Concrete tanks - Vertical Construction Joints in Walls 5

[vincentpa]

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,

 

[JedClampett]

I need some information. Are you talking about circular or rectangular (square) tanks? Also, what middle horizontal bars are you talking about? We usually have two curtains of reinforcing each way.
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.

 

[vincentpa]

We are doing both rectangular and round tanks for this job. We use two curtains of bars also; one on each face of the tank wall. The "middle bars" I am referring to are in the square tanks. They are the horizontal bars that will lap with the horizontal end bars at the corners of the tank walls. These are the bars in the "midspan" of the tank walls. I was told to put the vertical construction joints at or near the tank wall 1/3 points (along the width of the wall) to minimize shrinkage keeping the "middle bars" out of the first pour. Is this worth it?

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?

 

[DTGT2002]

JC,
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?

 

[miecz]

vincentpa-

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.

 

[JedClampett]

Vertical joints always rigorously installed at third points seem like overkill. I'd space them at 30 to 40 ft. Pour the corners first and wait for a while to pour the next adjacent sections. That will eliminate some shrinkage cracking at the corners.
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.

 

[hokie66]

This is always a contentious point, but i would like to vote against robust roughening of joint surfaces. Bush hammering and the like causes microcracking in the concrete which can cause durability issues. A clean surface is all that is required, based on a lot of reports of studies I have seen. High pressure water cleaning should do the job.

 

[miecz]

I'd agree with hokie66 here. The shear across the vertical joints isn't high at the joint locations, anyway. There so much vertical steel at the base of the wall, shear friction doesn't require it.

 

[minorchord2000]

Do you place waterstops in the vertical wall joints? We do here. The vertical waterstop is connected to the waterstop at the base of the wall using factory welded Tee joints.

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.

 

[miecz]

minorchord2000

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.

 

[vincentpa]

jmiec,
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.

 

[cedent]

Just to satisfy my curiosity (I don't design tank structures):

What do you mean by a 'starter wall'?

 

[miecz]

Contractors around here typically pour the corners first, and then the infill sections. You could limit the joint location to some minimum distance away from the corners.

 

[JedClampett]

A "starter wall" consists of a short portion of wall poured with the slab pour (we use a 2 1/2 inch tall section). It allows the waterstop (almost always 6 inches wide), not to interfere with the top horizontal steel (with 2 inches cover). The wall is then continued on top of it.
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.

 

[minorchord2000]

I will stick by my disdain for starter walls. We just lower the entire top mat of reinforcing bars in the base slab and bite the bullet on the reduced "d" for bottom slab negative moment. We do not bend the bars. Base slab concrete is much cheaper to place than the wall concrete.

 

[JedClampett]

Minorchord2000, I agree with you. But we're under constant pressure to reduce thicknesses. Even though it's cheaper to pour an extra 3 or 4 inches of slab, I get continuous sniping when my slab thicknesses increase. Someone will pull out a historical job and wonder why I'm gold plating this one. While I would never think of griping about pipe sizes or cable gauges, everyone seems to think they can spot an oversized slab. And it never stops, even after the project is complete.

 

[miecz]

Last job I did was "Value Engineered". The reviewer suggested reducing the thickness of the walls and mat away from the edges. He "saved" tens of thousands of dollars by reducing the volume of concrete. No thought was given to extra design time, cost of formwork, bending of rebars, special grading, and so on. Imagine designing this monster. I prepared a cost analysis to rebut the "savings", mostly with made up figures (how do I know the difference in cost between straight and bent #11s). Although I produced numbers showing the saving were offset by the construction cost, the project manager insisted we reduce the thickness of the base mat away from the walls, as a compromise with the value analysis!

 

[vincentpa]

Another difficult detail is in a multi-cell tank when the interior wall intersects the exterior wall - both cells need to be watertight and a PVC waterstop must be used (cell walls are lined and a Hydrotite waterstop cannot be used). The PVC waterstop will always conflict with the horizontal reinforcement of the exterior tank wall. The exterior wall reinforcement must be continuous here and cannot be bent. Therefore a starter wall or key must be used. Does anyone have another idea for this situation?

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?

 

[miecz]

I use diagonal bars for shear friction at outside corner intersections, but not at tee intersections at interior walls of multicell tanks.

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.

 

[JedClampett]

Anyplace you're worried about shear friction, you should also be worried about straight shear on your cross section. Don't forget, shear friction only applies at joints. The code also has significant reductions for sections under shear and tension, such a corner of a tank. See ACI 318-02 Section 11.3.2.3 for this reduction.
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.