Presented By:Presented By:

Engineer LATIF HYDER WADHOEngineer LATIF HYDER WADHO

Mehran University College of Engg: & Technology Khairpur Mehran University College of Engg: & Technology Khairpur

Civil Engineering MaterialsCivil Engineering Materials

11stst Term 1 Term 1stst Year B.E.Civil Year B.E.Civil

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CONCRETE

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It is a mixture of binding material, fine aggregates, coarse aggregates and water in suitable proportions.Cement and lime are generally used as binding materials where as sand, surkhi, etc are used as fine aggregates and crushed stones, gravel, broken pieces of bricks, etc and water free from organic impurities are used as coarse aggregates in the manufacturing of different types of concrete.

The required strength of concrete can be obtained by careful selection of its ingredients and their proper grading, accurate water measurements and adopting a good work man ship, proper mixing, finishing and curing of concrete during the construction work.

Concrete is a brittle material, it is strong in compression but weak in tension, so to over come that weakness steel bars are used in the R.C.C construction.

MANUFACTURING OF CONCRETE:

Following operations are mainly involved during manufacturing of Concrete.

Mixing:

For preparing concrete well washed fine aggregates, coarse aggregates and cement or lime are spread on pucca platform and then it is mixed thoroughly till uniformity is achieved. Machine mixing can be done, if the magnitude of work so demands.

Only that much concrete should be mixed which can be laid within two hours. If cement has been used in the mix then the concrete should be laid in position within 30 minutes after water has been added to it.

Placing:Laying and construction of concrete shall be paced within three hours of adding water in case of concrete prepared with lime mortar and within one hour in case lime cement mortar has been used.Concrete should be used in layers not exceeding 15 cm in thickness when consolidated. Next layer of concrete is laid only when the previous one has been thoroughly consolidated and it should not be dropped from heights as it would result in segregation of aggregates. If it is to be laid at a depth then chutes should be used.

Curing:

Concrete shall be well cured for a period of at least 07 days after its laying. Curing shall be done by spreading wet sand or gunny bags etc, and watering frequently.

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TYPES

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CONCRETE

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Different types of concrete are used depending upon requirements in different types of engineering works. Some well-known types of concrete are described as under:

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(1) Cement Concrete:

This is the plain concrete prepared by using Portland cement as the binding material and the remaining ingredients, as usual, are the fine aggregates-sand, coarse aggregates-broken stone, gravel and water. It is a concrete of high strength. For common constructional works, the ratio of cement. Fine aggregates, coarse aggregates by volume are 1:2:4 or 1:3:6. For a very high strength this ratio may be 1:11/2 : 3 and for a very ordinary work not requiring much of strength it may be 1:4:8.

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(2) Lime Concrete:This is another plain concrete which is prepared by using lime as the binding material and the remaining ingredients such as fine aggregates may be sand or surkhi, coarse aggregates may be broken bricks, broken stone or gravel and water. It is a cheap variety of concrete.

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(3) Reinforced Cement Concrete (R.C.C):

This is the cement concrete further reinforced by steel commonly known as R.C.C. The reinforced cement concrete has several advantages over plain concrete and structural steel.

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(4) Pre-stressed concrete:

It is that type of concrete in which steel as well concrete of very high strength are used, thus it can sustain excessively high loads.

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Except above mentioned Concrete types, there are some other types of concrete such as:

(5) Precast Concrete,(6) Mass Concrete,(7) Light Weight Concrete, (light

weight aggregates are used)(8) Non-fines Concrete (without

coarse aggregates)(9) Coloured Concrete.

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Uses and advantages of Concrete:Concrete as a constructional material has the following advantages:(1) Concrete is economically used in the long run as compared to other building materials.(2) Concrete possesses a high compressive strength and also resist well to weathering effects. When it is properly prepared its strength is equal to that of a hard natural stone.

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(3) Green concrete (fresh concrete) can be easily handed and moulded in any size and shape according to demand/specifications. The form work can be reused a number of times for similar jobs, resulting in economy.

(4) It is strong in compression and has unlimited structural applications in combination with reinforcement.

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(5) The concrete is strengthly used in the construction of foundations, walls, beams, lintels, roofs of buildings, air fields, water retaining structures, dams, bridges, etc.(6) It is durable and fire resistant and requires very little maintenance.(7) Concrete can be evenly spread on and filled in fine cracks, fissures for repairing the components of structures by grouting process.

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PROPERTIES OF CONCRETE IN PLASTIC STATE:

A freshly mixed concrete must possess the under mentioned properties.

(i) Good workability:

Workability of concrete can be defined as the ease with which it can be mixed, transported and placed in position in a homogeneous state.

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(ii) No segregation:

Segregation may be defined as the separation of coarser materials from other mass, if the quantity of water in the mix is more, the larger sized aggregates will be separated thus casing segregation, so concrete should be free from segregation.

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(iii) No bleeding:

Bleeding may be defined as separation of water or water sand cement from freshly mixed concrete. This is caused due to excess water content in concrete.

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PROPERTIES OF CONCRETE IN HARDENED STATE:A hardened concrete must possess the under mentioned properties:

(i) Adequate Strength:Strength is defined as the resistance of a hardened concrete to rupture under different loadings and is accordingly designated in different ways, i.e. tensile strength, compressive strength, flexural strength, etc. A good quality concrete in hardened state must possess the desired crushing strength.

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(ii) Durability:

Durability is defined as the period of time up to which concrete in hardened state withstands the weathering effects satisfactorily. This property is mainly affected by water cement ratio . A good quality concrete in hardened state must be durable.

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(iii) Impermeability:

The impermeability of hardened concrete may be defined as the property to resist entry of water. This property is achieved by using extra quantity of cement in concrete mix. A concrete in hardened state must be impermeable.

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(iv) Elasticity:

As hardened concrete is brittle material, it is desired that it should possess adequate elasticity.

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(v) Low shrinkage:

A hardened concrete should experience least shrinkage. This property is guided by water cement ratio. Shrinkage is less if the water cement ratio is less.

(vi) Least thermal expansion:

A hardened concrete should possess least coefficient of expansion.

Developed by:Nadeem Asghar 29

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