Some really nice weather arrived for the Easter weekend, so instead of sitting out on the deck drinking G ‘n’ Ts we got to finishing off our first little rammed earth wall.
From lessons learned on the first section, we removed the chamfer piece from the top of the form and tried freehand cutting a chamfer at the end instead.
We also discovered the origin of the bowing on the walers. It’s not the ramming of the dirt per se, but rather in the clamping. Our bolt clamps are outside the form, and were meant to provide a simple automagic method of holding the end of the form in place, against the rods. But the walers bend in at the ends ever so slightly as the rods are tightened, which translates to a noticeable bend outwards in the middle.
You can’t avoid having holes in the wall if you’re going to use a through-clamping method of holding the form. So the bolts need to go through the body of the form to prevent the walers from bending. We’ll do that next time.
Levelling the top of the wall was a lot easier without the chamfer pieces in the form.
We ran out of dirt at the end, and it seemed wasteful to make another batch just for a tiny corner. We left it as it was knowing we’d come back to do the last section the next day.
The bow in the wall is much more apparent with the form off. The slight curve also meant that where the two sections join the second section was slightly wider than the first section and there was a really obvious line on the wall that we don’t want going forward. The formwork has to be perfectly straight to make sure things like that don’t happen.
Chamfering was done with a mini mattock and a mallet, then smoothed with a steel ruler. All freehand. The chamfer is fine for the present purposes but it would be much better to do it against a guide that can keep it uniform along the length of the wall.
The dirt mix was a bit wet, too, and stuck to the form. Bits came away with the form. Again, they’re fine here but we’d want that to not happen going forward. We’ll try a drier mix and reduce the clay content.
The next day it was on to the last section. The PVC pipe we’d put into the second section had moved while ramming, though, and didn’t line up properly with the holes in the walers. We couldn’t get the rods through.
We’d decided that the big rods were too thick anyway, and had some thinner ones at hand. The thinner rods allowed for more play and we were able to get them through the holes. Again, having the through rods inside the edge of the form would make that a non-issue.
As we finished the last bit we realised that we didn’t have enough sifted local dirt. We’d calculated a 2:1 compression ratio from our test cylinders, and it turned out to be more like 2.2:1. Which is not bad considering the test cylinders were only 20cmx10cm.
It was no big deal to get the sifting table out and make some more dirt, though.
We were expecting the last shorter section to only take the morning, but what with the drama with the misaligned holes and running out of dirt it was 3:30PM by the time we had it done and everything tidied up.
We’d bought the cement for the wall some months previously, before knowing how long it would take to sort out the cement mixer. The bags had been sitting on the ground under the house. It’s dry down there, and the bags are plastic-lined inside, but moisture still got in and the outside layer of cement had gone hard. It broke into quite big lumps that the mixer couldn’t break up, so we ended up with lumps of cement in the last bits of wall. The sand was also quite sticky, and contained lumps, so there are intermittent areas of pure sand that aren’t good.
With all the formwork out of the way, we could put the bricks back the next day. We filled the little gap between the bricks and the wall with some concrete. This side of the wall will be under the deck eventually, but for the moment it’s nice to have it neatened off.
The middle section seems to be the best. The mix was drier and it hardened off very quickly. In a way, though, that worked against our favour. Five days later and the section where we’d run out of dirt is starting to crack and lift away. Because the lower portion had dried the top bit didn’t incorporate into it. It’s a relatively thin layer that dried faster than the rest. Leaving something to the next day is obviously another one of those mistakes that we won’t make again.
So, lessons learned:
- Make more rigid forms. We’re thinking a shorter form with formply on the outside of the waler as well as the inside, all screwed together.
- Through-rods inside the form. They only need to be finger-tight, too. The ramming will tighten them up.
- Use a drier mix.
- The big lump of wood rammer is good, but could be scraped down to make it more manageable.
- Make some kind of guide for cutting the bevel.
- Buy the cement only when we’re ready to ram.
- Use smaller through-rods.
- Always finish off a section for the end of a day’s work. It will be hard by the time you can get back to it the next day.
- Don’t work if it’s going to rain.
- Have covering ready for the amount of wall you have. It will want to be covered for some time after it’s complete if there’s any threat of rain.
- Use sand that doesn’t clump. The beach sand and builder’s sand test cylinders turned out pretty samey-looking, but the beach sand won’t clump.