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Cover Story

 
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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Posts: 6

PostPosted: Tue Mar 06, 2012 3:47 pm    Post subject: Cover Story Reply with quote

Dear All,

All of us know that provision of specified cover thickness is one of the key of getting durable concrete.

However, many a times, the quality of cover blocks is a neglected aspect on site. Even if correct cover thickness is provided, weak spots are available for the entry of water, chlorides, CO2 etc, if the cover blocks are porous or if the interface between the cover block and surrounding concrete allows the entry.

Thus, even if good quality impermeable concrete is made, it is of limited use as water ( and all other enemies of concrete) find its way through this weak link, defeating the purpose of otherwise good quality concrete.

Most of the times, the cover blocks are made of cement-sand mortar at site and at most of the places, specifications are  not provided in the tender for strength requiremenet as well as testing of cover blocks. It needs to be specified that the cover blocks shall be at least one grade higher than the concrete, so as to ensure the same level of impermeability. There are a few companies which provide high strength cover blocks which need to be promoted.

PVC cover blocks appear to be better in terms of porosity and strength, but there are some doubts raised about the quality of interface between them and concrete. Is some data available with somebody?

This cover story needs to be put on front page and given due attention.

REGARDS

Jayant Kulkarni

Consulting Structural Engineer
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Dr. N. Subramanian
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Joined: 21 Feb 2008
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PostPosted: Tue Mar 06, 2012 6:57 pm    Post subject: Re: Cover Story Reply with quote

Dear Er Jayant,

You are right. Most of the durability problems are due to the corrosion of reinforcement. Cover is the only defense against corrosion for normal reinforcement. Corrosion can not take place if there is no moisture- that is the reason why small cover is specified underneath the floors. Proper cover-thickness and quality is required to prevent corrosion.

As you have rightly pointed out use of plastic cover blocks or correct strength concrete blocks are a must for the durability . To my knowledge, I have not come across any study reg the bad effects of plastic cover blocks. (Of course I subscribe only to ACI structural Journal and Concrete Int. and ICJ - most of the matters related to materials are reported in ACI Materials Journal-though I read the abstracts in ACI Structural Journal.

In countries like USA and Germany where I have worked or lived for longer time, I find that they do not plaster the concrete surface as they use perfect formwork and good cover blocks. In Indian sites I have seen people using broken mosaic tiles and even gravel aggregate, which will not remain in place, or act as points for water seepage.

I have written a paper in ICJ, the contents of which are valid even today:
Concrete Cover for Durable RC Structures, ICJ, Apr 1997, pp. 197-201 which can be downloaded from http://www.cdcstruct.com/downloads/1997_04_Apr_Concrete%20cover%20NS%20and%20Geetha.pdf


Controlled permeability formwork can also be used to get better qualilty concrete in the elements. Several papers have been written on this subject and the following link provides these references(http://en.wikipedia.org/wiki/Controlled_permeability_formwork). In India I have mentioned about this in one of my papers based on the paper by Reddi, S.A. Permeable formwork for impermeable concrete. Indian Concrete Journal, January, 1992, pp 3135.

Increasing the cover thickness may not solve the corrosion problem. As mentioned by you or someone in this e-conf. increasing the cover beyond 50-60 mm result in more cracking of the cover. Use of Self Consolidating Concrete (I am happy to note that the use of SCC is increasing in India-see ICJ Aug 2009 issue) will also result in better quality of concrete and cover.

It is interesting to note that proper curing strengthens mainly the cover region of concrete member.

I take this opportunity to list some techniques used to access Rebar corrosion on existing structures :
1. Open circuit potential (OCP) measurements
2. Surface potential (SP) measurements
3. Concrete resistivity measurement
4. Linear polarization resistance (LPR) measurement
5. Tafel extrapolation
6. Galvanostatic pulse transient method
7. Electrochemical impedance spectroscopy (EIS)
8. Harmonic analysis
9. Noise Analysis
10. Embeddable corrosion monitoring sensor and
11. Cover thickness measurements
12. Ultrasonic pulse velocity technique
13. X-ray, Gamma radiography measurement
14. Infrared thermograph Electrochemical
15. Visual inspection

Best wishes and regards
Subramanian

Jayant Kulkarni wrote:
Dear All,

All of us know that provision of specified cover thickness is one of the key of getting durable concrete.

However, many a times, the quality of cover blocks is a neglected aspect on site. Even if correct cover thickness is provided, weak spots are available for the entry of water, chlorides, CO2 etc, if the cover blocks are porous or if the interface between the cover block and surrounding concrete allows the entry.

Thus, even if good quality impermeable concrete is made, it is of limited use as water ( and all other enemies of concrete) find its way through this weak link, defeating the purpose of otherwise good quality concrete.

Most of the times, the cover blocks are made of cement-sand mortar at site and at most of the places, specifications are  not provided in the tender for strength requiremenet as well as testing of cover blocks. It needs to be specified that the cover blocks shall be at least one grade higher than the concrete, so as to ensure the same level of impermeability. There are a few companies which provide high strength cover blocks which need to be promoted.

PVC cover blocks appear to be better in terms of porosity and strength, but there are some doubts raised about the quality of interface between them and concrete. Is some data available with somebody?

This cover story needs to be put on front page and given due attention.

REGARDS

Jayant Kulkarni

Consulting Structural Engineer
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lkjain.ngp
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Posts: 36

PostPosted: Wed Mar 07, 2012 6:46 am    Post subject: Cover Story Reply with quote

Dear sirs,

Kindly add to "cover block" remarks.

Cover block should be strong enough to bear a 100 kg (1kN) line load applied through a reinforcement bars, Normal block breaks by split tension. This may require a maximum w/c ratio of 0.40 and strength above M40. In case of concrete work of higher strength the strength of cover block material is needed to be higher than the concrete in consideration.

L. K. JAIN


On 6 March 2012 21:17, Jayant Kulkarni <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
  Dear All,

All of us know that provision of specified cover thickness is one of the key of getting durable concrete.

However, many a times, the quality of cover blocks is a neglected aspect on site. Even if correct cover thickness is provided, weak spots are available for the entry of water, chlorides, CO2 etc, if the cover blocks are porous or if the interface between the cover block and surrounding concrete allows the entry.

Thus, even if good quality impermeable concrete is made, it is of limited use as water ( and all other enemies of concrete) find its way through this weak link, defeating the purpose of otherwise good quality concrete.

Most of the times, the cover blocks are made of cement-sand mortar at site and at most of the places, specifications are not provided in the tender for strength requiremenet as well as testing of cover blocks. It needs to be specified that the cover blocks shall be at least one grade higher than the concrete, so as to ensure the same level of impermeability. There are a few companies which provide high strength cover blocks which need to be promoted.

PVC cover blocks appear to be better in terms of porosity and strength, but there are some doubts raised about the quality of interface between them and concrete. Is some data available with somebody?

This cover story needs to be put on front page and given due attention.

REGARDS

Jayant Kulkarni

Consulting Structural Engineer







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bsec
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PostPosted: Wed Mar 07, 2012 7:26 am    Post subject: Cover Story Reply with quote

Dear All,

I think the E-Conf is going great with so much of technical inputs from experts from all sides. I sincerely thank the organizers, moderators and the active participants for such a focused and lively discussions on the topic of Concrete Durability.

The opinions expressed in this forum so far are mostly from people with research / academic background or enlightened Consultants. I am yet to see the perspective on Durability from hard core practicing professional (say Contractor’s representative) and Client’s perspective. Views of those who are executing the jobs at site and supervising the same are extremely important.  

With best wishes

Alok Bhowmick

From: Dr N. Subramanian [mailto:forum@sefindia.org]
Sent: Wednesday, March 07, 2012 12:28 AM
To: econf@sefindia.org
Subject: [ECONF] Re: Cover Story

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basant_kukreti
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PostPosted: Wed Mar 07, 2012 9:53 am    Post subject: Cover Story Reply with quote

Dear Jayant :I do agree with you .please suggest narration of this item for site use.You will appreciate that No body reads voluminous document at site which is a essential part even rings are not fabricated in proper way.Tender documents are prepared n in hurry.
regards
Basant

On Tue, Mar 6, 2012 at 9:17 PM, Jayant Kulkarni <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
           Dear All,

All of us know that provision of specified cover thickness is one of the key of getting durable concrete.

However, many a times, the quality of cover blocks is a neglected aspect on site. Even if correct cover thickness is provided, weak spots are available for the entry of water, chlorides, CO2 etc, if the cover blocks are porous or if the interface between the cover block and surrounding concrete allows the entry.

Thus, even if good quality impermeable concrete is made, it is of limited use as water ( and all other enemies of concrete) find its way through this weak link, defeating the purpose of otherwise good quality concrete.

Most of the times, the cover blocks are made of cement-sand mortar at site and at most of the places, specifications are not provided in the tender for strength requiremenet as well as testing of cover blocks. It needs to be specified that the cover blocks shall be at least one grade higher than the concrete, so as to ensure the same level of impermeability. There are a few companies which provide high strength cover blocks which need to be promoted.

PVC cover blocks appear to be better in terms of porosity and strength, but there are some doubts raised about the quality of interface between them and concrete. Is some data available with somebody?

This cover story needs to be put on front page and given due attention.

REGARDS

Jayant Kulkarni

Consulting Structural Engineer
     



     


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Dr. N. Subramanian
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PostPosted: Wed Mar 07, 2012 3:25 pm    Post subject: Re: Cover Story Reply with quote

Dear All,

I want to appreciate the participation and valuable comments by my friend Er L.K.Jain, who is a reputed consultant from Nagpur and an expert on Liquid retaining structures. He is also the Past President of ICI.

I want to ask Er Jain whether he experienced any problem with plastic cover blocks- if we use them we need not worry about the grade of Concrete, is it not?

Regards
NS
lkjain.ngp wrote:
Dear sirs,

Kindly add to "cover block" remarks.

Cover block should be strong enough to bear a 100 kg (1kN) line load applied through a reinforcement bars, Normal block breaks by split tension. This may require a maximum w/c ratio of 0.40 and strength above M40. In case of concrete work of higher strength the strength of cover block material is needed to be higher than the concrete in consideration.

L. K. JAIN


On 6 March 2012 21:17, Jayant Kulkarni <forum@sefindia.org (forum@sefindia.org)> wrote:
Quote:
  Dear All,

All of us know that provision of specified cover thickness is one of the key of getting durable concrete.

However, many a times, the quality of cover blocks is a neglected aspect on site. Even if correct cover thickness is provided, weak spots are available for the entry of water, chlorides, CO2 etc, if the cover blocks are porous or if the interface between the cover block and surrounding concrete allows the entry.

Thus, even if good quality impermeable concrete is made, it is of limited use as water ( and all other enemies of concrete) find its way through this weak link, defeating the purpose of otherwise good quality concrete.

Most of the times, the cover blocks are made of cement-sand mortar at site and at most of the places, specifications are not provided in the tender for strength requiremenet as well as testing of cover blocks. It needs to be specified that the cover blocks shall be at least one grade higher than the concrete, so as to ensure the same level of impermeability. There are a few companies which provide high strength cover blocks which need to be promoted.

PVC cover blocks appear to be better in terms of porosity and strength, but there are some doubts raised about the quality of interface between them and concrete. Is some data available with somebody?

This cover story needs to be put on front page and given due attention.

REGARDS

Jayant Kulkarni

Consulting Structural Engineer







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