Concrete Tank with No Contraction Joints.

[strguy11]

I have a project where the contractor did not put contraction joints in the walls of a Wastewater treatment tank. The wall is 75 feet long and 14" thick. The horiz. wall reinf is #5 at 12 along each face. The design plans specifically called out for contraction joints to be spaced at 25' oc. max. It is no surprise that shrinkage cracks have developed in the walls. (they are about 6 to 8 feet on center, full height of the wall).

There are construction joints 12.5 feet from the ends of the walls, so there is about 50' of wall with no joints. (i.e. the contractor should have put one contraction joint in this area).

My question is, does anyone have any ideas on how to fix other than removal and replacement of the conrete? I have thought about epoxy injecting the cracks, but I think that new cracks will eventually form after some freeze thaw cycles.

My thought is to repair the cracks with epoxy, but in the center of the 50' section have the contractor cut out about a 6' or 8' wide section and dowel rebar into the wall install a proper contraction joint with a waterstop. Along the cut surfaces of the wall to remain, we would have them install an Rx type waterstop. Anyone got any better ideas?

 

[miecz]

We once had a similar situation. Long (50 ft) distances between joints, many cracks, leaking like a sieve. I lost a lot of sleep over these tanks. We went with epoxy crack injection and it worked just fine. I haven't done one of these in a while, but #5@12" seems a little light for a 14" thick wall.

 

[hokie66]

You are lucky that you have the contractor to blame. You have not provided enough reinforcement to control restraint shrinkage cracking in a water retaining structure. If you had the joints at 25' centers with that amount of reinforcement, you would still have the cracks at 8' centers to deal with, just one less. You can't prevent shrinkage cracks when walls are restrained, you can only control the cracking to acceptable widths.

 

[JedClampett]

I agree with miecz. I believe in the Hypocratic oath of structural engineering. First Do No Harm.
Let mother nature put in her contraction joints and seal them with epoxy. And #5 @ 12" horizontal is not enough steel. With no expansion joints, you should provide rho of .0052.
To really scare the contractor make him extend his warranty. If its one year, make him extend it to five years. It's meaningless, but it will really piss him off.

 

[Brad805]

What is the predicted crack width? If the estimated width is less than 0.5mm, Kryton and Xpex have surface applied products that will solve the problems. If your cracks are larger, then you need to look at some epoxy injection methods.

Brad

 

[strguy11]

I have looked that the ACI tank design guidelines and i am not sure why you guys are saying there is not enough horizontal steel. ACI 350 provides a chart showing the required steel ratio for a given spacing of shrinkage compensating joints. Our design called out for these to be 25 feet on center. Based on the figure, for 60 ksi reinforing, the ratio is about .0028. For a 14" wall, this equates to 0.44 in^2 of steel. #5 at 12" in each face is 0.62. What am I missing?

Our typical construction joint and contraction joint details only allow 50% of the horizontal steel to pass across the joint, creating a weak point. (The only difference in the joints is the type of waterstop used) We are not designing the wall to span horizontally.

 

[JedClampett]

Yeah, that's what the code says. But I always provide minimum of .0052 with additional bars at the bottom construction wall/slab joint. To be perfectly honest, I provide minimum bending steel most of the time (.0033 each face).
Steel=Cheap
Cracks=Expensive

 

[miecz]

Perhaps my take on this doesn't strictly follow the code. When I read "Movement Joint" in Article 7.12, I think "Expansion Joint", as in, no reinforcement crossing the joint, bulb type waterstop. I don't believe a construction joint with some or all reinforcement passing thru qualifies as a movement joint. So I provide construction joints at max. spacing 40 ft, and the maximum minimum reinforcement called for "where movement joints are not provided."

 

[hokie66]

Jed and I are on the same page with this. Codes can't tell you everything, and if the ACI code says .0028Ag is enough, it is wrong. My minimum for water retaining structures restrained at the base is .0060, and that has served me well. Spacing of contraction joints has little to do with it, as the cracks are going to form at regular centres of about 10 ft, no matter what you do. Controlling the width of the cracks is what matters.

 

[miecz]

strguy11-

Checked with the Repair Contractor, and it turns out that we didn't repair our cracked tanks with epoxy injection. We used a "Krystol Crack Repair System" by Kryton.

 

[Hemifun]

Strguy11, I'm curious, how high are your walls?

 

[ATSE]

Agree with Hokie.
Contraction joints in a straight wall for an environmental structure? Use of contraction joints relies on too many unknowns.
Use construction joints ("cold joints") at 30' to 50', and high horiz bar reinf ratio (at least 0.004bh) both between joints and across joints (i.e. continuous).
The 50% horiz steel continuous thru the contraction joint works about 50% of the time. Okay for concrete paving, not for retaining fluid.