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Concrete Production, Compaction of Concrete, Reducing Porosity in Concrete, compacting concrete, concrete compaction
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8/3/2014
1
CONCRETE : COMPACTION
BY
RAJESH JINDAL
15 Sep 2013
CTM : 2013
References
MS Shetty - Concrete Technology
Neville & Brookes - Concrete Technology
IS 456
www.concrete.org
Guide for consolidation of concrete : ACI
Committee Report 2006
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GEN OUTLINE
What is Compaction
Why is it required
Compacting Efforts
Manual
Mechanical
Certain Do’s & Don’ts
Compaction of Concrete What is Compaction
Process adopted for expelling the entrapped air to achieve dense mass
Constituent Entrapped Air
Before Compaction
After Compaction
Aggregates 60-75% 60-75%
Cement 25-40% 25-40%
Air 25-30% 1-2%
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Compaction of Concrete Why is Compaction required
To drive out entrapped air to densify fresh concrete During Mixing, Transportation & Placing – air gets
entrapped Quantity of air entrapped inversely proportional
to workability Entrapped air adversely affects quality & strength
To make fresh concrete conform intimately to shape of form
Increased homogeneity & uniformity
Why is Compaction Required
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Compaction of Concrete Adverse Effects of Entrapped Air
Low Strength & Durability Low resistance to abrasion Poor bonding with reinforcement and
low bond strength High Porosity Resulting in easy ingress of moisture,
chemicals Weakening of Concrete & reinforcement
CONSOLIDATING CONCRETE
Inadequate consolidation can result in: Honeycomb Excessive amount of
entrapped air voids (bug-holes)
Sand streaks Placement lines (Cold joints)
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Consolidating Concrete Improper Compaction
STRENGTH OF CONCRETE
The strength of a concrete specimen prepared, cured and tested under specified conditions at a given age depends on: w/c ratio Degree of
compaction
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Compaction of Concrete How to Achieve Full Compaction
Adequate Workability Mix not to be too wet – reduces strength For max strength – driest possible mix to
be used
Usage of Compacting Efforts
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Compaction of Concrete Compacting Efforts
Manual Compaction Rodding Ramming Tamping
Compaction by Vibrations By Vibratory rollers Compaction by Pressure and Jolting Compaction by Spinning
Compaction of Concrete Manual Compacting Efforts
Suitable For Unimportant Concrete work, small works Places where reinforcement is more &
does not permit use of vibrating equipment
Higher consistency Upto concrete thickness of 15 to 20 cms
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Compaction of Concrete Manual Compacting Efforts
Rodding Poking concrete with 2 m long, 16 mm dia
rod to pack concrete between rft, corners & edges
Ramming In foundations, unreinforced concrete on
ground Not permitted where rft may be disturbed
or formwork may fail
Compaction of Concrete Manual Compacting Efforts
Tamping Consists of beating the top fresh concrete
surface by wooden crossbeam of size 10 x 10 cm
When thickness is less Surface is more Not only compacts, but also levels top
surface
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Compaction of Concrete Compaction by Vibrations
Manual Compaction – may give satisfactory results; iff - Adequate Workability & Care
Strength low because high W/C ratio In case of High Strength Concrete, Low
W/C ratio, thick members, larger volumes – mechanical means of compaction - mandatory
Compaction of Concrete Compaction by Vibrations
Energy for compaction is supplied through oscillatory motion of vibrations (nearly SHM)
Vibrations are generated by means of rotating eccentric masses having Frequency Amplitude
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Compaction of Concrete Principle behind Compaction by Vibrations
Prior to compaction, concrete is mass of separate particles coated with mortar, held in pile by arching action of coarser particles.
Arching is result of friction between aggregate particles, surface tension & cohesive forces of cement paste.
Voids caused by arching – upto 30%
Compaction of Concrete Principle behind Compaction by Vibrations
Vibratory impulses liquefy mortar portion of concrete, thus help reduce friction resulting in consolidation under gravity
After vibrations, concrete internal friction is re-established, cohesion restores and strength increases
Velocity of compression waves generated is 45 m/sec in beginning of vibrations which increases to 240 m/sec at the end
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Compaction of Concrete Principle behind Compaction by Vibrations
Waves move air & water more than solids & generate hydraulic pressure with interstitial water filled space. Excess pressure results in pressure on pores, causes these to break, forces water out.
Pressure max in most constricted spaces & causes reduction in internal friction giving the paste temporary fluidity
Compaction of Concrete Principle behind Compaction by Vibrations
Consolidation takes place in two stages Stage – I Vertical settlement of coarse aggregates
takes place in a manner similar to packing of granular material
Shape of aggregates plays a major role, air voids up to 5% remains at this stage
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Compaction of Concrete Principle behind Compaction by Vibrations
Stage – II Concrete behaves like dense fluid Internal vibrations set particles in motion,
reducing internal friction, achieving temp liquefaction, thus enabling easy settlement
Air voids are removed forcing mortar to appear at the surface
This is one of the way to assess sufficiency achieved in of compaction by vibrations
Optimum Vibration Time
Timings of Vibrations in Seconds
Uni
t wei
ght
Optimum Time is directly proportional to
Size of Vibrator
Type of concrete
Size of concrete or
member
Type of vibrations i.e.
frequency, amplitude &
acceleration
Optimum Time
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Compaction of Concrete Advantages : Compaction by Vibrations
Efficient than Manual Concrete can be placed and compacted in
difficult sections Slump requirements – 4 cm with vibrations &
12 cms with manual method Facilitates use of less water – thus increased
strength & durability In essence, method of removal of entrapped
air is different
Compaction of Concrete Type of Vibrators
Internal Poker - immersed in concrete for compaction Poker can easily be removed & moved from
point to point
External External vibrators clamp direct to the
formwork requiring strong & rigid forms. Examples - Formwork, Table, Platform,
Surface & Vibratory Rollers type
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To aid in the removal of trapped air the vibrator head should be rapidly plunged into the mix and slowly moved up and down.
Internal Vibrators
The actual completion of vibration is judged by the appearance of the concrete surface which must be neither rough nor contain excess cement paste.
Immersion / Poker / Submersible / Spud Type
Flexible Drive Type
Motor in Head
Internal Vibrators
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Most Common Vibrations upto 12000 cps Dia of poker 20 to 175 mm Length of poker – 25 to 90 mm Needle can be replaced by flat needle
(blade) to compact congested areas Portable
Internal Vibrators
Internal Vibrators
d
1½ R
Vibrator
R - Radius of Action
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Char
acte
ristic
s &
App
licat
ions
of
Im
mer
sion
Vib
rato
rs
Internal Vibrators
Diameter of head, (mm)
Recommended frequency, (vib./min.)
Approximate radius of
action, (mm)
Rate of placement,
(m3/h) Application
20-40 9000-15,000 80-150 0.8-4 Plastic and flowing concrete in thin members. Also used for lab test specimens.
30-60 8500-12,500 130-250 2.3-8
Plastic concrete in thin walls, columns, beams, precast piles, thin slabs, and along construction joints.
50-90 8000-12,000 180-360 4.6-15 Stiff plastic concrete (less than 80-mm slump) in general construction .
Adapted from ACI 309
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Penetrate vertically to sufficient depth, hold stationary and remove slowly @ 7.5 m/sec
Vibrations at regular spacing to ensure compaction of all portions & overlap
Minimum 10 sec are required for complete compaction.
Execution of Vibrations
Execution of Vibrations
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Execution of Vibrations
R is Radius of action
D is Spacing
E is thickness of section
Execution of Vibrations 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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External Vibrators
Form vibrators Vibrating tables (Lab) Surface vibrators Vibratory screeds Plate vibrators Vibratory roller
screeds Vibratory hand floats
or trowels
External vibrators rigidly clamped to formwork so that form & concrete are subjected to vibrations.
Considerable work needed to vibrate forms. Forms must be strong and tied enough to prevent
distortion and leakage of the grout. Suitable for thin members or Pre-Cast units Frequency range – 1000-5000 cycles per min Finish quality – Good Consumes more power – less efficient than internal
External Vibrators
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Vibrating Table Used for small
quantities of concrete (lab & some precast elements)
External Vibrators Table Vibrators
Larger than table vibrators. Used for manufacture of Concrete Poles Railway Sleepers Prefabricated roofing elements
Platform vibrators may also be coupled for jerking and shock to achieve thorough compaction.
External : Platform Vibrators
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Also : Screed Board Vibrators Suitable for Roof slabs Floor Slabs Roads surfaces
Effective upto 15 cm depth Types : Pan Type, Vibrating Roller screed,
Vibratory Plate, Vibratory roller for pavements
External : Surface Vibrators
Other vibrators are not suitable
Used for mass concrete : Dams, pavements.
Heavy roller vibrates dry lean concrete Tech originated in Japan & now being
used worldwide. Example : Roller compacted M-10 grade
concrete used in Mathura Highway and Pune Expressway.
External : Vibratory Rollers
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External : Surface Vibrators
External : Surface Vibrators
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Combined action of Vibrations – Pressure & Jolting results in very good compaction & dense concrete
Very effective for very dry lean mixes Gen used for compacting hollow blocks,
cavity blocks, solid concrete blocks, railway sleepers etc.
Compaction of Concrete Vibrations with Pressure & Jolting
Efficient High frequency & Low amplitude
results in efficient compaction
Compaction by Vibrations
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Used for compacting concrete pipes. Recent technology Plastic concrete when spun at a very high
speed, gets well compacted by centrifugal force.
Used for concrete spun pipes, hume pipes etc.
Compaction of Concrete External : Spinning
Vibrators are powered by Electric motor either driving the vibrator
through flexible shaft or by motor situated in head of vibrator
IC engines driving the vibrator needle through flexible shaft.
Compressed air motor situated near the head of vibrator
Compaction of Concrete Guidelines for use of Vibrators
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Electricity driven or electric motor driven – most reliable & efficient Light & Compact Consistent Ease of Handling
Compaction of Concrete Guidelines for use of Vibrators
Care should be taken for Vibrating head – not to come in
contact with hard objects – Impact may damage bearings.
No sharp bends in flexible shaft drive Vibrators to conform to IS 2505 – 1963
Compaction of Concrete Guidelines for use of Vibrators
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Vibrators to be selected on appreciation of quantum & type of concreting
Concrete to be compacted must be stiff of high compaction factor of 0.75 to 0.85
Good and strong formwork
Compaction of Concrete Guidelines for use of Vibrators
Size of vibrating needle, Timings of vibration & time of introduction & removal of needle must be taken care of.
Joints of formwork to be tight to prevent squeezing out of grout and sucking in of air during vibrations.
Compaction of Concrete Guidelines for use of Vibrators
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Absence of precautions may lead to honeycombing.
Needles must be inserted vertical and at regular intervals.
Careful application of mould releasing agents.
Ensure bonding between layers.
Compaction of Concrete Guidelines for use of Vibrators
Internal vibrator not to be used to spread concrete.
Deposit concrete in advance and then vibrate.
No vibrations recommended near free end of concrete, usually not within 120 cms of it.
Compaction of Concrete Guidelines for use of Vibrators
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Restrict concrete layers upto 60 cms to ensure proper vibrations and removal of trapped air. Concrete upto 1m thick may require very powerful vibrators.
Depth of vibrations must facilitate bonding between layers.
Compaction of Concrete Guidelines for use of Vibrators
Ensure careful spacing and overlap for full compaction at all places.
Vibrations to be completed within an hour - initial setting time.
Grouping of reinforcement may be resorted to enable vibrations.
Over vibrations result in segregation.
Compaction of Concrete Guidelines for use of Vibrators
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Imperfections in Concrete Guidelines for use of Vibrators
Issue Appearance &
Description
Design Issues
Form & condition
Concrete & Placement
Compaction
Honey Combing
Stony, air voids
Narrow sections
Grout loss Free Fall, Low slump
Poor
Bug Holes Small Holes Excess Oil Lean, Low slump
Inadequate
Subsidence Cracking
Short cracks Plastic settlement
High W/C Inadequate
Form Offset Irregular Surface
Weak Form Non uniform
Cold Joints Discontinuity Poor Planning
Delayed Placing
Inadequate
Thank You