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How to reduce srinkage cracks in foundations

 
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Kaustubh Vaishnav
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Joined: 07 Apr 2010
Posts: 109

PostPosted: Thu Apr 08, 2010 6:09 am    Post subject: How to reduce srinkage cracks in foundations Reply with quote

Dear all
Please suggest methods for reducing shrinkage cracks
in foundations
whether use of polyster fibres will help in that?
if yes please explain.
or suggest any other alternative
Kaustubh
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suraj
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PostPosted: Thu Apr 08, 2010 3:29 pm    Post subject: Shrinkage Cracks Reply with quote

Shrinkage Cracks
Dear Eng
Kaustubh Vaishnav
Question is purely plain.
Is it for application on site or it is just for information purpose?
No details or other requirements have been explained.
Please, be specific about what type of foundations & what size as well as what type of concrete arw to be included on project?
Regards

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IntPE(India)Suraj Singh FIE Civil
Engineering & Arbitration

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Kaustubh Vaishnav
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Posts: 109

PostPosted: Fri Apr 09, 2010 4:26 am    Post subject: Re: Shrinkage Cracks Reply with quote

suraj wrote:
Shrinkage Cracks
Dear Eng
<A>
Kaustubh Vaishnav
Question is purely plain.
Is it for application on site or it is just for information purpose?
No details or other requirements have been explained.
Please, be specific about what type of foundations & what size as well as what type of concrete arw to be included on project?
Regards




Dear sir

Thanks for your reply



It is for application on site

This is basically for trapazoidal foundations for wind turbines

In which three stages of concrete we have designed

in first part M45 grade of concrete is there

in 2nd part M30 grade

& in third part again M45 grade of concrete is there

total concrete qnty is almost 250 cum.



As we are tring to minimise the time frame required for wind turbines

We have to go for early erections of tower.



Due to early loading cracks are developing on the surface.



I just want to give solution for execution team for the same problem



Dear sir plz suggest any solution to reduce shrinkage cracks in foundations



Kaustubh
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suraj
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PostPosted: Sat Apr 17, 2010 11:21 am    Post subject: Foundations Shrinkage Cracks Reply with quote

Foundations Shrinkage Cracks
250 cum is not a small quantity for one unit but, as stated, it is to be cast in three stages. Possibility of thick & massive members arise. Since, no drawing or section is shown, it is difficult to respond.
I hope that definition of massive member & thick member are known to you.
It is also hoped that GGBS or MS concrete may be used in this case.
What temperature controls are to be made?
According to requirement, massive concrete or thick members, should not allow more than 20 to 24 deg C concrete temperature with application of thermal curing for massive members.
Ambient temperature should also not exceed 32 deg C
Please write such posts on general discussion forum for an early response.
Regards

_________________
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IntPE(India)Suraj Singh FIE Civil
Engineering & Arbitration

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suraj
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PostPosted: Sat Apr 17, 2010 3:26 pm    Post subject: Crack prevention Reply with quote

Crack prevention <xml><o></o>1          Crack widths shall be controlled by an expeditious use of combinations of reinforcement sizes, spacing and cover. <o></o>
Crack widths shall be calculated using the applicable formula in BS 8007. <o></o>
2          The calculation shall be based on the long term, steady state loading. For durability it is not necessary to consider peak loadings although this may affect coating selection for the requirement for crack bridging and flexural performance. <o></o>
Crack widths apply at the surface of the concrete i.e. the full depth of cover shall be utilized in the calculation. <o></o>
3          Calculation of crack widths shall consider both load (flexural) and restraint (due to thermal and shrinkage effects) induced cracking. <o></o>
In order to reduce flexural cracking to acceptance limits, it will be necessary to use reduced allowable stresses in the reinforcement. <o></o>
4          Calculation of crack widths shall not use ‘deemed to satisfy’ options of BS 8007, i.e. do not calculate crack widths on PC and minimum reinforcement ratios. <o></o>
The minimum external restraint factor (R) shall be 0.5. <o></o>
5          Methods of calculating crack widths in relation to temperature and moisture effects are given in Appendix A - BS 8007. <o></o>
6          The minimum fall in temperature between hydration peak and ambient (T1) shall not be less than 31° C for walls and 21° C for ground slabs. <o></o>
Seasonal temperature fall (T2) shall be considered where continuous construction is used - BS 8007 - Table 5.1 - Option 1. This shall be not less than 30° C. <o></o>
Crack widths shall be limited as follows. <o></o>
Crack width shall be = 0.15 mm for all buried, submerged and exposed concrete. <o></o>
Crack width shall be = 0.30 mm for all concrete located in an air conditioned and sealed environment. <o></o>
Crack widths shall be = 0.10 for all liquid retaining structures. <o></o>
Reinforcement <o></o>1          Use smaller diameter re-bar at closer centers. <o></o>
For sections = 500 mm thick and for the outside 300 mm of large sections, reinforcement shall not be less than 0.35% of the applicable gross cross-sectional area of the concrete section. <o></o>
2          Maximum spacing of reinforcement shall be 150 mm in any direction. <o></o>
Use fabric reinforcement where possible (‘nested’ where necessary) as this gives better crack control. <o></o>
Do not bunch reinforcement or use in vertical or horizontal pairs. <o></o>
Reinforcement shall be adequately detailed to eliminate congested areas i.e. laps to be staggered. <o></o>
3          Place reinforcement nearest to the surface where it is the greatest restrained length which means horizontal wall reinforcement will be on the outside of the vertical reinforcement. <o></o>
4          Ensure additional diagonal reinforcement is placed at each re-entrant opening to prevent cracks emanating from corners. <o></o>
All reinforcement shall be fully detailed by the Designer on bar bending schedules BBS for fabrication. <o></o>
5          All concrete sections with a thickness of 250 mm or more, reinforcing bars shall be placed on both faces over the full section. In addition, minimum reinforcement shall be placed in the other two faces. <o></o>
6          Concrete Cover <o></o>
Adequate cover to the outside of all reinforcement is essential for resistance to corrosion for all types of sections & situations whatsoever.. <o></o>
  1. Minimum Concrete Cover (mm) Concrete cast against or permanently exposed to earth (all below grade structures) and all marine facilities over or in contact with water 75 <o></o><o> </o>
    1. Concrete exposed to weather (all above grade structures not enclosed by a temperature and humidity controlled building) 60 <o></o>
    2. Concrete not exposed to weather and located within a temperature and humidity controlled building 50 <o></o>
    3. Where any individual structural element falls within two or more categories then the most stringent criteria shall apply for the entire element. <o></o>
    4. Horizontal re-bar in walls and faces of large elements shall be on the outside of the vertical reinforcement for more effective crack control. <o></o>
    5. All concrete cover shall conform strictly with values given above unless noted otherwise on design documents or in the applicable standards. <o></o>
    6. Required covers shall not be reduced by provision of protective coatings, membranes or by membrane protective screed. <o></o>
    7. If fire resistance of more than 2 hours is required,  cover shall be as determined in Table 3.5 in BS 8110 Part 1 for the particular element under consideration. <o></o>
    8. Concrete Grades <o></o>
    9. Concrete shall have a minimum compressive strength as given in Specification for Concrete <o></o><o> </o>
      _________________
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      IntPE(India)Suraj Singh FIE Civil
      Engineering & Arbitration

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Kaustubh Vaishnav
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Joined: 07 Apr 2010
Posts: 109

PostPosted: Wed Apr 21, 2010 4:03 am    Post subject: Re: Foundations Shrinkage Cracks Reply with quote

Dear sir
Please explain Defination of massive member & thick member.
Kaustubh





suraj wrote:
Foundations Shrinkage Cracks
250 cum is not a small quantity for one unit but, as stated, it is to be cast in three stages. Possibility of thick & massive members arise. Since, no drawing or section is shown, it is difficult to respond.
I hope that definition of massive member & thick member are known to you.
It is also hoped that GGBS or MS concrete may be used in this case.
What temperature controls are to be made?
According to requirement, massive concrete or thick members, should not allow more than 20 to 24 deg C concrete temperature with application of thermal curing for massive members.
Ambient temperature should also not exceed 32 deg C
Please write such posts on general discussion forum for an early response.
Regards
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Kaustubh Vaishnav
...
...


Joined: 07 Apr 2010
Posts: 109

PostPosted: Wed Apr 21, 2010 4:06 am    Post subject: Re: Foundations Shrinkage Cracks Reply with quote

Dear sir
I dont no what is GGBS & MS concrete?
Please tell me in detail.
Kaustubh




suraj wrote:
Foundations Shrinkage Cracks
250 cum is not a small quantity for one unit but, as stated, it is to be cast in three stages. Possibility of thick & massive members arise. Since, no drawing or section is shown, it is difficult to respond.
I hope that definition of massive member & thick member are known to you.
It is also hoped that GGBS or MS concrete may be used in this case.
What temperature controls are to be made?
According to requirement, massive concrete or thick members, should not allow more than 20 to 24 deg C concrete temperature with application of thermal curing for massive members.
Ambient temperature should also not exceed 32 deg C
Please write such posts on general discussion forum for an early response.
Regards
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View user's profile Send private message
suraj
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Joined: 17 Apr 2008
Posts: 1981
Location: NCR Faridabad, E mail suraj_engineer@yahoo.co.uk

PostPosted: Thu Apr 22, 2010 4:10 am    Post subject: Concrete Reply with quote

Concrete
Pl refer to the links to know more.

<xml><o> </o>


<o> </o>


<o> </o>


GGBS stands for Ground granulated blast furnace slag
PFA stands for pulverised fuel ash
MS stands for Micro silica or silica fumes
CI stands for corrosion inhibitor
MS can replace GGBS
GGBS replaces OPC content
Upto 72% cement can be replaced.

Try to browse SEFI search & know more technicalities.
Regards

_________________
Thanks & Warm Regards
IntPE(India)Suraj Singh FIE Civil
Engineering & Arbitration

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