cast in place walls with internal insulation; thin walls/cols

[ae11]

In a single-family residential project, we're considering use of a cast in place concrete wall system where there are a couple of inches of rigid insulation embedded in the center of the wall forms. In effect, if we had a 8" total wall thickness, we'd have let's say 3" concrete face, then 2" rigid insulation, then another 3" concrete face.

Questions:
(1) general thoughts or experience with this system
(2) any way to get the system to behave more like an 8" solid wall, instead of two 3" concrete faces? The hope is that this system might be able to carry vertical loads from above.
(3) how to accommodate concentrated loads at the top of this wall, like where a concrete beam is supported at the top of the wall. One option might be to not use insulation immediately below the concentrated load, using 8" solid concrete all the way to the ground (could be a couple of feet in width). Another thought is to fill in a solid area immediately below the concentrated load, like how you might grout solid a few cells in masonry immediately around a concentrated load, but the solid area doesn't extend all the way to the ground).
(4) on a related question, how thin are people typically comfortable with for a solid concrete wall (ignoring the internal insulation question) for 2-story residential construction? Assume I have a better-than-average concrete contractor. Also assume I want to bear concentrated loads from concrete beams on top of the wall.
(5) how skinny/practical can I make a concrete column, assuming it's lightly loaded, not exceptionally slender, etc...8"x12"?? Of course I know it depends on the loads and slenderness ratio, but I guess I'm trying to determine what the practical limit might be for good quality residential construction.

Keeping my fingers crossed for not overly conservative feedback

 

[concretemasonry]

It sounds like you are looking at one of the ICF (Insulating Concrete Forms)systems.

There is a wide range of availability of technical support. One of the most common with a good technical, design and installation support is the Rewards system (no connection to me), but there are some other good ones. There are also some lower level form producers that concentrate on making meat trays for the Walmart meat department instead of technical/design support.

Dick










Engineer and international traveler interested in construction techniques, problems and proper design.

 

[vandede427]

this is done in tilt-up all the time.

 

[frv]

Precast as well.. in fact, you can add some shear ties (proprietary stuff) between the wythes to make it composite..

 

[concretemasonry]

Sorry - I got the layers reversed and came up with an incorrect system.

Sandwich panels are commonly used in the U.S. and internationally. The key is to have adequate ties between the two exterior layers to create a composite wall section.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[jike]

I recommend following the practices of precasters or tilt up contractors. When pouring panels horizontally, the control is much better and the panel can be thinner. If poured in the vertical position, air can easily get trapped or concrete can bridge over areas if to stiff or too much reinforcement. If pouring in the vertical position, concrete must be flowable so mid to high range water reducers are a must.

We poured a recent sandwich panel wall in Wisconsin in the vertical position for an industrial project and it came out well but you must use the proper ties and sufficient thickness to be able to properly place the concrete.

 

[Teguci]

When connecting the two wythes together consider the loss in insulation. Carbon fiber reinforcement will maintain your thermal break and connect the two thin concrete elements. For local loads consider bracing the load bearing wythe with carbon fiber reinforcement while taking the compression down the 2" concrete panel. Another option, of course, is a bump out.

 

[Maverick59]

I am very interested in this post as we are going to be building a Detention Center with 14' to 16' exterior walls, at 12" thick, and we are looking at placing 2" foam in the middle to raise the R-factor to 13. We are planning on using steel forms and pouring the inner 6" with the foam lacing the exterior of the form, and then after the concrete sets, moving the form along the extended ties to pour the outer 4" with the 2" foam on the middle wall, and pour in 100 foot sections. The question I have is, on a 16' foot wall using a medium water reducer, what is the recommendated rate of pour to be able to complete the section in one day? And are there any recommendations with this type of pour system?

 

[jike]

Have you gotten bids from precasters as an alternate?

I would discuss rates of pour with your ready mix supplier along with the admixture supplier. If you have never done this construction before, I recommend pouring a test panel to work out some of the issues first.

 

[Maverick59]

Local political direction excludes the use of precasters by wanting to keep the labor local, so we will pour in place. But yeah, we've never done a pour such as this, and will get with the ready mix company for the details, and thanks for the suggestion of a test panel.

 

[TXStructural]

For cast in place, I'd make one side (interior in Southern climates) thick enough to act as your bearing wall. Frequently, 4 inches is sufficient for supporting a light roof and normal wind loads. 6 inches for two-story, heavy roof, or high wind regions.

Look at PCA-100 (for ICF's) to get an idea of the capacity of such a wall without calcs.

There are sandwiched foam systems which have the shear ties, but you are now raising the cost, since you are buying a somewhat expensive system AND paying for forms.

The benefits of ICF construction includes the thermal mass being insulated from temperature swings, and the near absence of forming costs beyond the ICF itself.

Having the concrete mass of the exterior could be beneficial to avoid additional cladding costs if exposed concrete is the look you want. Exposed concrete could also as a passive heat collector for warm days and cold nights, but then insulating inside the concrete would negate the benefit.