Help & Advice
The ICF 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).
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.
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