Problems pouring high strength concrete

[christina2014]

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.

 

[Ingenuity]

Welcome to Eng-Tips!

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?

 

[rowingengineer]

We call them bugholes.

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/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[BUGGAR]

Set retarding admixture. Like Daratard.

 

[hokie66]

Agree that a retarder would help with the premature hardening, but perhaps that doesn't mesh with your "short window".

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?

 

[christina2014]

We are using a high quality paper faced ply, all forms are cleaned and sprayed with an environmentally friendly mould oil - the same as we use on countless other contracts with no problems. We are the subcontractor and the main contractor is supplying the concrete. We have been reporting problems from the first pour with the workability of this concrete, have been suffering delays between pours and from the time it gets to our operatives it is "going off" within 20-30 minutes. I have sought advice from the Concrete Society and shown them the concrete spec from the supplier and they think the microsilica content is too high and instead of a 40/50 strength it is probably more like 70. It should have S3 slump but on occasions our operatives have had trouble getting it out of the dumper bucket it's so dry.

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.

 

[Ron]

A couple of quick comments....

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.

 

[Ingenuity]

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.

@Ron: The quoted "consistency class" (UK terminology) is "S3", not 53mm. Class "S3" has a slump range of 100 - 150mm, with tolerance -20, +30.

 

[Ron]

Ingenuity...sorry, misread the OP. I saw the S3 consistency class on the mix design, but didn't know the terminology. If the slump were delivered in the S3 class as you have designated it would have been correct for the application; however, the OP noted that it was "dry"....leading me to believe a low slump.

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.

 

[christina2014]

Gentlemen - am so grateful for you taking the time to respond. Our operatives are quite sure the slump is nothing like an S3, at times they have struggled to get the mix through the steel reinforcing and this can't be right. The main contractor insists the concrete is being slumped on arrival, but I have heard otherwise. Would they have to record this? So far I have only received one delivery ticket. Have you any thoughts on the (I've been told by the Concrete Society) high microsilica content.

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!!

 

[LittleInch]

I fear this will not end well for you, but it seems clear from the comments above that the mix is not conducive to good workability. You need to start gathering evidence now for the inevitable fall out and finger pointing which is likely to ensue when faced with such an apparently obstructive main contractor.

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

 

[cvg]

you might try some independent QC to test slump and other properties of the concrete which it appears is wholly inadequate on the project. Also, you certainly should be able to get copies of all the delivery tickets and onsite tests which are being done. If they are not being done, then you have a legitimate complaint. I know you want to be a team player and not step on toes, but somebody is going to end up under the bus on this one. Better to raise a lot of red flags then to be the scapegoat on this project.

 

[christina2014]

One of specialists at the Concrete Society has looked at the spec and could see straight away why we are getting problems, you kind gentleman have also given me lots of information. I feel a lot more confident about tackling this head on now. The site manager's response to the detailed information given us by the Concrete Society was "well that does not tell us anything", but it told him everything. I'm now at the point where I will have to go way above his head. Is it our responsibility under our contract to bring any matters that might affect the works to their attention so we can work together to resolve the situation. Instead of this he simply won't acknowledge that there could be a problem with the concrete, but simply says it's coming as the spec, and continuing to send emails regarding the "bad quality of our works". I suspect however that in the background they are addressing this but will never pass the information on to me. I will request copies of the delivery tickets and slump tests, all I have at the moment is a handwritten list of slumps but no proof that these have actually been done. From speaking to me every day, since I sent the Early Warning Notice he has not spoken to me at all and his response to our contract manager on site was "I'm going to send you notices every day now". Not the actions of a professional site manager, more like a sulky child.

 

[LittleInch]

From what you've said I tend to agree, but only when the adults get involved will you resolve this. In the meantime you must start doing your own slump tests and taking some samples / photos / videos and recording them so that every time he sends you a notice saying the workmanship is poor, you can send him one back with details saying the concrete is poor, not the workmen....

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

 

[christina2014]

Hi Littleinch - am arranging as we speak............

 

[racookpe1978]

One needed factor for what seems to be a very "thin" section over existing concrete is to be sure the delivery pipe goes all the way to the bottom of the form, then is pulled up at the same rate as the form fills. Clearly, with the rebar in place and the very, very stiff mix, it won't flow sideways very far either.

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.