Slab on grade curing time for construction loads 1

[bhiggins]

Hi All,

I have a commercial project where an existing slab on grade will be replaced with a new 5" concrete slab, reinforced with #4 @ 12" O.C. each way. The concrete mix is 4000 PSI concrete with 25% flyash. The contractor wants to use the slab as soon as possible to perform the remodel construction. There will likely be forklift traffic (weight unknown right now) which will be carrying steel beams during construction. We told him 3 days cure time before using the slab. The contractor is wondering if there is a target concrete break strength and if there is a possibility to continue with construction any earlier.

Does anyone have any good advice for this situation? I'm thinking that the contractor should use a mix without flyash to attain higher early concrete strength. I'm wondering if using an accelerator would be suitable in this situation, but the concrete will be exposed so I'm worried if the accelerator will cause any finishing issues or other unforeseen issues. 3 day strength of a 4000 PSI mix is roughly 40% = 1600 PSI. Using different analysis methods, this strength seems adequate for heavy point loads, and strengths as low as 1000 PSI seem adequate also.

Here's my summary:

I don't think it's a good idea to use the slab prior to 3 days cure time with conventional concrete. Target strength prior to using the slab shall be +/- 1500 PSI. I don't think using a 5000 PSI or higher mix will help attain that strength in less than 3 days. I'm unaware of the consequences of using a chloride-free concrete accelerator.

Thoughts anyone?

 

[MIStructE_IRE]

I always just go by 3 days. Its impossible to predict what is going to drive on this, how many movements, turning maneuvers etc - so even if you had a target strength, I’m not sure how you would go about designing it.

Stick with 3 days. If he wants to go earlier than this he can - provided he pays for and rectifies any damage caused!

 

[JAE]

Yes - delete the fly ash if they want to get on earlier.

You can increase the cement content - but with floors that could mean higher levels of shrinkage and more cracking.

We do allow the contractor to take (and pay for) additional cylinders to test for strength at various times (3 day, 5 day, etc.)

With forklifts, I'd be wary about the high wheel loads and require the full 28 day strength before exposing the floor to the wheels.
Even then I'd want to check that the wheel loads are acceptable - based on normal slab-on-grade design using the applicable f'c, slab thickness, base stiffness, etc.

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[Ron]

Agree with JAE....a 5" slab is bare minimum for any wheel loading and forklifts can put significant loads on the slab. When you assessed the strength requirements were you only looking at shear or did you consider bending?

 

[Ingenuity]

I recently placed several thousand square feet of 5" UNreinforced concrete slab-on-grade at a quarry as part of a new mansand plant. 3,000 psi and 4,000 psi concrete placed (whilst equipment operational) and tele-handlers and skid-steers loaded the slab less than 24 hours after placement. We fully expected (and client did not care) it to crack, but, damn, could not find a single crack.

I don't recommend loading it like we did, but sometimes, with good ground prep and okay mix design, things work out. I certainly would not do it for a client who has different expectations.

 

[bhiggins]

I modeled the reinforced slab in RISA foundation, 100 PCF subgrade modulus, 1000 PSI concrete with two 12k point loads 3' apart returning acceptable results with #4 @ 16" O.C. each way.

I also checked the unreinforced slab using the equations in this paper which yielded a 24k point load at 1000 PSI:

https://www.structuremag.org/wp-content/uploads/2014/08/C-StructuralDesign-Azzi-Apr081.pdf

I'm not sure if these equations are calibrated for this concrete strength, but it appears to be OK.

 

[BridgeSmith]

The slab may have adequate strength for loading when the concrete hits 1000 or 1500 psf, but its resistance to tires, etc. scraping and gouging the surface may not be acceptable. As others have said, that will depend on the owner's expectations (and the equipment and the way it's handled on the surface).

 

[JAE]

Don’t you mean 100 PCI for the subgrade modulus?

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[MotorCity]

We don't specify a minimum time, we specify a minimum strength (usually 75% of f'c). You can still have poor concrete after 3, 7, or 28 days.