ACI Certification Aggregate Properties and Testing 3

Question 1

The absorption capacity of aggregate is important because it affects what aspect of concrete?

Accepted Answer

The effective water available for cement hydration — high-absorption aggregate soaks up batch water, reducing the water available for the cement reaction

Answer

If aggregate absorbs water from the mix, less water is available for cement hydration. Mix designs must account for aggregate absorption to ensure the correct water-cement ratio.

Question 2

What does petrographic examination (ASTM C295) reveal about an aggregate?

Accepted Answer

The mineralogical composition, physical characteristics, and potential durability issues of the aggregate through microscopic examination

Answer

Petrographic analysis uses microscopy to identify minerals, assess physical properties, and detect potentially reactive or harmful constituents that could affect concrete performance.

Question 3

Why is the soundness test performed on concrete aggregates?

Accepted Answer

To evaluate the aggregate's resistance to disintegration from repeated cycles of wetting and drying or freezing and thawing — simulating weathering

Answer

The soundness test (ASTM C88) simulates weathering by soaking aggregate in sodium or magnesium sulfate solution that crystallizes in pores, creating expansion pressure similar to ice crystal formation.

Question 4

How does aggregate shape and texture affect concrete performance?

Accepted Answer

Angular, rough-textured aggregates improve bond with cement paste (increasing strength) but reduce workability; rounded, smooth aggregates improve workability but may reduce strength

Answer

Aggregate shape and texture affect both fresh and hardened concrete: angular particles interlock better (stronger) but create more friction (harder to work), while rounded particles flow more easily.

Question 5

What is the significance of the amount of material passing the No. 200 sieve in concrete aggregate?

Accepted Answer

It measures the amount of very fine dust, silt, and clay particles that can coat aggregate surfaces, weaken the paste-aggregate bond, and increase water demand

Answer

Material passing the No. 200 sieve (75 μm) is mostly silt, clay, or stone dust. Excessive amounts weaken concrete by increasing water demand and interfering with cement paste bonding to aggregate.

ACI (American Concrete Institute Certification) Practice Test

ACI (American Concrete Institute Certification) Practice Test

ACI Certification Aggregate Properties and Testing 3