BBICF,
This is always a debated question here as to what is the best ICF mix. I use different mixes for different applications. If I am doing a short stem wall mono poured with a footing, then I typically use a simple footing mix of 5 to 5 1/2 sack with 3/4 rock at a 4 to 4 1/2 slump. Nothing fancy. If I have a high rebar issue or walls with lots of windows to pour under I will go with a more aggressive mix that is a 6 to 6 1/2 sack pea mix with a mid range water reducer, high range super plasticizer, viscosity modifier, slag and or fly ash, and air entrainment.
In my high performance mix the high cement does a few things first off it kicks fast, so only a qualified professional should use it, but the high cement also helps it to flow. The mid range water reducer does just that, reduces the amount of water we use which increases strength early but also increases the speed at which the concrete becomes unworkable. The high range super plasticizer helps with the flowability of the concrete but it also reduces the water needed in the mix. This is a double edge sword, it makes the concrete flow, but by reducing the amount of water it makes your mix "hotter" and can cause workability issuses if you don't watch and manage your concrete properly. The viscosity modifier is a great product, sometimes called liquid sand it makes your concrete flow like you are pumping a grout mix with 60 to 70 percent sand. It also keeps the mix from segregating when it hits the rebar, webs, and ties. This mix has very little water and we will pour it up to a 7 or 8 slump, but as soon as it stops flowing or moving it starts to set up. The fly ash and slag are cement replacements and depending on our requirements depends on what it does for your mix. Slag and fly ash are a round compound when looked a microscopically, cement is flat. Slag and fly ash act like little ball bearings and help with flowability. Now one of our mixes which is a "green" mix has 50 percent of the cement replaced with 40% slag and 10% fly ash. The action of the slag and fly ash in this mix retards the curing process dramatically. We had some 2 day test cylinders this year that broke at 800 psi then the 7 day was almost 4000 psi, when on the same pour we also poured the same mix only with 100% cement and the 2 day break was 3000 psi and the 7 day break was 4200 psi.
To make a long story short, if you order a 6 sack 100% cement with NO fly ash or slag, and with a mid range water reducer pea mix with 60% pea and 40% sand you should have a mix that reaches 3000 psi in less than 7 days. A couple of things to remember, the less water you add the higher and quicker you will reach strength and with a 6 sack I would find out how long it takes a truck to get to my site and add a retardant to the load for the road trip. This will ensure a good fresh batch of mud for you to play with.
ICF Contractor
PS as to your question on on the exterior use of Simpson ICFVLs. The ICFVL is not rated to be used with the new pressure treated lumbers so a few years ago Simpson quit recommending them for exterior use. Personally I think they would work with something like Ipa, structual cedar, or Silwood but would want to call Simpson and run it past them first.