Replaceable tendons provide bridge owners
the ability to address unanticipated poor in-service bridge performance.
With a little planning for future use,
replaceable tendons can also provide additional structural capacity.
Let's take a look at some of the unique components
of replaceable post-tensioned or PT bridge technology,
starting with those at the replaceable anchorage.
Lets strip away the concrete and reinforcing for a clearer view.
Here in the anchorage, we have a replaceable wedge plate,
a replaceable inner trumpet,
a replaceable coupler,
and a replaceable post-tensioning duct.
Lets now look at the non-replaceable anchor components.
We have the cast-in permanent outer trumpet, or Diablo form,
Stripping away the reinforcing and removing the grout cap and the cast-in permanent bearing plate.
Shown is an example of another anchor setup
that consists of a replaceable heat shrink sleeve,
metal pipe, trumpet,
and shim plate along with a cast-in permanent form, guide pipe, and bearing plate.
Let's move into the bridge to see the unique components
located at a tendon deviator.
Let's strip away the concrete for a clearer view.
Here, the external PT duct is shown
passing through the deviator by way of
the cast-in permanent Diablo form.
With this setup, the tendon is restrained by the deviator,
which generates the downward restraining forces
acting between the cast-in permanent Diablo form
and the external PT duct.
Now, let's consider the process of tendon replacement.
Taking a closer look at our bridge,
we discover a damaged tendon.
On the innermost tendon, there is a tear in the duct
and broken wires.
Upon removal of the duct section, we see that one tendon strand
is broken and light corrosion appears throughout
the other strands.
This tendon can be replaced using
the as-constructed replaceable PT tendon components and details.
Initial cuts are made between the anchorages
as indicated by the lines.
The cutting locations may not coincide
with the location of the damage.
The image shows a generic cutting sequence.
Sequencing is typically done from the midpoint
of the tendon working outward.
Using the cutting sequence,
the tendon is cut into segments and removed.
This process continues until the length of tendon
between the anchorages is removed.
The center pier diaphragm wall shows the cut tendon extending
from the wall face.
On the other side of the diaphragm wall,
the anchorage that needs to be replaced is shown.
First, the grout cap and its encasing PT grout are removed.
This frees the replaceable wedge plate from the cast-in bearing plate and allows for the removal of the anchorage,
leaving the cast-in permanent form
and bearing plate to accept the new anchor.
Both anchorages need to be removed
so let's move on to the left abutment of the bridge.
The abutment diaphragm wall shows the cut tendon
extending from the wall face.
The anchorage that needs to be replaced is shown on the other side of the diagraphm wall.
Similar to the central pier anchor,
the grout cap and its encasing PT grout are removed.
This frees the replaceable wedge plate
from the cast-in bearing plate and allows for removal
of the anchorage. need to be detailed with sufficient space
Note that abutments with replaceable PT tendons
behind the diaphragm wall to allow for tendon replacement.
There is enough space behind the diaphragm wall to remove
and replace the wedge plate, the inner trumpet,
and part of the post-tension tendon.
Depending on the space behind the diaphragm wall,
the tendon might need to be cut into sections to allow for removal.
Now that the damaged tendon is removed,
we can start the installation of the replacement tendon.
The bridge needs to be detailed to accommodate
the necessary installation processes and equipment.
The new PT tendon is installed in a similar manner as the original PT tendon.
The one piece of equipment that requires special
planning is the PT jack.
In our example, the jack is hoisted up under
the bridge and through an exterior access hatch.
It's then transported to the PT anchorage.
The jack is then hoisted onto
the scaffolding platform and used to
stress the new tendon. 0:05:17.000,0:05:20.000 Note that there needs to be enough room behind the anchorage
to accommodate the extended strands and the placement of the jack.
Next, the jack is removed.
The strands are cut, and the grout cap is installed.
The final step is to ground the replacement tendon.
Below the bridge, the ground pump is set up
and its grout hose supplies grout to fill the new PT tendon.
Another option is to set the grout pump on the top
of the bridge, coring a small diameter hole
in the deck slab for grout hose access.
This has been an overview of replaceable PT technology.
For more information on this technology and concrete bridges, visit the FHWA Bridge Website.
https://www.fhwa.dot.gov/bridge/concrete.