Organic Impurities Assessment Flashcards

Which ASTM standard governs the colorimetric test for organic impurities in fine aggregates for concrete?

Answer: ASTM C40

ASTM C40 is the standard test method for organic impurities in fine aggregates for concrete using the colorimetric method.

What does a color darker than the reference standard No. 3 organic plate indicate about a fine aggregate sample?

Answer: The aggregate contains harmful levels of organic matter

A color darker than the reference standard No. 3 organic plate indicates potentially harmful organic impurity levels requiring further investigation.

In the ASTM C40 test, what solution is added to the sand sample before comparing to the color standard?

Answer: 3% sodium hydroxide (NaOH) solution

A 3% sodium hydroxide (NaOH) solution is added to the sand and shaken, then allowed to stand before color comparison.

How long must the sand and sodium hydroxide mixture stand before the color comparison is made in ASTM C40?

Answer: 24 hours

The mixture must stand for 24 hours before the color of the solution is compared to the organic plate reference standard.

If a fine aggregate fails the ASTM C40 organic impurities test, what is the most appropriate next step?

Answer: Run ASTM C87 mortar strength test

ASTM C87 (mortar strength test) can determine if the organic impurities actually affect concrete strength, allowing the aggregate to potentially still be used.

What volume ratio of sand to sodium hydroxide solution is used in the ASTM C40 test?

Answer: 1 part sand to 2 parts solution

The test uses approximately 1 part sand to 2 parts of 3% NaOH solution by volume in a glass container.

Which type of organic compound is ASTM C40 most sensitive to detecting in fine aggregates?

Answer: Humic acids from decayed vegetation

ASTM C40 is primarily sensitive to humic acids derived from decayed plant matter, which impart a yellow-to-brown color to the NaOH solution.