Whether concrete or asphalt, before a pavement is placed, the surface to be paved has to be adequately “prepared.” Otherwise, the new pavement may fail. Surface preparation may entail either of the following: subgrade preparation for an entirely new pavement or preparation of an existing pavement for an overlay job.
Preparing the subgrade, or the granular base course, for a brand new pavement includes such activities as subgrade stabilization, over-excavation of poor subgrade, applying a prime coat and compacting the subgrade. Typically, the subgrade should be compacted to “adequate density” before the actual pavement placement. In industry parlance, “adequate density” is taken to mean as the relative density for the top 6 inches of subgrade of not less than 95-percent of a specified standard laboratory density. In fill areas, subgrade below the top 6 inches is often considered adequate if it is compacted to 90-percent relative density. If it is not that compacted to meet adequate density, the subgrade will continue to compress, deform or erode after the construction work, causing the placed pavement to eventually crack or even collapse.
All debris, large rocks, vegetation and topsoil from the area to be paved should be removed as they either do not compact well or they cause non-uniform compaction and mat thickness. The subgrade should treated with an industry-approved herbicide to prevent or at least retard future vegetation growth, which could affect subgrade support or lead directly to pavement failure.
For increased strength and stability, lime with portland cement or emulsified asphalt can be mixed in with the subgrade soil. Where the in situ subgrade soil is really extremely poor in quality, it can be excavated and replaced with better load-bearing filling material. A subbase course may be also added over the subgrade to further improve its load-bearing capacity. After fine-grading, the subgrade elevation should then continue to be compacted to conform closely with the specified subgrade elevation in the construction plan. Finally, the graded subgrade, or the top granular base layer, can be topped with a prime coat of hot mix asphalt mixture to promote bonding to the subsequent pavement layers. The prime coat also serves to fill the surface voids as well as preserve and protect the subbase material from moisture and other weather conditions.