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Deshuttering time and durability

 
This forum is locked: you cannot post, reply to, or edit topics.This topic is locked: you cannot edit posts or make replies. Thank Post    www.sefindia.org Forum Index -> E-conference on Durability of Concrete - Feb 27 to March 11 - 2012
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Jayant Kulkarni
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PostPosted: Fri Mar 09, 2012 5:42 pm    Post subject: Deshuttering time and durability Reply with quote

Dear Friends,

As we all know, and extensively reported in this forum, the cement has changed a lot in last 20-30 years, giving very early strength. Nowadays, we get 50-60% strength in three days and 80-85% strength in 7 days.

But the deshuttering time related clause in IS 456 remained unchanged since the 1964 edition.Is it correct to stick to these old limits? Is it OK if we deshutter early after ensuring the achievement of required strength for the expected load at that instant?

Discussions with a few senior colleagues raised a doubt about `E` value at early age. If E value at early age is less than that of 28-days( considered as fully matured) , the value of strain will not be as predicted, even if strength is achieved. This should lead to more crack width affecting long term durability.

Can any expert throw more light on this?

Jayant Kulkarni

Consulting Structural engineer
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basant_kukreti
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PostPosted: Sat Mar 10, 2012 2:57 am    Post subject: Deshuttering time and durability Reply with quote

Dear Mr Kulkari:Besides the test for 705 cube strength in my experience it is based on span temp from 7 days to 21 days ofcourse you can correc t me.In my limited experience  Ndt test can be better guide.
Regards
basant

On Fri, Mar 9, 2012 at 11:12 PM, Jayant Kulkarni <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear Friends,

As we all know, and extensively reported in this forum, the cement has changed a lot in last 20-30 years, giving very early strength. Nowadays, we get 50-60% strength in three days and 80-85% strength in 7 days.

But the deshuttering time related clause in IS 456 remained unchanged since the 1964 edition.Is it correct to stick to these old limits? Is it OK if we deshutter early after ensuring the achievement of required strength for the expected load at that instant?

Discussions with a few senior colleagues raised a doubt about `E` value at early age. If E value at early age is less than that of 28-days( considered as fully matured) , the value of strain will not be as predicted, even if strength is achieved. This should lead to more crack width affecting long term durability.

Can any expert throw more light on this?

Jayant Kulkarni

Consulting Structural engineer
     



     


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Dr. N. Subramanian
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PostPosted: Sat Mar 10, 2012 6:00 am    Post subject: Re: Deshuttering time and durability Reply with quote

Dear Er Jayant,

The following info posted on 16th Feb 2006 on Concrete Knowledge Centre of ACI will be of interest to you

"
There are some important general requirements related to stripping time in ACI 318-05, Building Code Requirements for Structural Concrete. Section 6.2.1 states:
“Forms shall be removed in such a manner as not to impair safety and serviceability of the structure. Concrete exposed by form removal shall have sufficient strength not to be damaged by removal operation.”

There are additional default requirements contained in 301-05, "Specifications for Structural Concrete," the primary one being in Section 2.3.2.5:
“Unless otherwise specified, leave formwork and shoring in place to support the weight of concrete in beams, slabs, and in-place structural members until concrete has reached fc′…”

ACI 347-04, “Guide to Formwork for Concrete,” Section 3.7.1, gives direction regarding who is responsible for determining when the form stripping operation can begin:
“Although the contractor is generally responsible for design, construction, and safety of formwork, criteria for removal of forms or shores should be specified by the engineer/architect.”

As stated in ACI SP-4, Formwork for Concrete:
“Since early form removal is usually desirable so that forms can be reused, a reliable basis for determining the earliest proper stripping time is necessary. When forms are stripped, there must be no excessive deflection or distortion and no evidence of cracking or other damage to the concrete, due either to removal of support or to the stripping operation. Supporting forms and shores must not be removed from beams, floors, and walls until these structural units are strong enough to carry their own weight and any approved superimposed load, unless provision has been made to allow for anticipated temporary construction loads, as for example in multistory work. In no case should forms and shores be removed from horizontal members before concrete has reached the strength specified by the engineer/architect for form removal.”

ACI 347.2R-05, “Guide for Shoring/Reshoring of Concrete Multistory Buildings,” provides detailed information regarding the practice of stripping formwork and placing reshores in multistory building construction."

Hence in ACI there is no specific time limit but the responsibility is with the contractor-who has to consult with the owner/site engineer and decide about deshoring.

But ACI 347.2R-05 gives specific recommendations for
Concrete Multistory Buildings
, which are not available in IS code.

Regards
NS  

Jayant Kulkarni wrote:
Dear Friends,

As we all know, and extensively reported in this forum, the cement has changed a lot in last 20-30 years, giving very early strength. Nowadays, we get 50-60% strength in three days and 80-85% strength in 7 days.

But the deshuttering time related clause in IS 456 remained unchanged since the 1964 edition.Is it correct to stick to these old limits? Is it OK if we deshutter early after ensuring the achievement of required strength for the expected load at that instant?

Discussions with a few senior colleagues raised a doubt about `E` value at early age. If E value at early age is less than that of 28-days( considered as fully matured) , the value of strain will not be as predicted, even if strength is achieved. This should lead to more crack width affecting long term durability.

Can any expert throw more light on this?

Jayant Kulkarni

Consulting Structural engineer
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gautam chattopadhyay
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Joined: 17 Feb 2009
Posts: 128

PostPosted: Sat Mar 10, 2012 11:06 am    Post subject: Deshuttering time and durability Reply with quote

Truly speaking 7 to 10 days is enough for deshuttering in case of buildings and the deshuttering always exceeds 28 days in bridges. At the time of removing scaffolds the structural element is subjected only to selfweight. 80 to 85% of characteristic strength I feel is enough for the element if not designed in wrong way. A beam with slenderness ratio 12 is not supposed to deflect excessively under selfweight. In case of bridges, the precast girders are erected after stressing that is after 21 to 28 days. Casting deck slab follows erection, then wearing course, hand rail etc and live load comes long after. A prestressed girder, by theory should not sag if the full section is in compression. However, present trend of using fly ash as an admixture, specially using HVFAC in bridge construction needs more attention and investigation. It is nice to see someone, for the first time has drawn our attention to value of E of concrete. That Fly ash retards setting time is an accepted phenomenon; investigation should be conducted on how presence of fly ash affects E of concrete. As we know, modulus of elasticity does not only affects deflection, it also affects moment of resistance in both working stress and ultimate stress design. Also stresses in reinforcements do also depend on corresponding value of E of concrete. Crack width, spacing of cracks do also depend on E. Thanks Jayant for raising the most vital and mostly forgotten aspect of structural engineering. 

On Fri, Mar 9, 2012 at 11:12 PM, Jayant Kulkarni <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear Friends,

As we all know, and extensively reported in this forum, the cement has changed a lot in last 20-30 years, giving very early strength. Nowadays, we get 50-60% strength in three days and 80-85% strength in 7 days.

But the deshuttering time related clause in IS 456 remained unchanged since the 1964 edition.Is it correct to stick to these old limits? Is it OK if we deshutter early after ensuring the achievement of required strength for the expected load at that instant?

Discussions with a few senior colleagues raised a doubt about `E` value at early age. If E value at early age is less than that of 28-days( considered as fully matured) , the value of strain will not be as predicted, even if strength is achieved. This should lead to more crack width affecting long term durability.

Can any expert throw more light on this?

Jayant Kulkarni

Consulting Structural engineer
     



     


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RKBhola
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Joined: 05 Dec 2009
Posts: 13

PostPosted: Sun Mar 11, 2012 10:58 am    Post subject: Deshuttering time and durability Reply with quote

Dear Friends,

Mr.Kulkarni has rightly mentioned that the deshuttering time depends upon the following two factors :


1.  The strength of the concrete at the time of deshuttering.  For most building applications, a strength of 70-80% of the 28 days strength should be sufficient to decide the deshuttering time from strength consideration.
2.  The second factor if the E value at the time of deshuttering.  This would impact the deflection during deshuttering, and the concern is regarding initiation of cracks that may lead to long term impact on the load carrying capacity and the durability of the member.


While it is relatively easy to assess the strength of the concrete during this period by testing cubes which are placed in the same condition as the poured concrete, it is important for the site to be able to test for the E value as well.  With the discussion regarding the  desirability of 'Performance Based' specifications for concrete, the design engineers can then specify the strength as well as E values at which the deshuttering can be permitted.  Can anyone recommend some simple site tests which can help to evaluate the E value?  


The above becomes especially important when you consider the ever increasing demand of Project Managers to reduce the deshuttering time (at times it is portrayed that the entire viability of the project is dependent upon it).  It is time that all the stakeholders in any project, right from the site supervisor to the project owner, realize the technical parameters and constraints involved in issues that directly impact the safety and the life of the building.  And it is the responsibility of the design engineers to make them aware. 


R.K. Bhola


On Fri, Mar 9, 2012 at 11:12 PM, Jayant Kulkarni <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear Friends,

As we all know, and extensively reported in this forum, the cement has changed a lot in last 20-30 years, giving very early strength. Nowadays, we get 50-60% strength in three days and 80-85% strength in 7 days.

But the deshuttering time related clause in IS 456 remained unchanged since the 1964 edition.Is it correct to stick to these old limits? Is it OK if we deshutter early after ensuring the achievement of required strength for the expected load at that instant?

Discussions with a few senior colleagues raised a doubt about `E` value at early age. If E value at early age is less than that of 28-days( considered as fully matured) , the value of strain will not be as predicted, even if strength is achieved. This should lead to more crack width affecting long term durability.

Can any expert throw more light on this?

Jayant Kulkarni

Consulting Structural engineer
     



     


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