Hi all – I'm not seeing much detail on the concrete and steel requirements for ICF systems. I think Fox Blocks said that bare minimum concrete strength was okay, something like 2,500 psi. I haven't seen any specs for the rebar. What strength of concrete and steel did you use in your builds? What grade of rebar do people typically use for ICF? Did any of you need to upgrade to a stronger concrete or steel to get approval from engineers for your house? I've seen some posts about ICF designs not being satisfactory to engineers or inspectors on some criterion or another, maybe some specialized needs. I've been thinking of speccing higher strength regardless of what's technically satisfactory. American concrete is strangely weak by world standards. 2,500 – 3,000 psi seems common in residential, but it's trivial to achieve 6,000 psi at this point, and higher profile projects are hitting 10,000, 20,000, and even higher strengths. (High strength would be easier to achieve if we forgot about blocks and just precast the concrete walls in a proper facility, and then wrapped the walls in insulation foam boards on both sides.) American rebar has gotten stronger in recent years, and I wonder if the house would benefit from using at least Grade 60. I wonder what the tangible benefits would be of a significantly stronger house. No version of an approved ICF house is going to fall down, so maybe the benefits would be limited to seismic resilience or something. Maybe the longevity of the home? It's inherently satisfying to build and live in a stronger house built to newer, higher standards as far as the concrete and steel grades. Have any of you tweaked or optimized your builds to make them stronger? What about more robust support for attachments, siding, etc.? I noticed that ThermaSteel uses steel studs turned sideways to eliminate thermal bridging, which is brilliant. Their panels are like SIPs, but with steel framing instead of OSB boards. It occurred to me that it would make sense to embed steel studs, straps, or "mini-studs" in the outsides of an ICF wall. I don't understand how the built-in plastic ties are supposed to work as far as attaching heavy loads like Hardie Plank or other fiber cement boards. I've seen the Fero ICF connectors, and I don't understand what we're supposed to do with their "V-Ties", how anything would screw into them. Steel studs would be straightforward, and they could be bolted into steel tracks at top and bottom. Has anyone done this or offered it? Thanks for your feedback.

Major ICF manufacturers have blanket engineering available that lists size, spacing, grade, etc of rebar along with thickness, strength, etc. of concrete for common applications. This information is also available with building codes in Canada and all/most of the US( (correct me if I am wrong, as we do not operate outside of Canada).

These designs carry a healthy safety factor, so increasing strength of concrete/rebar would be a redundancy. You can increase the structural design beyond what is recommended (ie, thicker concrete, more rebar, etc), but I don't feel this would be money well spent IMHO.

And most State building codes have a prescriptive design section for ICF and if you stay within limits, no PE stamp is required. And these designs meet all structural requirements with only 2,500 psi concrete. But there is nothing that prevents you from exceeding code requirements by going to a higher compressive strength concrete...other than higher expense. All ICF can easily hold Hardie siding so zero need to solve a problem that doesn't exist. Do some research...

Compared to stick built, an ICF building is practically a bank vault. Unless you are going for long spans or weird corner details, I doubt a high strength concrete would give any benefit except maybe improved hurricane/tornado performance.

Good enough seems to work quite well.

And seismic...we use 6000 psi concrete and more robust rebar to achieve 9.2 seismic design in our area. Not that this is required...more that it is desired... And again, just building to code only means you have crappiest house that can be legally built. One is allowed to exceed code if desired...