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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