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Plastic shrinkage cracks

 
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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vijaydshah
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Joined: 15 Nov 2011
Posts: 20

PostPosted: Sat Mar 03, 2012 6:39 pm    Post subject: Plastic shrinkage cracks Reply with quote

Dear Sefians,
Can any one throw light on what should be the exact de shuttering time for Beams and slabs in case of RMC concrete ? In the recent scenario of speedy construction contractors are wants early de shuttering as they want maximum repetitions of their formworks in short time. Does it also leads to reduce the durability of concrete? They generally rely upon cube strength which is not practically acceptable as the curing of cubes and site differs to a great extent.
I have seen at many site when RMC delievers at site it has already reached in plastic stage which leads to plastic shrinkage cracks which . As this has become a routine site persons have started to take it lightly. This can also reduce the durability.
We should also discuss on solutions of problems related to RMC like how and when to attend repairs to drying shrinkage or plastic shrinkage cracks. I am sure most of us are facing the same problems. Can any one suggest what is the best method to repair these cracks?  

Vijay shah

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suraj
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Joined: 17 Apr 2008
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Location: NCR Faridabad, E mail suraj_engineer@yahoo.co.uk

PostPosted: Wed Mar 07, 2012 6:01 am    Post subject: Plastic Shrinkage Cracks/Form Removal Reply with quote

Dear Eng Vijay Shah,
Plastic Shrinkage Cracks/Form Removal

  1. Specifc duration of keeping formwork in place, should be indicated on drawings & if not, it should be included somewhere else within project specifications. It is a responsibility of design engineer or project specifier to include such provision. Every issue cannot be reffered to relevant codes by site team.
  2. Forms cannot be removed prior to defined number of days even, if it visually appears so.
  3. Removal of formwork earlier than defined, adversely impacts on deflection that goes against durability criteria.
  4. Cube results are indicative one since, site concrete is not maintained on similar environment conditions as cubes are.
  5. Nowadays, various specifications started including mandating drilled concrete cores for set significant areas to ensure cube results. It is within discretionary power of contract engineer to ensure whether or not, site concrete reached specified level of strength.
  6. I understand that generally, minimum time for initial setting is 2 hours since PFA is used in all cements.
  7. Keeping in view loading, transporting, placing, compacting & finishing, this duration may or may not be adequate. This duration can be stretched by adding MS or other admixtures. Plastic cracks are caused due deficiency of hydration water. It is therefore, envisaged that curing must be controlled at a faster pace than it used to be in past.
  8. Curing compund can be sprayed immediately, after finishing or curing applied by covering with polythene sheet to be replaced by hesian cloth followed by, replacing on it with polythene sheet. Even after resorting to such measures, cracks are prone to be formed in hot wheather environment, immediate repairs of such cracks without addition of water but, cement grouting or other approved chemical grouts, has been tested effective provided, post repairs curing is managed properly on entire surface.
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vijaydshah
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Joined: 15 Nov 2011
Posts: 20

PostPosted: Wed Mar 07, 2012 12:36 pm    Post subject: Re: Plastic shrinkage cracks Reply with quote

Dear Members
Thanks a lot for the post.
Further to it Can we allow deshuttering after the required strength of concrete reaches at 70%. If we need strength of M20 and contractor uses M25 to gain early strength can we allow deshuttering after achieving 70 % strengh of M20?. Will not be concrete in Green Condition at this stage as per M25's material properties? Will this lead to adverse effects on durability of concrete ?

As many of conractors and builders uses such such theory should we allow this?


vijaydshah wrote:
Dear Sefians,
Can any one throw light on what should be the exact de shuttering time for Beams and slabs in case of RMC concrete ? In the recent scenario of speedy construction contractors are wants early de shuttering as they want maximum repetitions of their formworks in short time. Does it also leads to reduce the durability of concrete? They generally rely upon cube strength which is not practically acceptable as the curing of cubes and site differs to a great extent.
I have seen at many site when RMC delievers at site it has already reached in plastic stage which leads to plastic shrinkage cracks which . As this has become a routine site persons have started to take it lightly. This can also reduce the durability.
We should also discuss on solutions of problems related to RMC like how and when to attend repairs to drying shrinkage or plastic shrinkage cracks. I am sure most of us are facing the same problems. Can any one suggest what is the best method to repair these cracks?  

Vijay shah

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vijaydshah
SEFI Regulars
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Joined: 15 Nov 2011
Posts: 20

PostPosted: Wed Mar 07, 2012 12:42 pm    Post subject: Re: Plastic shrinkage cracks Reply with quote

Dear Members
Also the when cover is addresed please note that IS 456 has recommended more cover when concrete is below ground. The most important element are columns and i have seen very few contractors or engineers who ensures more cover below ground. Tis also is main cause of durability of the concrete as moisture starts penetrating through soil which leads to corrosion.


vijaydshah wrote:
Dear Members
Thanks a lot for the post.
Further to it Can we allow deshuttering after the required strength of concrete reaches at 70%. If we need strength of M20 and contractor uses M25 to gain early strength can we allow deshuttering after achieving 70 % strengh of M20?. Will not be concrete in Green Condition at this stage as per M25's material properties? Will this lead to adverse effects on durability of concrete ?

As many of conractors and builders uses such such theory should we allow this?


vijaydshah wrote:
Dear Sefians,
Can any one throw light on what should be the exact de shuttering time for Beams and slabs in case of RMC concrete ? In the recent scenario of speedy construction contractors are wants early de shuttering as they want maximum repetitions of their formworks in short time. Does it also leads to reduce the durability of concrete? They generally rely upon cube strength which is not practically acceptable as the curing of cubes and site differs to a great extent.
I have seen at many site when RMC delievers at site it has already reached in plastic stage which leads to plastic shrinkage cracks which . As this has become a routine site persons have started to take it lightly. This can also reduce the durability.
We should also discuss on solutions of problems related to RMC like how and when to attend repairs to drying shrinkage or plastic shrinkage cracks. I am sure most of us are facing the same problems. Can any one suggest what is the best method to repair these cracks?  

Vijay shah

Posted via Email
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suraj
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Joined: 17 Apr 2008
Posts: 1986
Location: NCR Faridabad, E mail suraj_engineer@yahoo.co.uk

PostPosted: Thu Mar 08, 2012 9:55 am    Post subject: Form Removal & Concrete Spaer Thickness below ground Reply with quote

Form Removal & Concrete Spaer Thickness below ground

  1. Yes, it is true that specified period of formwork removal can be governed by 70 % criteria of Charactreistics  strength but, how shall it be ensured on site?
  2. It should not be compared to laboratory results of crushing strength of cube since, site curing is not effected in similar conditions. Of course, should cubes be placed on site concrete member & similar conditions of curing applied, comparison could be effected to consideration of crushing strength of cubes.
  3. Moreover, formwork removal time does not exclusively depend on crusing strength of cubes but, span slab size as well, which is more significant from deflection & durability viewpoints.
  4. Concrete cover thickness is of course, increased where concrete is to go in contact with soil to provide protection to concrete from adverse effects of soil chemicals or what designers/specifiers may call waterproofing.
  5. Generally, concrete cover thickness in soil portion of concrete members should be 70 mm/60mm while, superstructural memebers may be 40/50 mm. Practically, this provision may pose site problems of construction execution since, structural drawing in general, do not indicate this provision in relevant sections. To contain this, some designers specifiy to increase section of stub columns or stub walls below ground level keeping intact the starter bars.
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