Placing and Finishing Concrete
04/13/20231
Placing and Finishing Concrete
Basic Requirements for Placing Concrete (1)
n Preserve concrete qualityu Water-cement ratiou Slumpu Air-contentu Homogeneity
n Avoid separation of aggregate and mortar
Placing and Finishing Concrete
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Basic Requirements for Placing Concrete (2)
n Avoid excessive horizontal movement
n Consolidate adequatelyn Maintain sufficient placement
capacityn Choose the right equipment for
the concretePlacing and Finishing Concrete
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Preparation Before Placing Includes:
n Trimming the subgrade
Placing and Finishing Concrete
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Preparation Before Placing Includes:
n Moistening the subgrade
Placing and Finishing Concrete
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Preparation Before Placing Includes:
n Compacting the subgrade
Placing and Finishing Concrete
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Depositing Concrete
n DO NOT —(a) disturb saturated subgrades so bearing capacity is maintained(b) deposit on frozen subgrade
n Deposit continuously and as near as possible to its final position
n Rate of placement should be such that previously placed concrete has not set when the next layer is placed upon it Placing and
Finishing Concrete
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Depositing Concrete
n Start placing along perimeter at one end with each batch discharged against previously placed concrete
n 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)Placing and Finishing Concrete
Slab Construction
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Depositing Concrete
Placing and Finishing Concrete
Discharging into previously placed concrete
Effective use of wheelbarrows
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Bleeding and Segregation
Placing and Finishing Concrete
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Fresh concrete is a mixture of solid particles with specific gravities ranging from about 2.6 (most aggregates) to 3.15 (Portland cement). After the concrete has been placed, the particles tend to settle and the water to rise.
Bleed water visible as a surface shininess or “sheen.”
Note reflections on water-covered surface.
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Placing and Finishing Concrete
Bleeding
Placing and Finishing Concrete
>Inability of solid constituents to hold mixing water as they settle down.>Remixing of bleeding water will result in weaker concrete.>If bleeding water evaporates at surface will cause shrinkage cracks.Partial evaporation may result in increase in strength.
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Control
of
BleedIng
Placing and Finishing Concrete
a. Increase fineness of cement.
b. Cement has high Aluminates.
c. Alkali and chloride are present.
d. Properties of fine aggregate , specially smaller than 150 microns
e. Rich mixes bleed lesser than lean mixes
f. Addition of pozzolansg. Air entrainment.04/13/2023 13
Segregation
Placing and Finishing Concrete
a. In dry mix coarser particle travel more along slope.b. In wet mix coarser particles settle down more .c. Vibration of concrete is allowed for longer duration CA moves down.
------------------------------------------ Addition of water in dry mix, reducing difference in specific gravity of aggregates and air entrainment in wet mixes will improve segregation04/13/2023 14
Incorrect Placement with Conveyor Belt
Placing and Finishing Concrete
Baffle
Mortar
Rock Mortar
Mortar
Rock
Shallow hopper
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Placement with Conveyor Belt
Placing and Finishing Concrete
Belt Scraper
No Separation
Provide at least 0.6 m (24 in.) headroom for downpipe, elephant trunk or equivalent
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Depositing Concrete
Pavement Slab
n Concrete deposited in front of slip form paver by dump trucks
n Concrete spread evenly across the subgrade by the paver before consolidation and finishing
Placing and Finishing Concrete
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Depositing Concrete
n Concrete deposited into hopper of slip form curb and gutter machine which then extrudes the concrete into the desired shape
Placing and Finishing Concrete
Curb/Curb and Gutter
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Depositing Concrete
n Deposit in horizontal layers of uniform thickness
u Reinforced — 150 mm to 500 mmu Mass — 375 mm to 500 mm
n Consolidate each layer before next is placed
n Timely placement & consolidation prevents flow lines and cold joints
Placing and Finishing Concrete
Walls
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Horizontal Construction Joint
Placing and Finishing Concrete
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Horizontal Construction Joints
Placing and Finishing Concrete
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Underwater Placement Methods
n Tremien Pumpn Bottom dump bucketsn Grouted preplaced
aggregate (specialized)n Toggle bagsn Bagworkn Diving bell
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Placing Concrete Under Water
n Water velocity 3 m (10 ft) / min.n Water temperature 5°C
(if below — test for strength gain)n w/c 0.45n Cementing materials content
390 kg/m3 (600 lb/yd3)n Slump range 150 to 225 mm (6 to
10 in.)
Placing and Finishing Concrete
Basic Recommendations
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Placing Concrete Underwater
Placing and Finishing Concrete
n Used: Tremie
n Advantages: Can be used to funnel concrete down through the water into the structure.
n Watch for: Discharge end always has to be buried in fresh concrete to ensure seal between water and concrete mass.
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Consolidation/ compaction of concrete
Placing and Finishing Concrete
Air gets entrapped while placing concrete. To achieve dense concrete for better strength air must be expelled. This process is called consolidation or compaction
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Consolidating Concrete
n Internal Vibration
n External Vibration
Placing and Finishing Concrete
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Internal Vibration
Placing and Finishing Concrete
d
R
1½ R
Vibrator
Radius of Action04/13/2023 27
Internal Vibrators
Diameterof 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)
8000-12,000180-360(7-14)
4.6-15(6-20)
Stiff plastic concrete (less than 80-mm [3-in.] slump) in general
construction . Placing and Finishing Concrete
Adapted from ACI 30904/13/2023 28
Systematic Vibration of Each New Lift
Placing and Finishing Concrete
CORRECTVertical penetration a few inches into previous lift (which should not yet be rigid) of systematic regular intervals will give adequate consolidationINCORRECTHaphazard random penetration of the vibrator at all angles and spacings without sufficient depth will not assure intimate combination of the two layers04/13/2023 29
Placing Concrete in a Sloping Lift
Placing and Finishing Concrete
CORRECTStart placing at bottom of slope so that compaction is increased by weight of newly added concrete. Vibration consolidates the concrete.INCORRECTWhen 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
n Form vibratorsn Vibrating tablesn Surface vibrators
u Vibratory screedsu Plate vibratorsu Vibratory roller screedsu Vibratory hand floats or
trowels
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External Vibrator
Placing and Finishing Concrete
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Consolidating Concrete
Inadequate consolidation can result in:u Honeycombu Excessive amount of entrapped
air voids (bugholes)u Sand streaksu Cold jointsu Placement linesu Subsidence cracking
Placing and Finishing Concrete
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Revibration of concrete
Placing and Finishing Concrete
Improved strength Improved qualityBetter bond Better impermeability
Reduced shrinkage Reduced creepReduced voids Reduced cracks
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Revibration of Concrete
Placing and Finishing Concrete
Density of concrete improves due to revibration
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Nuclear Gauges to Determine Subbase Compaction
Placing and Finishing Concrete
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Screeding (Strikeoff)
Placing and Finishing Concrete
The process of cutting off excess concrete to bring the top surface of a slab to proper grade
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Vibratory Screeds
Placing and Finishing Concrete
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Bullfloating
Placing and Finishing Concrete
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Darbying
Placing and Finishing Concrete
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Edging
n Required along all edge forms,isolation and construction jointsin floors and exterior slabs
n Cut concrete away from formsto a depth of 25 mm with apointed mason or margin trowel
n Edging may be required aftereach subsequent finishing operation for interior slabs
Placing and Finishing Concrete
Edging densifies and compacts concrete next to forms where floating is less effective
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Highway Straightedges
Placing and Finishing Concrete
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Floating (Power or Hand)
n To embed aggregate particles just beneath the surface
n To remove slight imperfections, humps, and voids
n To compact the mortar at the surface in preparation for additional finishing operations.
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Troweling
n Creates smooth, hard,dense surface
n Exterior concrete should not be troweled because: u it can lead to a loss of
entrained air caused by overworking the surface
u troweled surfaces can be slippery when wet.
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Brooming
Placing and Finishing Concrete
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Tining
Placing and Finishing Concrete
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Curing and Protection
n Cure for 7 daysPlacing and Finishing Concrete
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Placing on Hardened Concrete
Placing and Finishing Concrete
Preparing Hardened Concrete
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Bonding New to Previously Hardened Concrete
n Cement-sand grout
n Latexn Epoxy
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Isolation Joints
Placing and Finishing Concrete
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Contraction Joints
Placing and Finishing Concrete
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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
Placing and Finishing Concrete
Metric04/13/2023 52
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
Placing and Finishing Concrete
Inch-Pound04/13/2023 53
Making Contraction Joints
Placing and Finishing Concrete
Dry-cut sawing concrete
Grooving tool on bull-float
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Construction Joints
Placing and Finishing Concrete
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Joint Layout for Slabs
n Panels created by contraction joints should be approximately square
n Panel aspect ratio max. 1½ to 1n Contraction (control) joints should
only terminate at a free edge or at an isolation joint
n When joint spacing exceeds 4.5 m (15 ft), load transfer by aggregate interlock decreases significantlyPlacing and
Finishing Concrete
Basic Factors to Remember
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Typical Joint Layout
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Finishing Operations
n Consolidationn Strike-offn Edgingn Darbying or Bull Floating Lapse of Timen Edgingn Grooving (if desired)n Floating (power or hand)n Troweling (power or hand) Lapse of Timen Second Troweling (power or hand)n Final Troweling (hand)n CuringPlacing and
Finishing Concrete
Single Course Floors
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Finishing Operations - Exterior Slabs
n Consolidationn Strike-offn Depress aggregate with metal/wood strip
at joint location if hand tooledn Darbying or Bull floating Lapse of time
n Edgingn Jointing (optional if hand tooled)n Floating Lapse of time
n Texturing (brooming/swirl float finish)n CuringPlacing and
Finishing Concrete
Sidewalks, Driveways etc.
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Patching
Placing and Finishing Concrete
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Curing Patches
Placing and Finishing Concrete
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Cleaning Concrete Surfaces
n Cleaning methods:
u Water
u Chemical
u Mechanical
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Finishing Formed Surfaces
n Rough-form finishes
n Smooth off-the-form finish
n Smooth, rubbed finish
n Sand-floated finish
n Grout cleandown
(sack-rubbed finish)Placing and Finishing Concrete
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Special Surface Finishes
n Pattern and Textures
n Exposed Aggregate Concrete
n Colored Finishes
n Stains, Paints and Clear
Coatings
Placing and Finishing Concrete
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Placing and Finishing Concrete
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Placing and Finishing Concrete
Exposed aggregate finish
Bush Hammering Finish
Finishes
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Placing and Finishing Concrete
Craziness in Concrete
Spoils Surface.No harm to Structure.Happens due to shrinkage, carbonation or differential shrinkage between richer skin and parent concrete
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Working Safely with Concrete
Protect:
n Your Eyes
n Your Back
n Your Skin
Placing and Finishing Concrete
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Precautions
Placing and Finishing Concrete
http://www.cement.org/pdf_files/ms271.pdf
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.
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Videos 1/4
Placing and Finishing Concrete
Placing and Consolidating Vibration
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Videos 2/4
Placing and Finishing Concrete
Finishing I Finishing II
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Videos 3/4
Placing and Finishing Concrete
Contraction Joints Jointing
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Videos 4/4
Placing and Finishing Concrete
Improper Finishing
04/13/2023 73
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