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Sub-CONCRETE TECHNOLOGY

Topic-Production Of Concrete

Sem-6^th

By

ER.S.DAS/Er. Sasmita Gochhayat

HOD,CIVIL ENGINEERING

AY:2021-2022

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CIVIL ENGINEERING

PRODUCTION OF CONCRETE

Stages for production of concrete

• Batching
• Mixing�Transporting
• Placing
• Compacting
• Curing�Finishing

BATCHING OF MATERIALS

• A proper and accurate measurement of all the materials used in the production of concrete is called BATCHING .
• The factors affecting the choice of required batching system are
• Size of job
• Required production rate and
• Standards of batching performance
• Batching is of three types .
• Manual batching�Semi-automatic batching
• Automatic batching

MIXING �OF CONCRETE MATERIALS

• The process of creating a coating on the surface of all aggregates with cement paste and to blend all the ingredients into a uniform mass is termed as MIXING OF CONCRETE .
• Mixing process is of two types .
1. Hand mixing
2. Machine mixing
• Types of Machine mixing
• Tilting type mixer
• Non- tilting type
• Pan or stirring mixer

TRANSPORT OF CONCRETE

• The process of taking the concrete to the place of worksite as rapidly as possible without segregation and loss of ingredients is TRANSPORTATION OF CONCRETE .
• The process of transportation varies according to the size of the job . The principal methods of transporting concrete are as follows.
1. Barrows
1. Wheel barrows and handcarts�Power barrows or powered buggies or dumpers
2. Tippers and lorries
3. Truck mixers and agitator lorries
4. Dump buckets
5. The monorail system or trolley or rails

PLACING �OF CONCRETE

• The methods used in placing concrete in its final position have an important effect on its homogeneity , density and behaviour in service .
• Concrete should be placed in its final position rapidly so that it is not too stiff to work .
• When placing concrete , care should be taken to drop the concrete vertically and from not too height .
• While concreting in walls , footings and other thin sections of appreciable height , the concrete should be placed in horizontal layers not less than 150 mm in depth.

COMPACTION �OF �CONCRETE

• The process of removing the quantity of entrapped air during transportation of concrete is known as COMPACTION .
• The presence of even 5% of voids in hardened concrete left due to incomplete compaction may result in a decrease in compressive strength by about 35% .
• Methods of compaction
1. Hand rodding
2. Mechanical vibrations
3. Centrifugation or spinning
4. High pressure and shock

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CURING�OF CONCRETE

• The process of creation of an environment during a relatively short period of time immediately after the placing and compaction of concrete , favourable to the setting and hardening of concrete is termed as CURING .
• Methods of curing concrete can be broadly classified as
• The methods which replenish partly the loss of water by interposing a source of water�The methods preventing or minimizing the loss of water by interposing an impermeable medium between the concrete and the surrounding environment

FORMWORK

• The FORMWORK or SHUTTERING may be defined as moulds of timber or some other material into which the freshly mixed concrete is poured at the site and which hold the concrete till it sets .
• A good formwork should be strong , stiff , smooth and leak proof .
• The formwork includes the total system of support of freshly placed concrete , i.e., form lining and sheathing plus all necessary supporting members ,bracings , hardware and fasteners .
• Types of formwork .
1. Timber formwork
2. Plywood formwork
3. Steel formwork

COMPACTION OF CONCRETE

• INTRODUCTION
• PURPOSE OF COMPACTION�IMPORTANCE OF COMPACTION
• METHODS OF COMPACTION�CONSEQUENCES OF IMPROPER COMPACTION
• DEFECTS OF UNDER VIBRATION
• DEFECTS FROM OVER VIBRATION

INTRODUCTION

• During the manufacture of concrete a considerable quantity of air is entrapped and during its transportation there is a possibility of partial segregation taking place .If the entrapped air is not removed and the segregation of coarse aggregate not corrected , concrete may be porous , non-homogenous and of reduced strength .
• The process of removal of entrapped air and of uniform placement of concrete to form a homogeneous dense mass is termed as COMPACTION .

PURPOSE OF COMPACTION

• To remove entrapped air bubbles in concrete
• To achieve high density
• To improve strength and durability
• To eliminate honeycomb and other defects

IMPORTANCE OF COMPACTION

• Air voids increase concrete’s permeability . That in turn reduces it’s durability.
• If the concrete is not dense and impermeable, it will not be watertight. It will be less able to withstand aggressive liquids.
• Moisture and air are more likely to penetrate to the reinforcment causing it to rust .
• Proper compaction also ensures that the formwork is completely filled i.e. there are no pockets of honeycombed material and that the required finish is obtained on vertical surfaces

METHODS OF COMPACTION

The compaction of the concrete can be achieved in four ways .

1. Hand rodding
2. Mechanical vibrations
3. Centrifugation or spinning and
4. High pressure and shock

1) Hand Rodding

• Rodding is the process of ramming the concrete manually with a heavy flat faced tool .
• The rodding action is effective for a depth of concrete equal to five times the maximum size of aggregate .
• The compaction should continue until the cement mortar spreads on the surface of concrete .
• The main disadvantage of rodding is that it produces large pressure on the formwork .

2) Mechanical Vibrations

• Vibration is the commonly used method of compaction of concrete ,which reduces the internal friction between the different particles of concrete by imparting oscillations to the particles .
• The mechanical vibrations can be imparted by means of vibrators which are operated with the help of an electric motor or diesel engine or pneumatic pressure .

Types of vibrators

• Since concrete contains particles of varying sizes ,the most satisfactory compaction would perhaps be obtained by using vibrators with different speeds of vibration .
• The various types of vibrators are:
1. Immersion or Needle vibrators
2. External or Shutter vibrators�Surface vibrators
3. Vibrating table

2.1) Immersion or Needle vibrator

• Most commonly used vibrator
• Consists of steel tube with on end closed and rounded having and eccentric vibrating element inside it .
• Steel tube is called poker is connected to an electric motor or diesel engine through a flexible tube.
• Available sizes varying from : 40 to 100 mm dia�Frequency of vibration : upto 15000 rpm�Period of vibration : 30 seconds to 2 minutes

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2.2) External or Shutter vibrator

• These vibrators called form vibrators are clamped rigidly to the formwork at the predetermined points so that both the form and concrete are vibrated .
• Consume more power for a given compaction .
• Vibrators can compact upto 450 mm.
• Frequency of 3000 rpm to 9000 rpm

2.3) Surface vibrator

• Surface vibrators are placed directly on the concrete mass.
• These are best suited for the compaction of shallow elements and should not be used when the depth of concrete to be vibrated is more than 250 mm .
• These are used for compacting plain concrete or one –way reinforced concrete floors and road pavement .
• Frequency of this vibrator : 4000 rpm

2.4) Vibrating table

• The vibrating table consists of a rigidly built steel platform mounted on flexible springs and is driven by electric motor.
• Frequency of the vibration : 4000 rpm�The springs are so designed that they cause resonance .
• All vibrators should produce oscillations which are perfectly synchronised.
• Vibrating tables are most efficiently used in factories for testing specimens .

3) Centrifugation or Spinning

• This method is used in the production of elements which are circular in cross-section , such as concrete pipes , concrete lamp posts, etc .
• It comprises feeding the concrete into the horizontal mould spinning at a low speed .
• The centrifugation results in a water- tight product and hence is used in the manufacture of both pressure pipes for water supply and non-pressure pipes for sewage disposal and storm water drains .

4) High pressure and Shock

• This method comprises applying external pressure from the top and vibration from below the mould .
• The vibration tables can be used for this purpose .
• The excess water added during the mixing of forced out due to large pressure.
• The water-cement ratio at the end of vibropressing can be reduced to a value as low as 0.30 .

CONSEQUENCES OF IMPROPER COMPACTION

There are various problems and defects that could arise when concrete is not vibrated adequately .

• Honeycomb
• Excessive entrapped air voids�Sand streaks
• Cold joints
• Placement lines�Subsidence cracking

DEFECTS OF UNDER VIBRATION

• PLACEMENT LINES : These are dark lines between adjacent layers of concrete batches . It occurs when vibrators did not penetrate through under lying layers .
• EXCESSIVE ENTRAPPED AIR VOIDS : Under vibration leaves a lot of entrapped air in concrete which reduces strength.
• SAND STREAKS : When heavy bleeding washes mortar than a harsh mixture lefts behind that lacks workability. It is also caused by insufficient fine aggregates .

DEFECTS FROM OVER VIBRATION

• SEGREGATION : Heavier aggregates settle while lighter aggregates rise
• BLEEDING : Water comes out at surface due to excessive vibrations
• FORM DAMAGES : Over vibration may damage the formwork .

UNDER VIBRATION IS MORE OFTEN PROBLEM THAN OVER VIBRATION .