This week I have been mostly making….Concrete   Leave a comment

Shuttering for the concrete kiln base. The uneven stable floor meant there were gaps at the bottom of the shuttering in some places.

Shuttering for the concrete kiln base. The uneven stable floor meant there were gaps at the bottom of the shuttering in some places.

The wood fired kiln build has started. The first thing needed is a good level foundation. The design I’m basing my kiln on had a 4 inch (10cm) thick concrete slab. I happened to have some 14cm wide plywood strips which I exchanged with the lady who rents our paddock for a weeks rental of the paddock so my foundation was going to be 14cm deep. I created a 2 metre by 1.5 metre rectangle using the plywood as shuttering  and filled it about one third full with the hardcore which we find in the flower and vegetable beds. (The local soil is very stony and the house has been here for over 400 years and during that time it seems anything unwanted or broken was dumped somewhere in the garden.) The top edges of the shuttering were adjusted, using a spirit level, to make sure the concrete slab would be perfectly level.

After the first concreting session the gaps between the shuttering and the stable floor are sealed.

After the first concreting session the gaps between the shuttering and the stable floor are sealed.

The location of the stable means it is not feasible to get a concrete lorry to it so I had to mix my own concrete having wheel-barrowed the sand and cement from the road at the front of the house. I did it in two stages. The first session I mixed the concrete quite stiff and concentrated on filling the gaps between the bottom of the shuttering and the ground. The second session the mix was much wetter which allows the concrete to be levelled easily using a plank resting on the top edge of the shuttering.

The shuttering is full of concrete and the mixer is taking a rest

The shuttering is full of concrete and the mixer is taking a rest

 

 

 

 

 

 

 

small round lumps which appeared overnight in patches on the surface of the foundation slab

small round lumps which appeared overnight in patches on the surface of the foundation slab

Its a rule of DIY that nothing ever goes 100 percent to plan. When I came to look at the set concrete the following morning there were a few patches where the surface was lumpy. It certainly wasn’t lumpy when I left it the day before. On close examination it occurred to me that the lumps were all about the same size and shape. I wondered if bubbles had somehow formed in the wet cement, but a bit of prodding with a knife indicated they were solid. Fortunately concrete hardens slowly over a few days and I was able to investigate further by digging some of the lumps out. They were all approximately spherical and of very low density although quite hard and clearly mineral (rather than organic). I have no idea what they are but presume they were some sort of contaminant in the ballast I used to make the concrete with. Clearly they were light enough to float to the top of the liquid cement and protrude a couple of millimetres above the surface before the concrete began to set. Lumps on the surface are the last thing you need if you are going to lay bricks without mortar, which was the plan, so I investigated ways of removing the lumps.

Although not what you would call a smooth surface, the concrete was either a the right level or indented which meant my bricks would sit stably and levelly on it and not wobble

Although not what you would call a smooth surface, the concrete was either a the right level or indented which meant my bricks would sit stably and levelly on it and not wobble

Digging them out one by one was too slow and tedious but a trip to the tool shed reminded me I had a plasterer’s float that was designed for making large areas perfectly flat. It wasn’t intended to be used in the way I was going to use it, but since I last attempted to use it 30 years ago (unsuccessfully – plastering large areas is best left to the professionals unless you want a very rustic look) I felt it wouldn’t matter if it got a bit roughed up. It worked surprisingly well. As I moved it across the surface from a completely flat area to one of the bumpy areas its sharp metal edge would either knock the little spheres right out of the surface or it would cut them off level with the surface. It also levelled any raised areas around the sets of cat and bird footprints which too had appeared overnight. An hour later I had a foundation on which I could build the kiln – although it needed to harden for a few more days.

 

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