# Slip forming



## Rich D. (Oct 14, 2011)

Willy is said:


> Rich.....I'm sorry, but you are incorrect. In the classic sense, they are monolithic; it is one essentially continuous pour, one piece. Each layer, each pour is vibrated into the previous, wet poured onto wet.


So if the concrete is not cured but strong enough to take the weight and another layer is set on top of that it is still considered monolythic?

BTW I'm not trying to be a smart ass or sarcastic its a serious question..


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## KennMacMoragh (Sep 16, 2008)

Rich D. said:


> So if the concrete is not cured but strong enough to take the weight and another layer is set on top of that it is still considered monolythic?
> 
> BTW I'm not trying to be a smart ass or sarcastic its a serious question..


That's how I interpret it, monolithic meaning no cold joints. Even the times I've done a real monolithic pour where the footing and stem wall are formed into one, we still do it in layers but it doesn't sit for very long before we apply the next layer. If it sat for more than two hours between layers is when you might have a problem if you were going for no cold joints. Even if you vibrate it in to the previous layer, at that point you'd be disturbing the concrete after hydration, the point where it starts heating up and taking off, which is a no no.


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## Rich D. (Oct 14, 2011)

But don't the concrete need to be pretty hard for the slip for to be moved up to the next level?


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## KennMacMoragh (Sep 16, 2008)

Rich D. said:


> But don't the concrete need to be pretty hard for the slip for to be moved up to the next level?


I haven't seen it done in high rises, that's why I was curious about it. But no it doesn't take long for concrete to hold its shape, when it comes out of a curbing machine it's still wet, hasn't really begun it's hydration yet. Whenever I pour a slab in sections, I can remove the mid section screed and the concrete will stay there even if it's only been sitting for half hour. After about three hours, from the time it's mixed at the batch plant is the danger point, after that you don't want to be mixing or adding water to it, if you do it will lower the psi strength of the concrete. After two hours time it's questionable, three hours it will cause problems.


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## Willy is (May 20, 2010)

Rich, I going from memory and there could be varying practices from types of projects.

http://users.encs.concordia.ca/~zayed/JOURNALS/24.pdf

look at figure 1

You have a form, you fill it with concrete, say a few inches at a time. Gradually, the first poured concrete starts to set and is self supporting. The jacks are hit and the forms climb an inch. The self supporting concrete supports the wet concrete above it and keeps it from leaking out the bottom of the form. With every jack the form climbs up an inch, which creates additional space in the top of the form for wet concrete. As wet is added, steel reinforcing is added so there is both vertical and horizontal steel in the walls.

A "jackman" walks around with a probe, just a piece of rebar and jabs it into the wet concrete. Picture in a 42 inch form that the stuff on the top is wet; right out of a truck, the stuff coming out of the bottom of the form is hours old. It is still green, but self supporting and can be patched, brushed and finished as it exits the forms. 

And so yes..... the 8-12 inches of the top of the pour is still wet, get vibrated into subsequebnt layers of pours and so it is considered monolithic. In fact, there are no cold seams, it is one piece. In highway and tunnel construction the slip form also but i would venture a guess that there are expansion joints in those.

I did work a slip in which they did not have manpower to man an evening slip and so at the end of each day they jacked free and started anew each morning. Due to the batter of the forms, which are slightly wider at the base than at the top, this would leave a mild "shingle" at each cold joint. I wasn't there for that, but they had issues before I came, and had a lift in the initial pour of the day. The pour stuck to the forms and rode up with the forms causing a huge honeycomb, walls falling out, bent jackrods and several hundred feet of wall repair about 40 foot up in the air. Jackhammer time. : )

The rate at which we would pour would depend upon weather factors, but i recall getting 8-9 foot in a 12 hour shift (nights generally had slower setting concrete and did less footage). Some of these structures we did 135 feet in 6-7 days of continuous pouring

willy


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## Dirtywhiteboy (Oct 15, 2010)

I fuen hate slip forms They are used all the time over here for elevator shafts and the like greasy taper ties I almost died tied to a slip form, had some punk a$$ed local boi screaming at me I untied went down and almost kicked his a$$.


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## Willy is (May 20, 2010)

I'd like to buy a vowel.

(Translate that last one for me please) : )


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## Rich D. (Oct 14, 2011)

Thanks Willy. Interesting read. I always though concrete was pretty well set up. But now I understand its just hours old or less till the move the forms... Very interesting.. thanks for explaining :thumbsup:


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## concretemasonry (Dec 1, 2006)

.Once you start a large slip form job it is a 24/7 proposition and the concrete supplier is important to constantly provide a steady supply of concrete. When there are openings or cutouts, they have to be set before the forms get up to them. Any openings can cause some distortion and cause the ending structure to be inconsistent. For tall buildings, the walls are usually covered and it can be hidden. For silos, it is more critical because variations can cause bridging of materials (grain, aggregates, etc.)


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## Rich D. (Oct 14, 2011)

How are forms for opening placed and kept in place while the pour is going?


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## Willy is (May 20, 2010)

Rich D. said:


> How are forms for opening placed and kept in place while the pour is going?


I provided a link just above this thread. Look at figure 1
http://users.encs.concordia.ca/~zayed/JOURNALS/24.pdf
the brown is the wooden deck and form, the grey is concrete, the red and the blue represent the yokes which attach to the forms and provide rigidity to the structure, attach to the jacks and transmit lift from the jacks to the forms.The little blue bump on top of the horizonal yoke is the jack, and it climbs the jack rod. the bars to left and right represent vertical steel.

You can also see real life pictures of a deck in Nebraska in the 70's which used archaic electric jacks, but you can see the steel yokes and jacks in place and identify the jack rods as well.

http://www.contractortalk.com/f11/all-around-4000-so-what-then-118944/index3/


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