Performance of an asphalt mix is directly related to the condition of the surface on which the pavement is placed. When asphalt mixes are laid on top of existing pavements, the existing surface should be properly prepared before paving. In this edition of the RoadReady Newsletter, we are going to focus on leveling as preparation, more specifically the practice of milling, and talk about some factors to consider when making a choice on the best leveling preparation method to choose for a job.
Factors to Consider When Leveling Existing Surfaces for Paving
Elevations, clearances and drainage
When overlaying an existing pavement, you raise the entire surface by the thickness of the new layer, creating additional work to raise the surrounding structures like guardrails, shoulders and utilities. By milling the existing pavement, then inlaying new pavement, you can avoid modification of other structures on the job site. Properly performing this step also preserves clearances for bridges and overpasses and can maintain or corrects roadway water run-off and drainage.
Prevention of re-rutting and/or cracking
When an overlay is placed onto a pavement with rutting, you will experience faster re-rutting as well as potential rutting cracks after the road is subject to traffic. Even when you fill the ruts first, complications with density and proper compaction are possible, increasing the likelihood of cracks and re-rutting. A good way to ensure that a road doesn’t become re-rutted is to get rid of the old rutting by properly milling before the inlaying new pavement.
Structures like bridges, overpasses and on ramps are subject to increased stress from the weight of additional pavement. When you first mill and then inlay asphalt mix, you maintain the approximate weight that the structure was originally designed to withstand without needing to reduce weight limits for transport.
Complicated leveling courses
Milling eliminates the need for complicated leveling courses to fill existing pavement depressions. Leveling course quantities make it difficult to accurately estimate asphalt mix tonnage.
Consistent pavement density
Leveling course thicknesses are usually small, precluding the use of nuclear gauge density testing. Adequate mix density is therefore difficult to achieve and measure. Overlays also create inconsistent pavement densities because of aggregate segregation at the outside edge of mats. When you mill, you contain the mix, preventing aggregate segregation and density inconsistencies.
Reclaimed Asphalt Pavement
Reclaimed Asphalt Pavement (RAP) harvested from milling operations provides fiscal advantages as well as environmental benefits. Reclaimed asphalt is worth the virgin materials that it replaces. When you mill and use the RAP, fewer natural resources are ultimately consumed, leading to more sustainable pavements, in line with maintaining asphalt as America’s most recycled product.
There are other benefits that come when you mill pavements prior to inlaying new asphalt mix. In jobs where you must prepare a surface and allow traffic to drive before paving, milling provides a highly skid resistance surface for driving. Milling also increases the surface area of a pavement by 20-30%, which when applying a tack coat and placing new asphalt mix translates into more area to which the new mix can bond.