Problems pouring high strength concrete
Problems pouring high strength concrete
(OP)
Hi - would appreciate some advice - We are having considerable problems on a sea defence contract. We are encasing original walls with concrete and they are not vertical but slightly sloping. The concrete is C40/50 and is going off extremely quickly, within around 30 minutes and is curing extremely quickly.
On striking the shutters, despite very thorough vibration, we are being left with unsightly holes, and it almost looks like a sort of peeling away of the surface of the concrete in some areas, not just blow holes but 2" gaps and it looks a mess.
We have never had this problem before and are known for the quality of our works.
We are wondering if the concrete design could be the problem? We have never worked with concrete that "goes off" at this rate before and any short delay between loads is making it impossible to avoid a cold joint forming as the previous load has already gone off.
This is causing problems for us as we are the subcontractor and the finger is pointing at us. We are using good quality Ply for the formwork, we are making sure that the shutters are being thoroughly vibrated but are at a loss to know where the problem lies. I really don't think the problem lies with bad workmanship. We have a very short window to get the concrete placed before the tide comes in and presumably that is why the concrete has to cure very quickly.
If anyone could give me some advice I would be really grateful - trying to resolve as have a lot of concrete to place.
On striking the shutters, despite very thorough vibration, we are being left with unsightly holes, and it almost looks like a sort of peeling away of the surface of the concrete in some areas, not just blow holes but 2" gaps and it looks a mess.
We have never had this problem before and are known for the quality of our works.
We are wondering if the concrete design could be the problem? We have never worked with concrete that "goes off" at this rate before and any short delay between loads is making it impossible to avoid a cold joint forming as the previous load has already gone off.
This is causing problems for us as we are the subcontractor and the finger is pointing at us. We are using good quality Ply for the formwork, we are making sure that the shutters are being thoroughly vibrated but are at a loss to know where the problem lies. I really don't think the problem lies with bad workmanship. We have a very short window to get the concrete placed before the tide comes in and presumably that is why the concrete has to cure very quickly.
If anyone could give me some advice I would be really grateful - trying to resolve as have a lot of concrete to place.






RE: Problems pouring high strength concrete
Tell us a bit more:
Concrete properties: Portland cement-based concrete? Slump? Plasticiser? Etc
Environment: hot weather?
New wall face: what is the slope of the new formed face? Added wall thickness? What length and depth are the wall segments ( or how large a volume are you placing each pour?).
Exterior form vibration, or needle vibrators, or both?
Lots of rebar?
RE: Problems pouring high strength concrete
there are two causes to this issue that I commonly come across.
1. lots of vibration but not correct vibration. The contract vibrates the air out of the middle of the concrete but because of steel ect doesn't get the edge very well, air unlike water migrates vertically as well a horizontally. The air then sticks to the forms and now you have bugholes. This is normally with high strength concrete that has high cement contents and not much slump/bleed water. This can also be cause by vibrating the wrong areas first and last, always vibrate the forms last.
2. The forms are two good. The forms are put in place too well generally not allowing any air out, quite often there will be form realizing agents in place.
The first thing I do is start with vibration, I make the vibration person vibrate from the bottom up and they must come up with the air, rather than the normal level vibration that many think is fine. I also make sure they vibrate thru the cold joints. 2'nd is review the form work, maybe get some permeable form work or provide some air holes.
I recommend doing some test vertical surfaces at the concrete plant, seeing how long you can vibrate until segregation occurs and seeing how long vibration is required before air release, in high strength concrete this can be longer than you normal concrete. you will then have some numbers to tell the guy on the vibrator.
http://www.nceng.com.au/
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RE: Problems pouring high strength concrete
RE: Problems pouring high strength concrete
You didn't mention applying a form release agent.
You stated that the shutters are being vibrated. Does that mean you are relying on external vibration rather than internal?
How thick are the encasements?
RE: Problems pouring high strength concrete
This is a high profile contract and we want to do the best possible quality work that we are more than capable of, but we have no control over the concrete we are supplied with and our men are doing their best under difficult circumstances. I think I know more about concrete than the site manager running this project, as he has emailed me and said that "our men must stop allowing the concrete to cure and must keep it live". Not sure how he thinks they can do that!
Am attaching the Spec from the concrete supplier and also the spec from the Works Information, it is the C40/50 mix we're having trouble with. If you could take a look and see what you think. Are they the same? Our main contractor is not happy about the large voids and neither are we, and I can't understand why they won't listen to our concerns about this concrete? Possibly because the mix is out of spec? If it turns out that it's just bad workmanship then we will happily accept this and take responsibility, but I'm sure it is not and we do not want to be crucified financially.
RE: Problems pouring high strength concrete
The mix design appears to be "oversanded". The fine aggregate percentage of the total aggregate is 43%. That makes the mix "harsh" and more difficult to place. I typically shoot for around 38 percent fine aggregate. This much sand increases the water demand and the paste volume has to be higher to get coverage of the increased aggregate surface.
The slump is way too low for the application and there does not appear be any flowability aid provided. I would expect a 50mm slump for externally vibrated flatwork, but for formed concrete with internal vibration, I would prefer to see the slump in the 125mm range.
RE: Problems pouring high strength concrete
@Ron: The quoted "consistency class" (UK terminology) is "S3", not 53mm. Class "S3" has a slump range of 100 - 150mm, with tolerance -20, +30.
RE: Problems pouring high strength concrete
In either case, the mix design is not being properly controlled. If too dry at the discharge point, then it is likely that moisture compensation is not being properly applied at the plant and/or the aggregates are not in a saturated, surface-dry condition at the time of batching. An oversanded mix makes all of those conditions more difficult to deal with.
Ingenuity...thanks for the clarification.
RE: Problems pouring high strength concrete
Am in a difficult position here, as I feel I'm winding up our main contractor, who holds the purse strings! My only intention is to nip the concrete issue in the bud so we can produce quality works. This can surely only be of benefit for all as the concrete is the visible result. I sent an Early Warning notice very early on so we could get together to try and sort this out. I got quite a hostile reaction when I sent the Notice even though this is what our contract requires us to do.
I'm still not sure whether the Mix they have ordered from the concrete company is the same as Specified under the Works Information (sent as an attachment yesterday).
I've put our best, highly skilled operatives on this contract. I know what they are capable of, and apart from sometimes getting a small amount of the normal allowable small air holes, we have never produced works with these large unsightly voids. The fact that these coincide with the over dry mix cannot be a coincidence.
Instead of working with us to find a solution, the site manager has very obviously taken it as an insult that I have "dared" to have brought this up. He may be highly qualified for what he does, but he has not been out on site to look for himself, thinks our men should "keep the concrete live and stop allowing it to cure", and has never poured concrete in his life. The works are challenging enough as it is, as a good part of it is tidal, there have been unexpected storms in the last month where the steel reinforcing has been smashed to bits, formwork has had to be very quickly removed as the storms have brought the tide in very suddenly, newly laid concrete has been washed away, the main contractor has not ordered enough concrete so we have had to stop half way through a pour. and last week the concrete plant broke down for 2 days etc. etc. but all we are asking for is for them to give us concrete that is workable.
Sorry to go on, but very frustrating!!
RE: Problems pouring high strength concrete
Hence I would recommend that you study the contract very carefully to see what your rights and notices are, possibly employ your own inspector or competent engineer to monitor concrete on arrival, takes photos, notes, times possibly even take the odd sample for record purposes and send the notices the contract requires to show that you are not happy with the materials being provided by the main contract and that they are not being provided to specification. Sure you'll get some push back from the main contractor, but I suspect he knows deep down he's on shaky ground (no pun intended) and for some reason doesn't want to go after the other subcontractor (the concrete supplier) unless he is actually making it himself.
You might need to actually reject a couple of loads before anyone starts to take real notice, but records, more records and physical evidence will stand you in good stead if the lawyers letters start flying around. Keep notes up to date as you can forget things.
It certainly sounds like you're doing the right things to get a quality job - I assume the client has some inspectors / engineers looking at this? - You need to appraise him/her of the situation as you see if before the main contractor starts pouring all the blame onto you and your workforce.
Good luck and let us know how you get on.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Problems pouring high strength concrete
RE: Problems pouring high strength concrete
RE: Problems pouring high strength concrete
It may get worse before it gets better, but you need to safeguard your business and reputation.
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Problems pouring high strength concrete
RE: Problems pouring high strength concrete
So use four, five or six simultaneous delivery pipe locations where "normal practice" might be using two or three in that same length of form. Try using a higher delivery pressure - but that will likely be difficult.