Pouring 2 different slab levels 3

[Uhin]

Hello all,

How would you pour a concrete slab with a level difference of 1m. There is a beam running between them
Can you cast in 1 go or should it be done in 2 pours with a construction joint?

Can the second pour be done after 48 hours of the first pour after the initial setting?

Thanks

Section attached.

 

[BridgeSmith]

I would think you'd want to do it in 2 pours, with a construction joint at the top of the lower slab.

48 hours seems like an excessive amount of time to wait before pouring the wall and upper slab, though. I would expect 12 hours to be plenty of time for it to set. Most likely, 3-4 hours would be enough. Now that I think about it and did a little math, once the lower slab is firm enough to walk on, it should be firm enough to support the weight of the concrete in the wall and the upper slab. That's typically more like 1-2 hours.

 

[SlideRuleEra]

I would use construction joint as shown. For pour #2, wait at least 24 hours in warm weather, at least 48 hours in cold weather; don't want to risk breaking the rebar / concrete bond of bars projecting out of pour #1 by making pour #2 too soon.

 

[swazimatt]

definitely 2, if not 3 pours. single pour will bulge your slab where your 400 dimension isas there will be 1m head and vibrating will liquefy everything.

also need to consider the formwork. If you are unable to support the RHS of the beam you may only be able to install the vertical formwork once the lower slab has been cast, in which case it is good to cast in a 50mm kicker (blue line) to fix your formwork to, and green-cut/wirebrush the joint to remove any latents

 

[BridgeSmith]

don't want to risk breaking the rebar / concrete bond of bars projecting out of pour #1 by making pour #2 too soon.

I hadn't considered that possibility. If the 2nd pour was done when the 1st pour was just stiff enough so it wouldn't displace the lower slab concrete, would that still be a potential problem?

also need to consider the formwork. If you are unable to support the RHS of the beam you may only be able to install the vertical formwork once the lower slab has been cast,

That would be another consideration. The lower slab would definitely need to be a little stiffer if it has to support the extension of the upper. How much depends on the width of the support at the lower slab. At at 90mm wide, it would apply about the same pressure to the lower slab as the wall section (3.5 psi psi). If a 170 lb construction worker can walk on the slab in regular shoes, it's stiff enough to support the concrete and forms.

 

[SlideRuleEra]

BridgeSmith - Waiting at least over night in warm weather is good practice since there is more to this than just making pour #2. Properly constructed formwork for the build-down and possibly some of the pour #2 rebar will have to be placed... this will take time. That work alone should not be attempted over relatively "fresh" concrete. Also, if pour #2 was made too soon, the projecting rebar could be damaged during pour #2 concrete vibration inside the build-down.

The OP (wisely) does not seem to be in a hurry, just asking if 48 hr. is too long... IMHO, no, waiting 48 hours is good.

A qualified heavy construction or bridge contractor will have no problem constructing structurally adequate forms to resist fresh concrete pressure + internal vibration of a 1 meter build-down. As a former bridge contractor, we routinely designed, constructed, used plyform concrete forms that were 12 to 15 feet deep... photo of one of our overpass' crash walls adjacent to RR track shown in the photo:

 

[BridgeSmith]

One of the bridges I designed had wall piers that were 31' tall. Each one was poured in a single loooong day - 16 hours; partially because the ready-mix company only had a few trucks to bring the 235 CY of concrete to the site, but also because the concrete at the bottom had to stiffen up some so that the fluid pressure wouldn't blow out the forms.

 

[swazimatt]

Sliderulera and bridgesmith I am sure none of your pours also included the horizontal footing being cast at the same time as the vertical wall? that will be as quick as filling Dear Henry's bucket.

(PS @slideruleera I miss the days when we could balance on the top of formwork and hang off the rebar using pure common sense and darwinian survival LOL)