Why were ASTM specification sizes Numbers 6 and 67 accepted in addition to Number 7?

The acceptance of ASTM specification sizes Numbers 6 and 67 in addition to Number 7 is primarily due to changes in the quarrying and screening processes related to the highway industry. As quarries adjusted their screening methods to meet the demands of highway materials, the resulting aggregates (sizes 6 and 67) became viable alternatives to Number 7 for built-up roofing applications. This adaptation allowed for greater flexibility in sourcing materials while ensuring that the characteristics of the aggregates still provided suitable performance in roofing systems. Understanding these industry changes is crucial, as it highlights how materials evolve to meet both construction needs and standards.