How Aggregate Particle Shape and Gradation Affect Concrete Performance

• Workability and Flowability: Rounded particles reduce water demand and improve flow, while angular and elongated particles increase internal friction, requiring more paste or admixtures to maintain proper workability.
• Cohesion, Segregation, and Bleeding: Regularly shaped aggregates produce stable mixtures, resisting segregation and bleeding. Studies show that as the proportion of flaky and elongated particles increases, mixtures become harder to compact, with higher bleeding and separation risk.
• Compressive and Tensile Strength: Experimental studies indicate that concrete with fully spherical aggregates exhibits 5–28% higher 28-day compressive strength compared with mixes containing significant flaky or elongated particles. Angular particles improve interlock and strength, but excessive flaky material introduces weak planes that reduce tensile and flexural performance.
• Durability: Freeze–Thaw, Permeability, Carbonation: Flaky or elongated aggregates increase void content and reduce compactness, leading to higher permeability and lower freeze–thaw resistance. Research shows these mixes are more prone to microcracking and durability loss.

• “Aggregate Bridging” Effects: Excessive flaky particles tend to align parallel to casting direction, forming “bridging” zones with poor paste coverage and higher void content.
• Local Stress Concentration: Irregular, thin particles create stress concentration points under load, promoting early microcrack formation and accelerating structural degradation.
• Deviation from Design Strength: High proportions of flaky or elongated aggregates can lead to discrepancies between lab-tested mix designs and actual in-situ concrete performance, causing reduced strength, increased shrinkage, and compromised durability.

• Single-Size Gradation: Uses one dominant particle size. This results in high void ratios, low packing efficiency, and elevated paste demand. It is rarely used for structural concrete.
• Gap Gradation: Omits certain intermediate sizes. While it can enhance certain flow characteristics, it may cause bleeding, segregation, or weak zones if not carefully controlled.
• Continuous (Well-Graded) Systems: Contain a complete size range from fine to coarse particles. Well-graded aggregates achieve optimal packing, reduce paste volume requirements, and improve workability, compressive strength, and durability. Studies indicate that well-graded aggregates can reduce cement consumption by 10–15% per cubic meter compared with single-size systems.

• Void Content and Paste Demand: Poorly graded aggregates increase void ratio, requiring additional cement paste to fill gaps.
• Cement Consumption: Higher voids increase cement content and cost, while optimized gradation reduces material demand without compromising strength.
• Drying Shrinkage and Cracking: Excess voids and uneven particle distribution create stress concentrations, elevating shrinkage and crack formation risk.
• Pumpability and Construction Stability: Well-graded aggregates enhance flowability, minimize blockage in pumping systems, and maintain stability during placement. Improper gradation may cause segregation or localized weaknesses.

Relying solely on higher cement content or chemical admixtures to compensate for poor gradation is a common industry mistake. While it may temporarily achieve target workability or strength, long-term durability suffers due to higher shrinkage, cracking, and potential permeability issues. Sustainable concrete design requires controlling aggregate gradation at the source, not compensating downstream.

Different crushers produce distinct particle geometries and gradations:

• Jaw Crushers: Primary crushers break large rocks into coarse, blocky particles. They generally produce moderate angularity but high coarse fractions. Jaw crushers are essential for initial size reduction but cannot produce well-shaped aggregates suitable for high-performance concrete without secondary processing.
• Cone Crushers: Secondary or tertiary cone crushers improve particle shape and reduce flaky or elongated particles. Cone crushing produces more equidimensional particles and tighter gradation, enhancing packing density. Adjusting the crusher’s closed-side setting allows control over particle size and angularity.
• Impact Crushers / VSI (Vertical Shaft Impactors): These machines generate cubical fine aggregates with sharp edges and high sphericity. VSI crushers are widely used for manufactured sand and high-quality concrete aggregates. They can, however, generate excess fines if rotor speed and feed rate are not optimized, which may affect workability.

The crushing circuit configuration strongly affects final aggregate quality. Key considerations include:

• Multi-Stage Crushing: Employing primary, secondary, and tertiary stages ensures gradual size reduction, better shape control, and optimized gradation.
• Closed vs Open Circuit: Closed circuits with recirculation allow repeated crushing of oversized particles, producing uniform gradation and angularity. Open circuits may lead to over- or under-sized particles.
• Recirculation Ratio: Adjusting the proportion of particles returned to the crusher helps control shape uniformity but must be balanced to avoid excessive fines.
• Screening Efficiency and Mesh Size: Accurate screening removes undersized or oversized particles, stabilizing grading curves. High-efficiency screens reduce variability in coarse and fine fractions.
• Crusher Operational Parameters: Discharge settings, rotor speed (for impact crushers), and feed rate influence particle fracture patterns. Small adjustments can significantly alter particle shape distribution and flaky particle content.

Flaky and elongated particles are often generated when rocks break along natural cleavage planes or under excessive impact forces. Studies indicate that aggregates with a flakiness index above 30% can substantially reduce concrete strength and durability. Proper crusher selection, circuit design, and operational control can reduce the flakiness index to 10–15%, meeting high-quality concrete specifications.

Gradation stability depends on the interaction of crushing, screening, and recirculation. Well-designed circuits can produce continuous, well-graded aggregates suitable for dense packing, reducing voids and paste demand. Conversely, poorly designed circuits result in gap gradation, uneven particle size distribution, and variability that affects compressive strength, workability, and long-term durability.

• Quality Starts at the Crusher: Achieving predictable concrete performance depends more on controlled aggregate production than on downstream mix adjustments.
• Upstream Control Reduces Cost and Risk: Properly shaped and graded aggregates reduce cement and admixture demand, improve consistency, and minimize cracking and durability issues.
• Performance-Oriented Production: Modern concrete design increasingly evaluates aggregates based on expected structural performance, not just sieve-based compliance, emphasizing angularity, sphericity, and shape distribution.

High-strength and ultra-high-performance concretes are highly sensitive to aggregate characteristics. Angular, cubical particles enhance interlock and bonding, while even minor deviations in gradation can compromise compressive strength, modulus of elasticity, and durability. Flaky or elongated particles are particularly detrimental, increasing the risk of microcracking under high loads. Performance optimization in these mixes requires tightly controlled aggregate production.

For precast elements, surface quality, dimensional stability, and early demolding strength are critical. Aggregates with consistent shape and gradation ensure uniform compaction, reduce voids, and improve surface finish. Inconsistent aggregates can lead to uneven shrinkage, warping, or poor dimensional accuracy, which increases rejection rates and production costs.

Concrete incorporating recycled or low-carbon aggregates demands extra attention. Recycled aggregates often have irregular shapes, higher fines, and variable grading, which magnify the sensitivity of the mix to shape and particle size distribution. Optimized gradation and controlled shape are essential to achieve target performance while minimizing cement content and maintaining sustainability objectives.