Transfer Slab Horizontal Pour Break Question and sequence

[said the noob]

contractor has wanted to pour 1700mm dp transfer slab in two lifts, first lift being 900mm dp and second lift being 800mm dp, horizontal shear is minimmal as the horizontal shear plane is quite close to the COG of the entire 1700mm section.

but they had also mentioned that the slab will be destressed after the first lift, and then retensioned prior to the pouring of the second lift,

anybody know what is the purpose of the destressing and retensioning of the formwork pior to the second lift? they intend to pour the first lift in day 1 and then destress/retension, then day 2 will pour the second lift, shoring of the full 1700mm dp slab will be provided,

can anybody shed some light on why they are doing this? and is it allowed? it would seem the first lift 900mm dp slab would not have reached the required strengths to destress, at that point it would have to be self supporting of its weight during the brief moment is destressing/retenstioning.

TIA

 

[slickdeals]

The horizontal shear is maximum at the neutral axis. You will need to make sure that the interface shear is accounted for to enable full composite action between the two pours as you will have a horizontal cold joint through the mid-thickness of this member.

It looks like the contractor wants to pour the transfer slab in two stages to minimize the load that the formwork has to be designed for. The reason for decentering the formwork is to allow the first 900 mm to be self-supporting and relieve the load from the formwork. When they tighten the formwork again, then you are only carrying the weight of 800 mm of concrete.

 

[said the noob]

Thanks for your reply Slick,

May I ask why the horizontal shear is greatest at neutral axis? I think it is a relationship between the area above that you are considering with the COG of entire section,

I believe the shear flow equation is,

q=VQ/I, where Q=(Area above or below)*y, area is the area above or below the horizontal joint with respect to the COG of the entire composite section, so in our case it would be the 800mm dp second lift slab per width of section I am analyzing, y is 400mm (centroid of area above or below to COG of composite section. I would expect the further I am from the COG the greater the force I would need to design for, hence if my joint is very high or low along the depth I would expect to see higher stirrup requirements?

Also would it be too early to destress the first 900mm slab after one day? They do not plan on using high early str mix, slab is 40mpa

Tia

 

[KootK]

I agree with Slick that, if you're doing your VQ/It correctly, the maximum horizontal shear stress should occur at mid-depth. Moreover, it's prudent to consider a plastic state of stress in addition to an elastic one in reinforced concrete. That state of stress would produce a maximum & uniform horizontal shear stress at all cross section locations between the reinforcing plane and the compression block.

The first pour may well require more than a day's curing before de-stressing the shoring. It's something that deserves some careful consideration. Your low pour likely does not have top steel over the columns and, as such, may be prone to cracking and/or punching shear issues. Those cracks and stresses, such as they are, will be locked in when the second pour is placed. This may not be a big deal if the first pour thickness is so deep relative to the spans that, effectively, no slab cracking occurs.

A two way slab has very high local shear stresses near the supports. Ideally, you would be providing ties etc such that the demand at those peaks was met in kind. That can be pretty onerous, however, leading designers to cheat the elastic stresses a bit in favor of redistributing the horizontal shear about the slab plan. For the most part that's kosher. But one does not want to invite too much local slip potential between pours at the hot spots as, like the locked in low pour stresses, if complicates any estimate of slab deflection.

 

[said the noob]

Thank you guys for your replies you are correct we should see highest stresses near the neutral axis due to Q being large compared to y,

We have added a top mat over the first pour to take care of the negative moments that occurs with the first lift having to carry its self weight + construction live load and we also assumed it carries the 800mm do slab above also,

We beefed up our hairpins for certain areas where we see high shear forces such as near support columns and near transferring columns where we may see horizontal slip.

Cheers