Author Topic: New High rise buildings – Amazed how fast the concrete sets  (Read 2192 times)

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Offline Beachman

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I work in a high rise building in the city and across the street they are building a new high rise. At 10.30am they just finished pumping concrete to whatever level they are currently working on. At 2.30pm the crane is in full swing loading a couple of truckloads of scaffolding/supports for the next level on the freshly laid concrete.

I remember getting a driveway done many years ago and wasn’t allowed to walk on it for a couple of days, let alone put a couple of ton of building material.   

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #1 on: March 05, 2019, 04:26:40 PM »
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Offline Hairs

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #2 on: March 05, 2019, 04:54:33 PM »
Things are changing, I thought it had to cure for X amount of days.


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Offline GBC

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #3 on: March 05, 2019, 05:17:30 PM »
They will be pouring the next deck in 3-4 days with the high early strength concrete they use. The decks are all back propped. There is no getting around the fact it takes 28 days to reach max strength and the props will be left in place for quite a while while it cures but yes, the next props and Formwork will be up tomorrow with guys tying steel and running services the day after that if the weather holds. It’s is highly organised chaos.

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #4 on: March 05, 2019, 05:30:10 PM »
They will be pouring the next deck in 3-4 days with the high early strength concrete they use. The decks are all back propped. There is no getting around the fact it takes 28 days to reach max strength and the props will be left in place for quite a while while it cures but yes, the next props and Formwork will be up tomorrow with guys tying steel and running services the day after that if the weather holds. It’s is highly organised chaos.
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Offline GGV8Cruza

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #5 on: March 05, 2019, 08:25:36 PM »
Post tensioning in slabs also removes a lot of steel and gives an  early strength. Once fully stressed it holds up a lot of the building
 
The concrete we also use now is higher MPA so it gets up to strength a lot quicker.

I play with concrete most days of the week at the moment.

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Offline chester ver2.0

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #6 on: March 06, 2019, 07:56:21 AM »
As Cruza said

They also now stick temp probes in them and when the probe says the concrete is a certain temp then it is strong enough to support the next level
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Offline Livebait

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #7 on: March 08, 2019, 11:08:21 PM »
Most Highrise concrete slabs are poured using a 40Mpa or 50Mpa Post Tension (PT) mix, this mix will allow for a high early strength of between 22 - 25Mpa in 3days from slab pour, this allows for the stressing of the PT cables within the slab.  If its less than 22Mpa then generally they are delayed by a day as the slab should not be stressed until after22Mpa.  After the initial early strength the concrete will slow down and should be at full strength at 28 days.
As for loading of the decks after a slab has been poured, this happens quite alot as once the slab has been finished and can be walked on then they generallly will load Pans and formwork on it to save time the next day.  it doesnt matter to much if they leave a few foot prints in the surface as they all get covered anyway.  only podium and carpark levels will they require a faultless finish as it does not get any floor coverings over it.
These days the Columns are poured using 65-80 or 100Mpa.  this allows for a higher strength earlier which means the props and formwork can be removed sooner as the columns are strong enough for load bearing.

Driveways and houseslabs are generally 20-25Mpa and after 7days they can still be as low as around 12-18Mpa, escpecially during winter months when there is a cold night time air temperature.  Concrete requires temperatures of  6Degrees or abouve  so the air can cause the hydration process and remove the moisture form the poured concrete, any lower and the concrete curing slows right down.

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Offline #jonesy

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #8 on: March 09, 2019, 06:21:28 AM »
Okay, another builder type question.
How long can the timber framework remain exposed? 

I get that while it's being put up the frame and timber flooring can get rained on and would be okay. But near work there is a few townhouses where obviously the builder went belly up, just before the roof went on.
They have sat there for about a year fully exposed and all the frame is a nice grey colour. No idea what the floor would be like.
Someone has obviously come in with the money and they are off again as if nothing happened.
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Offline GBC

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #9 on: March 09, 2019, 06:34:47 AM »
We have taken on and finished a number of projects for qbsa and banks like what you describe. It all comes down to exactly what studs and flooring was used. If it is lightweight units with yellow tongue flooring it may well be a bin job after a year. There are floor sealing compounds which may have been used as well, you never know. We will cut holes in some floors to drain them if we know a week of rain is coming. Unfortunately most jobs that go to the wall have woeful work practices in the first place though.

Offline Rumpig

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #10 on: March 09, 2019, 07:51:26 AM »
Okay, another builder type question.
How long can the timber framework remain exposed? 

I get that while it's being put up the frame and timber flooring can get rained on and would be okay. But near work there is a few townhouses where obviously the builder went belly up, just before the roof went on.
They have sat there for about a year fully exposed and all the frame is a nice grey colour. No idea what the floor would be like.
Someone has obviously come in with the money and they are off again as if nothing happened.
being you seem to be in Victoria I'll assume the frame has seen a decent amount of rain in that 12 months?, I wouldn't be buying one of those units with my hard earned $$$$. Any treatment on the framing timbers (assuming they used something like H2 treated?) will have been effected, to what degree is anyone's guess as it really depends on how much water it's seen....who knows what bottom plates have sat in water pooling on the slab or wet area step downs and for how long also, what rot has started in those cases if that occurred?. The same goes with flooring as mentioned above,  depending on how much rain it's seen it's highly likely the particalboard floor (yellow tounge etc) if used will be rooted, manufacturer says 3 months left exposed is it's limit. Whatever the case the carpenter's planer and coilgun will get a workout getting studs / walls straight ready for sheeting, but that's just cosmetic stuff, the other stuff I mentioned is the real concern further down the track.
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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #11 on: March 09, 2019, 06:35:50 PM »
We even did some slabs with tension sensors cast into them and they got checked over the following months

Offline SJindustries

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #12 on: March 11, 2019, 08:11:31 AM »
I play with Grey Gold most days of the week at the moment.

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Offline Beachman

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #13 on: March 11, 2019, 11:57:59 AM »
Okay, another builder type question.
How long can the timber framework remain exposed? 

Couple of years ago in our street an old house was knocked down in favour of a new house, Frame goes up but for whatever reason the house was put on hold for 3 months and during that time we had a lot of rain. (No roof)

Builder comes back after 3 months and finishes it off and put it on the market. Nice house so sells fast. 4 months later my sparky mate was asked to disconnect some light/power switches as the new owners have lodged a claim under the builders insurance.

Apparently that length of time with the frame being exposed has now meant the frame has now dried and shrunk. The walls of the hallway were no longer straight, large percentage of plasterboard had cracked, tiles in the bathrooms falling off. 

Offline Beachman

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #14 on: March 11, 2019, 12:02:12 PM »
Also thanks to everyone who replied as very interesting how times have changed.

Offline #jonesy

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #15 on: March 12, 2019, 04:30:42 AM »
Took a pic if of the building. I doubt the line was in any way treated given how black it is. With the speed that the works have restarted I doubt they have replaced any timber.
It is one of those bulldoze a house, squeeze in as many units and flog them off as soon as you can. 
When the first builders were putting on the roof trusses on the third floor the scaffolding was only as high as the second floor and no harness
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Offline GBC

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #16 on: March 12, 2019, 05:22:12 AM »

When the first builders were putting on the roof trusses on the third floor the scaffolding was only as high as the second floor and no harness

Welcome to my apprenticeship haha. We used to build units around Moorooka 3 floors, 10m span trusses, no crane, no scaff. Someone had to climb up and nail off the first brace.... all good fun.

Im not passing judgement on your pic. Frames which have sat for that long need care to make sure they aren’t squeaky. I’ve seen worse than that finished off though.
It's not the sort of work you want to send inexperienced subbies to do though, there is a lot to look for and test. Looks like it might be a tile roof which will kill a few squeaks pretty quick.
The scaff has stayed on site all that time? They must have owned it.
« Last Edit: March 12, 2019, 07:29:22 AM by GBC »

Offline #jonesy

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Re: New High rise buildings – Amazed how fast the concrete sets
« Reply #17 on: March 12, 2019, 02:49:02 PM »
I had to crop the photo pretty hard to remove the company names at the top and bottom. Some of the scaffolding has only been recently added. I cant recall how much was there for the year of inactivity
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