Reinforced Concrete - Trent GlobalIt facilitates mixing, placing and compacting of the fresh concrete. Used for washing the aggregates. For curing purposes. Amount of water content

Reinforced Concrete

Dr S.Sreenath Phd, MSc (Engg), MSc (ICM), MBA

Concrete

Concrete is mainly composed of three materials:

Cement

Water

Aggregate

Admixture (Sometimes added)

Title of presentation

Concrete: Advantages

Good compressive strength

Protection of embedded steel

Flexibility of application


Concrete: Disadvantages

Low tensile strength

Rather high density

Prone to chemical attack


Cement

Powder form

Mix with water, a chemical reaction takes place.

Produces a very hard and strong binding medium

for the aggregate particles.


Hydration of cement

When cement comes in contact with water, the

water molecules react with individual compounds.

By time produce a firm mass which give cement its

bonding properties, strength and hardness.

This process is termed hydration.


Types of cement

Ordinary Portland Cement (OPC)

Rapid – hardening Portland Cement (RHPC)

Low-heat Portland Cement (LHPC)

Sulfate – resisting Portland Cement (SRPC)

Portland Blastfurnance Cement (PBFC)

Waterproof and Water Repellant Cement


Cement Storage and Preparation

Kept dry at all times.

Shed should have water proofed roof and walls.

Raised floor. By elevating on bricks or blocks.

Concrete batching plants, cement is stored in silos.


Aggregates

Aggregates are inert particles bound together by

the cement paste to form mortar.

The performance of concrete depends largely on

the physical, thermal and chemical properties of

its aggregates.

Also called as filler materials as 60 to 80% of the

volume of concrete is made up of aggregates.


Classification of Aggregates

Heavyweight aggregate

Normal aggregate

Lightweight aggregate


Functions of Aggregates

Reduce costs

Modify properties of concrete

Reduce dry shrinkage


Water in Concrete

React with cement and cause it to set and harden.

It facilitates mixing, placing and compacting of the

fresh concrete.

Used for washing the aggregates.

For curing purposes.

Amount of water content is very important to decide

the properties of fresh and hardened concrete.

Tap water is acceptable for mixing water.


Admixtures

To improve the properties of fresh or hardened

concrete.

Make changes in the properties of hydration,

liberation of heat, formation of pores and the

development of the gel structure in concrete.

To increase workability without increasing water

content or to decrease the water content at the

same workability.

To retard or accelerate both initial and final setting

times.


Concrete Admixtures usuage

To improve the workability of the fresh concrete.

To reduce water content for a given workability

thereby increasing the strength.

To increase durability of hardened concrete.

To retard setting or to increase it.

To impart colour to concrete.

To maintain volume stability by reducing or

offsetting shrinkage during concreting.

To increase concrete resistance to freezing and

thawing.


Different types of Admixtures

Air-entraining agents

Accelerating agents

Retarders

Water reducers or plasticizers

Super plasticizers

Bonding admixtures

Water repelling agents


Composite Action of Reinforced

Concrete Concrete does not resist tensile forces.

Reinforcement is designed to carry these tensile

forces, which are transferred by bond between the

interface of the two materials.

For adequate bonding, some reinforcement bars

are ribbed or twisted.

Proper concrete mixes provide adequate

impermeability of the concrete against water

intrusion and bar corrosion.

Close values of thermal efficient coefficient of steel

and concrete.


Stress – Strain Relations

Load applied, strain increases linearly with stress.

Concrete behaves as an elastic material.

After further loading, the curve is no longer linear

and the concrete behaves like plastic material.

In the plastic range, if the load is removed, the

deformation would not recover.


Stress Strain Curve for Concrete in

compression


Compressive strength of Concrete

Grade of Concrete

Minimum compressive strength

N/mm2 at 7 days

Specified characteristic compressive strength (N/mm2) at 28 days

M15 10 15

M20 13.5 20

M25 17 25

M30 20 30

M35 23.5 35

M40 27 40

M45 30 45


Compressive strength of Concrete

Age Strength percent

1 day 16%

3 days 40%

7 days 65%

14 days 90%

28 days 99%


Compressive strength of concrete


Applications of Reinforced Concrete

An important construction material.

Versatility, adaptability, resistance to fire and

corrosion result in negligence maintenance costs.

Better quality cement resulted in stronger and

more durable concrete.

Ideal for column, beam, slab construction of

residential and commercial structures.

The durability of concrete is influenced by the

exposure conditions, the concrete quality, the cover

to the reinforcement and the width of cracks.


Revision - Hydration

The reaction with water is termed "hydration". This

involves many different reactions, often occurring at

the same time. As the reactions proceed, the

products of the hydration process gradually bond

together the individual sand and gravel particles,

and other components of the concrete, to form a

solid mass.

Title of presentation 23 of 20

Revision - Hydration

The main product involved in the binding of cement

and water is heat, which is released when the

concrete is being hardened. This process is known

as the heat of hydration. Heat of hydration can

be managed while designing and pouring concrete,

during the process of curing and hardening.

However, if designers do not allow for the heat, it

can cause serious issues with cracking and can

also disturb the structural integrity of the concrete.


Temperature rise of mass concrete under adiabatic condition

(Adapted from ACI 2007.2R-07)


Setting time of Cement

Initial setting time is that time period between the

time water is added to cement and time at which 1

mm square section needle fails to penetrate the

cement paste, placed in the Vicat’s mould 5 mm to

7 mm from the bottom of the mould.

Final setting time is that time period between the

time water is added to cement and the time at

which 1 mm needle makes an impression on the

paste in the mould but 5 mm attachment does not

make any impression.


Vicat Apparatus


Setting Time


Workability

Workability is the ability of a fresh (plastic) concrete

mix to fill the form/mold properly with the desired

work (vibration) and without reducing the concrete's

quality.

Workability depends on water content, aggregate

(shape and size distribution), cementitious content

and age (level of hydration) and can be modified by

adding chemical admixtures, like superplasticizer.

Raising the water content or adding chemical

admixtures increases concrete workability.


https://en.wikipedia.org/wiki/Hydration_reaction

Concrete Slump Test

The test measures consistency of concrete in that

specific batch.

It is performed to check consistency of freshly

made concrete.

Consistency refers to the ease with which concrete

flows.

It is used to indicate degree of wetness.

Consistency affects workability of concrete.


Concrete Slump Test

Metal mould, in the shape of the frustrum of a cone, open at

both ends, and provided with the handle, top internal

diameter 4 in (100 mm), and bottom internal diameter 8 in

(200 mm) with a height of 1 ft (300 mm). A 2 ft (600 mm)

long bullet nosed metal rod, 5/8 in (16 mm) in diameter.

The test is carried out using a mould known as a slump

cone. The cone is placed on a hard non-absorbent surface.

This cone is filled with fresh concrete in three stages. Each

time, each layer is tamped 25 times with a rod of standard

dimensions. At the end of the third stage, concrete is struck

off flush to the top of the mould. The mould is carefully lifted

vertically upwards with twisting motion, so as not to disturb

the concrete cone.



Specification of concrete

Concrete strength is identified by grade.

Grade 25 has a characteristic cube crushing

strength of 25 N/sq mm after 28 days of curing.

Normal concrete grades are 30,35 and 40.

The concrete mix is classified as designed or

prescribed.

Designed mix – Contractor responsible for the mix

proportion to achieve the required strength and

workability.

Prescribed mix – The C&S Consultant specifies the

mix proportions.


Design Mix



Batching and Mixing of Concrete

Batching refers to the process of measuring the

correct amounts of the constituent materials.

Done either by weight or volume. But weight

recommended to be more accurate.

Measurement equipment regularly calibrated.


Batching of cement

Cement is batched either by bag or from a silo.

Bulk cement is cheaper than bagged cement.

Bulk cement, changes in mix proportions are made

more easily.


Advantages of using bulk cement

Bulk cement is cheaper than bagged cement.

A gang of men are not needed to off-load the

cement.

The trouble of ensuring adequate protection of the

cement during storage is avoided.

The mixer can always be used to capacity without

the risk of errors which may result from splitting

bags.

Changes in mix proportions are made more easily.


Batching of Aggregate

Weigh buckets with balanced mechanical lever and

load cell systems are widely used.

Allowance to be made when the aggregate is moist.


Batching of Water

Water can be measured by weight or volume.

Allowance are made at the time of batching by

deducting the water in the aggregate.


Batching of Admixtures

Follow the recommendation of the manufacturer.

Dosage to finalize based on design mix.

Use of reliable and accurate dispersing equipment

is important.


Mixing of Concrete

The objective of mixing is to coat the surfaces of all

aggregates with cement paste and to blend all the

ingredients of concrete to give a uniform

consistency.

Mixing may be performed by machine or by hand.

Machine mixing is generally more economical and

convenient.


Mixing Time

Mixing should continue until the concrete is uniform

in colour and consistency.

Concrete are discharged from hopper into the mixer

truck.

Mixing time depends on manufacturer’s

specification and the plant’s performance.


Transporting of Concrete

Transport must be rapid to ensure concrete does

not dry out or lose its workability during the time it

elapses between mixing and placing.

Segregation must be reduced to a minimum in

order to avoid non-uniform concrete.

The transport should be organized.

No additional water should be added to the

concrete.


Time Limit

In the case of ready mixed concrete, the usual

specified requirement is that concrete shall be

discharged from a truck mixer within two hours after

the time of loading.


Methods of Transportation

Truck mixers

Wheel barrows

Dumpers

Steel skips and buckets

Cranes

Concrete pumps and pipe lines


Placing of Concrete

Concrete shall be placed in a proper manner to

yield optimum result.


Reinforcement and Formwork

Is the formwork erected properly?

Is the formwork correctly aligned and levelled?

Can the formwork be struck without damaging the

concrete?

Has the release agent be applied? Is it the right

one?

Has the reinforcement the right cover? Are there

enough spacers?

Are the forms clean and free from rubbish or odd

bits of timber or metal?


Concreting Time

The interval between adding the cement to the

aggregate and the completion of the placing of the

concrete should not exceed 30 minutes.

When the mix is transported by purpose made

agitators, operating continuously, such interval may

be extended to a maximum of 2 hours and the

placing of concrete should not exceed 20 minutes.

When retrading admixtures are used, such intervals

may be further extended.


Earth Foundation

Before placing concrete in the foundation, all loose

earth must be removed from the bed.

Any roots of trees passing through the foundation

should be cut to prevent further growth which may

pierce the concrete at a later date.

The surface of the earth, if dry, should be made

moist, so that earth does not absorb water from the

concrete.

However foundation is wet and rain soaked, the

water and slush should be removed completely.


Placing

Careless placing of concrete may cause movement

and damage of the formwork.

Concrete to be deposited as near as practicable to

its final position.

Concrete should not be deposited in a large

quantity at any point.

Concrete should be deposited in horizontal layers.

Each layer should be compacted throughly before

the succeeding layer is placed.

Concrete should be carried out continuously.


Compaction of Concrete

The objective of compaction is to remove the

unwanted entrapped air.

The amount of entrapped air is related to the

workability.

Effective way of compacting is by use of vibration.

Vibration by internal or external vibrator.

Over vibration will cause segregation.


Finishing of Concrete

Common methods:

1. Screeding and levelling.

2. Brooming, Floating and Steel trowelling

Screeding and levelling when the concrete is still in

its plastic stage. Screeding is the process of cutting

off excess concrete to bring the top surface of a

slab to proper grade.

Levelling is to estimate high and low spots and to

embed large aggregate particles to produce desired

finish.


Finishing concrete

Floating can be done with wood or metal hand float

or with a finishing machine using float blades.

Floating is done to embed aggregates particles just

beneath the surface.

Floating is done to remove slight imperfections ,

humps and voids.

Floating is done to compact the mortar at the

surface.


Thank you


Documents

Reinforced Concrete - Trent GlobalIt facilitates mixing, placing and compacting of the fresh concrete. Used for washing the aggregates. For curing purposes. Amount of water content