The way the screed is set up and adjusted has a major impact on a range of mat quality issues. Primarily, screed settings affect mat finish and texture. Regardless of the mix type, the paving width or the paving depth, the screed should be set to produce a uniform mat with a tight finish. The screed should be examined and set up at the start of each paving shift. As needed during the paving operation, adjustments may be required.
Nose job
The angle of attack for the main screed and screed extensions, if so equipped, is determined when the screed is set on the starting reference. When the screed is properly set up, the nose of the screed runs with a slight "nose up" attitude. The correct angle of attack is 3 mm (1?8 in.) to 6 mm (1?4 in.).
If the main screed or a screed extension is running with a "nose down" angle of attack, the appearance of the mat will be an open texture behind the screed plate with low angle of attack. These types of mat defects can be eliminated only by correcting the angle of attack.
Many screeds have adjusters on the trailing edge of the main screed and the screed extensions. To decrease the angle of attack, loosen the jam nut and turn the adjuster one revolution counterclockwise. Before making any adjustments, mark the adjuster in a way that lets you see exactly when you have made one revolution. Then, lock the jam nut.
Adjust each of the adjusters across the trailing edge the same amount. This will slightly raise the trailing edge and force the nose of the screed plate down a little. Repeat these steps until the mat is uniform with the correct tight, but not shiny, texture across the full paving width. To increase the angle of attack, follow the same procedure but turn the adjusters clockwise to pick up the nose of the screed and force the trailing edge down.
Heightened awareness
Another screed adjustment involves the height of the screed extensions. When the main screed and the screed extensions are in the same plane, the mat is uniform across the full width. When an extension is too high or too low in relation to the main screed, a longitudinal mark appears in the mat.
To correct this mat defect, raise the screed extension using the extension height switch on the appropriate screed control box. The extension will react immediately to height corrections, so make small corrections until the separation mark completely disappears.
When an extension is too high in relation to the main screed, a mark appears that is in line with the outer edge of the main screed. In this condition, there is a step up and the mat is thicker behind the screed extension. Lower the screed extension until the mark disappears.
The height of the extensions should be set when the screed is set down on the starting reference. For screeds with rear-mounted extensions, the extension height should be about 6 mm (1?4 in.) above the main screed. For screeds with front-mounted extensions, the extension height should be 6 mm (1?4 in.) below the main screed.
Check extension height every time the screed is set down on a starting reference. Screed personnel should adjust extension height, if necessary, within a short distance of the starting point. Once the height is set correctly, no further adjustments should be needed.
Crown moments
For many mix designs, there is no need for the screed to have a lead crown. In other words, the screed is flat across the leading edge as well as the trailing edge. Mix will pass under the screed in a uniform manner and the texture of the mat is tight and uniform, if all other screed adjustments are correct.
Some large stone mixes, however, may require putting in a lead crown. If the mat shows an open texture stripe in the center, this is an indication that a lead crown is needed. A thermal image will probably show that the open texture stripe is cooler, too.
To correct this mat defect, adjust the main screed so the leading edge has a positive 3-mm (1?8-in.) lead crown.
On the strike-off
On screeds with rear-mounted extensions the position of the strike-off that is located directly in front of the main screed plate affects the angle of attack and consequently the texture of the mat.
The strike-off is adjustable and, for most mix designs, a setting that is 25 mm (1 in.) above the screed plate works well. Check the height of the screed at the beginning of each paving shift and make adjustments depending on the texture of the mat being placed.
For mixes with large aggregates, the 25-mm (1-in.) setting may be too high. You would expect to see the entire mat exhibiting an open texture with the strike-off up 25 mm (1 in.). Large stone mixes tend to create more lift under the screed and the screed rides up. In an effort to compensate for the added lift, screed personnel usually decrease the angle of attack. In other words, they force the screed to ride on its nose. The open texture is created because only a portion of the screed plate is finishing the mat surface.
To overcome the added lift, the correct procedure is to lower the strike-off. With less of the nose bar exposed, lift is reduced and the screed should run at the correct angle of attack and maintain mat thickness and texture.
If the strike-off is set too low for the angle of attack, the screed runs on the trailing edge. You may see a tight, shiny surface, and the screed may act erratically.
The correct procedure is to raise the strike-off to allow more mix to pass under the nose bar. With added lift, the screed will run at the proper angle of attack and the mat surface will improve.
Turning cold
If paving begins before the screed or a portion of the screed has been heated to a temperature close to the temperature of the mix, mix will stick to the bottom of the screed plate.
The cold screed will push material in front of it instead of sliding over the mat surface. The screed will drop and will produce a scuffed texture. The screed plate will warm up as it is exposed to the hot mix, but the mat will be substandard while the screed is cold. The proper procedure is to have the screed heated sufficiently before beginning to pave.
