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NATIONAL INSTITUTE OF TECHNOLOGY AGARTALA

Presented By:- RAVI GUPTA (10UCE111) Department Of Civil Engineering NIT Agartala

Placing and Finishing Concrete


Basic Requirements for Placing Concrete (1)

Preserve concrete quality◦Water-cement ratio◦Slump◦Air-content◦Homogeneity

Avoid separation of aggregate and mortar


Preparation Before Placing Includes:

Trimming the subgrade



Moistening the subgrade



Compacting the subgrade


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


Depositing Concrete

Discharging into previously placed concrete

Effective use of wheelbarrows


Incorrect Placement with Conveyor Belt

Baffle

Mortar

Rock Mortar

Mortar

Rock

Shallow hopper


Placement with Conveyor Belt

Belt Scraper

No Separation

Provide at least 0.6 m (24 in.) headroom for downpipe, elephant trunk or equivalent


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


Underwater Placement Methods

TremiePumpBottom dump bucketsGrouted preplaced aggregate (specialized)Bagwork


Placing Concrete Under Water

Water velocity 3 m (10 ft) / min.

Water temperature 5°C(if below — test for strength gain)

w/c 0.45Cementing materials content

390 kg/m3 (600 lb/yd3)Slump range 150 to 225 mm (6

to 10 in.)

Basic Recommendations


Placing Concrete Underwatern 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.


Consolidating Concrete

Internal Vibration

External Vibration


Internal Vibration

d

R

1½ R

Vibrator

Radius of Action


Internal VibratorsDiameterof 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 .

Adapted from ACI 309


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 layers


Consolidating ConcreteInadequate consolidation can result in:◦Honeycomb◦Excessive amount of entrapped

air voids (bugholes)◦Sand streaks◦Cold joints◦Placement lines◦Subsidence cracking


Nuclear Gauges to Determine Subbase Compaction


Screeding (Strikeoff)The process of cutting off excess concrete to bring the top surface of a slab to proper grade


Vibratory Screeds


Darbying


Edging

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

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


Highway Straightedges


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.


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.


Brooming


Tining


Curing and Protection

Cure for 7 days


Placing on Hardened Concrete

Preparing Hardened Concrete


Bonding New to Previously Hardened Concrete

Cement-sand grout

LatexEpoxy


Contraction Joints


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


Making Contraction Joints

Dry-cut sawing concrete

Grooving tool on bull-float


Construction Joints


Patching


Curing Patches


Cleaning Concrete Surfaces

Cleaning

methods:

◦Water

◦Chemical

◦Mechanical


Finishing Formed Surfaces

Rough-form finishes

Smooth off-the-form finish

Smooth, rubbed finish

Sand-floated finish

Grout cleandown (sack-rubbed finish)


Special Surface Finishes

Pattern and Textures

Exposed Aggregate

Concrete

Colored Finishes

Stains, Paints and Clear

Coatings



Working Safely with ConcreteProtect:

Your Eyes

Your Back

Your Skin


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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