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Carbonate vs. siliceous aggregate 3

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jtstruct

Structural
Jun 20, 2003
21
We are trying to determine fire ratings for an existing concrete elevated floor (pan joists at 2'-1" on center with a 2.5 inch slab). The code seperates the concrete categories into cabronate base and siliceous base. Is a core sample and testing the only way to dermine which type of aggregate was used?

Thanks in advance.
 
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jtstruct...
The confirmative way is to core the concrete and look at the coarse aggregate. It should be readily apparent which type of aggregate you have.

If you don't want to core it, you can ask for an original mix design or previous test results. Also, most Ready Mix producers use aggregates indigenous to the area unless specifically requested to do otherwise.

You can also chip off the surface in an inconspicuous location to check the coarse aggregate.

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Ron,

Thank you for your response. Can you list some of the physical differences between the two types of aggregate and what we should look for?

Thanks.
 
jtstruct...
Carbonate or Calcareous aggregates are generally white to tan in color, sometimes porous, essentially amorphous, and are softer than siliceous aggregates. Siliceous aggregates may be any color or mottled (such as granite), are hard and non-porous. They can show distinct cleavage.

Carbonate aggregates are generally from sedimentary rock formations.

On the side of a core, look at the exposed coarse aggregate. Scratch it with the point of a pocket knife blade. If it scratches without gouging, it is relatively hard and is more likely to be a siliceous material. Carbonate aggregates will gouge under knifepoint.

Carbonate aggregates are also reactive to hydrochloric acid (you can use muriatic acid, a weak form of hydrochloric acid). Drop it directly on the aggregate. It will "fizz" if carbonate (be careful to drop it directly onto the aggregate and not on the cement paste as cement contains great amounts of calcium and will fizz as well. Siliceous aggregates are generally non-reactive to weak hydrochloric acid.

Hope this helps. Good luck.

Ron

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Thanks Ron for your thorough description. Very helpful!
 
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