Using Castable Refractory Concrete In The Field

Finding and choosing a castable refractory concrete appropriate for use in heater construction can be difficult. Requirements are: high density, large aggregate and high tolerance to thermal shock. The castable used here is Mount Savages Heatcrete 24 ESC (24 f. extra strength course.)
The below article shows the forming, pouring, and deforming, of the 4 castable refractory concrete modules used in the construction of the indirect upper chamber bake oven.
The article outlines techniques appropriate for custom work in the field. Workshop practices and equipment are surely superior.

The 4 moulds to be poured. Clockwise from top left. hearth slab, rear slab, top slab, and lintel.

The hearth mould is darker as it is made from plywood used especially for form work.

Once the moulds have been screwed in place they need to be sealed in order to stop the evaporation of water during the reaction, and to allow the slabs to be deformed with ease once cured. The moulds can be lined in polythene, or sealed with vegetable shortening and silicone. Both techniques are valid, though only the method using vegetable shortening is illustrated here. Polythene gives the finished modules a shiny mirror like finish on the cold sides which is easily cleanable. The lustre of this finish though may significantly impede the evacuation of mechanical water during heat up. The dull surface of the modules from moulds lined with grease is considerably more porous.

Before the pour all moulds are sealed. Silicone is applied to all the corners and places where the form meets the plywood. The surfaces of the moulds are thoroughly greased with vegetable shortening.

A strip of ceramic paper is placed in the bottom of the form for the load relieving lintel. This will form the recess that will take an identical piece of ceramic paper when the lintel is layed. The paper should be covered with a strip of polythene to stop the mix soaking in to it during vibration of the form.

The castable should be mixed ideally in a mechanical mixer. Many castables are almost impossible to mix by hand.
The manufacturers recommended quantity of water must be adhered to. With Heatcrete 24 the one and three quarter gallon measure of water per 100 lbs of dry material, seems to be too dry. Though after thorough mixing it vibrates into place perfectly.
Even a slight excess of water can considerably weaken the finished modules.

Water used should be clean.
Both water and dry material need to be relatively warm during mixing, and kept warm after the pour, before and during the reaction. A temperature between 15 and 20 C is ideal. If pouring in cold temperature which necessitates warming of material, and water, care must be taken not to over warm them as this will speed up the reaction causing the mix to jam before being vibrated into place.

Due to the mix being so stiff it is essential to work fast.
The concrete is placed in the mould. It is better to over fill the mould and scrape off excess than to under fill and add.
The concrete should then be pushed into place with a trowel, before being vibrated. The below image shows the concrete after vibrating for one minute. Though, up until this point, the concrete has seemed way too dry, it should run in to the forms perfectly once vibrated.

Castable refractory concrete #1 (Video, 11 secs)

Castable refractory concrete #2 (Video, 12 secs)

The moulds are all nailed to a sheet of plywood which rests upon another sheet of plywood. This makes vibrating more effective, especially if working on a concrete floor.
Vibrating is done with a rotary hammer or hammer drill.
Placing the drill bit on a scrap piece of wood the form is vibrated around its periphery causing the concrete to settle in to place and then for trapped air bubbles to be driven up to the surface.
The way these three moulds are arranged means that the middle piece and the two inner portions of the outer pieces cannot be easily vibrated. Special attention should be given to placing and vibrating these areas.

Vibrating settles the concrete and expels air, but it also causes larger aggregate to fall towards the bottom of the form pushing lighter aggregate upwards. As this results in an imbalance in the desired ratio of aggregate through the piece. The mould should not be vibrated for longer than necessary.

The top surfaces of the modules, which are the hot surfaces, should be left rough and not finished by trowel. After pouring the forms should all be tightly covered with plastic, and all air expelled by running a hand over the plastic as it rests on the wet concrete. It is advisable to staple these plastic covers in place to avoid the corners being lifted by sly overnight winds.

Moulds lined with polythene.

The above moulds covered with polythene after pouring.

Curing: Curing is of high importance to final strength of the finished piece. The work area should be kept warm during curing.
The hydraulic reaction, driven by the exothermic setting of the concrete, will start several hours after pouring, depending on quantity of water and temperature of materials.
The reaction should cause the material to become quite hot as it continues for several hours. It is important that at this point that the work remains tightly covered, stopping water from being lost through evaporation during the reaction. Though I deform and use the modules the day after pouring, once they have cooled down, they could ideally be left in their forms for an additional two days. If deformed the day after it is better to keep the modules humid for a couple of days.

Lintel steaming from the exothermic reaction. (Video, 18 secs)

It is necessary to work accurately when making the forms. The surfaces of the modules that are gasketed are gasketed with eighth of an inch ceramic paper and so have to be straight and square to make good contact with each other as they are assembled.

The cold surface of the oven hearth was cast on to lightly greased form wood.
It is probably preferable to cast this piece on to polythene as it will provide a smoother shinier surface that is less permeable and easier to clean.

A hearth slab poured into a polythene lined mould.

The load relieving lintel showing the recess into its bottom face, caused by the piece of ceramic paper inserted into the mould before pouring.

Heat Up: Up to 50 % of the water addes during mixing may remain after the reaction has finnished. Therefore extreme care should be taken to bring the refractory core up to operating temperature slowly. This will allow mechanical water trapped in the modules to escape gradually through the materials natural porosity. Contact with flame should be avoided during the first fires, as much as possible. Though in this application it is almost impossible. Bringing the heater up to temperature too fast will cause this residual water to have a mechanical action on the material as it vaporises and expands.

A saw cut cross section through a piece of 'green' heat creat 24 ESC. The largest aggregate are approx 10 mm.

Quick And Easy Mould Lining.

Update 2011

Using grease is messy. Lining the molds with strips of polythene is time consuming and difficult, especially at the corners. Even when held in position with silicone, during vibration, the edges of the polythene strips tend to rise into the concrete. This results in polythene being trapped in the hardened concrete, which can cause the corners of the finished module to brake away. And potentially melt and smell which is problematic if the modules are oven components.

The following three images illustrate what may be the fastest, easiest and best method of lining moulds in the field.

The wood to make the moulds, after being cut to size is covered with waterproof tape on the side that will form the inner surface of the mould.

The mould is assembled on to a sheet of polythene larger than the mold. Excess polythene is cut away from outside the mould to stop it being accidentally moved during pouring and vibration. The polythene can also be stuck to the floor with silicone, though this is not necessary.

Finally a bead of silicone is applied to the vertical corners of the mould, and the edges of the taped wood where it meets the polythene Passing over the silicone bead with a finger will assure that there are no leaks between the plastic and mould.

Note: The reaction between uncured silicone and liquid castable will cause the surface of the castable at the interface to be powdery when cured. Though only superficial and not an actual problem this looks unsightly, and so smearing excessive amounts of silicone on to the inner surfaces of the mould should be avoided.

Marcus Flynn

Montreal 2007

Articles by Marcus Flynn
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