Early loading of RCC slab

[pkap]

Can someone share with me the criteria or requirement for early loading of concrete slab while it is still under curing period. The early loading is primarily due to movement of crafts, formwork table for next level of slab, temporary storage of rebar, wooden planks etc. Appreciate your help. Thank you. Regards

 

[ishvaaag]

I would say the later the better. However, this is contrary to your intent. Whilst the deformation is enough restrained by the underlying shoring, one should be able to carry operations as soon as the concrete is really set; following the first rule, if later, better.

It is also important to clarify what is your visualized scenario ... "moving of formwork" might mean entire inmediate removal or underlying formwork, and of course if the slabs get unsupported at an early age the scenario needs be carefully considered. The actual procedures need be ported to the necessary analyses to ensure that nor danger nor lack of serviceability will result from them.

 

[pkap]

Thanks Ishvaag, Ofcourse the later is better but sometime unavoidable due to schedule constraint or delays. By the way, we are achieving full concrete strength after 3 days. Does it give some assurance that early loading well within the design strength (shear and moment capacity) of newly cast slab would be ok?

 

[ishvaaag]

Certainly if the fromwork schedule does not overload some slab, attaining full strength in 3 days is very good for your intent; you trade later strength gain (normally) for a quicker schedule of construction.

In any case, mantaining "some" shoring is always good to the longterm defelection performance, and attainable through (only) some shoring systems (and not always compatible with what to be made in the to be by then initilly unshored floors.

Effect of progress of construction is analyzable, and I think to have seen some articles on it mainly in the context of composite columns and beams construction, since through the advance of the steel parts more floors can be loading in a number of shored or partially shored slabs. From the technical standpoint for RC it is not much a challenge (more of patience), it is a matter of making the model for each of the stages of the construction, be it by themselves or in the context of one program able to deal with staged construction. When as you have the design strength as early you might even use final concrete properties (if met by the schedule) to make such "construction time" analyses.

 

[dik]

The early creep of the concrete is usually a bigger problem with greater long term deflections.

Dik

 

[pkap]

Thanks Ishvaag and Dik for the insight.

 

[cvg]

suggest using concrete maturity meters to monitor in real time the strength gain of the slab and base your decision on the data you collect.

[link ]

http://www.transportation.njit.edu/..._OF_CONCRETE_MATURITY_METERS.htm#_Toc42508288

[/url]

 

[NS4U]

You need to be really careful with this. At early ages, concrete stiffness doesn't following strength.... that is, you may have your 28 day strength after 1 or 2 days, but you don't have the stiffness of a 28 day concrete. I think this is what dik is elluding to. There are papers out there on this, I can't put my hands on them though at the moment.

 

[dik]

That's exactly the problem NS4U... I have several papers on this, and will try to dig them up... Green concrete deforms more plastically.

Dik

 

[dik]

Should have added that a maturity meter doesn't help much...

Dik

 

[DST148]

Ref: ACI seminar - Troubleshooting Concrete Forming & Shoring - Flexural strength of the young concrete slabs can be taken conservatively as proportional to the comressive strength development. If the factored load capacity of the slab is 250 psf, then the load capacity of the slab at 75% f'c might be 0.95 * 250 ~ 238 psf and not 188 psf.
Deflection & shear strength is proportional to sqrt(f'c).
Formwork is typically removed at a compressive strength of 75% f'c.
For form removal and reshoring purposes, the key is determining the strength of concrete in the actual structure. ACI 318 & ACI 301 allow various test methods like field-cured cylinders, cast-in-place cylinders, penetration resistance, pullout strength, maturity factor etc. Although field-cured cylinders typically under estimate strength of in-place concrete, it is suggested for simplicity and reliability.

 

[dik]

... for small percentages of steel, as normally encountered in a slab, the concrete strength has very little influence on the flexural strength... it has an impact on the shear capacity. The big thing is the increased creep that can occur if the slab is not sufficiently reshored.

Dik

 

[rapt]

The two main problems will be reduced concrete modulus at time of loading (at very early loading it is doubtful if the reduction is linear) and significantly higher creep effects which definitely are not linear, both resulting in significantly higher deflections. If loaded at 7 days you could expect about 50% more creep deflection than if loaded at 28 days. If loaded earlier, this would be significantly worse.

If you are achieving full concrete strength in 3 days, you obviously are not using a standard concrete mix. You would need to do tests on that mix to determine all concrete properties (creep, shrinkage, modulus etc) and do the deflection calculations based on those properties. You cannot assume that you have 28day properties just because strength is achieved.

 

[shobroco]

I might point out that temperature has an effect as well. I had a slab that more than met design criteria even with 3rd party analysis after the fact but it deflected about 4 times what was acceptable and the only conclusion that we could agree on was an elevated temperature during initial cure. Creep ain't pretty!!

 

[pkap]

Thank you all for your important/ critical input. In a nut shell, creep behaviour shall also be considered in recently cast concrete slab when it comes to early loading (and not just the compressive strength). Regards

 

[dik]

pkap... you bring up another matter... it's not only the re-shoring that is important, but also the type of load the structure is subject to... for most offices, the actual live loading is only maybe 20% of the design load for the typical bay sizes... if a structure, be nature, is subject to a relatively hight percentage of the actual design load, then the same applies... actual loading should be delayed as long as possible.

Dik

 

[rapt]

Shobroco,

I know this is a different topic, but how did early high temperature increase the deflections by that much? Yes, it will introduce restraint stresses when the temperature reduces, and can cause some ugly effects in columns due to excessive shortening of long lengths of slab/beams, but I would not expect its effect on deflection to be any where near 4 times (then 4 times acceptable does not mean 4 times expected I suppose, depending on the calculation method used to calculate the expected)!