PT Slab Edge Protection During Controlled Demolition

[MacGruber22]

Does anyone else specify bent plates at live tendon anchors to control blow out during de-tensioning? I proposed it to a demo contractor and they liked the concept. 16" long x 1/8" thick 6063-T6 channel shape bolted top and bottom with total of four HUS-EZ screws. We were going to use ballistic blankets but we couldn't find a version with a way to reasonably anchor to the slab. How do you guys protect your live ends?

-Mac

 

[Canuck65]

We used to specify plywood and sandbags. Lots of sandbags. There were a couple of instances where they were actually used (live ends had been jackhammered out by mistake) and the sandbags performed very well. I can't remember how many bags, but about a dozen or so on each anchorage. The live ends were still encapsulated in the concrete as well.

Of course, there was the time that the sandbags were on the wrong set of anchorages ... the end of the tendon ended up on the roof of an adjacent building.

 

[Ingenuity]

Typically not. Depends on the sheath type. If paper-wrapped or heat-sealed, we typically do nothing except take safety precautions for persons in close proxiity. I have detensioned thousands of 'live' tendons and 99% of them are very uneventful. But we plan for the 1% event.

How are they doing the demo - excavator with concrete muncher attachment, hydraulic impact attachment?

Nature (corrosion) did this:

The grout plug was still attached to the strand.

Found the wedges on the street about 50 feet away.

 

[MacGruber22]

That sounds very tiring lugging sand bags around.

-Mac

 

[dik]

Ingenuity... what procedure do you normally use for detensioning?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[MacGruber22]

I agree we are protecting the 1% chance. But, it is about risk not just probability. Our contractor scope is limited to edge protection and cutting tendons. Another sub is handling shoring and demo. Not a fan of this but we are not running the show. We are at a preliminary stage till our client has their proposal approved. Here is a concept de-tensioning they submitted with their proposal.

-Mac

 

[EZBuilding]

Mac - I'd recommend editing your image to preclude the building name and location.

 

[dik]

I've not been involved with this, but do you put the protective 'end cap' in place and then core down, or use a small chipping hammer, to cut the tendons?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Ingenuity]

dik:

We usually GPR scan the slab/beam at high point (least concrete cover), concrete chip the top sover and create a small opening to expose the tendon/s, then use a 5" angle grinder to sever the strands. We only open up a length of exposed tendon/s to get the angle ginder into position. If you make the opening too long then when the strand wires are severed they tend to untwist/unravel and open up violently and can cause hand injuries to the person doing the cutting.

 

[Canuck65]

On our projects a covermeter is used to identify the tendon locations at a high point. Contractor then uses a quick cut with a concrete blade to cut perpendicular to the tendon. Takes less than a minute to cut down to the tendons and sever a 3 strand bundle. You know when you are starting to hit the tendons when the sparks start flying ... then pop, pop, bang.

 

[dik]

Thanks... cutting with the blade without exposing the strand seems to be a safer way to do it.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Ingenuity]

Cutting without chipping and locally exposing the tendons first can result in cutting rebar too (and/or elctricial). If it is a demo job not a big deal, if it is a repair project = big deal.

 

[Canuck65]

Top rebar was also located and marked in a separate colour paint. However, this was a unique project where the top rebar was also removed (sawcut and jackhammered slots) and replaced at the same time as the tendons.

 

[dik]

Thanks for the caution...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik