Placing and finishing of concrete

Placing and Finishing Concrete

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Basic Requirements for Placing Concrete (1)

n Preserve concrete qualityu Water-cement ratiou Slumpu Air-contentu Homogeneity

n Avoid separation of aggregate and mortar

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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

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n Moistening the subgrade

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n Compacting the subgrade

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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

Discharging into previously placed concrete

Effective use of wheelbarrows

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Bleeding and Segregation

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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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Bleeding

>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

BleedIng

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

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

Baffle

Mortar

Rock Mortar

Mortar

Shallow hopper

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Placement with Conveyor Belt

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

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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

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

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Horizontal Construction Joint

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Horizontal Construction Joints

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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.)

Basic Recommendations

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Placing Concrete Underwater

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

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

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Internal Vibration

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

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

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

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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

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Revibration of concrete

Improved strength Improved qualityBetter bond Better impermeability

Reduced shrinkage Reduced creepReduced voids Reduced cracks

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Revibration of Concrete

Density of concrete improves due to revibration

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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

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

Edging densifies and compacts concrete next to forms where floating is less effective

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Highway Straightedges

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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

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Tining

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Curing and Protection

n Cure for 7 daysPlacing and Finishing Concrete

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Placing on Hardened 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

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Contraction Joints

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Spacing of Contraction Joints in Meters

Slab thickness,

Maximum-size aggregate

less than 19 mm

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

Metric04/13/2023 52

Spacing of Contraction Joints in Feet

Slab thickness, in.

Maximum-size aggregate less than ¾ in.

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-Pound04/13/2023 53

Making Contraction Joints

Dry-cut sawing concrete

Grooving tool on bull-float

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Construction Joints

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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

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Curing Patches

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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

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Exposed aggregate finish

Bush Hammering Finish

Finishes

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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

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Precautions

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 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

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