Advanced Aggregate Characterization Flashcards

According to CSA A23.2-15A, what is the primary objective of conducting a petrographic analysis on a coarse aggregate sample?

Answer: To identify the mineralogical composition and physical characteristics of the rock types present.

CSA A23.2-15A outlines the procedure for the petrographic examination of aggregates. The primary purpose is to identify the constituent rock and mineral types, determine their physical and chemical characteristics, and detect the presence of potentially deleterious materials. This information is crucial for predicting the aggregate's performance in concrete or asphalt.

A laboratory performs a Magnesium Sulfate Soundness test (CSA A23.2-9A) on a new source of coarse aggregate. The result is a weighted average mass loss of 15%, which exceeds the project specification limit of 12%. What does this high percentage loss primarily indicate about the aggregate?

Answer: The aggregate has poor resistance to weathering and is susceptible to degradation from freeze-thaw action.

The Magnesium Sulfate Soundness test is an accelerated weathering test designed to estimate an aggregate's resistance to degradation by natural freeze-thaw cycles. The sulfate solution crystallizes within the aggregate's pores, creating internal pressures that simulate ice formation. A high mass loss indicates the aggregate has a weak internal structure and is not durable against this type of weathering.

A petrographic examination of a granite aggregate source reveals the presence of strained quartz and chert, which are known to be potentially alkali-silica reactive. To further quantify the risk of this aggregate causing deleterious expansion in concrete, which of the following tests would be most appropriate?

Answer: Accelerated Mortar Bar Test (CSA A23.2-14A).

The Accelerated Mortar Bar Test (referenced in CSA A23.2-14A for concrete prisms) is specifically designed to rapidly identify the potential for an aggregate to undergo deleterious expansion due to alkali-silica reaction (ASR). Mortar bars made with the aggregate are stored in a sodium hydroxide solution at an elevated temperature to accelerate the reaction, and their length change is measured over time.

What property of a fine aggregate is primarily assessed by determining its uncompacted void content, as described in CSA A23.2-11A?

Answer: The particle shape, angularity, and surface texture.

The uncompacted void content test measures the void space in a loosely packed sample of fine aggregate. Aggregates that are more angular and have a rougher surface texture will not pack together as tightly, resulting in a higher uncompacted void content. This property is critical for predicting performance characteristics in asphalt and concrete mixes.

The Micro-Deval abrasion test (CSA A23.2-29A) is performed on an aggregate sample to assess which of the following properties?

Answer: Hardness, toughness, and durability against abrasion and degradation.

The Micro-Deval test simulates the abrasive action and degradation that aggregates experience during processing, handling, and in-service use. It measures the mass loss of a sample after it has been tumbled with steel balls in the presence of water, which is an indicator of its resistance to abrasion and overall durability.

A technician is determining the percentage of particles with crushed faces in a coarse aggregate sample according to CSA A23.2-12A. Which of the following correctly describes the basis for this calculation?

Answer: The mass of particles meeting the specified crushed face requirement (e.g., one or two faces) as a percentage of the total mass of the test fraction.

CSA A23.2-12A, similar to ASTM D5821, requires the technician to manually separate particles that meet the definition of a crushed face. The percentage is then calculated based on the mass of these separated particles relative to the total mass of the test portion being examined. It is not based on volume, particle count, or surface area.