Concrete compaction [compatibility mode]

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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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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


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%


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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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


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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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


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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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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


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


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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


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.


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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.


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.


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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.


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.


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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

Engineering

Concrete compaction [compatibility mode]