Lateral Pressure for Design of Wall Forms 6

[GalileoG]

I'm looking at a table based on ACI Committee 347 Pressure Formulas for placement of concrete at 10 ft/hr or less. The maximum lateral pressure that I have found for the form work in question is 407 psf or 19.5 kPa.

Does this not seem a bit overboard? I have never done forms before so I can't really trust my judgment on this one.

Also, will 407 psf be applied on the forms on each side of the wall regardless of the variation of thickness of the wall along its length? Or is my understanding of the concept completely wrong?

Thanks!

 

[SlideRuleEra]

ftp://imgs.ebuild.com/woc/C02j043.pdf

Trust the numbers, not judgment. The pressures are "real" and the consequences of form failure are expensive.

Yes, both sides of the form, whether the wall is 6" or 6' thick. Same principle as hydrostatic pressure.

http://www.SlideRuleEra.net

 

[hokie66]

The 407 psf is less than the pressure from 3 feet of fluid concrete, so rate of placement needs to be strictly controlled. Pressure depends on depth, not on a horizontal dimension. As SlideRule says, you don't want a form failure. Not a pretty sight.

 

[Lion06]

Yes, this will be applied on both sides of the wall regardless of variation of wall thickness. This is nothing more than a fluid pressure check. Simple fluid mechanics says that pressure is dependent on the depth of the fluid column only.

 

[DTGT2002]

The formulas presented by committee 347 reflect years of experience and experiment to determine appropriate pour pressures based on pour rates AND concrete temperture at time of placement.

Does your table reflect the effects of admixtures and concrete weight? If no, your table is not based on the latest recommendations for formwork design.

Please review ACI-347-04 for additional details or ACI document SP-4 for more details.

SP-4 is a very useful handbook of formwork that can serve as a good introduction to formwork design. It is not the optimal solution for all formwork cases, but is a good reference that will provide a safe product when followed.

Concrete behaves hydrostatically at initial placement, but begins to harden thereafter, deviating from true fluid behavior. The equations provided by ACI define this behavior. In recent years, the introduction of many admixtures and cement blends has led to a need to revise the standard equations to reflect these changes.

As such, Cc, the chemistry coefficient must be applied to the equations to account for the changes in fluid properties and "setting time" due to admixtures.

If you want to be painfully conservative but undeniably safe, designing for full liquid head is the most basic wall forming approach. This approach is currently recommended for SCC concrete, as the behavior of SCC varies with design mix and admixture makeup. It is possible to see pressures well below hydrostatic. However, site specific or project specific studies will be required to give fair data at this point in time. Resaerch is ongoing to provide a good and sound method of estimating SCC pressures.

Sorry for the long winded answer. Please post if you still have concerns or review the concrete forming forum.

Daniel Toon

 

[GalileoG]

Thanks everyone!

For DTGT2002:

I pulled my 407 psf value from a book that is more than 30 years old (if memory serves me right.)

What I will do is include a statement that says "Set retarding admixtures shall not be allowed for the retaining wall concrete mix. Maximum slump at the point of placement shall not exceed 75mm"

 

[DRC1]

Not to complicate matters, but form pressure is also dependent on temperature.

 

[DTGT2002]

DRC1, I think it is not a complication - it is a necessity to stress the temperature is key, just like the rate of placement.

I would think it would be a good idea to purchase ACI347-04 if this standard is appropriate for your area, or determine what guidline should be adhered to in your location.

The theory is all there in the post by SlideRuleEra and I have saved it to my files, as I was not aware of that article and in fact thought that ACI did not officially present Dr. Johnston's (et al) work until a later version of the 347 guideline.