KAS-2012 1

CONCRETE HANDLING IN THE

FIELD

By K.Shah B.E.(Civil Engg) NIT Rourkela,India

M.Sc. (Environment), University of Leeds, UK

Ex-GM(Civil & Environment) – MNC

Currently Guest faculty-College of Technology &

Engineering

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Stages of producing concrete.

(1) Batching

(2) Mixing

(3) Transportation

(4) Placing

(5) Compacting

(6) Curing

(7) Finishing

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(1) Batching

(a) Volume batching

(b) weight Baching

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Volume batching

Volume batching is not good method

Moist sand in loose condition weights less

than the same volume of dry sand.

Practiced for small work.

For quality work ,weigh batching is

practiced.

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VOLUME BATCH

Gauge box

Various gauge boxes of different volumes are

used.

Length Width Depth Volume

33.3 cm 30 cm 20 cm 20 liters

33.3 cm 30 cm 25 cm 25 liters

Grade Cement- kg Sand -lts Coarse

Aggregate

lts

1:11/2:3 (M

200)

50 35 70

1:2:4 (M

150)

50 70 140

1:3:6 (M100) 50 105 210

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Weigh Batch Machine

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Weigh batching

Weigh batching is correct method

Facilitates accuracy, flexibility & simplicity

Different batching machine are available :

(a) manual machines & (b) Automatic machines

Manual machine :

Has two buckets

Buckets mounted on common spindle about which they rotate.

One is loaded while other is discharged in mixer.

Spring loaded dials indicate the weight.

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Automatic weigh batch

For large works

Over head hopper and discharges into

mixer.

Useful in ready mix concrete plant

Recorders for weight

Calibration is required from time to time.

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(2) MIXING

Mixing of cement,sand aggregates should

ensure that:

The mass is homogeneous

Uniform in color

consistent

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MIXING METHODS :

(1) Hand mixing

(2) Machine mixing

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Hand mixing

Practiced for small scale work (small house, repairing of house etc)

10 % extra cement is added to compensate inferior concrete produced by this method.

Spread fine & coarse aggregate in alternate layer

Spread cement over it

Mix with shovel till uniform color is achieved

Water is sprinkled over it

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Machine mixing

Medium & large scale work use machine mixing

Mixing is efficient, economical & produce quality concrete.

Type of mixer:

(a) Batch mixer : batch by batch with time interval

(b) Continuous mixer: continuously mixed & discharged (in dam construction)

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CONCRETE MIXER

(1) Pan type

(2) drum Type:

(a) tilting

(b) Non –tilting

(c) Reversing

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PAN MIXER

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PAN MIXER

A forced movement pan mixer has blades

that are fixed to an assembly that agitates

the concrete throughout the pan as the

vertical shaft rotates.

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DRUM MIXER

As per IS: 1791-1985 mixers are

designated by number which shows

capacity (liters) of batch:

a) Tilting : 85 T, 100T, 140 T, 200T

b) Non tilting : 200 NT,280 NT, 375 NT, 500

NT, 1000 R

c) Reversing : 200 R, 280 R, 375 R,500 R,

1000 R

T= Tilting, NT =non tilting, R=Reversing

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TILTING MIXER

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TILTING MIXER

Internal blades lift and tumble the

ingredients onto itself.

Two primary types exist:

horizontal (one end has and opening for

charging and a different end for

discharging)

single drum (materials are charged and

discharged through a single opening).

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NON TILTING MIXER

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NON TILTING MIXER

Single drum rotating about a horizontal

axis.

Fixed blades work the concrete towards

the discharge end of the mixer, in order to

provide a rapid rate of discharge.

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REVERSING MIXER

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REVERSING MIXER

The entire drum rotates around its axis as materials are loaded through a charge chute at one end of the drum and exit through a discharge chute at the opposite end of the drum.

Mixing blades are mounted on the inside surface of the drum and as the drum rotates the blades mix by lifting and dropping the materials during each rotation.

Once the materials are sufficiently mixed the rotation of the drum is reversed and the blade arrangement pushes the concrete through to the discharge end of the mixer.

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Sequence of charging drum

First half quantity of coarse aggregate is

placed in skip

Over it half quantity of sand

On that full quantity of cement

Over it balance quantity of coarse & fine

aggregates is place.

This prevents spillage of cement in air

while discharging in drum

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25 % Water is placed in drum and then

mix from skip is discharged in the drum

This prevents sticking of cement on blades

75 water is immediately poured after

placing mix material (cement sand etc) in

drum.

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Mixing time

In small machine, mixing time varies

between 1-2 minutes

In Ready Mix Cement mixer – 15-30

seconds

RPM of drum : 15-20

Compressive strength of concrete

increases with increase in mixing time but

after 2 minutes increase in compressive

strength is not significant.

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If concrete is not used after mixing it may

set

But when concrete is agitated on time to

time in drum setting time rule does not

follow.

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Retempering of concrete :

Some time concrete from RMC plant is not delivered to site due to traffic congestion

Concrete becomes stiff and becomes unworkable

Site engineers can reject the concrete if delay is more

If it can be of used then small volume of water is added and again agitated in the drum. This is called RETEMPERING OF CONCRETE.

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MANUFACTURING OF

CONCRETE

With same material if care is not taken,

resulting concrete will be bad concrete

What are good rules to make good quality

concrete.

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TRANSPORTATION OF CONCRETE

Precaution in concrete transportation:

Homogeneity of conc. Mass is maintained

Movement of hand trolly or truck on rough

road surface makes vibrations

This results in deposition of heavy

aggregates at bottom of truck

Water & cement slurry comes on top.

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METHODS OF TRANSPORTATION

1. Mortar Pan

2. Wheel barrow

3. Truck Mixer & dumpers

4. Crane, Bucket & rope way

5. Belt conveyors

6. Chutes

7. Skip & hoist

8. Transit Mixer

9. Pump & pipeline

10. Helicopter

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MORTAR PAN

Common method in India

More labour required

Segregation of concrete is less

Greater surface area of concrete is

exposed to sun, concrete dries.

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WHEEL BARROW

When transportation of concrete is at

ground level.

Movement of wheel on rough road

surface, segregates concrete.

Some wheel barrows have pneumatic

wheel to reduce vibration

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CRANE

Used for transporting concrete above

ground level.

For high rise buildings.

Cranes are fast

Can move horizontally & vertically

Concrete in skip discharge from bottom

In bucket concrete is discharged by tilting.

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BUCKET & ROPEWAY

Use for construction in:

Valley

Bridge pier in river

Dam

Advantage:

Concrete is not exposed to sun or air & no

loss of water.

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Truck Mixer & dumpers

Used for large concrete works.

Can travel any part of site.

Dumpers - 2-3 M3 Capacity

Trucks – 4 M3 Capacity

Bottom surface of truck is kept wet

Top of truck is covered to prevent

evaporation

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BELT CONVEYORS

Limited use in construction

Advantages:

Can transport large volume

Very quick

Can go where access is limited

Disadvantages :

On steep slope concrete segregates.

Exposed to sun for long time.

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CHUTE

For transporting from ground level to lower

level. (basement etc).

Used where labour can not reach due to

less space in trench etc.

Made of metal

Slope should not be < 1 vertical : 2.5

horizontal.

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SKIP & HOIST

Labour can go upto 3rd or 4th floors.

So skip is used for transport vertically up

(in multistory building).

Skip travels on vertical rail.

Skip can discharge manually or

automatically.

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TRANSIT MIXER

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TRANSIT MIXER

Used for long distance travel in RMC plant.

Concrete is continuously agitated in truck drum

(2 – 6 rpm).

Also transported mix in dry condition and water

is added on reaching the destination.

Wet Mix in truck must reach site in 1- 1.5 hours.

Pumps are also fitted on truck mixer to

discharge concrete.

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PUMPS & PIPELINE

Most popular method

Reliable & good quality pumps are used.

Mostly operated by diesel.

Concrete is placed in collecting hopper.

Rotating blades in hopper pushes concrete towards pipe.

Vacume in hose pipe (600 mm Hg)

Rotating rollers in pump chambers squeeze the concrete in pipe and flow of concrete is started.

Concrete is discharged from other end of hose pipe.

Concrete can be pumped upto 400 m height and 2000 m distance.

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SECTION OF PUMP

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PIPELINE

Pipeline should :

Have correct diameter as per pump

pressure. (generally 125 mm)

Have sufficient thickness

Good couplings

Poor pipeline can cause blockage.

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PIPELINE

Thumb rule : For 30 M3 /hr concrete and

200 m length, dia should be 100 mm.

Length > 500 m then dia = 150 mm.

Dia = 3 to 4 times the size of aggregate

Leaky pipe & coupling result in escape of

water /air & finally block the concrete.

Vertical pipe should good otherwise

difficult to change at height.

Pump is kept at distance from building

about 15 % of vertical length.

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PUMPABLE CONCRETE

Concrete which can be pushed through a pipeline is

called pumpable concrete.

Friction between pipe wall and concrete is less.

Concrete flows in the form of plug which is separated

from pipe wall by a thin layer of lubricating cement paste.

Flow resistant must be < pump pressure.

If the concrete is more wet then water comes out of mix

which makes more resistance to flow.

Stiff and also very wet concrete is not pumpable.

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Design of pumpable concrete

Concrete Mix is so designed that all material remain

together.

Mix must make redial movement of grout to maintain

lubricating paste.

Mix should be deformed at bends

Cement & fine particles (0.25 mm size) are important for

good flow.

350 to 400 Kg/ M3 of fine particles are necessary for

flow.

Slump of pumpable concrete is above 75 mm.

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PROBLEMS IN PUMPING

Blockage in pipe

Pipe should be cleaned after each day

operation

Blockage can be cleaned by forward-

backward pumping.

Tapping pipe with hammer

Clean pipe with rod or sponge ball pushed

by compressed air.

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PLACING CONCRETE

Must be placed in systematic manner.

Can be placed with following methods:

Within earth mould : Foundation

In timber plank formwork : Road, airport

slab.

Steel shuttering : Dam

Under water

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Concrete in Foundation

In foundation, ground is made wet.

Plastic sheet are laid between ground & slab

Concrete is dumped not poured.

No heap and dragging

Placed in layers of 35 – 40 cm in mass concrete

Avoid cold joints between 2 layers

Surface of previous layer is cleaned with wire brush

Sometime, cement slurry is placed on old surface

Top of previous layer kept rough for good bond.

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Concrete on Road, airport, floor slabs

Placed in alternate bays (allow shrinkage)

with contraction joints :

contraction joints

Bays

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Concrete in Beams & Column

Reinforcement correctly placed.

Correct cover required

Joints of shuttering to be plugged.

Mould releasing agent inside formwork

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STRIPPING TIME Form work should not removed until good

strength has come.

Type of form work Time

Vertical wall, columns 16-24 hrs

Soffit of slab (Prop should be refixed immediately) 3 days

Soffit of beam (Prop should be refixed immediately) 7 days

Props to slab Span < 4.5 m 7 days

Props to slab Span > 4.5 m 14 days

Props of beam (Span < 6 m) 14 days

Props of beam (Span > 6 m) 21 days

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UNDER WATER CONCRETE

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UNDER WATER CONCRETE

Tremie (means hopper) is used.

Funnel on top

Pipe of 200 mm size

Pipe bottom is plugged

Fill pipe with concrete

Lift pipe or jerk to release the plug

Keep bottom of pipe inside concrete

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Underwater Concrete

No compaction required as hydrostatic pr

of water compacts concrete.

Concrete of 0.3 W/C ratio can be placed

with Tremie.

Used for Pile or well foundation

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SLIP FORM TECHNIQUE

In this method, concrete is continuously placed, compacted & form work is pulled up for next layer of concrete.

Vertical slip form for Tall structure like silo, chimney

Horizontal slip form paver machine (HSFP) for road construction.

Concrete is dumped in front of HSFP machine by dumpers.

Compaction by vibrator installed inside HSFP machines.

Finishing of surface by HSF paver.

Operation of road alignment, gradient, curve are controlled by Computerized Laser Control system.

Speed of construction is 1 mt /min.

1 km of concrete road of 3.75 mt width is built in one day (16 hrs work).

Mumbai-Pune Expressway was constructed by this machine.

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VERTICAL SLIP FORM

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HORIZONTAL SLIP FORM ROAD PAVER

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COMPACTION OF CONCRETE

Compaction is a process of expelling the entrapped air inside concrete mass.

During mixing, transporting & placing the concrete, air gets trapped in concrete mass.

If this air is not removed, concrete will not get strength.

5 % of air voids reduces strength by 30 %

10 % of air voids reduces strength by 50 %

Durability of concrete is also reduces with air voids.

Insufficient compaction increases permeability of concrete.

Results in entry of aggressive chemicals in solution.

Chemicals attack concrete & reinforcement and life of concrete is reduced.

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METHODS OF COMPACTION

1. Hand compaction

2. Compaction by vibration

3. Compaction by pressure &

jolting

4. Compaction by spinning

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(2) COMPACTION BY VIBRATION

a) Internal Vibrator

b) Formwork Vibrator

c) Table Vibrator

d) Platform Vibrator

e) Surface Vibrator

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(1) HAND COMPACTION

Hand Rodding

Poking inside concrete with 1-2 mt long steel rod

Ramming

Unreinforced foundation & ground floor work

Tamping

Wooden beam is used to beat concrete (low thickness slab, road slab)

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COMPACTION BY VIBRATION

In hand compaction w/c ratio is more so

we get less strength in concrete.

In mechanical vibrator w/c can be kept low

so we get good strength.

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INTERNAL VIBRATOR

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(a) INTERNAL VIBRATOR

Most common in use

Called, Needle Vibrator or Immersion vibrator or Poker vibrator.

Consists : electrical/diesel power supply, Needle and shaft.

Frequency of vibrations can be 12000 cycles vibration per minute.

Needle diameter 20 to 75 mm

Length 25 to 90 cm.

Portable.

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FORMWORK VIBRATOR

Used for columns, walls, precast slab

Vibrator is clamped to formwork

Vibration is given to formwork

Vibration is transferred from formwork to

concrete

Useful in thin wall where reinforcement

obstruct the needle type vibrator.

Efficiency is lower then needle vibrator

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TABLE VIBRATOR

Vibrator is clamped to table

Used for concrete test cubes

Cubes are kept on table to get vibrations

Also used for small prefabricated slab

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TABLE VIBRATOR

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PLATFORM VIBRATOR

Similar to table vibrator but of large size

Used for long concrete electrical pole,

railway sleeper,prefabricated roofing

element

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PLATEFORM VIBRATOR

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SURFACE VIBRATOR

Known as Screed Board Vibrator

Used for thin roof slab where needle

vibrator can not be used

Not effective if slab thickness is more then

15 cm.

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SURFACE VIBRATOR

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COMPACTION BY PRESSURE & JOLTING

Used for hollow blocks, solid concrete

blocks

Stiff concrete is vibrated, pressed & given

jolts

Stiff concrete is compacted to get dense

form & good strength is achived.

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VIBRATION BY SPINNING

New method

Used for concrete pipes

Concrete when spun at high speed gets

compaction by centrifugal force

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VIBRATORY ROLLER

Road rollers has vibrating system

Roller while moving on raod slab gives

vibrations

Used for Lean concrete (M10) for road

base

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ROLLER VIBRATOR

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PRECAUTIONS IN VIBRATING CONCRETE

Vibrator gets damaged if comes in contact with hard object (Formwork, hard concrete)

Switch on when needle is inside fresh concrete mass

Should conform to IS 2505-1963

Degree of compaction can be recognized from rising air bubbles & formation of thin film on top

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HYDRATION OF CEMENT

Cement is made by Cao, SiO2, Al2O3, Fe2O3, MgO, K2O,

SO3

After burning in Kiln following products are made:

3Cao. SiO2 (C3S) - Tri calcium silicate

2Cao. SiO2(C2S) – Di cacium silicate

3Cao. Al2O3 (C3A) – Tri cacium aluminate

4Cao. Al2O3. Fe2O3(C3AF) -Tricalcium alumino ferrite.

On addition of water in cement reaction of C3S, C2S, C3A

& C3AF liberate heat. Heat is suside by addition of water.

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CURING OF CONCRETE

Curing is defined as “ making satisfactory moisture

content & favourable temperature” in concrete after

placing the concrete. So that hydration may continue

until the strength is developed.

Curing is required immediately after placing concrete.

During hydration, heat of hydration is released.

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CURING OF CONCRETE

Concrete delivers its strength by the hydration of cement particles.

Hydration is continuous & long time process.

Rate of hydration is fast immediately after making the concrete

Theoretically 0.23 w/c ratio required for hydration

0.15 w/c ratio required for filling the voids in gel.

Total 0.38 w/c ratio is optimum

In field condition, water evaporates & available water quantity reduced for hydration

Extra water is given by curing

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METHODS OF CURING

a) Water curing

b) Membrane curing

c) Application of heat

d) Other methods

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WATER CURING

a) Immersion : Slab is kept in water tank

b) Ponding: Roof slab is filled with water

c) Spraying : water spary on concrete wall is

d) Wet covering : Wet gunny bags on wall

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MEMBRANE CURING

Concrete surface is covered by plastic membrane

It is used where water availability is less.

Plastic sheet reduces evaporation in concrete

Membrane is applied after 2 days of water curing

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APPLICATION OF HEAT

Spraying of steam on concrete provides heat & moisture.

Higher temperature accelerates hydration rate & strength

of concrete is attended

Early strength of structure is obtained

Steam application possible at precast factory only.

Precast prestressed concrete girders of bridge are cured

with steam

Fast construction of bridge

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HIGH PRESSURE STEAM CURING

Superheated steam at high pressure (8.5 kg/cm2) & high

temperature(175 deg C) is applied on concrete.

This process is called “Autoclaving”

28 days strength of concrete is achieved in one day

Concrete becomes sulphate resistant

Low shrinkage in concrete

Used in production of Cellular concrete products(

Siporex, Celcrete)

FINISHING OF CONCRETE

Finishing is last operation of concrete making.

Finishing of top surface is required in roads, airport

strip, home floor

Methods of Finishing:

a) Form work Finish

b) Surface Treatment

c) Applied Finishes

SHAH K.A. 86

FORMWORK FINISH

Concrete obeys the shape of formwork

Grooves & lining on formwork plate gives

makes grooves & lining on concrete

Prefabricated tiles can be made of any

design

SURFACE TREATMENT

Domestic floor should be smooth, wear resistant, crack

free.

Mix should have good proportion without excess “Matrix”

Exposed aggregate finish : Colored pebbles on top layer

of wall

Bush Hammering : Electrically operated Brush with teeth

when applied on concrete removes top cement layer,

exposes aggregates and makes shining aggregates.

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APPLIED FINISH

Rough cast finish : mixture of cement, sand, round

gravel is applied on wall

Non slip finish: Railway platform & walkway around

pool are given non slippery finish by mixing large size

sand particles in floor concrete.