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Last Updated Feb 20, 2010

HEBEL concrete blocks for Garden Railways

'Hebel' is the trade name of aerated concrete blocks that are very lightweight. They consist of many small holes (1-2mm dia) in a white concrete.They are a CSR product - see their website here for distributors
It may be called "Thermalite" or "Celcon" in the UK.

You can buy Hebel blocks in sizes 600mm x 200 x thick, where the thickness can be 50,75, 100, 150 or 200mm. (That's 2'x8" x 2", 3", 4", 6" or 8")
They cost from $5-$10 per block. They are fine outdoors and are perfect for Garden Railway buildings and structures (eg bridges, piers)
The 50mm thick ones are relatively brittle - I broke a few in the car boot, just by laying then down flat on the drive home from the hardware store. So treat them gently!
The pic at left shows the rough texture you get on the 'big' sides of the blocks and the smoother finish on the tops and ends

Here's a couple of my structures: The left pic shows the church with the main wall rough finish and the buttresses and window surround with a 'plaster' finish. The church is a hollow building made from 50mm thick blocks, glued together with water based silicone. The station building is two, 150mm blocks sitting in top of one another, without any glue.
stone viaduct (34K) stone viaduct (34K)

stone viaduct (34K) The big viaduct is the most imposing hebel structure I've made. It took 6 months and lots of dust to make! Each arch is a 200x200mm block. The arches were 'cut' out by drilling many holes through from each side, then chiselling and angle grinding. Other people have had success by making curved templates from pineboard and using the bush saw.
The piers are 75mm blocks. The parapet is individual 25x25mm blocks glued on with hebel cement.

To cut it you can use normal wood saws, but they will get blunt pretty quickly. I now use a bush saw (bow saw?) It's like a big hacksaw with a replaceable blade. I find that even after a blade is no longer sharp enough to cut tree limbs etc, it still rips through Hebel!
You can drill it with wood drill bits but they too get blunt, so I use masonary bits where possible. But if I don't have the masonary bit size I need then I use the wood bit. I've used wood hole saws too.
An angle grinder is the best tool for getting rough shapes, but it makes lots of dust, so wear a mask! You can chisel it with sharp wood chisels. Then a rasp and files for finishing and even sandpaper.
You can cut it down to 10mm thick if you are very careful, but 30mm is about the min for handling without breaking accidently.

You can drive nails into it. I use galvanised nails and sometimes predrill smaller holes so I don't need as much hammering force.
You can buy special Hebel cement but it only comes in 20kg bags at around $20 a bag. It is good stuff, like very smooth, sticky concrete and takes colour the same as the Hebel so joints don't show up much. But the large quantity goes off in the bag so I have only ever used it the once.
Normal concrete can be used OK. I have also used 'normal' exterior grade silicone as used in bathrooms etc. But you have to make sure not to get ANY on visible surfaces.
But the best I have found is Fullers Ultra Clear water based sealant. It is white when it comes out of the tube, but dries clear. When dry it is waterproof. It's a bit like very thick PVA white glue!

The surface can be carved out with a 1” wood chisel or even a steel ruler. You don’t need a hammer, just push ‘scratch’ along the surface. This can leave parts ‘standing out’ for relief. This is quite fast to do - the front wall of the station (pic at top) took less than 30 minutes to do all the fancy quions around the windows!
Lines can be carved into the surface to represent stone blocks. Use a hacksaw blade or old screwdriver. Even a knife blade for narrow lines such as for brickwork, but this is getting a bit ‘fine’ for the size of the air holes and they tend to get ‘lost’. Scribing lines takes longer especially the short vertical lines.

The left pic shows scribing 'blocks' onto the surface with a hacksaw blade. The right one shows scrapping the surface down to leave some parts higher - eg around windows.
I have also used a very watery wash of plaster to fill in the little holes where I want a smoother surface. This can be coloured with cement oxide or painted.

I have never painted my Hebel. I use the oxides (used for concrete colouring). I’ve got black, brown, red (very dark) and yellow. Just mix in water and paint on with an old brush, scrubbing it well into the little holes. It weathers nicely (some runs off in the first rain), but may need recoating after 5 years or so. Horizontal surfaces like the tops of walls, do not last as long as vertical surfaces.

These are examples of the finishes possible. The left window 'frames' have a thick plaster coat. The wall is the 'chiselled' finish and the buttress had a thin wask of plaster and then the blocks carved with a knife. The bridge had no plaster, just a rasped surface and lines scribed with the old tile cutter.