Why Concrete cube test done on 7 and 28 days

[Srinivasan]

Can any one explains the true reason behind checking the concrete for compressive strength on 28th day.why not 27th 29th or 30th day.i am not looking for a answer like it attain strength of 99%on 28th day etc.it should have valid reasons.even though I am more than 40 years experience in construction I am not aware the reason
Today I read a article on the above subject.i want to cross check the same with our learned members

 

[Jayant Joshi]

Dear Sir.
Good question, I have also started thinking the same.
But what I believe structural designer designs RCC grade with reference of strength after 1 week (7 days) and after 4 week (28 days), which is normally checked at site.
If after 1 week the strength is OK and after 4 week the strength should be this, and if OK concrete is acceptable.
I think this time duration is again decided to decide total time line for any project. ( After 4 week you can remove shuttering etc.)

This is my opinion not supported by any document.

But you are right some can can start new data for testing after 5 days and 25 days, with proportionate strength reduction.

 

[Dnyan Deshmukh]

The main reason behind this is human mentality to forget about things (natural phenomena).

May be masters when they are deciding on at what age we should test the cube, they might had discuss on this natural phenomena too.

The thing is that each week have 7 days and every Monday comes after 7 Days, 14 Days, 21 Days and 28 Days etc and continue so on ( you may add 35 days 56 days so on)

The basic principle is, There is holiday once in a week and most probably the works are not done on Sunday (May not be true in Today's scenario)

So if we cast cube on any working day if frequency of testing is not 7 Days and 28 Days, the testing of cube might fall on Sunday and if it gets skip or forgot, it may cause issue in statistical analysis of cube test report which is used further for doing concrete mix design optimization and dealing with causes of failure and other research related thing.

So to make life easier and to ensure all day to day works are done (as there is no excuse that i forgot or miss that and mostly all people do their routine works without fail) cube testing activity might have set in pattern of repeated week days).

In some places mostly small house project where homes are built by small contractor or house owner they do not have the testing facilities and might not be strong in doing calculation, to facilitate de-shuttering this might help like if we cast slab on this Monday, we can de-shutter it only on next Monday or after that (here doing calculation of age of slab by them might go wrong)

When i was a Trainee engineer, i too asked same question to my VP of concrete cell during a seminar and he explained me in same way, how i written here and this make some sense of common thing.

When you ask anyone on construction project where, de-shuttering of slab is not done - when this slab was casted?
Everyone including Engineer will tell you, this was casted on last Monday (its easier to remember rather than memorizing the dates)

Best Practice
Always do de-shuttering only after getting desired cube results on specified days as there is continuous changes in process and we are Today doing concrete with lot of other admix such as Flyash, GGBS etc to make concrete more economical, sustainable, durable etc.

Hope it make some sense for why 7 and 28 days, its not about engineering but something related to ease of work (derived days obviously meet the requirement base on the testing done by those master at that time and still stands true to mostly produce concrete as per conventional mixes and to max extent with design mixes excluding mixes which are design with high volume admix which meets the requirement for which it is designed)

 

[Dnyan Deshmukh]

Dear Sir.
Good question, I have also started thinking the same.
But what I believe structural designer designs RCC grade with reference of strength after 1 week (7 days) and after 4 week (28 days), which is normally checked at site.
If after 1 week the strength is OK and after 4 week the strength should be this, and if OK concrete is acceptable.
I think this time duration is again decided to decide total time line for any project. ( After 4 week you can remove shuttering etc.)

This is my opinion not supported by any document.

But you are right some can can start new data for testing after 5 days and 25 days, with proportionate strength reduction.

To achieve early de-shuttering
Special type of concrete is designed to get higher strength at early age or simply higher grade concrete is used to get early strength.

Also the special type of Formwork systems are used such as Aluminium Formwork which have lesser span where props are removed after 7 days and we can remove shutters without disturbing the load carrying props.

We use to de-shutter after 36 or 48 hours base on achieved cube test strength by comparing it with desired strength for doing de-shuttering.

Such type of projects are tight scheduled and we need to work for more time and need to keep eye on everything (on Holidays too).

Normally on such Projects staff are in double shift and everyone knows about their duties and have proper co-ordination with each other to avoid mistakes in faster construction (Only System driven companies able to make such progress with Quality, Man driven systems tend to fail during their absence)

 

[Srinivasan]

Why do we test concrete on 28th day? why not on 27th day or 29th day?

This question remained unanswered to me until I got into final year of graduation. I explored a lot to get the answer by approaching various text book sources from the college library (remember there was NO google in April 1992), civil engineering associations and institutions from UK, Australia, etc. But the answer came from an unexpected source in my hometown, Mr Damodharan Nair, a retired draftsman of Thanjavur Municipality. Before we go to the answer, we have to peep into the history of civil engineering. As we all know civil engineering and concrete technology were developed and practiced in England in ancient times and now US is leading in advancing the technology further.

Concrete achieves its maximum material strength over its life time and depreciated by exposure and other environmental conditions. However, engineers with experience observed that about 99% of expected design strength was achieved in about 30 days and also observed that the rate of gaining strength was reducing thereafter making the concrete achieving its 100% design strength in a year or two.

The Church was so powerful those days that it was compulsory for every citizen to attend Sunday prayer without fail. The defaulters were punished severely. If they had to test concrete on the 30th day, it may fall on a Sunday. In order to escape the punishment by the Church, the engineers had two choices - they have to test concrete cube on 28th day (4x7) or 35th day (5x7). Since 35th day was far away from 30 days, the engineers started to test the concrete on 28th day. And the 28th day will never fall on a Sunday because they would never go to work and cast the concrete on Sunday. This custom was later made as part of British Standards when it evolved. Most of the Indian standards are based on the corresponding British standards and hence we also follow the same 28th day benchmark to test the concrete strength.

Now you can relate this information to our practice to test concrete on 7th, 14th and 28th day. (all in multiples of 7). While it is interesting to know the bit of history of concrete testing, its now time to re-think the age at which we test concrete. India in the past installed many coal fired thermal power plants of which the fly ash was a major by-product. The fly ash accumulated over years posed a big source of air pollution. So the authorities after many discussions with experts have given direction to cement plants to add increased percentage of fly ash in the cement manufacturing. As a result, the rate at which the concrete use to achieve strength has reduced and in many cases, 99% strength was achieved beyond the benchmark of 28 days. We have been constantly persuading the Bureau of Indian Standards to increase it from 28 days to 36 days. Of course, this is not good news for the Developers who are driven with promises of 7 day slab cycle in high rise projects.

The purpose of this article is to pacify the feeling of chill running down your spine when a concrete cube test fails next time. If the cubes fail on 28th day test, no need to panic and call for a NDT. You can wait and test another set of cubes on 36th day or 45th day. It is better to plan well and cast few additional sets of cubes at the time of casting.

Take your concrete seriously....n it will pay back in long run.

 

[Dnyan Deshmukh]

Why do we test concrete on 28th day? why not on 27th day or 29th day?

This question remained unanswered to me until I got into final year of graduation. I explored a lot to get the answer by approaching various text book sources from the college library (remember there was NO google in April 1992), civil engineering associations and institutions from UK, Australia, etc. But the answer came from an unexpected source in my hometown, Mr Damodharan Nair, a retired draftsman of Thanjavur Municipality. Before we go to the answer, we have to peep into the history of civil engineering. As we all know civil engineering and concrete technology were developed and practiced in England in ancient times and now US is leading in advancing the technology further.

Concrete achieves its maximum material strength over its life time and depreciated by exposure and other environmental conditions. However, engineers with experience observed that about 99% of expected design strength was achieved in about 30 days and also observed that the rate of gaining strength was reducing thereafter making the concrete achieving its 100% design strength in a year or two.

The Church was so powerful those days that it was compulsory for every citizen to attend Sunday prayer without fail. The defaulters were punished severely. If they had to test concrete on the 30th day, it may fall on a Sunday. In order to escape the punishment by the Church, the engineers had two choices - they have to test concrete cube on 28th day (4x7) or 35th day (5x7). Since 35th day was far away from 30 days, the engineers started to test the concrete on 28th day. And the 28th day will never fall on a Sunday because they would never go to work and cast the concrete on Sunday. This custom was later made as part of British Standards when it evolved. Most of the Indian standards are based on the corresponding British standards and hence we also follow the same 28th day benchmark to test the concrete strength.

Now you can relate this information to our practice to test concrete on 7th, 14th and 28th day. (all in multiples of 7). While it is interesting to know the bit of history of concrete testing, its now time to re-think the age at which we test concrete. India in the past installed many coal fired thermal power plants of which the fly ash was a major by-product. The fly ash accumulated over years posed a big source of air pollution. So the authorities after many discussions with experts have given direction to cement plants to add increased percentage of fly ash in the cement manufacturing. As a result, the rate at which the concrete use to achieve strength has reduced and in many cases, 99% strength was achieved beyond the benchmark of 28 days. We have been constantly persuading the Bureau of Indian Standards to increase it from 28 days to 36 days. Of course, this is not good news for the Developers who are driven with promises of 7 day slab cycle in high rise projects.

The purpose of this article is to pacify the feeling of chill running down your spine when a concrete cube test fails next time. If the cubes fail on 28th day test, no need to panic and call for a NDT. You can wait and test another set of cubes on 36th day or 45th day. It is better to plan well and cast few additional sets of cubes at the time of casting.

Take your concrete seriously....n it will pay back in long run.

Church may had laid foundation for Holy Day and later become holiday which is Sunday. No one would like to work on Holiday, do you?

First of all compulsory attendance in Church was until 1791 (check wikipedia)

It may be wrong but i got this information.

Now lets go back in history of concrete

First Portland Cement was invented by Master Joseph Aspdin in 1824 at England.

First Cube testing was done in 1836 at Germany.

In 1849 Reinforced concrete was developed by Master Joseph Monier

In 1854 first home was constructed in RCC by Master William Wilkinson.

In 1903 the first high rise (The Ingalls Building) of 16 story constructed in Cincinnati, Ohio, United States.


In 1913 first Ready Mix happen in Baltimore, Maryland.

This is what i got in History, i may be wrong but above all are facts.

 

[Dnyan Deshmukh]

@Srinivasan Sir, You are the most senior person at here and started this question and also you did some reply to it.

Request you to close this thread by choosing write answer for it from one of the reply, if you think there is answer in any of the replied post.

Your question is very nice and can make people to think and remember something which always make smile on there face when someone ask this question to them again in future.

 

[Tejas Jain]

This answer was given by IIT Bombay faculty.
The system of cube curing and testing was started by British Government. In that era Saturday and Sunday used to be holdiay. To avoid the testing date to coincide with Saturday or Sunday, cube testing used to be scheduled on 28 days.
Similar answer is suggested by Deshmukh sir.

 

[Manohara64]

For 7days ⅔ of strength will be achieve means 65 -70% of strength ..

 

[Demir]

İt May be an excuse for Church visitors,what about those with various beliefs and creeds?

 

[veera1234]

For cube test, 65% of strength achieved with in the 7 days of casting date makes so early to gain the strength but later 99% strength takes more than 2 weeks approximately 21 days later achieving strength takes lot of time after 7 days and it gains 99 for 28 days, so only we check for 7 days n 28 days for cube test results.