Determination of Concrete Plank Tendons 1

[Berniedog]

A Client of mine want to add load to a concrete plank floor system. No one has plans of the existing planks. Has anyone ever exposed the tendons and measured the diameter and used that info to estimate the strength of the planks. Then grout the hole back up?

 

[KootK]

I've seen it done with post pensioned cables in CIP slabs.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ingenuity]

I have NDT scanned and exposed pre-tensioning strands to precast structural products many times.

We always SPR (radar) scan the work area first. In your case of a plank, you will need to gain access to the bottom of the plank, and scan with radar or even a pachometer to locate the strands, then you know where to undertake the chipping. Whilst radar or a pachometer can locate strands (and tendons) non-destructively, they are not accurate in determining diameter of strand (or rebar either), hence why invasive proving is necessary.

Use the scanning to locate the center spacing of the strands within the planks and use invasive probing to determine strand diameter AND measured concrete clear cover. All do a visual check on the condition of the strand whilst you are there. The number of probes required will depend on the number of different planks in the structural system (spans etc), but we usually go for a 3 probe minimum, and check for consistency/variability amongst the results of the probing.

Overhead chipping lends itself to the use of lightweight electric chipping hammers, and caution must be exercised in NOT damaging the strands, even the individual wires should be damage-free.

In my work region, older precast structural products were manufactured from either 3/8" or 1/2" diameter 7-wire strands, so it is 'never a given' that they will be 1/2" ∅, which is prevalent today in 99% of precast products.

The attached photo is one of a precast I-joist that we had to expose the strand configuration to calculate the capacities given no existing drawings were available.

Patching of the bottom of plank, in your case, if best done with a quick-setting, per-manufactured, overhead cementitious repair mortar.

 

[hokie66]

Can you get at the ends of the slabs? The strands can be exposed there.

 

[KootK]

Nicely done Ingenuity. I love the photo. If I had a high resolution copy, I might consider having it framed as artwork.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ingenuity]

If I had a high resolution copy, I might consider having it framed as artwork.

You are a geek...but so too am I...

 

[Ingenuity]

Here is a photo of a plank soffit with red crayon marks representing the SPR scan results of strand locations. Two longitudinal strands were exposed to directly measure the diameter and the concrete bottom cover. The slightly surface-corroded lateral strand in the bottom (running E-W) is actually a piece of bent strand used as a lifting hook, that was prevalent in the day.

Also, by using SPR scanning first, you can often detect any plank voids (assuming hollow-core) before concrete chipping.

 

[Berniedog]

Thank you all for your help.