Post on 10-Apr-2018
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CONCRETELEARNING OBJECTIVE: Upon completingthis section, you should be able to describethe
finishing process for the final concretesurface.The finishing process provides the final concretesurface.
There are many ways to finish concretesurfaces, depending on the effect required.Sometimes you
only need to correct surface defects,fill bolt holes, or clean the surface. Unformedsurfaces may require
only screeding to proper contourand elevation, or a broomed, floated, or trowelledfinish may be
specified.SCREEDINGThe top surface of a floor slab, sidewalk, orpavement is rarely placed at the exact
specifiedelevation.Screeding brings the surface to therequired elevation by striking off the excess
concrete.Two types of screeds are used in concrete finishingoperations:the hand screed and the
mechanicalscreed.Hand ScreedHand screeding requires a tool called a screed.This is actually a templet
(usually a 2-by-4) having astraight lower edge to produce a flat surface (or acurved lower edge to
produce a curved surface).Move the screed back and forth across the concreteusing a sawing motion, as
shown in figure 7-44. Witheach sawing motion, move the screed forward an inchor so along the forms.
This forces the concrete builtup against the screed face into the low spots. If thescreed tends to tear the
surface, as it may onair-entrained concrete due to its sticky nature, eitherreduce the rate of forward
movement or cover thelower edge of the screed with metal. This stops thetearing action in most cases.You
can hand-screed surfaces up to 30-feet wide,but the efficiency of this method diminishes
onsurfaces more than 10-feet wide. Three workers(excluding a vibrator operator) can
screedapproximately 200 square feet of concrete per hour.Two of the workers work the screed while the
thirdpulls excess concrete from the front of the screed,You must screed the surface a second time to
removethe surge of excess concrete caused by the firstscreeding.Figure 7-44.-Screeding operation.7-24
Mechanical ScreedThe mechanical screed is being used more andmore in construction for striking off
concrete slabs onhighways, bridge decks, and deck slabs. This screedincorporates the use of vibration and
permits the useof stronger, and more economical, low-slumpconcrete. It can strike off this relatively
dry materialsmoothly and quickly.The advantages of using avibrating screed are greater density and
strongerconcrete. Vibrating screeds give abetter finish, reducemaintenance, and save considerable time
due to thespeed at which they operate. Vibrating screeds arealso much less fatiguing to operate than hand
screeds.A mechanical screed (figure 7-45) usuallyconsists of a beam (or beams) and a gasoline engine,or an
electric motor and a vibrating mechanismmo unted in the center of the beam. Most mechanicalscreeds are
quite heavy and usually equipped withwheels to help move them around. You mayoccasionally encounter
lightweight screeds notequipped with wheels. These are easily lifted by twocrewmembers and set back
for the second pass ifrequired.The speed at which the screed is pulled is directlyrelated to the slump of
the concretethe less theslump, the slower the speed; the more the slump, thefaster the speed. On
the finishing pass of the screed,there should be no transverse (crosswise) movementof the beam; the screed
is merely drawn directlyforward riding on the forms or rails. For a mechanical screed, a method is provided
to quickly start or stopthe vibration. This is important to prevent overvibration when the screed might be
standing still.Concrete is usually placed 15 to 20 feet ahead ofthe screed and shoveled as close as possible to
its finalresting place. The screed is then put into operationand pulled along by two crewmembers, one at
eachend of the screed. It is important that sufficientconcrete is kept in front of the screed. Should
theconcrete be below the level of the screed beam, voidsor bare spots will appear on the concrete surface
asthe screed passes over the slab. Should this occur, ashovel ful or so of concrete is thrown on the bare
spot,and the screed is lifted up and earned back past thisspot for a second pass. In rare cases, the screed
crewwill work out the void or bare spot with ahand-operated bull float, rather than make a
secondpass with the screed.The vibration speed will need to be adjusted forparticular mixes and different
beam lengths.Generally, the stiffer the mix and the longer the beam, the greater the vibration speed required.
The speed atwhich the screed is moved also affects the resultingfinish of the slab. After a few minutes of
operation, asatisfactory vibration pulling speed can beestablished. After the vibrating screed has
passedover the slab, the surface is then ready for broom orburlap finishing.W here possible, it is advisable
to lay out orengineer the concrete slab specifically for use of avibrating screed. Forms should be laid out
in lanes ofequal widths, so that the same- length screed can beused on all lanes or slabs. It should also be
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planned, ifpossible, that any vertical columns will be next to theforms, so that the screed can easily be
lifted ormaneuvered around the column.There are four important advantages of using avibrating finishing
screed. First, it allows the use of low-slump concrete, resulting in stronger slabs.Second, it reduces and
sometimes eliminates thenecessity of hand tamping and bull floating. Third, itincreases the density of the
concrete, resulting in asuperior wearing surface. And fourth, in the case offloor slabs, troweling can begin
sooner since driermixes can be used, which set up more quickly.7-25