Concrete Tank w/o Joints-ACI 350 2

[ARLORD]

Anyone with large water retaining concrete tank design experience, is it good practice to have 30ft high walls, between 150 and 300 ft long without movement joints. According to ACI 350, without movement joints the reinforcement ratio should be at least 0.005. If this requirement is met, is it OK to omit movement joints for water retaining structures. What are your thoughts.

 

[JedClampett]

I would stay stay from the longer spans without any movement (expansion) joints. I could go up to 200 ft without any, but over that is risky. We use .005 reinforcing ratio for 40 ft spacing of movement joints. It would be a huge leap to assume that amount of reinforcing eliminates the need for movement joints for a 300 ft. length.
Expansion joints are a pain to locate and likely to leak, but they're better than the alternative.

 

[Hemifun]

My rule of thumb is to place contaction joints at a max. spacing of 1.5 to 2 times the wall height. You WILL get shrinkage cracks so you might as well plan for them and try and control where they form. Make sure your joint detail includes a notch on both sides of the wall to create a point of weakness and a waterstop.

 

[ARLORD]

Yes, joints 45'-60' on center in wall and base mat, that's what I thought. Do you guys use two different movement details. One Contraction and one Expansion.

The Expansion having a gap of about 1" filled solid, and with backer bar and joint sealant each face.

A Contraction joint with reinf stopped, smooth bar, greased one end.

Stop reinf and dumbbell-type waterstops at each of course. What are you thoughts.

 

[JedClampett]

We only use contraction joints in very rare occasions. We've had problems with them and manage to avoid them whenever possible.
I'd place construction joints at 30 ft, pour the walls and slabs in an alternating pattern and save the expansion joints for 100 to 200 ft. spacing.
I know that other outfits use contraction joints and don't have any issues. And we use them in very specific cases. But any joint without reinforcing crossing the joint is risky and the less of these you use, the better.

 

[Hemifun]

Quite honestly, I have never used an expansion joint in a water retaining structure. However, I have never designed one 300 ft. long either. As far as the contraction joints are concerned, I always run at least 50% of the horizontal steel thru the joint. Another tip is to try and locate the CJ at the ends of laps, this being a location where there is an abrupt change in steel area therefor a prime location for a shrinkage crack to occur.

 

[ARLORD]

JedClampett,

I plan to use one expansion joint. Is it OK to have an expansion joint in the wall and just a construction joint in the 30 mat foundation at the same location. I've never seen a mat foundation with an expansion joint.

 

[JedClampett]

If you have an expansion joint in a wall, you need to continue it in the mat. Otherwise, there's a problem with compatibility at the location where they meet.

 

[hokie66]

Why would you need an expansion joint? Concrete doesn't do much expanding. It shrinks.

 

[BAretired]

hokie,

I used to believe as you do, but I have seen concrete expand considerably in the hot sun. In one instance about 15 years ago, a concrete pavement between a building and a roadway buckled up on a hot day. The pavement actually lifted off the ground and failed in compression. After the failure, the slab was so uneven that you could not easily walk over the surface. I was amazed because I had never seen anything like this before. I guess this is why folks put expansion joints in sidewalks.

BA

 

[hokie66]

BA,

But this discussion is about a water tank. I wouldn't think that expansion would be an issue there, unless the water is heated.

As to pavements, I agree that they expand in the sun. But expansion joints don't really help with that longterm unless they are properly maintained. If left on their own, they deteriorate, fill up with debris, and grow as seasonal temperature variation allows for the joints to collect more debris and widen.

 

[ARLORD]

hokie66,

Why mention expansion joints, only because ACI lists expansion and contraction joints as movements joints. ACI 350 recommends movement joints in water retaining structures. I have a 30ft high retaining/tank wall that will be 292ft long. One of ACI codes recommends expansion joints in walls more than 150ft long.

I've never seen a subject in structural engineering where opinions vary so much for such an important matter. I can see how the spacing of joints can be hard to pin down, but I can't determine when to use an expansion and/or contraction joint. Or just use construction joints.

I like how the codes are vague enough to mention each type of joint for each application, walls, slabs, etc, but they do not specify when to use them.

 

[ARLORD]

I have a 280'x120'x30' deep buried water retaining tank, inside dimensions. My preliminary design: The walls width will be stepped, 3ft think at the base for 8ft, 2ft thick for 16ft, and the top 6ft will be 15" thick. The slab will be 30"-36" thick, approx 292'x132'.

I currently plan to use construction joints in the slab at 40ft oc. One expansion joint in the long walls at mid length, 146', and contraction joints in the wall at 40 ft oc. No expansion joint in the slab. Still not sure if I want to stop the reinf at the contraction joints, leaning toward not doing so, which will make them construction joints at 40'oc. I will be using the max reinf ratio of 0.005 per ACI 350 in walls and slab.

 

[miecz]

Hokie66 is right, there won't be any expansion in a tank of unheated liquid. I think ACI350 is recommending a more effective joint for long tanks, and used the common term "Expansion Joint" when they really meant was Isolation Joint, which is the same thing. I think sometimes we nit pick semantics on this forum.

 

[JedClampett]

If you could build the entire vessel, instantly fill it, leave it full and never take it out of service, you wouldn't need any expansion joints. But the tanks are subject to temperature deltas until they are filled and when they are occasionally taken off line for cleaning or repairs.
And knowing some members of the ACI 350 committee, they don't use any loose semantics. When they say expansion joints, they want a joint with a defined gap that can accomodate growth.

 

[miecz]

JedClampett

Good point. Tanks can be down for repairs for months.

 

[fa2070]

While we're at it, can anyone explain the difference between construction and expansion joints?
Expansion joints are designed to accommodate growth from temperature deltas whereas construction joints come from scheduled interruptions in concrete pouring (e.g. slabs), but at the end of the day both types of joints represent discontinuities in the slab that must be waterproofed and sealed. So, if the figure shows the typical expansion joint, how different is a construction joint from a detailing point of view?

Thanks.

 

[ARLORD]

In a construction joint all the reinforcing would continue thru the joint. You also would not have the gap with filler in a constr joint. I wouls also add a key in the constr jt. The water stop would be included in a constr jt.

 

[fa2070]

So, a construction joint is very similar to a contraction joint. Right?
I found this FAQ on PCA's website: Q: Joints: What Type are Used and Where?

 

[hokie66]

In the joint in fa2070's post pictured above, how would you get concrete under that waterstop? The detail might work for walls, but not for slabs.