Determining present pre-stressing

[gwynn]

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?

 

[Lion06]

Are they post-tensioned, or pre-stressed/precast? The post states pre-stressed, so I'll make the assumption. Pre-stressed members have the steel tensioned prior to casting the concrete and then the tendons are cut. The ends typically get covered with some kind of coating for corrosion purposes, but if you can get access to the end you can probably get the strand area.

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.

 

[Zambo]

There seems to be a cofusion over terminology here. Prestressed members can be stressed through either pre-tensioning or post-tensioning. From the original post we don't know which.

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?

 

[Lion06]

Zambo-
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.

 

[gwynn]

To clarify, they are post-tensioned. I have heard pre-stressing as both a way to specify pre-tensioned and to specify pre-stressed in general. *shrugs*

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.

 

[rapt]

On the off topic subject of definitions,

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!

 

[Zambo]

Lion 6, in my experience rapt's explanation is entirely correct, although it is a moot point whether we are describing the act of stressing the steel tendons or the act of application of a compressive force to concrete member, in effect the same.

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.

 

[gwynn]

Just to close the loop here, after a fair bit of effort we were able to track down a set of the as-builts that were in better condition which solved the problem.

 

[hokie66]

gwynn,
Just curious...was this structure as old as you thought? Bridge girders? If a building, that time frame is very early. Bonded or unbonded?

 

[gwynn]

It is early sixties, which is about what was expected. They are bridge girders with bonded tendons. I can't say I have ever seen a bridge girder with unbonded tendons in these parts.

 

[hokie66]

That's good. Unbonded in buildings is bad enough, would be disastrous in bridges.

 

[graybeach]

Gwynn,
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.

 

[gwynn]

Graybeach,

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.