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curing in high rise building

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kier legard
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Hi,

1, Does curing time have any effect on the floor cycle time in high rise building?

2.After casting of the slab in high rise building does curing time have any effect on the start of successive works like col, walls, or can this work starts immidiately after the concrete is dry?

Replies

Larry Bjorn
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kier

You can strike vertical shutters the day after is has been poured. Hence, 2 sets might be a little excessive.

Generally, 2/3 Design Strength will suffice to strike the decking. 2/3 Design Strength is usually reached within a week. Note that some systems allow you to strike decking panels earlier without disturbing the back-propping.

The amount of back-propping is not as straight forward and will required an engineers input. However, from my experience, 2-3 floors will be necessary.

If you are doing a costing exercise, please remember that decking panels need to be replaced as they wear.

Regards,
Larry
Armando Moriles
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Clive,

Your explanation and share of techniques based on your wide range of experiences are very well appreciated.I’m sure that such would help us a lot not only in the improvements of our technical know how, but also helps to open up the door for everybody to openly participate in the discussion without hesitation in fear of outright rejection of thoughts and ideas...Thank you and keep it up...

Kier,

I think you have enough information already with regards to the effect of curing period for your programme. 6-7 days cycle time period is very logical I think. Such that if you are looking at the 28 days curing period, you might need at least 3 sets of system table formworks for slabs and 2 sets for your vertical structures like columns, elevator shafts, shear walls, etc...However, I would like you to look at the availability of your tower crane and man lifts for your programme. A 60 storey building is a tall and huge one. You might need to conduct a detailed tower crane analysis in order to make sure that you have enough tower crane hours available to support your 6 day cycle time period in consideration to other activities like the MEPF and architectural works that are of equally important as far as tower crane and man lifts utilizations are concerned.

Hope it helps...

Regards,

Arman
kier legard
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Hi Larry,

How much formwork/backpropping should be used for say 60 storey?

Regards,
kier legard
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Hi Clive,

Thanks, you enlighten me.

Can you give a sample sequence of work of typical floor say 5 or 6 day cycle for the structural works(corewall,col, wall,slab).

Say day 1 casting of slab
day2 col/ wall steel works
day 3......
day 4....

Thanks,
kier
Clive Randall
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Kier
Just caught up with this one
Curing: Does it affect the cycle in a high rise building
The answer is not clear cut and is dependent on a number of factors.
1. What is the strength of the concrete, if it is a high grade 60mpa or above particuarly in vertical elements the initial set is often delayed due to considerations of heat of hydration. To limit this retarders are often used so that the initial set is prolonged and thus the heat generated at the maximum can be limited. You will also find that the 3 day strength devlops at a slower percentage rate than say a 45mpa concrete. So dependent on how quickly you are going striking formwork on vertical elements this may be an issue. Coupled with the high strength concrete issue is one of heat differentials within the concrete element. To avoid micro cracking the differentials at any point within the element must be controlled this often leads in vertical elements to either chilling down the internal section of the element and or insulating the formwork. Both may delay subsequent installation of the decking to the floor above. What is important here is that curing does not only involve the strength of concrete.
2. Floor slab curing may not be a controlling factor in the normal course of events and subject to sufficiently robust falsework the need for the concrete slab to have achieved any level of strength can be eliminated. However there is a catch here that if you leave the formwork in place for say three floors you are seriously likely to be well over the design live load of the floor. Backpropping can overcome this to an extent but only if you can get the loads back into the permanent structural vertical elements without distressing the slabs. This is by no means easy. However you do not have to wait for 28 days to gain the design strength. As a rule of thumb design strengths of 70
% are sufficient to carry structural loading ie the dead load of the structure above. 70% strength can at the very outside be expected after 7 days. Where designers specify minimum cement content of around 300+kgm3 you can expect the strength to be achieved after 3 days. However to prove this you need to site cure your cubes and test before striking. A double check on punching shear of props is worthwhile carrying out as is the need to check what the wet concrete load is from the floor being concreted on the floor being struck.
3. Be carefull of post tensioning, flat plate slabs with post tensioning often require the PT to be stressed prior to concreting the floor above, while a three day strength of 25mpa is required this is achievable do not use accelerators to achieve it as the tendon bond strength development will not have developed with the strength of the concrete. Again site curing and testing is essential. So where post tensioning is specified take advice on the strength development.
4. I see no problem with a 4 day cycle if the project is well planned there is no reason whatsoever that when concrete reached its initial set vertical elements cannot be progreed, by utilising jump forms for vertical elements these can be removed from the critical path leaving floors as the major problem.
In conclusion I would suggest that curing is often not a major cause of delays to a cycle however each case as can be seen above will require consideration.
I trust this helps.
Clive
Karim Mounir
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What i meant is that u can start the erection of steel rft. of the Rc columns of the 2nd slab directly after pouring the 1st slab, u can’t put shoring/decking (of the 2nd slab) directly over the 1st slab until the concrete (of the 1st slab) reaches its designed compressive strength.

Regards.
eduardo padul
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Hi Karim,

You mentioned "but u can’t start the next RC slab until the RC reaches its designed strength.


If the the floor cycle of a building is 6 days and it takes 3 days to complete the next slab form/steel concrete.
then concrete should reached its designed strength in 3 days in order to complete next slab.

Regds

eduardo
Karim Mounir
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kier,

Concrete gets its compressive strength in 28 days, u should refer the specs. of the project.
kier legard
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Hi Karim,

How long does the concrete used in high rise usually attained strength 2? 3? days. Floor cycle can be done in 5 to 6 days right? You mean the floor cycle is dependent on the early attainment of concrete strenght?

Best Regard,
kier
Karim Mounir
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Kier,

u can start columns & walls activities after pouring the RC slab directly (eg. without having to wait for curing time), but u can’t start the next RC slab until the RC reaches its designed strength.

Regards.
Larry Bjorn
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Kier,
From my experience, curing time does not affect the floor cycle time as long as the frame contractor has enough decking & back propping material available for the subsequent areas/floors. You cannot strike the back propping until the slab has gained sufficient strength. Also, on a tight programme, the backpropping timing is likely to dictate when the following trade can start.
You can start working on the slab the day after it has been poured.

Regards,
Larry