Date: Mon Aug 31 16:29:43 2009 Back to Contents ------------------------------------------------------------------------

Author: Matt Lowry

Subject: Re: Bed of Nails -- RE: High speed video


I had a similar experience a few years ago. I had mistakenly selected a block which had spent a long time outside, and upon doing the demonstration it wouldn't break - ouch! Finally, after calling it quits, I broke the thing open by smashing it *really* hard (harder than is safe for the demo) and found the same thing: it was hardened inside due to the moisture it had absorbed while being stored.

That said, Bill's advice is good. Make sure you get blocks that have been stored inside in a nice dry environment.

Cheers - Matt

-----Original Message-----
From: [] On Behalf Of Bill Alexander
Sent: Monday, August 31, 2009 11:31 AM
Subject: Re: [tap-l] Bed of Nails -- RE: High speed video

For a construction job I was doing at home I had to purchase a pallet of cinder blocks, and went to the local factory and bought them there. I got to see how they are made (at least here). A slightly moist mixture of sand, cement, and water is dropped into the form, then it is hydraulically compressed to make maximum grain to grain contact. Now remember, this is a crumbly, almost dry mixture. At this point, the form pulled away, and the intact block was carefully lifted off onto a pallet. Just a few seconds start to finish, no cure time.

I mention this because one time we ran out of blocks here, and I brought in one from home that had sat outside for a couple of years. The prof told me it almost killed him (exaggeration) because it was much tougher than the new ones. Over time and moisture the cement had completed its reactions and really hardened. I now purchase them the week before the demo, and if possible from a local hardware store that stores them inside.

Bill Alexander
Humboldt State University
Arcata, CA 95521

Any job worth doing is worth buying a new tool.

From Mon Aug 31 16:29:43 2009