CEMENT CONCRETE

PROPERTIES

High compressive strength

Lower tensile strength

Negligible change when temp changes

Develops cracks over time

COMPOSITION

Cement

Water

AggregatesReinforcement

Admixtures

Requirements of water Used in mixing and curing. Potable water should be used. Sea water is not permitted. pH value of water should not be less

than 6.

Disadvantages of using more water and

advantages of using less water

Disadvantages of using more water

Creates a diluted paste.

Excess water bleeds out onto the surface.

This affects the compressive and tensile strengths.

Loss of cohesiveness and homogeneity.

Harmful to strength and durability.

Advantages of using less water

Increased strength. Lower permeability. Increased

resistance to weathering.

Better bond between concrete and reinforcement.

Reduced shrinkage and cracking.

Less volume change from wetting and drying.

AggregatesFINE AGGREGATE COARSE AGGREGATE

Size more than 4.75mm.

Includes gravels and crushed stones.

Obtained by crushing various types of granites, hard lime stones and sand stones.

Harsh concrete.

Size less than 4.75mm.

Includes natural sand or crushed stone with most particles passing through a sieve.

Obtained from pits, lake, river or sea-shore.

Thoroughly washed before use.

Reinforcement Reinforced concrete is a composite material

 in which concrete's relatively low tensile strength and ductility are counteracted by the inclusion of reinforcement having higher tensile strength and/or ductility.

. Reinforced concrete may also be permanently stressed (in compression), so as to improve the behaviour of the final structure under working loads. In the United States, the most common methods of doing this are known as pre-tensioning and post-tensioning.

Properties of reinforcement

For a strong, ductile and durable construction the reinforcement needs to have the following properties at least:

High relative strength.High toleration of tensile strain.Good bond to the concrete, irrespective of pH, moisture, and similar factors.Thermal compatibility, not causing unacceptable stresses in response to changing temperatures.Durability in the concrete environment, irrespective of corrosion or sustained stress.

CharacteristicsTwo physical characteristics give reinforced concrete its special properties:

The coefficient of thermal expansion of concrete is similar to that of steel, eliminating large internal stresses due to differences in thermal expansion or contraction.

When the cement paste within the concrete hardens, this conforms to the surface details of the steel, permitting any stress to be transmitted efficiently between the different materials. Usually steel bars are roughened or corrugated to further improve the bond or cohesion between the concrete and steel

Common failure modes of steel reinforced concrete:

Mechanical failure

Carbonation

Chlorides

Alkali silica reaction

Conversion of high alumina cement

Sulphates

CARBONATION:

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Cement

Production, Composition &

Types

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Raw Materials in Cement

PRODUCTION OF CEMENT

1. Acquisition of raw materials.2. Preparation of raw materials.

3. Burning.4. Addition of Gypsum and Distribution.

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Compounds in Cement

CEMENT TYPES General Purpose Ordinary Portland Cement Sulphate Resisting Cement High Early Strength Low Heat Cement Air Entraining Cement White Cement

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ORDINARY PORTLAND CEMENT

Variety of artificial cement. On hardening , its color resembles to the

rocks near Portland in England. Main constituents : Lime , Silica ,

Alumina.

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CHEMICAL COMPOSITION OF CEMENT :

Lime63%Silica 22%Alumina 06%Iron Oxide 03%

Gypsum 01-04%

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FUNCTIONS OF CEMENT MANUFACTURING CONSTITUENTS

Lime : Sufficient quantity forms di-calcium silicate and tri-calcium silicate. Silica : Gives strength.

Alumina : Sets cement quickly ; lowers clinkering temperature.

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Iron Oxide : gives color to cement. Magnesia : helps in giving color to cement. Gypsum : added to increase the setting of

the cement.

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SULPHATE RESISTING CEMENT

Modified form of O.P.C. Used when O.P.C. is liable to

disintergrate. Uses : Hydraulic structures , canals.

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HIGH EARLY STRENGTH CEMENT

Provides strength at early stage. Reason : finer grinding , burning at high temperature , increased lime

content. Strength by Rapid Hardening Cement

in 4 days = strength by O.P.C. in 14 days.

Uses : Highway Slabs.Suitable in Cold weather areas.

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LOW HEAT CEMENT

Heat of hydration is reduced by tri-calcium aluminate. Less % of Lime.

Uses : Mass Concrete Works : Damns.

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AIR ENTRAINING CEMENT

Produced by grinding minute air entraining materials with clinker.

Improves workability , durability. Uses : Natural raisins , fats , oils.

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WHITE CEMENT

Keeps the content of Iron Oxide as low as possible.

Costlier than O.P.C. Uses : Architectural finishing in buildings.

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CHARACTERISTICS OF CEMENT

Proportion of sand and cement decides the strength of mortar.

Richer mixture than 1:3 proportion is prone to shrinkage.

When water is added to initiate the cement and sand mixture , hydration

starts and binds sand particle and surface of masonry and concrete.

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ADVANTAGES

Strong.

Creates large structures easily.

Has high thermal mass.

DISADVANTAGES

Subjected to cracking.

Difficult to provide idoneous curing conditions.

Not ideal when settlement is expected.

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ADMIXTURES

CONCRETE MIXTURES:Concrete admixtures are used to improve the behavior of concrete under a variety of conditions.

There are two main types of Admixtures:

1. Chemical admixture2. Mineral admixture

CHEMICAL ADMIXTUES:reduce the cost of construction,

modify properties of hardened concrete,

used to improve the quality of concrete during mixing, transporting, placement and curing.

They fall into the following categories:Air entrainers

Water reducers

Set retarders

Set accelerators

Super plasticizers

Specialty admixtures: which include corrosion inhibitors, shrinkage control, alkali-silica reactivity inhibitors, and coloring.

MINERAL ADMIXTURES:Mineral admixtures affect the nature of the hardened concrete through hydraulic or Pozzolanic activity.

Pozzolans are cementations materials and include natural Pozzolans (such as the volcanic ash used in Roman concrete), fly ash and silica fume.

They can be used with Portland cement, or blended cement either individually or in combinations.

Properties of mineral admixtures:make mixtures more economical,

reduce permeability,

increase strength,

influence other concrete properties

CONCLUSIONS:Plasticizers, super plasticizers , & Air-entraining admixtures is mostly used.

Super plasticizers is widely used all over the world .

India is catching up with the use of super plasticizer in the construction of high rise buildings, long span brides because it reduced cement content, increased workability, & increased strength.