PSN-L Email List Message

Subject: Re: Concrete testing done
From: beezaur beezaur@..........
Date: Mon, 05 May 2003 21:09:43 -0700

Daryl P. Dacko wrote:
> At 04:16 PM 5/5/03 -0700, you wrote:

> I tried casting a large table out of a very simular mix, for some holography
> experiments, and had a lot of problems with flexure during the cure.
> It finally settled down after a few months, but I still see some seasonal
> flexure due to temperature and humidity diffrences.
> I think that a pre-stressed approach would be a lot better, (. . .)
> For what it's worth,
> Daryl

Exactly.  The section I am using is similar to a post-tensioned 
prestressed box beam bridge section.  It is a triangle-hexagon that is 
hollow with tension rods running both inside and out, but not in contact 
with the concrete, to keep all parts of the section under axial 
compression.  There will be eight 1/4" rods running parallel to the 
beam, attaching to angles (Ls) at the beam end, clamping the beam 
axially.  The Ls will be about 2 inches on a side, ~3/16 inch thick.

This is a lousy way to describe a shape, but the following are the 
coords of its vertices and rod centers in inches:

Rods @.............................................. for neg. x]}

Ls (4 of them) go at the top and bottom of each end of the beam. 
Leveling screws go through the bottom Ls. A little funky, but cheap, 
strong, and rigid.

You may have been seeing some aggravated creep in your table.  Most 
concretes creep significantly for several years, depending on the load.

I will be interested to see if my section will be stable enough.  One 
thing that worries me is the trapped air - there was an awful lot of it 
in my test cylinders.  Another is various behaviors of steel vs. 
concrete, like thermal expansion and reaction to humidity.  Time will tell.

A day without math is like a day without sunshine.


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