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CHAPTER 2

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RESIDENTIAL BUILDINGS

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COMMERCIAL BUILDINGS

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INDUSTRIAL BUILDINGS

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INTRODUCTION

Concrete is most widely used as construction material.

Obtained by mixing cement, water + aggregates(sometimes admixtures in required proportions)

These mixture then placed in forms and allowed to cureand becomes hard like stone.

The hardening is caused by chemical reaction betweenwater and cement and its continues for a long time, andconsequently the concrete grows stronger with age.

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INTRODUCTION (cont.)

The strength, durability and other characteristics ofconcrete depend:o

Properties of its ingredientso Properties of the mix

o Method of compaction

o Other controls during placing, compaction and curing

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CLASSIFICATION OF CONCRETE

It is usual to specify a particular concrete by the proportion (byweight) of the constituents and their characteristics.

Example, 1:2:4 concrete refers to particular concrete mixed of

cement, sand and coarse aggregate in a ratio 1:2:4 (with aspecified type of cement, water cement ratio & max. size ofcoarse aggregate).

Concrete can be classified either:

o Prescribed mix concrete oro Designed mix concrete

-Depending on the specifying requirements of the desirableproperties of concrete (strength, workability, durability andpermeability.

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CLASSIFICATION OF CONCRETE (cont.)

Prescribed mix concrete:o Suitable for small jobs or use initially before designed mixes

have been established

o Cube strength test is not required by specification but may

needed for other purposes

Designed mix concreteo Used for economy of material content

o Must be sufficient similar concrete batches to set up testingregime based on statistical interpretation of results.

o Cubes will normally be tested at 28 days

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PROPERTIES OF CONCRETE

Concrete has to satisfy performance requirements in theplastic state and also hardened state.

Plastic state concrete should be workable and free from

segregation and bleeding. Segregation

Is the separation of coarse aggregate

Bleeding

Is the separation of cement paste from the main mass

Hardened state concrete should be strong, durable andimpermeable, & have min. dimensional changes.

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GRADES OF CONCRETE

Concrete generally graded according to its compressivestrength at 28 days.

Various grades of concrete are grouped into 9 categories as

stipulated in codes of practice BS 8110 best known basedon their characteristics strength in N/mm2

Table below shows the tabulation of concrete grade based

on BS 8110.

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GRADES OF CONCRETE (cont.)

GradeCharacteristic

Strength(N/mm2 )

Lowest grade suitable for specific purposes

7

10

7.0

10.0 Mass concrete

15 15.0 Reinforced concrete using Light weight aggregate

2025

20.025.0

Reinforced concrete using Heavy weight aggregate

30 30.0 Prestressed Post-tensioned concrete

405060

40.050.060.0

Prestressed Pre-tensioned concrete

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GRADES OF CONCRETE (cont.)

Concrete of grades 7 & 10 suitable for lean concrete basesand for mass concrete and these need not be designed.

Concrete of grade lower than grade 15 - not suitable for

reinforced concrete works.

Concrete of grade lower than grade 30 not to be used inthe prestressed concrete works.

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ADVANTAGES OF CONCRETE

Economical in long run as compared to other engineeringmaterials. Except cement, it can be made from locallyavailable coarse and fine aggregates.

Concrete possesses a high compressive strength andminimal of corrosive and weathering effects. Whenproperly prepared, its strength = a hard natural stone.

The fresh concrete can be easily handled and moulded intoany shape or size according to specifications. The formworkcan be re-used a number of times for similar jobs resultingin economy.

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ADVANTAGES OF CONCRETE (cont.)

Strong in compression and has unlimited structural applications incombination with steel reinforcement. The concrete and steel haveapproximately equal coefficients of thermal expansion. The concrete isextensively used in the construction of foundations, walls, roads,airfields, buildings, water retaining structures, docks and harbours,

dams, bridges, bunkers and silo etc.

Concrete can be sprayed on and filled into fine cracks for repairs by theguniting process.

Concrete is durable and fire resistant and requires very littlemaintenance.

Concrete can be pumped and hence it can be laid in the difficultpositions also.

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DISADVANTAGES OF CONCRETE

Concrete has low tensile strength and hence cracks easily.Therefore, concrete is to be reinforced with steel bars ormeshes.

Fresh concrete shrinks on drying and hardened concreteexpands on wetting. Provision for contraction joints has tobe made to avoid the development of cracks due to dryingshrinkage and moisture movement.

Concrete under sustained loading undergoes creepresulting in the reduction of prestress in the perestressedconcrete construction.

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DISADVANTAGES OF CONCRETE (cont.)

Concrete expands and contracts with changes in temperature.Hence, expansion joints have to provided to avoid the formationof cracks due to thermal movement.

Concrete is not entirely impervious to moisture and containssoluble salts cause efflorescence.

Concrete is liable to disintegrate by alkali and sulphate attack.

The lack of ductility inherent in concrete as a material isdisadvantageous with respect to earthquake resistant design.

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