How Does the Uncompacted PCS Surface Command Work for Paving Operations?

The uncompacted PCS surface command of TBC is a truly unsung hero that has rescued many paving projects over the years - it is used to create a model for a machine that corrects for a poor underlying surface - e.g. a milled surface that was not milled with 3D or a poorly prepared subgrade or a subgrade that was laid wrong because of a misadjustment of the machine (misadjustment or knocked out of a adjustment by a truck!) (For paving operations the Truck is the same as the earthmoving machine that knocks out the survey stakes and has an operator that jumps out and sticks them back in the ground …)

This is how it works

You have a milled surface or a prepared subgrade for instance that was not done with a 3D Mill / machine so it is not uniform depth below target Finished Grade of the Asphalt Mat / material mat you want to put on top of it. You have the Finished Grade Model for the Asphalt / material Mat. If you difference between the two surfaces there is a variable mat thickness. If you lay the asphalt to design or design + and then roll it, because the mat has variable thickness it will compact more where it is deeper and less where it is shallower - assuming that the amount of compaction is a % of depth. The end result will not be the finished grade that you wanted.

So what the command does is allows you to pick the milled surface / prepared subgrade surface and the finished grade surface and input a factor for the amount of compaction expected and then creates a 3rd surface that allows for the variable compaction amount that you expect.

The end result is a surface that when laid pre rolling will be high by varying amounts to the design, but once rolled it will be near perfect.

I have seen it used personally in several scenarios - here are a few of the ones I have seen / been involved with

  1. A well known football stadium project had several layers of material laid by a paver (not sure it was asphalt as it was under the field) and on one of the layers a truck hit the screed and knocked it out of alignment, they laid multiple runs of material up and down the field with the paver and then found that they had created a herringbone layer - this was not Layer 1 nor one of the last 2 layers - so instead of ripping out the layer they measured it with a TS and created the surface and then created the surface for the next layer and ran the command to create the Uncompacted PCS surface - they then paved the uncompacted PCS surface (this was kind of a double or quits situation - a real leap of faith) and then rolled it and it was spot on - they totally loved Trimble for it as it saved the project from what I remember - I don’t know who the contractor was off the top of me head but anyhow it really worked.

  2. I saw it on a Runway Project where they used it to compensate for poor subgrade preparation to lay a material layer with a paver and again it did the same thing

  3. I saw it on a Highways Project in Ireland where the subbase contractor screwed up the subgrade and rescued it with the paver using this command

So it really does work, I know the Trimble Paving guys have many stories of where it had to be used to “fix an issue” and for the contractor it has always been a leap of faith and when they see what comes out of the paver they get really nervous, but then the roller knocks it into shape and everyone is relieved and blown away by the results and then become real converts to use it when they get in a tough spot.

You need to prove it to yourself for sure and you need a good surveyed surface and a good FG surface for it to work well - you also have to trust in compaction and rolling and the fact that asphalt or material does compact uniformly based on thickness - thicker material will compact more than thinner material. The biggest issue is knowing the compaction factor to use -so a trial area is likely needed to compute that unless you know the factors already (you guys probably already know that I am sure)