How to Choose Good Quality Coarse Aggregates

Key Takeaways:

• Uncovering the importance of selecting high-quality coarse aggregates.
• Identifying the characteristics of good quality coarse aggregates.
• Tips for sourcing the best coarse aggregates for your projects.

The Critical Role of Coarse Aggregates

Coarse aggregates act as the backbone of concrete, composite pavements, and other building materials. They make up 60-75% of a typical concrete mix by volume. With poor quality aggregates, the concrete will be weak, prone to cracking and deterioration. But high-quality crushed stone and gravel ensure buildings and infrastructure remain sound for decades of service life.

Coarse aggregates encompass gravels, crushed stone, blast furnace slag, and other rocks that are larger than a 4.75 mm No. 4 sieve. Size classifications range from fine through coarse. Common standard sizes are 3/8”, 3/4″, 1″, 1.5” and larger. The largest aggregates may be 2” or more for mass concrete applications.

Choosing appropriate, high-performing coarse aggregates is essential to meet the durability, strength, appearance, weight, and sustainability goals of construction projects. Testing helps characterize essential aggregate properties that impact quality. With information on geological origin, together with lab analysis, engineers can select the optimal aggregates for their specific demands.

Characteristics of High-Quality Coarse Aggregates

What are the defining traits of quality coarse aggregates that influence concrete integrity and performance? Here are key characteristics and tests to evaluate them:

Strength and Hardness

Aggregates must be able to withstand crushing forces and abrasion within the concrete mix. Hard, competent rocks like granite resist the grinding action from loading and traffic without breaking down. Weaker, porous aggregates can disintegrate prematurely.

Strength and hardness testing includes:

• Crushing value – A crushed sample is sieved to measure fines produced from fracturing. Lower values indicate stronger aggregate.
• Los Angeles abrasion – Aggregate undergoes impact, crushing and abrasion in a rotating steel drum. The lower the weight loss, the stronger the material.
• Impact value – Measured by dropping a known weight onto a test sample and calculating fracture percentage.
• Mohs scale – A relative mineral hardness scale based on scratch resistance. Values range from 1 (talc) to 10 (diamonds).

Durability

Resistance to weathering and adverse environmental conditions determines aggregate durability. Durable aggregates retain their integrity when subject to moisture, freeze-thaw, and chemical exposures over time.

Testing includes:

• Absorption – Measuring water absorption percentage after 24 hour submersion indicates porosity.
• Freeze-thaw – Cycling a saturated sample from freezing to thawing temperatures tests weathering resistance. Weight loss determines durability.
• Soundness – Aggregate specimens are subjected to sodium or magnesium sulfate solution to evaluate mineral reactivity.
• Degradation – Exposing to abrasive, corrosive, or eroding environments simulates service conditions. Weight loss measures durability.

Shape and Size Distribution

Well-graded coarse aggregates with consistent particle shapes compact into a dense concrete matrix. Elongated and flat particles are prone to segregation which reduces strength.

Analysis includes:

• Sieve analysis – Determines size distribution and percentages retained on standard sieve sizes. Helps identify gaps or excesses.
• Fineness modulus – Calculated from sieve analysis results, it indicates overall aggregate size. Values between 2.3-3.1 are common.
• Particle shape evaluation – Aggregate imaging and software digitally measures form and angularity. Rounder aggregates have lower interlock.
• Specific gravity and voids – Determines relationships between aggregate density, porosity and void spaces in concrete.

Proper Composition

Harmful substances and reactive minerals can compromise aggregates:

• Petrographic analysis – Microscopic exam detects harmful grains, textures, and rock origins. It identifies reactive minerals like chert or pegmatite.
• Chloride ion content – Chlorides corrode steel rebar. Coal, lignite and shale may contain high levels from marine sources.
• Sulfate content – Soluble sulfates contribute to delayed ettringite formation, cracking and expansion.
• Organic impurities – Plant matter, silt and clay reduce concrete strength and durability. Washing removes them.

With comprehensive lab testing, you can uncover potential aggregate reactivity and contamination issues before they impact construction quality.

Call to Action: Our professional aggregate testing lab performs all essential tests to ASTM and AASHTO standards. Contact us today to test your coarse aggregates.

The Critical Role of Suppliers

Responsibly sourcing quality aggregates starts with partnering with reputable suppliers that prioritize consistent quality control.

Characteristics of reliable aggregate suppliers include:

• Long-standing geological and technical expertise in aggregate production and supply chains
• Quarry control and management practices that ensure consistent, uncontaminated aggregate reserves
• Sampling and testing programs that fully characterize new quarry sources before large-scale production
• Ongoing aggregate quality control testing during production such as monitoring moisture, gradation and contaminants
• Transparent data reporting and an openness to third party audits of their quality control practices
• Knowledge of local construction demands and capabilities to supply appropriate aggregates and gradations
• Environmentally responsible mining and management procedures to reduce waste, water use, dust, noise, vibration and visual impacts

By partnering with suppliers that make aggregate quality control a priority, you reduce project risk and ensure a reliable source of aggregates that meet target specifications over the long term.

Choosing the Best Aggregate for Your Project’s Needs

With quality control testing data in hand, you can select appropriate aggregates for your unique project requirements. Construction applications demand different aggregate properties. Consider factors like:

• Strength needs – Higher strength concrete requires stronger, more angular crushed aggregates. Smooth rounded stones are prone to segregation in structural applications.
• Appearance – Surface finishes like exposed aggregate require decorative, colored aggregates of certain shapes and textures. Aggregate color consistency is crucial for architectural concrete.
• Load levels – Heavy duty pavements and gravity dams require aggregates with high abrasion and impact resistance. Lighter applications can utilize less strong aggregates.
• Chemistry – Marine environments and acid exposures demand low-reactivity aggregates that resist chemical attack. Aggregates high in carbonates and silicates dissolve in acids.
• Constructibility – Harsh climates or remote locations may dictate using locally available aggregates, even if that means compromising on certain properties.
• Sustainability – Recycled concrete aggregate helps reduce landfill waste but may have higher water demands in new concrete. Locally-sourced aggregates lower transport emissions.

‍