Toughness and Hardness of Aggregate

The aggregate impact value (AIV) measures toughness, or an aggregate's resistance to failure by impact. Aggregates passing a half-inch sieve and retained on a three-eighths-inch sieve are placed in a cup and struck 15 times by a freely falling standard hammer. The fraction passing a No. 8 ASTM sieve afterward is weighed to calculate the AIV. Lower values indicate tougher aggregates.

The Los Angeles abrasion test assesses hardness by simulating wear processes. Aggregates and steel balls are placed in a rotating drum that pulverizes the material through attrition and abrasion. After rotation, aggregates passing a No. 12 ASTM sieve are weighed to calculate the Los Angeles abrasion value. Results correlate well with actual wear and concrete compressive strength.

Airport runways experience heavy impact forces and continuous wear from aircraft traffic. Aggregates must possess sufficient toughness to withstand impact loads and hardness to resist abrasion. These properties ensure concrete durability and long-term performance under demanding conditions, preventing premature deterioration and maintaining structural integrity.

Toughness testing uses aggregates passing a half-inch sieve and retained on a three-eighths-inch sieve, with results measured through a No. 8 ASTM sieve. Hardness testing employs aggregates sieved through a No. 12 ASTM sieve. These standardized sieve sizes ensure consistent sample preparation and comparable results across different aggregate sources.

Lower aggregate impact values indicate greater toughness and better resistance to impact failure. Since the AIV quantifies the fraction of aggregates that fragment under hammer blows, a lower percentage passing the No. 8 sieve demonstrates superior toughness. This inverse relationship makes AIV a reliable quality indicator for selecting aggregates in impact-prone applications.

Steel balls are placed inside the Los Angeles abrasion machine drum alongside aggregates. As the drum rotates, the steel balls tumble and pulverize the aggregates, simulating real-world wear and attrition processes. This mechanical action degrades the aggregate material, allowing measurement of hardness through the Los Angeles abrasion value calculation.

Toughness prevents aggregate fracture under impact loads, while hardness resists surface wear from traffic abrasion. Together, these properties ensure concrete pavements maintain structural integrity and surface quality over extended service life. Aggregates with poor toughness or hardness degrade faster, compromising pavement performance and requiring premature maintenance or replacement.