Evaluation of the Crushing Characteristics of Slate Coarse Aggregate Used for Asphalt Mixture

The relict bedding and slaty cleavage structure in slate directly influences the crushing characteristics and strength properties of slate aggregates. When slate aggregates are used in asphalt concrete, it may have risks of insufficient resistance to crushing and uncertain long-term durability. In order to investigate the crushing behavior of slate coarse aggregates in asphalt mixtures, a comparative study was conducted using limestone and basalt aggregates as reference. Various tests were carried out including crushing value tests, single-particle compression crushing tests, Marshall compaction resistance tests, and gyratory compaction resistance tests. The crushing patterns, crushing strength, and gradation changes of slate aggregates after crushing were systematically examined. Based on the Weibull distribution function, the statistical distribution of single-particle crushing strength was analyzed. Additionally, the particle distribution patterns were studied for single-sized aggregates, blended aggregates, and asphalt mixtures after these were subjected to crushing under Marshall compaction and gyratory compaction. The test results indicated that the crushing value of slate coarse aggregates was 9.2%, which indicates superior crushing resistance compared to traditional limestone and basalt. After long-term exposure to water immersion at 60 °C, high-pressure steam treatment, and heating at 220 °C, the increase in crushing value of slate coarse aggregates was less than 1.5%, indicating excellent water and heat resistance. The two-point and four-point crushing strengths of single particles of slate coarse aggregates were higher than those of limestone and basalt coarse aggregates, and the single-particle compression crushing strength followed the Weibull distribution pattern. Both single-sized and blended slate aggregates exhibited lower proportions of crushing during Marshall and gyratory compaction compared to basalt and limestone aggregates. Asphalt mixtures prepared with slate coarse aggregates also demonstrated better crushing resistance than those made with basalt and limestone, confirming that the bedding structure of slate does not cause excessive crushing in asphalt mixture. The obtained findings were limited to the tested slate aggregates from a single quarry and thus necessary performance verification should be conducted on slate aggregates from other sources before practical engineering applications.