ICF walls concrete pour completed

Polarwall ICF walls concrete pour completed

Well, after about 6 weeks of working evenings and weekends, we were ready for the concrete pour to fill the walls.

The run up to the pour was pretty nerve wracking for us. As anyone who has watched the drama series 'Grand Designs' will know, every single house build by ICF has a 'burst', which is where the polystyrene blocks burst open and all the concrete pours out.  Even all the self build guides cannot mention ICF without referring to a burst. For us, this would be even more of a problem as we are building inside an exiting structure - any bursts on the outside would be impossible to contain. Alan fom Polarwall assured us that if we had built it properly, there would not be a burst.


Craig took a week off work and built most of the walls. There were a lot of intricate details, particularly around the parts were the floor levels are different and where we had to build 'up and over' the internal existing concrete walls. Prior to the pour, we had to get the concrete mix right. Most people have no ide who complex and interesting concrete is.  We ordered a C40 CEM1 with 120mm target slump oversanded pump mix. Easy, yes?!

To give you an idea :

'Slump' - this is how much the concrete will collapse under it's own weight (when fresh) - the lower the number, the less this happens. The higher, the more watery the concrete. The problem with a high number is that there is more water, and this increases the pressure when you pour it into the walls, increasing the chances of a burst. The lower, the less workable the concrete is, which means it will not find it's way into every corner and crevice, round all the ties and reinforcing bar. Polarwall wanted a 90m slump, but because we were pumping through a long hose, the pump company wanted a higher slump nearer 150mm. We settled for a target of 120mm.

Compressive strength - Concrete is measured in difference strengths - this means how much force can be applied to it before it breaks. We were going to go with a C35, but on speaking to the Labs at Breedon (the readymix company), I identified that a higher compressive strength would 'go off' quicker - i.e start to set. This is because of the higher cement content, creating a reaction that would generate more heat, thus start to set more quickly. We settled on C40.

Cement content - readymix companies nowadays will generally sell you a CEM2 mix - what this means is that whilst there is a lot of cement, they also use by-products of other industries that have cemetious propertie e.g fly ash. This is cheaper for them (And us) and more environmentally aware. However, Alan from Polarwall wanted a CEm1 mix, which has more cement, so we went with that.

Because we were pumping concrete through a 2inch hose, we needed to make sure that the mix was wet enough and fluid enough not to cause blockages. To facilitate this, we asked for a oversanded mix, which adds more and with lightly less stone. Because we're adding more sand, we need to add more cement too...you can see how complicated it all becomes! The chap at the ready mix company was great and helped me understand it all and work out the best mix. I remember  long time ago reading that in many cases, the concrete for a job is often wrong and I can see how. If i had not paid so much attention, we could have ended up with too wet a mix, and lots of bursts, too dry a mix, and lots of voids in the walls, too thick a mix and lots of blockages....

So, Tuesday arrived, the pump guys setup their line up to the water tank. If you are wondering - we couldn't use one of these big booms you see because our house is too far back from the road. The guys were quite muted, but as they were never normally the chattiest, I didn't bother. The first load of concrete arrive and we started the pour. When the second turned up, I started to work out where the next load should get to and realised we wouldn't have enough. I then identified that I had miscalculated the concrete volume - I had ordered 20m3, but needed 23m3. I had not taken into account the fact that the walls actually started 300mm under the floor level.  A frantic call to Breedon ensued and thankfully they managed to find a mixer that was free and could get back up.

Everything went well and there was no drama. The pump guys completed the pour up to wall height and we put in the wall plates. We finished up at 5pm before heading to the airport to drop Alan from Polarwall.

When the pour was complete, the pump crew told us they had been dreading the pour - the do a lot of ICF for one company and there are always bursts, so they assumed ours would be even worse. That pleased us!

And so some pictures...