Understanding Uncompacted PCS Surfaces

A course of asphaltic concrete is compressed a predictable percentage when compacted. In order for the top of the compacted surface to match a design surface, the paving control system (PCS) must be set higher than the design to consider the amount of compaction. Typically the thickness of the new course needs to vary because the previous course is not a consistent depth from the design surface. These varying depths and the needed compaction are resolved by measuring the existing course, creating an existing surface, comparing the existing surface to the design surface, and then creating a target surface that properly considers the varying depth and percentage of compaction. This target surface is called the "uncompacted PCS surface". The method used to calculate the isochore/difference model used in generating the uncompacted surface is described in the help topic called Understanding Earthwork Volume and Isochore Calculations. Here are the basic steps:

The difference between the base course surface and the finish grade surface (specified in the command) is calculated using the prismoidal method to create a temporary isochore.
That isochore is multiplied by the rolling compaction factor (specified in the command) to create a thicker isochore (because under normal conditions, it is always a fill operation).
That isochore is added to the top of the base course surface to calculate/create the uncompacted surface for PCS higher than the finished design surface.

This field process, and how it it handled in the software, is illustrated in the images below.

In the field,

In the software,

you start with an underlying surface, potentially with longitudinal waves,

you specify the underlying surface,

you then lay down varying amounts of material, using a differential design and accounting for the compaction factor of the material,

you then specify the finished design surface and the compaction factor of the material,

and finally you compact it to the finished grade, using differential compaction.

to create an uncompacted surface which can be used by PCS machines in the field (stage 2 shown in the "In the field" column on the left).

Note: The paving industry refers to this subject matter as "differential compaction", but this is a misnomer; the compactive effort does not vary and the amount of compaction (by volume) is effectively constant. It is only the thickness reduction caused by the compaction that varies, and hence "differential thickness reduction" is a more appropriate descriptive term.

Longitudinal vs. Lateral Road Waves

The images above illustrate the accommodation of longitudinal road waves. Pavers can easily be operated to vary the laid mat thickness on either side of the screed as the machine progresses along the road (thereby accommodating longitudinal road waves as illustrated independently on either side), but screeds are not typically designed to dynamically adjust their cross-sectional shape to accommodate lateral road waves.

This software produces an idealized surface, which reflects the differences in elevation between the underlying surface and the design surface. The uncompacted PCS surface that is created represents the elevation of the top of the laid asphalt mat, as required to accommodate the expected volumetric reduction when the asphaltic concrete is compacted. A commonplace result will be a surface that reflects a combination of both longitudinal and lateral waves.

PCS400 software uses this idealized surface, controlling the screed in a manner that accommodates the distinction between longitudinal and lateral road waves that is consistent with the capabilities of the machine being employed. In forming the idealized surface, no accommodation is made for the paver's capabilities or limitations.

Material Compaction Factor

In creating an uncompacted PCS surface, you are required to enter a compaction factor for the material to represent the volumetric reduction that is expected as a result of the rolling and compaction of a course of asphaltic concrete. When expressed as a multiplication factor, this property states the compacted material's thickness as a multiplier of the laid material thickness prior to compaction. As an example, a rolling compaction property multiplication factor of 0.75 results in a compacted in place mat thickness that is 75% of the thickness laid in forming the uncompacted PCS surface. That is, it has shrunk by 25%.

Paving Courses and Surface Classification

The surface on which the asphaltic concrete course is to be laid is called the "underlying surface". It may be the top of the base course or the top of an underlying pavement course. The design surface to be achieved in placing the subject pavement course is referred to as the "design top of pavement course"; this is distinguished from the finished roadway surface, even though it might coincide with that design surface as it is the top of the particular pavement course currently being laid and not necessarily the top most course. Paving operations typically involve multiple courses of asphaltic concrete.

The uncompacted PCS surface has properties specific to asphaltic concrete paving. By default, its surface classification is set to unclassified.

Surface Dependencies

An uncompacted PCS surface is dependent on the specified underlying surface, the design top of pavement course surface, and the designated compaction factor. The uncompacted PCS surface will be updated if either of those other surfaces is edited, or if the compaction factor is changed. If either of the designated underlying or design top of pavement course surfaces is deleted, the dependent uncompacted PCS surface will be deleted as well.