Reinforcing Bars - Stabbing into wet concrete
Reinforcing Bars - Stabbing into wet concrete
(OP)
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.





RE: Reinforcing Bars - Stabbing into wet concrete
RE: Reinforcing Bars - Stabbing into wet concrete
RE: Reinforcing Bars - Stabbing into wet concrete
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!
RE: Reinforcing Bars - Stabbing into wet concrete
How can it be determined if the new bar is lapped properly or in close contact with the old bar?
RE: Reinforcing Bars - Stabbing into wet concrete
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?
RE: Reinforcing Bars - Stabbing into wet concrete
Don't forget that end bearing rebar is acceptable if section 8.5.7.4 of Building Code Requirements for Masonry Structures are followed.
RE: Reinforcing Bars - Stabbing into wet concrete
RE: Reinforcing Bars - Stabbing into wet concrete
RE: Reinforcing Bars - Stabbing into wet concrete
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.
RE: Reinforcing Bars - Stabbing into wet concrete
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.
RE: Reinforcing Bars - Stabbing into wet concrete
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.
RE: Reinforcing Bars - Stabbing into wet concrete
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.
RE: Reinforcing Bars - Stabbing into wet concrete
RE: Reinforcing Bars - Stabbing into wet concrete
RE: Reinforcing Bars - Stabbing into wet concrete
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.
RE: Reinforcing Bars - Stabbing into wet concrete
“Plunge Columns”
Pratt, A.; Alexander, S
Concrete [UK] Vol.37 No.4 April 2003 pp.8-12
www.concrete.org.uk