Insulated Concrete Formwork Help & Advice

Our insulated concrete formwork help & advice centre has been created as a reference and information hub for designing and building ICF structures in general, and Polarwall in particular.

Insulating Concrete Formwork Design

Insulated Concrete Formwork Concrete Specification 

Insulated Concrete Formwork Concrete Specification 

The Insulated Concrete Formwork concrete specification needs to have a predictable rate of cure. Otherwise there may be too much pressure from fluid concrete in the formwork. To achieve a predictable rate of set we need to ensure that the ready mix concrete company aren’t playing fast and loose with the concrete design.


The most expensive element in the concrete is the cement. Concrete companies love to reduce the cement and replace it with one of two waste products that are cementitious by nature. The two waste products are PFA (Pulverised Fuel Ash) and GGBFS (Granulated Ground Blast Furnace Slag). These concrete with these additives in will still reach the 28 day target compressive strength. However PFA and GGBFS will slow down the curing rate and the concrete will take longer to lose fluidity causing greater pressure within the formwork. Therefore,  the first thing to ensure is that the mix is a CEM1 mix (this denotes pure Portland Cement). 

Concrete Strength

The second thing with the ICF concrete specification is the ultimate strength of the mix. For above ground structures (that are not reinforced) the concrete needs to be a minimum strength of 25kN which is classed as C25. Reinforced walls must have a minimum compressive strength of 35kN so this is classed as RC35 (the”R” denotes reinforced). 


The ideal aggregate size for the pour is 10mm. This flows nicely and will usually compact better than the larger 20mm stones, which can cause hang ups around heavier reinforcement. 

Having a smaller aggregate also has advantages in the pumping. A 3 inch hose is ideal and most pump operators will not use a 3 inch hose with larger aggregates as this is more likely to block.

Target Slump

Next is the slump.  This needs to be wet enough to be pumped and to fill the voids under proper compaction. However if too wet it will create higher pressure in the formwork.  Concrete slump refers to the amount that the concrete inside a 300mm high cone will slump down when the cone is lifted up (think making sandcastles on the beach). 

Slump can be a target slump or a slump range. Slump ranges are specified as: 

  • S1 – 0mm to 50mm  
  • S2 -50mm to 90mm 
  • S3 – 100mm to 150mm 
  • S4 – 160mm to 200mm 
  • S5 – 210 and above 

After this level we are down to a different test by measuring  by how far it spreads out from the centre. 

The ideal slump for us is around 100mm.  This flows nicely and the very open nature of the Polarwall formwork means we get good compaction. Some systems complicated, convoluted shapes in their ties structure may need to have a higher slump to achieve adequate compaction. This specification is a guide for Polarwall which may be of some use to others. 

Read more about pouring concrete into insulated formwork.

If you have any questions about Insulated Concrete Formwork Concrete Specification get in touch today.

The various sections above cover the main topics for Insulated Concrete Formwork (ICF) construction both above and below ground. 

Do not use this as an alternative to what your chosen manufacturer recommends.  We can do things with Polarwall that ICF blocks can’t do and there may some things that you do with other ICFs that Polarwall can’t do. For example our instructions on vibrating the concrete may not be wise on some EPS block systems. 

We want to add to and develop this section, so we welcome all your questions, suggestions for topics and feedback.  If you want to go deeper into a topic please let us know.  This advice hub does not replace our training (or any other companies training) and you still need to avail yourself of our FREE training course.

Have a question or suggestion not listed here? Feel free to contact us