Check the Angle of Attack
BY John Ball
The screed is supposed to float along, smoothing out the mix in an even, beautiful black lane. When the screed tilts sharply to the front or back, it will affect material placement and experience wear and tear on the bottom of the screed plates. These are not good things to have happening if your goal is a top quality pavement.
First: You want the screed plate to start out level, flat, and almost parallel with the ground. There will be about an eighth of an inch of angle at the front of the screed to accommodate the mix.
You will begin with the tow point cylinders centered—at zero—and then adjust them up or down to get the correct angle of attack for the day’s paving. Once they are set, you shouldn’t have to change them. If making adjustments later in the paving day, do so carefully.
When you adjust the tow point cylinders up, the tow arms will lift slightly. This will cause the front of the screed (the nose) to rise. This will cause down-pressure on the back end of the screed (the trailing edge). Think of a water ski. When the front goes up, the back goes down into the water. If you adjust the tow point too quickly, making an erratic change, this causes the trailing edge to lower into the mat. It will drag and tear until the screed levels out.
When you adjust the tow point cylinders down, the tow arms will lower slightly. This will cause the front of the screed (the nose) to lower. This will cause the back end of the screed (the trailing edge) to rise or lift. You’re not screeding off the material now. There’s no pressure on the material, yet you want that compression. Having no pressure on the mat with the back third or two-thirds of the screed plates, the nose of the screed will dig in, and you won’t get uniform material coming off the back end of the screed.
These phenomena also cause uneven wearing on the front or back sections of the screed plate.
To protect the equipment and the mat, you want to double-check the angle of attack before you take off. You want to set the tow point cylinders so that the screed is positioned properly for the mat you’re going to lay. Also make sure the depth crank is leveled off.
Next: Get on your hands and knees and use a level, a straight edge, and a tape measure to double-check the screed plates are level, and to determine which section needs adjusting. If the nose is low, figure out why. Fix it. Clean it so you can tell if there’s been damage to the metal already. If the trailing edge is what’s low, again, figure out why and get it adjusted before you try to place the mat.
John Ball is the proprietor of Top Quality Paving and Training, Manchester, New Hampshire. He provides personal, on-site paving consulting services around the United States and into Canada. For more information, contact him at (603) 493-1458 or firstname.lastname@example.org.