fresh concrete - copy

8/7/2019 Fresh Concrete - Copy

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INDEX

SR.NO. TOPIC PAGE NO.

1 INTRODUCTION 42 WORKABILITY 5

3 SEGREGATION 8

4 BLEEDING 9

5 SETTING TIME 11


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INTRODUCTION

Fresh concrete:

Fresh concrete or plastic concrete is a freshly mixed material which can bemoulded into any shape. The relative quantities of cement, sand, aggregates

and water mixed together, control the properties of concrete in waste state as

well as in a hardened state.

The proportion of water plays an important role in making the state of theconcrete. Thus it is utmost necessary to maintain the water/cement ratio in the

concrete for its good workability.

Fresh concrete contains about 30% voids in its state.

Fresh concrete should have the property, which while compacting can easilyflow and move to attain the desired shape. Thus it should have proper

compatibility and mobility.


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Workability of concrete

The workability of concrete can be defined as the consistency of concretesuch that the concrete can be transported, placed and finished easily andwithout segregation.

A concrete which has high consistency and which is more mobile, need be ofright workability for a particular job. Thus every job requires particular

workability. A concrete which is considered workable for mass concrete

foundation is not workable for concrete used in roof construction. A concrete

considered workable when used in thick section is not workable when

required to be used in thin sections.

Therefore the word workability assumes full significance of the type of work,thickness of sections, extent of reinforcement and mode of compaction.

Factors affecting workability:

y Water contenty Amount and type of aggregatey Aggregate cement ratioy Weather conditionsy Use of admixturesy Sand to aggregate ratio


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I. Water content or Water Cement Ratio

More the water cement ratio more will be workability of concrete. Since by

simply adding water the inter particle lubrication is increased.

High water content results in a higher fluidity and greater workability. Increased

water content also results in bleeding. Another effect of increased water contentcan also be that cement slurry will escape through joints of formwork.

ii. Amount and type of Aggregate

More the amount of aggregate less will be workability.

Using smooth and round aggregate increases the workability. Workabilityreduces if angular and rough aggregate is used.

Greater size of Aggregate- less water is required to lubricate it, the extrawater is available for workability

Angular aggregates increases flakiness or elongation thus reducesworkability. Round smooth aggregates require less water and less

lubrication and greater workability in a given w/c ratio

Porous aggregates require more water compared to non absorbentaggregates for achieving same degree of workability.

iii. Aggregate Cement ratio

More ratios, less workability. Since less cement mean less water, so the paste isstiff.

iv. Weather Conditions

1. Temperature

If temperature is high, evaporation increases, thus workability decreases.

2. Wind:

If wind is moving with greater velocity, the rate of evaporation also increasereduces the amount of water and ultimately reducing workability.


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v. Admixtures

Chemical admixtures can be used to increase workability.

Use of air entraining agent produces air bubbles which acts as a sort of ball

bearing between particles and increases mobility, workability and decreases

bleeding, segregation. The use of fine pozzolanic materials also has betterlubricating effect and more workability

vi. Sand to Aggregate ratio

If the amount of sand is more the workability will reduce because sand has more

surface area and more contact area causing more resistance.

Measurement of workability

Some of the test measure the parameters very close to workability and providesatisfactory results and information.

The following tests are commonly employed to measure workability:

y Slump testy Flow testy Compacting testy Vee bee consistometer testy Kelly ball test


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Slump test

Slump test is used to determine the workability of fresh concrete. Slump test as

per IS: 1199 1959 is followed. The apparatus used for doing slump test are

Slump cone and Tamping rod.

Procedure to determine workability of fresh concrete by slump test.

i) The internal surface of the mould is thoroughly cleaned and applied with a

light coat of oil.

ii) The mould is placed on a smooth, horizontal, rigid and nonabsorbent surface.

iii) The mould is then filled in four layers with freshly mixed concrete, each

approximately to one-fourth of the height of the mould.

iv) Each layer is tamped 25 times by the rounded end of the tamping rod (strokes

are distributed evenly over the cross section).v) After the top layer is rodded, the concrete is struck off the level with a trowel.

vi) The mould is removed from the concrete immediately by raising it slowly in

the vertical direction.

vii) The difference in level between the height of the mould and that of the

highest point of the subsided concrete is measured.

viii) This difference in height in mm is the slump of the concrete.


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COMPACTING FACTOR

Compacting factor of fresh concrete is done to determine the workability of fresh

concrete by compacting factor test as per IS: 1199 1959. The apparatus used is

Compacting factor apparatus.

Procedure to determine workability of fresh concrete by compacting factortest.

i) The sample of concrete is placed in the upper hopper up to the brim.

ii) The trap-door is opened so that the concrete falls into the lower hopper.

iii) The trap-door of the lower hopper is opened and the concrete is allowed to

fall into the cylinder.

iv) The excess concrete remaining above the top level of the cylinder is then cut

off with the help of plane blades.

v) The concrete in the cylinder is weighed. This is known as weight of partiallycompacted concrete.

vi) The cylinder is filled with a fresh sample of concrete and vibrated to obtain

full compaction. The concrete in the cylinder is weighed again. This weight is

known as the weight of fully compacted concrete.

Compacting factor = (Weight of partially compacted concrete)/(Weight of

fully compacted concrete)


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SEGREGATION

Segregation can be defined as the separation of the constituent material ofconcrete. A good concrete is one in which all the ingredients are properly

distributed to make a homogenous mixture.

If a sample of concrete exhibits a tendency for separation of say, coarseaggregates from rest of ingredients, then the sample is said to be showing the

tendency of segregation.

Such concrete is not only good to be weak; lack of homogeneity is also goingto induce all undesirable properties in the hardened concrete.

Segregation may be of three types;y The coarse aggregate separating out of or setting down from rest of thematrix.y The paste or matrix separating away from coarse aggregate.y Water separating out from the rest of the material being a material of the

lowest specific gravity.

A well made concrete, taking into consideration various parameters such asgrading, size, shape, texture of the aggregates with optimum quantity of water

makes a cohesive mix. Such concrete will not exhibit a tendency of

segregation.

The conditions favourable for segregation are, as can be seen from the thebadly proportioned mix where sufficient matrix is not there to bind and

contain the aggregate. Insufficiently mixed concrete with excess water content

shows a higher tendency for segregation.


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Prevention of Segregation

y Good quality cement helps much in reducing the effect of segregation.y Segregation can be reduced by proper proportioning and uniform and

complete mixing.

y Using of standard quality aggregates also responsible in reducing thesegregation.

y Proper mixing of water in proper proportion can also avoids the segregation.


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BLEEDING

Bleeding is sometimes known as gain of water. It is a particular form ofsegregation in which some of the water comes out of the surface of the

concrete being of the lowest specific gravity among all the ingredients of

concrete.

Bleeding is seen in a highly wet mix, badly proportioned and insufficientlymixed concrete.

In thin members like roof slab or road slabs and when concrete is placed insunny weather shoe excessive bleeding.

Due to the excessive bleeding the water and cement comes to top and theaggregates goes down .This formation of the cement on the top is known asLaitance in such a case, the top surface of the slabs and the pavements will

not have good wearing quality.

This Laitance formed on the road produces dust in summer and mud in rainyseason.

Water while traversing from bottom to top makes continuous channels. If thewater cement ratio used is more than 0.7, the bleeding channels will remain

continuous and un segmented. These continuous bleeding channels are often

responsible for causing permeability of the concrete structures.While mixing water is in the process of coming up, it may be intercepted by

aggregates.

The bleeding water is likely to accumulate below the aggregates. Thisaccumulation of water creates water voids and reduces the bond between the

aggregates and the paste.

Bleeding rate increases with time up to about one hour or so and thereafter therate decreases but continuous more or less till the final setting time of cement.

Bleeding is not completely harmful if the rate of evaporation of water fromthe surface is equal to the rate of bleeding.


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Prevention of bleeding

y Bleeding can be reduced by proper proportioning and uniform and completemixing.

y Use of finely divided pozzolanic material reduces bleeding by creating alonger path for water to traverse.

y Air entraining agent is very effective in reducing the bleeding.y Bleeding can be reduced by using finer cement or cement with low alkali

content.


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SETTING TIME

The setting time of concrete is the time taken by the concrete to set to thehardened state. This state comes in action after the plastic state.

Setting time of the concrete depends upon the w/c ratio, temperatureconditions, and type of cement, use of mineral admixtures, and use of

plasticizers.

The setting time of the concrete is more of practical significance for siteengineers than setting time of cement.

When retarding plasticizers are used, the increase in setting time, the durationup to which concrete remains in plastic condition is of special interest.

Factors affecting setting:

Following are the factors that affect the setting of concrete.

1. Water Cement ratio

2. Suitable temperature

3. Cement content

4. Type of Cement

5. Fineness of Cement

6. Relative Humidity

7. Admixtures

8. Type and amount of Aggregate

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