Expansions and Contraction Joints 1

[wvu4646]

I work for a Gerneral Contractor that is currently building a denitrification building. There will be 35 filters 11.67' wide x 102' wide. The wier wall is 15.17 high. There are 17 filters on each side of the building. There are 36 filter slabs for 17 filters. All but 4 slabs are 26'x27'x5.75'. There are 8 24" pile per slab (4 rows and 2 columns). The location to the center of pile relative to the slab: 3.083' from left, .417' from right, and 3' from top and bottom. Each pile has an ultimate bearing resistance of 460 tons. The Y-Walls in the filter are centered over the columns of pile. There are only expansion and contraction joints between all of the slabs. There is a foundation keyway between all of the slabs that is .958' deep (T/6) and 1.917' wide (T/3). The expansions and contraction joints continue up between the walls. With no rebar through the joints and the slabs cantilevering over the pile it seems very unusual to have no construction joints. Our company has just finished a denitrification building that only had 2 expansions and contraction joints throughout the structure. Please provide any insight into the reason and logice for the slabs not being tied togeather. I have attached some drawings to reference.

 

[JedClampett]

It looks like the Design Engineer wanted to split up the concrete pours into approximately 30' square pieces, which is recommended for water bearing structures.
I'd like to see the details for his contraction joint. If the bars pass though them, then I would call it a construction joint and have no problem. If they stop I would like it less, because these joints are more likely to leak, but being on piles would minimize any movement, so it shouldn't be a problem either way. And not to criticize anyone else's design, but I would delete the shear keys. They're pretty much worthless.
My question is more on the first building you built. That seems like a lot of concrete with very few joints.

 

[wvu4646]

It is a true "contraction joint" with no rebar passing through. I have attached the standard details. We are concerned about leakage also. Each filter has to be tested in accordance with ACI 350.1 HST-100. We are planning to discuss less stringent test requirements with the Engineer. Testing Per ACI will add quite a bit of time to the project. I did not work on the prevous structure, but some of the engineers from that job are on the current job with me. That building only had a total of 7 filters. The structure we are currently building will be the largest of its kind in the world. There is also a biological airated filter building adjacent to the denitrification filters, but this is a future contract and we do not have the drawings yet. I have been reading quite a bit about shear keys being less favorable. Does the ACI manual adress this specifically? Thanks for the reply.

 

[JedClampett]

Well, the good news is it will be easier for you to build. Since the bars stop, you don't have to have them penetrating your formwork. But I doubt the engineer will eliminate the shear keys.
I still don't like it, and I think it's unnecessarily complicated with the shear keys and all, but having the whole building on piles should cure a lot of problems. I don't think it will leak unless you don't do a good job installing the waterstop.
I would put in construction joints instead of contraction joints, but besides that, I can't find a fatal flaw.

 

[hokie66]

I don't know how you would build those keyed joints without them being construction joints. They are mighty big keys...unless you taper them and break the bond, you risk breaking the key and changing the path of leakage.

 

[wvu4646]

One of the very first RFI's I asked was if it would be acceptable to use a tapered joint for all keys. The engineer approved are request to use tapered joints. I have attached the approved detail with tapered joints.

 

[JedClampett]

Well, it's better, but it's still unreinforced concrete. It's impossible to vibrate no matter what portion is poured is poured first. But some designers love them.
Somewhere ACI mentions that shear keys are overrated. It might be in the commentary. Or maybe it's in some textbook. I'll look for it tomorrow.

 

[wvu4646]

Thanks Jed. We have the 2010 Manual of Concrete Practice 6 volume set. I had to beg for them to get the hard copy for the job. I wish I had the electronic edition to expedite searching. If you you find something it would be much appreciated.

 

[hokie66]

Agree with Jed that the shear keys are not desirable. Because the slab is so stiff and supported on piles, the keys will see little if any load. If the engineer insists on keys, I see no reason why a 2" or 3" key would not be enough, and would not causes the constructability issues of the 11.5" keys you have now.

 

[wvu4646]

The other issue with the key is the location that the waterstop fabrication will be located to tie into the walls. The expansions and contraction continue throught the walls, and with the wall key only about 4" the center of the waterstop fab coming from the slab and from the wall are about 7.5" apart. We would like to propose to delete the shear key in the slab, but will need some back-up information to present to the Engineer. Any references would be helpful.

 

[hokie66]

It is very difficult for those of us on an internet site to give worthwhile advice to a contractor who is involved in an argument with another engineer. However, with what you have told us, it appears to me that there are an excessive number of construction/contraction joints in this structure, keyed or not. Some of the joints serve no purpose and would be better cast monolithically, with the reinforcement extending through.

Knowing nothing about the qualifications and experience of the design engineer, there are nevertheless clues given by the drawings. For example, there are cases where corner bars are shown around interior corners rather than extending to the far face. This is unacceptable practice, and is likely to cause breakout in those corners.

This may be a case where you need to retain the services of an independent engineer experienced in this type work to review the drawings and plead your case.

 

[wvu4646]

The Engineer issued a new corner detail early in the project. The outside face bars have no hook, the inside face bars extend to the outside face and hook, and there is a corner bar on the outside face. I am just looking for some good references to read up on. We feel that we just bring the issue to the attention of the owner and Enginee incase there were some errors in the design they could be addressed prior to the concrete being placed. We would rather address this now, even though a change to construction joints with rabar through the joint would add a great deal of time to the forming.

 

[dik]

The vertical bars at the joint are likely going to initiate any cracking and with corners, I prefer to have the reinforcing extend to the far side of the inside corner... just a couple of comments.

Dik

 

[hokie66]

I was suggesting that many of the construction joints can be deleted, so no formwork or waterstops at those locations.

 

[wvu4646]

That would be even better if they would eliminate some of the expansion/contraction joints. Each slab is around 138 cy. It would be nice to get rid of a few of the joints. I think this is very unlikely to happen at this point. One of the Engineer's made a comment that the structure was designed so it would not crack. We have a meeting with the structural engineer tomorrow morning to discuss this and some other items. Our goal is to have the depth of the keys throughout the structure to be universal. We were able to change the 9" starter wall with a key to just a 3" inverted key. This is applicable at all slab to wall construction joint locations. If we could have it 3" everywhere it would allow all of the waterstop fabrications from the slabs to match the keys in the walls. The way the detail is now will create the waterstop to be offset depending on T of the slab and wall. If we could eliminate the key it would make the forming easier. We could put some sugar paper on the bulckhead and just pressure wash the joint after stripping the bulkhead. I used sugar paper on the Hoover Dam Bypass and it eliminated a lot of labor to roughen the joint. I am curious to see what the Engineer has to say tomorrow.

There are two large structures on this project, and two different contracts. One of the contracts has not let yet. Our contract includes the pile, some grade beams and slabs for the adjacent structure. That structure was designed by a different firm. It is interesting to see how different the stuructures are designed because of different engineers. In the other project at slab to wall construction joints that include rebar there is no key and the rebar bends down to accomodate the waterstop. All of the grade beams in the structure we are building have the same expansion and contraction joints as the slabs we have been discussing. In the other structures there were no expansion or contraction joints in the 3 mudwell slabs (153'-10"x50'). The building only has 2 expnsion joints the the disrection that is 500'. Two very different designs for two similar structures. I attached the foundation for the other structure.

 

[hokie66]

The two structures have similar title blocks, which to me indicates the same firm, but different engineers. Is that correct?

 

[wvu4646]

I believe that the Engineer of Record subbed out the structural design for the BAF to AECOM. I usually call the structural Engineer at AECOM when I have queations on that portion of the contract. I think that RKK will be designing the rest of the structure.

 

[wvu4646]

We had a meeting with the Engineer today. The only problem was that the Structural Engineer was not available. We have sent in several RFI's and are hoping to meet with the Structural Engineer on Wednesday now.