What is the crown in road paving and where is it?

Answer 1

Alright... your interest in paving (and also grading, if you don't realize it) is a bit curious to me, but, to the point..

Roads aren't laid flat. They might appear flat at a glance, but they're not. A lot of factors go into engineering roadways, and that also includes anticipated rainfall/storm surge and drainage. Not laying these roads flat ensures that water runs off the roadways... if it didn't, then it could pool up on the roadways and make them very dangerous to drive. So the idea of not laying a road flat is to prevent things such as hydroplaning in the vehicles traveling on them.

So there'll be a high point in the roadway - that's called the crown. Where it is, exactly, depends on the roadway itself, as well as the factors which go into designing the roadway. Let's say you're driving on a roadway in a mountainous region... for the sake of argument, we'll say it's a road which runs north-south. On the west side of the roadway, you have high ground, while you'll have lower ground on the east side of the roadway. Well, knowing how gravity works, you'll know that the drainage is going to be to the east side of the roadway, so that roadway will be graded so that it slopes downward towards the east side. The crown is going to be the high point on the west side of the roadway in this instance.

Now, if you're traveling along a number of other roads... rural roads on flat land, city/subdivision streets, et. al, you might notice storm drains or ditches on both sides of the roadway. What this means is that drainage is diverted in both directions, so the crown will be more towards the center of the roadway. The caveat to this type of design being that that crown is going to be the weak point of the asphalt mat which is laid on it... if you go down a road with an older asphalt mat, you might notice that crews will come by and do crack seal - pouring tar into cracks which form in the roadway. And you'll often see long lines of this crack sealant right smack in the center of it... that's because a crown set up in this fashion is often the first point at which cracks will form in the asphalt mat.

A typical crown grade will be 2 - 3% - nothing very drastic. Setting up the roadway and crown (and grading out the quarter crown, as well) will be the job of dirt crews, rather than asphalt crews. The dirt crews will start by cutting a rough grade in the vicinity of where the roadway will be. Then they'll take the transit and shoot hubs.. the formula for this, I'm not going to get into right now, because I could show you much easier than I could explain it here. So they'd shoot the hubs, tap them into the grown at the point where the dirt should be, and the graders will cut down the dirt or fill it in and grade it to the top of those hubs, then the grader will cut the grade and ensure that the grade is the appropriate depth below the top of the curb face... so, if there's four inches of asphalt, the grade will be cut four inches below the top of the curb face, and the blade will be angled at whatever the grade of the crown is. The whole process of cutting a grade involves both the blade hand (who runs the grader), and a laborer who'll chase hubs.. they'll ensure each hub is uncovered, signal to the blade hand how far down they need to cut (or how far up they need to fill), etc. They'll also probably have to do various other tasks at given times, such as running the sheep's foot or combo roller, maybe even running a scraper, etc.