Concrete Wall Formwork Design 4

[bookowski]

When pouring a concrete wall(s) the worst case design pressure would be p = (w x h). If the wall is poured in lifts or at a slow rate then the concrete should be setting up and reduce that pressure, i.e. reduce the effective 'h'. The time required would vary by the particular mix, but are there any guidelines out there for this? ACI 347 notes 'for forms that can be filled rapidly before stiffening of the concrete takes place, h should be taken as the full height of the form'.

What would be a reasonable way to determine how long is required, or what pour rate, to not need the full 'h'?

(Assume that I can't provide ties to take the full w x h)

 

[Hoaokapohaku]

I know of no specific guidelines for this. It's important to consider what is referred to when waiting for the concrete to "set up" enough so the next lift can be poured. It is not initial or final set of the concrete; that would produce a cold joint. Rather, the setting needed before the next lift is placed is a product of false set; early stiffening of the concrete due to plaster rehydrating into gypsum or ettringite formation. The timing and amount of false set that occurs is a function of many variables including cement chemistry, time, temperature, w/c ratio, etc. The number of variables in play makes it nearly impossible to determine or predict how much time is needed between lift placement. Because of this, normally the contractor's foreman or the masons themselves make the determination by observation of the in place concrete. Generally, they are very good at it. They have learned the hard way what the concrete looks and feels like when the next lift can be placed.

 

[slickdeals]

If you look in ACI 347, there are formulas to calculate maximum formwork pressure based on rate of placement (R). This can be found in Section 2.2.2.1. As you rightly said, you can reduce the rate of pour to reduce your formwork lateral pressure.

 

[bookowski]

Thanks.

I have seen some formwork design aids that provide pressure based on pour rate (feet per hour) but they seem to vary and I haven't seen much documentation on where they get their numbers.

 

[slickdeals]

Attached is a good article on Formwork Pressures by Mary Hurd

http://www.concrete.org/tempComDocs/107394-6023/ci2906hurd.pdf

Also see attached for some class notes on temporary works

http://courses.washington.edu/cm420/

 

[PMR06]

APA has some good design guides for formwork design. Here's a reference I've used many times:

http://www.g-lumber.com/pdf/APA-Concrete-Formwork.pdf

 

[bookowski]

Thanks slickdeals and PMR, good info - just what I was looking for.