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Repairing Corroded Slab Top Rebar where it extends thru wall

Repairing Corroded Slab Top Rebar where it extends thru wall

Repairing Corroded Slab Top Rebar where it extends thru wall

(OP)

In the case of top bars in a parking garage slab, that extend thru the supporting shear wall, the rebars are corroded completely away at about 6" south of the shear wall, but are ok at the north face of the shear wall. If bars are drilled into the 12" thick shear wall, and epoxied in with Hilti adhesive, is the depth of the hole and the distance that the bars must extend into the shear wall governed by the anchor Tables in the Hilti Technical Manual for anchors, or is it governed by the procedure for "splicing" bars using the Hilti adhesive? The latter gives much longer required lengths.

RE: Repairing Corroded Slab Top Rebar where it extends thru wall

You would need a splice on both the north and south side to make the top bars effective.

RE: Repairing Corroded Slab Top Rebar where it extends thru wall

(OP)
That is what I thought. Thanks Hokie66. I believe that Hilti has a PROFIS program for splicing bars with adhesive.

If the existing bars to the north can be developed within the shear wall (I am not sure that they can), and the bars to the south can be developed with new dowels that are adhesive anchored into the shear wall and extended southward a lap length with the existing bars to the south far enough to where they are not corroded, would that be ok?

RE: Repairing Corroded Slab Top Rebar where it extends thru wall

Not in my opinion. But then, I can never get how a bar can be developed in an epoxied hole in a shorter length than in good concrete.

RE: Repairing Corroded Slab Top Rebar where it extends thru wall

Quote (ajk)

If the existing bars to the north can be developed within the shear wall (I am not sure that they can), and the bars to the south can be developed with new dowels that are adhesive anchored into the shear wall and extended southward a lap length with the existing bars to the south far enough to where they are not corroded, would that be ok?

With the method proposed, you would develop some degree of continuity across the wall but I wouldn't expect that to be quite the continuity that one would get from the unmolested original rebar. What's your goal with reinstating the top steel? Do you actually need the flexural capacity? Do you need shear friction transfer from slab to shear wall? Often times, these bars are just "detailing reinforcement" for crack control. And that cat's probably out of the bag already.

Quote (hokie66)

I can never get how a bar can be developed in an epoxied hole in a shorter length than in good concrete.

Same thing bothers me. I have a theory (surprise):

1) Epoxy goo bonds to the bar better than concrete. So you're in good shape right at the surface of the bar.

2) At the goo to concrete interface, your diameter and surface area has grown some such that your goo to concrete interface isn't critical.

3) The statements above assume that you're in RC concrete mode where your bar tension is ultimately restrained by a concrete strut of some sort. Where that's not the case, such as a bar being pulled from an unreinforced concrete blob by the hand of god, concrete breakout would govern and there's no way in hell that the drill and epoxy, shallow embedment rebar would perform as well as the rebar in good concrete.

4)Because the usual case is rebar restrained by a concrete strut of some sort, the geometry of the joint matters a great deal. And a shallower embedment makes critical things even more critical in this regard. It's another way in which the drill and epoxy anchorage will often produce a weaker overall connection if the embedment is reduced relative to Ld. Most designers don't seem to understand this well, however, and the adhesive manufacturer's certainly don't go out of their way to advertise it.

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.

RE: Repairing Corroded Slab Top Rebar where it extends thru wall

(OP)
To Kootk: your points 1 and 2 are what I believe is generally given as the reason that a bar anchored into a hole with adhesive can be developed in a shorter length than when cast in new concrete. So while you commendably came up with that explanation on your own, it is in fact the way that I have always heard it explained and there is nothing about that explanation that seems unbelievable.

In any event, the Hilti data is based on actual load testing of the anchors, so the proof of the pudding is in the eating. I am not so sure about their American competitors. In any event, we (and many others) have proof load tested adhesive anchors on site and can verify that they can sustain at least 2 times the working load, when properly installed and I have no reason to doubt that they would not approach or surpass 4 times the design working load in 90% of the cases, unless they were not installed properly.

I don't think Hilti would be publishing wrong data for so may decades, unless they were another certain car company...and I don't think that they are or things would be falling down all over the place.

The big problem with adhesive anchors is that the worker has to be properly trained by the company in their installation, particularly in the cleaning of the hole, and carry it out assiduously. If the job is rushed, or if the worker lacks patience, is he going to do that?

As to the continuity issue, I recognize that since this building was designed in 1956 by WSD, we do not have to develop 100% of every bar, but we do have to develop some of their capacity. As a general rule, I like to restore the original strength if it is not to difficult to do so. A possible solution is to drill thru the wall just above the slab, then insert new bar thru this hole so that it extends out the wall each side of the wall and is embedded into the new topping, and make the topping structurally bonded. I don't like to do structurally bonded toppings because I think it is dicey as to whether the bond is achieved, but this might be a case where it is worth doing to get some reserve strength.

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