Placing and Finishing Concrete

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Placing and Finishing Concrete
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Basic Requirements for Placing Concrete (1)

• Preserve concrete quality
• Water-cement ratio
• Slump
• Air-content
• Homogeneity
• Avoid separation of aggregate and mortar

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Basic Requirements for Placing Concrete (2)

• Avoid excessive horizontal movement
• Consolidate adequately
• Maintain sufficient placement capacity
• Choose the right equipment for the concrete

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Preparation Before Placing Includes

• Trimming the subgrade

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Preparation Before Placing Includes

• Moistening the subgrade

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Preparation Before Placing Includes

• Compacting the subgrade

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Depositing Concrete

• DO NOT
• (a) disturb saturated subgrades so bearing
  capacity is maintained
• (b) deposit on frozen subgrade
• Deposit continuously and as near as possible to
  its final position
• Rate of placement should be such that previously
  placed concrete has not set when the next layer
  is placed upon it

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Depositing Concrete
Slab Construction

• Start placing along perimeter at one end with
  each batch discharged against previously placed
  concrete
• Do not
• (a) dump in separate piles then level and
  work together
• (b) deposit in large piles then move
  horizontally into position
• These practices result in segregation
• (mortar flows ahead of coarser material)

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Depositing Concrete
Effective use of wheelbarrows
Discharging into previously placed concrete
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Incorrect Placement with Conveyor Belt
Baffle
Mortar
Mortar
Rock
Shallow hopper
Rock
Mortar
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Placement with Conveyor Belt
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Depositing Concrete

• Pavement Slab
• Concrete deposited in front of slip form paver by
  dump trucks
• Concrete spread evenly across the subgrade by the
  paver before consolidation and finishing

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Depositing Concrete
Curb/Curb and Gutter

• Concrete deposited into hopper of slip form curb
  and gutter machine which then extrudes the
  concrete into the desired shape

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Depositing Concrete
Walls

• Deposit in horizontal layers of uniform thickness
• Reinforced 150 mm to 500 mm
• Mass 375 mm to 500 mm
• Consolidate each layer before next is placed
• Timely placement consolidation prevents flow
  lines and cold joints

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Horizontal Construction Joint
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Horizontal Construction Joints
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Underwater Placement Methods

• Tremie
• Pump
• Bottom dump buckets
• Grouted preplaced aggregate (specialized)
• Toggle bags
• Bagwork
• Diving bell

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Placing Concrete Under Water
Basic Recommendations

• Water velocity ? 3 m (10 ft) / min.
• Water temperature ? 5C(if below test for
  strength gain)
• w/c ? 0.45
• Cementing materials content ? 390 kg/m3 (600
  lb/yd3)
• Slump range 150 to 225 mm (6 to 10 in.)

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Placing Concrete Underwater

• Used Tremie
• Advantages Can be used to funnel concrete down
  through the water into the structure.
• Watch for Discharge end always has to be buried
  in fresh concrete to ensure seal between water
  and concrete mass.

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Consolidating Concrete

• Internal Vibration
• External Vibration

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Internal Vibration
Radius of Action
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Internal Vibrators
Diameter of head, mm (in.) Recommended frequency, vibrations per minute Approximate radius of action, mm (in.) Rate of placement,m3/h (yd3/h) Application
20-40 (3/4-1½) 9000-15,000 80-150 (3-6) 0.8-4 (1-5) Plastic and flowing concrete in thin members. Also used for lab test specimens.
30-60 (1¼-2½) 8500-12,500 130-250 (5-10) 2.3-8 (3-10) Plastic concrete in thin walls, columns, beams, precast piles, thin slabs, and along construction joints.
50-90 (2-3½) 8000-12,000 180-360 (7-14) 4.6-15 (6-20) Stiff plastic concrete (less than 80-mm 3-in. slump) in general construction .
Adapted from ACI 309
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Systematic Vibration of Each New Lift
CORRECT Vertical penetration a few inches into
previous lift (which should not yet be rigid) of
systematic regular intervals will give adequate
consolidation
INCORRECT Haphazard random penetration of the
vibrator at all angles and spacings without
sufficient depth will not assure intimate
combination of the two layers
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Placing Concrete in a Sloping Lift
CORRECT Start placing at bottom of slope so that
compaction is increased by weight of newly added
concrete. Vibration consolidates the concrete.
INCORRECT When placing is begun at top of slope
the upper concrete tends to pull apart especially
when vibrated below as this starts flow and
removes from concrete above.
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External Vibration

• Form vibrators
• Vibrating tables
• Surface vibrators
• Vibratory screeds
• Plate vibrators
• Vibratory roller screeds
• Vibratory hand floats or trowels

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Consolidating Concrete

• Inadequate consolidation can result in
• Honeycomb
• Excessive amount of entrapped air voids
  (bugholes)
• Sand streaks
• Cold joints
• Placement lines
• Subsidence cracking

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Nuclear Gauges to Determine Subbase Compaction
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Screeding (Strikeoff)

• The process of cutting off excess concrete to
  bring the top surface of a slab to proper grade

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Vibratory Screeds
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Bullfloating
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Darbying
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Edging
Edging densifies and compacts concrete next to
forms where floating is less effective

• Required along all edge forms,isolation and
  construction jointsin floors and exterior slabs
• Cut concrete away from formsto a depth of 25 mm
  with apointed mason or margin trowel
• Edging may be required aftereach subsequent
  finishing operation for interior slabs

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Highway Straightedges
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Floating (Power or Hand)

• To embed aggregate particles just beneath the
  surface
• To remove slight imperfections, humps, and voids
• To compact the mortar at the surface in
  preparation for additional finishing operations.

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Troweling

• Creates smooth, hard,dense surface
• Exterior concrete should not be troweled because
  
• it can lead to a loss of entrained air caused by
  overworking the surface
• troweled surfaces can be slippery when wet.

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Brooming
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Tining
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Curing and Protection

• Cure for 7 days

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Placing on Hardened Concrete
Preparing Hardened Concrete
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Bonding New to Previously Hardened Concrete

• Cement-sand grout
• Latex
• Epoxy

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Isolation Joints
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Contraction Joints
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Spacing of Contraction Joints in Meters
Slab thickness, mm Maximum-size aggregate less than 19 mm Maximum-size aggregate 19 mm and larger
100 2.4 3.0
125 3.0 3.75
150 3.75 4.5
175 4.25 5.25
200 5.0 6.0
225 5.5 6.75
250 6.0 7.5
Metric
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Spacing of Contraction Joints in Feet
Slab thickness, in. Maximum-size aggregate less than ¾ in. Maximum-size aggregate ¾ in. and larger
4 8 10
5 10 13
6 12 15
7 14 18
8 16 20
9 18 23
10 20 25
Inch-Pound
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Making Contraction Joints
Grooving tool on bull-float
Dry-cut sawing concrete
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Construction Joints
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Joint Layout for Slabs
Basic Factors to Remember

• Panels created by contraction joints should be
  approximately square
• Panel aspect ratio max. 1½ to 1
• Contraction (control) joints should only
  terminate at a free edge or at an isolation joint
• When joint spacing exceeds 4.5 m (15 ft), load
  transfer by aggregate interlock decreases
  significantly

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Typical Joint Layout
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Finishing Operations
Single Course Floors

• Consolidation
• Strike-off
• Edging
• Darbying or Bull Floating
• Lapse of Time
• Edging
• Grooving (if desired)
• Floating (power or hand)
• Troweling (power or hand)
• Lapse of Time
• Second Troweling (power or hand)
• Final Troweling (hand)
• Curing

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Finishing Operations - Exterior Slabs
Sidewalks, Driveways etc.

• Consolidation
• Strike-off
• Depress aggregate with metal/wood strip at joint
  location if hand tooled
• Darbying or Bull floating
• Lapse of time
• Edging
• Jointing (optional if hand tooled)
• Floating
• Lapse of time
• Texturing (brooming/swirl float finish)
• Curing

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Patching
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Curing Patches
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Cleaning Concrete Surfaces

• Cleaning methods
• Water
• Chemical
• Mechanical

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Finishing Formed Surfaces

• Rough-form finishes
• Smooth off-the-form finish
• Smooth, rubbed finish
• Sand-floated finish
• Grout cleandown
• (sack-rubbed finish)

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Special Surface Finishes

• Pattern and Textures
• Exposed Aggregate Concrete
• Colored Finishes
• Stains, Paints and Clear Coatings

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Working Safely with Concrete

• Protect
• Your Eyes
• Your Back
• Your Skin

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Precautions
WARNING Contact with wet (unhardened) concrete,
mortar, cement, or cement mixtures can cause SKIN
IRRITATION, SEVERE CHEMICAL BURNS (THIRD-DEGREE),
or SERIOUS EYE DAMAGE. Frequent exposure may be
associated with irritant and/or allergic contact
dermatitis. Wear water-proof gloves, a
long-sleeved shirt, full-length trousers, and
proper eye protection when working with these
materials. If you have to stand in wet concrete,
use waterproof boots that are high enough to keep
concrete from flowing into them. Wash wet
concrete, mortar, cement, or cement mixtures from
your skin immediately. Flush eyes with clean
water immediately after contact. In-direct
contact through clothing can be as serious as
direct contact, so promptly rinse out wet
concrete, mortar, cement, or cement mixtures from
clothing. Seek immediate medical attention if you
have persistent or severe discomfort.
http//www.cement.org/pdf_files/ms271.pdf
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Videos 1/4
Placing and Consolidating
Vibration
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Videos 2/4
Finishing I
Finishing II
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Videos 3/4
Contraction Joints
Jointing
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Videos 4/4
Improper Finishing