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amit_haridas SEFI Member
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发布:2月2日结婚9, 2012 5:52 amPost subject: Crucial issues in respect of durability of Concrete |
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混凝土通常是基于其魅力racteristic strength in our country, Most contracts, we would only find a virtual �copy � paste� of IS 456 specifications, which mention the grade of concrete, Min�m cem�t contents, W/C ratio etc. However the aspects affecting durability are often overlooked both in practice and on paper.
The present day race for completing projects on time, at lower costs is driving people to take shortcuts which probably weren�t being taken before. We may today have better construction materials, than before, but the conviction in utilising these for improvement has considerably lowered as profit booking is the main vision. At many construction sites, the site engineers want very high slump concrete, as it reduces the effort of placing the concrete, vibrator needles are used to spread concrete into beams and on slabs, instead of shovels.
Most often we see that the utmost attention is given to a concrete test specimen, which is only supposed to last 28 days, than the concrete in structure itself, which is supposed to last for ages. Curing of concrete structure is negligible, which results in lower strength and in long term affects permeability resistance of concrete. I have seen many projects where shuttering materials for next slab/columns, are dropped on a virtually one day old concrete cast the earlier day, wouldn�t the concrete suffer from internal stresses / micro cracks which will show its effect in the long run.
当我们添加如粉煤灰火山灰材料concrete, we believe that the durability and long term strength will increase, but the irony is that we do not cure the concrete, and expect the secondary reaction to take place by magic. Most consultants here in India, are averse to Flyash / GGBS mixes, as reportedly they do not give the required performance. However, rather than correcting site practices, we tend to blame these materials for lack of performance.
Being in ready mixed concrete industry for few years, meeting different customers, some consultants, it is clear, that price is more important than mix design, Adherence to specifications on paper is more important than to ensure the practices are followed in place.
Corruption in construction industry is also quite high, coupled with lack of training and knowledge in handling of concrete, the concrete placement often is substandard.
Durability of concrete can only be achieved, with stringent checks, Policies and overall conviction by all in the fray. As long as we do not educate everybody right from top to bottom, i.e. the decision makers to the executing and support team, Until then the durability aspect would only be a subjective matter.
With warm regards,
Amit Haridas |
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发布:2012年2月29日,我们上午7:15Post subject: Crucial issues in respect of durability of Concrete |
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For durability of concrete the following are to be taken care :-
a. cement - good quality cement shall be used. The cement older than the stipulated period shall not be used. Before incorporating the cement in the works, at least the strength shall be tested.
Proper care shall be taken that already set cement shall not be used.Supervisers or engineers at site should take care of this.
Placing and compacting of concrete shall be done with in initial setting time. This is not followed even in ready mix concrete.
coarse and fine aggregate-- The grading is very essential. normally the coarse aggregate as it comes from crusher is used without sive analysis,size shape etc.
b.water - quality of water is not checked.water cement ratio is not correctly adhered resulting in lose of strength.
c. Form work - form work should be such that the slurry should not escape through the gaps and avoid honey combing. Good quality shuttering materials not used.
d.Reinforcement - steel reinforcement from approved manufacturers only shall be used and re rolled steel shall not be used.The engineer at site/structural engineer� should check the placing/spacing of reinforcement in accordance with structural drawing. Now a days only single knot is used instead of double knot which makes the reinforcement for displacement. Rusted steel reinforcement will expand and the concrete will get cracks.
e. Curing - No care is being done for curing in accordance with codal provisions especially for high rise building
testing of concrete shall be done at least at 7days and 28 days strength to ensure compressive strength.
f.For easy workability quantity of admixture is increased resulting in cracks in concrete.
k c francis
On Wed, Feb 29, 2012 at 11:41 AM, admin forum@www.buonovino.com)> wrote:
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[edited] 混凝土通常是基于其魅力racteristic strength in our country, Most contracts, we would only find a virtual �copy � paste� of IS 456 specifications, which mention the grade of concrete, Min�m cem�t contents, W/C ratio etc. However the aspects affecting durability are often overlooked both in practice and on paper.
The present day race for completing projects on time, at lower costs is driving people to take shortcuts which probably weren�t being taken before. We may today have better construction materials, than before, but the conviction in utilising these for improvement has considerably lowered as profit booking is the main vision. At many construction sites, the site engineers want very high slump concrete, as it reduces the effort of placing the concrete, vibrator needles are used to spread concrete into beams and on slabs, instead of shovels.
Most often we see that the utmost attention is given to a concrete test specimen, which is only supposed to last 28 days, than the concrete in structure itself, which is supposed to last for ages. Curing of concrete structure is negligible, which results in lower strength and in long term affects permeability resistance of concrete. I have seen many projects where shuttering materials for next slab/columns, are dropped on a virtually one day old concrete cast the earlier day, wouldn�t the concrete suffer from internal stresses / micro cracks which will show its effect in the long run.
当我们添加如粉煤灰火山灰材料concrete, we believe that the durability and long term strength will increase, but the irony is that we do not cure the concrete, and expect the secondary reaction to take place by magic. Most consultants here in India, are averse to Flyash / GGBS mixes, as reportedly they do not give the required performance. However, rather than correcting site practices, we tend to blame these materials for lack of performance.
Being in ready mixed concrete industry for few years, meeting different customers, some consultants, it is clear, that price is more important than mix design, Adherence to specifications on paper is more important than to ensure the practices are followed in place.
Corruption in construction industry is also quite high, coupled with lack of training and knowledge in handling of concrete, the concrete placement often is substandard.
Durability of concrete can only be achieved, with stringent checks, Policies and overall conviction by all in the fray. As long as we do not educate everybody right from top to bottom, i.e. the decision makers to the executing and support team, Until then the durability aspect would only be a subjective matter.
With warm regards,
Amit Haridas
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发布:2月2日结婚9, 2012 7:44 amPost subject: E-Conference on Durability of Concrete |
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Dear Sefians,
I would like to congratulate organisers SEFI in bringing out this conference on a very important topic concerning durability of concrete structures. The excellent introductory notes by Mr. V.R. Kulkarni and Dr. N. Subramanian have set the proper tone for the deliberations of this conference. I have few comments and suggestions to make on the topic which are written hereunder:
1.
Corrosion of reinforcement
a)
It is one of the main reasons to cause cracking and spalling of concrete which affects durability of concrete very much in the long run. A good quality and dense concrete is the pre-requisite against reinforcement corrosion, but the quality of steel rebars itself has a significant influence on it. It is well established that mild steel (MS) bars are more corrosion resistant as compared to cold-twisted deformed (CTD) bars or TMT bars, as the level of induced stresses in CTD and TMT bars are much higher than those in MS bars which enhances the potential of initiating corrosion. Besides, CTD and TMT bars have inherent deficiency in its manufacturing process to slow down initiation of corrosion in bars. In CTD bars, a part of the residual strain is withheld which initiates the corrosion faster.
b)
Many have mentioned in this forum about how the older structures have stood test of time without having any durability problem. Prior to the introduction of high strength CTD bars in India in 1968, only mild stars were used as reinforcement. The concrete mix used then were usually 1:2:4 (equivalent to M20) for most structures, and 1:1
�
:3 (≈ M25) for more important or special structures. The difference between then and now is the use of CTD and TMT bars replacing MS bars, and more aggressive environment we are living in now. Although higher concrete grades are being used now for important structures, the durability problem is still there in some of them because of the poor quality construction practices combined with the use of CTD and TMT bars.
c)
In the case of liquid-retaining structures where durability is of utmost importance, the design practice by working stress method considers lower permissible stress in reinforcement combined with limitation on permissible tensile stress in concrete as an uncracked section. The limit-state method for design of these structures has stringent requirement on permissible crack width under service load condition.
d)
Considering the above phenomena, in severe environmental exposure conditions and/or in situations where the quality of concrete is not guaranteed due to lack of quality construction and supervision, it would be prudent to use only MS bars instead of CTD and TMT bars. Although this may not be an economical proposition initially, but in the long run it would pay off. For any structural engineer to adopt this proposition in practice, there should be necessary support by way of codal provisions.
e)
Similarly, for stirrups in beams and links in columns which form the outermost reinforcement and closer to the outer face of concrete, only MS bars are to be used to safeguard against early corrosion. If the design demands, closer spacing of stirrups and links can be provided.
2.Cover:
a)
There is a tendency to specify too much cover to reinforcement in the name of durability. Any cover more than 75mm requires extra nominal reinforcement by way of mesh reinforcement with about 50mm cover near the surface. These are required, particularly in structural elements having large surface area such as floor and ground slabs, retaining and shear walls, to avoid surface cracks due to initial thermal and shrinkage effects. Such provisions are there in some of the international codes, and our codes should also incorporate such provisions.
b)
The concrete grade of concrete blocks used to provide the necessary cover should be of the same grade as that of the main concrete of the structural member. If the concrete blocks are of poor quality, they would become weak spots for early deterioration. A better proposition is to use PVC cover blocks which do not rust thus eliminating blemishes on the concrete surface.
3.Slip form construction:
a)
Slip-form method of construction is very much used for the construction shafts like structures, like tall chimneys, shafts of over-head tanks, bridge piers, cable-stayed bridge pylons, shear walls for multi-storied buildings etc., as it has the advantage of speedy construction. The use of ready-mixed concrete (RMC) is also in vogue these days for this type of construction. The speed of slip-form construction varies in the range of 2.5m to 5m height per day. When the concrete is mixed at site, retarders are usually added as admixtures to concrete to slow down the setting time of concrete, after doing initial trials to suit the volume of concrete and height at which it is to be placed. While using RMC for such constructions, extra care is required as there have been cases of construction failures due to formation of cold separation joints in the shaft leading to transfer of whole load on the reinforcement alone and eventual failure.
I trust you will find the above observations useful.
With best wishes,
Yours truly.
N. Prabhakar
Chartered Structural Engineer
Vasai (E)
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Posted: Sat Mar 10, 2012 5:29 amPost subject: Re: E-Conference on Durability of Concrete |
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Dear Er Prabhakar,
Thank you for your kind words and for your valuable comments.
As you have said mild steel bars are more corrosion resistant and due to the lower strength may result in less cracking. But I feel that they are not the only causes for our older bulidngs to show less cracking. It might be that they were built with Grade 33 cement, cured properly and not subjected to construction loads when they the concrete had not attained proper strength.
Some research papers papers from USA suggest that if the detailing rules are followed the cracks in concrete will be within permissible levels and those cracks (All Concrete members will have cracks unless they are prestressed) will not result in cracking unless the structure is in severe environment.
Moreover using Mild steel will not be economical and muuch of mild steel available in the market is of bad quality(rerolled-esp. the smaller dia. bars). Don't you think the TMT bars are better to resist corrosion?
I agree with you on other points.
Thanks again for sharing your thoughts.
Regards Subramanian
N. Prabhakar wrote: |
Dear Sefians,
I would like to congratulate organisers SEFI in bringing out this conference on a very important topic concerning durability of concrete structures. The excellent introductory notes by Mr. V.R. Kulkarni and Dr. N. Subramanian have set the proper tone for the deliberations of this conference. I have few comments and suggestions to make on the topic which are written hereunder:
1.
Corrosion of reinforcement
a)
It is one of the main reasons to cause cracking and spalling of concrete which affects durability of concrete very much in the long run. A good quality and dense concrete is the pre-requisite against reinforcement corrosion, but the quality of steel rebars itself has a significant influence on it. It is well established that mild steel (MS) bars are more corrosion resistant as compared to cold-twisted deformed (CTD) bars or TMT bars, as the level of induced stresses in CTD and TMT bars are much higher than those in MS bars which enhances the potential of initiating corrosion. Besides, CTD and TMT bars have inherent deficiency in its manufacturing process to slow down initiation of corrosion in bars. In CTD bars, a part of the residual strain is withheld which initiates the corrosion faster.
b)
Many have mentioned in this forum about how the older structures have stood test of time without having any durability problem. Prior to the introduction of high strength CTD bars in India in 1968, only mild stars were used as reinforcement. The concrete mix used then were usually 1:2:4 (equivalent to M20) for most structures, and 1:1
�
:3 (≈ M25) for more important or special structures. The difference between then and now is the use of CTD and TMT bars replacing MS bars, and more aggressive environment we are living in now. Although higher concrete grades are being used now for important structures, the durability problem is still there in some of them because of the poor quality construction practices combined with the use of CTD and TMT bars.
c)
In the case of liquid-retaining structures where durability is of utmost importance, the design practice by working stress method considers lower permissible stress in reinforcement combined with limitation on permissible tensile stress in concrete as an uncracked section. The limit-state method for design of these structures has stringent requirement on permissible crack width under service load condition.
d)
Considering the above phenomena, in severe environmental exposure conditions and/or in situations where the quality of concrete is not guaranteed due to lack of quality construction and supervision, it would be prudent to use only MS bars instead of CTD and TMT bars. Although this may not be an economical proposition initially, but in the long run it would pay off. For any structural engineer to adopt this proposition in practice, there should be necessary support by way of codal provisions.
e)
Similarly, for stirrups in beams and links in columns which form the outermost reinforcement and closer to the outer face of concrete, only MS bars are to be used to safeguard against early corrosion. If the design demands, closer spacing of stirrups and links can be provided.
2.Cover:
a)
There is a tendency to specify too much cover to reinforcement in the name of durability. Any cover more than 75mm requires extra nominal reinforcement by way of mesh reinforcement with about 50mm cover near the surface. These are required, particularly in structural elements having large surface area such as floor and ground slabs, retaining and shear walls, to avoid surface cracks due to initial thermal and shrinkage effects. Such provisions are there in some of the international codes, and our codes should also incorporate such provisions.
b)
The concrete grade of concrete blocks used to provide the necessary cover should be of the same grade as that of the main concrete of the structural member. If the concrete blocks are of poor quality, they would become weak spots for early deterioration. A better proposition is to use PVC cover blocks which do not rust thus eliminating blemishes on the concrete surface.
3.Slip form construction:
a)
Slip-form method of construction is very much used for the construction shafts like structures, like tall chimneys, shafts of over-head tanks, bridge piers, cable-stayed bridge pylons, shear walls for multi-storied buildings etc., as it has the advantage of speedy construction. The use of ready-mixed concrete (RMC) is also in vogue these days for this type of construction. The speed of slip-form construction varies in the range of 2.5m to 5m height per day. When the concrete is mixed at site, retarders are usually added as admixtures to concrete to slow down the setting time of concrete, after doing initial trials to suit the volume of concrete and height at which it is to be placed. While using RMC for such constructions, extra care is required as there have been cases of construction failures due to formation of cold separation joints in the shaft leading to transfer of whole load on the reinforcement alone and eventual failure.
I trust you will find the above observations useful.
With best wishes,
Yours truly.
N. Prabhakar
Chartered Structural Engineer
Vasai (E)
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Posted: Sat Mar 10, 2012 3:07 pmPost subject: Crucial issues in respect of durability of concrete |
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Dear Dr. Subramanian,
谢谢你的对我的观察和评价posting in the E-conference, which is the only one that has been written on my posting so far. Not a word was mentioned about it in the daily summary also. Most of the views in the conference are related to cement and concrete only, and some of them even suggesting to make ordinary construction sites turn into a highly sophisticated cement plant or concrete testing lab. By reading all these views, I felt that the conference was more on the durability of plain concrete and not for reinforced concrete, and my views on reinforcement was completely out of place.
My suggestion of using mild steel bars is only for the case of severe environmental exposure conditions and/or in situations where the quality of concrete is not guaranteed due to lack of quality construction and supervision, although it may be uneconomical. To prove my point on the question of having reduced stresses in steel, I had written an analogy about the design practice for liquid-retaining structures where durability requirements are more stringent. As with other types of steel bars, the quality of mild steel bars used should be a good one with proven test certificates from the steel mill.
No doubt the quality and strength of steel has improved over years, mainly because it is made in a factory in the quality conscious environment. The same thing can be said about concrete in a small number of large projects in the public sectors like bridges, flyovers, thermal and nuclear power projects, as there is proper supervision of work and quality check at such sites. But, the same thing cannot be said about the case of majority of small and medium size constructions in the private sector. Although, most of the design for reinforced concrete section is based on the theory of cracked section, the strain in steel bars is to be kept low to avoid concrete cracking in service load condition. At present, there is certainly a mismatch in the use of high strength steel bars with concrete of low grade. To use CTD and TMT bars, the grade of concrete should also be high, something like M30 and above. For any structural designer to adopt this proposition in practice, there should be necessary support by way of codal provisions.
With kind regards and best wishes,
N. Prabhakar
Chartered Structural Engineer
Vasai (E)
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Er.Vithal Jadhav SEFI Member
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Posted: Sun Mar 18, 2012 12:04 pmPost subject: Crucial issues in respect of durability of Concrete |
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Dear Er, Prabhakar, I also agree with your point that M.S is more resistant to corrosion compared to Tor steel. The addtion of more carbon no doubt gives more strength, but at the same time changes the inherent property of steel to corrode more. So while gaining the strenght we are loosing corrosion resistant property of steel. TMT thermo mechanically reated bars no doubt have good tensile strength but I do not think these steels are less corrosive compared to other steels like CTD, TISCON TATA & KAMADHENU. The old structures not only are safe with mild steel but also with huge thickness of walls with less bearing stress and also lesser grade of cement used with full and proper curing before the structures are loaded.
Er.Vithal Jadhav
OnSat, 10/3/12, Dr N. Subramanianwrote:
Quote: |
From: Dr N. Subramanian Subject: [ECONF] Re: Crucial issues in respect of durability of Concrete To:econf@www.buonovino.com Date: Saturday, 10 March, 2012, 5:30 AM
Dear Er Prabhakar,
Thank you for your kind words and for your valuable comments.
As you have said mild steel bars are more corrosion resistant and due to the lower strength may result in less cracking. But I feel that they are not the only causes for our older bulidngs to show less cracking. It might be that they were built with Grade 33 cement, cured properly and not subjected to construction loads when they the concrete had not attained proper strength.
Some research papers papers from USA suggest that if the detailing rules are followed the cracks in concrete will be within permissible levels and those cracks (All Concrete members will have cracks unless they are prestressed) will not result in cracking unless the structure is in severe environment.
Moreover using Mild steel will not be economical and muuch of mild steel available in the market is of bad quality(rerolled-esp. the smaller dia. bars). Don't you think the TMT bars are better to resist corrosion?
I agree with you on other points.
Thanks again for sharing your thoughts.
Regards Subramanian N. Prabhakar wrote:Dear Sefians,
I would like to congratulate organisers SEFI in bringing out this conference on a very important topic concerning durability of concrete structures. The excellent introductory notes by Mr. V.R. Kulkarni and Dr. N. Subramanian have set the proper tone for the deliberations of this conference. I have few comments and suggestions to make on the topic which are written hereunder:
1.
Corrosion of reinforcement
a)
It is one of the main reasons to cause cracking and spalling of concrete which affects durability of concrete very much in the long run. A good quality and dense concrete is the pre-requisite against reinforcement corrosion, but the quality of steel rebars itself has a significant influence on it. It is well established that mild steel (MS) bars are more corrosion resistant as compared to cold-twisted deformed (CTD) bars or TMT bars, as the level of induced stresses in CTD and TMT bars are much higher than those in MS bars which enhances the potential of initiating corrosion. Besides, CTD and TMT bars have inherent deficiency in its manufacturing process to slow down initiation of corrosion in bars. In CTD bars, a part of the residual strain is withheld which initiates the corrosion faster.
b)
Many have mentioned in this forum about how the older structures have stood test of time without having any durability problem. Prior to the introduction of high strength CTD bars in India in 1968, only mild stars were used as reinforcement. The concrete mix used then were usually 1:2:4 (equivalent to M20) for most structures, and 1:1
½
:3 (≈ M25) for more important or special structures. The difference between then and now is the use of CTD and TMT bars replacing MS bars, and more aggressive environment we are living in now. Although higher concrete grades are being used now for important structures, the durability problem is still there in some of them because of the poor quality construction practices combined with the use of CTD and TMT bars.
c)
In the case of liquid-retaining structures where durability is of utmost importance, the design practice by working stress method considers lower permissible stress in reinforcement combined with limitation on permissible tensile stress in concrete as an uncracked section. The limit-state method for design of these structures has stringent requirement on permissible crack width under service load condition.
d)
Considering the above phenomena, in severe environmental exposure conditions and/or in situations where the quality of concrete is not guaranteed due to lack of quality construction and supervision, it would be prudent to use only MS bars instead of CTD and TMT bars. Although this may not be an economical proposition initially, but in the long run it would pay off. For any structural engineer to adopt this proposition in practice, there should be necessary support by way of codal provisions.
e)
Similarly, for stirrups in beams and links in columns which form the outermost reinforcement and closer to the outer face of concrete, only MS bars are to be used to safeguard against early corrosion. If the design demands, closer spacing of stirrups and links can be provided.
2.Cover:
a)
There is a tendency to specify too much cover to reinforcement in the name of durability. Any cover more than 75mm requires extra nominal reinforcement by way of mesh reinforcement with about 50mm cover near the surface. These are required, particularly in structural elements having large surface area such as floor and ground slabs, retaining and shear walls, to avoid surface cracks due to initial thermal and shrinkage effects. Such provisions are there in some of the international codes, and our codes should also incorporate such provisions.
b)
The concrete grade of concrete blocks used to provide the necessary cover should be of the same grade as that of the main concrete of the structural member. If the concrete blocks are of poor quality, they would become weak spots for early deterioration. A better proposition is to use PVC cover blocks which do not rust thus eliminating blemishes on the concrete surface.
3.Slip form construction:
a)
Slip-form method of construction is very much used for the construction shafts like structures, like tall chimneys, shafts of over-head tanks, bridge piers, cable-stayed bridge pylons, shear walls for multi-storied buildings etc., as it has the advantage of speedy construction. The use of ready-mixed concrete (RMC) is also in vogue these days for this type of construction. The speed of slip-form construction varies in the range of 2.5m to 5m height per day. When the concrete is mixed at site, retarders are usually added as admixtures to concrete to slow down the setting time of concrete, after doing initial trials to suit the volume of concrete and height at which it is to be placed. While using RMC for such constructions, extra care is required as there have been cases of construction failures due to formation of cold separation joints in the shaft leading to transfer of whole load on the reinforcement alone and eventual failure.
I trust you will find the above observations useful.
With best wishes,
Yours truly.
N. Prabhakar
Chartered Structural Engineer
Vasai (E)
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