Rebar Cover Over Cold Joint

[mogug]

I am reviewing a design for a new concrete beam being placed on top of an existing concrete slab. The design proposes to place the new beam bottom reinf directly on the existing slab (zero cover under bars). Shear reinf is provided with upside down u-shaped bars epoxied into the slab.

My gut tells me the bars can't rest directly on the existing slab. I would think there might be bond issues. I realize the beam/slab can be treated as monolithic because shear transfer is provided. Do I have a valid concern about the cover? Is this covered anywhere in ACI 318?

 

[hokie66]

Although it may not be a valid concern, I would also want the bottom bars chaired, say 20 mm off the slab.

As to the U-bars used for shear reinforcement, I wouldn't depend on the available anchorage. My approach would be to hang the slab with drilled in bars, but place rectangular stirrups under the new beam bottom bars. This give your cover to the bottom bars, and anchors the stirrups.

 

[Once20036]

I'd be concerned about bond/developement length without concrete around 100%/360deg of the bottom bars.

 

[DST148]

Here are few things I would consider --(May be you might have already done these.)

We need to know all the info about the existing slab - type of slab, direction of spanning, reinforcing etc. The behavior of the existing slab should not be significantly altered by adding this beam. The effectiveness of the new longitudinal bars will depend to a large extent on the thickness of the existing slab and the depth of the new up stand beam - location of longitudinal bars in relation to neutral axis.
ACI 318-05, section 12.2.3, Cb - factor required for determining development lengths - one criterion is the distance from the center of bar to the nearest concrete surface. Nearest is the key. Although drilling stirrups ensures composite action at the interface between the existing slab and the new beam, the existing concrete slab is not much of a help in determining the cb factor since there is no real bonding between the bar and the top of the existing slab. Of course this should not be too much of a concern if the bars are not to be spliced within the span of the beam and properly hooked at the supports or if the reinforcing in the existing slab is adequate to take the entire bending. I would definitely go with the suggestions proposed by hokie66.

 

[DST148]

This is further to my previous response.
mogug - the subject heading is misleading. If it is a cold joint, the stirrups would be in-place and zero cover would not matter.

 

[ErZaZee]

Mind if I ask, what will this new beam carry or what is the purpose of this new beam?

What kind of slab is this?

 

[TXStructural]

In order to assure that the new longitudinal bars act compositely with the concrete, they need to be fully embedded in concrete. I work expect a crack to exist along the new concrete-old concrete interface, which would reduce or eliminate any bond between the to. The primary reason deformed bars develop composite action in tension is through the deformations. If a splitting plane exists along the new bars (the new-old interface and the small void inevitable along the bottom radius of the bar) I would count the bar as largely unbonded.

As to the stirrups installed with adhesive, be sure design adheres to 318-11 App D. When I have designed this type of repair, I thru-bolted the shear reinforcement, and the existing slab was just dead weight.