Friday, January 29, 2010

Rammed Earth Testing - Large Sample

The success of the small rammed earth samples meant it was time to build a larger mold for a larger sample. Some leftover block-board, a couple of trips to the wood shop and 2m of 20mm threaded rod later, we had our large sample mold. It consists of four panels and produces a 25x25cm column with 45⁰ chamfered corners.

Out on the site, we dug a fresh hole approximately where the earth pit would be, so as to test the actual soil that we hope to use. After digging about a meter to get bellow the organic top soil, we hit nice orange sub-soil.  friendly neighbor came over to investigate, and ended up helping out (he is a mason and is now part of the team working on the foundations). The soil had about the right moisture content as it came from the ground, so we used it as is. We laid the freshly dug soil in 4” courses and rammed it down with our improvised rammer.

There was a bit of a struggle releasing the nuts due to the lack of a wrench, but after a couple of well placed blows the threaded rods came lose. This is defiantly something that will need to be addressed in the design of the full sized formwork.

The surface finish and strength were both better than I had hoped for – the sides were incredibly smooth and free of defects. The layering was subtle but is apparent enough to add some visual interest to any walls that we leave unfinished – which will be the default.

After a week sitting unprotected in the field and a number of heavy rainstorms, the Large Sample showed little sign of damage. The top surface had some minor pitting from the rain, but the sides were untouched. Like the small samples the block has cured into something resembling sandstone and is reassuringly solid. The sample suggests that, assuming good coverage of the tops of the walls and window sills, the walls made from this earth could survive the rains without the need for stabilizers.

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