Free floating screed
Encyclopedia
The free floating screed is a device pioneered by Barber Greene Company in the 1930s that revolutionized the asphalt
paving
process.
The device is designed to flatten the material (e.g. concrete
or asphalt
) below it, which is also known as screed.
, allow the depth behind the screed to be constant.
The angle
at which the tow arm pull is exerted on the screed also contributes to the motion
; 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
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
. 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.
Asphalt
Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...
paving
Pavement (material)
Road surface or pavement 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 In the past cobblestones and granite setts were extensively used, but these surfaces have mostly been replaced by asphalt or concrete. Such...
process.
The device is designed to flatten the material (e.g. concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
or asphalt
Asphalt
Asphalt or , also known as bitumen, is a sticky, black and highly viscous liquid or semi-solid that is present in most crude petroleums and in some natural deposits, it is a substance classed as a pitch...
) below it, which is also known as screed.
Description
The free floating screed has a number of forces acting on it that, when in equilibriumMechanical equilibrium
A standard definition of static equilibrium is:This is a strict definition, and often the term "static equilibrium" is used in a more relaxed manner interchangeably with "mechanical equilibrium", as defined next....
, allow the depth behind the screed to be constant.
- Tow arm pull: the force exerted on the screed by the paver dragging it
- MassMassMass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
: 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 viscosityViscosityViscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
and mass.
The angle
Angle
In geometry, an angle is the figure formed by two rays sharing a common endpoint, called the vertex of the angle.Angles are usually presumed to be in a Euclidean plane with the circle taken for standard with regard to direction. In fact, an angle is frequently viewed as a measure of an circular arc...
at which the tow arm pull is exerted on the screed also contributes to the motion
Motion (physics)
In physics, motion is a change in position of an object with respect to time. Change in action is the result of an unbalanced force. Motion is typically described in terms of velocity, acceleration, displacement and time . An object's velocity cannot change unless it is acted upon by a force, as...
; 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
Horizontal plane
In geometry, physics, astronomy, geography, and related sciences, a plane is said to be horizontal at a given point if it is perpendicular to the gradient of the gravity field at that point— in other words, if apparent gravity makes a plumb bob hang perpendicular to the plane at that point.In...
(angle of attack) will control the amount of material extruded
Extrusion
Extrusion is a process used to create objects of a fixed cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section...
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
Base course
Base Course in pavements refers to the sub-layer material of an asphalt roadway and is placed directly on top of the undisturbed soil so as to provide a foundation to support the top layer of the pavement...
. 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.