By Brian Hall
Too many times, on the job site I hear problems such as “I can’t get my screed to do what you told me me it would do”. Many times this can be corrected by an easy screed setup or just a few twists and turns. Every screed, no matter who the manufacturer, reacts in much the same way as the other. Whenever you see a change in depth or consistency, it can easily be remedied by looking at what’s happening to the asphalt as it passes through the paver.
Now, I know what you are thinking. He’s not talking to me. And you’re right. I’m talking to those other guys that don’t read these trade magazines or go into paving classes on a consistent basis. But just humor me for a while if you don’t mind.
What we’ll discuss this month is a few of the things that happen when the mix hits the paver. Let’s start with what we need to do before the asphalt even hits the jobsite.
Screed Setup: Every paver manufacturer has a specific procedure for bringing the screed back to 0, as I call it. That is to say making the screed ready to pave each day. That’s right, screed setup should be done every day before you pave. You wouldn’t start out on a trip with the tire pressure in your car varying 30-50% between tires, so why start with your screed out of alignment? The steps may vary between manufacturers, but the end result is the same. Consult your Dealer for this procedure.
Now let’s discuss all the factors that affect the screed. The paver and screed operator must understand what happens to the mat every time he makes a change in speed, angle of attack and head of material. Let’s touch on each one of these so you can better understand.
Paving Speed: Many of today’s pavers are equipped with electronically controlled steering, which helps in governing your travel speed, but many still have the old reliable cable controlled steering. At a constant speed, the “shear factor” or the process that allows the asphalt to travel under the screed remains constant, thus your depth remains constant. When you increase your speed, your shear factor decreases and your screed begins to “dive”. On the other hand, when you slow down, the shear factor increases, causing your depth to increase. As you can imagine, when the paver operator is constantly varying his speed, the result is a surface that is unacceptable, with a rough ride and the tendency to hold water.
Head of Material: When I am speaking of head of material, I am talking about the amount of material that is allowed to gather in front of the screed. What is the right amount? Well, it varies, but not by much. The contractors that I work with most typically say around 2/3 of the way up the front of the screed produces an acceptable finished product. If you “starve” your screed, the screed will tend to dive and the opposite is certainly true as well. This is the most challenging when you are operating a tilt hopper design paver, where you are constantly running out of material. The trick is with this type of paver is to stop the process before the screed starts running out of material. Today’s pavers are sometimes equipped with automatic conveyors and augers which play a huge part in governing the head of material.
Angle of Attack: Here is the big one. Have you ever seen our friend “Windmill Johnny” on the paving crew? He’s the guy who can’t take his hands off of the depth screw. He turns it one way, looks down the paver and turns it back the other way again and again. What is he accomplishing? Absolutely nothing except putting ripples in the mat. Here’s the rule: No matter whose paver you are using, it takes 5 tow arm lengths to realize a change in depth. 65% of the change occurs in the first tow arm length and the rest over the next 4. I always look at Mr. Johnny and wonder “What does he see in the area to be paved that makes him think that he needs more depth?” My rule of thumb is this – If there is no change in the base, speed of the paver or head of material, there is no reason to adjust the depth screw. Let the paver do its job.
Now, this is a condensed version of how a screed works, but I hope you get the picture. It’s all geometry and physics and has been the same since the first screed hit the market, just brighter logos and shinier paint. But it’s certainly not rocket science!
Brian Hall is a territory manager at ST Engineering LeeBoy, Inc. You can contact him at email@example.com