The mechanical screed is being used more and
more in construction for striking off concrete slabs on
highways, bridge decks, and deck slabs. This screed
incorporates the use of vibration and permits the use
of stronger, and more economical, low-slump
concrete. It can strike off this relatively dry material
smoothly and quickly.
The advantages of using a
vibrating screed are greater density and stronger
concrete. Vibrating screeds give abetter finish, reduce
maintenance, and save considerable time due to the
speed at which they operate. Vibrating screeds are
also much less fatiguing to operate than hand screeds.
A mechanical screed (figure 7-45) usually
consists of a beam (or beams) and a gasoline engine,
or an electric motor and a vibrating mechanism
mounted in the center of the beam. Most mechanical
screeds are quite heavy and usually equipped with
wheels to help move them around. You may
occasionally encounter lightweight screeds not
equipped with wheels. These are easily lifted by two
crewmembers and set back for the second pass if
The speed at which the screed is pulled is directly
related to the slump of the concretethe less the
slump, the slower the speed; the more the slump, the
faster the speed. On the finishing pass of the screed,
there should be no transverse (crosswise) movement
of the beam; the screed is merely drawn directly
forward riding on the forms or rails. For a mechanical
screed, a method is provided to quickly start or stop
the vibration. This is important to prevent over
vibration when the screed might be standing still.
Concrete is usually placed 15 to 20 feet ahead of
the screed and shoveled as close as possible to its final
resting place. The screed is then put into operation
and pulled along by two crewmembers, one at each
end of the screed. It is important that sufficient
concrete is kept in front of the screed. Should the
concrete be below the level of the screed beam, voids
or bare spots will appear on the concrete surface as
the screed passes over the slab. Should this occur, a
shovelful or so of concrete is thrown on the bare spot,
and the screed is lifted up and earned back past this
spot for a second pass. In rare cases, the screed crew
will work out the void or bare spot with a
hand-operated bull float, rather than make a second
pass with the screed.
Figure 7-45.-Mechanical screed.
The vibration speed will need to be adjusted for
particular mixes and different beam lengths.
Generally, the stiffer the mix and the longer the beam,
the greater the vibration speed required. The speed at
which the screed is moved also affects the resulting
finish of the slab. After a few minutes of operation, a
satisfactory vibration pulling speed can be
established. After the vibrating screed has passed
over the slab, the surface is then ready for broom or
Where possible, it is advisable to lay out or
engineer the concrete slab specifically for use of a
vibrating screed. Forms should be laid out in lanes of
equal widths, so that the same- length screed can be
used on all lanes or slabs. It should also be planned, if
possible, that any vertical columns will be next to the
forms, so that the screed can easily be lifted or
maneuvered around the column.
There are four important advantages of using a
vibrating finishing screed. First, it allows the use of
low-slump concrete, resulting in stronger slabs.
Second, it reduces and sometimes eliminates the
necessity of hand tamping and bull floating. Third, it
increases the density of the concrete, resulting in a
superior wearing surface. And fourth, in the case of
floor slabs, troweling can begin sooner since drier
mixes can be used, which set up more quickly.