At 02:38 PM 7/9/98 -0400, you wrote:
>
><SNIP>
>Note the difference in tension and compression for the rock-types.
>This is why rebar is used in concrete, to add tensile strength for a
>better composite building material. Fiberglass is another example of this.
>The resin has compressive strength, the cloth has the tensile strength.
>
> The tensile and compressive strengths of metals are much more evenly
>matched, but can still vary.
>
>
> - - ... MARK_N1LO...- -
>
Thanks for the data on the strengths of the various materials, Mark. I have
a permanent "references" email folder for data as this.
Let me add something that had really puzzled me until I happened to take a
short course on "Properties of Engineering Materials" (or something like
that) that my employer paid for years ago.
If you look up the data, you will notice that the tensile strength of
composites such as concrete&rebar or glass_strands&epoxy actually EXCEEDS
the tensile strength of the components.
How could that be?
Turns out that when one measures the strength of a piece of rebar, or an
individual glass fiber, one is actually determining the strength of the
weakest point in the sample measured. The strength of a perfect fiber
would be quite a bit higher.
When one "glues together" lots of fibers, as in fiberglass, the "glue"
serves to bridge across the small weak spots of the fibers, transferring
the load that might otherwise break that individual fiber to neighboring
fibers and giving the composite a tensile strength somewhat higher than the
components, nearer to the tensile strength one would see in a "perfect"
sample of the fiber. Same with rebar-reinforced concrete. Also, the gravel
in the concrete mix serves the same purpose - it helps to give the mix a
tensile strength more like the gravel than the sand/cement portion (at
least, so said this Clemson University professor, but it looks like your
data may not support this).
I also learned why a "pre-stressed" concrete structure is so much stronger
than a conventional one in tensile strength. Your data shows how the
tensile strength is MUCH lower than compressive strength for concrete . By
pre-stressing the concrete structure (adding compression to it in
manufacture), one "translates" some of that compressive strength capability
into tensile strength capability. Picture a tube made of plain concrete. It
might yield in tension at say 1K pounds, but could support say 10K pounds
in compression. Now run a steel cable through this tube and stretch that
steel cable to say 5K pounds tension and anchor it to the tube ends. The
concrete tube is now under 5K pounds compression, and will yield at an
additional 5K pounds compression, so the compressive strength of the
structure has been reduced, BUT - it can now support 6K in tension, because
the steel cable will "absorb" 5K pounds, and the structure will not yield
until the additional 1K of tension is applied to the concrete itself,
giving an effective 6K pounds tensile strength to the structure.
Hope I remembered all this stuff correctly.
Jerry W4UK
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