Elevated Slab - Concrete Strength? 2

[mjohan]

Prior to setting interior load bearing walls on an elevated slab constructed with composite steel floor joists, what is an accepted minimum compressive strength for the concrete?

TIA

 

[JAE]

In general terms, concrete is usually adequate for continuing construction operations at about 75% of f'c.

However, for your case with load-bearing walls, you, as the engineer, should check on the adequacy of the whole system (slab and beams) to receive load-bearing wall loads at various levels of f'c.

So the answer to your question is not a general one, but is specific to your project and the loads applied. You are the only one who knows this so you are tasked with checking the flexural and shear capacity of all affected members and deciding on what f'c level is appropriate.

I would guess that you would want at least 90% of the f'c (if not all 100%) in this case.

 

[MotorCity]

mjohan,

Because the slab supports load bearing walls, I would DEFINITELY recommend performing a structural analysis if loads are going to be applied before the 28 day strength is achieved. Also, since the joists are composite you should check the design for whether it is based on a shored or unshored condition. Even if your calculations indicate otherwise, I would still specify that 75% of the strength be achieved before loading.

Good luck.

 

[CSEllc]

I agree with both previous answers.

As minimum you should have 70-75% of f'c before any work is being preformed on the slab. But you need to check the entire system to determine what you really need.

You might find that at the reduced concrete strength, the deflection of the steel joists might exceed that maximum allowed by MSJC of L/600 or 0.3".

 

[mjohan]

Thanks for your advice.

At this point I could use some advice on defending a 75% strength requirement. Any codes or publications which specify this minimum number?

A joist company (supplier of the floor system for my project) has indicated that construction may continue the day after the concrete has been poured. They state that a 1000 psi concrete strength is sufficient for construction loading with a recommendation to exercise care when working (i.e. storage of materials, bundled mesh, etc.) Is this practical, responsible and/or safe?

Your thoughts are appreciated.

 

[SlideRuleEra]

The 28 day compressive strength is typically specified by the engineer of record and is based on calculations. You may find recommendations based on previous (successful) projects. However don't just accept what a supplier tells you without verification.
Take a look at the following thread for a similar discussion: thread507-90025

 

[mjohan]

Thanks SlideRuleEra,

I took a look at the link and the "rule of thumb" is evident in everyone's posts on this forum.

The problem I seem to have is that the joist supplier is also the slab designer. The overall system is a composite system. Many of their design concepts, including construction loading, is based on testing. Because this slab is a proprietary system, I cannot really run numbers on their system and tell them it does, or does not, work.

The criteria specified on their slab system is to strip the forms when concrete reaches 500 psi, and construction may continue at 1000 psi.

My issue is that the entire engineering world feels differently.

 

[JAE]

Well, here's another take on it - as my earlier post assumed (incorrectly) that you had a fully cast-in-place system - while it appears that you have a steel joist system that uses the concrete slab compositely.

In this case, the weight of the joist itself, and the wet concrete, can be successfully carried by the joist itself, with no composite action required. Once the concrete has hardened, then the joist/concrete begin to act compositely and the system is much stiffer. The ultimate strength of the system is based on a full f'c and this limit state is based on a fully loaded floor (100% live load, with factors).

During construction you do not really approach this load level and the floor system engineers may be correctly stating that the dead load of the floor, and subsequent "construction live load" of perhaps 10 to 20 psf will not hurt the concrete as the stresses are low. So the 1000 psi concrete limit may actually work just fine.

The other consideration is the ability of the concrete slab to span between joists - this also should be looked at in addition to the joist/slab beam system.

 

[cap4000]

To put in some perspective on the slab f'c values. At 24 hrs after the pour typically it's 20 percent of f'c. At 3 days after it's 50 percent of f'c at 7 days it's 70 percent and at 14 day's after it's 90 percent of f'c. The 500 psi you mentioned is called the "vibration limit" whereby you cannot re-vibrate the concrete any longer and plastic shrinkage is virtually gone. My issue would be the entire "World" feels differently. I suggest you get the joist/slab designer to give it to you in Writing that he is responsible for any construction related problems at 1000psi. Don't hold your breath!!! Perhaps some shoring or high early strength concrete would be my prescription if he can't wait at least 36hrs. Proceed with caution!! Good Luck!!

 

[jheidt2543]

There was a time when concrete poured on a metal deck/steel joist had to be shored until the concrete reached strength. Now, with labor rates as high as they are, designers tend not to use that method and the deck/joist are designed to carry the wet weight of the concrete. In that case, getting on the new slab the next day is no problem structurally, but you may have to protect the finish of the slab from construction damage.