Aggregate Gradation and Properties Flashcards

A laboratory technician performs a sieve analysis on a fine aggregate sample. The results show a Fineness Modulus (FM) of 3.4. According to CSA A23.1, what does this value indicate about the aggregate?

Answer: The aggregate is coarser than the typical acceptable range for fine aggregate.

The Fineness Modulus (FM) is an index of the fineness of an aggregate. For fine aggregate (sand), CSA A23.1 typically requires the FM to be between 2.3 and 3.1. An FM of 3.4 is higher than this range, indicating that the sand is relatively coarse.

During a sieve analysis of a coarse aggregate sample according to CSA A23.2-2A, the technician fails to ensure the sieves are clean before starting the test. Some finer particles from a previous test are present in the lower sieves. How would this MOST likely affect the calculated results for the current sample?

Answer: It would falsely increase the calculated percentage passing the finer sieves.

If finer particles from a previous test are already in the lower sieves, the mass of material collected on these finer sieves during the test will be artificially high. This leads to a calculation that shows more material passing through the upper sieves and being retained on the lower ones than is actually present in the sample, thus increasing the percent passing for those finer sieves.

A concrete mix design requires a well-graded aggregate. Which of the following descriptions best characterizes a well-graded aggregate?

Answer: An aggregate containing a wide and continuous distribution of particle sizes.

A well-graded aggregate has a good representation of all particle sizes from coarse to fine. This distribution allows the finer particles to fill the voids between the larger particles, resulting in a dense, strong, and economical concrete mix with minimal paste requirement.

According to CSA A23.1, which of the following is considered a deleterious substance that could negatively impact the bond between cement paste and aggregate if present in excessive amounts?

Answer: Clay lumps or coatings on the aggregate particles.

Deleterious substances are contaminants that can harm the properties of concrete. Clay lumps and coatings are specifically identified as deleterious because they can interfere with the bond between the aggregate surface and the cement paste, leading to reduced strength and durability.

A technician is preparing a coarse aggregate sample for a Flat and Elongated Particles test (CSA A23.2-13A). The specification requires testing particles using a 3:1 ratio. What does this ratio represent?

Answer: The ratio of the aggregate's length to its width, or width to its thickness.

In the context of the Flat and Elongated Particles test, the ratio (e.g., 3:1, 5:1) defines the dimensional criteria for a particle to be classified as flat or elongated. For example, a particle is considered elongated if its length is more than three times its width, and flat if its width is more than three times its thickness.

In a sieve analysis performed according to CSA A23.2-2A, the sum of the masses retained on all the sieves plus the pan is found to be 0.8% less than the initial dry mass of the sample. What is the most likely cause of this discrepancy?

Answer: Loss of fine dust particles during the sieving operation.

It is common to lose a small amount of the sample during the sieving process, especially fine dust particles that can become airborne. CSA A23.2-2A notes that if the final total mass differs from the initial mass by more than 0.3%, the test results should be considered with caution and the test may need to be repeated. A loss of 0.8% indicates a significant loss of material, likely fine particles.