Concrete Column Splice 1

[aggman]

I have a situation where I have two 30" X 42" concrete columns cantilevered up from a footing 25'. The contractor wants to pour the columns in 6' increments, leaving 3 construction joints in the footer. I looked in the ACI code and could not find anything that specifically said you could not do this, but my gut says that this would hinder the shear transfer in the column since the concrete would have a horizontal crack all the way through. The column sees realatively high shear and bending, while the compressive loads are small compared to the column size. This also makes me uneasy, due to the friction between the concrete faces would not be overly high, forcing the shear into the vertical bars. Reinforcement consist of 22 #8 bars vertical and #4 ties @ 10". What are your opinions on this? Please respond as soon as you can, as they are breathing down my back to give them an answer.

Thanks,

 

[SlideRuleEra]

With proper reinforcing you should be ok. Many reinforced concrete structures subjected to high shear and bending loads have horizontal construction joints. Examples are chimneys and silos that are jump formed, and tall columns in bridge substructures.

1. Put a depressed key in the top of each lift.
2. Strip the forms as soon as practical, clean laitance.
3. Soak top of the concrete with water for several hours to improve bond between old and new concrete (I worked one job - rebuild of a badly damaged electric generating station tubine-generator pedestal - with a respected national concrete "expert" who insisted that the "thoroughly soaked" concrete will create a bond equal to or better than epoxy compounds - based on the results, I believe him.
4. Remove the "soaking water" and place the next lift - do this as rapidly as practical (the bond between the concrete lifts will be better the "younger" the concrete.
5. Wet cure everything, ALL the time - from the start, except when actual work is being performed.

Do it right, don't skip any steps, and relax

 

[structuresguy]

If you are really concerned about the shear, you can cast a shear key, as suggested by SlideRuleEra, or cast into the column a shear tab made of steel. Your columns are large enough that you could cast a W-shape into the column center to act as the shear tab. Just size the W-shape to take the shear load, or some justifiable portion of it. After all, you say you have 22 #8 bars. These will transfer something like 1000 kip at ultimate alone.

 

[aggman]

Just to follow up on this. I appreciate your responses. After I wrote the post, I continued to look in the ACI code, and found section 11.7. This section is used to design shear transfer reinforcement for interfaces between two concretes cast at different times. More or less what the code says happens is that as the shear is applied to the crack, the concrete has a tendency to lift or seperate, which could cause the the perimeter (bending steel) to yield. To offset this additional reinforcement is added to handle the seperation and shear forces. Thanks for you quick responses.

 

[flamby]

How much time interval your contractor wants between successive pours? I think it should be less than 24 hours so that the top surface can be washed and fines can be cleaned with a water jet. As SlideRuleEra notes, in bridge and dam piers, we do it all the time and cast the concrete in several lifts without any special treatment. But we are required to wash the fines off the surface and expose course aggregates before fresh concrete is going to be placed. However if the concrete is so set (Oh we are already near the weekend ) that fines can not be removed, you can use mechanical methods (chisel & hammer) to create uneven surface.

Placing a shear key in area crowded with rebars is probably difficult from contractor's point of view. Hammer and chisel may be as good as shear key.

 

[SAMO]

In addition to what SlideRuleEra suggested, I'd request to use concrete with higher strength about 5" bellow and above the joint. Does this make any sense?
Thanks

 

[Taro]

Doesn't make much sense to me, SAMO. Sounds like a very expensive proposition to change concrete strengths intermittently through the height of two columns. And I don't think it would produce appreciably better results.