379th Expeditionary Civil Engineer Squadron Placing Asphalt

The free floating screed is a device pioneered in the 1930s that revolutionized the

asphalt Asphalt most often refers to: * Bitumen, also known as "liquid asphalt cement" or simply "asphalt", a viscous form of petroleum mainly used as a binder in asphalt concrete * Asphalt concrete, a mixture of bitumen with coarse and fine aggregates, u ...

paving process. The device is designed to spread and smooth out, or

screed Screed has three meanings in building construction: # A flat board (screed board, floating screed) or a purpose-made aluminium tool used to smooth and to "Wikt:true#Verb, true" materials like concrete, stucco and plaster after they have been p ...

, the material (e.g.

concrete Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...

) below it.

Description

The screed connects to the tractor portion of the paving machine via its tow arm. Paving material is transferred from the hopper at the front of the tractor to the screed, and augers spread it across the width of the screed. The material then flows out across the width of the screed at the desired depth. A screed operator stands on a platform at the center of the screed and controls the placement of the paving mixture. Adjusting screed settings will change the placement depth and width, as well as amount of material being placed. Many modern screeds can be run in an automatic mode, too. Because the only connection between the asphalt paver and the screed is the tow arm, the screed can "float" vertically relative to the paver. This allows the paver to traverse uneven ground while the screed floats over the material placed in front of it. The free floating screed has become standard because of the smoothing or averaging effect it can have on the existing

base course The base course or basecourse in pavements is a layer of material in an asphalt roadway, race track, riding arena, or sporting field. It is located under the surface layer consisting of the '' wearing course'' and sometimes an extra ''binder cour ...

. The free floating screed has a number of

forces In physics, a force is an influence that can cause an object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the magnitude and directi ...

acting on it that, when in

equilibrium Equilibrium may refer to: Film and television * ''Equilibrium'' (film), a 2002 science fiction film * '' The Story of Three Loves'', also known as ''Equilibrium'', a 1953 romantic anthology film * "Equilibrium" (''seaQuest 2032'') * ''Equilibr ...

, allow the depth behind the screed to be constant. * Tow arm pull: the force exerted on the screed by the paver dragging it *

Mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

: the weight of the screed * Resistance of the head of material: the opposing force exerted on the screed by the pile of material in front of the screed. This force depends in turn on the material's

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

and mass. The

angle In Euclidean geometry, an angle can refer to a number of concepts relating to the intersection of two straight Line (geometry), lines at a Point (geometry), point. Formally, an angle is a figure lying in a Euclidean plane, plane formed by two R ...

at which the tow arm pull is exerted on the screed also contributes to the

motion In physics, motion is when an object changes its position with respect to a reference point in a given time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed, and frame of reference to an o ...

; its resultant force is either added or subtracted from the mass of the screed. If each of these forces is constant, altering the angle of the screed to the horizontal (angle of attack) will control the amount of material

extruded Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex ...

behind the screed. Increasing the angle of attack will cause the screed to climb higher through the pile of material, and therefore raise its trailing edge, increasing the amount of material extruded behind the screed. Similarly, reducing the angle of attack will reduce the amount of material extruded.

Patent

The free floating screed was first developed by Harry H. Barber, one of the founders of the Barber-Greene Co., in 1933 for which he received patent number three years later. Barber Green dominated the market for free floating screed equipment until the patent expired in 1955, and now all major asphalt paver manufacturers use this design principle in their equipment.

References

External links

Pavement Interactive: Paving Equipment
{{DEFAULTSORT:Free Floating Screed Pavements Building engineering

Description

Patent

See also

References

External links