Concrete Beam - Horizontal Cold Joint
Concrete Beam - Horizontal Cold Joint
(OP)
We often get asked by contractors if its acceptable to pour deep concrete beams in two pours. This usually occurs when the slab ties into the side of the beam. (Top of slab elevation is lower than top of beam elevation) And GC wants to pour the slab to outside edge of beam in one pour.
Is this something others are allowing? Typically we do not, however, this has been met by much resistance.
If you were to allow a cold joint in a concrete beam - what is the primary failure mode that you'd need to check. I understand longitudinal shear at the cold joint may be an issue. However, if the hooks are embedded in the lower section of the beam you will have rebar continually crossing the cold joint. In addition, the concrete can be left rough at both faces. Concrete friction and your #3 hooks (or similar) should provide enough strength to resist this load.
Another issue for exterior beams may be water penetration - if a cold joint is left in the beam, water could easily find its way into the beam and corrode the rebar.
Does anyone have any references or design suggestions for me on this topic? What should I be checking to approve this method?
I greatly appreciate your input.
Is this something others are allowing? Typically we do not, however, this has been met by much resistance.
If you were to allow a cold joint in a concrete beam - what is the primary failure mode that you'd need to check. I understand longitudinal shear at the cold joint may be an issue. However, if the hooks are embedded in the lower section of the beam you will have rebar continually crossing the cold joint. In addition, the concrete can be left rough at both faces. Concrete friction and your #3 hooks (or similar) should provide enough strength to resist this load.
Another issue for exterior beams may be water penetration - if a cold joint is left in the beam, water could easily find its way into the beam and corrode the rebar.
Does anyone have any references or design suggestions for me on this topic? What should I be checking to approve this method?
I greatly appreciate your input.






RE: Concrete Beam - Horizontal Cold Joint
I have used such a detail on occasion with no problems.
DaveAtkins
RE: Concrete Beam - Horizontal Cold Joint
Mike McCann
MMC Engineering
RE: Concrete Beam - Horizontal Cold Joint
RE: Concrete Beam - Horizontal Cold Joint
For a 4 1/2" slab or such it is impossible to provide this development and you would therefore ignore the top, second pour and count on the lower portion of beam alone.
RE: Concrete Beam - Horizontal Cold Joint
The other thing to consider is that the upstand will shrink later than the beam below, so some additional crack control reinforcement close to the joint should be considered.
JAE, I don't see that this joint involves shear friction. It is just horizontal shear.
RE: Concrete Beam - Horizontal Cold Joint
Vs + Vc shear values found in chapter 11 deal with shear in beams with flexure, an entirely different shear behavior than that produced by horizontal shear.
Since the shear plane is due to concrete cast at different times, and the surface condition can vary a lot (troweled surface, as-cast surface, or intentionally roughened surface) shear friction is the way to go.
The spacing of stirrups across this horizontal shear plane would be dependent on the shear and these would be added to the flexural shear stirrups.
Thoughts?
RE: Concrete Beam - Horizontal Cold Joint
RE: Concrete Beam - Horizontal Cold Joint
However, the ACI Code has a specific section on horizontal shear. You don't follow the shear friction section of the Code for this situation.
DaveAtkins
RE: Concrete Beam - Horizontal Cold Joint
JAE - can you tell me where this statement the "ACI code requires the reinforcement to be developed to achieve full fy." in ACI?
I agree with JAE regarding checking shear friction. Per ACI 11.7.1 "Provisions of 11.7 are to be applied where it is appropriate to consider shear transfer across a given plane such as an existing or potential crack.... OR an interface between two concretes cast at different times"
Based on the shear friction design method Vn=A*fy*coef friction. (Friction Coef for normal weight roughened concrete = 1.0 per ACI 11.7.4.3)
For this condition I simply plan to check VQ/Ib<Vn.
Does anyone disagree with the above?
RE: Concrete Beam - Horizontal Cold Joint
Badger - I'm away from my office right now but in ACI 318 there is a paragraph that states something like this: "When the reinforcement is required to develop full yield, then the provisions for As(req'd)/As(provided) cannot apply." I'll look it up later and post it here unless someone beats me to it.
RE: Concrete Beam - Horizontal Cold Joint
It does reference minimum ties in accordance with 11.5.6.3 (which is the minimum stirrups for beam shear). Does this mean the stirrups can do double duty?
RE: Concrete Beam - Horizontal Cold Joint
RE: Concrete Beam - Horizontal Cold Joint
If you look in the shear friction section, 11.7.8 states that you must use reinforcing that is developed to fy.
StructuralEIT - I think 17.5 probably does apply here more-so than the shear friction section. But it looks like they are similar. 17.5.3.4 actually refers you back to shear friction when Vu exceeds a set limit.