Building construction & maintanance

7/27/2019 Building construction & maintanance

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Plastic State

Setting State

Hardening State


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Concrete solidifies and hardens after mixingwith water and placement due to chemicalprocess known as Hydration.

The water reacts with the cement, whichbonds the other components together,eventually creating a stone like material.


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Mix Design is the term used to define howthe concrete will be mixed and how it will beplaced to form the structure.

Batching is the process of measuringconcrete mix ingredients either by volume orby mass and introducing them to mixture.Traditionally Batching is done by volume.


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GRADE RATIO CEMENT(in bags) SAND(in cubes) METAL(in cubes)15 1:3:6 13 0.53 0.9220 1:2:4 18 0.50 0.8825 1:1:3 23 0.42 0.8240 1:1:2 31 0.44 0.96


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1400 to 16000C


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Coarse aggregates Fine aggregatesAbove 5mm dia. Below 5mm dia.

E.g-Gravel E.g-Sand


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selection of aggregates is an importantprocess.

consideration when selecting aggregates

gradingdurabilityparticle shape and surface textureunit weights and voids


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Accelerators Retarders

Plasticizers

Water reducing


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Accelerators are added to concrete either toincrease the rate of early strengthdevelopment or to shorten the time ofsetting or both

Usage of Accelerators are depend onTemperature of the mix


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Retarders slow down the hydration of cement.In simple words is decrease the setting timeof the concrete.


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The mixing operation consists of rotation andstirring, the ingredients of the concrete touniform mass.

This uniformity must not be disturbed by theprocess of discharging from the mixer.

There are several types of Concrete Mixersare used to mix concrete.

Some of them are Batch Mixers, Tilting DrumMixers, Pan Type Mixers & ContinuousMixers.


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The efficiency of the Concrete Mixer can bemeasure by the viability variability of thesamples from the mix.

Mixing time depends on the type and size ofthe Mixer and on the speed of rotating.

Capacity of the Mixers are varying from 1Cubic Yard to 10 Cubic Yards.


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If instead of being batched and mixed on site,concrete is delivered for placing from aCentral Plant. Such Central Plants are knownas Batching Plants.

The concrete produced at Batching Plants arenormally referred as Ready Mix Concrete orPre-Mixed Concrete.

Ready mix concrete produce in such BatchingPlants are transported to the site by usingTruck Mixers.


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This type of concrete is used extensively whenconstructing Multi storied structures and offersnumerous advantages in comparison with othermethods of manufacture. Some of the advantages areas follows.

1. Close quality control of batching which reducesthe variability of the desired properties ofhardened concrete.

2. Use on congested sites where there is little spacefor a mixing plant and aggregate stock pile.

3. Use of Truck Mixers to ensure care in

transportation, thus prevent segregation andmaintain workability.

4. Convenience when small quantities of concrete orintermittent placing is required.


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There are TWO major Tests are used tocontrol the quality of the concrete.

They are Slump Test

Cube Tests.

Slump Test is used to control the Workabilityof the concrete.

Cube Test is used to ascertain the requiredstrength of the concrete.


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The operation of placing ensures the requirementof Strength, Impermeability and Durability ofharden concrete in the actual structure.

As far as placing is concerned, the main objective

is to deposit the concrete as close as possible toits final position so that segregation is avoidedand the concrete is fully compacted.

The aim of the good concrete placing can be

stated as, is to get the concrete into position at aspeed and in a condition, that allow it to becompacted properly.


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Once the concrete has been placed , it isready to be compacted.

The purpose of compaction is to get rid ofthe air voids that trapped in loose concrete.

It is important to compact the concrete fullybecause air voids reduce the strength of theconcrete.

Air voids increase concretes permeability.

That in turns reduces its durability. If the concrete is not dense and impermeable,

it will not be watertight.


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Moisture and air are more likely to penetrateto the reinforcement causing it to rust.

Honey Combs may occur due to the airvoids.

To compact concrete you apply energy to itso that mix becomes more fluid. Air trappedin it can then rise to the top and escape.

As a result, the concrete becomeconsolidated and left with a good densematerial


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RODDING: Rodding is a manual techniqueand are only suitable for work of foundationsand columns.

VIBRATION: is the quickest and best methodto compact concreter.

Porker Vibrators are used for this purpose.Porkers with diameter ranging 25 and 75 mmare readily available and these are suitable formost reinforced concrete work.


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External Vibration


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The length of time it takes for a Porkervibrator to compact concrete fully dependson: The WORKABILITY of the concrete: The less

workable the mix, the longer it must be vibrated. Bigger Vibrators do faster job in compaction.

The depth of the concrete: Thick sections of theconcrete take longer for compaction.


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To achieve proper placing and compactionfollowing rules should be kept in mind. The Concrete should placed in uniform layers not in

large heaps or slopping layers.

The rate of Placing and of Compaction should be equal.

On no account should water ever be added to concretein setting.

Each layer should be fully compacted before placing thenext one, and each subsequent layer should be placed

whilst the under lying layer is still plastic so thatmonolithic construction is achieved.

Collision between concrete and formwork orreinforcement should be avoided


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Care needs to be taken to properly CUREconcrete and to achieve best Strength andHardness.

This happens after Concrete has been placed.

Cement requires a moist, controlledenvironment to gain strength and hardenfully.

Cement paste Hardened over a time and in

around 3 weeks , over 90% of the finalstrength is typically reached , though it maycontinue to strengthen for decades.


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