforcement for the abutment diaphragms, bursting reinforcement over the
bridge bearings and holding-down reinforcement for the expansion joint, as well as
transverse deck reinforcement. This accumulation of sets of reinforcement frequently
gives rise to excessive congestion of the section which makes it very diffi cult to cast
and compact the concrete. However, the most important prerequisite for a successful
anchor is the presence of sound concrete immediately behind the bearing surfaces
that transfer the tendon force. It is thus very important that the design of the primary
anchor reinforcement is carried out carefully, and that it is not over-conservative. It
is also essential to prepare large-scale drawings of all the reinforcement in the anchor
zone, where the bars are shown at their correct thickness, with two lines.
The designer should always avoid placing anchorages at the minimum distance
from concrete edges, or adopting the minimum distance between anchors, unless it is
essential to the project. The failure rate for anchors rises as edge distances are reduced.
When it is essential to use minimum distances, extra care is required in the design and
in the reinforced concrete detailing.
Clearly, the safe edge distance also depends on the strength of the concrete when the
tendons are stressed. More caution is required for cast-in-situ structures, particularly
if they are being stressed as early as possible, than for precast structures where the
concrete is likely to be considerably stronger than its nominal cube strength when the
tendons are stressed.
5.24.3 Spalling
Adjacent to the anchor are areas of so-called spalling tension, Figure 5.22. Although
these areas of tension cannot in theory compromise the strength of the member, they
may cause the concrete cover to break away exposing the bursting reinforcement,
and they may delay the job as explanations are demanded and repairs planned. The
cause of these areas of tension may be understood by referring again to Figure 5.22.
The compressive force of the anchor is carried between the stress trajectories. The
concrete within these trajectories shortens under the effects of the high compressive
stresses that exist behind the anchor. However, the concrete immediately outside the
outermost trajectory is not subjected to this compression. Consequently, there is a
strain discontinuity between these two zones that sets up shearing and tensile stresses
in the concrete adjacent to the anchor.
The forces involved in spalling are weak, and these areas need to be reinforced by
small-diameter bars, typically not larger than 10 mm, which may be bent to tight radii
and thus can be fi xed into the corners of the concrete member. Using large-diameter
bars, which have large bending radii and require considerable bond lengths to develop
their working force, is pointless.
