Shoring removal time for concrete Slab 1

[Italo01]

Hello, I've read this old thread about Formwork removal and i'm interested in this topic since i'm designing a composite steel-concrete building for the fabricator/erector and he wants to reduce the shoring removal time.

https://www.eng-tips.com/viewthread.cfm?qid=414461

Someone nicknamed NS4U said that "Modulus gain lags strength gain " and others agreed. My concrete professor at the time said this also, but the brazilian code has a equation to predict the time dependency of the elastic modulus based on CRB-FIP model(1990) that shows that the increase in the elasti codulus is faster than the increase in strength.

I'm attaching the CRB-FIP model relevant part .

Using the equation, at the time that the concret has 75% of strength at 28 days, it has 86,6% of modulus of elasticity at 28 days. Can anyone provide a source which contradicts this.

Thank you.

 

[Enable]

As far as I know most of the code relationships between modulus of elasticity, and the concrete strength, are empirically derived with E ~ sqrt(fc`). Naturally this means if you double your fc` you'll have less of an increase in E (fc` from 10 -> 20 results in an increase in E of only 41%)

Early age concrete is different. The empirical relationships dont quite hold and need to be used with extreme caution. That said early age concrete in this context is a day old, and so for the time frames we tend to deal with, the relationships will suffice.

You should check out the ACI-U course on shoring and early concrete behavior. It's something like $30 and well worth it. Here are some slides from the lecture on early age concrete behavior.

Even the code relationships are not so great even for concrete that has aged some. It's best to use a good deal of caution when relying on them for mission critical elements. The Canadian standard has it written right in the code that the expected range is between 80-120% of that predicted by the equations.

EDIT - Should also be mentioned that the slide below is for relatively high strength concrete. I believe the code equations are better for conventionally ranging mixes (20-40 mPa) but cant access anything to show that right now. Does anyone have a comparison table between prediction / experimental from various code equations for conventional mixes?

 

[Italo01]

Thanks Enable.

I'm thinking about something along 4 to 7 days, so this ACI clause does not apply. Also, the concrete has a conventional strength.

Do you know if the ACI prrdiction of time dependent Modulus of elasticity is similar to the onr of CEB-FIP that i posted?