Reinforcing Bars - Stabbing into wet concrete 2

[DWP]

Are there any circumstances when a contractor may "stab" reinforcing bars or dowels into wet concrete instead of securing the bars before the pour? I have a contractor who wants to "stab" bars into a wet concrete footing and subsequently slip-form a traffic barrier. I have not found anything in the ACI or AASHTO Codes on this subject. Thanks for your help.

 

[jheidt2543]

It is done all the time

 

[Ron]

DWP...I agree with jheidt2543. One caution is the timing of this "stabbing". If done too late, too much aggregate displacement occurs that does not "remold" around the rebar, particularly if you are using a hook or other bent bar.

 

[Ron]

....finishing answer. I'm forum impaired this morning!!>>>

ACI 318 does give you a bit of help here. The code states that reinforcement must be accurately placed and adequately support BEFORE CONCRETE IS PLACED.

The Concrete Reinforcing Steel Institute (CRSI) Manual of Practice has a similar statement but goes a bit further stating that the reinforcing steel should be tied and secured against displacement prior to placing concrete.

Technically, it is not correct to place the rebar after concrete placement and would be construed as a code violation, since it deviates from ACI 318 and CRSI Manual of Practice. It is; however, as jheidt2543 pointed out, commonly done in shallow strip footings for dowels. That doesn't make it right...just common!

 

[Rjeffery]

What is industry practice is not neccessarly right. The dowels which will continue a wall up from a footing are bent for a reason. They should be tied to the footing steel so the load transfer (compression or shear) can be transmitted to that steel. the same applies to masonry construction. the brick layer would rather not have to lift a buttered block over his (or her to be PC) head to be able to place the next course when he or she can 'stick' the new bar in the grout later (and OBTW use it to consolidate the grout without using a vibrator.)

How can it be determined if the new bar is lapped properly or in close contact with the old bar?

 

[jheidt2543]

Rjeffery,

Regarding rebar lap in masonry, to be considered a lap the rebar do not have to be in contact. In fact, the lap bar can be in the adjacent grouted core and still be considered an acceptable lap. Why this is I don't understand, but it is correct.

Also, I've recently seen plans that call out masonry rebar laps of 8' to 10' for #8 & #9 bars. Now, how do the designers expect a mason (or masonette) to lift the bar that high?

 

[Rjeffery]

sounds like they want cast in place concrete walls for a masonry price with the entertainment value of a three ring circus thrown in for free.

Don't forget that end bearing rebar is acceptable if section 8.5.7.4 of Building Code Requirements for Masonry Structures are followed.

 

[concretedoc]

For some data comparing pullout strength of stabbed bars and bars tied in position before the concrete was placed, go to

http://www.worldofconcrete.com.

Click on "problem clinic" and then on "search problem clinic." Use "stab" and "rebar" as your search terms. You'll find a problem clinic item with limited data showing that stabbed bars had the same pullout strength as pre-placed bars. If you have trouble finding this information on the Internet, see Concrete Construction magazine, December 2000, page 57.

 

[BryanStein]

Concretedoc is right. Its done all the time and there is nothing wrong with it, IF, IF, IF, there is proper developement length embedded on both sides of the joint. Many times, slabs or foundations are not thick enough to get adequate embeddment. Thus, the bars must be bent 90 degrees to get the proper development length on both sides of the joint.

 

[JAE]

concretedoc-
I haven't looked at your site - but I would still be very cautious about stabbing. I would guess that the stabbing that they did was under controlled circumstances where the concrete was still quite plastic and the paste was able to re-contact itself to the bar.

I've seen cases where contractors stabbed bars in concrete that was on its way to setting and where the bars were hammered into the concrete. This absolutely left voids in and around the bars which would ultimately diminish any development.

That said, for small bars like #3's, and where the strength or developement of the bar isn't a critical issue, maybe its OK.

But unless you can be there, control the timing of the stabbing, and ensure the concrete can receive the bars and still reform to make plastic contact - I wouldn't allow it. I know - its done all the time...but that doesn't make it a preferred choice.

 

[jheidt2543]

Let's think about this again! "Stabbing" a rebar into wet concrete is certainly "better" than inserting a rebar into a drilled hole in solid concrete - which is also done all the time. The drilled in dowel relies totally on friction for pull out. There must be some testing some where that compares "stabbed" rebar to drilled in rebar in pull out strength.

But, what really is the purpose of the rebar dowel? Isn't MAINLY loaded in shear? Very seldom is a footing dowel in tension, hence development length is not a critical issue. I wouldn't use "stabbed" rebar in a retaining wall footing where you could get tension, but in normal foundation walls I don't see any problem.

 

[JAE]

jheidt2543 -

I understand your point on shear vs. tension....but the ACI conceptualization of how shear works is that the dowel holds the separate concrete parts together so that their roughness/friction is properly developed across the joint. The bar itself isn't utilized in shear - it is acting as a TENSION element, holding the parts together.

In fact, the ACI code requires that bars used for shear friction be fully developed - and the As(req'd)/As(provided) reduction is not even allowed.

 

[jheidt2543]

JAE,

I follow the point you make and I agree that the FAILURE MODE of a dowel is based on concrete shear however, what about the case of load transfer dowels in a floor slab? They transfer the load via dowel shear and don't have much developement length, particularly when they are either a smooth dowel or a greased dowel.

In the case of a footing, many continous footings are bullfloated off, thus little surface roughness. Or how about the case of a shear key in the footing, which works in shear only?

I guess my point is, as I'm sure you know, the ACI code still has many confusing and CONFLICTING provisions (the newly published ACI Masonry Code is another good example). The connection of a "normal" foundation wall to a continous footing is VERY seldom in tension, it is in compression from the dead weight of the wall and loading from the structure above. That is why, in my opinion, a straight dowel in a continous footing is OK. Now, dowels from a heavily loaded concrete column into a spot footing are entirely different. They could be pushed through the footing so, a straight bar pushed into the footing is not acceptable. The footing has to be investigated for punching shear.

This discussion started out asking whether "stabbing" of rebar is acceptable practice. I think, in MOST cases yes, for the reasons I've stated above. I do agree, that in some cases, it is not acceptable. Design knowledge and field experiance tell us the difference.

 

[Taro]

We can debate the pros and cons of wet-stabbing rebar (capacity, quality assurance, etc.) ad infinitum. But one of the main issues that hasn't been mentioned yet is liability and risk exposure. ACI 318 Section 7.5.1 clearly prohibits wet-stabbing reinforcing for structural concrete. I personally would not deviate from the legally adopted building code unless I had a damn good reason (convenience is not a good enough reason). To do otherwise could be professional suicide.

 

[rater57]

as an inspector, i see dowels stabbed into concrete footings all the time - and i don't see it done well most of the time. here is what i see. the installer rarely measures on center spacing accurately. they will measure the first bar from the end then lay a tape along the form or on the ground then stabb all the other bars from there. if the first bar is off, then all the other bars are off. if the first bar is placed accurately, all the other bars can still be off a little. that little catches up and most times i will find several dowels bent over, then back again to get its position in the cmu hole correct. the bending process damages the concrete footing right around the point of bar entry into the footing and bends the bar excessively. also, the installer does not mark the bar for depth of stabb into the concrete. this is all eyeballed. all too often they stabb too far and either punch through the bottom of the footing or are too close to the bottom. if they pull the bar back up some, then there will be a void space under the bar. then too, there is the contractor who uses they least depth of footing possible - in some cases, 6 inches. that leaves only three inches of stabb depth for the straight dowel in order to maintain the required 3 inches clearance to the ground. is three inches really enough? especially if the dowel is stabbed in just a little too late into conrete that is about to set? and finally, there is talk here of no tension on the bar. that may be correct in a static situation, but what about when that big earthquake strikes. if the ground rolls in an undulating wave form, the foundation will be placed in tension for a second on the back side of the passing wave. if the stabbed bar pulls out even slightly wouldn't that create a problem for the next big one?

 

[rater57]

the second major issue of this topic i would like to address is that of liability. as an independent inspector (not working for any government agency and thus very shallow pockets and no staff attorney to fall back on) who knows what the code says - the new international residential code states the dowel is to be tied in place - and what aci318, etc. says, if i allow the dowels to be stabbed in place, am i not taking on the liability in the event of failure? even if this is industry practice, at the very least i would have to try to prove this is court. which means a whole lot of expense for a small business man to prove to the judge and maybe a jury that what i did is in fact common. but, for me, the major problem is other code inspectors who are not well versed on the aci code or in structural matters (maybe they came from a plumbing background, etc.) and therefore see the industry common practice of wet setting dowels as entirely correct rather than the incorrect industry standard. standing there on site arguing what the code says to a builder who has never done it any other way but wet stabbing is just asking for a physical resolution - if you know what i mean.

if wet stabbing dowels is ok, then let's get code language to say so too so there is no conflict - with codes and common practice or people on site.

 

[dbuzz]

The following paper may be if interest:

“Plunge Columns”
Pratt, A.; Alexander, S
Concrete [UK] Vol.37 No.4 April 2003 pp.8-12

http://www.concrete.org.uk