Concrete tanks - Vertical Construction Joints in Walls
Concrete tanks - Vertical Construction Joints in Walls
(OP)
When we design concrete tanks walls, we use vertical construction joints at approximate third points. I try to make the joints coincide with the end of the middle horizontal bar that would lap with the corner bar. This reduces the horizontal shrinkage effects. I have problems isolating the middle horizontal bar for smaller tanks. Where do you place vertical construction joints in concrete tanks?
Where the base slab meets the tank wall, we use either a starter wall to accomodate a PVC waterstop or we roughen the surface and don't uses a starter wall or key when we use a Hydrotite waterstop. In both cases we rely on shear friction to transfer the shear from the wall to the base. However, we use keys in the vertical wall construction joints. How do you detail your vertical wall construction joints? Do you use keys? Do you roughen the joint and rely on shear friction? I think roughening the joint on a vertical surface would be too labor intensive. I'm interested in your input.
Thanks,
Where the base slab meets the tank wall, we use either a starter wall to accomodate a PVC waterstop or we roughen the surface and don't uses a starter wall or key when we use a Hydrotite waterstop. In both cases we rely on shear friction to transfer the shear from the wall to the base. However, we use keys in the vertical wall construction joints. How do you detail your vertical wall construction joints? Do you use keys? Do you roughen the joint and rely on shear friction? I think roughening the joint on a vertical surface would be too labor intensive. I'm interested in your input.
Thanks,






RE: Concrete tanks - Vertical Construction Joints in Walls
As far as keys in the starter wall joint, we don't use them. They're hard to build and create limited improvement of shear capacity. I'd also eliminate them in the vertical joint. Roughening the joint isn't that hard. I'd make them do it.
Note that shear friction only applies specifically at the joint. You still need sufficient thickness of your wall to carry the shear load. This will usually control.
RE: Concrete tanks - Vertical Construction Joints in Walls
For the key, no key, or roughened joint sitution for the vertical construction joints, I am asking about both rectangular and round tanks. I wanted to use a roughened joint but I was a little concerned about the edges of the joint. Will they chip due to feathering or is this not a problem? Is the joint roughened inside the rebar curtains or along the entire surface?
RE: Concrete tanks - Vertical Construction Joints in Walls
What do you use to judge the acceptablility of the 1/4" amplitude surface roughening?
The only in depth resource I have ever known has been the ICRI references, but these speak to preparing a surface for coatings, not necessarily for adjacent concrete pours.
Other than engineering judgement, which is not usually found in inspectors, how do you determine what is acceptable?
RE: Concrete tanks - Vertical Construction Joints in Walls
We don't detail the location of the vertical joint. The location is driven by the form work material availablitly. We only specify that the length between joints not exceed 40 feet.
We use keys in our vertical construction joints. It facilitates the construction of the vertical waterstop. We do not use a key at the base of the wall. We rely on shear friction. In lieu of roughening the surface, we use mu=0.6.
RE: Concrete tanks - Vertical Construction Joints in Walls
Have them bush hammer or some similar roughening for the vertical joints. I wouldn't worry about feathering. If you don't look too close, you won't see it.
RE: Concrete tanks - Vertical Construction Joints in Walls
RE: Concrete tanks - Vertical Construction Joints in Walls
RE: Concrete tanks - Vertical Construction Joints in Walls
We do not use starter walls, as they are difficult to construct. We lower the top bar in the base slab to accommodate the embedded waterstop without conflicting with the top horizontal bar in the base slab.
RE: Concrete tanks - Vertical Construction Joints in Walls
We place waterstops in the vertical joints, as you do. Do you mean that you lower the top bars everywhere or bend them down at the perimeter? We bend our bars at the perimeter.
RE: Concrete tanks - Vertical Construction Joints in Walls
I am hesitant to allow the contractor to choose a construction joint location. They might pick a corner of the tank. I have a small tank 20'x20'x14' and I don't want to put control joints at the third points but I don't necesarily like to put the control at midspan either.
hokie66,
As for roughening the joint, a 15 pound hammer does not create significant micro-cracking. Anything above a 15 pound hammer will start to crack the concrete. For the base slab, they usually dont finish the concrete where it is specified to be roughened. That is usually good enough. What reports are you talking about for just a clean surface? If you are doing concrete repair, the surface is already roughened usually.
minorchord2000,
I don't like to bend the bars down because those bars are resisting the negative moment induced by the wall. I don't think that a 5-1/2" starter wall is that difficult to build. ACI recommends starter walls in ACI 350.2R-04 Concrete Structures for Containment of Hazardous Materials. I've never met a contractor that didn't gripe about something he's done or not done all his career.
As for shear friction, I don't like the fact that ACI states that you don't have to increase the shear friction reinforcement, i.e. the flexural reinforcement is good enough. It doesn't feel right for a tank wall. My question is, how can the flexural steel and the shear friction steel be the same? I know ACI states the shear friction will be greater on the compressive side of the flexural element. I don't buy it. Have they done studies in walls for this? I always add shear friction reinforcement at the base of the wall and at the corners if there is no reserve capacity in my flexural reinforcement or I just increase the size of the flexural reinforcement.
RE: Concrete tanks - Vertical Construction Joints in Walls
What do you mean by a 'starter wall'?
RE: Concrete tanks - Vertical Construction Joints in Walls
RE: Concrete tanks - Vertical Construction Joints in Walls
Contractors hate it. It's hard to pour and vibrate without having the concrete level itself off. But the alternatives are worse for varying reasons.
RE: Concrete tanks - Vertical Construction Joints in Walls
RE: Concrete tanks - Vertical Construction Joints in Walls
RE: Concrete tanks - Vertical Construction Joints in Walls
RE: Concrete tanks - Vertical Construction Joints in Walls
Another thought on shear friction:
I can see using the method as described in ACI at the base of the tank wall for vertical reinforcement. ACI states that the flexural reinforcement does not need to be increased because the increased compressive resultant will balance the forces. But at the corners and for intersecting walls, I don't necessarily buy that. It seems to me that the "shear friction" is behaving more like a dowel and there would be no increase in compressive resultant (especially at the intersecting walls) or at least the increase is not as large perhaps redistributing due to the more flexible wall. In the ACI detailing manual and in older drawings of tanks, I have seen diagonal bars (when looking at the tank in plan) at the corners and at intersecting walls. These diagonal bars appear to be shear friction bars. Does anyone still use diagonal bars at the corners and at intersecting walls? Or do you rely on shear friction as described in ACI?
RE: Concrete tanks - Vertical Construction Joints in Walls
At the base of these interior walls, I bend the bars down, same as at the exterior walls. It forms an X under the interior wall. The bend occurs at the face of the wall, and the bar then extends its development length.
RE: Concrete tanks - Vertical Construction Joints in Walls
Vincentpa, as far as your question about wall to wall joints, don't let them make a construction joint at the corner. Lots of bad things are happening there. Move the joint some distance away. This removes the joint from the highest stressed region and allows the waterstop to be installed traditionally.
RE: Concrete tanks - Vertical Construction Joints in Walls
I wouldn't let them put a construction joint at the corners but I am still hesitant about putting the construction joint in the middle of wall vertically. I wouldn't put a construction joint in a beam at midspan. But if I don't put construction joints at the middle of the wall/span, there will be too many pours for a small tank, at least it seems like too many pours. Do you put vertical construction joints in the middle of the span/wall? Do you feel comfortable doing this?
RE: Concrete tanks - Vertical Construction Joints in Walls
RE: Concrete tanks - Vertical Construction Joints in Walls
If the wall is designed vertically, either cantilevered or simply supported, it does not matter if you locate a vertical joint at mid length of the wall.
RE: Concrete tanks - Vertical Construction Joints in Walls