Aggregate Pop-outs
LONG WINTERS IN MINNESOTA. WHAT YOU NEED TO KNOW ABOUT AGGREGATE POP-OUTS ON YOUR PROJECT.
Written by Brandon Brever, P.E.
What are Aggregate Surface Pop-outs?
Aggregates are the small, granular materials such as crushed stone, gravel, sand, and slag that are mixed with bitumen to create asphalt pavement. When a piece of aggregate becomes dislodged from the pavement surface due to various reasons, it creates a cavity or pop-out. Pop-outs can range in size from the smallest to the largest aggregate used in the asphalt mixture.
Why Do They Occur?
Aggregate pop-outs can occur due to various reasons, including the use of softer aggregates, moisture damage, snowplow damage, and multiple freeze-thaw cycles. Pavements that contain lower quality aggregates or minerals that are soft or weak may be more prone to surface aggregate dislodgement more quickly than those with harder, higher quality aggregate. Heavy traffic can also accelerate pop outs. In general, aggregate popouts occur gradually throughout the first several years after the pavement has been placed. However, in severe winters, where we experience many snow and snowplowing events with many freeze-thaw cycles it is possible that close to 100% of pop-outs will occur in the first winter.
Do surface pop-outs affect the overall pavement life?
No! Pop-outs on paved surfaces do occur in Minnesota. Based on pavement history, surface pop outs have not shown to affect the long-term structural integrity of asphalt mixes. With our current mix specifications asphalt cement is able to coat all aggregates in the mix to provide adequate protection against freeze-thaw damage.
When surface pop-outs do occur, we are often asked if water will infiltrate in those exposed impressions and cause further damage. Due to the voids being unconfined, the water does not appear to cause any further freeze-thaw damage. Asphalt pavements contain air voids to allow for water to expand/contract as well as evaporate and escape through the pavements.
Sustainability of using local aggregates.
Local aggregates are those that are sourced from nearby quarries or sources. Using local aggregates reduces the transportation and energy costs associated with transporting aggregates from far-off locations.
Some areas of our state have limited aggregate resources and have been utilizing softer aggregates successfully for the last 50+ years.
What can be done?
Surface pop-outs are commonly thought of as an aesthetic surface defect that occur throughout the first several years after the pavement has been placed. The projects we have seen with this type of distress do not require immediate attention and do not have long term pavement damage occurring. Depending on the severity of the pop-outs, there are several options that can be done through pavement preservation methods if aesthetics are a concern for your project. Some locals have had success with chipsealing, fog sealing, and other methods. We encourage you to contact the MnDOT Pavement Preservation office for more information to meet your needs.
Our seasonal fluctuations are challenging in northern climates. Asphalt pavements have a long history of providing an excellent quality surface for the traveling public in Minnesota. We will continue to provide the smoothest and most economical roads in Minnesota.