The free floating screed is a device pioneered in the 1930s that revolutionized the
asphalt
Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term ...
paving process.
The device is designed to flatten the material (e.g.
concrete
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wid ...
or
asphalt
Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term ...
) below it, which is also known as
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 "true" materials like concrete, stucco and plaster after they have been placed on a surface ...
.
Description
The free floating
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 "true" materials like concrete, stucco and plaster after they have been placed on a surface ...
has a number of
forces acting on it that, when in
equilibrium, 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 property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different ele ...
: 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
The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water.
Viscosity quantifies the int ...
and mass.
The
angle
In Euclidean geometry, an angle is the figure formed by two rays, called the '' sides'' of the angle, sharing a common endpoint, called the ''vertex'' of the angle.
Angles formed by two rays lie in the plane that contains the rays. Angles ...
at which the tow arm pull is exerted on the screed also contributes to the
motion
In physics, motion is the phenomenon in which an object changes its position with respect to time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed and frame of reference to an observer and m ...
; 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 c ...
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.
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 ...
. Since 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.
Patent
The company
Barber Greene
Barber-Greene Company was a company founded in 1916 by American mechanical engineers Harry H. Barber and William B. Greene. It was formed to sell standardized material-handling machines to mechanize small manual tasks in an economical way. Thoug ...
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.
See also
*
Road surface
A road surface (British English), or pavement (American English), is the durable surface material laid down on an area intended to sustain vehicular or foot traffic, such as a road or walkway. In the past, gravel road surfaces, hoggin, cob ...
References
External links
History of Asphalt, National Asphalt Pavement Association
{{DEFAULTSORT:Free Floating Screed
Pavements
Building engineering