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Help ... concrete expert required.
plentywahalla - 20/12/13 at 07:54 PM

I am no expert in building practise but I have mixed concrete before and never had this problem, although the application was a bit unusual.

We were attempting to effect a temporary repair to a sunken rivetted iron barge. We had re-floated it using a 4" salvage pump to overcome the leaks and towed it onto a slip to make a repair good enough to get it to a ship repair yard. We only had about 7 to 8 hours between tides. The proven method for patching is to build a box around the hole and fix some rebar to the surrounding structure and pour in concrete. Canvas is put down first and this acts as caulking as it takes up when wetted and the weight of the 8 inch thick concrete slab and the rebar holds it in place.

Well it all went wrong. The owner of the slip wants the barge shifted as soon as possible and it's costing money. Also with xmas coming up the tides were far from ideal. We poured the concrete yesterday at about 3 pm and mixed it with Febspeed accelerator to bring the setting time down to 2 to 3 hours. This morning the concrete has cracked all over, it appeared swollen and had come away from the wooden box and could be easily broken up with a pick and could be crumbled by hand.

So what did we do wrong? it could be one of three things I think.

First frost damage, although the night wasn't that cold but there was frost on the deck this morning. There is a gap under the hull and there could have been wind chill on the wetted iron hull.

Secondly I could have got the mix ratio of the accelerator wrong. The mix was a quite strong 25kgs cement and 100 kgs ballast. To this I added 2.5 litres Febspeed straight into the mix. Was this mix ratio right and also should it be added to the water first?

Thirdly we were unable to get the bilge completely dry and ended up adding the concrete into about half an inch of salt water. Would salt water affect the contrete?

Help ... I can't afford to get it wrong twice!

steve m - 20/12/13 at 08:03 PM

I can not answer about your mix etc, but in sussex this morning it was +1 at 0630, and my car was icier than the polar cap
so an educated guess, was frost damage/or the water content freezing, especially if it was out in the open

steve

nearly done - 20/12/13 at 08:07 PM

Few things spring to mind, what was the weather like last nigh? up here in Derbyshire minus 3 more than enough to cause havoc with any wet conny secondly the mix is well to strong - concrete is usually 5:1:1 not as you mixed. The accelerator should have mixing guides on the pack as to what ratio to add it too. I think you may have "over egged the cake" as it were.

Cheers, Sean.

mookaloid - 20/12/13 at 08:12 PM

I'ts a few years since I got my city and guilds in Concrete Technology, but the usual thing to go wrong with this sort of special mix is too much water being added. If the water/cement ratio is too high then the additive won't make any difference.

you should ad enough water to just about be able to work the concrete but it should more or less stand up on it's own. If you know what slump is then I would be looking for a low slump reading of around 20mm or so I think.

If I was doing the mixing, I would dry mix the aggregate and cement, then put the additive in followed by just enough water to achieve the above.

After placing the concrete - preferably using a vibrating poker to get all the air out, you need to protect it from the cold. The warmer it is the better. If it did freeze then it would kill the reaction which causes the cement to set.

Hope that helps

Mark

MP3C - 20/12/13 at 08:35 PM

You should never mix concrete below 5 degrees otherwise it doesn't set properly. Also does that feb have any guidance when being in a salty enviroment, could have changed something making it produce a gas and blow out the mix.

jacko - 20/12/13 at 08:37 PM

Don't think salt water would help you at all

emsfactory - 21/12/13 at 01:09 PM

I would go with cold temp. I always took the mixing amounts as a minimum amount for concrete etc. so cant see it being too much cement. As far as im aware concrete sets better underwater than in air. Although pouring in a puddle would just water it down more. Problems with that explained above.

Dont know about the accelerator, not used that before. Generally try and only do concretey stuff in the summer.

jossey - 21/12/13 at 07:24 PM

Cracks are water vapour freezing.

Bare - 22/12/13 at 03:26 AM

a Sunken Riveted Iron Barge.. Really!? That thing is at best Victorian!
And you've wasted Time AND money on this.
It's Older than Moses and every bit as useful
Caveat Emptor Kiddies.

Peteff - 22/12/13 at 10:38 AM

Could you drill and plate it temporarily to keep the water away from the concrete. It's all to do with the chlorides reacting with hydroxides in the cement from what I remember. Here you are
Chlorides
Chlorides, particularly calcium chloride, have been used to shorten the setting time of concrete.[1] However, calcium chloride and (to a lesser extent) sodium chloride have been shown to leach calcium hydroxide and cause chemical changes in Portland cement, leading to loss of strength,[2] as well as attacking the steel reinforcement present in most concrete.

It's not had time to attack the rebar but it will have affected the cement component in the concrete.

MikeR - 22/12/13 at 12:33 PM

quote:
Originally posted by Bare
a Sunken Riveted Iron Barge.. Really!? That thing is at best Victorian!
And you've wasted Time AND money on this.
It's Older than Moses and every bit as useful
Caveat Emptor Kiddies.


another helpful post. if youve bothered to follow the site youll know salvage is what the poster does for a living.

plentywahalla - 22/12/13 at 08:40 PM

Thanks all (but one!) for helpful advice. I know what went wrong so next time we will be better prepared. I shouldn't be pressured by the yard owner to work in less than ideal conditions.

Quick note to the feckwit above .... 70 tonnes heavy iron worth about £0 stuck out in the mud. Worth about £10,000 in a shipbreakers!