Bleeding

-Some water from the concrete comes out to surface being of the lowest Specific gravity among all the ingredients

-Usually observed in highly wet mixes, badly proportioned mixes, and improperly mixed concrete

-Water Comes up & accumulates at surface

-Sometimes cement also comes to the surface

-The aggregates goes down

-Formation of cement paste at the surface is Laitance

-Bleeding won't be harmful if rate of evaporation of water from surface is equal to the rate of bleeding

-Removal of water after it had played its role in providing workability from body of concrete by way of bleeding will do good to the concrete

Segregation

-Separation of constituents of the heterogeneous mixture

-Difference in size of particles are causes of segregation

-Suitable grading & by care in handling minimizes segregation

-Two forms of segregation

-1.Coarser particles tends to separate out

-2.Separation of Grout

Hand Mixing Of Concrete

-Cement is then spread over the aggregates

-Dry Materials are mixed by turning over from one end of the try to the other (turning 3 times is usually required)

-Water is slowly added so that neither water by itself nor with cement can escape

-Mix is turned over again usually 3 times until it appears uniform in color & consistency

Mixing Time

-Often a tendency to mix concrete as rapidly as possible

-Mixing time should be enough to get concrete uniform in composition and satisfactory strength

-It is not the mixing time but the number of revolutions of mixer that is the criteria of adequate mixing

-Generally about 20 revolutions are sufficient

Pan Mixers

-Consists of circular pan rotating about its axis with 1or2 stars of pedal rotating about a vertical axis

-Relative movement between the pedal & the concrete is same

-Concrete in every part of the pan is thoroughly mixed

-Scraper blades prevent concreting sticking to the sides of the pan & height of the pedals can be adjusted to prevent the coating of concrete at the bottom of the pan

Non-Tilting Mixers

-Axis of the mixer is always horizontal

-Discharge is obtained by inserting a chute or by reversing the direction of the drum or rarely by splitting of drum

-Slow rate of discharge can cause segregation of concrete

-Larger size of aggregate tend to stay in the mixer & the discharge sometimes starts as a mortar and finishes as a collection of coated coarse aggregate particles

Tilting Mixers

-Mixing chamber known as the drum is tilted for discharging

-Usually bowl shaped drum with veins inside

-Discharge is good because concrete can be tipped out rapidly as an unsegregated mass as soon as the drum is tilted

-Preferable for low workability and for large size aggregates

General Types Of Mixers

-Tilting Mixers

-Non-Tilting Mixers

-Pan Mixers

Concrete Mixers

-Achieve uniformity of the mix

-Should discharge the mix without disturbing that uniformity

-Method of discharging is one of the basis of classification of concrete mixers

Mixing Of Concrete

-Ingredients should be properly mixed

-Surface of All Aggregate Particle is coated with cement paste & which is homogeneous and macro scale and therefore possessing uniform properties

Consistency & Slump

-The concrete should flow freely around corners & between the reinforcing bars

-Slump test is used to determine the consistency of concrete



 Recommended Slumps :

Concrete Construction	Maximum inches(mm)	Minimum inches(mm)
RCC Fdn. Walls & Footings	3(76)	1(25)
Plain Footing & Substructure Walls	3(76)	1(25)
Beams & Reinforced walls	4(102)	1(25)
Building Columns	4(102)	1(25)
Pavement & Slabs	3(76)	1(25)
Mass Concrete	2(51)	1(25)

Types Of Slump