Concrete: setting time ?

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Concrete: setting time ?

Postby Shastadad » Sat May 05, 2012 9:27 am

There are several threads here regarding curing of concrete but I would like to know what is the minimum time that is needed for concrete to "set"

They have finally torn down and are redoing the perimeter wall of my development, this time using an inverted T design foundation, and I have been praying that they would finish before the "summer" rains started

Yesterday they filled in the upper forms at about 3 PM and it started raining around 7 AM this morning

Was that enough time for the concrete to set up ?

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Re: Concrete: setting time ?

Postby cooked » Sat May 05, 2012 10:24 am

yes the concrete will be ok. They cast concrete in tidal areas where it gets covered by sea water twice a day. On larger castings when it is very hot contractors often use sprinklers during hot weather. The concrete is setting, a chemical reaction, the water combines with the cement to form concrete. Many people refer to this process as drying, well you don't want it to dry out, you want it to set.
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Re: Concrete: setting time ?

Postby sirineou » Sat May 05, 2012 3:27 pm

There are two, concepts in concrete, setting.
one is the set time that can be as little as a couple of hrs, and there is the curing time that can be years.
Where I work, which is the concrete high-rise construction industry in NYC, when we have a small floor plan, we work on a two day cycle, That is we will erect a floor every other day. After we pour a floor,we could be walking on it and erecting the forms for the next floor, in a couple of Hrs, that means the concrete is Set enough to walk on, but has not developed self supporting strength yet.once the concrete has set, a little rain is nor bad, in fact it is a good thing in the summer time when it is dry and you want to keep the concrete hydrated.
The next day, the forms can be stripped, but screw Jack re-shoring must be placed to support the floor ,every 8 ft the re-shore must stay in place at least 3 weeks,by that time, the concrete has cured to sufficient strength to support it's self with out deflecting.
The length of time it requires for concrete to fully cure, depends on many factors, some of which are, Thickness, concrete composition, and environment, and can vary from a couple of years to tenths of years.
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Re: Concrete: setting time ?

Postby MGV12 » Sat May 05, 2012 4:06 pm

sirineou wrote:There are two, concepts in concrete, setting.
one is the set time that can be as little as a couple of hrs, and there is the curing time that can be years.
Where I work, which is the concrete high-rise construction industry in NYC, when we have a small floor plan, we work on a two day cycle, That is we will erect a floor every other day. After we pour a floor,we could be walking on it and erecting the forms for the next floor, in a couple of Hrs, that means the concrete is Set enough to walk on, but has not developed self supporting strength yet.once the concrete has set, a little rain is nor bad, in fact it is a good thing in the summer time when it is dry and you want to keep the concrete hydrated.
The next day, the forms can be stripped, but screw Jack re-shoring must be placed to support the floor ,every 8 ft the re-shore must stay in place at least 3 weeks,by that time, the concrete has cured to sufficient strength to support it's self with out deflecting.
The length of time it requires for concrete to fully cure, depends on many factors, some of which are, Thickness, concrete composition, and environment, and can vary from a couple of years to tenths of years.


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Re: Concrete: setting time ?

Postby geordie » Sat May 05, 2012 4:39 pm

MGV12 wrote:
sirineou wrote:There are two, concepts in concrete, setting.
one is the set time that can be as little as a couple of hrs, and there is the curing time that can be years.
Where I work, which is the concrete high-rise construction industry in NYC, when we have a small floor plan, we work on a two day cycle, That is we will erect a floor every other day. After we pour a floor,we could be walking on it and erecting the forms for the next floor, in a couple of Hrs, that means the concrete is Set enough to walk on, but has not developed self supporting strength yet.once the concrete has set, a little rain is nor bad, in fact it is a good thing in the summer time when it is dry and you want to keep the concrete hydrated.
The next day, the forms can be stripped, but screw Jack re-shoring must be placed to support the floor ,every 8 ft the re-shore must stay in place at least 3 weeks,by that time, the concrete has cured to sufficient strength to support it's self with out deflecting.
The length of time it requires for concrete to fully cure, depends on many factors, some of which are, Thickness, concrete composition, and environment, and can vary from a couple of years to tenths of years.


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Definately on the button concrete is curing all of its life ?? As a teenager i was asked what is the lifespan of concrete ? i guesed at 50 to a 100 years but was informed that 100 years was the correct answer ? " it stops curing after that "" its deteriating then !! there are still good examples of roman concrete around i have always thought if it was levelled before submesrsion it would be fine and in most cases a good thing
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Re: Concrete: setting time ?

Postby cooked » Sat May 05, 2012 10:08 pm

hmmm... I suppose setting is a part of the curing process. The concrete initially increases in strength rapidly. Concrete increases in strength throughout its life as the last chemical reactions slowly occur. An engineer calculates concrete dimensions on the basis of its strength at 28 days. As long as the rebars don't become exposed, the water is clean and there are no pollutants in the aggregates, there is no reason why it shouldn't live for ever.
From a contractors point of view of course, there are indeed two (three) concepts, that is: 'can't walk on it yet', 'can walk on it, but be careful' and 'you can take the form work away now'.
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Re: Concrete: setting time ?

Postby Mike Judd » Sun May 06, 2012 7:53 am

I have mentioned it on another Post, and as having a great deal to do with so called "Concrete Cancer" in Sydney, where water gets through to the Rebars , which expands as it rusts, blowing the concrete away in big chunks. A large university in the U.S. did a long term study on exposed concrete and found that moisture penetrated concrete on a average of 1m.m. per year and when it reached the Rebar it took 1 year to go around it and start the expansion. When you realise that the average cover is a max of 25m.m. sometimes a lot less with poor placement or movement while pouring the concrete itself.(Thailand is not the only place where shoddy workmanship occurs) You can imagine the drama with lumps of concrete raining down on Sydney streets a few years back, that was because the trend in the 60's and 70's was for all the High Rises to be clad with Precast panels usually about 100 m.m. to 150 m.m. thick and lots just bare concrete finish. Obviously the closer the steel to the surface, the sooner the problem occurred.
Solution ? either a great deal of cutting out and patching or on the more prestige buildings, cladding and sealing the whole building with aluminium panels to stop any further water penetration. You will notice that most High Rises are now finished off in aluminium and glass Curtain Walling, it's the most efficient way of covering the out-side of buildings.
Concrete will last a very long time, it's the steel that gives it it's tensile strength that's it's weakness in the end ,depending on the amount of cover.
I would always recommend stopping water penetration to start with by at least painting exposed concrete columns/beams etc;
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Re: Concrete: setting time ?

Postby jazzman » Sun May 06, 2012 8:58 am

The formwork/trenches for the ground beams can be lined with impermeable PVC foil that stays in place for extra protection from ground water. Adding waterproofing agent (namya ganseum) to the mix of any concrete that is going to be permanently or regularly exposed to damp and the elements will also help. It's not expensive.
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