ASPHALT ANSWERS
Daily screed flatness check is a must for a quality job
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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 bhall @ leeboy . com www . callape . com [ 28 ] 1.800.210.5923