Thursday 11 October 2012

Mix too wet

Half way through the hemp walls it was a real blow to our confidence and morale to find out that our hemp mixes have been too wet. Correspondence with Klara from the Australian Hemp Masonry Company pointed to my cracking and pulling away cracking problems being caused by my mixes being too wet and therefore having a higher amount of shrinkage and also possibly insufficient wetting down between layers. On reflection I have noticed that I have been making wetter mixes as the days have got much hotter.

So we have reduced the amount of water in our mix to one 9 litre bucket for each mix (5.5kg hemp, 9kg binder and 8.5kg sand) with just a tiny bit more water added if the sand is really dry. With our dryer mix recipe in hand we did one days work hemping and will be more cautious and see how it goes before doing more walls. We used 9 litres of water only for these mixes and I was worried that they looked too dry as the sand did not seem to be held to the hemp and binder but looked like it was sitting on the outside, but as my partner pointed out I got it wrong the first time, so he got his way and we used 9 litres only. But just to check I made our last mix using 10 litres of water and was much happier with how the sand was bound in.

The dryer mixes looked a little different. They stuck more around the top of the mixer, but when emptied out left the inside of the mixer looking cleaner, they poured out of the buckets easier, but were not any different to tamp.

When I took the form work off these dryer sections the next day I could hear some sand fall out when I removed the formwork, and had a little bit of hemp fall out when I removed the spacers. I thought the wall was well tamped but the hemp falling out happened on a few but not all spacers, so whether it was due to insufficient tamping or too dry a mix I could not tell. To try to ensure more even wetting down, particularly when starting a new rise, where the old layer can be quite dry we have used a children's watering can. Due to other commitments we can not work on the walls for a couple of weeks, so we will see how they look when we return to site by which time they will have had a chance to dry out.
The dryer mix
My friend Martin, who helped out with the build a few weeks ago but has no background in hemp building, seemed to be able to pick the cause of my problems and commented in response to my complaint about cracks in my walls -  "They look like the shallow, narrow, non-structural cracks that you get in cement as it drys, if you mix it up a bit too wet. Isn't that what the render is for? To cover up the imperfections and hold the whole show together?" I only hope he is right about the render being able to cover up such imperfections.

Following my worries about my mix now being too dry, I turned to Klara again for advice. She said "The mix should be as dry as it can be and still clump together." But if with 9 litres of water the mix was still too dry I could add up to a quarter of a bucket of water. So after my earlier debate with my partner it appears that the mix I used with 10 litres is still within the acceptable limits. I estimate that in the old mixes I was using about 12 to 14 litres of water (as my one bucket then was about 4-5 cm from the top), so clearly that was too much water.  

We will finish the rest of the house using dryer mixes and then see what we can do about the back section with the cracks, perhaps we will do some test render patches to see if that will cover them, before we contemplate the possibility of having to pull some sections down and redo them.

Kitchen wall up to window height
Uneven layer due to noggins alternately being at differnt heights at top of previous rise
No new wall, but formwork moved up ready for the next weekend of work

No comments:

Post a Comment