[Strawbale] Abusing tempered glass (was Re: Natural, inexpensive kitchen cabinets)

[Rob Tom]

On Wed, 16 May 2007 19:25:09 -0400, Chris Green <pojeros at telus.net> wrote:

> skylin wrote:
>> How about inserting a slab of recycled tempered glass into a concrete  
>> counter top. Could possibly be inserted as the concrete was setting?


The thing to do would be to cast the countertop upside down.

Place whatever inserts you're going to include, in the bottom of the form  
and then dump the concrete in.

Doing it the other way (face up) is problematic because the inserts would  
interfere with floating & trowelling operations.

When casting face-down, the concrete acquires the texture of whatever was  
used to line the bottom of the form. If the liner is something like  
polyethylene sheet (or sheet of glass for that matter) the finished  
surface of the concrete emerges glassy-smooth/shiny, straight from the  
form-- no muss, no trowelling fuss. To get the same finish casting face  
up, would require a LOT of elbow grease (and time and sweat and skill).

As for the co-efficients of linear expansion of glass and concrete, a  
quick Google will tell you that the two materials are very dissimilar, so  
a glass insert would probably require a control joint around the perimeter  
to accommodate the differential rate of movement.

Steel rebar works so well with concrete because their coefficients of  
thermal expansion are very similar.  Glass works well only as an aggregate  
(ie small chunks) in "glasscrete".

> The surfaces of tempered glass are under compression. That means the
> outside surface is squeezing the glass on the inside. If the glass hits
> a sharp stone, or is scratched by a sharp object like a knife, it could
> shatter into a lot of tiny pieces,

I've had large sheets of tempered glass get picked-up (by wind) and tossed  
several feet and land on boulders, sometimes more than once, and the glass  
survived intact.

Having seen how tough tempered glass is, once when building my home I was  
very casual in securing a stack of 40 sheets of tempered glass in the back  
of my truck, using only a 2x2 stick jammed up against the side of the  
stack and the other side of the truck to keep the stack in place. (On  
previous occasions I had built an A-frame rack to support the glass on  
edge and lined all the surfaces with carpet.) I took an approach ramp to  
the 416 expressway way too fast and the 2x2 prop slid out and the entire  
stack of glass flopped over onto the other side of the truck. No damage.

OTOH, I recall at university , being in the studio during late afternoon  
when the setting summer sun was shining onto the large fixed glass windows  
set into punched masonry openings and the tempered glass just shattered  
spontaneoulsy.

The stresses in a sheet of tempered glass are concentrated at the edges so  
it is the edges that are fragile. ie If you bashed a piece of glass in the  
face, it's likely nothing would happen (as with the wind &  
cornering-too-fast incidents I've described above).

If a worker installed a tempered glass unit so that upon thermally-induced  
expansion of the sheet of glass, an edge was subjected the the protruding  
head of a screw so that it would act as an anvil (as with the university  
window example) or happened to subject an edge to impact against something  
like a hard, sharp corner the glass would likely explode/shatter.

I use sheets of salvaged tempered patio door glass as cutting boards when  
cutting large sheets of paper or matte board to ensure a clean cut. As  
Beel would say, "No problema".


=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at   C h a f f Y a h o o  dot  C a  >
(winnow the chaff from my edress in your reply)