ATT Organic Impurities Assessment 2 — Questions and Answers
Question 1: Which ASTM standard governs the colorimetric test for organic impurities in fine aggregates for concrete?
- ASTM C33
- ASTM C40 (Correct answer)
- ASTM C117
- ASTM C136
Correct answer: ASTM C40
ASTM C40 is the standard test method for organic impurities in fine aggregates for concrete using the colorimetric method.
Question 2: What does a color darker than the reference standard No. 3 organic plate indicate about a fine aggregate sample?
- The aggregate is free of organic impurities
- The aggregate contains harmful levels of organic matter (Correct answer)
- The aggregate meets ASTM C33 requirements without further testing
- The aggregate needs only sieve analysis
Correct 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.
Question 3: In the ASTM C40 test, what solution is added to the sand sample before comparing to the color standard?
- Hydrochloric acid solution
- 3% sodium hydroxide (NaOH) solution (Correct answer)
- Distilled water
- Sulfuric acid solution
Correct 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.
Question 4: How long must the sand and sodium hydroxide mixture stand before the color comparison is made in ASTM C40?
- 15 minutes
- 1 hour
- 24 hours (Correct answer)
- 72 hours
Correct answer: 24 hours
The mixture must stand for 24 hours before the color of the solution is compared to the organic plate reference standard.
Question 5: If a fine aggregate fails the ASTM C40 organic impurities test, what is the most appropriate next step?
- Reject the aggregate immediately
- Run ASTM C87 mortar strength test (Correct answer)
- Increase the water-cement ratio
- Reduce the aggregate gradation size
Correct 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.
Question 6: What volume ratio of sand to sodium hydroxide solution is used in the ASTM C40 test?
- 1 part sand to 1 part solution
- 1 part sand to 2 parts solution (Correct answer)
- 2 parts sand to 1 part solution
- 3 parts sand to 1 part solution
Correct 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.
Question 7: Which type of organic compound is ASTM C40 most sensitive to detecting in fine aggregates?
- Chlorides
- Humic acids from decayed vegetation (Correct answer)
- Sulfates
- Alkali silica compounds
Correct 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.
ATT Organic Impurities Assessment 2
Which ASTM standard governs the colorimetric test for organic impurities in fine aggregates for concrete?