Concrete Quality: How Bad Can It Get Before Structural Compromise?
Hello all,
Just a quick one to spark some debate and follows on from Ed’s the other day. I have been involved with the vertical elements of my project site which include columns and outrigger core walls. The pours have been going well but on opening the forms up today a number of elements have had issues. Multiplex have referred these issues to the consulting engineers and are waiting on their response for the required remedial action or acceptance as they see fit. Some of the elements where poured on different days using a mixture of kibbling and pumping. All are 80mpa which generally is being reached well before the 28 day mark with the columns feelably hot. The mix is designed to be self compacting with no requirement for vibration but the supplier has been having issues with producing consistently accurate mixes. These elements are on level 7 of 82.
Column 1 – Evidence of very small honeycombing and cold pour joints
I would suggest that no rectification is required for this column. Your thoughts?
Column 6 – Evidence of small honeycombing and cold pour joints
Again I would suggest that no rectification is required for this column. Your thoughts?
Column 7 – Evidence of cold pour joints and large scale honeycombing which is exposing reinforcement.
My feeling on this one is that rectification will be required. I think that as a minimum they will ask for the concrete to patched to cover over the steel work but that more extreme remediation could well be asked for.
Column 10 & Outrigger Core Wall 8 – Evidence of small honeycombing and cold pour joints isolated to a single batch of concrete. This was kibbled and you can clearly see the flow of concrete away from the column, which was the insertion point, into the rest of the wall.
As before I would suggest that no rectification is required for this column. Your thoughts?
Multiplex should be getting the consultants response shortly so I will update the post with this detail once received and you will be able to compare your thoughts to that of what actually happened.
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I’ve no idea
I would like to know more about the mix design for self compacting concrete
I can see that, if the elements are very large ( like the column section you show, vibrating might be difficult. I presume the mix has a fairly uniform aggregate and some kind of pasticiser?
On the wall elements I can’t see why you wouldn’t vibrate.
The bubbles you see I know are sometimes associated with the inability to move bubbles out of the mix. You see this in vertical elements close to the top of the pour. In the past I’ve asked for re vibrating in the plastic state. People raise their eyebrow when you ask but we’ve done it for ages
The acceptability of the finish, if not compromising the fire resistance/ structural integrity, is largely dependent upon the client’s/ architects requirements aesthetically. We used 80 mpa concrete recently on the tree columns that I have mentioned in a previous post. Also self compacting and it gave an excellent finish as per the clients demands (I’ll find the mix design for you). However we also used it in the slipform and you saw the result of that… not so good. Despite the requirement for extensive repairs, we weren’t so bothered about aesthetics on the cores as they will mostly be covered up. I imagine that may the case with your column too.
Just thinking about the reasons for your air entrapment. Because I have a concrete fetish I did a bit of reading on placing of self compacting concrete. To ensure good quality finish, I read that even though it is fluid the rate of pour has to be controlled to allow entrapped air to escape. Ideally it should be placed with minimal free fall to minimise the entrapment of air. Was it poured quickly or dropped from a height?
Hi Ben,
I’d be intrigued to know the outcomes for these issues.
I think you are right about columns 1 & 6 (the first 2). In the UK I would expect the contractor to ‘bag off’ i.e. a labourer rubs a lean mortar mix over the surface using a hessian sack to fill the blow holes if finish is important. Column 7 should have been broken back and made good if it is being relied upon as part of the stability system unless there is substantial redundancy wherupon perhaps patching up might suffice, after all there is a gamma factor to fall back on for just this reason. again I suspect you are correct for column 10 and the associated wall.
Eds comments on SC concrete above are worth a read and I would only add that there can be merit in vibrating either with a poker or through extrnal vibrators on the form just to aid compaction.