When it comes to Durability of the structure, what we understand is structure is termed as durable when it can carry all load throughout its service life without failing. it means it gives the same performance till its end of design life as it perform Today.
The durability of the structure is taken very seriously by designers of structure and designers of concrete. when it comes to designing of something, there are norms which gets followed to get a specific characteristic in design product.
But in actual who governs the durability of the structure, who can certify that structure will perform as same at its end of the life, its still a question which i have on my mind as there is no technology Today known to human being which can predict the life of the structure to which it can last and by looking at any structure no one can predict what is going on inside in terms of integrity. even if there is any technology available in market it can not be popular to be used by any house owner to check the remaining life of its home.
Structural designer did the job of designing a structure to a specified life by following codes in respective countries, Concrete designer did the designing of concrete to ensure the concrete is durable and is not a permeable, also the other attacks which can spoil concrete are also taken care during designing process of concrete by doing trials and test on it.
Up to this it looks OK and all the parameters are considered to ensure the durability or serviceability of structure for design life. After the designing is over, the next job is to execute the project and complete the structure to make it serviceable for intended users of it.
When i ask questions to executors of the job mostly a Civil Engineers who works at actual field, what is the durability of the structure, in most of the cases i found that, this is the subject which is not given to them for understanding in details.
A general answer which mostly everyone could get is, when the reinforcement bars get corrode, concrete gets cracked the durability of the structure get reduced, to achieve the durability one need to protect the reinforcement bars from rusting.
I do not find any difficulty in understanding above lines to understand the durability of the structure, but those lines are only lines and in practical application those are got forgotten or given second priority in work schedule because nobody put thoughts on it in greater details to make it understand in more practical way.
Basically durability can be achieved by ensuring there is no rusting of bars or degradation of concrete due to any kind of attack in the service life of structure. if it happens, structure will remain durable for its intended service life and beyond.
The corrosion of reinforcement bars is electro mechanical process where one end of the bar is act as a cathode and another as anode, during the medium of water or air, ends get charged with negative and positive charge which accelerate the process of corrosion of reinforcement steel bars. to make this happen the medium need to break the concrete.
Breaking of concrete or disintegration in concrete could happen due to various reasons like attacks from air, water etc. concrete is not compacted and have porousness inside of it or do have honeycombing which creates direct contact of medium with reinforcement bars.
No material on this earth is immortal, every material disintegrates over time according to exposure conditions, it would take several thousands of years to make steel as soil. only the longest surviving material which we see is stone and its a main reason why ancient civilization used it to build there structures and keep written records of their writings.
Coming back to the subject, to ensure the durability of the structure one should ensure the reinforcement bars are not getting rusted and concrete is not susceptible to any kind of attack during construction of the structure.
So in my opinion to make a durable structure, supreme responsibility lies with the Civil Engineers who are actual executing those job at construction site to build a structure. I will put some lights on basic things which could improve the durability of structure.
When a structural designer do designing of the structure, base on the exposure conditions, decides on how much cover should be given to reinforcement, so that it can survive till its design life. In case when you do not find the specifically mention cover to reinforcement bars on structural drawing, you should ask your structural consultant for details of covers to be provided. For a smaller projects or individual houses structural drawing given have notes on cover to be provided to reinforcement bars which has 3 or more tables which prescribe on cover to each member of structure as per exposure condition. If you are building such project, you should also need to take the details of cover in writing from structural consultant to avoid customer dissatisfaction in later stage.
Now i know what cover to be provided to reinforcement bars, when a structure is actually getting built, are we serious about the intensity of what happens when there is less cover than the specified, is i am reducing the life of structure? or we just so ignorant that we make it as a its OK to have less cover.
Here we fail to understand the structural designers point of view on having cover to reinforcement bars, this will inordinately going to lead in reduction of durability and by the time we may or may not be on this earth to see its refercations. So after understanding it, i Hope every Civil Engineer will give importance to give adequate cover to reinforcement bars.
We should also check for the correct size of the cover blocks used for doing construction of the project.
Cover block should be of same strength as of concrete which is being poured in structure, using weaker cover blocks will also lead to healthy and faster attacks on reinforcement bars which will reduce the durability of structure.
It is now trend to use PVC cover block due to its cheaper price but we should also think from the durability point of view to use those PVC cover Blocks or not. PVC never stick to cement, over the time it will form a separation cracks and make access to reinforcement bars.
After doing concreting, it is important to do construction joint preparation to ensure good bonding between old and new concrete we pace in structure. if the lattice not removed from the concrete surface, it may lead to weaker bond in concrete.
The smooth surface at the top of the concrete could be cause due to over vibration or fine particle accumulation during process of compaction of the concrete. It is important to remove all those lattice from the top of the concrete. this can be achieved by various ways, such as using water jet, roughening the concrete surface by making aggregate exposed without dislodging or you can press the 20mm size aggregates halfway into top surface to form a mechanical key for bonding of the concrete.
Same way construction joints in slab are also important and they are weaker sections in structure from where durability of the structure can get reduce. the joint should be made in a straight line and at locations as specified by the structural consultant only. it do happens that concrete get flowed beyond the stopper which was placed to arrest the concrete. in such cases all concrete which flowed beyond the stopper should be removed when its fresh or by breaking on very next day.
The bond between the concrete and reinforcement steel bar is very important part from durability point of view, during practical application, it is commonly observed that, some concrete do get stuck to reinforcement bars during concreting activity. it should be removed from steel bars to ensure proper bonding of concrete with reinforcement steel bars.
It is also observed that, during application of oil to shutters, some oil do also get spilled on slab and reinforcement bars get coated with it, we should no allow oiling of shutter on slab top once the reinforcement work is started. in case oil spillage observed over reinforcement bars, we should get it clean with detergent by washing.
When it comes to top slab reinforcement bars, we often use rebar chair / ghodi and it do have many local names, there is a possibility that rebar chair is directly placed on shutter as the bar bender do think that it will give more stability to keep top reinforcement bars in place but it will have negative impact on durability of the structure, the rebar chairs should be always kept on bottom reinforcement bars of slab. for tying each lag of chair should be tied to 2 reinforcement bars of the slab which will ensure no toppling of chair during movement.
The most prone area for all kind of loads on structure is the column beam junction which are most busiest part of the structure doing actively load transfers. we should ensure the lapping of column reinforcement bars should never fall in this junction which will increase the life of the structure.
Also, during tying of reinforcement bars the cage of beam reinforcement is made above the slab level and then lowered in the position. for columns which have slabs and beams all around and shuttering work is done, it is practically not possible to tie all stirrups of column in that junction. we should ensure those stirrups in column beam junction are tied properly as per spacing as this is most prone zone to cracking.
Low or inadequate cover to reinforcement bars lead to exposed reinforcement bars after concreting of structural members as the concrete does not pass due to low space. it can create another problem of honey combing too.
Honeycombing and exposed reinforcement bars in column do receive a patching with either cement paste or mortar or mortar with aggregate depending on the depth, in most of the cases those patching jobs are done by mason and mostly in hurry to ensure no one looks at it and make issue. also those patch wont get cure properly which can developes cracking and the source for reducing the durability of structure, if we do ensure proper covers are installed to reinforcement bars, those issues gets eliminated and durability of the structure is ensured.
Exposed reinforcement bars can also be observed in slab bottom when the adequate cover to reinforcement bars are not installed.
Exposure of reinforcement bars could also happen due to the workability of the concrete, if the concrete when received at site do lack in required amount, it will also create a problems of rebar exposure and honeycombing in structure due to no able to pass and reach each corner with efforts taken for doing compaction of concrete.
It is important to ensure the concrete does not lack in workability which is required to pour the concrete in structure. pressurizing the workforce to do the concrete pouring with concrete lacking in workability is not only asking for trouble but begging for it to come.
Ensuring the compaction of concrete and cover to reinforcement bars is the prime responsibility of execution engineer to ensure the durability of structure is achieved. Concrete which is not compacted properly will also lead to have porous structure in it which will lead to weaker zone which can initiate rusting of reinforcement bars.
After placing of the concrete and finishing the surface to desired finishing details, it is very much important to start early curing on the concrete surface by sprinkling water and covering with hessian cloth or plastic sheets. else will lead to shrinkage cracking in the concrete, shrinkage cracks can also go through the whole thickness of structural member. shrinkage cracks creates the entry for mediums to do corrosion of reinforcement bars.
Those cracks will also take one of the medium which is water from top to down through it and make a permanent way for it which could actively do corrosion.
As i have written many articles on concrete, i have not gone in details of concrete from durability point of view.
You might have noticed during reading the article, many times i written reinforcement steel bars. because it is important to protect it to ensure the durability of the structure.
After reading article, you might have understood that, who is responsible for ensuring durability of the structure which is actual executor of that job in field and they should therefore must certify the durability of the structure.
Wait a minute, how could they certify the durability?
Well it is the basic process in any construction company and comes in Total Quality Management.
Civil Engineers do certifies the durability of the structure by filling and signing the checklist for executed works by them, this articles is just for them to understand, what they do fill and sign in checklist is not just a point or not just a formality its a Responsibility of them.
I hope after reading this article, when you work at site and see a situation like any above point discussed, you will first get struct with the durability point of view and later the schedule time for completion.
I would also request other members to do post on points to be considered and look after during execution from durability point of view.
Also would like, many scholars to make this subject as your Project to be submitted in collage and try to understand the practical things and how we can improve it to meet the requirement of Owners, Designers and Build the structures for intended users which could last beyond the design life.
The durability of the structure is taken very seriously by designers of structure and designers of concrete. when it comes to designing of something, there are norms which gets followed to get a specific characteristic in design product.
But in actual who governs the durability of the structure, who can certify that structure will perform as same at its end of the life, its still a question which i have on my mind as there is no technology Today known to human being which can predict the life of the structure to which it can last and by looking at any structure no one can predict what is going on inside in terms of integrity. even if there is any technology available in market it can not be popular to be used by any house owner to check the remaining life of its home.
Structural designer did the job of designing a structure to a specified life by following codes in respective countries, Concrete designer did the designing of concrete to ensure the concrete is durable and is not a permeable, also the other attacks which can spoil concrete are also taken care during designing process of concrete by doing trials and test on it.
Up to this it looks OK and all the parameters are considered to ensure the durability or serviceability of structure for design life. After the designing is over, the next job is to execute the project and complete the structure to make it serviceable for intended users of it.
When i ask questions to executors of the job mostly a Civil Engineers who works at actual field, what is the durability of the structure, in most of the cases i found that, this is the subject which is not given to them for understanding in details.
A general answer which mostly everyone could get is, when the reinforcement bars get corrode, concrete gets cracked the durability of the structure get reduced, to achieve the durability one need to protect the reinforcement bars from rusting.
I do not find any difficulty in understanding above lines to understand the durability of the structure, but those lines are only lines and in practical application those are got forgotten or given second priority in work schedule because nobody put thoughts on it in greater details to make it understand in more practical way.
Basically durability can be achieved by ensuring there is no rusting of bars or degradation of concrete due to any kind of attack in the service life of structure. if it happens, structure will remain durable for its intended service life and beyond.
The corrosion of reinforcement bars is electro mechanical process where one end of the bar is act as a cathode and another as anode, during the medium of water or air, ends get charged with negative and positive charge which accelerate the process of corrosion of reinforcement steel bars. to make this happen the medium need to break the concrete.
Breaking of concrete or disintegration in concrete could happen due to various reasons like attacks from air, water etc. concrete is not compacted and have porousness inside of it or do have honeycombing which creates direct contact of medium with reinforcement bars.
No material on this earth is immortal, every material disintegrates over time according to exposure conditions, it would take several thousands of years to make steel as soil. only the longest surviving material which we see is stone and its a main reason why ancient civilization used it to build there structures and keep written records of their writings.
Coming back to the subject, to ensure the durability of the structure one should ensure the reinforcement bars are not getting rusted and concrete is not susceptible to any kind of attack during construction of the structure.
So in my opinion to make a durable structure, supreme responsibility lies with the Civil Engineers who are actual executing those job at construction site to build a structure. I will put some lights on basic things which could improve the durability of structure.
When a structural designer do designing of the structure, base on the exposure conditions, decides on how much cover should be given to reinforcement, so that it can survive till its design life. In case when you do not find the specifically mention cover to reinforcement bars on structural drawing, you should ask your structural consultant for details of covers to be provided. For a smaller projects or individual houses structural drawing given have notes on cover to be provided to reinforcement bars which has 3 or more tables which prescribe on cover to each member of structure as per exposure condition. If you are building such project, you should also need to take the details of cover in writing from structural consultant to avoid customer dissatisfaction in later stage.
Now i know what cover to be provided to reinforcement bars, when a structure is actually getting built, are we serious about the intensity of what happens when there is less cover than the specified, is i am reducing the life of structure? or we just so ignorant that we make it as a its OK to have less cover.
Here we fail to understand the structural designers point of view on having cover to reinforcement bars, this will inordinately going to lead in reduction of durability and by the time we may or may not be on this earth to see its refercations. So after understanding it, i Hope every Civil Engineer will give importance to give adequate cover to reinforcement bars.
We should also check for the correct size of the cover blocks used for doing construction of the project.
Cover block should be of same strength as of concrete which is being poured in structure, using weaker cover blocks will also lead to healthy and faster attacks on reinforcement bars which will reduce the durability of structure.
It is now trend to use PVC cover block due to its cheaper price but we should also think from the durability point of view to use those PVC cover Blocks or not. PVC never stick to cement, over the time it will form a separation cracks and make access to reinforcement bars.
After doing concreting, it is important to do construction joint preparation to ensure good bonding between old and new concrete we pace in structure. if the lattice not removed from the concrete surface, it may lead to weaker bond in concrete.
The smooth surface at the top of the concrete could be cause due to over vibration or fine particle accumulation during process of compaction of the concrete. It is important to remove all those lattice from the top of the concrete. this can be achieved by various ways, such as using water jet, roughening the concrete surface by making aggregate exposed without dislodging or you can press the 20mm size aggregates halfway into top surface to form a mechanical key for bonding of the concrete.
Same way construction joints in slab are also important and they are weaker sections in structure from where durability of the structure can get reduce. the joint should be made in a straight line and at locations as specified by the structural consultant only. it do happens that concrete get flowed beyond the stopper which was placed to arrest the concrete. in such cases all concrete which flowed beyond the stopper should be removed when its fresh or by breaking on very next day.
The bond between the concrete and reinforcement steel bar is very important part from durability point of view, during practical application, it is commonly observed that, some concrete do get stuck to reinforcement bars during concreting activity. it should be removed from steel bars to ensure proper bonding of concrete with reinforcement steel bars.
It is also observed that, during application of oil to shutters, some oil do also get spilled on slab and reinforcement bars get coated with it, we should no allow oiling of shutter on slab top once the reinforcement work is started. in case oil spillage observed over reinforcement bars, we should get it clean with detergent by washing.
When it comes to top slab reinforcement bars, we often use rebar chair / ghodi and it do have many local names, there is a possibility that rebar chair is directly placed on shutter as the bar bender do think that it will give more stability to keep top reinforcement bars in place but it will have negative impact on durability of the structure, the rebar chairs should be always kept on bottom reinforcement bars of slab. for tying each lag of chair should be tied to 2 reinforcement bars of the slab which will ensure no toppling of chair during movement.
The most prone area for all kind of loads on structure is the column beam junction which are most busiest part of the structure doing actively load transfers. we should ensure the lapping of column reinforcement bars should never fall in this junction which will increase the life of the structure.
Also, during tying of reinforcement bars the cage of beam reinforcement is made above the slab level and then lowered in the position. for columns which have slabs and beams all around and shuttering work is done, it is practically not possible to tie all stirrups of column in that junction. we should ensure those stirrups in column beam junction are tied properly as per spacing as this is most prone zone to cracking.
Low or inadequate cover to reinforcement bars lead to exposed reinforcement bars after concreting of structural members as the concrete does not pass due to low space. it can create another problem of honey combing too.
Honeycombing and exposed reinforcement bars in column do receive a patching with either cement paste or mortar or mortar with aggregate depending on the depth, in most of the cases those patching jobs are done by mason and mostly in hurry to ensure no one looks at it and make issue. also those patch wont get cure properly which can developes cracking and the source for reducing the durability of structure, if we do ensure proper covers are installed to reinforcement bars, those issues gets eliminated and durability of the structure is ensured.
Exposed reinforcement bars can also be observed in slab bottom when the adequate cover to reinforcement bars are not installed.
Exposure of reinforcement bars could also happen due to the workability of the concrete, if the concrete when received at site do lack in required amount, it will also create a problems of rebar exposure and honeycombing in structure due to no able to pass and reach each corner with efforts taken for doing compaction of concrete.
It is important to ensure the concrete does not lack in workability which is required to pour the concrete in structure. pressurizing the workforce to do the concrete pouring with concrete lacking in workability is not only asking for trouble but begging for it to come.
Ensuring the compaction of concrete and cover to reinforcement bars is the prime responsibility of execution engineer to ensure the durability of structure is achieved. Concrete which is not compacted properly will also lead to have porous structure in it which will lead to weaker zone which can initiate rusting of reinforcement bars.
After placing of the concrete and finishing the surface to desired finishing details, it is very much important to start early curing on the concrete surface by sprinkling water and covering with hessian cloth or plastic sheets. else will lead to shrinkage cracking in the concrete, shrinkage cracks can also go through the whole thickness of structural member. shrinkage cracks creates the entry for mediums to do corrosion of reinforcement bars.
Those cracks will also take one of the medium which is water from top to down through it and make a permanent way for it which could actively do corrosion.
As i have written many articles on concrete, i have not gone in details of concrete from durability point of view.
You might have noticed during reading the article, many times i written reinforcement steel bars. because it is important to protect it to ensure the durability of the structure.
After reading article, you might have understood that, who is responsible for ensuring durability of the structure which is actual executor of that job in field and they should therefore must certify the durability of the structure.
Wait a minute, how could they certify the durability?
Well it is the basic process in any construction company and comes in Total Quality Management.
Civil Engineers do certifies the durability of the structure by filling and signing the checklist for executed works by them, this articles is just for them to understand, what they do fill and sign in checklist is not just a point or not just a formality its a Responsibility of them.
I hope after reading this article, when you work at site and see a situation like any above point discussed, you will first get struct with the durability point of view and later the schedule time for completion.
I would also request other members to do post on points to be considered and look after during execution from durability point of view.
Also would like, many scholars to make this subject as your Project to be submitted in collage and try to understand the practical things and how we can improve it to meet the requirement of Owners, Designers and Build the structures for intended users which could last beyond the design life.
