Soils and Aggregates Flashcards

Which compaction test method uses a 10-lb hammer dropped 18 inches and is commonly specified for highway subgrade?

Answer: Modified Proctor (AASHTO T 180)

The Modified Proctor test uses a 10-lb (4.54 kg) hammer with an 18-inch drop and higher compactive effort, simulating modern heavy compaction equipment.

The plasticity index (PI) is calculated as:

Answer: Liquid limit minus plastic limit

Plasticity index equals liquid limit minus plastic limit and represents the range of moisture content over which a soil behaves plastically.

A nuclear density gauge used for field compaction testing measures:

Answer: In-place wet density and moisture content simultaneously

Nuclear density gauges use gamma radiation and neutron scatter to simultaneously measure in-place wet density and moisture content of compacted material.

What is the primary purpose of lime stabilization in subgrade soils?

Answer: Reduce plasticity and improve strength of high-plasticity clays

Lime reacts with clay particles through pozzolanic and cation exchange reactions to reduce plasticity index and improve strength and stability.

During a sieve analysis, the cumulative percent retained on a sieve is used to calculate:

Answer: Fineness modulus and percent passing

Cumulative percent retained values are used to compute percent passing each sieve and to calculate the fineness modulus for fine aggregates.

Which aggregate characteristic is most important for resistance to polishing under traffic in surface course applications?

Answer: High polish stone value or resistance to abrasion

Aggregates with high resistance to polishing maintain surface texture and skid resistance under repeated traffic loading in pavement surface courses.

A relative compaction of 95% means the field dry density is:

Answer: 95% of the maximum dry density from Proctor test

Relative compaction is expressed as the ratio of field dry density to maximum dry density from the Proctor test, multiplied by 100%.