ACI-318 Development Length using SikaGrout 212 1

[lostit40]

I would like to use a non-shrink grout (SikaGrout 212) to embed a rebar into a concrete slab, to reduced the embedment length. The code have a maximum bond stress function of 8.3MPa, but SikaGrout 212 bond strength is 13.1 MPa (Flowable) which is higher than the function sqrt f'c. Reason for using SikaGrout 212 is due to the higher bond strength to reduce the embedment length. Which value should one use?

 

[JedClampett]

This is not a code approved application. You're going off into uncharted territory. It might work, but why not use a Hilti or Simpson epoxy instead?

 

[CANPRO]

This comment may be way off the mark, just my initial thought after reading the OP....you will have an increased bond stress between the bar and the grout, but doesn't the grout still have to bond to the concrete? You still have to develop the tension into the slab.

 

[TXStructural]

Jed and CANPRO are correct. You usually cannot decrease the development length where the bars have to be fully developed. What you describe has long been done with headed anchors for shear applications, but I would not spec this for standard rebar, especially in tension, without some test data.
First issue is that, save for some friction, bar develops based on the deformations, not surface bond interaction. This may be assumed to account for 30%, based on the factor for epoxy-coated bar which has no surface bond with concrete. Making a hole for a grouted embedment will probably result in a relatively smooth hole.
Second, a high strength adhesive or grout could effectively increase the diameter (and thus surface area) which would be useful to decrease development length. For this to be useful, the interior of the hole would need to be rough, with an amplitude similar to that of the bar. That said, it has been found that simply increasing the height of deformations on bars does NOT effectively decrease development length.
Third, getting nearly 100% embedment with a grout will be difficult, if not impossible. Doing this with an adhesive is similarly difficult, but the relatively higher strength and narrowness of gap between the bar and the surrounding concrete mean that small voids have a lesser effect on strength of the bond.
Finally, it is not only the development of the bars you intend to embed, but also how the other bars inside the concrete interact with the concrete. It is insufficient to connect the new bars to the concrete if the concrete cannot get the forces where they need to be. ACI 318-11 App D has guidance on dealing with this.

 

[lostit40]

Thanks for the comments, let me shed more light on the matter. i am checking a precast wall panel using dowel connections a Flexible metallic conduit is cast into the foundation/wall and wall, then a grade 60 rebar is inserted and grouted. they (precast manu.) want to change to SikaGrout 212 to reduced the length of rebar. being the overall eng. on the project was to check connections supplied by the precast company. i know the 2 scenarios (1) interface between rebar and grout, and (2)interface between corrugated pipe and concrete.

 

[KootK]

I think that most of TX's comments still apply. Base your bar development improvement on f'c and assess concrete breakout around the conduit somehow. I don't know how one goes about checking the interface between the conduit and the surrounding concrete. PCI must have something to say about that.

@TX: could you elaborate on the headed anchors for shear applications? I don't get it but it sounds interesting.

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.

 

[Brad805]

PCI has a commentary about the use of corrugated conduit. PG6-8 of Rev 6

http://www.splicesleeve.com/

is another option in lieu of post tensioning ducts.

 

[TXStructural]

As to to the OP's dowels, what is the real force involved?
If the dowels are to transmit tension forces, you should tread carefully since you will be on your own, from a liability perspective. If they are for engagement of shear friction across the joint, they have to be fully developed according to the code - in which case, I would recommend using a different method of keying the components together.
Also, the cost of the rebar is irrelevant, so there must be another reason for them to want this connection. Find out why they really want to reduce the connection length and go from there.

As to the grout pockets and shear studs, it is reasonably common to core holes above composite steel beams to post-install additionally shear studs. The pockets are then grouted around the studs once they are welded into place on the beam.

 

[CELinOttawa]

Groutting in a drossbach duct like this is standard practice in a lot of places, but I don't know of anyone who uses an increased grout strength to try to reduce the development length...