Chapter 7 – Procedures for finishing slabs-on

Procedures for finishing
Chapter 7
Chapter Topics
Striking off (screeding) the concrete
Bull floating or darbying
Highway-type straightedge
Waiting period
Edging and jointing
Chapter Topics
• Machine float and trowel direction and
• Floating-to-troweling sequence
• Finishing when setting time varies from load to
• Floating edges
• Walk-behind basics
• Hand floating and troweling
• Blade tilt
Overview of Finishing Steps
Finishing to Obtain F-numbers
Striking Off (Screeding)
• Screeding is the act of striking off the surface of
the concrete to a predetermined grade.
• Immediately done after spreading and
• Types of screeds
– Hand held
– Vibrating
– Laser-guided
Rigid vs. Wet Screeds
• Support for the screeding process
• Rigid screeds
– Temporary pipe or rail, edge forms, bulkheads, adjacent
hardened concrete
• Wet screed
– A ribbon or pad of fresh concrete used as guides in
screeding the rest of the fresh concrete
• Use wet screeds for FF20/FL 20 or lower and use rigid
screeds for higher F-number
Bull Floating or Darbying
• Smooths the surface removing high and low spots.
• Done immediately after screeding before bleed
water appears.
• Bullfloat used on large areas.
• Darby used for congested areas.
• Use only one and not both (bull float or darby).
Highway Straightedging
• Use to improve flatness and levelness.
• Use immediately after screeding before bleed
water appears.
• Use in cutting and filing operation.
• Use multiple times in different directions.
Waiting Period
• Further finishing can’t begin until bleeding has
stopped and concrete is firm enough to start
– No visible bleed water on surface
– Footprint indentation of ¼ inch or less
• Finishing can’t be done with bleed water; so
remove it if necessary.
– Drag heavy rubber hose across surface
– Use squeegee to push water off surface
• Commonly done on sidewalks, driveways and steps.
• Leaves a neater, slightly rounded edge that is less likely
to chip under traffic or when forms are removed.
• Before using edges, good practice to use a pointed
trowel (vertical sawing motion) to loosen the concrete
from the side form.
• Place edger flat and then tilt up slightly in the direction
the tool is moved.
• Jointer or groovers are used to make contraction joints in
exterior flatwork; driveways and sidewalks.
• Mark the location of the joint with a string line or chalk line
on opposite side forms or on the concrete surface.
• To start the joint, push the groover into the concrete and
move it forward while applying pressure to the back of the
• After the joint is cut, turn the groover around and pull it
back over the grooved joint to produce a smooth finish.
• Most common final treatment for exterior flatwork; provides slip
resistant surface.
• The rough texture is made in the surface by running the broom
crosswise (perpendicular) to the direction of travel.
• Several textures can be produced by using various types of brooms
and different amounts of pressure while the concrete is stiffening.
• Rough brooming typically follows the bull floating pass.
• If a fine broom finish is required, the concrete is often floated and
lightly troweled.
• Gently place the broom on the opposite edge of the surface and
pull the broom toward you.
• When the broom is close to the near edge, lift it up and return it to
the far edge.
• Let the next pass slightly overlap the previous pass.
• Always pull the broom, never push it. This allows more of the
broom face to contact the concrete surface and gives better control.
• Pushing the broom may cause surface paste to build up on the
broom that can damage the surface during the push.
• The broom should be dampened but not dripping with water.
• After the waiting period, floating is done to:
– Embed the large aggregate beneath the surface
mortar layer;
– Remove slight humps or other imperfections and
produce a plane, level surface; and
– Compact and further consolidate the surface in
preparation for other finishing.
• Float when bleed water sheen is gone and
footprint indentation of ¼ inch or less.
• Immediately after floating.
• No troweling should ever be done unless the surface has first
been floated.
• Troweling produces a hard, smooth surface that will be easy
to clean and maintain.
• For first troweling, a blade at least 4-3/4 in. wide is preferred,
and it should be kept as flat against the surface as possible.
• For additional trowelings, smaller trowels are used, and these
are tilted slightly up in the direction of travel to increase the
pressure on the trowel.
Machine Direction and Patterns
• Use power float and trowels in controlled patterns.
• Start the first float pass perpendicular to the direction of last
finishing procedure. Typically that last procedure is bull floating, so
the first float pass is perpendicular to that.
• Subsequent float passes are perpendicular to previous float passes.
Each pass in the alternating direction refines the flatness of the floor
until the concrete stiffens to a point where it can no longer be
• Floating or troweling in only one direction creates a wave, and
repeated floating or troweling in only one direction accentuates the
• These waves lower the floor flatness and levelness and are difficult
to remove during restraightening if that procedure is used.
Machine Direction and Patterns
• For double-riders, the weight of the machine is centered over
each rotor that is attached to the blades.
• This concentrates the weight at the center of the blades, with
less weight (and pressure) at the outer edge of the blade. This
creates a trough at the center of each rotor.
• Overlapping each pass by ½ minimizes the depth of the
trough left by the previous pass.
• After the first pass, center the rotor of the machine over the
mortar ridge.
• Each pass erases the mortar ridge left by the previous pass.
Floating Edges
• If done properly, machine floating the slab edges at
least once before the hand finisher begins edge work
can save a lot of hand labor.
• If the edges are low and more concrete must be
brought to them, use a ride-on or walk-behind
machine on a forward pass along the edge, with the
float blades or pan on or over the edge form.
• If the edges don’t need more concrete, or if you aren’t
sure, make a backward pass with the ride-on or walkbehind machine.
Walk-Behind Basics
• Use the handle to guide the machine.
• To move left, apply a slight upward pressure on the handle, to move
right, press down slightly.
• To move forward, twist the handle clockwise.
• To move backward, twist the handle counterclockwise.
• To rotate in one spot, hold the handle in a neutral position.
• Do not stay in one spot for too long. Move the machine across the
slab in a sweeping motion.
• Wipe out footprints during each pass.
Blade Tilt
• For both floating and troweling operations, the angle of the blade is
important .
• For floating, the blade must be absolutely flat to prevent tearing or
gouging of the surface.
• On each successive troweling pass, the angle of the trowel blade is
slightly increased to exert greater pressure on the concrete surface.
• When a trowel blade is tilted or pitched at too great an angle, an
objectionable “washboard” or “chatter” surface will result. If this
occurs, reduce the tilt and continue troweling until the surface is
smooth and level, after which the tilt may again be slightly increased.
• The tilt is excessive if the leading edge of the blade is more than 1 in.
above the concrete surface.

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