ACI Certification Aggregate Properties and Testing 2 — Questions and Answers

Question 1: What is the primary purpose of the sieve analysis test for concrete aggregates?

To measure aggregate color
To determine the particle size distribution (gradation) of the aggregate, which affects concrete workability and strength (Correct answer)
To test aggregate hardness
To measure moisture content

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

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.

Sieve analysis (ASTM C136) is one of the most fundamental aggregate tests in concrete technology. The test involves: shaking a dried aggregate sample through a nested stack of sieves with progressively smaller openings. The weight retained on each sieve is recorded and used to calculate the percentage passing each sieve size. The resulting gradation curve reveals: (1) Whether the aggregate meets the specified grading requirements (ASTM C33 for concrete aggregates). (2) The fineness modulus for fine aggregate — a single number summarizing the gradation. (3) Gaps in gradation that could affect concrete workability, segregation resistance, or packing density. (4) Whether the aggregate is too coarse (harsh, hard to work) or too fine (requires more water and cement). A well-graded aggregate — one with a good distribution of particle sizes — produces concrete that is more workable, denser, and stronger than concrete made with poorly graded aggregate.

Question 2: The specific gravity of an aggregate affects which property of concrete?

Color only
Unit weight (density) of the concrete — higher specific gravity aggregate produces heavier concrete (Correct answer)
Setting time
Slump

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

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.

Specific gravity (ASTM C127 for coarse aggregate, C128 for fine) measures how dense the aggregate particles are compared to water. Normal-weight aggregates have specific gravities around 2.4-2.9. This property matters because: (1) Concrete unit weight — directly affects the dead load in structural design. Normal-weight concrete is ~145-150 lb/ft³. (2) Mix design calculations — specific gravity is used to calculate batch weights and absolute volumes in concrete mix proportioning. (3) Yield verification — knowing the specific gravity helps calculate the theoretical yield of a concrete mix, which is compared to actual yield. (4) Aggregate classification — lightweight aggregates (SG < 2.0) produce structural lightweight concrete (~90-120 lb/ft³). Heavyweight aggregates (SG > 4.0) produce radiation-shielding concrete. (5) Absorption — specific gravity testing also determines absorption capacity, which affects the water-cement ratio (free water available for cement hydration vs. water absorbed by the aggregate).

Question 3: What does the Los Angeles Abrasion test measure?

How smooth the aggregate surface is
The resistance of coarse aggregate to degradation by abrasion and impact, indicating its toughness and durability (Correct answer)
The aggregate's color change when rubbed
How well aggregate bonds with cement paste

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

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.

The Los Angeles Abrasion test (ASTM C131/C535) is a durability test that simulates the wear and impact aggregate experiences during concrete mixing, placing, and in-service use. The procedure: place a specific weight of coarse aggregate in a steel drum with a specified number of steel spheres (charge). Rotate the drum 500 revolutions at 30-33 RPM. The steel balls impact and grind the aggregate. After testing, the material is sieved — the percentage that has broken down to pass the No. 12 sieve is the 'percent loss.' Typical specifications limit the loss to 40-50% for most concrete applications, though some uses (heavy traffic pavements, aggregate base) require lower limits. Aggregates with high LA Abrasion loss values are soft or friable and will produce concrete that wears quickly under traffic or abrasion. This test is particularly important for: concrete pavements, industrial floors, parking structures, and any application where the concrete surface is subject to wear.

Question 4: Why is the moisture content of aggregate important to measure before batching concrete?

It affects the color of the concrete
Aggregate moisture contributes to the total water in the mix, affecting the water-cement ratio, which directly controls concrete strength and durability (Correct answer)
Moisture only matters for aggregate stored outdoors
It doesn't affect concrete — only aesthetics

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

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.

Aggregate moisture content (ASTM C566) directly impacts concrete quality: (1) Water-cement ratio — the most critical factor in concrete strength. The mix design specifies a target w/c ratio (e.g., 0.45). If aggregate carries 5% surface moisture and this isn't accounted for, the actual w/c ratio could be 0.50 or higher, reducing strength by 10-15%. (2) Four moisture states — aggregates exist in four conditions: oven-dry (OD), air-dry (AD), saturated surface-dry (SSD — the reference condition for mix design), and wet (carries free surface moisture). (3) Batch adjustments — before every batch, measure aggregate moisture. Subtract the SSD absorption value to get the free moisture. Add this free moisture to the batch water to get total water. Reduce the added batch water by the free moisture amount. (4) Practical impact — on a rainy day, sand can carry 5-8% surface moisture. For a 6-yard truck with 5,000 lbs of sand, that's 250-400 lbs of extra water if not adjusted. (5) Automated plants — modern batch plants have moisture probes in aggregate bins that continuously adjust water additions.

Question 5: What is 'alkali-silica reactivity' (ASR) and why is it tested in aggregates?

A test of aggregate strength
A harmful chemical reaction between certain silica minerals in aggregate and alkalis in cement that causes cracking and expansion in concrete over years (Correct answer)
A measure of how quickly aggregate dissolves
A test of aggregate color stability

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

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.

Alkali-Silica Reactivity (ASR) is one of the most destructive long-term deterioration mechanisms in concrete. The chemistry: certain forms of silica in aggregate (opal, chalcedony, strained quartz) react with sodium and potassium hydroxides (alkalis) in cement paste. The reaction produces a gel that absorbs moisture and swells, creating internal pressure that cracks the concrete. The process is slow — often taking 5-20 years to show visible damage. Testing methods: (1) ASTM C1260 (Accelerated Mortar Bar Test) — submerges mortar bars in hot NaOH solution for 16 days. Fast but sometimes gives false positives. (2) ASTM C1293 (Concrete Prism Test) — measures expansion of concrete prisms over 1-2 years. More reliable but very slow. (3) Petrographic examination (ASTM C295) — a geologist examines the aggregate under a microscope to identify reactive mineral forms. Mitigation: use supplementary cementitious materials (fly ash, slag, silica fume) that consume alkalis, use low-alkali cement, or avoid reactive aggregates entirely.

Question 6: What does the 'fineness modulus' of fine aggregate indicate?

How fine the cement is ground
A single number representing the average particle size of sand — higher values indicate coarser sand, lower values indicate finer sand (Correct answer)
The percentage of material passing the No. 200 sieve
The specific gravity of fine aggregate

Correct answer: A single number representing the average particle size of sand — higher values indicate coarser sand, lower values indicate finer sand

Fineness modulus (FM) is calculated from sieve analysis results by summing the cumulative percentages retained on specific sieves and dividing by 100. It provides a quick way to characterize sand gradation.

Fineness modulus is a shorthand for sand gradation: FM = (sum of cumulative % retained on sieves No. 100, 50, 30, 16, 8, 4, and 3/8") ÷ 100. Typical range: 2.3-3.1 for concrete sand. FM = 2.3-2.5 = fine sand, FM = 2.5-2.8 = medium sand, FM = 2.8-3.1 = coarse sand. Why it matters: (1) Mix design — the FM is used in the ACI 211.1 mix design method to determine the proportion of coarse aggregate to fine aggregate. Higher FM sand requires less coarse aggregate to fill the mix. (2) Water demand — finer sand (lower FM) has more surface area, requiring more water to coat all particles, which increases the water-cement ratio unless additional cement is added. (3) Consistency — monitoring FM over time helps detect changes in the sand supply that could affect concrete quality. A change of more than 0.2 from the average may require mix adjustments. (4) Workability — very fine sand (FM < 2.3) produces sticky, hard-to-finish concrete. Very coarse sand (FM > 3.1) produces harsh, hard-to-pump concrete.

ACI Certification Aggregate Properties and Testing 2

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