Dear all,

I would like to know the any graph or relation ship between form work stripping time vs compressive strength of concrete.

As per IS 456: 2000 Amendment 5 mentioned the form work stripping duration for different type of structures.

But I want the relation ship between stripping time vs compressive strength.

## Gopal Mishra

The strength of concrete increases with age. The table shows the strength of concrete at different ages in comparison with the strength at 28 days after casting.

AgeStrength percentSo, the minimum stripping time of concrete should be such that it can gain sufficient compressive strength to withstand its self-weight.

If there is any construction to be carried out above the member, then that should only be carried out till the concrete has gained sufficient strength to withstand the working load, self-weight, and weight from construction above it. Till the concrete gains that much strength, the formwork should not be removed and no construction should be carried out above it.

Now we can not relate the compressive strength in MPa with the stripping time, as different members may have different requirements, and also it may have different grades of concrete. If we see the compressive strength gained in percent, then we can co-relate somehow based on the actual requirement of the structural member.

## Gopal Mishra

Dear Seshachalam, the compressive strength of concrete does not depend on the formwork stripping time. Formwork stripping time is specified in order to prevent deflection and stability of the structural member.

Compressive strength of concrete mainly depends on the quality control during mixing, transporting, placing, compacting, and curing of the concrete.

Early removal of formwork from concrete section may lead to:Kindly refer this article: Concrete Formwork Removal Time, Specifications and Calculations## Seshachalam Chamarthy

Dear Gopal Mishra,

Thanks for your reply.

I saw your answer and refer to the article; both are representing general information. It is good. But my requirement is the relationship between the compressive strength of concrete vs. stripping time of formwork.

Some relation is there either theoretically or experimentally. IS code 456 amendment 5 mentioned in a tabular format for stripping time for different structures. That means some relation was already derived. That is my requirement.

Based on the relationship & factors, we may improve the same further. That means reduces the stripping time and saving the construction time.

Thank you for your reply.

Seshachalam ch.

## APRAMEYANAGARAJA

Dear Seshachalam,

Formwork stripping time solely based on adequate strength gain, and associated capability of the structure to withstand the self-weight and any immediate availing superimposed loads. Further, it should be capable of sustaining any oncoming live loads in any form for which the member is designed. We can estimate the probable strength of concrete at any given instant of time, as depicted in the equation as in cl—5.1.1 of SP:24.

This gives a guideline on the estimation of strength gain at a given instant of time. This cannot be considered as a substitute for validating the compressive strength (cube strength) by testing it under compressive testing machine, the required compressive strength as prescribed for the structure has to be met. I hope this helps.

## Seshachalam Chamarthy

Dear APRAMEYANAGARAJA,

I saw your answer and IS Code SP 24, and both are representing gaining compressive strength of concrete with time. The equation of compressive strength with respective time is good. But my requirement is the relationship between the compressive strength of concrete vs. stripping time of formwork like relationship between stress vs. stripping time.

“As per IS 456:200 Clause 11.3 Stripping time – Forms shall not be released until the concrete has achieved a strength of at least twice the stress to which the concrete may be subjected at time of removal of formwork.”

Some relation is there either theoretically or experimentally. IS code 456 amendment 5 mentioned the tabular format for stripping time for different structures. That means some relation was already derived. That is my requirement.

Based on the relationship and factors, we may improve the same further. That means reduces the stripping time and saving the construction time.

Thank you for your reply.

Seshachalam ch.

## Kuldeep Singh

Hello there Me. Chamarhy! I read the complete thread and I understand that you haven’t exactly got the answer you are looking for. Well, that’s partly because you are slightly misinterpreting cl. 11.3 of IS 456 (2000). It says that

formwork must not be removed until concrete gains at least twice the strength it needs to stand the stresses it will experience during formwork removal.Now, if you look carefully, this doesn’t mean that the code has given this advice with concrete’s final compressive strength in view. It is solely there for thesafety of the structural member during formwork removal.The table within that clause, says that for ordinary conditions (OPC is used, ambient temperatures >= 15°C, adequate curing is done), these stripping times for the concrete to reach that above mentioned strength will be so and so for so and so types of formwork.

It is not talking about the compressive strength of concrete.Apparently there is indeed no specific mathematical relationship between the formwork stripping time and the

final (28 days) compressive strength.This guideline is solely for the protection of concrete during formwork removal. Obviously concrete gains strength the better the better it’s cured. So, if someone wishes to strip early and begin curing early, the code is trying to advice against it. Furthermore, concrete indeed attains strength in direct proportionality with the quality control in production, the quality of the materials and the adequacy of curing. As far as I think, there could be only a little implication that concrete gains strength well if it retains heat in very early age and formwork does insulate the concrete. But that’s not much important because you can’t wail too long to strip formwork so that you can start curing when it’s the right time.Now, the next point is the theoretical or experimental evidence that you are seeking. I’d like to humbly tell you that the table in 11.3 which claims that these are the times for achieving double the strength requirement for formwork stripping must indeed be based on experimental data. It is easy to model the stresses induced at the time of stripping. Now, the strength of concrete within formwork can also be estimated via pull out test and a few non destructive tests. You can Google ‘concrete strength tests during maturity period’ and it’ll pop up with a number of papers and articles. The pull out test for instance, corellates the compressive strength to the force required to pull out a core of specific shape and size, in a specific way from the maturing concrete.

To conclude, the code is not talking about a relationship between removal time and the compressive strength. It’s only talking about the corelation between removal time and the strength

at that very time of formwork removal, for which, experimental evidence has long been found.Sorry for a long explanation. But, I’ll be more than pleased if it clears your doubt. Thank you!