presentation properties fresh concrete
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DAMTRANSCRIPT
PowerPoint Presentation*
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PROPERTIES OF FRESH CONCRETE
This presentation contains certain forward looking statements concerning future business prospects and profitability of HCC, which are subject to a number of risks and uncertainties and actual results could materially differ from those in such forward looking statements, important developments that could affect the company's operations include a downtrend in the infrastructure sector, significant changes in political and economic environment in India, tax laws, labor relations, litigation etc. The company does not undertake to make any announcement in case any of these forward looking statements become materially incorrect in future or update any forward looking statements made from time to time by or on behalf of the company.
DISCLAIMER
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Concrete
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Mixing of concrete
It is essential that the mix ingredients are mixed properly so as to produce fresh concrete in which all the aggregate particles is coated with cement paste and therefore possessing uniform properties
Types of mixers:
Mixing of concrete (contd.)
Mixing time for producing a good mix in a batching plant generally varies between 30 to 90 secs
Lesser mixing time than recommended will not produce a uniform mix, but prolonged mixing time than required will have no significant improvement
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Workability
Generally implies the ease with which a concrete mix can be handled from mixer to it’s finally compacted shape
Consistency – fluidity
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Method
Concrete put in cone in 4 layer, each layer tamped 25 times
Not suitable for dry mixes
Top struck off and cone carefully lifted off
Slump measured
Mixed concrete put in top hopper
Allowed to fall into 2nd hopper then cylinder
Suitable for all mixes
Method
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Other Tests
Vebe Test – time for standard cone to be compacted flat by glass plate on vibrating table eg., for workable concrete the Vebe time = approx. 3 sec
Flow Test – the measured spread in mm of a standard cone on a dropping table (40mm, 15 times)
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75
25
75
25
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Segregation
Segregation is defined as separation of the constituents of a heterogeneous mixture so that their distribution is no longer uniform
It is a tendency for
sand-cement mortar to separate from coarse aggregates
Cement mortar to separate from fine aggregate
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Poor grading
High workability
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Tendency of water to rise to the surface
This will cause weakness or dustiness of the surface of finished concrete or a line of weakness between pours
Bleeding is affected largely by the properties of cement. It can be avoided by
A finer cement
Using air-entraining admixture
Proper grading of aggregates is required to get desired workability
Main factors governing desired grading are:
Surface area of aggregates which determine the amount of water necessary to wet all the solids
Relative volume occupied by the aggregates
Workability of mix and tendency to segregation
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Maximum strength of the placed concrete
Maximum durability
Maximum density
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Vibration of concrete
The object in vibrating the concrete is to mobilise it sufficiently, so that it becomes plastic enough to enable air voids to be removed and aggregate particles to gravitate together to from a homogeneous mass
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Heat of Hydration
The exothermic reaction during setting can cause a significant temperature rise in large concrete pours
This causes expansion, then setting, then contraction
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Heat of Hydration (contd.)
The temp. of concrete has to be lowered so that the temp. differential between surface and core of concrete is not high
Methods to cool concrete
Use of chilled water
Air content in Fresh Concrete
Mixing and handling can significantly alter the air content of fresh concrete
Various test methods:
It is the most widely used method
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Page *
Curing
If left in contact with water, concrete will continue to gain strength for many months
Otherwise all free water evaporates or is used up in the hydration process and no further hydration can continue
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Membrane curing using curing compound
Application of heat (steam curing)
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Self Compacting Concrete
Self-compacting concrete is the concrete that is able to flow under its own weight and completely fill the formwork, even in the presence of dense reinforcement, without the need of any vibration, while maintaining homogeneity
No vibration needed
Slump flow
PROPERTIES OF FRESH CONCRETE
*
PROPERTIES OF FRESH CONCRETE
This presentation contains certain forward looking statements concerning future business prospects and profitability of HCC, which are subject to a number of risks and uncertainties and actual results could materially differ from those in such forward looking statements, important developments that could affect the company's operations include a downtrend in the infrastructure sector, significant changes in political and economic environment in India, tax laws, labor relations, litigation etc. The company does not undertake to make any announcement in case any of these forward looking statements become materially incorrect in future or update any forward looking statements made from time to time by or on behalf of the company.
DISCLAIMER
*
*
*
*
*
*
*
*
Page *
Concrete
*
Mixing of concrete
It is essential that the mix ingredients are mixed properly so as to produce fresh concrete in which all the aggregate particles is coated with cement paste and therefore possessing uniform properties
Types of mixers:
Mixing of concrete (contd.)
Mixing time for producing a good mix in a batching plant generally varies between 30 to 90 secs
Lesser mixing time than recommended will not produce a uniform mix, but prolonged mixing time than required will have no significant improvement
*
Page *
Workability
Generally implies the ease with which a concrete mix can be handled from mixer to it’s finally compacted shape
Consistency – fluidity
*
*
Method
Concrete put in cone in 4 layer, each layer tamped 25 times
Not suitable for dry mixes
Top struck off and cone carefully lifted off
Slump measured
Mixed concrete put in top hopper
Allowed to fall into 2nd hopper then cylinder
Suitable for all mixes
Method
*
Other Tests
Vebe Test – time for standard cone to be compacted flat by glass plate on vibrating table eg., for workable concrete the Vebe time = approx. 3 sec
Flow Test – the measured spread in mm of a standard cone on a dropping table (40mm, 15 times)
*
*
75
25
75
25
Page *
Segregation
Segregation is defined as separation of the constituents of a heterogeneous mixture so that their distribution is no longer uniform
It is a tendency for
sand-cement mortar to separate from coarse aggregates
Cement mortar to separate from fine aggregate
*
Poor grading
High workability
*
Tendency of water to rise to the surface
This will cause weakness or dustiness of the surface of finished concrete or a line of weakness between pours
Bleeding is affected largely by the properties of cement. It can be avoided by
A finer cement
Using air-entraining admixture
Proper grading of aggregates is required to get desired workability
Main factors governing desired grading are:
Surface area of aggregates which determine the amount of water necessary to wet all the solids
Relative volume occupied by the aggregates
Workability of mix and tendency to segregation
*
*
Maximum strength of the placed concrete
Maximum durability
Maximum density
*
Vibration of concrete
The object in vibrating the concrete is to mobilise it sufficiently, so that it becomes plastic enough to enable air voids to be removed and aggregate particles to gravitate together to from a homogeneous mass
*
Heat of Hydration
The exothermic reaction during setting can cause a significant temperature rise in large concrete pours
This causes expansion, then setting, then contraction
*
Heat of Hydration (contd.)
The temp. of concrete has to be lowered so that the temp. differential between surface and core of concrete is not high
Methods to cool concrete
Use of chilled water
Air content in Fresh Concrete
Mixing and handling can significantly alter the air content of fresh concrete
Various test methods:
It is the most widely used method
*
Page *
Curing
If left in contact with water, concrete will continue to gain strength for many months
Otherwise all free water evaporates or is used up in the hydration process and no further hydration can continue
*
Membrane curing using curing compound
Application of heat (steam curing)
*
Self Compacting Concrete
Self-compacting concrete is the concrete that is able to flow under its own weight and completely fill the formwork, even in the presence of dense reinforcement, without the need of any vibration, while maintaining homogeneity
No vibration needed
Slump flow
PROPERTIES OF FRESH CONCRETE