DCC has clarified with its Materials Laboratory why the Pebble Beds (PBs) have apparently become more durable (LAA, 5.18). PSV is a measure of how quickly a sample of aggregate will become polished, i.e. on a road surface how quickly it will lose its resistance to skidding. Aggregates specified for roads on approaches to pedestrian crossings and traffic lights may require a PSV of 68 or more, whereas for an infrequently used minor road a PSV of 50 would be acceptable.
Only a limited number of quarries produce high PSV material in the UK. The PBs have previously tested in the mid 50s and Straitgate at 53. The 'Mat Lab' has however recently measured the effective PSV or ePSV of the PBs at 60-64 using "a mix of particle sizes from 3mm to 8mm, which provides greater friction levels than a single sized particle due to the greater surface area (with the smaller particles filling the gaps between the larger ones), and is also more durable and resistant to wear from turning vehicles". DCC confirmed that any extra demand for the PBs created by this re-rating would be relatively minor. Most higher risk locations require PSVs of more than 65.
