Combined Cast-In & Post Installed

[CaliEng]

Potentially looking at a unique detail for an anchor. Due to various site conditions, I am considering an anchoring system which utilizes both post-installed (adhesive) for the lower portion of the anchor embedment, and then cast-in the remaining portion of the anchor. Obviously this is a-typical, but does actually seem reasonable at first glance. However, running the numbers is a bit tricky potentially. Can consider this one cohesive slab, or analyze the embeded potions separately and add the capacities. Anyone looked at anything like this before

 

[Ingenuity]

You are going to be on your own on this to get ICC-type tabulated capacities, unless you just conservatively utilize the capacity of the post-installed embedment and neglect the cast-in embedment length.

How many anchors are we talking about: 4 or 400?

Proof testing, maybe, or if justified run some tests to failure, divide by 5 and get a working load!

I did this recently (tests to failure) to screw anchors to a CMU wall where the contractor and glazing sub needed failure loads of anchors to arrive at a working capacity.

 

[KootK]

I think that we could probably use a bit more context here. I assume that we're talking about a cold joint here? Wall / slab?

Some preliminary thoughts:

1) I feel as though the combo of cast in and post installed anchors could probably share tension quite well.

2) In the absence of welded washers etc, I feel that all of the shear should probably go to either the cast in anchors or the post in stalled anchors, depending on the situation. Sharing of shear seams optimistic.

3) The elephant in the room, as I feel you've implied, is whether or not concrete breakout frustums that cross a cold joint can still be assumed to retain their normal geometries. In many, but not all cases I feel that assumption would be questionable. Again though, it would help to know what we're looking at here in greater detail.

 

[CaliEng]

Thanks for the insight.
Right, context.
Just looking at single anchor into a thin slab (hence trying to add more embedment depth).
KootK - that's interesting you think the combo would share tension (which is the intent), but not so sure about the breakout geometry (which is understandable since its an odd setup). I dont suppose this has been tested in a controlled setting?

 

[KootK]

Disregard what I wrote earlier. I totally misunderstood your situation.

- How thick is your slab?

- How thick will the new concrete be?

- Will the new concrete be a bonded topping or a localized pier/wall attached to the slab with dowels?

I would think that you could reliably mobilize either:

1) The slab concrete alone (obviously) or;

2) The new concrete alone pushed upon by the breakout frustum of the existing concrete. The new concrete will likely contain shrinkage cracks owing to the restraint provided by the existing concrete.

The big question is whether or not you can mobilize #1 and #2 simultaneously. My gut feel is that you can but, I agree, it would be nice to see some testing on that.

 

[CaliEng]

Interesting insight.
For this instance, current slab 6", new concrete 6" and localized to the anchor, with sufficient edge distance for the load. And the new would actually have a hairpin around the anchor to tie to and adjacent raised slab (but obstructed above). Im sure a detail would help here..
I suppose they should be bonded together with some weldcrete

By mobilize do you mean failure?

So I put together a sort of generic detail for this. Hope this helps. Obvs the anchor will be connected to something else, but left that out, just considering the embedment into the two differnt potions.

Currently thinking (A) considered as one slab (B) Considered as two slab with tension load distributed to representative embedment length or (B) considered separately with specific embedment depths, then sum capacities and apply a reduction factor of say 0.75

For reference:
I actually found a thread considering the combination of post-installed & cast in (though in a very different way) on EngTips (

https://www.eng-tips.com/viewthread.cfm?qid=488537).

One reply referenced the Sept 2021 issue of StructureMag (

https://www.structuremag.org/?page_id=18686)

- again not completely relevant, but related and interesting

 

[KootK]

By mobilize do you mean failure?

Mobilize = develop the presumed capacity of.

In your situation, I don't see the pullout resistance being appreciably better than that of the original slab alone.