Determining present pre-stressing
Determining present pre-stressing
(OP)
I am dealing with a project which involves evaluating the capacity of existing pre-stressed concrete beams. As expected, the 50-60 year old as-builts are not in the best of shape. Usually, through a combination of shop drawing and design drawing review, we can come up with all the relevant information. But, in this case we cannot determine the size of the strand present (the strands are stressed to 0.7fpu, not a specific force unfortuneately.
I've dealt with non-destructive/invasive methods for determining the location of strand before, but not the size. Are there any easy methods out there?
I've dealt with non-destructive/invasive methods for determining the location of strand before, but not the size. Are there any easy methods out there?






RE: Determining present pre-stressing
I would also get in touch with a precaster and see if they can shed any light on what a typical strand would have been for the time frame you're looking at.
RE: Determining present pre-stressing
Lion 6 has a idea which should work, it could be possible to expose the end of a tendon - but proceed with caution. Do you have details of the tendon ends, are they accessible?
RE: Determining present pre-stressing
My experience has been that while both approaches are methods of prestressing (verb), that members case with the tedons already stressed are typically referred to as "prestressed" and members stressed after the concrete is cast are referred as "post-tensioned".
Is your experience different? I wasn't trying to imply that one was prestressed (again, verb, not adjective) and one wasn't.
RE: Determining present pre-stressing
The ends of the girders are inaccessible. Otherwise, I would agree that chipping out the mortar over the ends of a cable could be done.
We do have the details for the ends of the tendons, is there anything specific you are looking for?
The girders appear to be a non-standard, or at least uncommon, girder section for the time period so I would not rely on what was commonly used.
RE: Determining present pre-stressing
Prestressing is the act of stressing the steel before it is bonded to the concrete. There are 2 forms.
Pre-tensioning, where the stressing is done before the concrete is poured
Post-tensioning where the stressing is done after the concrete is poured.
But they are both prestressing!
RE: Determining present pre-stressing
gwyn,
you ask what we are looking for in exposing the ends. Now that you have confirmed that the beams are post-tensioned then the answer is the anchorage of the tendons. You will see how many strands per tendon and dicsover the size of the strands. Or to get right down to basics you can find out the number of wires per strand and the diameter of the wire.
But as you cannot access the ends trying to expose any part of a strand is a bit too much of a job to consider immediately. Can you try to find out the type of strand that was used in your location at the time of construction of the beams. You seem to be part way there if you have the anchorage details.
RE: Determining present pre-stressing
RE: Determining present pre-stressing
Just curious...was this structure as old as you thought? Bridge girders? If a building, that time frame is very early. Bonded or unbonded?
RE: Determining present pre-stressing
RE: Determining present pre-stressing
RE: Determining present pre-stressing
How are the tendons holding up? Do you have any cracks or spalls that follow the tendon profile? This can indicate corrosion of the tendon which would affect the prestressing force.
RE: Determining present pre-stressing
There are no cracks following the tendon profile, nor spalling. In any case, it looks like the overload is a fair bit beyond what the girders are capable of withstanding with the tendons intact. Nonetheless, thanks for the tip.
Hokie,
I agree. Fortunately, it seems bridge designers picked that up years ago as well.