Impact of Asphalt Lift Thickness on Pavement Density and Durability

Project summary:

A key factor in achieving adequate density is the ratio of the asphalt lift thickness to the size of the aggregate in the mixture. Generally, it is considered that lifts need to be thick enough to provide room for the aggregates to reorient and densify, but not so thick that the bottom of the lift does not feel the compaction force and therefore does not densify. In recent publications, researchers recommended the thickness to nominal maximum aggregate size (t/NMAS) ratio be at least 3:1 for fine graded mixes and at least 4:1 for coarse graded mixes; however, there is no consensus on what value is best. In a limited number of projects, pavements were built with higher lift thicknesses. Since most of the work was of empirical nature, the results were mixed in terms of achieving desired density levels and meeting International Roughness Index (IRI) requirements.

The research group at the University of Minnesota has developed a rational approach to asphalt compaction that has led to better guidelines for designing Superpave 5 mixtures and for modelling the compaction process in asphalt mixtures. The work included analyzing the compaction curves of gyratory cylinders of various heights, and the model predictions were very good.

The research team proposes to use this new approach, validated in previous research on Superpave 5 mixtures, to design asphalt mixtures that can achieve desired densities, and also desired surface roughness, at higher lift thickness values. Anecdotal evidence indicates that using higher lift thickness, which has better heat retention, has resulted in better densities compared to traditional lift thickness values; however, the increase in surface roughness has deterred contractors from using this cost-saving method. It becomes then very important to better understand the factors responsible for the increase in surface roughness and, most importantly, to determine if more durable and more cost-effective asphalt pavements can be built using higher lift thicknesses, in spite of higher initial roughness values.