SEGREGATION AND BLEEDING 1 SEGREGATION The separation of

SEGREGATION AND BLEEDING 1

SEGREGATION The separation of coarse aggregate from the concrete mix in plastic stage is called segregation. 2

Concrete is not a homogenous material but rather a mixture of materials of different specific gravities. So there is always a tendency for the coarser and heavier particles to settle down and for lighter materials to rise to the surface. Segregation reduces the strength and durability of concrete. 3

Causes of segregation. Segregation takes place when: (i) there is too much of water in the mix. (ii) there are badly graded aggregates. (iii) leakage of mortar from formwork. (v) concreting is done under-water. 4

Prevention of segregation. (i) The mix should be designed correctly and minimum amount of water should be used for mixing. (ii) The height of free fall of concrete should not exceed 1 m in any case. (iii) The air-entraining agents should be used for reducing segregation as these reduce the quantity of mixing water. . 5

BLEEDING The appearance of water along with some particles of cement and very fine sand on the surface of freshly placed concrete is called bleeding. A good concrete should be free from bleeding. The term water gain is also sometimes used instead of bleeding when water rises to the surface, flow channels are formed in concrete mass.

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Causes of bleeding. (i) presence of excess water. (ii) Deficiency of fine aggregate. (iii) Too much finishing. Prevention of bleeding can be prevented by: (i) controlling the water- content ratio (ii) providing finer grading of fine aggregates. (iii) using finely ground cement. (iv) The air-entraining agents should be used to prevent bleeding. 8

MIXING AND VIBRATION IN CONCRETE 9

CONCRETE MIXER A concrete mixer (also commonly called a cement mixer) is a device that homogeneously combines cement, aggregate such as sand or gravel, and water to form concrete. A typical concrete mixer uses a revolving drum to mix the components. For smaller volume works portable concrete mixers are often used so that the concrete can be made at the construction site, giving the workers ample time to use the concrete before it hardens. An alternative to a machine is mixing concrete by hand. This is usually done in a wheelbarrow. 10

CONCRETE MIXING MACHINE 11

VIBRATION IN CONCRETE ► When a bucket of low slump concrete is dumped into a form, the concrete is in a honeycombed condition. From 10 to 30 percent of the concrete is in the form of irregularly distributed entrapped air depending on the mix, size and shape of the form, amount of reinforcing steel, and method of depositing the concrete in the form. Contractors know they must overfill a form because concrete should shrink when it is compacted. 12

HOW VIBRATOR WORKS A vibrator causes a violent agitation of the particles in the mix. This eliminates the friction between the particles which enabled the concrete to support itself in a honeycombed condition. The mix now becomes unstable and starts to flow under the simultaneous effect of vibration and gravity. The concrete moves tightly against the form and around the reinforcing steel. The mix particles rearrange themselves. The mortar fills the voids between the coarse aggregate particles, and the entrapped air rises to the top of the layer. 13

TYPES OF VIBRATORS There are two basic ways to vibrate concrete: internally and externally. In internal vibration, the vibrator is immersed in the interior of the concrete and the vibratory waves carry outward to the surfaces. In external vibration, the vibration is applied to the surfaces and the vibratory waves penetrate the interior. On any construction job the vibrators should match reasonably well the concrete mix being vibrated. 14

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Thanking you 17

By: - Tarundeep Singh 140378 D 2 C 3 2 nd Year ( b-tech. C. E) 18