Casting the hearth slab

My oven design has a 5 cm thick slab of refractory concrete cast over the Foamglas insulation, on this we will lay the hearth bricks set into a thin layer of adobe mortar.  The reasons for casting this slab are to provide a flat level base to form the hearth, to increase the effective thickness of the hearth to 5 cm more than the walls and roof of the oven and to create a structurally strong base to the oven.  The ridged slab will spread the heavy load of the oven masonry, protecting the Foamglas, it also has a higher thermal conductivity and thermal capacity compared to the hearth bricks, that will improve the thermal characteristics of the oven.

The slab is made from castable refractory reinforced with stainless steel fibers.  Seen below, these are drawn fibers, approximately 25 mm long.  This type of reinforcing is used instead of regular mild-steel bar. Reinforcing using construction steel bar expands at a very different rate to the refractory and will cause the cast component to crack and fail.

This mix we are using consists of;

1 part refractory cement (Secar 70)

4 parts crushed metamorphic rock

2% by weight stainless fibres

12% Water

3/4 of the water, crushed rock and fibers are mixed using a power drill with egg-beater style attachment for 5 minutes.  The Cement is then added along with the remaining water and mixed for a further 3 minutes.

The refractory concrete is then spread out in our form work in a thin thin layer.  This will produce a much stronger slab than if one thick layer is used.  Using this method forces all the fibers into a  flat 2 dimensional matrix, aligned in the direction that will give the slab the most strength.  In this photograph you can see how the form-work was made by using relatively flimsy plywood strips, leveled in the corners on little wedges and then stiffened by the addition of blocks aver 15 cm attached temporarily with hot-melted glue.  This is a very fast way of making an accurate form that does not require lots of ridged material.  The form-work is then sealed against the Foamglas around the edge using a general purpose silicone. This will be removed later along with the form.

This shows the progress after 4 layers have been built up.  The mix is quite dry and requires quite a lot of working to remove any voids.  A dryer mix is preferred to putting in too much water.  If lots of water is used then there is more to evaporate and there will be more shrinkage and potentially cracking.

After checking the flatness of the slab with a straightedge run along the form the surface is given a careful smoothing.  

The slab is then covered in a thin plastic wrap in order to slow the rate of drying.  The Curing process will only continue while there is moisture in the concrete.  As it is very cold here at the moment, probably -4 C tonight, we have erected a smaller tent over the slab and placed an electric heater on a low setting.  The temperature of the drying slab will be maintained at a minimum temperature of about 4 degrees C for the next few days until the slab is fully cured.

The slab should be hard enough my the weekend so that I can start laying the hearth bricks.