ACI Certification Aggregate Properties and Testing 2

Question 1

What is the primary purpose of the sieve analysis test for concrete aggregates?

Accepted Answer

To determine the particle size distribution (gradation) of the aggregate, which affects concrete workability and strength

Answer

Sieve analysis separates aggregate into size fractions by passing it through a series of increasingly fine sieves. The resulting gradation curve indicates whether the aggregate meets specification requirements.

Question 2

The specific gravity of an aggregate affects which property of concrete?

Accepted Answer

Unit weight (density) of the concrete — higher specific gravity aggregate produces heavier concrete

Answer

Specific gravity measures the density of aggregate particles relative to water. Higher specific gravity aggregates produce heavier concrete, which matters for structural calculations and mix design.

Question 3

What does the Los Angeles Abrasion test measure?

Accepted Answer

The resistance of coarse aggregate to degradation by abrasion and impact, indicating its toughness and durability

Answer

The LA Abrasion test (ASTM C131) tumbles aggregate with steel balls in a drum, measuring the percentage of material that breaks down. Higher loss = softer, less durable aggregate.

Question 4

Why is the moisture content of aggregate important to measure before batching concrete?

Accepted Answer

Aggregate moisture contributes to the total water in the mix, affecting the water-cement ratio, which directly controls concrete strength and durability

Answer

Aggregate typically carries surface moisture that adds to the batch water. Without adjusting for this moisture, the actual water-cement ratio will be higher than designed, reducing concrete strength.

Question 5

What is 'alkali-silica reactivity' (ASR) and why is it tested in aggregates?

Accepted Answer

A harmful chemical reaction between certain silica minerals in aggregate and alkalis in cement that causes cracking and expansion in concrete over years

Answer

ASR causes internal gel formation that absorbs water and swells, creating internal pressure that cracks the concrete from the inside out. Affected structures may need expensive repair or replacement.

ACI (American Concrete Institute Certification) Practice Test

ACI (American Concrete Institute Certification) Practice Test

ACI Certification Aggregate Properties and Testing 2