Very First Concrete Test Article
To test a concrete insulator for holding the extruder's heater (see also High Temperature Materials), a test cylinder was made from 5 parts cement premix and 3 parts damp greenhouse vermiculite/perlite mix together with just enough water (quantity not measured). A woodscrew and 3mm knitting needle were placed in the concrete.
The concrete was kept in place by a thick cardboard tube 37mm I/D and blocked at one end with gaffer (duct) tape The tube was dampened before use to avoid drying out the concrete. The components were taped in place to stop them sinking, and the whole assemblage left in shady surroundings. All cats were dissuaded from entering the experimental zone.
Time started, 4:30pm. Ambient temp 21.5C, gentle breeze, some cat hair.
After 1 hour the needle was removed by pulling upwards. This resulted in some damage to the upper surface of the block and the coating on the tip of the needle. Pushing downward may have been a better idea, or perhaps a more comprehensive mould is required.
10am next day, the concrete is just starting to show small grey patches on the upper surface. Initial curing is expected to be complete within an hour or so, but the test article will be left alone for at least 24 hours.
It is already becoming apparent that the exit hole left by the needle is substantially larger than 3mm. A sneak peek reveals that the hole at the other end is much closer to 3mm.
After 48hrs, the cylinder had cured to the point where it could be handled and worked. It shed a lot of sand, but the overall integrity was good. It is mechanically not dissimilar to pummice, and a firm rub can easily dislodge surface material.
The concrete had a great many voids in it, and some of this may need removing. Perhaps a sander or similar vibration system could settle the concrete more thoroughly?
The screw embedded in the concrete comfortably supports a 12N load. A firm, steady pull just using fingers failed to dislodge the screw.
The central 3mm hole had rough sides, but was well-formed with little debris.
-- Main.VikOlliver - 11 Feb 2006