Paint Post-tensioned cables

[slickdeals]

Folks,
I am trying to specify on the drawings to spray paint the P/T cables location on the forms with a spray paint prior to placing concrete so that the locations can be seen from below after the slab forms are stripped.

Has anyone specified this before and if yes, is there a particular paint product that was used?

 

[swivel63]

spray paint the drape and tendon layout on the tables? because i figure you can just look at the slab edge to see where they're at. i've never seen this specified, but that doesn't mean anything.

 

[racookpe1978]

But would spray paint be right way to go? Seems it would dry very fast (to the bottom of the form), then - if dry - the paint would not "stick" to the wet concrete - so when the forms are stripped away, the concrete would not retain the painted lines.

So, would a slower-drying brushed on latex be better?

A string laid on the form would be "picked up" by the concrete - but would probably be moved too if it weren't tied at both ends by a nail. The string would leave a "line" in the concrete so you could avoid drilling through the rebar/tensioning wires.

 

[msquared48]

What about using lampblack instead of the paint? No problem with drying as it is a powder. It just discolors the concrete placed against it.

Mike McCann
MMC Engineering

 

[ron9876]

Have seen paint and chalk spec'ed. Need to be placed just before placing concrete. It will leave a mark on the bottom of the slab but contractors don't want to screw up their forms. We stopped using it.

 

[structuresguy]

We have the contractor spray the forms all the time with PT slabs. The paint can be there for days (or weeks) and still stick to the slab. As for spec'ing it, just tell them right in the specs to locate all tendon runs by marking the forms with either spray paint or chalk.

Just going by the slab edge to locate tendons is very innaccurate. The tendons may curve laterally, and typically spread out at the ends. Also, the slab edge is patched and finished over the end caps, so in the finished product, you won't see them anyway. But if you have drop ceiling, the paint will always be visible.

If you are planning to have exposed unfinished concrete, say in parking garage, then you may want to use chalk lines, as I think these may be easier to pressure wash off down the road.

 

[structuresguy]

You can even have them paint the forms before the tendons are in place, to use as a guide for the tendon installed. Paint = this is where tendon goes.

 

[swivel63]

can someone tell me what the purpose is? so that the iron workers will know where to place the cables? or is there something else? doesn't this fall in means and method?

 

[slickdeals]

@swivel63,
Many a time the MEP guys want to install something overhead. From time to time we have had them hit the low point of a cable group and caused all sorts of headaches.

We want to be able to locate the tendons so that people who are trying to drill from below know where the cable is.

 

[civilperson]

Cast sleeves in the beam to protect the tendons and strands from other penetrations.

 

[hokie66]

civilperson,

They are talking about marking slab tendons so they can be located in the future, when additional penetrations may be required.

structuresguy,

You want the markers to stay in place, not able to be removed by pressure washing. But I suppose that doesn't stop the owner from covering them up by painting the soffit.

slickdeals,

The staples used to hold the ducts in place are usually found quite easily, and painting on the form helps to define the line. I have seen plastic tape used, at several points along the tendon, but it doesn't stick well to oiled forms, so sometimes gets dislodged. If it floats up in the concrete, the thin layer of concrete under the tape spalls off, so I don't like that approach.

 

[dik]

We used to try marking the locations with limited success. With todays low cost cover meters, I think that the contractors can locate the cables pretty easily. Just note it on the drawings and make it part of the procedure.

Dik

 

[RHTPE]

When you all come up with the perfect way to permanently leave evidence of the P/T cable locations in the slab soffit, please let me know.

I presume we are talking about unbonded tendons NOT installed a ducts.

Spray-painted lines on the top of the forming deck work, and the paint will transfer to the concrete. Should be reapplied for the next level. In a ceiled area the paint lines should not be objectionable, but I can't say how permanent they will be. In an exposed situation (such as a garage) I suppose a pressure-washing would remove the paint.

Tape, string, etc. are not reliable as they will get dislodged during concrete placing opperation.

Perhap a 1/4" thick X 3/4" wide reveal stapled to the formwork directly below the cable would be ideal. It'll leave a groove in the slab soffit that will be visible forever, yet is shallow enough to not severely impact cover. But I can hear the contractors screaming already about labor involved to install.

Having spent a few decades in the concrete construction industry I felt compelled to comment.


Ralph
Structures Consulting
Northeast USA