huangyhg

i c f blocks
we have received a number of reqests to design and certfy house foundations using icf blocks ( 6 or 8 inch concrete wall sandwiched by styrofoam sheets.) what do you guys think of that type of construction. i have made cut tests and found honeycombing ... that makes me nervous. it seems that workmanship is a big factor. contractors here do not require liscence do build them in sone cities. some juridictions requier engineered drawings .... some dont. they seem to be marketed on the claim for "fantastic" insulating value and heat cost savings.

you have hit on the biggest fault with an icf system. they are frequently used by diyers or people that claim to be contractors. the bracing of the walls is critical.
even more critical is the placement of the concrete (rate and methods). all too often, it is too wet and just dumped where access is convenient and allowed to flow and then topped off for that area.
unfortunately, the forms are left in place and there is not way to spot the typical problems of cold joints and honeycombing. under/around windows and near corners are biggest problem areas.
good specifications and enforcement during construction are the keys. a good local testing lab with non-destructive equipment can catch many of the classic errors after the fact.
it is ironic that they seem to be popular below grade where it is easy to do it wrong and where the insulation value is not as critical because of the moderating effects of the soil. the below grade waterproofing problems are whole other world.
above grade, the structures are usually used by more qualified contractors and the concrete is pumped, so slower vertical pour rates and lifts are easier to do and control. it is much easier to make one set-up and pump multiple lifts in rotation and bulkheads can be used.
some of the icf block suppliers have excellent engineers on staff and technical support. others have noting and are only qualified to make foam trays for the meat department at walmart.
dick
from an engineering standpoint, make sure you know that the icf product the owner/contractor wants to use is in compliance with the codes (aci for example). we have run across ansi "approved" icf products that have the plastic reinforcment "holders" where you snap the rebar into place that did not meet the clearance requirments of aci 7.7. when we told the manufacturer, there only response was "we are ansi approved".
the other hitch is that many of the products have the plastic reinf. holders formed into the foam (i.e. you can't remove them) which leads to a problem when you want to cut the form down to fit a custom dimension. the best way around this is to just make sure your fdn dimensions match the block coursing layout.
and lastly, we have seen products with a 7 5/8" concrete core that the contractor wants to substitute at the last minute. unfortunately, most residential walls are designed for 8" thickness, so from a design standpoint we had to tell them to pay us more $ to redesign for a 7 5/8" wall, or to find a forming system that provided a true 8" core.
the honeycomb problem can be overcome by using a high slump concrete mix, but like you mentioned, one will never know because you can't see through the forms.
i do quite a few icf homes. i always spec the splump, a 3/8 aggregate and a vibration. i only like the flat wall systems, the conical or waffle systems make me very nervous. pay attention to your top plate connection at the concrete interface (if they are above grade walls). in high wind regions there have been failures of the top plate in cross bending, anchor bolts are not enough, you need some type of steel bearing plate.
i've done a few icf projects and have developed a program for plotting the interaction diagram for axial load and moment and have used them for substantial basement areas both commercially and residentially. also developed several cad blocks for standard conditions.
there are a couple of design manuals, one prescribed and the other a design manual. i don't recall if they were aci or pca (the latter, i think) that are pretty good.
the reinforcing shown in the manufacturer's literature is generally excessive, but not always.
dik

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